There has been progress in the identification of factors that confer important risk for the development of breast cancer. The factors include: heritable mutations in susceptibility genes; exposure to therapeutic radiation during breast development (as for Hodgkin's disease survivors who received therapeutic radiation to the chest); and histologic lesions, including LCIS and atypical hyperplasias.
Testing for mutations in the BRCA1 and BRCA2 breast ovarian cancer susceptibility genes has become part of the established care of breast cancer patients. Genetic information from BRCA1/2 testing is used to help healthy at-risk women to avoid breast and/or ovarian cancer, and ultimately to avoid death from those cancers. Data accumulated over the past decade have provided evidence that breast cancer surveillance can be improved with the addition of breast MRI, that prophylactic oophorectomy substantially reduces the risk of ovarian cancer and, when performed before menopause, can reduce the risk of breast cancer as well, and that prophylactic mastectomy reduces the risk of breast cancer by more than 90%.
It has been observed that approximately 80% of BRCA1-associated breast cancers are negative for ER, PR and HER2 (so-called triple negative) and cluster with basal-like breast cancers by DNA microarray, while 80% of BRCA2-associated breast cancers are ER+ and PR+, but HER2 negative, and luminal. These data are surprising given the close relationships between these genes in their DNA repair activities, and raise some concern that hormonal interventions will not successfully reduce the risk of BRCA1-associated breast cancers. Other strategies may be necessary to reduce breast cancer risk for this group.
Genetic information has been shown to have important implications for women with breast cancer as well. Women with strong family histories of breast and/or ovarian cancer, and women diagnosed before age 40 may consider testing at the time of breast cancer diagnosis if they would use the information to make treatment decisions. Some women choose bilateral mastectomies over breast-conserving treatment if they learn that their risk of second primary breast cancer exceeds 50%, and if their prognosis from the original breast cancer is good. Some women opt for oophorectomy as part of the management of their ER+ breast cancer if they are premenopausal mutation carriers (and could participate in TEXT).
Recently, novel agents, the PARP inhibitors, have been shown to be effective in the phase II trials in women with BRCA1 or BRCA2 mutations and metastatic ovarian or breast cancer. These drugs target DNA repair pathways that are particularly vulnerable in women with BRCA1/2 mutations. The agents may also be effective in women with sporadic breast cancer, and are currently in trials in Europe and the United States alone and in combination with cytotoxic agents.