Abnormalities of 53BP1 in Basal-Like Breast Cancer.

Abstract

Background: Abnormalities in the DNA repair mechanism have been proposed to play a central role in the development of breast cancers. However the specific mechanism underlying DNA repair defects in sporadic breast cancer are mostly unknown. As 53BP1 plays a critical role in DNA repair response and has been identified as a potential tumor suppressor, its expression in basal-like breast cancer cell lines and clinical samples was investigated.Experimental Design: mRNA expression of 53BP1 was investigated in publicly available gene expression data sets. Protein expression of 53BP1 was evaluated in a large set of clinically annotated breast cancer specimens by immunohistochemistry. Analysis of 53BP1 and related proteins was performed in a set of human and mouse breast cancer cell lines.Results: Gene expression analysis demonstrated that a subset of basal-like breast cancer had decreased expression of 53BP1. Analysis of over 400 breast cancers in a tissue microarray showed that loss of 53BP1 staining was almost exclusively found in tumors that lacked ER, PR and HER2 expression; this "triple-negative" phenotype is characteristic of BLC. Approximately 42% of "triple-negative" breast cancers have abnormally low levels of 53BP1, while <2% of tumor that express either ER or HER2 lack 53BP1 expression. Tumors that have low 53BP1 expression are associated with worse distant metastasis free survival when compared to tumors with normal 53BP1 expression. A set of breast cancer cell lines with abnormal 53BP1 expression was also identified. 53BP1 abnormalities were present in some tumors and cell lines with known BRCA1 mutations.Discussion: These data suggest that a subset of basal-like breast cancers lack normal 53BP1 expression and imply that these tumors may have profound defects in DNA repair/check point function that could be exploited therapeutically. As 53BP1 abnormalities were found in some tumors with known BRCA1 mutations, this suggests that loss of 53BP1 may cooperate with abnormalities in the BRCA1-related DNA repair pathways in the pathogenesis of basal-like breast cancers. Data regarding the molecular mechanisms underlying 53BP1 abnormalities in breast cancer cell lines and samples will be presented. Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 1122.