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Carcinogenesis Advance Access originally published online on August 11, 2007
Carcinogenesis 2007 28(9):1954-1959; doi:10.1093/carcin/bgm141
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© The Author 2007. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

Interactions among GSTM1, GSTT1 and GSTP1 polymorphisms, cruciferous vegetable intake and breast cancer risk

S.E. Steck6,*, M.M. Gaudet1, J.A. Britton2, S.L. Teitelbaum2, M.B. Terry3, A.I. Neugut3,4, R.M. Santella5 and M.D. Gammon1

Department of Nutrition
1 Department of Epidemiology, School of Public Health, University of North Carolina, Chapel Hill, NC, 27599 USA
2 Department of Community and Preventive Medicine, Mount Sinai School of Medicine, New York, NY, 10029 USA
3 Department of Epidemiology, Mailman School of Public Health
4 Department of Medicine, College of Physicians and Surgeons
5 Department of Environmental Health Sciences, Mailman School of Public Health, Columbia University, New York, NY, 10032 USA
6 Present address: Department of Epidemiology and Biostatistics, Arnold School of Public Health, University of South Carolina, 2221 Devine Street, Room 231, Columbia SC 29208, USA

* To whom correspondence should be addressed. Tel: +1 803 734 4431; Fax: +1 803 734 5505; Email: stecks{at}gwm.sc.edu


   Abstract
Top
 Abstract
Introduction
 Materials and methods
 Results
 Discussion
 Funding
 References
 
Isothiocyanates are anticarcinogenic phytochemicals found in cruciferous vegetables that both induce and are substrates for the gluthatione S-transferases (GSTs). The GSTs are phase II metabolizing enzymes involved in metabolism of various bioactive compounds. Functional polymorphisms in GST genes have been identified and may interact with cruciferous vegetable intake to affect cancer risk. We examined this hypothesis using data from the Long Island Breast Cancer Study Project, a population-based case–control study conducted in Long Island, NY, from 1996 to 1997. Cruciferous vegetable intake in the previous year was assessed via modified Block food frequency questionnaire. DNA was extracted from blood samples (n = 1052 cases and n = 1098 controls) and genotyped for GSTM1 deletion, GSTT1 deletion and GSTP1 Ile105Val using multiplex polymerase chain reaction and Taqman assays. Unconditional logistic regression was used to estimate adjusted odds ratios (ORs) and 95% confidence intervals (CI). We found an 86% increase in the OR for breast cancer among carriers of the GSTM1 null, GSTT1 null and GSTP 105Ile/Ile genotypes (OR = 1.86, 95% CI = 1.12, 3.08) and a 36% decrease in the OR among carriers of GSTM1 present, GSTT1 null and GSTP1 105Ile/Val + Val/Val genotypes (OR = 0.64, 95% CI = 0.42, 0.97) compared with GSTM1 present, GSTT1 present and GSTP1 105Ile/Ile carriers. We found no joint effects among GST polymorphisms and cruciferous vegetable intake and breast cancer risk. In conclusion, we found associations between specific combinations of three GST gene polymorphisms and breast cancer risk but these did not modify the association between cruciferous vegetable intake and breast cancer. Additional studies are needed to confirm the associations observed.

Abbreviations: CI, confidence interval; GST, glutathione S-transferase; OR, odds ratio


   Introduction
Top
 Abstract
 Introduction
Materials and methods
 Results
 Discussion
 Funding
 References
 
Cruciferous vegetables, such as broccoli, cabbage and cauliflower, contain glucosinolates which are metabolized into isothiocyanates and indoles, two phytochemicals that are anticarcinogenic in animal models (1,2). We previously found a borderline significant inverse association between cruciferous vegetable intake and breast cancer risk among post-menopausal women in the Long Island Breast Cancer Study Project {odds ratio (OR) for six or more servings per day compared with zero to one servings per day was 0.80 [95% confidence interval (CI): 0.60–1.05]} and a positive association among pre-menopausal women [OR for six or more servings per day compared with zero to one servings per day was 1.76 (95% CI: 1.18–2.61)] (3). Other case–control studies have reported reduced risk of breast cancer with increased cruciferous vegetable intake in post-menopausal women (4) or with increased broccoli intake in pre-menopausal women (5). However, no association of cruciferous vegetable intake and breast cancer risk was observed in the Pooling Project of combined analyses of seven cohort studies (6) or in a case–control study of early-stage breast cancer (7). Fowke et al. (8) found reduced risk of breast cancer in Chinese women in the highest quartile of urinary isothiocyanate (used as a biomarker for cruciferous vegetable intake) as compared with the lowest quartile, but no association for cruciferous vegetable intake assessed by a food frequency questionnaire in the same population.

Possible explanations, among others, for inconsistent findings across studies may be different misclassification rates or the presence of modification by biologically relevant genotypes such that an association between diet and breast cancer risk is masked when examined in mixed genotype populations. Isothiocyanates are both substrates for and inducers of the glutathione S-transferases (GSTs) (9), a family of phase II metabolizing enzymes involved in carcinogen detoxification and metabolism of various bioactive compounds. Functional polymorphisms in three of the most well-studied GSTs (GSTM1, GSTT1 and GSTP1) have been identified that may interact with cruciferous vegetable intake to modify cancer risk (reviewed in ref. 10). To examine this hypothesis, we studied whether the association of cruciferous vegetable intake and breast cancer risk was modified by polymorphisms of GSTM1, GSTT1 and GSTP1 in a large case–control study. Two previous studies of breast cancer did not find significant interaction among individual GST genotypes and cruciferous vegetable intake (or the biomarker urinary isothiocyanate levels) and breast cancer (5,8). However, only a few studies have had a large enough sample size to examine combined effects of more than two GST gene polymorphisms on breast cancer risk (11–14) and none has reported the joint effects of cruciferous vegetable intake and combined polymorphisms in relation to breast cancer risk which was the aim of this study.


   Materials and methods
Top
 Abstract
 Introduction
 Materials and methods
Results
 Discussion
 Funding
 References
 
Institutional Review Board approval was obtained by all collaborating institutions. The Long Island Breast Cancer Study Project has been described in detail previously (15). Briefly, a large, population-based case–control study of breast cancer was conducted in Nassau and Suffolk counties on Long Island, NY, between 1 August 1996 and 31 July 1997. Breast cancer cases (n = 1508, 82.1% of eligible cases) were identified through pathology/cytology records of 33 institutions. Population-based controls (n = 1556, 62.7% of eligible controls) were identified using random digit dialing for women under the age of 65 years and by Center for Medicare and Medicaid Services (formerly known as Health Care Financing Administration) rosters for women 65 years and older. Controls were frequency matched to the expected age distribution of cases by 5-year age group.

A total of 73% of both case and control respondents donated a blood sample from which DNA was extracted (15). Stored DNA samples were available for 1052 cases (70% of participating cases) and 1098 controls (71% of participating controls). Genotyping was performed by BioServe Biotechnologies, Ltd (Laurel, MD). Positive and negative controls were included with each batch, and 10% random samples were repeated to confirm results with >97% concordance. GSTM1 and GSTT1 genotypes (presence and absence of the allele defined as ‘present’ and ‘null’ genotype, respectively) were determined by a multiplex polymerase chain reaction method, with the constitutively present gene ß-globin as an internal positive control. Polymerase chain reactions were performed with 50 ng of genomic DNA using 30 pmol of each GSTM1 and GSTT1 primer and 10 pmol of each ß-globin primer in a method adapted from Bell et al. (16,17). The forward primers for GSTM1, GSTT1 and ß-globin were each modified on the 5' end by the addition of a fluorescent dye tag (Beckman, Fullerton, CA). GSTP1 Ile105Val (rs1695) was determined by high-throughput matrix-assisted laser desorption/ionization time-of-flight. We classified the presence of a GSTP1 Val allele in the same category as the GSTM1 and GSTT1 null genotypes when examining the joint effects because there is evidence that the Val allele has lower specific activity toward the major isothiocyanate found in broccoli, sulforaphane (18).

An interviewer-administered in-person questionnaire obtained information on potential covariates (15). Among the respondents who completed the main questionnaire, 98.2% of breast cancer cases and 97.6% of controls also completed a self-administered modified Block food frequency questionnaire, previously validated in another population (19), which assessed diet in the year prior to interview. We calculated intakes of cruciferous vegetables in servings per week as described previously and the main effects of cruciferous vegetable intake by quintile and breast cancer risk have been published previously (3). We replicated these results for the subgroup of the population with GST genotyping data and found almost identical ORs but with a wider CI due to the smaller sample size (post-menopausal women, OR = 0.80, 95% CI = 0.57, 1.13 and pre-menopausal women, OR = 1.74, 95% CI = 1.07, 2.83 for women consuming six or greater half-cup servings of cruciferous vegetables per week compared that with the referent of zero to one half-cup servings per week). For the current analyses, cruciferous vegetable intake was dichotomized as zero to five half-cup servings per week versus six or greater half-cup servings per week because no other categories of intake besides six or greater servings per week were associated with breast cancer risk. We defined menopausal status using several variables: the date of last menstrual period reported by the participant, prior surgical information on hysterectomy or oophorectomies, cigarette smoking status and use of hormone replacement therapy (15).

Unconditional logistic regression was used to calculate ORs and 95% CIs (20). Effect modification on the multiplicative scale was evaluated by comparing the ORs for cruciferous vegetable intake across strata of GST genotypes and testing for heterogeneity. Interaction terms were created between the cruciferous vegetable intake variable and GST genotypes, and a likelihood ratio test was conducted comparing the model with the interaction terms to the reduced model containing only the individual variables. To examine deviations from the additive scale of interaction, joint effects were evaluated by creating indicator variables such that individuals with the highest hypothesized risk (low cruciferous vegetable intake and high activity GST genotype) were the common referent group, and ORs and 95% CIs were calculated for all other groups in comparison (21). Stratified analyses based on estrogen receptor (ER)/progesterone receptor (PR) status were also conducted. Statistical significance was set at P < 0.05. All analyses were conducted using SAS 9.1.


   Results
Top
 Abstract
 Introduction
 Materials and methods
 Results
Discussion
 Funding
 References
 
Demographic characteristics of the study population have been described in detail previously (15). The main effects of cruciferous vegetable intake and breast cancer risk have also been published previously (3). The GSTM1 and GSTT1 null genotypes were observed in 48 and 21%, respectively, of cases, and 45 and 22%, respectively, of controls. In all, 40% of cases and 43% of controls were heterozygous and 9% of cases and 8% of controls were homozygous variant for the GSTP1 Ile105Val polymorphism. No substantial associations between GSTM1, GSTT1 and GSTP1 genotypes and breast cancer risk were observed when genes were examined individually [for GSTM1, OR = 1.12 (95% CI = 0.94, 1.34) for the null genotype as compared with the referent present genotype; for GSTT1, OR = 0.97 (95% CI = 0.79, 1.21) for the null genotype as compared with the present genotype and for GSTP1, OR = 0.91 (95% CI = 0.77, 1.08) for the Val allele carriers as compared with the referent Ile/Ile genotype carriers].

We examined the main effects of the genes in combinations of two (Table I) or three (Table II) genes. We observed increased risk of breast cancer for women with the GSTM1 and GSTT1 null genotypes as compared with GSTM1 and GSTT1 present genotypes, and this was confined to post-menopausal women (Table I). Additionally, only in post-menopausal women, we found decreased risk of breast cancer for GSTM1 present and GSTT1 null carriers as compared with GSTM1 present and GSTT1 present carriers. In the three-way gene analyses, increased risk of breast cancer was observed for women with the combination of GSTM1 null, GSTT1 null and homozygous wild-type (105 Ile/Ile) for GSTP1 compared with women with GSTM1 present, GSTT1 present and homozygous wild-type for GSTP1 (Table II) which, again, appeared to be confined to post-menopausal women. Decreased risk of breast cancer was observed for women with GSTM1 present, GSTT1 null and GSTP1 Ile/Val or Val/Val genotypes compared with GSTM1 present, GSTT1 present and homozygous wild-type for GSTP1 and this was evident for both pre- and post-menopausal women. All other combinations of these genotypes were not associated with breast cancer in this population.


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  		Table I. ORa and 95% CI for two-way combinations of GSTM1, GSTT1 and GSTP1 genotypes and risk of breast cancer

 


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  		Table II. ORa and 95% CI for three-way combinations of GSTM1, GSTT1 and GSTP1 genotypes and risk of breast cancer

 
No joint effects of cruciferous vegetable intake and GST polymorphisms on breast cancer risk were observed either for each gene separately (Table III) or for the two-way combinations of genotypes (Table IV). For GSTT1, highest risk for pre-menopausal women was observed in those consuming high cruciferous vegetable intakes with the null genotype as compared with low consumers with the gene present, though the interaction P-value was not significant. The increased risk observed for GSTM1 null and GSTT1 null genotype carriers as compared with GSTM1 present and GSTT1 present carriers persisted, even among individuals consuming higher amounts of cruciferous vegetables [OR for high consumers of cruciferous vegetables with GSTM1 and GSTT1 null genotypes was 2.06 (95% CI = 0.94, 4.52) as compared with low cruciferous vegetable consumers with GSTM1 and GSTT1 present genotypes]. We did not stratify the two-way gene and diet interaction analyses by menopausal status or examine the joint effects of the three-way combinations of genes and cruciferous vegetable intake due to the small cell sizes when stratifying by four variables in this study population.


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  		Table III. Joint effectsab of GST genotype and cruciferous vegetable intakec on risk of breast cancer in the Long Island Breast Cancer Study Project 1996–1997

 


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  		Table IV. Joint effectsab of two-way combinations of GSTM1, GSTT1 and GSTP1 genotypes and cruciferous vegetable intakec on breast cancer risk

 
We also examined the genotype and genotype x diet associations with breast cancer risk stratified by ER/PR status. Among women with ER–/PR– and ER–/PR+ tumors, carriers of the GSTP1 105Val allele were at increased risk (OR = 1.38, 95% CI = 0.95, 1.99 and OR = 2.33, 95% CI = 1.06, 5.12, respectively, for Val carriers as compared with Ile/Ile carriers), but among women with ER+/PR+ tumors, an inverse association was observed (OR = 0.78, 95% CI = 0.62, 0.98 for Val carriers as compared with Ile/Ile carriers). No substantial associations were observed across ER/PR status categories for the GSTM1 or GSTT1 deletions. There were no joint effects of cruciferous vegetable intake and GST genotypes on breast cancer risk stratified by ER/PR status; i.e. the increased risk among women with ER–/PR– and ER–/PR+ tumors remained for the GSTP1 105Val carriers even with high intake of cruciferous vegetables (data not shown).


   Discussion
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
Funding
 References
 
We examined joint effects of GSTM1, GSTT1 and GSTP1 polymorphisms and cruciferous vegetable intake in relation to breast cancer risk in a large population-based case–control study. We previously found a non-significant inverse association between cruciferous vegetable intake and breast cancer in post-menopausal women and a positive association among pre-menopausal women (3). Examination of the main effect of genotypes revealed an increased risk associated with the combination of GSTM1 null, GSTT1 null and GSTP1 homozygous wild-type (Ile/Ile) as compared with GSTM1 present, GSTT1 present and GSTP1 homozygous wild-type. In comparison with the same referent group, decreased risk of breast cancer was observed for women with GSTM1 present, GSTT1 null and GSTP1 heterozygotes + homozygous variant genotypes. Contrary to our hypothesis, we found no evidence for modification of the effect of cruciferous vegetable intake on breast cancer by GST genotype, either in combination or individually.

The majority of previous studies have reported no main effect of GST genes individually on breast cancer risk. A pooled analysis of seven studies published up to June 1999 which examined GSTM1, GSTT1 and GSTP1 and breast cancer revealed no association among individual polymorphisms and breast cancer risk (14). Examination of a combination variable comparing GSTM1 null, GSTT1 null and GSTP1 105Val carriers to GSTM1 present, GSTT1 present and GSTP1 105Ile/Ile carriers also revealed no association with breast cancer. However, the sample size for these combined polymorphism analyses was limited as only two (22,23) of the seven studies assessed the GSTP1 Ile105Val polymorphism. A strength of the present study is the large sample size which, for example, had five times as many cases and twelve times as many controls in the combined category of GSTM1, T1 and P1 deficient as compared with the pooled study. The increased risk we observed for the combination of GSTM1 null, GSTT1 null and homozygous wild-type (Ile/Ile) for GSTP1 has been similarly observed in two other studies, though not statistically significantly (11,12) and in a third study using a different referent group (13). When we used the same referent group as the Mitrunen et al. (13) study, namely GSTM1 present, GSTT1 present and GSTP1 Val allele, the OR was similarly increased (OR = 1.89, 95% CI = 1.14, 3.12) for carriers of GSTM1 null, GSTT1 null and GSTP1 Ile/Ile genotypes.

Examination of interaction among these three genes is complicated because it is unclear whether the GSTP1 105Val allele or the Ile allele should be considered the "deficient" group. There is some evidence that the Ile allele has reduced activity toward detoxifying diol epoxides such as that of benzo(a)pyrene as compared with the Val allele (24), whereas other evidence suggests that the Val allele has lower specific activity toward the major isothiocyanate found in broccoli, sulforaphane (18). Given our interest in the interaction with cruciferous vegetables, we chose to classify the presence of a Val allele in the same category as the GSTM1 and GSTT1 null genotypes when examining the joint effects.

Given the involvement of GSTs in deactivating and detoxifying carcinogens, deletions in GSTM1 and GSTT1 resulting in no enzyme activity may compromise an individual's ability to deactivate carcinogens, thus increasing risk of cancer. This is supported by our two-way gene interaction analyses showing increased risk with the GSTM1 and GSTT1 null genotypes as compared with the combined wild-type genotypes. As mentioned above, the direction of the functional change of the GSTP1 Ile105Val polymorphisms is unclear, but there is evidence to suggest that the Ile allele confers reduced activity toward carcinogen detoxification. As such, one would expect the highest risk group to be carriers of the GSTM1 null, GSTT1 null and GSTP1 Ile/Ile genotypes which is also what we observed. The consistency of these findings across other studies that examined combinations as well as the biologic rational should be interpreted with caution given that this combined GSTM1 null, GSTT1 null and GSTP1 Ile/Ile group represented one of the smallest numbers of women of all eight combination groupings. In addition, the decreased risk that we observed associated with the GSTM1 present, GSTT1 null and GSTP1 Val allele carriers was among one of the smallest groups of combined genotypes, and it is unclear biologically why this group would have decreased risk of breast cancer as compared with the GSTM1 present, GSTT1 present, GSTP1 Ile/Ile carriers.

Previous clinical studies reported an inverse association between GSTP expression and ER and PR status of breast cancer tumors (i.e. higher GSTP expression was found in ER– and PR– tumors) (25,26). Thus, we examined the associations among individual GST genotypes and breast cancer risk stratified by ER/PR status. Among cases with ER–/PR– and ER–/PR+ tumors, carriers of the Val allele of the GSTP1 Ile105Val polymorphism had increased risk of breast cancer as compared with Ile/Ile carriers. In contrast, among cases with ER+/PR+ tumors, Val allele carriers had reduced risk of breast cancer. If the Val allele confers increased expression of the GSTP1 gene, then our findings of increased risk for ER–/PR– and ER–/PR+ tumors among Val allele carriers would be consistent with the previous clinical studies showing that increased GSTP expression was more prevalent in ER– and PR– tumors. From experimental studies, there is evidence that phase I enzymes and GSTP1 interact in estrogen metabolism (27), and GSTP1 is the predominant GST expressed in breast tissue (28). The reason for the difference in risk conferred by the GSTP1 105Val allele between different ER/PR status categories is unclear and warrants further exploration.

Isothiocyanates from cruciferous vegetables inhibit tumor formation in animals and have beneficial effects in cancer cell culture models (1). Isothiocyanates are metabolized by the GSTs prior to being excreted in the urine. A carrier of GSTM1 and GSTT1 null genotypes and the less active genotype of GSTP1 Ile105Val would hypothetically be less able to metabolize and eliminate isothiocyanates, thus reducing their risk of cancer due to an enhanced pool of isothiocyanates in the body (10). Contrary to this hypothesis, in a recent broccoli feeding study, Gasper et al. (29) found GSTM1 null individuals excreted more sulforaphane metabolites after consumption of broccoli than GSTM1 present individuals. Similarly, we found a higher proportion of GSTM1 null individuals had high total isothiocyanate excretion after broccoli consumption compared with the proportion of GSTM1 present with high isothiocyanate excretion and that specific GSTT1, GSTP1 and GSTA1 polymorphisms were not related to total isothiocyanate excretion (30). These findings suggest that perhaps alternative routes of metabolism exist for isothiocyanates from broccoli.

Previous literature has found contrasting modifying effects among cruciferous vegetable intake, GSTM1 polymorphisms and cancer risk depending on the geographic location of the population under study. Cancer protection from high cruciferous vegetable intake appears to be stronger in individuals with the GSTM1 null genotype as compared with the GSTM1 present genotype in studies in Asia (31,32) and central and eastern Europe (33). However, in studies in USA, the protective effect of cruciferous vegetable intake is stronger in individuals with the GSTM1 present genotypes as compared with GSTM1 null genotype carriers (34–36). In contrast, several studies have found no interaction between cruciferous vegetable intake and GSTM1 genotype in relation to cancer (37–39), including one study of breast cancer in USA population (5) and one of breast cancer in an Asian population (8) which are consistent with our results.

A possible explanation for the varying effects across studies in different countries is the difference in types and amounts of cruciferous vegetables consumed (29). In USA, broccoli makes up approximately 30 to 50% of total cruciferous vegetables consumed (5,34,38). In contrast, in China, the major cruciferous vegetable in the diet is cabbage (32). Cruciferous vegetables contain varying amounts of different isothiocyanates, with broccoli being the largest source of the isothiocyanate sulforaphane and cabbage being primarily a source for 3-butenyl and 4-pentenyl isothiocyanates (29). Interestingly, sulforaphane was the poorest substrate for GSTM1, GSTP1, GSTA1 and GSTM2 enzymes, yet was the most potent inducer as compared with three other isothiocyanates in vitro (40). This suggests that it is sulforaphane's ability to induce the GSTs rather than it's role as a substrate for the GSTs that is important in cancer prevention which is consistent with the epidemiologic literature in some studies in USA showing that intake of cruciferous vegetables (the majority being broccoli) is more protective in individuals with the active forms of the enzymes as compared with individuals with the inactive forms of the enzymes.

Another difference across studies is that some of the previous studies have used urinary levels of isothiocyanate metabolites as a biomarker for cruciferous vegetable intake rather than relying on dietary recall. Urinary isothiocyanate metabolite concentrations are a more objective assessment of intake than dietary recall, but only represent the previous 1–2 days of intake which may not reflect the relevant time frame of exposure in breast cancer development. Inaccurate recall of dietary intake, whether differential or non-differential between cases and controls, can lead to biased results. Non-differential misclassification of either variable (gene or diet) can bias interaction parameter estimates away from or toward the null. We expect minimal misclassification of genotype data in our study given the >97% concordance among blinded duplicate samples in the analyses. In this situation, misclassification of dietary intake that is differential with respect to disease status may result in either no bias of the multiplicative interaction parameter or attenuation toward the null when dietary intake is independent of genotype among controls and when misclassification of diet is non-differential with respect to genotype (41). Using a chi-square test, we examined the association between GSTs and cruciferous vegetable intake among controls only and found no substantial associations for any of the GST genotype variables. Given that it is unlikely that misclassification of cruciferous vegetable intake is differential with respect to GST genotypes, and then it is unlikely that differential misclassification of dietary intake by disease status substantially biased our interaction parameter estimates in this study.

Using a traditional method for genotyping the GSTM1 and GSTT1 deletions, heterozygote and homozygote wild-type carriers were grouped together in the assay results such that we were not able to distinguish one from the other. New methodology for determining heterozygotes has recently been published and there is evidence that heterozygosity has differential effects as compared with homozygous wild-type (42,43). Incorporation of this methodology may aid future studies in explaining the inconsistent findings with regard to GSTs and breast cancer risk (44).

In conclusion, utilizing data from a large population-based sample of American women, we found an 86% increase in the OR for breast cancer among carriers of the GSTM1 null, GSTT1 null and GSTP 105Ile/Ile genotypes and a 36% decrease in the OR among carriers of GSTM1 present, GSTT1 null and GSTP1 105Ile/Val + Val/Val genotypes compared with GSTM1 present, GSTT1 present and GSTP1 105Ile/Ile carriers. However, we found no evidence to support our hypotheses of possible joint effects among cruciferous vegetable intake and multiple GST polymorphisms and breast cancer risk. Our analyses on the specific combinations of GSTM1, GSTT1 and GSTP1 polymorphisms and breast cancer risk need to be confirmed in other large studies of breast cancer.


   Funding
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 Funding
References
 
National Cancer Institute and National Institutes of Environmental Health Sciences (CA/S66572, P30ES10126, P30ES09089 and 1K07CA102640).


   Acknowledgments
 
Conflicts of Interest Statement: None declared.


   References
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 Funding
 References
 

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Received February 22, 2007; revised June 15, 2007; accepted June 18, 2007.


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