Results from this case-control study nested within a large, prospective cohort provide limited evidence that variants of the VDR gene may be related to risk of breast cancer. Haplotype GTCATTTCCTA in LD block B was observed to be associated with a reduced risk. Among individual SNPs, breast cancer risk was observed to be significantly associated with rs2544038 (block A) and rs11168287 (block C), and to have a marginally significant association with rs2239181 (block B).
To our knowledge, this study is the first to examine comprehensive haplotypes in the VDR
gene in relation to breast cancer risk. Most previous studies have examined individual polymorphisms, focusing primarily on the restriction fragment length polymorphisms BsmI
(rs7975232), and TaqI
(rs731236). Results of these studies have been inconsistent. The weight of evidence tends to be strongest for an association with FokI
. Among the studies that examined FokI
in relation to breast cancer risk (3
), several provided some evidence of increased risk among carriers of the ff
), with odds ratios ranging between 1.16 and 2.34. Apart from one case-control study that reported a significantly decreased relative risk of 0.71 among ff
), the remaining studies found no association. The reasons for this discrepancy remain unclear, although it may be due in part to differences across studies in sample size, race/ethnicity of study populations, or control selection. FokI
is not in LD with any other SNPs (32
). A meta-analysis that included most of the above studies (i.e., all studies published through October 2008) (62
) suggested a small, but significantly, increased risk of breast cancer among Fok1 ff
carriers compared to FF
carriers (summary OR=1.15; 95% CI = 1.03, 1.28). Our estimated OR of 1.2 was consistent in direction and magnitude with this summary OR, but was not statistically significant. This SNP did not depart from Hardy-Weinberg equilibrium among controls in our study and the minor allele frequency was similar to that of other white populations. An effect of this SNP is plausible, given that the f
allele results in production of a VDR protein that is less effective as a transcriptional activator (34
), the consequences of which would be expected to mimic that of lower vitamin D status.
Two studies that examined VDR
) used only a small number of polymorphisms that likely did not capture a large fraction of the variation in this gene (32
). One of these studies inferred haplotypes from only BsmI, ApaI, TaqI
, and a poly(A) repeat and found no association between any haplotype and breast cancer risk (44
). The other study inferred haplotypes from the FokI, TaqI, VDR
-5132, and Cdx2
polymorphisms, and found increased risk of breast cancer associated with the haplotype containing FokI F, TaqI t, VDR-5132
C, and Cdx2
). These results and those of the present study cannot be directly compared because of differences in haplotype definitions (32
). Moreover, while some in vitro
studies of VDR haplotypes have reported higher mRNA expression for the BsmI-ApaI-TaqI
haplotype BAt (rs1544410-A/rs7975232-A/rs731236-C) than for the haplotype baT (rs1544410-G/ rs7975232-C/rs731236-T), other studies have observed the opposite pattern not only for mRNA expression, but also for mRNA stability and transactivation, which could be due in part to differences in the cell lines used (63
In the present study, we identified one haplotype, GTCATTTCCTA in LD block B, that was significantly associated with a 50% reduced risk of breast cancer. While we are unaware of comparable haplotype data from other studies, a number of studies have investigated several individual SNPs within this block. Our results for SNPs within block B are consistent with a meta-analysis of commonly examined VDR
), which found no significant associations between Bsm1, Apa1
, or Taq1
, which are in this block, and risk of breast cancer. Reports published since this meta-analysis are also largely consistent with these findings (36
). Although we did not test BsmI
, we did test rs731236, which is in strong LD (r2
=0.97) with BsmI
among whites (64
) and which showed no association with breast cancer risk. In addition, a recent case-control study (40
) that included several of the less commonly-studied SNPs assessed in the present study also supports our findings of no effect associated with rs739837, rs1989969, rs2107301, and rs2238135, which are found in blocks B and C. Our haplotype results, if confirmed, would suggest that another polymorphism or polymorphisms in strong LD with rs2239181 may be responsible for the reduced risk associated with this haplotype.
We found only limited evidence of differences in risk by menopausal status or by ER/PR status of the tumor. Previous studies that investigated interactions between menopausal status and VDR
variants on breast cancer risk have found little evidence of effect modification (35
). At the same time, evidence is limited and inconsistent for a modification of effect of VDR
variants by ER or PR status. FokI ff
was found to have a marginally and nonsignificantly stronger association with ER−/PR− tumors than ER+/PR+ tumors in one study (39
) and the TaqI t
allele was associated with a significantly increased breast cancer risk only in ER+ tumors in another study (41
); however, no modification of effect by ER or PR status was found in other studies in which it was investigated (35
). We observed no differences in risk for FokI
by ER or PR status and we are unaware of other studies that examined modification of the association of rs2544038 and breast cancer risk by ER or PR status.
The heterogeneity of observed associations across studies may be due in part to differences in vitamin D status of study populations. Persons living on farms may receive more UV exposure and have different diets than the general population, which could influence circulating vitamin D levels during certain periods. In contrast to the static genetic variation investigated herein, these exposures are complex and time-varying. The interaction between markers of vitamin D status and VDR variants on breast cancer risk in this cohort will be investigated in detail in the future.
The observed association between smoking and breast cancer risk is consistent with the conclusions of a recent expert panel on tobacco smoke and breast cancer risk, which concluded that the relationship between active smoking and breast cancer is consistent with causality (65
). Results from two large prospective cohort studies published subsequent to this report (66
) support this conclusion. Although this association remains controversial, further discussion is beyond the scope of this paper.
Limitations of this study include the availability of DNA for only about 51% of the cases in the cohort. However, previous analyses suggest that this was unlikely to introduce selection bias (52
). The study sample size was relatively small, especially for stratified analyses, though we had sufficient power to detect modest associations in the full sample. Also, there were too few non-whites in this cohort to separately examine associations among these subjects. Exclusion of non-whites from analyses had minimal impact on risk estimates. Lastly, given the number of comparisons that were performed, we cannot exclude the possibility that the observed associations occurred by chance.
This study had a number of strengths. These include detailed data at baseline and at 5 years follow-up on potential confounding factors. The availability of data collected prior to disease diagnosis mitigated concerns about survival or reporting bias. Moreover, reliability of these data was shown to be good to excellent for a range of factors among cohort members who completed the same questionnaire at least one year apart (68
). We were able to capture and incorporate into our analyses much of the variation in the VDR
gene by using an extensive set of haplotype tag SNPs. Finally, we were able to compare results between both states in this cohort, which provided some evidence of internal consistency.
These results provide limited evidence that variation in the VDR gene may be associated with risk of breast cancer. The modest magnitude and borderline significance of most of these associations necessitate caution in their interpretation. Further years of follow-up of this cohort, with inclusion of additional incident cases and follow-up data in future analyses, should help to clarify the relationship between VDR variants and the risk of breast cancer.