In this family-based case-control study of CRC, we did not find evidence to suggest that variation in VDR and GC is associated with risk of CRC overall. Our data also suggested that associations between SNPs in these genes are not modified by calcium supplement use, family history of CRC, or total calcium intake estimated from an FFQ. We found only weak suggestions that dietary vitamin D could modify the association between SNPs in VDR and risk of CRC (), although these associations were not statistically significant after correction for multiple testing. We observed statistically significant associations between several SNPs in GC and MSI-H CRC; however, these results should be interpreted with caution due to the many comparisons we have made.
There are several mechanisms that may explain the inhibitory action of vitamin D on carcinogenesis, including reduced cellular proliferation (16
), induction of apoptosis (49
), inhibition of inflammation (54
) and angiogenesis (55
), and protection of colonic epithelium from the damaging effects of bile acids (56
). Given that VDR is expressed in a wide variety of tissues, including some with no known role in calcium metabolism (17
), it is logical to hypothesize that variation in VDR
may influence these potential anti-carcinogenic effects of vitamin D through altered transcriptional control of 1,25-(OH)2
. In addition, two SNPs in GC
(rs4588 and rs7041) have been shown to influence 25-OHD levels (33
), which suggests that variation in this gene could also influence the actions of vitamin D. We would also expect that the effect of variants in these genes may be most relevant in the context of vitamin D levels.
Whereas the majority of epidemiologic studies of VDR
variants and risk of CRC have found associations with the disease, the results for specific variants have not been entirely consistent. Inconsistency has been a common problem in genetic epidemiology studies (57
) and is not limited to studies of the VDR
gene. Potential explanations for heterogeneity in genetic association studies include inadequate power, bias, and population differences.
The FokI restriction site polymorphism (rs2228570) is the most frequently reported VDR
variant in epidemiologic studies of CRC (18
) and adenoma (31
). In studies of CRC, three of the seven studies that evaluated this variant found evidence for an association with risk. In contrast, this variant was not associated with adenoma risk in three studies (31
), although a statistically significant association for risk of large adenomas was observed in one of these (31
). In a recent meta-analysis, the summary OR for this variant was not associated with CRC, although there was significant heterogeneity among studies (27
). Unfortunately, we were unable to evaluate associations between the FokI variant and CRC risk in our study as this SNP did not pass quality-control checks.
In three studies, the BsmI variant (rs1544410) was not associated with risk of colorectal adenoma (30
); however, data from two of the studies suggested that the variant may modify the relationship between calcium and/or vitamin D intake and risk of adenoma (30
). In studies of CRC, this variant was associated with risk of CRC overall in three studies (19
) with evidence for modification by dietary intake of calcium and vitamin D (21
). However, two additional studies showed no association with CRC (18
). In a recent meta-analysis, the summary OR was borderline protective for this variant after one study contributing to heterogeneity in risk estimates was excluded (27
). In our analysis, we observed no overall association between the BsmI variant and CRC (). In addition, we observed no evidence of modification by calcium supplement use or by dietary calcium and vitamin D intake in the subset of individuals with FFQ data.
The TaqI variant (rs731236) was not associated with risk of colorectal adenoma or cancer in three studies (25
) but was associated with a borderline statistically significant risk of CRC in one (19
). We observed no overall association between this variant and risk of CRC (). Three previous studies have evaluated associations between the VDR CDX2
variant (rs11568820) and CRC risk, with two studies reporting no association between this variant and risk (18
) while the other study reported a statistically significant increased risk in individuals with the AA genotype (24
). We observed no association between this SNP and CRC in our study population ().
To our knowledge, this is the first epidemiologic study to evaluate the association between variation in GC and risk of CRC. Our data suggest that this gene is unlikely to play a major role in genetic susceptibility to this malignancy. We observed limited evidence that variants in GC may be associated with a reduced risk of MSI-H CRC, although these results should be interpreted with caution due to the many comparisons that we have made as well as the lack (at least to date) of a biologic rationale for a role of this gene only in MSI-H CRC.
This study has several strengths including the comprehensive evaluation of both VDR
, the availability of systematically collected data on tumor characteristics, and the case-unaffected sibling study design that controls for any potential confounding by ethnicity. This family-based design is more powerful than studies that include unrelated controls for detecting gene-environment interactions (62
Several limitations must also be considered. While the case-unaffected sibling design is more powerful for detecting gene-environment interactions, this design may have lower power for detecting genetic main effects because the study sample is often overmatched on genotype (62
). Because overmatching is an issue of power, rather than bias, evaluating the width of the confidence intervals can provide information regarding the overall power of the study. For the main effects of the genotypes (), the confidence intervals are quite narrow, especially for the more common variants. This would indicate that the lack of association in our study is not simply an issue of inadequate power. Detailed FFQ data were collected only for a subset of the participants in this study (32%), so we have limited power to detect heterogeneity by calcium and vitamin D intake. In addition, we did not collect information on sun exposure, so we are unable to account for endogenously synthesized vitamin D levels. Cases were interviewed up to five years after diagnosis, which introduces the potential for possible survival bias and increases the potential for exposure misclassification; however, when we restricted our analyses to subjects interviewed within the first two years after diagnosis, we observed very similar results. In addition, we have no strong a priori
hypothesis that any of these SNPs would be strongly associated with prognosis.
In conclusion, we did not observe any statistically significant associations between variants in VDR and GC and CRC overall in this family based analysis. After correction for multiple comparisons, we observed no indication that these associations were modified by calcium supplement use and dietary and total intake of calcium and vitamin D. We did observe associations between SNPs in GC and MSI-H CRC, although these results should be confirmed in additional studies.