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By Kornelia Polyak, MD, PhD, and Pankaj Seth, PhD Breast cancer is one of the most prevalent cancers in women and comprises 18% of all female cancers worldwide. The search for hereditary breast cancer genes has identified two high-penetrance cancer susceptibility genes, BRCA1 and BRCA2.1 Disappointingly, mutations of BRCA1 and BRCA2 are found very rarely in sporadic cases, which make up 90% of all breast cancers. The search to find additional high-penetrance genes is ongoing, but it appears that the majority of breast cancer cases may be due to multiple low-penetrance genes and the influence of environmental or endogenous carcinogen exposure (see Table). The identification of these genes represents a major challenge, and most of the low-penetrance cancer susceptibility genes have been identified based on a candidate approach. This requires prior knowledge of the gene sequence and function. This approach, therefore, precludes the identification of novel genes.
|Table-Low-penetrance breast cancer susceptibility genes|
|GSTM1||Altered detoxification of carcinogens||12,13|
|CYP1A1||Metabolism of estrogen and polycyclic aromatic hydrocarbons||14|
|SOD2||Metabolism of superoxide anions||15|
|CYP17||Metabolism of steroid hormones||16|
|ERa||Altered estrogen signaling||16,17|
|NAT2||Metabolism of aromatic and heterocyclic amines||18|
|AR||Altered androgen signaling||19|
|SULT1A||Metabolism of carcinogens and endogenous hormones||6|
|COMT||Metabolism of catechol estrogens||20,21|
|XRCC1, XRCC3||Base excision repair||22|
|H-ras-VNTR||Proto-oncogene/Altered transcription/Linkage disequilibrium||23|
Recently developed genomics technologies, including DNA arrays and serial analysis of gene expression (SAGE), coupled with the completion of the human genome sequence, will enable large, unbiased, population-based studies that may identify novel genes associated with modest increases in breast cancer risk.2 However, these studies require the analysis of large cohorts and are likely to take years to complete. However, a combination of these two different approaches can analyze downstream targets of known breast cancer risk factors or cancer preventive agents using genomics technologies.
Phenol sulfotransferases recently were identified as a new group of low-penetrance breast cancer susceptibility genes by analyzing the response of breast cancer cells to tamoxifen using SAGE, and they represent one of the first examples of the feasibility of this type of approach.
Hormonal Factors and Breast Cancer
Animal models and human epidemiological studies support the role of hormonal factors, particularly estrogen, in the development of breast cancer.3 In addition, one of the most important phenotypic and prognostic features of breast carcinomas is the presence or absence of hormone receptors. Clinical trials have shown that anti-estrogen (tamoxifen) therapy decreases the risk of second primary breast cancers in women with invasive breast cancer and decreases breast cancer incidence in high-risk patients, proving the importance of estrogens in tumor development and identifying tamoxifen as a breast cancer preventive agent.4,5
However, little is known about the mechanisms that account for the tumorogenic effects of estrogen and the cancer preventive effects of tamoxifen. The best documented biological property of estrogen is its ability to activate the transcription of genes. Therefore, determining the gene expression profiles following estrogen and tamoxifen treatment will enable a better understanding of the tumor-promoting effects of estrogen and the chemopreventive effects of tamoxifen.
Sulfotransferases and Breast Cancer
To determine the global cellular response of breast cancer cells to estrogen and tamoxifen in a comprehensive and unbiased way, Seth and associates generated SAGE libraries from an estrogen-dependent human breast cancer cell line (ZR75-1) prior to and following estrogen or tamoxifen treatment.6 Interestingly, the gene encoding SULT1A phenol sulfotransferase, a metabolic enzyme involved in the metabolism of environmental carcinogens and steroid hormones (e.g., estrogen), was identified as one of the tamoxifen-induced genes.
Sulfotransferases are enzymes involved in the metabolism of xenobiotics and endogenous chemicals (steroids, catecholamines, and iodothyronines).7,8 Sulfation is a common step in phase II metabolism and generally leads to detoxification, but certain compounds can become mutagenic once sulfonated. In humans, the phenol sulfotransferase family consists of three highly related genes (SULT1A1, SULT1A2, and SULT1A3) localized on chromosome 16p.9
Inherited differences in the enzymatic activity of drug-metabolizing enzymes (e.g., glutathione transferases and N-acetyltransferases) have been shown to influence cancer risk, and differences in sulfotransferase activity similarly may influence breast cancer risk. In fact, several animal and in vitro studies have found an association between high sulfotransferase activity and the risk of developing chemically induced cancers.10 Interestingly, exon 7 of the SULT1A1 gene contains a functionally relevant polymorphism that significantly influences its enzymatic activity.11 There are two major alleles (high-activity SULT1A1*1 and low-activity SULT1A1*2) that encode proteins with an approximately 10-fold difference in enzymatic activity. Similarly, there are several functionally relevant polymorphisms in the highly related, but much less active SULT1A2 gene. The induction of SULT1A by tamoxifen, a known breast cancer preventive agent, together with the known inherited variability in SULT1A enzymatic activity, led the authors to formulate a hypothesis that polymorphism in phenol sulfotransferase genes might influence the risk of breast cancer.
To test this hypothesis, the authors analyzed the distribution of the low- and high-activity SULT1A1 alleles in 444 breast cancer patients from three different cohorts and 227 controls (healthy blood donors, male and female) free of malignancy using a polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) approach. Two of the cohorts comprised 378 early-onset breast cancer patients (cohort 1: 280 cases, < 40 years of age at diagnosis; and cohort 2: 98 cases, < 57 years of age at diagnosis); the third cohort included 66 sporadic breast cancer patients (ages 24-89 years).
Most of these patients had no family history of breast cancer and some had been shown not to be carriers of mutant BRCA1 or BRCA2 genes. The authors found no difference in SULT1A1 allele or genotype frequency between breast cancer patients and healthy controls, indicating that polymorphism in SULT1A1 does not influence breast cancer risk in this cohort. However, these early-onset breast cancer patients already may have a strong predisposition to breast cancer due to some unidentified moderate or high cancer susceptibility genes, the effect of which may mask the milder effect of a SULT1A1 polymorphism. Thus, further studies are needed to determine whether SULT1A1 polymorphism influences breast cancer risk in unselected patient populations.
Next, the authors analyzed the age of onset of breast cancer in relation to SULT1A1 genotype. Interestingly, in cohort 1, homozygotes for the low-activity allele had an earlier onset, whereas in cohort 2, homozygotes for the high-activity allele had an earlier onset. The differing effect of SULT1A1 on the age of onset in the two patient populations could be due to differences in exogenous (exposure to various chemicals) or endogenous (estrogen levels) factors. Alternatively, SULT1A1 polymorphism may influence the penetrance of other breast cancer susceptibility genes. This later hypothesis is strongly supported by the finding that 27 patients with other cancer (in addition to breast cancer) were all homozygous or heterozygous carriers of the high-activity SULT1A1 allele. These additional cancers were of diverse origin and included ovarian, colorectal, thyroid, and basal cell carcinomas; osteosarcomas; melanomas; and Hodgkin lymphomas. None of these multiple cancer patients had Li-Fraumeni, Cowden, or Gorlin syndrome, and only one of the breast-ovarian cancer patients had a germline BRCA1 mutation. The high frequency of the high-activity SULT1A1 allele in patients with multiple cancers may indicate that high phenol sulfotransferase activity increases overall cancer risk or that SULT1A1 modifies the penetrance of certain high-penetrance cancer susceptibility genes. This latter hypothesis can be tested by analyzing the SULT1A genotype of known hereditary cancer syndrome patients (e.g., BRCA1, BRCA2, PTEN, APC, p53, and PTCH mutation carriers).
Another important point made in the paper is the suggestion that the patient’s SULT1A1 genotype may influence the effectiveness of certain breast cancer preventive agents like tamoxifen. First, tamoxifen may increase SULT1A protein levels and, therefore, enzymatic activity. Depending on the SULT1A genotype, this effect may be beneficial or harmful. Second, tamoxifen itself may be metabolized and presumably inactivated by SULT1A. Thus, in patients with high-activity SULT1A alleles, the biological dose of tamoxifen may be lower than in patients who are homozygous for low-activity SULT1A alleles. Testing the SULT1A genotype of patients participating in tamoxifen prevention trials and correlating this with clinical outcome is the best way to answer this question. This issue is particularly important because most of these patients are at high risk of breast cancer due to genetic or environmental factors, and the effects of both could be influenced by SULT1A.
Phenol sulfotransferases have joined the growing list of low-penetrance breast cancer susceptibility genes (see Table). These genes include glutathione transferases (GSTM1), cytochrome P450 family members (CYP1A1 and CYP17), N-acetyl-transferase 2 (NAT2), catechol-O-methyl transferase (COMT), manganese superoxide dismutase (SOD2), and components of the estrogen (ER) and androgen (AR) signaling pathways.
As this list indicates, most of these low-penetrance breast cancer susceptibility genes are involved in the metabolism of steroid hormones, environmental carcinogens, or both. This finding appears to confirm the hypothesis that the most important risk factor for sporadic cancers is lifestyle and its interaction with the individual’s genotype. Although patients cannot change their genetic composition, they clearly can change their behavior and significantly decrease their cancer risk. Future goals of molecular epidemiology include determining the individual’s susceptibility to cancer based on detailed genotyping, which will be administered like a routine blood test, and designing preventive therapies accordingly. It remains to be determined whether this is an achievable goal, but due to the recently developed genomics technologies and the complete sequence of the human genome, we now have the tools to start addressing these questions. (Dr. Polyak is Assistant Professor of Medicine, and Dr. Seth is Postdoctoral Fellow, Department of Adult Oncology, Dana-Farber Cancer Institute, Boston, MA.)
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