Background
Polycystic ovary syndrome (PCOS) is a common endocrine disorder affecting 9 to 18% of women of reproductive age according to different diagnostic criteria [
1‐
3]. While its etiology remains unclear, PCOS is considered a polygenic and multifactorial disease, with metabolic, endocrine, and reproductive alterations [
4]. In PCOS women, evidence suggests that vitamin D levels may be decreased and related to hormone and metabolic disturbances [
5,
6].
The vitamin D receptor (VDR) is expressed in many tissues and organs (such as those involved in calcium homeostasis mechanisms), in glucose metabolism, and in the reproductive system [
7], and modulates vitamin D action in these systems.
VDR gene (ID: 7421) polymorphisms have been investigated in PCOS as well as in disturbances of androgen secretion. A previous study has suggested an association between
VDR gene variants and precocious pubarche (PP) [
8]; in turn, data on PCOS risk are controversial, with a relationship between
VDR gene variants and PCOS detected by some [
9‐
12] but not all studies [
13‐
15]. Regarding endocrine characteristics,
VDR gene polymorphism has been associated with total testosterone in PCOS and PP populations [
8,
13], with estradiol levels in PP girls [
8], and with metabolic abnormalities in different non-PCOS populations [
16‐
23].
Therefore, the aims of the present study were to assess the genotypic and allelic distribution of Bsm-I (rs1544410), Apa-I (rs7975232) and Taq-I (rs731236) polymorphisms of the VDR gene and to determine whether these gene variants are associated with 25-hydroxyvitamin D [25(OH)D] levels and with metabolic abnormalities, including MetS, in women with PCOS in comparison to non-hirsute, ovulatory control women.
Discussion
In the present study, despite the similar vitamin D levels detected in PCOS and control participants, the CC genotype of Apa-I SNP of the VDR gene was specifically related to higher risk of MetS in PCOS participants. Moreover, this same genotype was associated with higher blood pressure, total cholesterol, and LDL-c in both PCOS and control participants. To the best of our knowledge, this is the first report to show an association between Apa-I VDR gene polymorphism and MetS in a PCOS population. This observation is relevant because it may help explain the meaning of vitamin D level variation, which may not play a role per se, but rather reflect a putative gene-environment interaction in different populations.
The few available studies analyzing the influence of Apa-I
VDR gene polymorphisms on metabolic variables in PCOS women have reported no association with insulin resistance [
10,
13] or glucose and lipid abnormalities [
10]. However, data from non-PCOS populations suggest that metabolic abnormalities, such as obesity, insulin resistance, low HDL-c, and type 2 diabetes are associated with the
VDR gene [
16‐
21]. In this sense, a recent meta-analysis comprising 9232 participants showed that the association between insulin resistance-related diseases and Apa-I and Bsm-I
VDR gene variants was more pronounced in dark-pigmented Caucasians and Asians than in Caucasians with white skin. In the sub-group analysis, Bsm-I (GG genotype) was associated with MetS, and the Apa-I variant (CC genotype) was associated with insulin resistance-related diseases in a population living in a mid-latitude zone (30°–60°) [
23], which is also the case of the present population (30°01′59”S).
While a functional role of
VDR gene polymorphisms has not yet been established, the association between Apa-I gene variant and MetS observed in the present study could be assumed to be linked to disturbed
VDR gene expression [
33]. The Apa-I polymorphism is located at the 3′ untranslated region (3′ UTR) of the
VDR gene, which has been recognized as being involved in the modulation of gene expression, especially through the regulation of mRNA stability and efficiency of protein translation [
34]. Moreover, the methylation levels of the
VDR gene appear to be altered according to race and presence of the polymorphisms of the 3’UTR region of the gene [
35]. Additionally, Apa-I is in strong linkage disequilibrium with other
VDR gene polymorphisms in different populations [
22,
36], which may be contributing to the general transcriptional activity of
VDR in different biological processes. Importantly, the
VDR gene regulates more than 200 genes, and mediates most effects of vitamin D on gene expression via formation of a heterodimer with the retinoid X receptor molecule, which binds to promoter regions of many target genes [
37,
38].
In our study, lower 25(OH)D levels were associated with MetS and with its isolated components in PCOS women, such as higher glucose, waist circumference and triglycerides. In this sense, the present results are in agreement with a meta-analysis reporting that women with PCOS and vitamin D deficiency are more likely to have dysglycemia compared to those without vitamin D deficiency [
5], and that in women with both PCOS and MetS, vitamin D levels are lower than in women with PCOS and without MetS [
39].
Similar vitamin D levels were detected in the present study in PCOS and control participants regardless of the presence of Apa-I SNP. Interestingly, while two meta-analyses [
5,
6] comprising 3182 and 2262 women respectively showed that serum 25(OH)D concentrations were lower in PCOS compared to controls, the reported standardized mean difference between the groups in both studies seems of little clinical relevance – only 0.74 ng/mL (95%IC: -1.26 to − 0.22) [
5] and 0.64 ng/mL (95%IC: -1.12 to − 0.15) [
6]. In turn, the fact that our PCOS patients with MetS had lower vitamin D levels and higher frequency of CC polymorphism compared to those without MetS suggests that the Apa-I gene variant might impact vitamin D levels in PCOS with MetS. In fact, vitamin D status is influenced by many factors, especially dietary pattern, season, and genetic traits [
40]. A better understanding of the genetic factors that may be involved in vitamin D level variation and metabolic disturbances could shed some light on hypothetical gene-environment interactions of vitamin D. Further studies with larger PCOS populations and higher proportion of MetS are needed in order to confirm this hypothesis.
We did not find any association between genotypes or haplotypes of
VDR gene variants in PCOS participants. Only a few studies are available in the literature assessing
VDR gene polymorphism and risk of PCOS, with uncertain conclusions, which vary according to the studied sample. While some studies show an association between at least one
VDR gene polymorphism and PCOS [
9‐
12,
41], others report similar distributions of Bsm-I, Apa-I and Taq-I polymorphisms in PCOS and control women [
13‐
15]. Also, regarding haplotypes of
VDR gene variants, no definitive data are available, with few reports of distinct haplotypes of
VDR gene polymorphisms presenting slightly higher frequency in PCOS women when compared to controls [
10,
12,
14]. These unclear data may be, at least in part, attributed to ethnic differences in the studied populations and to the polygenic condition of PCOS. Yet, other studies have reported an association of
VDR gene polymorphisms with PP [
8] and diabetes [
42‐
45].
One strength of our study is the focus on a less well represented ethnic group, PCOS women from southern Brazil, with assessment of gene variants which may be contributing to this polygenic and multifactorial disease. Furthermore, we evaluated polymorphisms found in a genomic position that plays an important role in the modulation of gene expression. Limitations of the present study are the relatively small sample size of 291 participants (191 PCOS and 100 controls) and the low frequency of MetS in the control group, precluding complementary analyses correlating VDR gene polymorphisms and MetS in that group. In addition, further studies on functional evaluation of VDR SNPs are needed in order to deepen the understanding of findings.