Introduction

The product of the TP53 gene is a transcription factor (p53) that activates or represses a large number of target genes that regulate a broad array of extremely important cellular functions, such as cell cycle, metabolism, DNA repair, senescence, and apoptosis. This factor is therefore essential for maintaining genome integrity [1]. In humans, p53 has 393 amino acids and the TP53 gene is located in the short arm of chromosome 17 [2]. Alterations of the TP53 gene or perturbations in the TP53 pathway are frequently correlated with carcinogenesis; more than 50% of human tumors carry mutations in this gene [3].

The steady-state levels of p53 are primarily determined by the rate at which it is degraded, rather than the rate at which it is produced. The TP53 gene is constitutively expressed in all cell types, but p53 does not accumulate in non-stressed cells, since it is rapidly degraded by the proteasome via ubiquitination [4, 5]. On the other hand, the p53 levels increase in response to various stress signals, such as UV irradiation, low oxygen concentrations (hypoxia), and exposure to high temperatures [6, 7, 8, 9].

There are many polymorphisms described for TP53, but a C→G non-synonymous substitution (rs1042522: c.215C>G, p. Pro72Arg; [10]) that promotes the amino acid change Pro→Arg at codon 72 of p53 is one of the most widely studied. This polymorphism has been described to be associated with an increased risk for developing cancer, since the p53-Pro allele is less active than p53-72Arg in inducing apoptosis, among other characteristics [11, 12].

Proper p53 transcriptional function is strongly linked to the activity of several other proteins encoded by the genes MDM2 (Mouse double minute 2 homolog; OMIM 164785), MDM4 (Mouse double minute 4 homolog, OMIM 602704), and USP7 (Ubiquitin-specific protease 7; OMIM 602519), also known as HAUSP (Herpesvirus-associated ubiquitin-specific protease). Another important gene in the so-called classical TP53 network [13] is LIF (Leukemia-inhibitory factor; OMIM 159540), which plays an essential role in the early phases of embryonic development in humans, and is regulated by p53 (S1 Fig).

The E3 ubiquitin-protein ligase, MDM2, mediates the activity of p53 by directing it to degradation by the proteasome [5, 14, 15]. MDM2 expression is also tightly regulated by p53 [16]. This auto-regulatory loop allows for the precise regulation of protein levels and activities of both p53 and MDM2 proteins [4, 17, 18].

The most well studied polymorphism in the MDM2 gene (rs2279744: c.14+309T>G) is located in its internal promoter. It consists of a single-nucleotide change from T→G, which increases the affinity of a sequence in MDM2 for the Sp1 transcription factor (Specificity protein 1; OMIM 189906). As a result, homozygotes for the G allele express more MDM2 than homozygotes for the T allele [19, 20]. In the presence of high levels of MDM2, there is a corresponding decrease of p53, causing a reduced response to cellular stress, impaired DNA repair, decreased apoptosis, and senescence [19]. Some studies have demonstrated that MDM2-309T and MDM2-309G alleles have different distributions in human populations [18, 21]. For instance, derived allele MDM2-309-G has higher frequency in European and Asian than African populations (average values: ∼0.35, ∼0.70, and ∼0.03, respectively; [22, 23, 24]). This allele may compensate for the higher apoptotic frequencies caused by the prevalence of allele p53-72Arg in Eurasians (∼0.56; [22, 23, 24]), suggesting adaptation [18].

The MDM4 protein, encoded by the MDM4 gene acts as a negative regulator of p53, inhibiting its transcriptional activity [25, 26, 27]. MDM2 and MDM4 form heterodimers with a high capacity for ubiquitination of target proteins, thus leading to degradation of targets, like p53 [28]. Deletion of either MDM2 or MDM4 induces p53-dependent early embryonic lethality in an animal model [16, 29]. The AA genotype for the single-nucleotide MDM4 polymorphism (rs1563828:g.204547449A>G) was associated with an increased risk for breast cancer [30].

Another important regulator of p53 is USP7, encoded by the USP7 gene, which deubiquitylates p53 and protects it from proteasome degradation [31]. The USP7 gene has a G→A substitution in intron 25 (rs1529916: g.8897333G>A), and derived allele A has been associated with endometriosis, female infertility, and prostate cancer [13, 32].

LIF is a cytokine expressed in various cell types, and its main function is to strengthen the blastocyst training of human embryos. In the very first days post-fertilization, LIF expression increases in the endometrium, creating a favorable environment for blastocyst implantation. Allele G of LIF (T→G transversion at the 3′ UTR region of the gene; rs929271: g.30242237T>G) is associated with female infertility [13]. LIF expression level is also known to be 2 times lower in cells bearing the p53-72Pro allele, compared to p53-72Arg, which can lead to the decrease of the implantation and fertility rate. In summary, several studies have strongly suggested that polymorphisms in the p53 signaling pathway play an important role in blastocyst implantation and are associated with recurrent pregnancy loss [13, 33, 34].

The genetic variability observed in contemporary human populations and the functionalities associated with the polymorphisms described above allow us to infer that a simple neutral model of mutation and drift is insufficient to explain the allelic distributions observed. Thus, it has been suggested that positive selection contributed to adaptation of Homo sapiens in different ecosystems. For example, the p53-72Arg allele (rs1042522) is more common in Europeans than in Africans, leading to the hypothesis that its distribution is dependent on latitude and maintained by selective pressures [35, 36]. On the other hand, Shi et al. [23] found that winter temperatures and UV radiation correlated significantly with the TP53 (rs1042522) and MDM2 (rs2279744) allele distributions in East Asian populations, indicating the possibility of adaptation to distinct environments.

America was the last continent occupied by humans in pre-colonial times. González-José et al. [37] and Bortolini et al. [38] suggested that an initial major dispersal began after 21,000 years before present, and that the biological and cultural characteristics of the first Americans that emerged, in part, were reshaped by recurrent trans-Beringian/circum-Arctic gene flow and important local population dynamics during a standstill period in Beringia. For example, Native Americans have experienced dramatic episodes of genetic drift and successive bottleneck events during migration across the continent. Furthermore, signals of positive natural selection associated to autochthonous environmental and cultural conditions have also been described [39, 40, 41].

Based on these findings, we hypothesize that the allele distributions of the classical TP53 pathway genes in Native American populations reflect adaptation, not only demographic and/or random events. To test our hypothesis, we determined the genotypes of the five above-mentioned SNPs in 282 unrelated individuals and compared the results to a large number of climate-related environmental variables, such as altitude, temperature, and seasonal mean UV radiation. Additional data regarding two of these SNPs (TP53-rs1042522 and MDM2-rs2279744) were compiled from the literature for a more extensive population analysis.

Materials and Methods

Results

Table 1 shows the derived allele frequency for each SNP investigated (individual genotypes can be seen in S2 File). Wide variations were observed in some allele frequencies in both population groups (highland and lowland). For instance, the frequency of MDM2-309-G is about five times higher in Guaraní Ñandeva than Guaraní Kaiowa, which may reflect genetic drift since the split of these two Guaraní partialities occurred less than 2,000 years ago [45]. On the other hand, several highland populations from Peru and Bolivia present similar distributions of MDM2-309-G. Most of these highland populations show deviations from the Hardy-Weinberg equilibrium (HWE), especially in Peruvian samples for the MDM2-309 locus (Table B in S1 File).

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Table 1. Derived¹ allele frequencies and AMOVA results.

https://doi.org/10.1371/journal.pone.0137823.t001

AMOVA analysis, using both data sets (Table 1), indicated that homogeneity and population structures could be seen in both highland and lowland populations. For instance, population structure measured by F_ST statistics (i.e. the among-populations component of genetic variance) is observed in the two groups considering TP53 rs1042522 (F_ST = 0.068 and 0.054, for highland and lowland, respectively), while for MDM2 rs2279744 homogeneity is observed in highland populations (F_ST = −0.020; p = 0.801) while high heterogeneity is observed in lowland populations (F_ST = 0.274; p < 0.001). Only the F_ST value observed for LIF rs929271 in the highland group (11.8%) is similar to the average estimated across the human genome (12%; [58]). For F_CT (between-groups component of variance), the variance is high (11%) for MDM2 rs2279744 data, indicating a remarkable and significant difference between the allelic distributions of the highland and lowland populations.

Correlation analyses

Correlation coefficients and their statistical significances are given in Table 2. In data set A, there were significant associations between the G allele of USP7 (rs1529916) and the annual mean of ultraviolet irradiance (rho = 0.760 p = 0.004) and PC1 (rho = −0.741, p = 0.006). This allele was also nominally associated with the mean of ultraviolet irradiance in the coldest semester (rho = 0.681 p = 0.015) and in the warmest semester (rho = 0.618 p = 0.032). MDM2 (rs2279744) T allele was nominally associated to longitude (rho = −0.587 p = 0.045) and the mean of ultraviolet irradiance in the coldest semester (rho = 0.605 p = 0.037), while LIF (rs929271) T allele was nominally associated to annual mean of ultraviolet irradiance (rho = 0.693 p = 0.013) and PC1 (rho = −0.664, p = 0.018).

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Table 2. Correlation analysis results.

https://doi.org/10.1371/journal.pone.0137823.t002

In data set B, there were significant associations between the T allele of MDM2 (rs2279744) and altitude (rho = 0.673, p = 0.003), the mean of ultraviolet irradiance in the coldest semester (rho = 0.827 p < 0.001), and PC1 (rho = −0.610 p = 0.009). This allele was also nominally associated to PC2 (rho = −0.567, p = 0.018).

Gene-gene interaction analyses

We used the MDR approach to search for gene-gene interactions (Table D in S1 File). These analyses were intended to explore differences between highland and lowland populations in genotype combinations among the SNPs investigated since gene networks, such as those investigated here, can be sources of epistasis. Significant two- (p = 0.004) and three-locus interactions (p = 0.004) were identified in data set A. However, an analysis of IG based on entropy measures revealed that these effects were not explicated by epistasis (negative values in the branches among nodes; Fig 1A). On the other hand, IG values of both USP7 (7.26%) and LIF (4.23%) indicated that both genes have a large main effect in a scenario where altitude is considered, corroborating our previous analysis. Regarding data set B, the largest main effect was observed for MDM2 (IG = 9.51%), which contrasts with the low value for TP53 (IG = 0.41%). A potential synergism (epistasis) between the two loci was also found, but it is apparently weak (IG value of only 1.54%; Fig 1A), at least when it is compared with the potential mechanism of action on MDM2. On the other hand, it is 3.7 times greater than the main effect of TP53. It is noteworthy that independent of the TP53 genotype, the genotype MDM2-TT is always favorable and most commonly found in highlands (Fig 1B). In other words, MDM2 showed the greatest contribution to adaptation to hostile environments, such as those found in the highlands.

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Fig 1. Summary of the multifactor dimensionality reduction (MDR) interaction models.

(A) Interaction graphs comprised of nodes with pairwise connections between them. Values in nodes represent information gain (IG) of individual genes (main effect), while values between nodes are the IG of each pairwise combination (interaction effects). Positive entropy (plotted in red) indicates interaction (epistasis) and negative entropy (plotted in green or blue) indicates redundancy. Independence is represented by the gold color. (B) The MDM2-TP53 interaction associated with altitude in data set B. High-frequency genotype combinations in individuals who inhabit highlands (≥ 2,500 meters) are depicted as darkly shaded cells and low-frequency combinations in those individuals as lightly shaded. For each cell, the left bar indicates the absolute number of individuals who inhabit highlands and the right bar the absolute number of individuals who inhabit lowlands (< 2,500 meters).

https://doi.org/10.1371/journal.pone.0137823.g001

Discussion

More than 60,000 scientific studies have been published in the last 30 years concerning the roles of TP53 network genes, as well as of their variants, in human susceptibility to cancer and other pathological conditions. Special issues in scientific journals, dedicated to these topics, have also been published (see, as example [59]). This overwhelming number of studies contrasts with the rarity of studies of an evolutionary context, which are indispensable for explaining differences in the TP53 network allele distributions along human populations, which often cannot be understood as simply a result of stochastic processes. Our goal here was to help fill this gap, providing information about five polymorphisms of the classical TP53 network in Native American populations and how their variability patterns could be explained.

Our analysis of data set A, which included original information of 5 SNPs in 12 Native American populations, suggests a well-known role of genetic drift in those groups, illustrated by wide difference in MDM2-G allele frequencies between the two Guaraní sub-groups. However, other instigating results can be associated to adaptation to environmental conditions in Native American populations. Alleles USP7-G (rs1529916) and LIF-T (rs929271) were correlated with ultraviolet irradiance and index of temperature and precipitation, variables comprising PC1. Additionally, examining variables with the highest representation in the PC1 components (> 0.90), it is possible to see that in regions where the annual mean temperatures, minimum temperatures of the coldest month, mean temperatures of the driest quarter, mean temperatures of the coldest quarter, and precipitation are low, the presence of ancestral alleles G and T are significantly higher. In other words, our analysis as whole reveals that alleles USP7-G and LIF-T are more highly represented in stressful environments (low temperature, arid climate, wide temperature range during the day, and high levels of UV radiation), which is typical of high altitudes. It is noteworthy that derived alleles of these SNPs have been associated with cancer susceptibility, infertility, and endometriosis [13, 32], so that the alleles USP7-G and LIF-T could be considered as protective factors against the consequences of harsh environmental stress.

Human populations living at high altitudes are likely to have developed specific adaptations to support both the harsh conditions described above and low oxygen concentrations (hypoxia; [41]). Monge in 1948 [60] proposed that the hypoxia could reduce fertility in humans. However, recent studies have shown that the reproductive functioning of populations indigenous to high altitudes is adapted to hypoxia and other extreme conditions [61]. Our results with USP7 (rs1529916) and LIF (rs929271) polymorphisms could be connected with adaptation of the reproductively successful ancestors of modern Andes populations.

In examining data set B, we found the ancestral MDM2-T allele is strongly correlated with winter mean UV radiation, altitude, and PC1. The highest representations in the PC1 components (> 0.90) are annual mean temperature, minimum temperature of coldest month, minimum temperature of coldest quarter, and annual precipitation. Allele T is significantly more frequent in communities located at high altitudes experiencing extreme environmental conditions, such as high UV radiation and dry and cold climate. In addition, the binary logistic regression analysis showed that MDM2-TT individuals are more frequently found in highlands. MDM2-TT homozygotes express typical steady-state levels of MDM2, maintaining an adequate level of p53 [20], and consequently can appropriately respond environmental stresses. An important confounding factor could be admixture with Europeans, which is more important in Andean than in the lowland populations considered here [42, 62, 63]. However, any effect of admixture would be in the opposite direction, since MDM2-G frequency is relatively high in Spaniards (0.39; [24]).

The inverse correlation between MDM2-T frequencies and winter UV radiation is consistent with the findings of Shi et al. [23], which showed that low levels of UV are significantly correlated with genotype MDM2-GG in Han Chinese populations, similarly deviating from HWE. These authors suggested that MDM2-GG is selected for in areas of low UV activity (at high altitudes, the thinner atmosphere will filter less UV radiation; consequently for every 1000 m increase in altitude, the UV radiation level will increase ∼12%; http://www.weather.gov.hk/radiation/tidbit/201012/uv_e.htm [64]). Natural selection can be evoked to explain these results, although the HWE test is considered too weak to detect this phenomenon.

As mentioned above, native Andean populations have successfully adapted to environments with low oxygen concentrations. One gene that contributes to hypoxia adaptation is EPAS1 (Endothelial PAS domain-containing protein 1, also known as HIF-2α, Hypoxia-inducible factor—alpha 2 (OMIM 603349)), which acts by preventing toxicity promoted by hypoxia. This gene plays an important role in both the classical and the expanded TP53 network. For instance, the alpha subunit of EPAS1 regulates p53 activity, including through prevention of damage-induced degradation and nuclear export of MDM2, stabilizing nuclear p53 [65]. Foll et al. [41] confirmed the action of positive selection on EPAS1 in both Tibetans and Andeans. Furthermore, several studies have revealed a role for p53 and its regulation in physiological and metabolic processes resulting from environments with low oxygen concentrations [8, 66, 67]. Recently, Eichstaedt et al. [68] studied an indigenous population living in the Argentinean Andes (Colla) and identified signatures of positive selection in genes involved in cellular hypoxia, including TP53. Importantly, hypoxia induces p53 accumulation through down-regulation of MDM2 [66]. These results reinforce our suggestion that individuals with the MDM2-TT genotype represent an adaptation to the environmental stresses of high altitudes. In addition, the interaction analysis performed by the MDR method using both data sets (A and B) revealed the potential for the MDM2, LIF, and USP7 genes to play an additional central role in a high altitude setting. Thus, taken together, our results demonstrate that variation of the p53-activating stressors could not be directly correlated with p53-Pro72Arg alleles, but with frequencies of the other functional polymorphisms examined, such as USP7-G (rs1529916), LIF-T (rs929271), and MDM2-309, as well as synergic interactions between them.

Under neutral model conditions, South Amerindians living in lowlands present higher levels of population structure when compared to those seen in indigenous Andean communities [62, 69]. However, not all F_ST values obtained in our study were consistent with this expectation (Table 1). Positive selection disturbs the patterns of genetic variation expected under a standard neutral model [70]. Additionally, it is possible to see that some derived alleles, such as MDM2-G, have high frequencies in Asian populations with putative common ancestry (0.57–0.82; [22, 23, 24]), but a surprisingly low distribution in Andeans (average value: ∼0.13). An excess of unexpectedly low and/or high frequencies of derived alleles can also be considered a marker of positive selection [70]. Thus, the distributions of the classical TP53 pathway alleles in Native American populations could be under selective pressure. Sucheston et al. [22] investigated 52 worldwide populations from the HGDP-CEPH-panel for the prevalence of p53-Pro72Arg and MDM2-309 polymorphisms, but found no significant association with climate variables. However, the Native American samples in the Sucheston et al.’ study [22] were much smaller than the present study (see Table 1), which may explain the divergent results.

Finally, government surveys in Peru indicate that the rate of gestational and postpartum complications in Aymara regions is lower than the national average (1.8% and 5% respectively; http://www.dge.gob.pe/publicaciones/pub_asis/asis26.pdf, p. 165; [71] http://www.dge.gob.pe/portal/docs/intsan/asis2012.pdf, p. 76 [72]). These same official sources also indicate differences in the cancer incidences between lowland localities and some regions situated at high altitude (for example in the Puno state, where the Anapia community is located; http://www.dge.gob.pe/portal/docs/asis_cancer.pdf, p. 64 [73]). These findings are in agreement with our genetic results. However, only additional and specific studies can accurately relate our evolutionary findings with those related to the health of contemporary Andean populations.

A well-regulated p53 network is crucial for maintaining genomic integrity. Several polymorphisms in this pathway have been described, and the different allele frequencies among human populations have been interpreted as the result of selective pressure. Humans occupied high-altitude locations in the Andes as early as 12,800 years ago, providing a sufficient period of time for the initiation of organismal selection and developmental functional adaptation ([74] and references therein). Here we are suggesting that natives from Andes, who are subjected to low temperatures, arid climates, wide temperature ranges during the day, high levels of UV radiation, and hypoxia, among other environmental insults, are protected by a selected combination of alleles/genotypes of the TP53 pathway. The present study identifies for the first time the potential role of the MDM2, LIF, and USP7 in the adaptation of the Andean populations.

Supporting Information

Acknowledgments

We are very grateful to the individuals who donated the samples analyzed here and to the Fundação Nacional do Indio (Brazil) for logistic support. We thank René Vasquez for his assistance in the sample collection in Bolivia and Sidia Maria Callegari Jacques for statistical support. We are grateful to professor David Comas for his careful review of this manuscript.