Nady Roodi

Activation of 17beta-estradiol (E2) through the formation of catechol estrogen metabolites, 2-OH-E2 and 4-OH-E2, and the C-16alpha hydroxylation product, 16alpha-OH-E2, has been postulated to be a factor in mammary carcinogenesis. Cytochrome P450 1B1 (CYP1B1) exceeds other P450 enzymes in both estrogen hydroxylation activity and expression level in breast tissue. To determine whether inherited variants of CYP1B1 differ from wild-type CYP1B1 in estrogen hydroxylase activity, we expressed recombinant wild-type and five polymorphic variants of CYP1B1: variant 1 (codon 48Arg-->Gly), variant 2 (codon 119Ala-->Ser), variant 3 (codon 432Val-->Leu), variant 4 (codon453Asn-->Ser), variant 5 (48Gly, 119Ser, 432Leu, 453Ser). The His-tagged proteins were purified by nickel-nitrilotriacetic acid (Ni-NTA) chromatography and analyzed by electrophoresis and spectrophotometry. We performed assays of E2 hydroxylation activity and quantitated production of 2-OH-E2, 4-OH-E2, and 16alpha-OH-E2 by gas chromatography/mass spectrometry. Wild-type CYP1B1 formed 4-OH-E2 as main product (Km, 40+/-8 microM; k(cat) 4.4+/-0.4, min(-1); k(cat)/Km, 110 mM(-1) min(-1)), followed by 2-OH-E2 (Km, 34+/-4 microM; k(cat), 1.9+/-0.1 min(-1); k(cat)/Km, 55 mM(-1)min(-1)) and 16alpha-OH-E2 (Km, 39+/-5.7 microM; k(cat), 0.30+/-0.02 min(-1); k(cat)/Km, 7.6 mM(-1)min(-1)). The CYP1B1 variants also formed 4-OH-E2 as the main product but displayed 2.4- to 3.4-fold higher catalytic efficiencies k(cat)/Km than the wild-type enzyme, ranging from 270 mM(-1)min(-1) for variant 4, to 370 mM(-1)min(-1) for variant 2. The variant enzymes also exceeded wild-type CYP1B1 with respect to 2- and 16alpha-hydroxylation activity. Thus, inherited alterations in CYP1B1 estrogen hydroxylation activity may be associated with significant changes in estrogen metabolism and, thereby, may possibly explain interindividual differences in breast cancer risk associated with estrogen-mediated carcinogenicity.

BACKGROUND: In breast cancer patients, about two thirds of the tumors are estrogen receptor (ER)-positive and one third are ER-negative. The molecular mechanisms leading to the ER-negative phenotype are poorly understood. Nearly all ER-negative and about 40% of ER-positive cancers are resistant to endocrine therapy. PURPOSE: In this study, we examined the entire coding region of the ER gene in ER-positive and ER-negative primary breast tumors to determine whether deletions/insertions or point mutations might account for the ER-negative phenotype. METHODS: We amplified exons 1 through 8 of the ER gene in 118 ER-positive and 70 ER-negative primary breast tumors and searched for mutations by single-strand conformation polymorphism analysis, denaturing gradient gel electrophoresis, and DNA sequencing. RESULTS: Both ER-negative and ER-positive tumors contained neutral polymorphisms in codons 10 [TCT-->TCC (Ser)], 87 [GCG-->GCC (Ala)], 243 [CGC-->CGT (Arg)], 325 [CCC-->CCG (Pro)], and 594 [ACA-->ACG (Thr)]. There was no correlation of any of the polymorphic alleles with the ER phenotype or other clinicopathologic parameters including tumor type, size, grade, or stage. However, the polymorphism in codon 325 showed a strong association with a family history of breast cancer (P = .0005). This association was observed both in premenopausal and postmenopausal patients. Despite extensive searching in exons 1 through 8, we found no deletions/insertions and only two missense mutations in codons 69 [AAC (Asn)-->AAG (Lys)] and 396 [ATG (Met)-->GTG (Val)] of the same ER-negative tumor. Thus, only 1% of the primary breast cancers had point mutations in the ER gene. CONCLUSIONS: In the majority of primary breast cancers, the ER-negative phenotype is not the result of mutations in the coding region of the ER gene, but is due to deficient ER expression at the transcriptional or post-transcriptional level. IMPLICATIONS: The correlation reported previously, as well as our current findings, suggest that further investigations are warranted to understand the possible linkage of the ER gene locus to hereditary breast cancer.

The oxidative metabolism of 17beta-estradiol (E2) and estrone (E1) to catechol estrogens (2-OHE2, 4-OHE2, 2-OHE1, and 4-OHE1) and estrogen quinones has been postulated to be a factor in mammary carcinogenesis. Catechol-O-methyltransferase (COMT) catalyzes the methylation of catechol estrogens to methoxy estrogens, which simultaneously lowers the potential for DNA damage and increases the concentration of 2-methoxyestradiol (2-MeOE2), an antiproliferative metabolite. We expressed two recombinant forms of COMT, the wild-type (108Val) and a common variant (108Met), to determine whether their catalytic efficiencies differ with respect to catechol estrogen inactivation. The His-tagged proteins were purified by nickel-nitrilo-triacetic acid chromatography and analyzed by electrophoresis and Western immunoblot. COMT activity was assessed by determining the methylation of 2-OHE2, 4-OHE2, 2-OHE1, and 4-OHE1, using gas chromatography/mass spectrometry for quantitation of the respective methoxy products. In the case of 2-OHE2 and 2-OHE1, methylation occurred at 2-OH and 3-OH groups, resulting in the formation of 2-MeOE2 and 2-OH-3-MeOE2, and 2-MeOE1 and 2-OH-3-MeOE1, respectively. In contrast, in the case of 4-OHE2 and 4-OHE1, methylation occurred only at the 4-OH group, yielding 4-MeOE2 and 4-MeOE1, respectively. Individual and competition experiments revealed the following order of product formation: 4-MeOE2 > 4-MeOE1 >> 2-MeOE2 > 2-MeOE1 > 2-OH-3-MeOE1 > 2-OH-3-MeOE2. The variant isoform differed from wild-type COMT by being thermolabile, leading to 2-3-fold lower levels of product formation. MCF-7 breast cancer cells with the variant COMT 108Met/Met genotype also displayed 2-3-fold lower catalytic activity than ZR-75 breast cancer cells with the wild-type COMT 108Val/Val genotype. Thus, inherited alterations in COMT catalytic activity are associated with significant differences in catechol estrogen and methoxy estrogen levels and, thereby, may contribute to interindividual differences in breast cancer risk associated with estrogen-mediated carcinogenicity.

A key enzyme involved in the production of potentially carcinogenic estrogen metabolites and the activation of environmental carcinogens is cytochrome P450 1B1 (CYP1B1), the predominant member of the CYP1 family expressed in normal breast tissue and breast cancer. Because of the preeminent role of CYP1B1 in mammary estrogen/carcinogen metabolism, we examined the CYP1B1 gene to determine whether genetic differences could account for interindividual differences in breast cancer risk. We focused on exon 3, because it encodes the catalytically important heme binding domain of the enzyme, and discovered three polymorphisms of which two are associated with amino acid substitutions in codons 432 (Val-->Leu) and 453 (Asn-->Ser), designated as m1 and m2, respectively. Approximately 40% of Caucasian women have the m1 Val allele compared with nearly 70% of African-American women (P < 0.0001). The allele frequency also differs significantly in m2, with the rare Ser allele being present in 17.4% of Caucasians but only in 3.4% of African Americans (P < 0.0003). To determine whether the polymorphic CYP1B1 alleles hold implications as potential breast cancer risk factors, we compared the CYP1B1 genotypes in 164 Caucasian and 59 African-American breast cancer cases with those in age-, race-, and frequency-matched controls. Odds ratio calculations failed to show a significant association between any of the genotypes and breast cancer. Because CYP1B1 is known to be involved in mammary estrogen metabolism, we investigated whether the estrogen receptor status is influenced by the CYP1B1 genotypes. Caucasian patients with the m1 Val/Val genotype have a significantly higher percentage of estrogen receptor-positive (P = 0.02) and progesterone receptor-positive breast cancers (P = 0.003). There was no correlation with the m2 genotypes. These data suggest that the CYP1B1 polymorphisms in exon 3 are not associated with increased breast cancer risk but that the m1 polymorphism may be functionally important for steroid receptor expression in breast cancer of Caucasian patients.