MTR and MTRR Polymorphisms, Dietary Intake, and Breast Cancer Risk
- 1Division of General Internal Medicine and Public Health, Department of Medicine and Vanderbilt-Ingram Cancer Center, Vanderbilt University, Nashville, Tennessee and 2Department of Epidemiology, Shanghai Cancer Institute, Shanghai, China
- Requests for reprints:
M.J. Shrubsole, Center for Health Services Research, Vanderbilt University Medical Center, Nashville, TN 37232-2587. Phone: 615-936-0812; Fax: 615-322-1754. E-mail: martha.shrubsole{at}vanderbilt.edu
Introduction
Methionine, the precursor for the universal methyl donor, S-adenosylmethionine, is produced through the irreversible transfer of a methyl group from 5-methyltetrahydrofolate. This reaction is regulated by two enzymes, methionine synthase (MTR) and methionine synthase reductase (MTRR). MTR is polymorphic at nucleotide 2,756 (A-to-G) and has been associated with decreased plasma homocysteine levels (1-3). MTRR is polymorphic at nucleotide 66 (A-to-G) and the variant has a lower affinity for MTR (4) and is inconsistently associated with homocysteine level (5-7), although it is a risk factor for neural tube defects (8) and Down syndrome (9) in conditions of higher homocysteine. There is no report on either MTR or MTRR in relation to breast cancer risk. In an extension of our previous reports that folate intake was inversely associated with breast cancer risk (10) and that this association was particularly strong among women with the methylenetetrahydrofolate reductase (MTHFR) 677TT genotype (11), we investigated whether these associations may be modified by MTR and MTRR genotypes.
Materials and Methods
The Shanghai Breast Cancer Study is a large population-based case-control study conducted in urban Shanghai, China. Detailed study methods have been published previously (12). In brief, cases ages between 25 and 64 years were identified through a rapid case-ascertainment system supplemented by the population-based Shanghai Cancer Registry. Controls were identified from the Shanghai Resident Registry and frequency matched to the expected age distribution of cases by 5-year age groups. All subjects completed an in-person interview. Dietary intakes were assessed using a 76-item food frequency questionnaire that captured >85% of food intake in Shanghai (13). Blood samples were collected from 1,193 (82%) cases and 1,310 (84%) controls and used in this study for genotyping assays. MTRR Ile22Met (A66G, rs1801394) genotyping was done using the Taqman 5′-Nuclease Assay (C_3068176_10; Applied Biosystems, Foster City, CA). MTR Asp919Gly (A2756G, rs1805087) genotyping was done by BioServe Biotechnologies Ltd. (Laurel, MD) using Masscode assay (14). The consistency rate of quality control samples was 100% for MTR A2756G and 96% for MTRR A66G.
All dietary intake analyses only included cases (92.1%) and controls (91.3%) who did not use alcohol regularly or take vitamin supplements. Unconditional logistic regression models were used to calculate odds ratios (OR) and their 95% confidence intervals (95% CI) after adjusting for potential confounding variables. Diet was categorized into tertiles based on the control distribution. Energy was adjusted using the standard multivariate method (15). Stratified analyses were used to evaluate the potential modifying effect of MTHFR genotypes and folate and folate cofactor intakes.
Results
The frequencies of the MTR A2756G and MTRR A66G alleles were 0.10 and 0.24, respectively, among the controls who were not statistically different from the cases (data not shown). The genotype distributions among both cases and controls did not differ from the predicted distribution under Hardy-Weinberg equilibrium. Risk of breast cancer did not differ statistically by the MTR or MTRR genotypes either overall or by menopausal status nor did the genotypes modify the null association with MTHFR C677T genotype (Table 1). Likewise, there were no clear differences in risk for breast cancer and the joint MTHFR-MTR-MTRR genotypes.
MTR, MTRR, and MTHFR genotype frequencies and adjusted OR (95% CI) for breast cancer among Chinese women, Shanghai Breast Cancer Study, 1996 to 1998
The joint associations of MTR and MTRR genotypes and dietary folate and folate cofactor intake with breast cancer risk are presented in Table 2. Low intake of folate was associated with an increased risk among all genotypes, and the strength of the association did not differ by genotype. Risk associated with the genotypes was not statistically significantly different than one within all strata of methionine, vitamin B12, and vitamin B6 intakes.
Joint association of MTR and MTRR genotypes and folate and folate cofactor intake with breast cancer risk among Chinese women, Shanghai Breast Cancer Study, 1996 to 1998
Discussion
We found that there was no statistically significant association between the risk for breast cancer and MTR A2756G or MTRR A66G genotypes. We further found that this association was not modified by MTHFR C677T genotypes or intakes of folate, methionine, vitamin B12, or vitamin B6. This is the first report of MTR and MTRR genotypes in relation to breast cancer risk. MTR has been associated with a reduced risk for both colorectal cancer (16, 17) and acute lymphoblastic leukemia (18) and increased risk for malignant lymphoma (19) and not associated with cancer risk for non-Hodgkin's lymphoma (18, 20) and uterine cancer (21). In the only two studies that have evaluated the MTRR A66G genotype in relation to cancer risk, there was no observed association with acute lymphoblastic leukemia, non-Hodgkin's lymphoma, or gastric cardia cancer (20, 22), although there was an increased esophageal cancer risk among those who did not consume alcohol (22).
MTR and MTRR are critical enzymes responsible for the biosynthesis of methionine, the precursor for methylation reactions, and the regeneration of tetrahydrofolate for nucleotide biosynthesis. Under conditions of adequate methionine, ∼40% of homocysteine is remethylated to methionine through the activity of these enzymes (23). Alterations, therefore, in the function of these enzymes could have important effects on DNA methylation, synthesis, and repair. The MTRR A66G variant has a 3- to 4-fold lower affinity for MTR (4) and has been associated with altered blood or plasma levels of homocysteine, folate, or vitamin B12 in some but not all studies (5-7, 24, 25). Likewise, reports of MTR A2756G and homocysteine are conflicting (7, 26). We did not find any statistically significant associations between these genotypes and breast cancer risk even among conditions of replete and low intake. Nor did we observe any associations when the low-activity MTHFR, the rate-limiting enzyme for the methionine cycle, was either present or absent. If these particular variants of these critical enzymes do indeed have important functional consequences, our data suggested that these consequences do not seem likely to alter one-carbon metabolism sufficiently to affect risk for breast cancer.
This case-control study is one of the largest and most comprehensive evaluations of genetic variants in enzymes involved in the remethylation of homocysteine. Potential biases are limited in this study because both cases and controls had very high participation rates (>90%) and high blood donation rates (>80%). Fruit and vegetable intake, the major contributors to folate, methionine, and vitamin B6 intakes, did not significantly differ between cases and controls, and recall of diet would unlikely be related to genotype. We also observed little confounding when we carefully adjusted for known risk factors and any possible residual confounders would need to be very strong to alter the null associations observed in this study. We cannot exclude the possibility that these genotypes may be related to risk of breast cancer in an older population; however, the null association was not modified by menopausal status. Particular strengths of our study include the population-based design, the estimation of folate and cofactor intake in a population of nonusers of alcohol and vitamin supplements, and the large sample size that facilitated examination of modifying effects.
In summary, we found that MTR and MTRR genotypes are not likely to play an important independent role in breast cancer etiology. This is the first evaluation of these genotypes with breast cancer risk and future studies are warranted in populations with different nutrient intake.
Footnotes
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Grant support: National Cancer Institute, USPHS grants RO1CA64277 and RO1CA090899 and grant DAMD17-02-1-0606 (M.J. Shrubsole).
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The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
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- Accepted January 6, 2006.
- Received August 3, 2005.
- Revision received November 23, 2005.
