Vitamin D receptor gene polymorphisms, serum 25-hydroxyvitamin D levels, and melanoma: UK case–control comparisons and a meta-analysis of published VDR data

Abstract

We have carried out melanoma case–control comparisons for six vitamin D receptor (VDR) gene single nucleotide polymorphisms (SNPs) and serum 25-hydroxyvitamin D₃ levels in order to investigate the role of vitamin D in melanoma susceptibility. There was no significant evidence of an association between any VDR SNP and risk in 1028 population-ascertained cases and 402 controls from Leeds, UK. In a second Leeds case–control study (299 cases and 560 controls) the FokI T allele was associated with increased melanoma risk (odds ratio (OR) 1.42, 95% confidence interval (CI) 1.06–1.91, p = 0.02). In a meta-analysis in conjunction with published data from other smaller data sets (total 3769 cases and 3636 controls), the FokI T allele was associated with increased melanoma risk (OR 1.19, 95% CI 1.05–1.35), and the BsmI A allele was associated with a reduced risk (OR 0.81, 95% CI 0.72–0.92), in each instance under a parsimonious dominant model. In the first Leeds case–control comparison cases were more likely to have a higher body mass index (BMI) than controls (p = 0.007 for linear trend). There was no evidence of a case–control difference in serum 25-hydroxyvitamin D₃ levels. In 1043 incident cases from the first Leeds case–control study, a single estimation of serum 25-hydroxyvitamin D₃ level taken at recruitment was inversely correlated with Breslow thickness (p = 0.03 for linear trend). These data provide evidence to support the view that vitamin D and VDR may have a small but potentially important role in melanoma susceptibility, and putatively a greater role in disease progression.

Introduction

High penetrance genes such as CDKN2A and CDK4 underlie susceptibility to melanoma in rare families.¹ A number of phenotypic factors that are in part genetically determined, including number of common and atypical nevi, red or blonde hair, skin type and tanning ability, have been identified as risk factors for melanoma in the general population.² Recent large-scale association studies have shown that the genetic determinants of some of these phenotypes, such as red hair and freckles,³ are also risk factors for melanoma.4, 5 These genes include the melanocortin 1 receptor (MC1R) gene, which was previously identified as a melanoma susceptibility gene in case–control association and familial melanoma studies.5, 6, 7, 8, 9 The large-scale association studies have also provided strong evidence to support the role of other pigment genes such as the agouti signalling protein (ASIP) locus and tyrosinase (TYR)4, 10 as melanoma susceptibility genes. There are some reports of other associations with candidate genes and risk but the studies are small and sometimes conflicting.

25-Dihydroxyvitamin D₃ (1,25(OH)₂D₃), the active metabolite of the steroid hormone vitamin D₃, is a potent regulator of cell growth and differentiation¹¹ and moderates gene transcription by binding to the vitamin D receptor (VDR). 25-Hydroxyvitamin D₃ is also implicated in cell death, tumour invasion and angiogenesis,11, 12 making it an important candidate for moderating both risk of melanoma and prognosis from melanoma. In this study we have looked at inheritance of variants in the VDR gene and susceptibility to sporadic melanoma.

Five VDR polymorphisms (Cdx2, rs11568820; FokI, rs2228570, previously known as rs10735810; BsmI, rs1544410; ApaI, rs7975232 and TaqI, rs731236), primarily chosen based on predicted changes in receptor activity and/or expression based on in vitro and in vivo evidence reviewed by Uitterlinden,¹³ have been studied by other groups. The functional consequences of the Cdx2 and FokI variants have been extensively characterised, whilst the functionality of the common BsmI–ApaI–TaqI haplotypes is less clear, despite the publication of a very large number of studies looking at the association of these haplotypes with many different diseases. More recently, an additional promoter polymorphism has been described in a melanoma population, 1012 bp upstream of the exon 1a transcription start site (A-1012G, also known as GATA; rs4516035).¹⁴

Previous studies have reported associations between some or all of these VDR polymorphisms and risk and/or outcome of a number of malignancies, including prostate, colon, breast and renal cancers.15, 16, 17, 18, 19 However, there have been a limited number of small studies examining the role of VDR polymorphisms in melanoma.14, 20, 21, 22, 23, 24 In 2008, one of these studies was updated in the literature, with increased numbers of cases, and inclusion of BsmI for the first time.²⁵ Recently two meta-analyses were reported that support the view that BsmI²⁶ and both BsmI and FokI,²⁷ respectively, are associated with melanoma risk. Here, we looked at these six single nucleotide polymorphisms (SNPs) in two Leeds case–control data sets to investigate the effect on risk of melanoma and on Breslow thickness.

Sunburn and intense intermittent sun exposure are associated with an increased risk of melanoma.28, 29 The lack of risk for most body sites associated with occupational sun exposure at most latitudes however has led to the view that sun exposure has a complex relationship with melanoma risk, and that chronic sun exposure might even be protective for melanoma in some low sun exposure countries as a result of enhanced vitamin D synthesis in the skin.²⁸ Clarification of the potential association of vitamin D and the VDR with melanoma risk remains therefore of importance.

Vitamin D levels are known to be lower in obese subjects,30, 31, 32 and we have therefore also explored the relationship between body mass index (BMI), serum 25-hydroxyvitamin D₃ levels (henceforth referred to as serum vitamin D levels) and melanoma risk. Increased BMI has been reported to be associated with melanoma risk. The observation was first made by Thune et al.³³ and subsequently by a series of other groups.34, 35, 36, 37, 38 The underlying biological explanation for this association is not understood, although one study provided some evidence that high fasting glucose was associated with melanoma risk.³⁹

In two Leeds case–control series, we have looked at inheritance of the aforementioned six single nucleotide polymorphisms (SNPs) in VDR to explore susceptibility to sporadic melanoma. We carried out a meta-analysis of all published SNP data including our new Leeds data. We looked at the effect of these SNPs and of serum vitamin D levels taken at study recruitment on Breslow thickness as a marker of tumour progression.