The Role of CtIP in BRCA1-Mediated Tumor Suppression

Familial cases of breast and ovarian cancer are often attributed to germline mutations of the BRCA1 tumor suppressor gene. Although the mechanisms of BRCA1 tumor suppression are poorly understood, its protein product has been implicated in multiple aspects of the DNA damage response. As such, BRCA1 may suppress tumor development through its role in the maintenance of genomic integrity. The C-terminus of BRCA1 contains two tandem BRCT motifs that form a single phosphoprotein binding motif that mediates the interaction of BRCA1 with at least three distinct phospho-ligands; Abraxas, BACH1, and CtIP. We recently reported that the tumor suppression activity of BRCA1 is dependent on the phospho-recognition function of its BRCT motifs. Of the three known phospho-ligands, the CtIP repair protein is intriguing because its interaction with BRCA1 is ablated by tumor-associated missense mutations in the BRCT domain. Accordingly, CtIP may be a critical mediator of the genome maintenance and tumor suppression functions of BRCA1. Here we evaluate the role of CtIP in these BRCA1-dependent processes using murine cells expressing Ctip polypeptides (Ctip-S326A) that fail to interact with Brca1. Surprisingly, we demonstrate that the BRCA1-CtIP interaction is dispensable for mammalian cell viability, critical aspects of BRCA1 function in genome stability, and BRCA1-mediated tumor suppression. Given that CtIP plays a diverse role in maintaining genome integrity, we also assessed whether CtIP has functions relevant to tumor suppression independent of its interaction with BRCA1. To test this hypothesis, we generated mice carrying a conditional-null Ctip (CtipCo) allele and used Cre recombination to inactivate the gene specifically in mammary epithelial cells. Furthermore, since we recently demonstrated that the genome maintenance and tumor suppression functions of BRCA1 do not depend on its individual interaction with the BRCT phospho-ligands Abraxas, BACH1, or CtIP, we examined whether the interaction of BRCA1 with two or more of these phospho-ligands mediates these functions.