A meta-analysis of the association of ApaI, BsmI, FokI, and TaqI polymorphisms in the vitamin D receptor gene with the risk of polycystic ovary syndrome in the Eastern Mediterranean Regional Office population

Abstract Background The results of case-control studies on the association between vitamin D receptor gene (VDR) polymorphisms and polycystic ovary syndrome (PCOS) are inconclusive. Objective We aimed to more precisely evaluate the correlation between the ApaI, BsmI, FokI, and TaqI VDR gene polymorphisms and PCOS susceptibility. Materials and Methods PubMed, Scopus, Science Citation Index, and Google Scholar databases were searched to retrieve related reports released up to the end of 2020. To evaluate the association strength of the VDR gene polymorphisms with PCOS risk, pooled odds ratios (OR) with a 95% confidence interval were determined. Results In total, 1,119 subjects (560 PCOS cases and 559 controls) from 7 studies were included which met the inclusion criteria. A statistically significant association between the TaqI polymorphism and PCOS susceptibility was found in the Eastern Mediterranean Regional Office population (T vs. t: OR = 0.715; TT vs. tt: OR = 0.435, p < 0.001; TT vs. Tt+tt: OR = 0.696, p = 0.01; tt vs. TT+Tt: OR = 1.791, p < 0.001). It was found that the ApaI variant was a risk factor in the dominant inheritance model (AA vs. Aa+aa: OR = 1.466, p = 0.01) and the FokI polymorphism was a protective factor in the recessive inheritance model (ff vs. FF+Ff: OR = 0.669, p = 0.04). The VDR BsmI polymorphism did not show association with PCOS susceptibility. Conclusion Our meta-analysis revealed that the VDR ApaI in the dominant model, VDR FokI in the recessive model, and VDR TaqI polymorphisms in all genetic models are associated with vulnerability to PCOS. However, further studies with a larger sample size are required.


Introduction
Polycystic ovary syndrome (PCOS) is one of the most common syndromes in reproductive-age people, distinguished by clinical characteristics such as prolonged anovulation, menstrual dysfunction, and polycystic ovaries (1,2).
According to the National Institutes of Health 1990 criteria and the Rotterdam 2003 criteria, the combined prevalence of PCOS is ∼4-21% worldwide (3). Women with PCOS have a high probability of type 2 diabetes mellitus and cardiovascular disease (4,5). Patients with PCOS have an impaired glucose tolerance, insulin resistance, hyperinsulinemia, and obesity (6,7). However, the exact etiology and underlying pathological mechanisms of PCOS are not clear.

Interactions
between genetic and environmental factors are believed to play a significant role in the incidence and progression of PCOS (8,9,10). Insulin resistance and hyperinsulinemia are recurrent metabolic disturbances in PCOS. Research demonstrates that levels of vitamin D could be related to hormonal and metabolic conditions (11). Vitamin D is converted to 1, 25-dihydroxycholecalciferol in the kidneys and liver (12,13). Lines of evidence suggest a significant relationship of vitamin D levels with the pathogenesis, signs, and symptoms of PCOS (14). A recent meta-analysis identified more considerable variation in serum 25-hydroxyvitamin D, total cholesterol, serum insulin, low-density lipoprotein cholesterol, and triglycerides in women with PCOS than in healthy subjects (15).
The vitamin D receptor (VDR), a liganddependent transcription factor belonging to the steroid/thyroid hormone receptor superfamily, is responsible for vitamin D's effect. VDR is commonly found in many tissues of the female reproductive system and regulates the biological effects of vitamin D (16,17). As is illustrated in figure 1, VDR and unoccupied retinoid X receptor form a heterodimer transcription unit which regulates the transcription through binding to the vitamin D response element in the promoter region of target genes (18,19). About 3% of the human genome, including genes essential to glucose metabolism, are regulated by VDR (13).
The VDR gene is located on chromosome 12q13.11 and contains 14 exons that provide instruction for a 427-amino-acid protein (20).

Data extraction
The eligible studies were selected according to the following criteria by 2 independent authors (A.Az and A.SH), and any disagreements were decided by a third reviewer. The first author of each study, publication year, country of origin, the sample size of cases and controls, and data of the frequency genotype of distribution were recorded.

Ethical considerations
The

Statistical analysis
Using

Characteristics of eligible studies
The studies used in this meta-analysis were selected using the preferred reporting items for systematic reviews, and meta-analyses flow diagram, as seen in figure  case-control studies, respectively. Table I   Reports assessed for eligibility (n = 27) Reports excluded: Reviews (n = 5) Studies on the correlation of PCOS with VDR gene polymorphisms in non-EMRO population (n = 12) Insufficient data (n = 3) Studies included in review (n = 7) Reports of included studies (n = 7) Studies on ApaI (7) Studies on BsmI (4) Studies on FokI (6) Studies on TaqI (6) Identification Screening Included Figure 2. Flow chart of the literature retrieval and selection process. Table II shows

VDR ApaI variant and PCOS risk
This review included 6 studies on the relationship between the VDR ApaI rs7975232 (A > C) polymorphism and risk of PCOS in the EMRO population. In all reports, the heterogeneity test showed no substantial heterogeneity. Therefore, the fixed effects model findings from the Mantel-Haenszel system were applied. As illustrated in figure 3, the correlation of ApaI SNP in the VDR gene with vulnerability to PCOS was statistically significant only in the dominant genetic model in the total populations (AA vs. Aa+aa: OR = 1.466, 95% CI = 1.093-1.9671, p = 0.01). No significant relationships of the VDR ApaI polymorphism with susceptibility to PCOS were observed in the other models (allelic, homozygote, and recessive genetic models, p > 0.05).

VDR BsmI variant and PCOS susceptibility
studies on the relationship between the VDR BsmI polymorphism and PCOS susceptibility were included in the metaanalysis (Table I)

VDR FokI variant and PCOS susceptibility
The current meta-analysis included a total of 8 reports on the relationship between the VDR FokI polymorphism and PCOS susceptibility, as seen in table I. We used the findings from the randomeffects model on the grounds that heterogeneity was significant in certain model contrasts (allelic and homozygote models). A significant association of decreased risk was observed in the recessive genetic model (ff vs. FF+Ff: OR = 0.669, 95% CI = 0.455-0.982, p = 0.04) (Figure 4). In the allelic (F vs. f), homozygote (FF vs. ff), and dominant (FF vs. Ff+ff) genetic models, the pooled analysis found no significant association between this locus and PCOS risk (p > 0.05).   (Table II, Figures 5 and 6).

Publication bias
As indicated in table II, the Begg and Egger trials and the Begg funnel plots were carried out in all comparisons. No significant asymmetrical evidence was found, indicating that there was no publication bias in the meta-analysis.

Discussion
In this meta-analysis, we presented the evidence Consistent with our results, a meta-analysis disclosed that the T allele of the VDR TaqI SNP was significantly associated with PCOS risk.
However, a correlation of VDR ApaI and BsmI with susceptibility to PCOS was not found in any of the included studies (43). In contrast to our findings, a meta-analysis published in 2019 found a link between ApaI and BsmI, and PCOS vulnerability International Journal of Reproductive BioMedicine Shahmoradi et al. in the Asian population. However, VDR TaqI did not show a connection (22). In a similar study, a group of researchers discovered an essential link between ApaI and PCOS vulnerability but no link between TaqI and risk of PCOS (14). However, they found only a small increase in PCOS risk associated with the BsmI A/G polymorphism in Asians based on ethnic diversity. Overall, this study's results are aligned with some studies (24,26,28,32) and are in contrast to others (44)(45)(46), and might shed some Another of the VDR variants is FokI, which is located on exon 2 and results in a protein of different sizes. While this SNP has not been shown to interact with ligand binding, heterodimer formation with retinoid X receptor, or DNA binding, most studies have indicated that the shorter type of the protein (424 aa) is somewhat more active than the longer form (427 aa). However, it tends to be a gene-and cell-type-specific phenomenon, indicating that specific genes and cell types could be more susceptible to the polymorphism's effect than others (40,47).
According to some research, this polymorphism controls mRNA expression and may lead to different diseases (48). Although our study found that VDR FokI in the recessive genetic model (ff vs. FF+Ff) can influence PCOS, the other genetic models did not reveal significant protective/susceptible effects. Similar to our findings, 2 other meta-analyses also showed that VDR FokI (rs2228570) did not reveal a relationship with PCOS susceptibility (14,22). The association of the FokI polymorphism with PCOS susceptibility has been examined in several case-control studies with varied results (23,29,31). These and the relatively small sample size.

Conclusion
In summary, the current meta-analysis provided