This meta-analysis, the first to our knowledge on this topic, indicated that the available evidence is in favor of a significant association between the VDR BsmI polymorphism and the risk of developing cutaneous melanoma (Fig. 1). By contrast, the analysis of pooled data ruled out the melanoma-promoting role of the FokI polymorphism that was hypothesized by some investigators. In addition, available evidence was not sufficient to draw any conclusion regarding the role of the other 3 SNPs (TaqI, EcoRV, and Cdx2) that have been investigated to date. To interpret our findings, we need to briefly summarize the current knowledge regarding the VDR gene organization and the functional effects of the 5 VDR polymorphisms considered.
VDR Gene Organization and Polymorphisms
Located on chromosome 12q12-q14, the VDR gene contains 5 promoter regions, 8 protein-coding exons, and 6 untranslated exons; and all of these regions are spliced alternatively. At least 196 VDR SNPs have been described,29, 30 and 64 of them lie in the promoter region, 32 lie in the 3′ and 5′ untranslated regions, and 2 synonymous SNPs and 2 nonsynonymous SNPs lie in the coding region.5, 31 The 5 SNPs that were studied for their association with melanoma risk usually are grouped into 2 categories: a cluster of tightly linked polymorphisms at the 3′ end (BsmI, intron 8; TaqI, silent site of exon 9) and 3 polymorphisms at the 5′ end of the gene (FokI, exon 2; EcoRV and Cdx2, promoter region).
FokI is the only SNP that leads to a different gene product: The T-C transition at this restriction site produces an ATG start codon, initiating translation 10 base pairs upstream and, thus, resulting in a 3 amino-acid, extended VDR protein.32, 33 The f allele, which results in this longer protein and was associated with increased melanoma risk in 2 of 4 studies (Table 2), appears to be less effective at activating the transcription of a VDR reporter construct,34 thereby indicating that this SNP is relevant functionally. However, the meta-analysis of available data, which was possible because of the homogeneity among the 4 studies, ruled out a significant association between the FokI polymorphism and melanoma risk.
For this SNP, which is in mutual tight linkage disequilibrium with the other polymorphisms at the 3′ end of the VDR gene,5, 31, 35 there is no evidence of a functional effect on VDR activity.36 However, it is believed that the 3′-end polymorphisms (with particular regard to BsmI and TaqI) affect messenger RNA stability and VDR gene transcription regulation5, 37; and the net effect of bb, ff, and tt genotypes may be envisaged as a reduction in the cellular activity of the vitamin D system. Indeed, in vitro functional studies have demonstrated that the baT (BsmI/ApaI/TaqI) haplotype inserted into transfection constructs resulted in lower reporter gene activity compared with BAt38 and was associated with low VDR messenger RNA levels39: These findings, along with ours, are in line with the hypothesis that a reduction in vitamin D system activity may lead to an increased melanoma risk. Otherwise, the BsmI polymorphism may be in linkage disequilibrium with other functionally relevant polymorphisms that will require further investigation.
This SNP of the VDR promoter (A-1012G) may modulate the docking of a transcription factor: in fact, this polymorphism is within the core sequence of a putative glutamyl-transfer RNA amidotransferase subunit A 3 (GATA-3) binding site in the A allele, whereas this binding site is not present in the G allele.40 Because GATA-3 plays a pivotal role in the polarization of naive T cells into T-helper 2-type lymphocytes41—which are believed to hamper an effective anticancer immune response42—it has been suggested that the EcoRV polymorphism may tip the balance toward the phenomenon of tumor immune escape.43 Results from the 3 studies that investigated the relation between this SNP and the risk of melanoma are conflicting; and, unfortunately, the between-study heterogeneity did not support the use of available data for meta-analysis. Therefore, further work is warranted to elucidate the role of this VDR variant in the determinism of melanoma development.
The functional effect of this RFLP at the 3′ end of the VDR gene is similar to that of BsmI. In analogy to the EcoRV polymorphism, the available data are conflicting, and meta-analysis was not indicated because of heterogeneity. Consequently, in this polymorphism, further work is warranted to assess whether the VDR variant is associated with melanoma risk.
Cdx2, which is a caudal-related homeodomain transcription factor, is important during the development of the intestine and, in adults, it reportedly regulates VDR expression in the small bowel.44 Polymorphism at the Cdx2 binding site alters the transcriptional activity of the VDR promoter region: in particular, the A allele is associated with higher VDR gene transcription activity.45 The available evidence (lack of association) comes from a single study: consequently, further investigation will be required before ruling out a significant impact of this SNP on the risk of melanoma.
A significant limitation of the current meta-analysis is that the results could not be adjusted by patient characteristics (eg. age, sex), environmental factors (eg, sun exposure, dietary vitamin D intake), or any other melanoma risk factors (eg, phototype, skin/eye color). Conversely, only an individual patient data meta-analysis could address this issue, and no study reported such detailed information, which calls for the development of publicly available databases aimed at the collection and analysis of biologic information on a single-patient basis.46, 47
It must be noted that, as shown in Table 2, the authors of all series in our meta-analysis adjusted their associated findings by age and sex, 3 groups22, 23, 26 analyzed the impact of known risk factors (ie, phototype, skin/eye color), and 2 groups22, 26 took into consideration environmental factors that potentially influenced the results (ie, sunburn history and vitamin D intake, vitamin D plasma levels). After these corrections, the results differed from the unadjusted data in only 1 case (the BsmI polymorphism).22
With regard to the BsmI polymorphism—the only one that was associated significantly with the risk of melanoma in the current meta-analysis—the 3 available studies performed 3 different genotype comparisons while adjusting the results (BB vs bb,26 bb vs BB,23 and Bb + BB vs bb22), which did not allow us to meta-analyze the adjusted OR by using the generic inverse variance method. Clearly, well designed, population-based, large, multi-institutional studies are warranted to address the issue regarding whether any VDR polymorphism is associated independently with melanoma risk.
In conclusion, current evidence is in favor of the association between 1 VDR gene polymorphism (BsmI) and the risk of melanoma development, although further work will be necessary to validate the risk identified in the current meta-analysis. Although the functional effect of the VDR BsmI SNP has not been elucidated fully, these findings indirectly support the hypothesis that sun exposure may have an antimelanoma effect through activation of the vitamin D system, as proposed for other cancer types.48, 49