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Departments of Epidemiology [I. D., S. E. H., G. A. C., D. J. H.] and Nutrition [D. J. H.], and the Harvard Center for Cancer Prevention [I. D., G. A. C., D. J. H.], Harvard School of Public Health, and the Channing Laboratory [I. D., S. E. H., L. L., G. A. C., D. J. H.], Department of Medicine, Brigham and Womens Hospital, and Harvard Medical School, Boston, Massachusetts 02115
| Abstract |
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| Introduction |
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hydroxylation. The CYP1B1 enzyme predominately catalyzes the formation of 4-OH-E2 (1, 2, 3)
, the most carcinogenic estrogen in animal models (4)
. Unlike the 2-OH-E2 derivative, 4-OH-E2 induces uterine adenocarcinoma (5)
and can induce DNA single-strand breaks (6)
. In one study, human breast cancer tissue had a significantly higher ratio of 4-OH-E2/2-OH-E2 compared with adjacent normal tissue (7)
. In human breast cancer cell lines, the formation of 4-OH-E2 is inducible by dioxin, a common environmental contaminant (8)
. Although CYP1B1 is expressed in a wide variety of tissues, expression is particularly high in the breast, prostate, and uterus (9
, 10)
, supporting a role for CYP1B1 in hormone-mediated cancer. These findings underscore the importance of the CYP1B1 with regard to metabolism of environmental carcinogens and estrogens and its potential role in the initiation of tumors in estrogen-responsive organs, like the breast. Two polymorphisms have been examined in relation to breast cancer risk, the m1 allele (Val to Leu at codon 432) and the m2 allele (Asn to Ser at codon 453). Recently, biochemical studies determined that the Val allele and the Asn allele had higher catalytic efficiency for the 4-hydroxylation of estradiol compared with their wild-type counterparts (11 , 12) . Changes in 4-hydroxylation of 17ß-estradiol are of particular interest because of the potential carcinogenicity and estrogenic activity of the 4-OH-E2.
In a case control study, Bailey et al. (13) , found no association with the m1 and m2 alleles and breast cancer risk. They did, however, find an association between the m1 Val/Val genotype and Caucasian breast cancer patients who had ER-positive breast cancer (P = 0.02); no correlation with the m2 allele was noted (13) . In a second case control study of 186 Asian breast cancer cases and 200 Asian controls, the authors found that women with the m1 Leu/Leu genotype had a 2-fold elevated risk of breast cancer compared with women with the Val/Val genotype (14) . This lack of consistency may be attributable to ethnic differences among studies. These authors were unable to evaluate receptor status.
In this study, we evaluated, among primarily Caucasian women, the relationship between the CYP1B1 alleles and breast cancer risk in a nested case control study within the NHS cohort. Given the role of CYP1B1 in estradiol metabolism, we also evaluated the relationship between the m1 and m2 alleles and circulating estrogen levels.
| Materials and Methods |
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Molecular Analysis.
All analyses were conducted with laboratory personnel blinded to case status. DNA was extracted from buffy coat fractions using the Qiagen QIAamp Blood Kit (Qiagen, Inc., Chadsworth CA). Genotyping was performed by automated DNA sequencing on the ABI 377X using BigDye-terminator Cycle Sequencing Ready Reaction Kit (PE Biosystems, Foster City, CA). Because of the close proximity of m1 to m2 within 60 bp, we were able to amplify both polymorphisms within one amplicon. PCR amplification of m1 and m2 was generated using primers 5'-CCAACACCTCTGTCTTGGGA-3' and 5'-GCTCATTTGGGTTGGCCCTG-3'. Heterozygotes were called at positions where the secondary peak height was
4550% of the primary peak height in both forward and reverse sequence reads for a subset of the samples. After establishing the criteria for base calling, the forward PCR primer was used for all subsequent sequencing reactions. All of the genotyping was performed by laboratory personnel unaware of case control status. For quality control, a random 10% of the samples were inserted to validate genotyping identification procedures; concordance for blinded samples was 100%.
Statistical Methods.
ORs and 95% CIs were calculated using conditional and unconditional logistic regression. In addition to the matching variables, we adjusted for the following breast cancer risk factors: BMI (kg/m2) at age 18 (continuous), weight gain since age 18 (<5 kg, 5-<20, and
20), age of menarche (<12 years, 12, 13, and
13), parity/age at first birth (nulliparous, one to two children/age at first birth
24 years, one to two children/age at first birth
24, three children/age at first birth
24 years, and three or more children/age at first birth
24 years), first-degree family history of breast cancer (yes/no), history of benign breast disease (yes/no), and duration of postmenopausal hormone use (never; past, <5 years and
5; and current, <5 years and
5). We also adjusted for age at menopause (continuous in years) in analyses limited to postmenopausal women. Indicator variables for CYP1B1 m1 and m2 were created using the Val/Val and Asn/Asn as the reference category in the regression models. These reference categories were used to be consistent with studies published previously (13
, 17)
. Genotype was also evaluated using dichotomous variables, Val/Leu + Leu/Leu combined and Asn/Ser + Ser/Ser combined, as a gene dosage effect on breast cancer risk was not apparent. Unconditional multivariate models controlling for the matching factors enabled all controls to be included in analyses, limiting the cases to specified histopathological characteristics. Interactions between genotypes and breast cancer risk factors were evaluated by including interaction terms in multivariate logistic regression models. The likelihood ratio test was used to assess the statistical significance of these interactions.
Mixed regression models were used to evaluate the association between genotype and circulating hormone levels among postmenopausal controls not currently using postmenopausal hormones, controlling for BMI at blood draw and the matching variables. Hormone fractions were measured in three different batches; laboratory batch was treated as a random effect in all hormone analyses. We calculated least square mean plasma steroid hormone levels to evaluate differences in hormone levels between the genotypes. The natural logarithm of the plasma hormone values was used in the analyses to reduce the skewness of the regression residuals. We used the SAS statistical package for all analyses (Ref. 18 ; SAS Institute, Inc.).
| Results |
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The prevalence of the m1 Val allele was similar to previous reports in Caucasian women (45% NHS cases and 42% NHS controls compared with 40%; Ref. 13
). The prevalence of the m2 Ser allele was also similar to reports published previously in Caucasian women (16% NHS cases and 18% NHS controls compared with 17.4%; Ref. 13
). The calculated frequencies for the m1 alleles (Val and Leu) were 0.46 and 0.54, respectively, in the cases and 0.42 and 0.58 in the controls. For the m2 alleles (Asn and Ser), the frequencies were 0.84 and 0.16 in the cases and 0.81 and 0.19 in the controls, respectively. Genotype frequencies for the m1 alleles were in Hardy-Weinberg equilibrium for cases (
2 = 0.43, Df = 1, and P = 0.51) but slightly out of Hardy-Weinberg equilibrium for the controls (
2 = 4.1, Df = 1, and P = 0.04). This finding is most likely attributable to chance as no genotyping errors were detected among the blinded quality control samples, and data for the cases were in equilibrium. For the m2 alleles, the genotype frequencies for the cases (
2 = 0.58, Df = 1, and P = 0.45) and the controls (
2 = 0.09, Df = 1, and P = 0.76) were in Hardy-Weinberg equilibrium. The m1 and m2 polymorphisms are not in linkage disequilibrium. We found no significant associations between any genotype frequencies and risk of breast cancer (Table 1)
. The results remained unchanged after stratifying by menopausal status, hormone use, and age at menarche for all genotypes. We did not find any significant associations between any of the genotypes and specific strata of breast cancer defined by receptor status (Table 2)
. However, we did observe an association with receptor status and genotype. Women who had ER-positive tumors were more likely to have the Val/Val genotype (
2 = 4.87, P = 0.03) compared with women with ER-negative tumors (24 versus 10%). These findings are consistent with those reported by Bailey et al. (13)
. Additionally consistent with Bailey et al. is the lack of an association with ER status for the m2 genotypes.
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| Discussion |
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Our results suggest that despite a potential association with estradiol levels, neither the V432L nor the A453S polymorphisms in the CYP1B1 gene alone or in combination are sufficient to substantially influence breast cancer risk in Caucasian women.
| Acknowledgments |
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| Footnotes |
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1 Supported by NIH Grants CA65725, CA87969, and CA49449 I.D. is partially supported by a grant from the American Cancer Society (RPG-00-061-01-CCE). ![]()
2 To whom requests for reprints should be addressed, at Channing Laboratory, 181 Longwood Avenue, Boston, MA 02115. E-mail: Devivo{at}channing.harvard.edu ![]()
3 The abbreviations used are: E2, 17-ß-estradiol; ER, estrogen receptor; CYP1B1, cytochrome P450 1B1; NHS, Nurses Health Study; OR, odds ratio; CI, confidence interval; BMI, body mass index. ![]()
Received 10/12/01; revised 2/15/02; accepted 2/24/02.
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