Genetic polymorphisms in breast cancer risk and prevention

PL01-01 Breast cancer is a common but complex disease that is of concern to all women. In the United States, a woman is diagnosed with breast cancer about every 2 minutes. Although numerous advances have been made many women, estimated in excess of 40,000, will die this year. Prevention represents the ultimate goal and clear progress has been made with the use of selected estrogen receptor modulators (SERMs) tamoxifen plus raloxifene in that they have shown the ability to reduce invasive breast cancer by about one-half when given to high risk women. However, in the three-quarters of women without any identifiable genetic risk, the currently used models are not robust. For example, in the large P-1 trial of tamoxifen versus placebo, about 56 women were treated with tamoxifen for 5 years to prevent invasive breast cancer in a single patient. Improved determination of risk is crucial in order to more effectively utilize currently available treatments and decrease the number needed to treat. A minority of cases have one of the known predisposition mutations such as BRCA 1 or BRCA 2. Identification of additional genetic abnormalities would not only identify patients at risk but also give direction to new strategies for prevention. This represents a formidable task given the large number of genes (~30,000+) and number of single nucleotide polymorphisms (SNPs) (7M?). Breast cancer is a complex disease but to date the emphasis for prevention and treatment strategies has been on interference with estrogens either by blocking their interaction with the estrogen receptor such as with tamoxifen or lowering estrogen concentrations with ovarian function suppression in premenopausal women or aromatase inhibitors in postmenopausal women. Aside from mutations in predisposition genes, the strongest indicators of risk have been hormonally mediated factors such as early menarche, late menopause and levels of estrogens in the blood. The prevailing opinion is that this relationship between estrogens and breast cancer risk has been estrogen receptor (ER) mediated in that estrogen stimulates cell growth leading to accumulation of mutations. Evidence is increasing for an alternative and possibly additional mechanism of estrogen-related breast carcinogenesis in which certain estrogen metabolites are genotoxic producing depurinating adducts, apurinic sites in DNA, error-prone base excision repair leading to mutations and subsequently breast cancer. The estrogen genotoxicity hypothesis provides another area for study of genetic differences between patients that could identify patients at increased risk and potential strategies for prevention. This review will focus on new information relating to polymorphisms in a gene related to tamoxifen biotransformation, arguably the most important drug to date for women (and men) with breast cancer. Emerging information relating to polymorphisms in the aromatase gene CYP19 will be considered as the aromatase inhibitors are a class of drugs that have become established as being a part of optimal management of women in the adjuvant setting following removal of an early stage breast cancer and are the main class being studied in the prevention setting for postmenopausal women. Given the great importance of estrogens in breast cancer risk, studies examining genes in the estrogen metabolic pathway will be considered. In selection of therapy for women with breast cancer, the focus has been almost exclusively on the characteristics of the tumor, e.g., ER and HER-2. Until recently, essentially no attention has been paid to the host and her genetic makeup as it relates to the metabolism of different drugs. The first real clinical application of pharmacogenetics in breast cancer management relates to tamoxifen’s biotransformation to active anti-cancer metabolites. New information has arisen on the metabolism of tamoxifen to the active metabolite, 4 hydroxy-N-desmethyl-tamoxifen (endoxifen). Endoxifen is a metabolite with antitumor activity and affinity for the ER that is similar to 4-hydroxy-tamoxifen, but one that is normally present in substantially higher concentrations. CYP2D6 plays a central role in the metabolism to endoxifen and one published study in the adjuvant setting in early breast cancer shows that genotypic differences in CYP2D6 and use of CYP2D6 inhibitors has an impact on outcomes of women treated with tamoxifen. These findings also suggest that polymorphisms in the other genes encoding enzymes involved with drug metabolism should be investigated. There are clear implications of this information on tamoxifen and CYP2D6 for women considering tamoxifen in the prevention setting. This is particularly the case since completion of the STAR (Study of Tamoxifen And Raloxifene) in which the 2 agents were shown to be equivalent in terms of reducing the incidence of invasive breast cancers. Thus, if it was to be confirmed that CYP2D6 metabolism was important in the prevention setting then raloxifene, which is not metabolized by CYP2D6, would be preferable in the woman who was found to be a CYP2D6 poor metabolizer. The aromatase inhibitors represent a major class of drugs in the armamentarium against breast cancer. The aromatase gene has been resequenced and functional genomics have been performed on the identified non-synonymous coding SNPs showing significant decreases in levels of activity. These finding are consistent with a hypothesis that genetic variation in the CYP19 gene might be important in the activity of aromatase inhibitors. Currently the emphasis is on examining multiple genes (thus pharmacogenomics) in pharmacodynamic and pharmacokinetic pathways in women receiving aromatase inhibitors for breast cancer. This information has the potential for great importance in patient management because of differences in efficacy and striking differences in tolerability with some patients developing severe side-effects, especially musculoskeletal complaints. The aromatase inhibitors are currently a major focus of research in the prevention setting in postmenopausal women. The estrogen genotoxicity hypothesis states that certain metabolites of estrogen, specifically catechol estrogen quinones, can react with DNA and lead to mutations. This estrogen genotoxic mechanism does not require the presence of the ER. The presence of pathways for deactivation of the catechol estrogen quinones would be expected to decrease their reactions with DNA. The Mayo group under the direction of Richard Weinshilboum has performed a study of catechol O-methlytransferase (COMT) utilizing a comprehensive approach of (1) COMT resequencing, (2) breast cancer association studies (involving 1482 patients), (3) functional genomic studies and (4) replication association studies. They found 2 SNPs in the COMT distal promoter region that were associated with breast cancer risk reduction. The 2 SNPs were found by functional studies to up-regulate transcription and alter DNA-protein binding and these findings are consistent with the hypothesis that deactivation of the catechol estrogen quinones would decrease breast cancer risk. These functional studies provide biologic plausibility for the decreased risk observed in the Mayo association study. The Mayo group has also examined polymorphisms in the CYP19 (aromatase) gene and risk of breast cancer in an association study also involving 1483 patients. This study was based on resequencing of the CYP19 gene in which 88 polymorphism were identified. Two haplotyp-tagging approaches were utilized (methods of Carlson et al. and Stram). No association with breast cancer risk was detected for individual variants, tagSNPs or haplotype-tag SNPs despite 80% power to detect odds ratios as low as 1.49 for minor allele frequency of 0.10. Thus, there was no indication that CYP19 variants were associated with risk of developing breast cancer. The complexity of the vast number genetic interactions has led to a growing sophistication in the methodology for examining many hundreds of thousands of SNPs in association studies. Such genome wide association studies (GWAS) will allow the identification of moderate risk alleles. This approach does not require any knowledge regarding position or function of the SNP thereby giving leads as to what genes to study on a functional basis and opening up new avenues of research into the biology of breast cancer and identification of new strategies of risk assessment and prevention.