MS pathogenesis seems to involve both genetic susceptibility and environmental risk factors. Three sequential factors are implicated in the environmental risk. The first acts near birth, the second acts during childhood, and the third acts long thereafter. Two candidate factors (vitamin D deficiency and Epstein-Barr viral infection) seem well suited to the first two environmental events.
A mathematical Model for MS pathogenesis is developed, incorporating these environmental and genetic factors into a causal scheme that can explain some of the recent changes in MS-epidemiology (e.g., increasing disease prevalence, a changing sex-ratio, and regional variations in monozygotic twin concordance rates).
This Model suggests that genetic susceptibility is overwhelmingly the most important determinant of MS pathogenesis. Indeed, over 99% of individuals seem genetically incapable of developing MS, regardless of what environmental exposures they experience. Nevertheless, the contribution of specific genes to MS-susceptibility seems only modest. Thus, despite HLA DRB1*1501 being the most consistently identified genetic marker of MS-susceptibility (being present in over 50% of northern MS patient populations), only about 1% of individuals with this allele are even genetically susceptible to getting MS. Moreover, because genetic susceptibility seems so similar throughout North America and Europe, environmental differences principally determine the regional variations in disease characteristics. Additionally, despite 75% of MS-patients being women, men are 60% more likely to be genetically-susceptible than women. Also, men develop MS at lower levels of environmental exposure than women. Nevertheless, women are more responsive to the recent changes in environmental-exposure (whatever these have been). This explains both the changing sex-ratio and the increasing disease prevalence (which has increased by a minimum of 32% in Canada over the past 35 years). As noted, environmental risk seems to result from three sequential components of environmental exposure. The potential importance of this Model for MS pathogenesis is that, if correct, a therapeutic strategy, designed to interrupt one or more of these sequential factors, has the potential to markedly reduce or eliminate disease prevalence in the future.
Citation: Goodin DS (2009) The Causal Cascade to Multiple Sclerosis: A Model for MS Pathogenesis. PLoS ONE 4(2): e4565. https://doi.org/10.1371/journal.pone.0004565
Editor: Enrico Scalas, University of East Piedmont, Italy
Received: September 30, 2008; Accepted: December 22, 2008; Published: February 26, 2009
Copyright: © 2009 Goodin. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: The author has no support or funding to report.
Competing interests: The author has declared that no competing interests exist.
Human diseases such as multiple sclerosis (MS) are often chronic and have complex etiological bases . Both the genetic background and environmental events are critical. For example, an individual from northern Europe or northern North America has a life-time MS risk of about 1–2 per 1,000 population . Risk for individuals with an affected family member increases in proportion to the shared genetic information between themselves and the proband –. Monozygotic-twins of an MS proband have approximately 200 times the risk in the general population –. Nevertheless, it is clear that genes are not the only disease determinants. Otherwise, the proband-wise concordance-rate for monozygotic-twins would be closer to 100% than the reported 20–30% in these northern populations , –. This conclusion is even more apparent in southern populations where the concordance-rate for monozygotic-twins is approximately half that in the north –. Consequently, that there must be important environmental or epigenetic factors involved in MS pathogenesis. Although the present paper explores, through a mathematical Model, the relationship of genetic predisposition and environmental exposure to recent changes in MS epidemiology, in many respects, the Model is also applicable to other complex human diseases.
The Nature of the Environmental Exposure
Important observations regarding MS pathogenesis relate to the absence of micro-environmental contributions to MS risk. Thus, studies in adopted individuals, conjugal couples, brothers and sisters of different birth order, and in siblings and half-siblings raised together or apart, have demonstrated that micro-environmental influences don't contribute measurably to MS risk , , –. Consequently, the relevant environmental events must act at a population level. Moreover, if, in addition to a genetic predisposition, one or more population level environmental events need to occur in order for MS to develop, then it is only natural to enquire as to how many such events are necessary and whether it is important for these events to occur at any particular time or in any particular order. Three sets of epidemiological observations bear on these issues.
A Very Early Environmental Influence.
The first set relates to a maternal effect in MS . This maternal effect is supported by three epidemiological observations. The first is that half-siblings, who are concordant for MS, are twice as likely to share the mother compared to the father , , indicating that MS susceptibility is being passed from mother to child through something other than nuclear genes. An environmental exposure, occurring either in the intrauterine or in the early post-natal period, is one possibility. After the child becomes independent of their mother, however, such a maternal effect would be unexpected from an environmental factor.
This maternal effect, however, need not be environmental. It could also be due to mitochondrial genes, from genetic-imprinting favoring expression of certain maternal genes, or from other epigenetic factors . With respect to these other possibilities, there has been an interesting debate about the existence of a so-called “Carter effect” in MS, whereby paternal transmission would be favored for certain genetic traits , . One report found weak evidence (p = 0.032) for preferential paternal transmission of MS risk  whereas a larger Canadian study found no such effect . Importantly, however, neither study showed the preferential maternal transmission expected if mitochondrial genes, genetic imprinting, or epigenetic factors explained the maternal effect in MS . An environmental cause for the maternal effect, by contrast, would not be observed in these studies because the maternal intra-uterine and post-natal environments are the same regardless of which parent transmits MS risk.
The second observation is that the MS concordance-rate for fraternal-twins seems to be greater than that for full-siblings , . Thus, in the large Canadian cohort , the MS concordance-rate in full-siblings was reported to be 2.9% with a standard-error (SE) of 0.6%, compared to the concordance-rate in dizygotic-twins, which was 5.4% (no statistical comparison provided). A recent review  reached the same conclusion. Such a disparity cannot be explained by mitochondrial inheritance, genetic imprinting, or epigenetic factors because, on average, these should be similar for siblings and fraternal twins sharing the same biological parents. Rather, this discrepancy must be due to environmental factors acting during the shared intra-uterine or early post-natal period.
The third observation relates to the month-of-birth effect in MS reported in Canada and northern Europe –. In a study combining patients from Canada, Denmark, and Sweden, significantly more MS patients were born in May and fewer in November, compared to background rates during the rest of the year . Another study found more RRMS patients born in May than November . Finally, in 67 Canadian patients, born in the southern hemisphere, this month-of-birth effect seemed reversed . Whether such a reversal is generally found in the southern hemisphere must await further study.
These observed month-of-birth effects provide clear evidence for an early environmental influence, involved in MS pathogenesis, time-locked to birth. As the time interval between the birth and any environmental event increases, the coupling between birth and the event should become increasingly less precise and, thus, the birth-signal should become increasingly less distinct. The fact that this signal is so clear , suggests that the relevant environmental event occurs very near to the birth itself. Moreover, this environmental event is periodic and appears coupled to the solar cycle . Of possible relevance to this circa annum periodicity for MS susceptibility, is the fact that mothers of May babies (who spend much of their intrauterine life during the winter months) will experience less sun exposure during their pregnancies compared to mothers of November babies. It is tempting to speculate (and has been speculated), therefore, that this maternal environmental event is related to a low maternal sun-exposure (perhaps resulting in low levels of vitamin D) while the child is in utero , .
Childhood/Adolescent Environmental Influences.
The second set of observations relates to an environmental influence in persons who migrate from one region to another , –. When individuals move (prior to approximately age 15 years) from a high MS prevalence area to a low prevalence region (or vice versa), their MS risk is similar to that of the region to which they moved. By contrast, when they make the same move after age 15, their MS risk is similar to that of the region from which they moved. Similarly, children of immigrants from low prevalence regions (born in high-prevalence areas) have an MS risk similar to their birth country rather than their country of ethnic origin . These observations implicate an environmental event, involved in MS pathogenesis, which acts sometime between birth and young adulthood (∼15 years) but does not act thereafter.
Adult Environmental Influences.
The third set of observations relate to the fact that the initial clinical symptoms in MS are typically delayed by many years (often decades) after this critical early-period when the maternal factor and the migratory factor operate , –. Perhaps, only these critical early events are necessary although the apparently low MS penetrance is difficult to explain, at least with respect to some candidate environmental factors. Thus, despite the fact that Epstein-Barr viral (EBV) infection, especially when it causes symptomatic mononucleosis, has been consistently linked to MS –, fewer than one in 900 individuals with an EBV infection and only a small fraction of patients with mononucleosis will ever develop MS (Table 1). Consequently, it seems most likely that subsequent environmental events dictate the timing of symptom onset.
Causal Factors in MS Pathogenesis.
Many factors, including small pox, typhoid, EBV, other infections, vitamin deficiencies, low-sunlight, cosmic-rays, occupational hazards, living with domesticated animals, dietary habits, trauma, stress, and toxic exposures have all been postulated to be linked to MS pathogenesis . Of these possibilities, however, vitamin D deficiency and EBV infection have been gaining the greatest support for their potential role in MS pathogenesis. Moreover, each of these factors seem particularly well suited to the epidemiological scheme outlined above.
The in vivo production of active vitamin D in humans requires the conversion of 7-dehydro-cholesterol into vitamin D3 –. The first of this two-step transformation (to pre-vitamin D3) is catalyzed by the exposure of 7-dehydro-cholesterol in the skin to ultraviolet B (UVB) radiation (i.e., electromagnetic radiation having a wavelength of 280–320 nm). Subsequently, stable Vitamin D3 is formed by an internal rearrangement of the double bond structure of the pre-vitamin D3 molecule. Vitamin D3 is then hydroxylated, firstly to 25(OH) D3 by 25-hydroxilase (mostly in the liver) and secondly by 1-α hydroxilase in the tissues (including the kidney) to form active vitamin D (1,25(OH)2D3). Dietary intake of vitamin D3 can bypass the UVB-dependent part of this pathway and permits maintenance of normal vitamin D serum levels, even in the absence of adequate UVB radiation. Unfortunately, however, there are few natural dietary sources of vitamin D3. It is only found in certain animals  such as oily fishes and reindeer, which derive it or its precursors from their diet (from phytoplanctonic algae in the case of fish and from lichen in the case of reindeer). Interestingly, two human populations with a notably low risk for MS – are the Inuit or Eskimos (who consume oily fish on a daily basis) and the Sami or Lapps (for whom reindeer meat is a staple). In both of these populations, their main source of active vitamin D comes from their diet . For many other human populations, by contrast, exposure of the skin to the UVB radiation is of paramount importance for maintaining adequate active vitamin D levels throughout the year.
In this context, it is of considerable interest that, as latitude increases (both north and south of the equator), the amount of effective UVB radiation decreases due to an increase in the angle of the sunlight reaching the planet surface (i.e., traveling a longer distance through the Earth's atmosphere). In central and north-east Africa (where Homo sapiens evolved) plenty of UVB radiation is available for vitamin D synthesis throughout the year –. However, when humans migrated out of Africa approximately 50,000 years ago and began to inhabit the temperate regions of the globe, adequate UVB exposure throughout the year was no longer guaranteed. In fact, it has been estimated that the levels of UVB radiation at the latitude of the US-Canadian border are insufficient to produce an adequate amount of active vitamin D during most months of the year, especially in winter –. Moreover, as has been noted by many authors and consistent with a role for vitamin D deficiency in MS pathogenesis, world-wide distribution maps of reduced UVB availability are strikingly similar to world-wide distribution maps of MS prevalence , , , , .
Vitamin D acts in the body through binding to its receptor (VDR). Once such binding takes place, this ligand-receptor complex is internalized, forms a heterodimer with the retinoid X receptor (the nuclear receptor for 9-cis retinoic acid), and this heterodimer is translocated to the nucleus where it acts as a transcription factor, binding, often together with co-factors, to vitamin D response elements (VDREs) in the promoter region of several nuclear genes . Interestingly, and perhaps very informatively, VDRE has recently been identified in the promoter region adjacent to the HLA DRB1*1501 allele, an allele and a portion of the genome that has been consistently linked to MS pathogenesis in Caucasian populations , . The binding of vitamin D to the VDRE regulates the expression of multiple genes, acting either positively or negatively depending upon the specific gene involved.
The roles of vitamin D in calcium homeostasis and the maintenance of bone health are widely recognized , . Less well known is its role in other processes, including anti-neoplastic actions and a variety of immune functions such as cell proliferation, differentiation, and immunomodulation , , –. Nevertheless, VDR is widely expressed throughout the body, including on activated T and B lymphocytes, and on macrophages , ,  and vitamin D has been implicated in the maturation of dendritic cells and in the modulation of antigen-specific immune responses in vivo –. During gestation, human decidual cells synthesize active vitamin D, particularly in early pregnancy, and this has led to the suggestion that vitamin D may help to regulate both acquired and innate immune responses at the fetal-maternal interface . Finally, vitamin D deficiency has been implicated in the likelihood of developing a variety of autoimmune diseases including insulin dependent diabetes mellitus, rheumatoid arthritis, experimental autoimmune encephalomyelitis (EAE), and inflammatory bowel disease , , . Because these autoimmune diseases are more prevalent in females, this raises the possibility that there might be differences in the physiological responses to vitamin D between males and females and, in fact, such gender-specific differences in vitamin D metabolism have been reported , . Interestingly, in one of these studies  vitamin D supplementation was found to confer protection against EAE only in intact female mice but not in males or in ovariectomized females.
As a result of these considerations, vitamin D deficiency seems to be a good candidate for the early (maternal) event noted above. Not only is this maternal factor (like vitamin D) coupled to the solar cycle in temperate regions , vitamin D is known to be involved in immune development and maturation , , –, its deficiency is associated with autoimmunity , , , the world-wide distribution of reduced UVB radiation mirrors that of MS prevalence , –, extreme northern populations with high dietary intake of vitamin D such as the Inuit and Sami people have a very low MS prevalence –, and there are interactions between the physiological effects of vitamin D and gender in some mammals , . This last aspect of vitamin D physiology might provide insight to why MS prevalence is gender-specific and, perhaps, also, to why MS incidence is increasing, especially among women –.
EBV is a member of the herpes family of double-stranded linear DNA viruses, which is also referred to as the human herpes virus 4 (or HHV-4). It is one of the most common viral infections of humans, with over 90% of individuals becoming infected at some time during their life . As maternal antibody protection disappears after birth, infants become susceptible to infection by EBV. In many parts of the world the initial EBV infection typically occurs during early childhood and is either asymptomatic or causes only mild symptoms indistinguishable from numerous other childhood infections. In developed regions such as the North America and Europe, however, the initial infection is often delayed until adolescence or young adulthood (possibly related to better hygiene), in which case infectious mononucleosis (glandular fever) develops in 35 to 50% of instances. The principal targets of the initial viral infection are the epithelial cells of the oropharynx and the B-lymphocytes.
Once a cell is infected, the viral genome becomes circularized and persists within the cell as an episome, executing distinct genetic programs, which result in either a lytic or a latent infection. Lytic infections produce a large number of free viral particles, which then can infect other B lymphocytes. Latent infection ultimately predominates (probably due to the host immune response) and the latent phase genetic programs cause the infected B-lymphocytes to proliferate and to be directed to the sites (e.g., the bone marrow) where the virus persists indefinitely. Such persistence is made possible by the virus turning off most of its genes, only occasionally to be reactivated, to produce fresh virions, and to cause further cell lysis.
With an acute infection, antibodies to antigens associated with viral replication, the viral capsid antigen (VCA) and the diffuse and restricted early antigens (EA), appear during either the late incubation period or the acute illness . Antibodies to the VCA are initially of the IgM class. However, this response lasts only 1–2 months, after which the anti-VCA antibody response shifts to the IgG class. These antibodies persist for the lifetime of the individual. Antibodies to EA are of the IgG class and often drop to undetectable levels after 3–6 months. Nevertheless, although EA antibodies are a sign of active infection, approximately 20–30% of individuals will have persistent titers for years and these antibodies are also found in patients with chronic active infections or with secondary complications such as Burkitt's lymphoma or nasopharyngeal carcinoma. Antibodies to one or more of the EBV nuclear antigens (EBNA 1 to 5) that are expressed in latently infected B lymphocytes, appear 3–6 weeks after the initial infection. These antibodies persist for the lifetime of the individual. Other than MS, EBV infection has also been implicated as playing some role in the pathogenesis of certain malignancies (e.g., nasopharyngeal carcinoma, EBV-positive Hodgkin lymphoma, and Burkitt's lymphoma) as well as in several autoimmune disorders such as Sjogren's syndrome, rheumatoid arthritis, and systemic lupus erythematosus . In these conditions, persistent titers to EBV are also found, suggesting that chronic immune activation against EBV may actually be participating in the pathophysiological process .
In contrast to vitamin D, however, EBV cannot be the maternal factor because infection does not typically occur either in utero or during the early post-partum period. Rather, EBV seems a much better candidate for the second environmental event. Nevertheless, vitamin D deficiency could also act during childhood and, indeed, some of the direct data supporting a role of vitamin D in MS pathogenesis actually suggests an effect in childhood or early adulthood –. Despite this, however, the evidence for EBV involvement in MS pathogenesis is compelling , , –, . Thus, even though EBV infection occurs in over 90% of populations matched for age and sex with MS patients –, –, the evidence for prior EBV infection in adult onset MS is essentially 100% and significantly more likely than controls (Table 1). Even in those rare MS patients who test negatively for prior EBV exposure, this result could easily be a false negative finding because, in every such case, the antibody response hasn't been measured to the entire set of EBV antigens (Table 1). Also, the prior nature of the EBV infection seems clear both by the presence of IgG (not IgM) antibodies to EBV antigens and by the unequivocal evidence (when it has been possible to measure) of infection long before the advent of clinical MS symptoms –, , , –. Such a strong association is very hard to ignore.
Although this apparent high prevalence of prior infection in MS could conceivably be the consequence of either false negative tests within the general population or false positive tests in MS patients, both possibilities seem unlikely. Thus, the near 100% prevalence cannot be due to a general hyper-immune state in MS patients because their antibody responses to other common pathogens (e.g., cytomegalovirus, measles, mumps, chicken pox, herpes simplex, etc.) are not similarly increased –, , . Moreover, a recent pathological study found evidence of EBV infection in a substantial proportion of those B lymphocytes infiltrating the central nervous system in 21 of 22 MS cases examined at postmortem . In addition, evidence of viral reactivation seemed to be restricted to ectopic B-cell follicles in the meninges and in acute lesions . Finally, the increased risk of MS, either with delayed exposure to EBV , , ,  or following symptomatic mononucleosis , , , , strongly suggests that the association between MS and this particular pathogen is genuine. Taken at face value, this near 100% association indicates that EBV is a necessary (but not a sufficient) condition for adult MS to develop and, therefore, that EBV must be a part of the causal pathway leading to MS. If EBV infection is permissive in this way then, like the second environmental factor in MS pathogenesis, it probably acts during childhood or adolescence (when both late infection and mononucleosis occur). There are no leading candidates for the third (or other) environmental factor or factors.
Changing Environmental Influences.
It is also of note that MS epidemiology has changed in important ways over the past several decades. Thus, the incidence (prevalence) of MS is increasing –, –, especially in women –. As a consequence of this, the sex-ratio has been altered  and, recently, a switch in the latitude gradient for MS incidence has been reported . Because MS genetics seems unlikely to have shifted in so short an interval, these observations presumably relate to a change in the environmental determinants of MS. Although many wide-spread environmental changes are known to be taking place (e.g., increasing atmospheric concentrations of CO2, CH4, and other pollutants; increasing global temperatures; a depletion of stratospheric ozone; a greater dietary consumption of trans-fats, etc.), one recent change (potentially relevant to the possible role of vitamin D deficiency) is that people are increasingly encouraged to avoid prolonged sun-exposure and to use sun-block to prevent skin cancer . Nevertheless, sun-block with sun-protective-factor (SPF)-15 blocks approximately 94% of the incoming UVB radiation and higher SPF levels block even more . As a result, any wide-spread use of sun-block and sun-avoidance will exacerbate any population deficiency of vitamin D synthesis and, presumably, will increase the likelihood of diseases related to vitamin D deficiency. By contrast, the pattern of EBV infection seems to have changed little over this interval .
Results and Methods
The Causal Model
Thus, there seems to be clear epidemiological evidence for at least three distinct environmental events contributing to MS pathogenesis. Consequently, it is unnecessary to choose between the vitamin D and the EBV hypotheses. They may both be correct. Nevertheless, even if these two environmental events are part of a pathway to adult MS, they may not be on the same or the only pathway. Indeed, assuming that each is part of some causal pathway, there are several possible arrangements for how these environmental-events might produce MS (Figure 1). No pathway can be excluded entirely although, if prior EBV infection is necessary for adult MS (Table 1), then pathways 1 and 2 (Figure 1) must occur rarely, if at all. Similarly, if pathway 4 were the major pathway, an observable maternal effect would not be anticipated. Consequently, only pathway 3 (implying sequential environmental events) seems to form a necessary part of causal cascade to adult MS. The first two events may be an appropriately-timed vitamin D deficiency and an appropriately-timed EBV infection (as in the Figure). However, these particular associations with the first and second environmental events are not a necessary component of the Model and, if it turns out that these two factors are not relevant, then two suitable alternatives would simply need to be substituted into the equations without any alteration of the Model itself.
The other environmental factors along the different pathways to MS have been designated O1–O4 because these relevant (but unknown) factors need not be the same along each path. In the Figure, the first two environmental events are represented as an appropriately‐timed vitamin D deficiency and an appropriately‐timed EBV infection. However, the association of these specific events with the first and second environmental events (discussed in the text) is not a necessary part of the Model itself.
Definitions for the terms used in the Model are presented in Table 2. The life-time probability of getting MS (PMS) will be equated with the prevalence of MS in the general population (Table 2). This is an approximation, which is accurate only if disease incidence is unchanging, mortality is unaffected, the population size is stable, and the diagnostic sensitivity (over the entire life-span) is unchanged. None of these conditions pertain exactly. Nevertheless, this assumption seems reasonable as a rough approximation, especially because the error introduced by increased mortality (in the estimate for individuals currently aged 35–40 years) will be opposite in direction from the others. The probability of genetic susceptibility (PG) will refer to the probability of an individual possessing any of possibly several “susceptible” genotypes and will refer to the susceptibility conferred by genetic determinants present at conception. Alterations of genetic expression subsequently will be considered environmental events although the genotypes that permit such alterations to occur would still be included within the group of “susceptible” genotypes. Because mitochondrial genes linked to MS have not been found  and because, as discussed above, preferential maternal transmission has not been observed , , genetic susceptibility will be attributed to nuclear genes.
Initially, the probability (PE) will be used to describe the combined occurrence of an entire set of environmental events (taking place at appropriate times) that could cause MS in, at least, some genetic backgrounds. Such an occurrence will be referred to as an individual experiencing a “sufficient” set of environmental events. Here, it is understood that it is possible that every “sufficient” set of environmental events may not be “sufficient” for every “susceptible” genotype. However, the set of environmental events does need to be “sufficient” for, at least, one such genotype. Under these conditions, the probability that any individual will develop MS is described by the equation:(1)where (PE|G) is the conditional probability of an individual experiencing a “sufficient” set of environmental events given that they are genetically susceptible to MS. The last term (PMS|G, E) is the conditional probability of MS developing in an individual having both an appropriate genetic make-up and experiencing a “sufficient” set of environmental-events. As such, this last term allows for different probabilities of developing MS in persons with different combinations of “susceptible” genotypes and “sufficient” environmental exposures. It also allows for a purely stochastic factor, in which only a fraction of individuals will actually develop MS, even when they are genetically susceptible and when they experience an environmental exposure “sufficient” for MS to develop given their particular genotype. Importantly, however, Equation (1) is neither speculative nor controversial and it does not limit the environmental or genetic possibilities. It is simply the definitional statement for these various conditional probabilities.
Nevertheless, although it is clear that genetic susceptibility plays a key role in MS pathogenesis, theoretically, it could also turn out that everyone in the population might become susceptible to MS in response to some very special environmental circumstances, regardless of their genetic make-up. In this case, Equation (1) would require some modification (see Appendix S1). Despite this possibility, however, the available experimental evidence suggests that the large majority (perhaps all) of MS is due to the effect of specific environmental events acting on genetically susceptible individuals and, thus, that Equation (1) adequately characterizes the causal pathway leading to MS (see Appendix S1 for an expanded discussion of these issues).
For a monozygotic-twin of an MS proband, (PG) should be close to 100%. Even though gene copy numbers can vary and epigenetic factors may differ to a degree between monozygotic-twins, especially as the individuals age , , in general, any monozygotic-twin of an MS proband will be genetically susceptible if this is conferred solely by nuclear genes inherited at conception. Moreover, as discussed in Appendix S1, for a monozygotic-twin of an MS proband, the lifetime probability of developing MS (PMS) is equal to the proband-wise concordance-rate of MS (CRMZ). Thus, in this special case, where (PG≈1) and where (PMS≈CRMZ), Equation (1) simplifies to:(2)
Thus, the product of the conditional probability of experiencing a “sufficient” environmental exposure and the probability of an appropriate outcome from a random (stochastic) process is equal to the proband-wise monozygotic-twin concordance-rate (which is also equal to the penetrance of the complex genetic trait). This relationship is independent of whether any specific factor is in the causal path to MS and the value of [(PE|G)(PMS|G, E)] can be determined regardless of whether genetic susceptibility, environmental exposure, and any stochastic processes are independent of each other. As a result, if both (PMS) and (CRMZ) are known for any particular region, the prevalence (probability) of genetic susceptibility (PG) in the population for that region can always be approximated as:(3)
Note that in deriving Equation (3), beyond ascribing susceptibility to nuclear genes, no assumptions have been made with respect to the environmental events, the genetic variables or their interactions. As noted, Equation (1) follows directly from the definition of conditional probability and Equations (2) and (3) are immediate consequences of this. Moreover, each of the potential errors (discussed earlier), which arise from equating (PMS) with the prevalence of MS in the general population, will also arise from equating (CRMZ) with the prevalence of MS in monozygotic-twins of an MS proband. Therefore, in Equation (3), these errors (whatever they are) should largely cancel.
As an example, for far northern populations –, the range of estimates for (CRMZ) is 20–30% and for (PMS) is 0.1–0.2%. Therefore, (PG) in these regions can be approximated as 0.3–1.0% (Table 3). In more southerly regions of North America and Europe, both rates are approximately half those in the north –, implying that (PG) is in the same range (Table 3). In fact, applying Equation (3) to every region in which (PG) can be calculated from the available epidemiological data, (PG) seems to be in a very similar range everywhere –, , , without any obvious north-south gradient (Table 3). It may be (as is often suggested) that the probability of genetic susceptibility is different in some ethnic populations than in others although, at present, based on the apparent absence of any difference in susceptibility between the ethnically different populations of Europe and North America (Table 3), such a possibility is pure speculation. Thus, all that can be said at the moment is that throughout Europe and North America, the probability of being genetically susceptible is remarkably consistent (Table 3).
Nevertheless, it is clear that a person's genetic make-up is, by far, the most important determinant of MS risk, despite the fact that the contribution of individual genes to that risk seems to be small and that several genome-wide screens have not provided evidence for strong associations other than at the HLA DRB1 locus . Thus, by this probabilistic analysis, over 99% of individuals seem to be genetically incapable of getting MS, regardless of what environmental events they experience during their lives. Paradoxically, however, because the (PG) term is so similar in different areas (Table 3), it is the environmental events (i.e., the PE term) that determine the observed regional variations in MS epidemiology. In addition, the mechanisms underlying genetic susceptibility to MS are likely to be quite complex. Thus, even though the HLA DRB1*1501 allele has the largest and most consistent association with MS susceptibility of any genetic marker identified to date , approximately 99% of the individuals who carry this susceptibility allele are not even genetically susceptible to getting MS (Table 3).
For the purposes of this Model, only two time-periods from the Canadian study will be considered in detail although, in fact, the observed change in sex-ratio seems steady and consistent over the entire study-interval  and the parameter estimates derived from each these different sex-ratio data points (see Appendix S1) is quite similar (Table 4). The first interval considered is (1941–1945) because, of the older data points, this is the oldest with a very narrow confidence interval . The second is (1976–1980), which will be considered “current” because this is the youngest 5-year cohort whose members have lived long enough (i.e., 35–40 years) for MS to declare itself. Also, this time-period is the most likely to match-up with “current” estimates of the monozygotic-twin concordance-rates. Obviously, these point estimates, are associated with error terms , , so that estimates derived from them are only valid within the limits set by these potential errors. Nevertheless, knowing that the sex-ratio in the (1976–1980) time-frame is 3.2  and using an estimate of 0.1% for MS prevalence in Canada , , a gender-specific MS prevalence of 152.4 and 47.6 per 100,000 population can be calculated for women and men respectively (Table 3). Moreover, the proband-wise monozygotic-twin concordance-rate for women (0.34) is significantly larger (p<0.001) than the same rate (0.065) for men .
From Equation (3), and as shown in Table 3, men seem to be 60% more likely to be genetically susceptible to getting MS than are women (i.e., PGM>1.6 PGW). This result is independent of the actual prevalence of MS in Canada and, consequently, the greater current MS prevalence in women, presumably, is due to the fact that the [(PE|G)(PMS|G, E)] term in Equation (1) is presently larger for women than for men.
This gender-specific environmental effect might reflect a true difference in exposure (e.g., maybe women use more sun-block or sun-avoidance than men, maybe they spend less time out of doors, or maybe they have better hygiene as children and therefore acquire EBV later). It could also reflect gender-specific differences in vitamin D metabolism, which causes men and women to experience a deficiency at different absolute exposure levels , . It may also reflect women having a greater probability of actually developing MS once the necessary environmental and genetic events have come together or it could be that some combination of these factors contributes to the observed gender-specific differences.
In the Model, these gender-specific terms [(PEW|GW)(PMS|GW, EW) and (PEM|GM)(PMS|GM, EM)], will be referred to (collectively) as the probability of an “effective” exposure (i.e., an exposure that actually produces disease in a susceptible individual). These gender differences also necessitate the use of gender-specific monozygotic-twin concordance-rates (Zw and Zm), as defined in Table 2.
MS epidemiology in Canada has been changing in the 35 year interval between (1941–1945) and (1976–1980). First, MS prevalence may have doubled (in which case C would be equal to 0.5) and the sex-ratio of women-to-men has increased from 2.2 to 3.2 , , . From these two pieces of information, and assuming both that PGW and PGM are unchanged and that the current MS prevalence is 0.1%, it follows that the gender-specific MS prevalence in (1941–1945) was 68.8 and 31.2 per 100,000 population for women and men respectively and that the gender-specific term for the probability of “effective” exposure during this period (Table 2) is (0.307*C = 15.3%) for women and (0.085*C = 4.3%) for men (see Appendix S1). Moreover, knowing these values permits the relationship between the probability of an “effective” exposure and the actual exposure level of the population to be defined rather precisely (see Appendix S1).
If the hazard-rate for “effective” exposure is constant and the same for men and women, which seems plausible for a stochastic process related only to the actual exposure level received by the population (see Appendix S1 for a discussion of these issues), the most general equations describing the approach to maximum probability of “effective” exposure are exponential and are given by:and
In these equations, a and b are positive constants (≤1), which represent the maximum probability of “effective” exposure for men and women, x (≥0) represents the actual exposure level received by the population, and r (≥0) is the hazard-rate for “effective” exposure. The threshold exposure level at which disease becomes possible and is defined as the exposure-level (rx) such that:
By virtue of a few basic epidemiological observations , , we can specify two points on these gender-specific response curves and, therefore, each curve can be defined within narrow limits (see Appendix S1). Thus, based upon the current proband-wise monozygotic-twin concordance-rates , allowing for an accuracy (±1 SE), and based upon the observed changes in sex-ratio , it can be shown that (b>a), that (0.018<a<0.154), that (0.335<b<0.576), that (2.7<b/a<26.1), and that (λ<−0.1). It is therefore apparent that women are more responsive to the environmental changes that have taken place (whatever they are) than men. Despite this, however, men have a lower threshold of actual environmental exposure for the disease to develop than women. Such a circumstance might explain why the earliest reports of MS were often in men ,  and why a 1922 study reported that of 363 MS patients from the United States and of 1,142 cases from Europe, approximately 58% (in both regions separately) were men . Moreover, it can be shown that, at a minimum, there must have been a 32% increase in the prevalence of MS in Canada over the 35 year interval of study. Some increase in MS prevalence might be expected from better diagnostic techniques although, because this minimum increase in MS prevalence depends only upon the observed change in the sex-ratio (see Appendix S1), this explanation seems unlikely. Indeed, assuming that MS prevalence has doubled over the 35 year interval , , and that the “current” estimates for Zw2, Zm2, R2, and R1 are accurate, every parameter can be determined precisely [i.e., b = 0.449, a = 0.078, b/a = 5.751, and λ = −0.373] and the theoretical response curves constructed exactly (Figure 2). Finally, these parameter estimates are quite stable for all of the changes in sex-ratio that have been observed over time in Canada (Table 4).
These curves are based upon the “current” proband-wise monozygotic-twin concordance-rates for men and women in Canada  and upon the change in the sex-ratio observed in Canada between the two time-periods . However, in deriving this set of curves, two further assumptions have been made. First it has been assumed that the currently observed values for Zw2 and Zm2 are accurate . Second, it is assumed that C (defined in Table 2) is equal to one half (i.e., that the prevalence of MS has doubled between the two time intervals). Of course, the actual level of environmental exposure (whatever this represents) is unknown. Nevertheless, the environmental exposure has been scaled such that the difference in the actual exposure level between these two time-periods (whatever this is), multiplied by the unknown hazard rate, is assigned the arbitrary value of one exposure unit (see Table 2). In these curves, the probability of a “sufficient” exposure to the entire set of environmental events (PE) is assumed to have changed (in some manner) as the actual level of exposure (x) of the population has increased between the two time intervals of study. This change in (PE), however, could be due to a change in only one, in some, or in all of the relevant environmental factors. NB: Although the two time-periods of (1941–1945) and (1976–1980), which were used for parameter estimation, are shown along the x-axis, this axis represents an increasing (but unknown) environmental exposure. It is not a time-axis.
Thus, there can be no doubt that the environmental factors have been changing over the past several decades and probably for much longer. Nevertheless, it is possible that only some of these implicated environmental factors have changed and, thus, only these particular factors may be responsible for the changes that have taken place in MS epidemiology over the past several decades. If, as discussed earlier, the sequential pathway 3 (Figure 1) plays the dominant role in adult MS pathogenesis, then it follows that the “total” environmental term (PE) can be re-written as:(4)
In this equation, (PVD) is the probability of a “sufficient” exposure to vitamin D deficiency, the term (PEBV|VD) is the conditional probability of a “sufficient” EBV exposure given a “sufficient” vitamin D exposure, and (PO|VD, EBV) is the conditional probability of the other “sufficient” environmental exposures given the fact that the individual has already experienced “sufficient” vitamin D and EBV exposures.
If individuals are equally likely to receive a “sufficient” exposure to each of these three environmental events (VD, EBV, and Other), if (PMS|G, E) is 100%, and if the conditional probabilities are approximately equal to the probabilities themselves, then it follows that 63% of northern European and northern North American populations ,  have experienced what would have been a “sufficient” exposure to each of the three implicated environmental events in a genetically susceptible individual. Thus, in this circumstance:
Even in the more southerly regions of these continents , a “sufficient” exposure to each of the events would still be experienced by 53% of the population. It is only because genetic susceptibility is so infrequent that the disease is uncommon. Moreover, if (PMS|G, E) is less than 100% (as seems likely), these numbers will only increase further as this probability declines. Consequently, the necessary environmental exposures in the causal pathway to MS seem likely to be very common events. Indeed, because both vitamin D deficiency and EBV infection are both very common population-wide events, this conclusion is fully consistent with these factors being the first two environmental events involved in MS pathogenesis. Thus, vitamin D deficiency (at least to some degree) is anticipated in the large majority of individuals living in low sun-exposure regions  and EBV is an extremely prevalent pathogen in human populations (Table 1).
If, as suggested by the above Model, the causal pathway leading to adult MS involves sequential environmental events or factors (pathway 3; Figure 1) then, theoretically, modification or disruption of any one of these factors has the potential to completely prevent the disease . The simplest factor to modify in this way would be vitamin D deficiency because vitamin D can be supplemented easily and cheaply. However, due to the low incidence of MS, any test of such a therapeutic strategy will be very difficult to undertake. First, it will require following thousands of individuals for many (>30) years. Second, the cost of using a randomized, placebo-controlled design in this setting is prohibitive, even disregarding the ethical and logistical problems of using a placebo for such a prolonged period. And finally, any study designed to enrich the study-cohort for high-risk individuals by including only first-degree relatives of MS probands  will fail if the critical time for environmental exposure occurs in utero, during the early post-natal period, or even during childhood. Thus, by the time that MS probands are identified, most of their brothers and sisters will have already passed their window of therapeutic opportunity.
Therefore, in order to test this hypothesis, it will require the use of an open-label, observational study design that includes all women who want to become pregnant. Moreover, it will need to employ statistical methods for minimizing bias in the analysis of non-randomized data [e.g.], [ 96]–. Adequate vitamin D supplementation will need to be recommended both before and during pregnancy for the mother and thereafter for both the mother and child. However, because the amount of supplementation necessary to achieve adequate blood-levels of vitamin D (i.e., in the normal range) may be quite high, it should be anticipated that some (perhaps many) obstetricians or pediatricians will be unwilling to recommend such large dosages of vitamin D either to their pregnant or to their pediatric patients. Such a circumstance, however, does not detract from the study design. In fact, a large variation in physician and patient behavior (with respect to the use and magnitude of vitamin D supplementation) will actually make the final data analysis easier –. As a result, in order to conduct such a trial, there is no need to encourage reluctant physicians to recommend supplementation, especially when large numbers of MS patients around the world are already consuming large quantities of oral vitamin D on a daily basis.
The current FDA recommendations suggest that 400 international units (IU) of vitamin D per day is an adequate amount of supplementation, although this recommendation is derived mostly from the effects of vitamin D on calcium homeostasis. The requirements for immune modulation, however, are likely to be higher . For example, healthy men have been estimated to use between 3,000 and 5,000 IU/day . Moreover, even with calcium supplementation, doses of vitamin D between 7,000 and 40,000 IU/d seem to be safe and unaccompanied by toxicity, including elevations in serum calcium concentrations or in the calcium-to-creatinine ratios , . Consequently, dosages of between 1,000 and 10,000 IU/d day would not seem unreasonable or unsafe.
However, if physicians do recommend supplementation (at whatever level), they should monitor their patients' blood-levels to ensure that these don't exceed the range of normal. Importantly, however, the hypothesis that early vitamin D supplementation influences the subsequent likelihood of developing MS can be tested solely by acquiring observational data and does not require anyone to agree to any specific treatment plan. Nevertheless, as mentioned earlier, the conduct of such a study will require patients and their children to be followed prospectively for long periods of time – a design that, realistically, is only feasible in situations where large population-based centralized medical records are available (e.g., countries with universal health care) or in non-mobile communities where complete ascertainment and longitudinal follow-up is possible. Physicians (and their patients) could be informed about the nature of the hypothesis being tested (possibly through the MS societies or other outlets) and about the ranges of vitamin D dosages being considered (including current FDA guidelines). Such a study would be cost-effective (only information already available needs to be captured), it would be easy to accomplish (everyone can participate and can take whatever supplementation they choose), and it would pose no ethical dilemmas (each person and each physician is free to choose what they feel is best for themselves or for their patients). Moreover, this seems like an extremely important study to begin, especially in health care systems where persons and their offspring can be easily tracked. Certainly, such a study seems to carry minimal risk, it requires little cost, and, if successful, it would provide inestimable benefit for future patients. However, because it will take up to 30 years to arrive at an answer, the prospective acquisition of this kind of data should begin now.
Conceived and designed the experiments: DSG. Performed the experiments: DSG. Analyzed the data: DSG. Contributed reagents/materials/analysis tools: DSG. Wrote the paper: DSG. Derived the mathematical model: DSG.
- 1. Rothman KJ, Greenland S (1998) Modern Epidemiology, 2nd Edition. Philadelphia: Lippincott Williams & Wilkins. KJ RothmanS. Greenland1998Modern Epidemiology, 2nd EditionPhiladelphiaLippincott Williams & Wilkins
- 2. Compston A, Confavreux C, Lassmann H, et al. (2006) McAlpine's Multiple Sclerosis, 4th Edition. London: Churchill Livingston. A. CompstonC. ConfavreuxH. Lassmann2006McAlpine's Multiple Sclerosis, 4th EditionLondonChurchill Livingston
- 3. Ebers GC, Sadovnick AD, Risch NJ, and the Canadian Collaborative Study Group (1995) A genetic basis for familial aggregation in multiple sclerosis. Nature 377: 150–151.GC EbersAD SadovnickNJ Rischand the Canadian Collaborative Study Group1995A genetic basis for familial aggregation in multiple sclerosis.Nature377150151
- 4. Sadovnick AD, Ebers GC, Dyment DA, Risch NJ, and the Canadian Collaborative Study Group (1996) Evidence for genetic basis of multiple sclerosis. Lancet 347: 1728–1730.AD SadovnickGC EbersDA DymentNJ Rischand the Canadian Collaborative Study Group1996Evidence for genetic basis of multiple sclerosis.Lancet34717281730
- 5. Robertson NP, Fraser M, Deans J, et al. (1996) Age-adjusted recurrence risks for relatives of patients with multiple sclerosis. Brain 119: 449–455.NP RobertsonM. FraserJ. Deans1996Age-adjusted recurrence risks for relatives of patients with multiple sclerosis.Brain119449455
- 6. Compston A, Coles A (2002) Multiple sclerosis. Lancet 359: 1221–31.A. CompstonA. Coles2002Multiple sclerosis.Lancet359122131
- 7. Ebers GC, Sadovnick AD, Dyment DA, et al. (2004) Parent-of-origin effect in multiple sclerosis: observations in half-siblings. Lancet 363: 1773–1774.GC EbersAD SadovnickDA Dyment2004Parent-of-origin effect in multiple sclerosis: observations in half-siblings.Lancet36317731774
- 8. Nielsen NM, Westergaard T, Rostgaard K, et al. (2005) Familial risk of multiple sclerosis: A nationwide cohort study. Am J Epidemiol 162: 774–778.NM NielsenT. WestergaardK. Rostgaard2005Familial risk of multiple sclerosis: A nationwide cohort study.Am J Epidemiol162774778
- 9. Mumford CJ, Wood NW, Kellar-Wood H, et al. (1994) The British Isles survey of multiple sclerosis in twins. Neurology 44: 11–15.CJ MumfordNW WoodH. Kellar-Wood1994The British Isles survey of multiple sclerosis in twins.Neurology441115
- 10. Willer CJ, Dyment DA, Risch NJ, Sadovnick AD, Ebers GC, and the Canadian Collaborative Study Group (2003) Twin concordance and sibling recurrence rates in multiple sclerosis. Proc Natl Acad Sci (USA) 100: 12877–12882.CJ WillerDA DymentNJ RischAD SadovnickGC Ebersand the Canadian Collaborative Study Group2003Twin concordance and sibling recurrence rates in multiple sclerosis.Proc Natl Acad Sci (USA)1001287712882
- 11. Islam T, Gauderman WJ, Cozen W, et al. (2006) Differential twin concordance for multiple sclerosis by latitude of birthplace. Ann Neurol 60: 56–64.T. IslamWJ GaudermanW. Cozen2006Differential twin concordance for multiple sclerosis by latitude of birthplace.Ann Neurol605664
- 12. French Research Group on Multiple Sclerosis (1992) Multiple sclerosis in 54 twinships: Concordance rate is independent of zygosity. Ann Neurol 32: 724–727.French Research Group on Multiple Sclerosis1992Multiple sclerosis in 54 twinships: Concordance rate is independent of zygosity.Ann Neurol32724727
- 13. Ristori G, Cannoni S, Stazi MA, et al. and the Italian Study Group on Multiple Sclerosis in Twins (2006) Multiple sclerosis in twins from continental Italy and Sardinia: A Nationwide Study. Ann Neurol 59: 27–34.G. RistoriS. CannoniMA Staziand the Italian Study Group on Multiple Sclerosis in Twins2006Multiple sclerosis in twins from continental Italy and Sardinia: A Nationwide Study.Ann Neurol592734
- 14. Ebers GC, Yee IML, Sandovnick AD, Duquette P, and the Canadian Collaborative Study Group (2000) Conjugal multiple sclerosis: Population-based prevalence and recurrence risks in offspring. Ann Neurol 48: 927–931.GC EbersIML YeeAD SandovnickP. Duquetteand the Canadian Collaborative Study Group2000Conjugal multiple sclerosis: Population-based prevalence and recurrence risks in offspring.Ann Neurol48927931
- 15. Sadovnick AD, Yee IML, Ebers GC, and the Canadian Collaborative Study Group (2005) Multiple sclerosis and birth order: A longitudinal cohort study. Lancet Neurol 4: 611–617.AD SadovnickIML YeeGC Ebersand the Canadian Collaborative Study Group2005Multiple sclerosis and birth order: A longitudinal cohort study.Lancet Neurol4611617
- 16. Bager P, Nielsen NM, Bihrmann K, et al. (2006) Sibship characteristics and risk of multiple sclerosis: A nationwide cohort study in Denmark. Am J Epidemiol 163: 1112–1117.P. BagerNM NielsenK. Bihrmann2006Sibship characteristics and risk of multiple sclerosis: A nationwide cohort study in Denmark.Am J Epidemiol16311121117
- 17. Dyment DA, Yee IML, Ebers GC, Sadovnick AD, and the Canadian Collaborative Study Group (2006) Multiple sclerosis in stepsiblings: Recurrence risk and ascertainment. J Neurol Neurosurg Psychiatry 77: 258–259.DA DymentIML YeeGC EbersAD Sadovnickand the Canadian Collaborative Study Group2006Multiple sclerosis in stepsiblings: Recurrence risk and ascertainment.J Neurol Neurosurg Psychiatry77258259
- 18. Bartolomei MS, Tilghman SM (1997) Genomic imprinting in mammals. Annu Rev Genet 31: 493–525.MS BartolomeiSM Tilghman1997Genomic imprinting in mammals.Annu Rev Genet31493525
- 19. Kantarci OH, Barcellos LF, Atkinson EJ, et al. (2006) Men transmit MS more often to their children vs women: The Carter effect. Neurology 67: 305–310.OH KantarciLF BarcellosEJ Atkinson2006Men transmit MS more often to their children vs women: The Carter effect.Neurology67305310
- 20. Herrera BM, Ramagopalan SV, Orton S, et al. (2007) Parental transmission of MS in a population-based Canadian cohort. Neurology 69: 1208–1212.BM HerreraSV RamagopalanS. Orton2007Parental transmission of MS in a population-based Canadian cohort.Neurology6912081212
- 21. Willer CJ, Dyment DA, Sadovnick AD, et al. and the Canadian Collaborative Study Group (2005) Timing of birth and risk of multiple sclerosis: population based study. Br Med J 330: 120–124.CJ WillerDA DymentAD Sadovnickand the Canadian Collaborative Study Group2005Timing of birth and risk of multiple sclerosis: population based study.Br Med J330120124
- 22. Templer DI, Trent NH, Spencer DA, et al. (1992) Season of birth in multiple sclerosis. Acta Neurol Scand 85: 107–109.DI TemplerNH TrentDA Spencer1992Season of birth in multiple sclerosis.Acta Neurol Scand85107109
- 23. Sadovnick AD, Duquette P, Herrera B, et al. (2007) A timing-of-birth effect on multiple sclerosis clinical phenotype. Neurology 69: 60–62.AD SadovnickP. DuquetteB. Herrera2007A timing-of-birth effect on multiple sclerosis clinical phenotype.Neurology696062
- 24. Chaudhuri A (2005) Why we should offer routine vitamin D supplementation in pregnancy and childhood to prevent multiple sclerosis. Med Hypotheses 64: 608–618.A. Chaudhuri2005Why we should offer routine vitamin D supplementation in pregnancy and childhood to prevent multiple sclerosis.Med Hypotheses64608618
- 25. Dean G, Kurtzke JF (1971) On the risk of multiple sclerosis according to age at immigration to South Africa. Br Med J 3: 725–729.G. DeanJF Kurtzke1971On the risk of multiple sclerosis according to age at immigration to South Africa.Br Med J3725729
- 26. Alter M, Kahana E, Loewenson R (1978) Migration and risk of multiple sclerosis. Neurology 28: 1089–1093.M. AlterE. KahanaR. Loewenson1978Migration and risk of multiple sclerosis.Neurology2810891093
- 27. Elian M, Nightingale S, Dean G (1990) Multiple sclerosis among United Kingdom-born children of immigrants from the Indian subcontinent, Africa, and the West Indies. J Neurol Neurosurg, Psychiatr 53: 906–911.M. ElianS. NightingaleG. Dean1990Multiple sclerosis among United Kingdom-born children of immigrants from the Indian subcontinent, Africa, and the West Indies.J Neurol Neurosurg, Psychiatr53906911
- 28. Esther Kahana E, Nelly Zilber N, Abramson JH, et al. (1994) Multiple sclerosis: Genetic versus environmental aetiology: Epidemiology in Israel updated. J Neurol 241: 341–346.E. Esther KahanaN. Nelly ZilberJH Abramson1994Multiple sclerosis: Genetic versus environmental aetiology: Epidemiology in Israel updated.J Neurol241341346
- 29. Cabre P, Signate A, Olindo S, et al. (2005) Role of return migration in the emergence of multiple sclerosis in the French West Indies. Brain 128: 2899–2910.P. CabreA. SignateS. Olindo2005Role of return migration in the emergence of multiple sclerosis in the French West Indies.Brain12828992910
- 30. Ascherio A, Munger KL, Lennette ET, et al. (2001) Epstein-Barr virus antibodies and the risk of multiple sclerosis: A prospective study. Am Med Assoc J 286: 3083–3088.A. AscherioKL MungerET Lennette2001Epstein-Barr virus antibodies and the risk of multiple sclerosis: A prospective study.Am Med Assoc J28630833088
- 31. Ponsonby AL, van der Mei I, Dwyer T, et al. (2005) Exposure to infant siblings during early life and risk of multiple sclerosis. Am Med Assoc J 293: 463–9.AL PonsonbyI. van der MeiT. Dwyer2005Exposure to infant siblings during early life and risk of multiple sclerosis.Am Med Assoc J2934639
- 32. Sundström P, Juto G, Wadell G, et al. (2004) An altered immune response to Epstein-Barr virus in multiple sclerosis: A prospective study. Neurology 62: 2277–2282.P. SundströmG. JutoG. Wadell2004An altered immune response to Epstein-Barr virus in multiple sclerosis: A prospective study.Neurology6222772282
- 33. Serafini B, Roiscarelli B, Franciotta D, et al. (2007) Dysregulated Epstein-Barr virus infection in multiple sdclerosis. J Exp Med 204: 2899–2912.B. SerafiniB. RoiscarelliD. Franciotta2007Dysregulated Epstein-Barr virus infection in multiple sdclerosis.J Exp Med20428992912
- 34. Goldacre MJ, Wotton CJ, Seagroatt V, Yeates D (2004) Multiple sclerosis after infectious mononucleosis: record linkage study. J Epidemiol Community Health 58: 1032–1035.MJ GoldacreCJ WottonV. SeagroattD. Yeates2004Multiple sclerosis after infectious mononucleosis: record linkage study.J Epidemiol Community Health5810321035
- 35. Thacker EL, Mirzaei F, Ascherio A (2006) Infectious mononucleosis and risk for multiple sclerosis: a meta-analysis. Ann Neurol 59: 499–503.EL ThackerF. MirzaeiA. Ascherio2006Infectious mononucleosis and risk for multiple sclerosis: a meta-analysis.Ann Neurol59499503
- 36. Farrell PJ (2007) Role for HLA in susceptibility to infectious mononucleosis. J Clin Invest 117: 2756–2758.PJ Farrell2007Role for HLA in susceptibility to infectious mononucleosis.J Clin Invest11727562758
- 37. Ascherio A, Munger KL (2007) Environmental risk factors for multiple sclerosis. Part I: The role of infection. Ann Neurol 61: 288–299.A. AscherioKL Munger2007Environmental risk factors for multiple sclerosis. Part I: The role of infection.Ann Neurol61288299
- 38. Nielsen TR, Rostgaard K, Nielsen NM, et al. (2007) Multiple sclerosis after infectious mononucleosis. Arch Neurol 64: 72–75.TR NielsenK. RostgaardNM Nielsen2007Multiple sclerosis after infectious mononucleosis.Arch Neurol647275
- 39. Holick MF (1998) Vitamin D requirements for humans of all ages: new increased requirements for women and men 50 years and older. Osteoporos Int 8: Suppl 2S24–29.MF Holick1998Vitamin D requirements for humans of all ages: new increased requirements for women and men 50 years and older.Osteoporos Int8Suppl 2S2429
- 40. Hayes CE, Nashold FE, Spach KM, Pedersen LB (2003) The immunological functions of the vitamin D endocrine system. Cell Molec Biol 49: 277–300.CE HayesFE NasholdKM SpachLB Pedersen2003The immunological functions of the vitamin D endocrine system.Cell Molec Biol49277300
- 41. Nagpal S, Na S, Rathnachalam R (2005) Noncalcemic actions of vitamin D receptor ligands. Endocr Rev 26: 662–687.S. NagpalS. NaR. Rathnachalam2005Noncalcemic actions of vitamin D receptor ligands.Endocr Rev26662687
- 42. Lips P (2006) Vitamin D physiology. Prog Biophys Molec Biol 92: 4–8.P. Lips2006Vitamin D physiology.Prog Biophys Molec Biol9248
- 43. Gillie O (2006) A new government policy is needed for sunlight and vitamin D. Br J Dermatol 154: 1052–1061.O. Gillie2006A new government policy is needed for sunlight and vitamin D.Br J Dermatol15410521061
- 44. Sinclair H (1977) Polyunsaturated fatty acids in multiple sclerosis. Br Med J 2: 1217.H. Sinclair1977Polyunsaturated fatty acids in multiple sclerosis.Br Med J21217
- 45. Koch-Henderson N (1995) Multiple sclerosis in Scandinavia and Finland. Acta Neurol Scand 161: 55–59.N. Koch-Henderson1995Multiple sclerosis in Scandinavia and Finland.Acta Neurol Scand1615559
- 46. Grønlie SA, Myrvoll E, Hansen G, et al. (2000) Multiple sclerosis in North Norway, and first appearance in an indigenous population. J Neurol 247: 129–133.SA GrønlieE. MyrvollG. Hansen2000Multiple sclerosis in North Norway, and first appearance in an indigenous population.J Neurol247129133
- 47. Jablonski NG, Chaplin G (2000) The evolution of human skin coloration. J Hum Evol 39: 57–106.NG JablonskiG. Chaplin2000The evolution of human skin coloration.J Hum Evol3957106
- 48. Jablonski NG, Chaplin G (2002) Skin deep. Sci Am 287: 74–81.NG JablonskiG. Chaplin2002Skin deep.Sci Am2877481
- 49. Kimlin MG, Olds WJ, Moore MR (2007) Location and vitamin D synthesis: Is the hypothesis validated by geophysical data? J Photochem Photobiol B: Biology 86: 234–239.MG KimlinWJ OldsMR Moore2007Location and vitamin D synthesis: Is the hypothesis validated by geophysical data?J Photochem Photobiol B: Biology86234239
- 50. Adams CWM (1989) A Colour Atlas of Multiple Sclerosis & Other Myelin Disorders. IpswichSuffolk, UK: Wolfe Medical Publications. CWM Adams1989A Colour Atlas of Multiple Sclerosis & Other Myelin DisordersIpswichSuffolk, UKWolfe Medical Publications101
- 51. Rosati G (2001) The prevalence of multiple sclerosis in the world: an update. Neurol Sci 22: 117–139.G. Rosati2001The prevalence of multiple sclerosis in the world: an update.Neurol Sci22117139
- 52. Ebers G, Ramagopalan SV, Maugeri NJ, et al. (2008) Expression of the Multiple Sclerosis associated MHC Class II Allele HLA-DRB1*1501 is Regulated by Vitamin D. Ann Neurol 64: suppl 12S66.G. EbersSV RamagopalanNJ Maugeri2008Expression of the Multiple Sclerosis associated MHC Class II Allele HLA-DRB1*1501 is Regulated by Vitamin D.Ann Neurol64suppl 12S66
- 53. Cantorna MT (2000) Vitamin D and autoimmunity: Is vitamin D status an environmental factor affecting autoimmune disease prevalence? Proc Soc Exp Biol Med 223: 230–233.MT Cantorna2000Vitamin D and autoimmunity: Is vitamin D status an environmental factor affecting autoimmune disease prevalence?Proc Soc Exp Biol Med223230233
- 54. Deluca HF, Cantorna MT (2001) Vitamin D: Its role and uses in immunology. FASEB J 15: 2579–2585.HF DelucaMT Cantorna2001Vitamin D: Its role and uses in immunology.FASEB J1525792585
- 55. Griffin MD, Lutz W, Phan VA, et al. (2001) Dendritic cell modulation by 1alpha,25 dihydroxyvitamin D3 and its analogs: A vitamin D receptor-dependent pathway that promotes a persistent state of immaturity in vitro and in vivo. Proc Natl Acad Sci 98: 6800–6805.MD GriffinW. LutzVA Phan2001Dendritic cell modulation by 1alpha,25 dihydroxyvitamin D3 and its analogs: A vitamin D receptor-dependent pathway that promotes a persistent state of immaturity in vitro and in vivo.Proc Natl Acad Sci9868006805
- 56. Evans KN, Nguyen L, Chan J, et al. (2006) Effects of 25-Hydroxyvitamin D3 and 1,25-Dihydroxyvitamin D3 on Cytokine Production by Human Decidual Cells. Biol Reprod 75: 816–822.KN EvansL. NguyenJ. Chan2006Effects of 25-Hydroxyvitamin D3 and 1,25-Dihydroxyvitamin D3 on Cytokine Production by Human Decidual Cells.Biol Reprod75816822
- 57. Cantorna MT, Mahon BD (2004) Mounting evidence for vitamin D as an environmental factor affecting autoimmune disease prevalence. Exp Biol Med 229: 1136–1142.MT CantornaBD Mahon2004Mounting evidence for vitamin D as an environmental factor affecting autoimmune disease prevalence.Exp Biol Med22911361142
- 58. Suarez F, Rossignol C, Garabédian M (1998) Interactive effect of estradiol and vitamin D receptor gene polymorphisms as a possible determinant of growth in male and female infants. J Clin Endocrinol Metab 83: 3563–3568.F. SuarezC. RossignolM. Garabédian1998Interactive effect of estradiol and vitamin D receptor gene polymorphisms as a possible determinant of growth in male and female infants.J Clin Endocrinol Metab8335633568
- 59. Spach KM, Hayes CE (2005) Vitamin D3 Confers Protection from Autoimmune Encephalomyelitis Only in Female Mice. J Immunol 175: 4119–4126.KM SpachCE Hayes2005Vitamin D3 Confers Protection from Autoimmune Encephalomyelitis Only in Female Mice.J Immunol17541194126
- 60. Hernán MA, Olek MJ, Ascherio A (1999) Geographic variation of MS incidence in two prospective studies of US women. Neurology 53: 1711–1718.MA HernánMJ OlekA. Ascherio1999Geographic variation of MS incidence in two prospective studies of US women.Neurology5317111718
- 61. Koch-Henriksen N (1999) The Danish Multiple Sclerosis Registry: a 50-year follow-up. Mult Scler 5: 293–296.N. Koch-Henriksen1999The Danish Multiple Sclerosis Registry: a 50-year follow-up.Mult Scler5293296
- 62. Celius EG, Vandvik B (2001) Multiple sclerosis in Oslo, Norway: prevalence on 1 January 1995 and incidence over a 25-year period. Eur J Neurol 8: 463–469.EG CeliusB. Vandvik2001Multiple sclerosis in Oslo, Norway: prevalence on 1 January 1995 and incidence over a 25-year period.Eur J Neurol8463469
- 63. Barnett MH, William DB, Day S, et al. (2003) Progressive increase in incidence and prevalence of multiple sclerosis in Newcastle, Australia: a 35-year study. J Neurol Sci 213: 1–6.MH BarnettDB WilliamS. Day2003Progressive increase in incidence and prevalence of multiple sclerosis in Newcastle, Australia: a 35-year study.J Neurol Sci21316
- 64. Sarasoja T, Wikström J, Paltamaa J, et al. (2004) Occurrence of multiple sclerosis in central Finland: a regional and temporal comparison during 30 years. Acta Neurol Scand 110: 331–6.T. SarasojaJ. WikströmJ. Paltamaa2004Occurrence of multiple sclerosis in central Finland: a regional and temporal comparison during 30 years.Acta Neurol Scand1103316
- 65. Orton SM, Herrera BM, Yee IM, et al. and the Canadian Collaborative Study Group (2006) Sex ratio of multiple sclerosis in Canada: A longitudinal study. Lancet Neurol 5: 932–936.SM OrtonBM HerreraIM Yeeand the Canadian Collaborative Study Group2006Sex ratio of multiple sclerosis in Canada: A longitudinal study.Lancet Neurol5932936
- 66. Henle W, Henle G, Andersson J, et al. (1987) Antibody responses to Epstein-Barr virus-determined nuclear antigen (EBNA)-1 and EBNA-2 in acute and chronic Epstein-Barr virus infection. Proc Natl Acad Sci (USA) 84: 570–574.W. HenleG. HenleJ. Andersson1987Antibody responses to Epstein-Barr virus-determined nuclear antigen (EBNA)-1 and EBNA-2 in acute and chronic Epstein-Barr virus infection.Proc Natl Acad Sci (USA)84570574
- 67. Vaughan JH (1995) The Epstein-Barr virus in autoimmunity. Springer Semin Immunopathol 17: 203–230.JH Vaughan1995The Epstein-Barr virus in autoimmunity.Springer Semin Immunopathol17203230
- 68. Van der Mei IAF, Ponsonby A-L, Dwyer T, et al. (2003) Past exposure to sun, skin phenotype, and risk of multiple sclerosis: Case-control study. Br Med J 327: 316–322.IAF Van der MeiA-L PonsonbyT. Dwyer2003Past exposure to sun, skin phenotype, and risk of multiple sclerosis: Case-control study.Br Med J327316322
- 69. Munger KL, Zhang SM, O'Reilly E, et al. (2004) Vitamin D intake and incidence of multiple sclerosis. Neurology 62: 60–65.KL MungerSM ZhangE. O'Reilly2004Vitamin D intake and incidence of multiple sclerosis.Neurology626065
- 70. Munger KL, Levin LI, Hollis BW, Howard NS, Ascherio A (2006) Serum 25-hydroxyvitamin D levels and risk of multiple sclerosis. Am Med Assoc J 296: 2832–2838.KL MungerLI LevinBW HollisNS HowardA. Ascherio2006Serum 25-hydroxyvitamin D levels and risk of multiple sclerosis.Am Med Assoc J29628322838
- 71. Van der Mei IA, Ponsonby AL, Dwyer T, et al. (2007) Vitamin D levels in people with multiple sclerosis and community controls in Tasmania, Australia. J Neurol 254: 581–90.IA Van der MeiAL PonsonbyT. Dwyer2007Vitamin D levels in people with multiple sclerosis and community controls in Tasmania, Australia.J Neurol25458190
- 72. Levin LI, Munger KL, Rubertone MV, et al. (2005) Temporal relationship between elevation of Epstein-Barr virus antibody titers and initial onset of neurological symptoms in multiple sclerosis. Am Med Assoc J 293: 2496–2500.LI LevinKL MungerMV Rubertone2005Temporal relationship between elevation of Epstein-Barr virus antibody titers and initial onset of neurological symptoms in multiple sclerosis.Am Med Assoc J29324962500
- 73. Sumaya CV, Myers LW, Ellison GW (1980) Epstein-Barr virus antibodies in multiple sclerosis. Arch Neurol 37: 94–96.CV SumayaLW MyersGW Ellison1980Epstein-Barr virus antibodies in multiple sclerosis.Arch Neurol379496
- 74. Bray PF, Bloomer LC, Salmon VC, Bagley MH, Larsen PD (1983) Epstein-Barr virus infection and antibody synthesis in patients with multiple sclerosis. Arch Neurol 40: 406–408.PF BrayLC BloomerVC SalmonMH BagleyPD Larsen1983Epstein-Barr virus infection and antibody synthesis in patients with multiple sclerosis.Arch Neurol40406408
- 75. Larsen PD, Bloomer LC, Bray PF (1985) Epstein-Barr nuclear antigen and viral capsid antigen antibody titers in multiple sclerosis. Neurology 35: 435–438.PD LarsenLC BloomerPF Bray1985Epstein-Barr nuclear antigen and viral capsid antigen antibody titers in multiple sclerosis.Neurology35435438
- 76. Sumaya CV, Myers LW, Ellison GW, Ench Y (1985) Increased prevalence and titer of Epstein-Barr virus antibodies in patients with multiple sclerosis. Ann Neurol 17: 371–377.CV SumayaLW MyersGW EllisonY. Ench1985Increased prevalence and titer of Epstein-Barr virus antibodies in patients with multiple sclerosis.Ann Neurol17371377
- 77. Shirodaria PV, Haire M, Fleming E, Merrett JD, Hawkins SA, Roberts SD (1987) Viral antibody titers: Comparison in patients with multiple sclerosis and rheumatoid arthritis. Arch Neurol 44: 1237–1241.PV ShirodariaM. HaireE. FlemingJD MerrettSA HawkinsSD Roberts1987Viral antibody titers: Comparison in patients with multiple sclerosis and rheumatoid arthritis.Arch Neurol4412371241
- 78. Munch M, Riisom K, Christensen T, Møller-Larsen A, Haahr S (1998) The significance of Epstein-Barr virus seropositivity in multiple sclerosis patients? Acta Neurol Scand 97: 171–174.M. MunchK. RiisomT. ChristensenA. Møller-LarsenS. Haahr1998The significance of Epstein-Barr virus seropositivity in multiple sclerosis patients?Acta Neurol Scand97171174
- 79. Myhr KM, Riise T, Barrett-Connor E, et al. (1998) Altered antibody pattern to Epstein-Barr virus but not to other herpesviruses in multiple sclerosis: a population based case-control study from western Norway. J Neurol Neurosurg Psychiatry 64: 539–542.KM MyhrT. RiiseE. Barrett-Connor1998Altered antibody pattern to Epstein-Barr virus but not to other herpesviruses in multiple sclerosis: a population based case-control study from western Norway.J Neurol Neurosurg Psychiatry64539542
- 80. Wagner HJ, Hennig H, Jabs WJ, et al. (2000) Altered prevalence and reactivity of anti-Epstein-Barr virus antibodies in patients with multiple sclerosis. Viral Immunol 13: 497–502.HJ WagnerH. HennigWJ Jabs2000Altered prevalence and reactivity of anti-Epstein-Barr virus antibodies in patients with multiple sclerosis.Viral Immunol13497502
- 81. Haahr S, Plesner AM, Vestergaard BF, Höllsberg P (2004) A role of late Epstein-Barr virus infection in multiple sclerosis. Acta Neurol Scand 109: 270–275.S. HaahrAM PlesnerBF VestergaardP. Höllsberg2004A role of late Epstein-Barr virus infection in multiple sclerosis.Acta Neurol Scand109270275
- 82. Serafini B, Roiscarelli B, Franciotta D, et al. (2007) Dysregulated Epstein-Barr virus infection in multiple sclerosis. J Exp Med 204: 2899–2912.B. SerafiniB. RoiscarelliD. Franciotta2007Dysregulated Epstein-Barr virus infection in multiple sclerosis.J Exp Med20428992912
- 83. Freedman DM, Mustafa M, Alavanja MCR (2000) Mortality from multiple sclerosis and exposure to residential and occupational solar radiation: A case-control study based on death certificates. Occup Environ Med 57: 418–421.DM FreedmanM. MustafaMCR Alavanja2000Mortality from multiple sclerosis and exposure to residential and occupational solar radiation: A case-control study based on death certificates.Occup Environ Med57418421
- 84. Celius EG, Vandvik B (2001) Multiple sclerosis in Oslo, Norway: prevalence on 1 January 1995 and incidence over a 25-year period. Eur J Neurol 8: 463–469.EG CeliusB. Vandvik2001Multiple sclerosis in Oslo, Norway: prevalence on 1 January 1995 and incidence over a 25-year period.Eur J Neurol8463469
- 85. Sundström P, Nyström L, Forsgren L (2003) Incidence (1988–97) and prevalence (1997) of multiple sclerosis in Västerbotten County in northern Sweden. J Neurol Neurosurg Psychiatry 74: 29–32.P. SundströmL. NyströmL. Forsgren2003Incidence (1988–97) and prevalence (1997) of multiple sclerosis in Västerbotten County in northern Sweden.J Neurol Neurosurg Psychiatry742932
- 86. Ranzato F, Perini P, Tzintzeva E, et al. (2003) Increasing frequency of multiple sclerosis in Padova, Italy: a 30 year epidemiological survey. Multiple Sclerosis 9: 387–392.F. RanzatoP. PeriniE. Tzintzeva2003Increasing frequency of multiple sclerosis in Padova, Italy: a 30 year epidemiological survey.Multiple Sclerosis9387392
- 87. Emmons KM, Colditz GA (1999) Preventing excess sun exposure: It is time for a national policy. J Natl Cancer Inst 91: 1269–1270.KM EmmonsGA Colditz1999Preventing excess sun exposure: It is time for a national policy.J Natl Cancer Inst9112691270
- 88. Müller AMS, Ihorst G, Mertelsmann R, Engelhardt M (2005) Epidemiology of non-Hodgkin's lymphoma (NHL): trends, geographic distribution, and etiology. Ann Hematol 84: 1–12.AMS MüllerG. IhorstR. MertelsmannM. Engelhardt2005Epidemiology of non-Hodgkin's lymphoma (NHL): trends, geographic distribution, and etiology.Ann Hematol84112
- 89. Fraga MF, Ballestar E, Paz MF, et al. (2005) Epigenetic differences arise during the lifetime of monozygotic twins. Proc Nat Acad Sci 102: 10604–10609.MF FragaE. BallestarMF Paz2005Epigenetic differences arise during the lifetime of monozygotic twins.Proc Nat Acad Sci1021060410609
- 90. Bruder CEG, Piotrowski A, Gijsbers ACJ, et al. (2008) Phenotypically Concordant and Discordant Monozygotic Twins Display Different DNA Copy-Number-Variation Profiles. Am J Hum Genet 82: 763–771.CEG BruderA. PiotrowskiACJ Gijsbers2008Phenotypically Concordant and Discordant Monozygotic Twins Display Different DNA Copy-Number-Variation Profiles.Am J Hum Genet82763771
- 91. Kuusisto H, Kaprio J, Kinnunen E, et al. (2008) Concordance and heritability of multiple sclerosis in Finland: Study on a nationwide series of twins. Eur J Neurol 15: 1106–1110.H. KuusistoJ. KaprioE. Kinnunen2008Concordance and heritability of multiple sclerosis in Finland: Study on a nationwide series of twins.Eur J Neurol1511061110
- 92. Hansen T, Skytthe A, Stenager E, et al. (2005) Concordance for multiple sclerosis in Danish twins: An update of a nationwide study. Mult Scler 11: 504–510.T. HansenA. SkyttheE. Stenager2005Concordance for multiple sclerosis in Danish twins: An update of a nationwide study.Mult Scler11504510
- 93. Murry TJ (2000) The history of multiple sclerosis. In: Burks JS, Johnson KP, editors. Multiple Sclerosis: Diagnosis, Medical Management, and Rehabilitation. New York: Demos Medical Publishing. pp. 1–35.TJ Murry2000The history of multiple sclerosis.JS BurksKP JohnsonMultiple Sclerosis: Diagnosis, Medical Management, and RehabilitationNew YorkDemos Medical Publishing135
- 94. Wechsler IS (1922) Statistics of multiple sclerosis. Arch Neurol Psychiat 8: 59–75.IS Wechsler1922Statistics of multiple sclerosis.Arch Neurol Psychiat85975
- 95. Bodnar LM, Simhan HN, Powers RW, et al. (2007) High prevalence of vitamin D insufficiency in black and white pregnant women residing in the northern United States and their neonates. J Nutr 137: 447–452.LM BodnarHN SimhanRW Powers2007High prevalence of vitamin D insufficiency in black and white pregnant women residing in the northern United States and their neonates.J Nutr137447452
- 96. Rosenbaum PR, Rubin DB (1983) The central role of propensity score in observational studies for causal effects. Biometrica 70: 41–55.PR RosenbaumDB Rubin1983The central role of propensity score in observational studies for causal effects.Biometrica704155
- 97. Trojano M, Pellegrini F, Fuiani A, et al. (2007) New natural history of interferon-beta-treated relapsing multiple sclerosis. Ann Neurol 61: 300–306.M. TrojanoF. PellegriniA. Fuiani2007New natural history of interferon-beta-treated relapsing multiple sclerosis.Ann Neurol61300306
- 98. Goodin DS, Jones J, Ebers GC, et al. Establishing long-term efficacy in chronic disease: The use of recurrent partitioning and propensity score adjustment to estimate long-term outcome in multiple sclerosis (submitted)DS GoodinJ. JonesGC EbersEstablishing long-term efficacy in chronic disease: The use of recurrent partitioning and propensity score adjustment to estimate long-term outcome in multiple sclerosis (submitted)
- 99. Goodin D, Ebers G, Reder A, et al. (2008) Early treatment with interferon beta-1b associated with improved long-term outcome in multiple sclerosis. Multiple Sclerosis 14: Suppl 1S44.P52D. GoodinG. EbersA. Reder2008Early treatment with interferon beta-1b associated with improved long-term outcome in multiple sclerosis.Multiple Sclerosis14Suppl 1S44P52
- 100. Hayes CE, Nashold FE, Spach KM, Pedersen LB (2003) The immunological functions of the vitamin D endocrine system. Cell Molec Biol 49: 277–300.CE HayesFE NasholdKM SpachLB Pedersen2003The immunological functions of the vitamin D endocrine system.Cell Molec Biol49277300
- 101. Heaney RP, Davies KM, Chen TC, Holick MF, Barger-Lux MJ (2003) Human surum 25-hydrocholecalciferol response to extended oral dosing with cholecalciferol. Am J Clin Nutr 77: 204–210.RP HeaneyKM DaviesTC ChenMF HolickMJ Barger-Lux2003Human surum 25-hydrocholecalciferol response to extended oral dosing with cholecalciferol.Am J Clin Nutr77204210
- 102. Kimball SM, Ursell , O'Connor P, Vieth R (2007) Safety of vitamin D3 in adults with multiple sclerosis. Am J Clin Nutr 86: 645–651.SM KimballUrsellP. O'ConnorR. Vieth2007Safety of vitamin D3 in adults with multiple sclerosis.Am J Clin Nutr86645651
- 103. Witte JS, Carlin JB, Hopper JL (1999) Likelihood-Based Approach to Estimating Twin Concordance for Dichotomous Traits. Genetic Epidemiol 16: 290–304.JS WitteJB CarlinJL Hopper1999Likelihood-Based Approach to Estimating Twin Concordance for Dichotomous Traits.Genetic Epidemiol16290304
- 104. McGuigan C, Dunne C, Crowley J, et al. (2005) Population frequency of HLA haplotypes contributes to the prevalence difference of multiple sclerosis in Ireland. J Neurol 252: 1245–1248.C. McGuiganC. DunneJ. Crowley2005Population frequency of HLA haplotypes contributes to the prevalence difference of multiple sclerosis in Ireland.J Neurol25212451248
- 105. Fisher LD, van Belle G (1993) Biostatistics: A methodology for the health sciences. New York: John Wiley & Sons. LD FisherG. van Belle1993Biostatistics: A methodology for the health sciencesNew YorkJohn Wiley & Sons