In this study, we examined polymorphisms in the rate-limiting enzyme, MTHFR and a polymorphism in an associated enzyme, MTR. Both enzymes are important in one-carbon metabolism. None of these polymorphisms, however, were associated with an alteration in breast cancer risk in either pre- or postmenopausal women. We did find some evidence of an interaction of MTHFR C677T and alcohol with risk of breast cancer among postmenopausal women with the homozygote variant genotype (TT), but there was no evidence of that interaction for the other two polymorphisms.
The biochemical reactions involved in one carbon metabolism are necessary for the synthesis of purines and pyrimidines as well as for the formation of S-adenosylmethionine (SAM), important for the methylation of substrates such as DNA and therefore essential to gene regulation (4
). The fundamental acceptor molecule for one carbon metabolism is folate (4
). Alcohol consumption can affect the intake, absorption, activation and storage of folate and other nutrients that are methyl contributors (8
). Abnormal folate status as a result of alcohol ingestion could potentially adversely affect methylation, both globally and at specific CpG sites in promoter regions of genes as well as nucleotide synthesis and consequently DNA synthesis and repair.
Consistent with our findings regarding the association of MTHFR
C677T genotype and breast cancer risk, a recent meta-analysis that included seventeen case control studies, found no association of MTHFR
677 genotype and breast cancer risk (22
); the reported summary OR for TT homozygotes compared to CC homozygotes was 1.04 (95% CI=0.94–1.16) (22
). More recently, however, Suzuki et al, reported an increase risk of postmenopausal breast cancer in Japanese women with the MTHFR
677 TT genotype (OR = 1.83, 95% CI: 1.08–3.11) (35
A1298C, a number of epidemiological studies have found, as did we, no association of genotype with risk (23
). The variant allele for MTHFR
A1298C may have less impact on enzyme activity than the MTHFR C677T
variant allele. It is also possible that the variant genotype for this polymorphism is only important in the presence of the variant genotype for the MTHFR
677 polymorphism (41
). In our study, we did not have any participants who had both variant alleles.
There are few studies examining MTR
and breast cancer risk (23
) and results are inconsistent. We did not find any alterations in risk by MTR
genotype for either pre or postmenopausal women in our population.
Gene-gene-environment interactions may also be important in understanding the role that one carbon metabolism plays in breast carcinogenesis. We did an exploratory examination of the significance of having one or more than one variant genotype in comparison to having the common genotype for either polymorphism for MTHFR C677T, MTHFR A1298C and MTR genotype. Additionally, we looked at the number of variant alleles for all three polymorphisms in relation to risk for breast cancer. Our study does not support an association for combined genotype or for multiple alleles of these specific genes.
It is possible that genetic variation in the enzymes involved in one carbon metabolism may not influence the development of breast carcinogenesis independent of other factors such as alcohol intake There is approximately a 10% increase in risk of breast cancer with consumption of one drink of alcohol per day (2
), yet there have been only a few studies examining MTHFR C677T
and breast cancer risk that included an examination of interactions with alcohol consumption (23
). Additionally, in these studies the assessment of alcohol was not comprehensive and often limited to recent intake; results are not consistent (23
). Although we did not find evidence for a multiplicative interaction, we found a significant increase in breast cancer risk among postmenopausal women with the TT genotype for MTHFR
C677T whose lifetime total intake in ounces was above the median which is consistent with our hypothesis. Additionally, we saw an increase in risk for these women whose lifetime drinks per drinking day was above the median, about 1.91 drinks per drinking day, even with adjustment for total intake of alcohol. It is possible that there may be additive interaction without multiplicative interaction which may be of greater interest in the case of disease prevention.
When we examined alcohol consumption within decades, it appeared that drinking at younger ages among postmenopausal women with the TT genotype was more associated with risk possibly because of higher alcohol consumption in those years. These findings suggest that one carbon metabolism may play a role in the observed association of alcohol with breast cancer risk among postmenopausal women. We have no explanation for the significant findings among nondrinkers and low drinkers with the CT genotype other than that they may be spurious findings from small cells in subgroup analyses.
A strength of this study was the detailed assessment of lifetime alcohol consumption. Unlike most studies which assess only recent or usual alcohol intake, we were able to assess alcohol intake throughout life. Further, we assessed quantity and frequency separately, allowing us to examine both total consumption and intensity of consumption.
As with any case control study, there are limitations that need to be considered in the interpretation of the findings. There is a possibility of measurement error, particularly for measurement of lifetime alcohol. Nondifferential error in measurement of intake would likely bias results to the null. Despite this potential for error, our methods for measuring alcohol consumption were more comprehensive and detailed than previous assessments of alcohol intake (46
). While recall bias regarding report of alcohol may also be a problem, there is evidence that bias in recall of alcohol consumption does not substantially alter results regarding alcohol (47
To aid us in assessment of selection bias, we conducted a short interview with a subset of those refusing to participate and those who had agreed to participate in the case-control study. There was some tendency for both participating cases and controls to drink more than non-participants who agreed to the short interview; differences were small and not significant. There was some tendency for participating cases to have lower stage disease than non-participants. It may be that our results are not generalizable to later stage disease. Among participants, we did not find important differences between those who were genotyped and those who were not. It is unlikely that genotype was related to participation in the study. We examined whether or not subjects, who were eliminated from this study due to the absence of DNA or diet information, were different based on alcohol consumption and whether or not this could have contributed a bias. We compared the risk estimates for breast cancer of those in the parent study with those included in this study for alcohol intake and risk estimates were the same between the groups for lifetime total ounces of alcohol intake and lifetime number of drinks per drinking days stratified by menopausal status.
In summary, we found that greater lifetime alcohol consumption among postmenopausal women with the TT variant genotype of MTHFR C677T was associated with increased risk of breast cancer. Further, we found that intensity of consumption may also contribute to this association. These findings indicate that one carbon metabolism may be important in the pathway leading to carcinogenesis and may explain, at least in part, the observed association of alcohol and breast cancer among postmenopausal women.