There is considerable, but not complete, overlap between the triple-negative and basal-like subtype cancers, both of which are common in BRCA1-associated breast cancer,[214,215] particularly in women diagnosed before age 50 years.[74,75,76] A small proportion of BRCA1-related breast cancers are ER-positive, which are associated with later age of onset.[216,217] These ER-positive cancers have clinical behavior features that are "intermediate" between ER-negative BRCA1 cancers and ER-positive sporadic breast cancers, raising the possibility that there may be a unique mechanism by which they develop.
It has been hypothesized that many BRCA1 tumors are derived from the basal epithelial layer of cells of the normal mammary gland, which account for 3% to 15% of unselected invasive ductal cancers. If the basal epithelial cells of the breast represent the breast stem cells, the regulatory role suggested for wild-type BRCA1 may partly explain the aggressive phenotype of BRCA1-associated breast cancer when BRCA1 function is damaged. Further studies are needed to fully appreciate the significance of this subtype of breast cancer within the hereditary syndromes.
The most accurate method for identifying basal-like breast cancers is through gene expression studies, which have been used to classify breast cancers into biologically- and clinically-meaningful groups.[212,219,220] This technology has also been shown to correctly differentiate BRCA1- and BRCA2-associated tumors from sporadic tumors in a high proportion of cases.[221,222,223] Notably, among a set of breast tumors studied by gene expression array to determine molecular phenotype, all tumors with BRCA1 alterations fell within the basal tumor subtype; however, this technology is not in routine use due to its high cost. Instead, immunohistochemical markers of basal epithelium have been proposed to identify basal-like breast cancers, which are typically negative for ER, progesterone receptor, and HER2, and stain positive for cytokeratin 5/6, or epidermal growth factor receptor.[224,225,226,227] Based on these methods to measure protein expression, a number of studies have shown that the majority of BRCA1-associated breast cancers are positive for basal epithelial markers.[74,205,226]
There is growing evidence that preinvasive lesions are a component of the BRCA phenotype. The Breast Cancer Linkage Consortium initially reported a relative lack of an in situ component in BRCA1-associated breast cancers, also seen in two subsequent studies of BRCA1/BRCA2 carriers.[228,229] However, in a study of 369 ductal carcinomain situ (DCIS) cases, BRCA1 and BRCA2 mutations were detected in 0.8% and 2.4%, respectively, which is only slightly lower than previously reported prevalence in studies of invasive breast cancer patients. A retrospective study of breast cancer cases in a high-risk clinic found similar rates of preinvasive lesions, particularly DCIS, among 73 BRCA-associated breast cancers and 146 mutation-negative cases.[231,232] A study of Ashkenazi Jewish women, stratified by whether they were referred to a high-risk clinic or were unselected, showed similar prevalence of DCIS and invasive breast cancers in referred patients compared with one-third lower DCIS cases among unselected subjects. Similarly, data about the prevalence of hyperplastic lesions have been inconsistent, with reports of increased[234,235] and decreased prevalence. Similar to invasive breast cancer, DCIS diagnosed at an early age and/or with a family history of breast and/or ovarian cancer is more likely to be associated with a BRCA1/BRCA2 mutation.