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Short Communication |
Oncology and Molecular Endocrinology Research Center, Laval University Medical Center (CHUL), Faculty of Pharmacy, Laval University, Quebec G1V 4G2, Canada [C. G.]; Department of Biology and Center for Cancer Research, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 [C. G., D. E. H.]; Channing Laboratory, Department of Medicine, Brigham and Womens Hospital and Harvard Medical School, Boston, Massachusetts 02115 [I. D. V., S. E. H., D. J. H.]; Departments of Epidemiology [I. D. V., S. E. H., C. A. H., D. S., D. J. H.] and Nutrition [D. J. H.], Harvard School of Public Health, Boston, Massachusetts 02115; and Harvard Center for Cancer Prevention, Boston, Massachusetts 02115 [C. A. H., D. J. H.]
| Abstract |
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genotype, estrone and estradiol levels tended to vary by UGT1A1 genotypes. The results presented do not support a strong association between the UGT1A1 promoter polymorphism and the risk of breast cancer. | Introduction |
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Interindividual variation in UGT1A1 expression is explained by a polymorphic alteration in the atypical TATA-box region of the UGT1A1 gene (10, 11, 12) . This polymorphic site is characterized by a variation in the number of TA repeats in the A(TA)nTAA sequence of the promoter. The most common allele (UGT1A1*1) contains six TA repeats, whereas the principal variant allele (UGT1A1*28) contains seven TA repeats. Two other UGT1A1 alleles, the UGT1A1*33 [A(TA)5TAA] allele and the UGT1A1*34 [A(TA)8TAA] allele, have been found almost exclusively in the African-American population (13 , 14) . In a previous study, we investigated the association between breast cancer and A(TA)7TAA and A(TA)8TAA UGT1A1 alleles, which have been shown to have lower transcriptional activity in vitro (8) . In the CBCS study, the alleles A(TA)7TAA and A(TA)8TAA were associated with a 2-fold increase in the risk of developing breast cancer in premenopausal African-American women.
In the present study, we evaluated, among primarily Caucasian women, the relationship between UGT1A1 alleles and breast cancer risk in a nested case-control study within the NHS cohort. Because decreased transcription of the UGT1A1 gene has been found to be associated with increased plasma levels of substrates of the UGT1A1 protein, e.g., bilirubin, we also evaluated the relationship between the UGT1A1*28 allele and circulating estrogen levels and the interaction between UGT1A1 and CYP17 polymorphisms.
| 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., Chatsworth, CA). DNA samples from cases and controls were genotyped for the dinucleotide insertion/deletion present in the promoter region of the UGT1A1 gene using previously described methods (8)
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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: (a) BMI (kg/m2) at age 18 (continuous); (b) weight gain since age 18 (<5, 519.9, and
20 kg); (c) age of menarche (<12, 12, 13, and
13 years); (d) 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 years; three children/age at first birth
24 years; and three or more children/age at first birth
24 years); (e) first-degree family history of breast cancer (yes/no); (f) history of benign breast disease (yes/no); and (g) duration of postmenopausal hormone use (never; past, <5 and
5 years; current, <5 and
5 years). We also adjusted for age at menopause (continuous in years) in analyses limited to postmenopausal women. Indicator variables for all three genotypes were created using the UGT1A1*1/UGT1A1*1 hypothesized low-risk genotypes as the reference category in the multivariable models. Genotype was also evaluated using a dichotomous variable with the UGT1A1*1/UGT1A1*28 and UGT1A1*28/UGT1A1*28 subjects combined, because a gene dosage effect on breast cancer risk was not apparent. Unconditional multivariable 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 controls, controlling for BMI at blood draw and the matching variables (16) . Hormone fractions were measured in two to three different batches; the laboratory batch was treated as a random variable in all hormone analyses except for dehydroepiandrosterone sulfate among never-users of hormone replacement, where a batch effect was not observed. We calculated least-square mean plasma steroid hormone levels to evaluate the differences in hormone levels between the genotypes; this was limited to postmenopausal women. The UGT1A1*1/UGT1A1*1 group was used as the reference category. To examine the interactive effect of the UGT1A1 gene and the CYP17 gene on hormone levels, we used the SAS Proc Mixed procedure. 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 (SAS Institute, Inc.).
| Results |
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2 = 1.49; degrees of freedom (df), 1; P = 0.22) and controls (
2 = 0.80; df, 1; P = 0.37). The prevalence of the UGT1A1*33 and UGT1A1*34 alleles was extremely low in the NHS, which was consistent with the fact that most women self-reported their ethnicity as Caucasian, with very few Asians (0.3%), Hispanics (0.1%), and African Americans (0.3%). The prevalence of the lower-transcriptional activity alleles was slightly higher in the African-American population (0.40 in the CBCS control group; Ref. 8
) compared with the Caucasian population (0.32 for the NHS control group).
There was no significant association between the low-activity allele, UGT1A1*28 [A(TA)7TAA], and breast cancer (Table 1)
. Compared with women homozygous for the UGT1A1*1 allele, the estimated RR for the UGT1A1*28/*28 genotype was 0.80 (CI, 0.491.29). A slightly but nonsignificantly higher RR was observed among premenopausal homozygotes (UGT1A1*28/*28 versus *1/*1; RR = 1.28; CI, 0.384.64) compared with postmenopausal women (RR = 0.80; CI, 0.471.36). No association was revealed in subgroups defined by age at menarche, menopausal status, and clinical and pathological characteristics. We observed that having both polymorphisms in UGT1A1 (UGT1A1*28) and CYP17 (A2 polymorphism) did not affect the risk of developing breast cancer.
In the analysis of the relationship of genotype with plasma steroid hormone levels, we calculated least-square geometric mean levels for each UGT1A1 genotype among postmenopausal women not currently using postmenopausal hormones (Table 2)
. Compared with women with the UGT1A1*1/UGT1A1*1 genotype, women with the UGT1A1*1/UGT1A1*28 and UGT1A1*28/UGT1A1*28 genotypes had nonsignificant elevated levels of E1: 4.4% (P = 0.38) and 8.0% (P = 0.15), respectively. Because the synthesis of estrogens occurs principally in the adipose tissue in postmenopausal women, stratification was done according to BMI at blood draw (BMIs
23 kg/m2, >23 kg/m2 and
27 kg/m2, and >27 kg/m2). Comparing women with low (BMI
23 kg/m2) versus high (BMI >27 kg/m2) estrogen production, the elevation of circulating E2 levels was greater in women who had at least one UGT1A1*28 allele (87.9%) compared with UGT1A1*1 homozygous women (52.4%). Within BMI subgroups, women who had at least one UGT1A1*28 allele in the highest stratum of BMI (>27 kg/m2) had a nonsignificant increase in the level of E2 (17.5%; P = 0.09) compared with UGT1A1*1 homozygous women (Table 2)
. Furthermore, we evaluated the combination of UGT1A1 and CYP17 genotypes on hormone levels. In analyses limited to women with a BMI >27 kg/m2 who had at least one UGT1A1*28 and one CYP17 A2 allele (n = 30), there was a marginally significant increase in the levels of E1 (33.2%; P = 0.07) and a nonsignificant elevation in E2 (27.9%; P = 0.18) compared with UGT1A1*1 and CYP17 A1 homozygous carriers (n = 20).
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| Discussion |
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In the analysis of hormone levels and UGT genotype among postmenopausal women not taking hormone replacement therapy, we observed a moderate increase in the levels of E1 and E2 associated with the UGT1A1*28 allele alone and in combination with CYP17 A2 polymorphism only in overweight women (BMI >27 kg/m2). However, this modest elevation in the plasma levels of estrogenic hormones in postmenopausal women may be insufficient for us to detect an altered risk of breast cancer. It is also well known that different UGT enzymes possess overlapping substrate specificity, including reactivity for estrogenic hormones (5) . In the case of E2 glucuronidation, although UGT1A1 is the major UGT reported to date to actively conjugate E2 (8 , 9) , other existing or not yet isolated UGTs may have a higher specificity and/or efficacy toward this steroid molecule (2 , 21) . In that case, a partial deficiency in UGT1A1, such as the one studied here, would have a limited impact on E2 metabolism and, consequently, on breast cancer risk. The modest additional elevation of E2 among the heaviest women who are carriers of at least one UGT1A1*28 allele is suggestive of a possible contribution of the glucuronidation pathway, and especially of UGT1A1, in the maintenance of hormone homeostasis in situations where estrogens are present at higher concentrations.
Our results suggest that the decrease in transcription of the UGT1A1 gene caused by the TA polymorphism is not sufficient to alter breast cancer risk in Caucasian women.
| Acknowledgments |
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| Footnotes |
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1 Supported by the Medical Research Council of Canada (to C. G.) and NIH Grants CA40356, CA49449, and CA65725. ![]()
2 To whom requests for reprints should be addressed, at Oncology and Molecular Endocrinology Research Center, Laval University Medical Center (CHUL), Faculty of Pharmacy, Laval University, Quebec G1V 4G2, Canada. E-mail: chantal.guillemette{at}crchul.ulaval.ca ![]()
3 The abbreviations used are: UGT, UDP-glucuronosyltransferase enzyme; NHS1, Nurses Health Study 1; OR, odds ratio; CI, confidence interval; CYP17, cytochrome p450c17
; CBCS, North Carolina Breast Cancer Study; E2, estradiol; E1, estrone; BMI, body mass index; RR, relative risk. ![]()
Received 11/17/00; revised 3/23/01; accepted 4/ 9/01.
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