Figure 1.
Strategy for SNP selection using genome-wide association and candidate gene studies.
Figure 2.
Genetic variation in the vitamin D synthesis and metabolic pathway.
Skin exposure to ultraviolet B (UVB) radiation initiates the conversion of 7-dehydrocholesterol to previtamin D3. 7-dehydrocholesterol reductase (DHCR7) encodes the enzyme 7-dehydrocholesterol reductase, which converts 7-dehydrocholesterol to cholesterol, thereby removing the substrate from the synthetic pathway of vitamin D3. The previtamin D3 in turn gets converted to vitamin D3 in a heat-dependent process. Vitamin D (represents D2 or D3) is transported to the liver, where it is converted by vitamin D-25-hydroxylase (CYP2R1) to 25-hydroxyvitamin D [25(OH)D]. This is the major circulating form of vitamin D that is used by clinicians to determine vitamin D status. This form of vitamin D is biologically inactive; it is bound to the vitamin D-binding protein (GC), transported to the kidneys and converted by 25-hydroxyvitamin D-1α- hydroxylase (1-OHase) (CYP27B1) to the biologically active form 1,25-dihydroxyvitamin D3 (Calcitriol). Calcitriol increases the expression of 25-hydroxyvitamin D-24- hydroxylase (24-OHase) (CYP24A1) to catabolise 25(OH)D to the water-soluble, biologically inactive calcitroic acid, which is excreted in the bile. DHCR7 and CYP2R1 function upstream of the production of 25(OH)D and hence, termed as 25(OH)D synthesis indicators, while GC, CYP27B1 and CYP24A1 function downstream of the 25(OH)D production and hence, termed as 25(OH)D metabolism indicators.
Figure 3.
The selection of vitamin D SNPs for the use as instruments in Mendelian Randomization (MR) analysis.
Table 1.
Association of SNP with ln 25-hydroxyvitamin D adjusted for sex.
Figure 4.
Association between the SNPs, synthesis, metabolism and metabolismGWA allele scores and ln 25(OH)D with and without adjustment for biomarkers, dietary and lifestyle indicators.
The bars are the 95% CI. Biomarkers: coagulation markers- von Willebrand factor, tPA and D-dimer; Inflammatory markers- fibrinogen and CRP; Lipid marker- Triglycerides, low density lipoproteins, high density lipoproteins and total cholesterol; Lung function marker- FEV; Cardiovascular disease related factors- diastolic and systolic blood pressures, IgE, IGF1 and HbA1c). Dietary and lifestyle markers: time spent outside, sun cover, oily fish consumption, vitamin D supplements, season, smoking, alcohol consumption, PC/TV time, recreational MET hours, social class, body mass index, abdominal obesity and geographical region.
Figure 5.
Associations of the five SNPs and allele scores with geographical region, social, dietary and lifestyle factors.
The bars are the 99.6% CI. The effects of the allele scores and the individual SNPs for each lifestyle factor can be identified based on the intensity of the coloured boxes.
Table 2.
Association of Allele Scores with ln 25-hydroxyvitamin D concentrations adjusted for sex.
Figure 6.
Power and sample size to detect the 5% decrease in blood pressure by 10 nmol/l increase in 25(OH)D observed in the 1958 British birth cohort using genetic proxy indicators (significance level α = 0.05).
The curves in (A) from the bottom to the top of the graph are in the order of min effect size with CYP27B1 (short dash), CYP24A1 (long dash), DHCR7 (dash dot), CYP2R1 (dash), GC (dot). The curves in (B) from the bottom to the top of the graph are in the order of min effect size with Synthesis score (dash dot), MetabolismGWA score (long dash) Metabolism score (dash), both scores (dot). The horizontal black line and attached vertical dashed lines indicate the sample size required for a study with 80% power using the genetic proxy.