Abstract Breast cancer is the most prevalent cancer in women worldwide and mutation in breast cancer susceptible gene 2 (BRCA2) is one of the genetic factors conferring highest risk (40-70%) of developing breast cancer. Although BRCA2 proteins are known to maintain genomic stability mainly by homologous recombination-mediated DNA repair, a detailed mechanism of how BRCA2 loss induces tumorigenesis remains unclear. Interestingly, BRCA2 null mice display embryonic lethality, while inactivation of both alleles of BRCA2 in tumor cells renders them able to survive and are predisposed to tumorigenesis only when there are mutations in BRCA2 genetic interactors. In the current study, we tried to identify such genes using a mouse embryonic stem cell (mESC)-based insertional mutagenesis screen. In mESCs with one knockout allele of Brca2 and the other allele flanked by loxP sites, cell death can be induced by Cre-mediated loss of the conditional allele. To identify genes whose up-regulation can rescue cell death induced by loss of BRCA2, we transduced these mESC with murine stem cell retrovirus (MSCV) expressing Cre recombinase. We obtained Brca2-null mESCs that were rescued by the viral insertion and used them to clone the viral insertion sites. We identified several candidate genes that were up-regulated in viable mESCs. One of these genes encodes a PDZ domain-containing protein, GIPC3, and expression of Gipc3 mRNA was elevated 3 to 4-fold in the rescued clone. We were able to rescue lethality of Brca2-null mESC by overexpressing GIPC3. The rescued cells lost RAD51 foci formation in response to irradiation and were hypersensitive to DNA damaging agents and exhibited an overall increase in genomic instability. Mechanistically, GIPC3 can bind to APPL1 and APPL2, two adaptor proteins involved in AKT-mediated cell survival pathway. We found an increased level of the active form of AKT in GIPC3-rescued cells, and a knock down of APPL1 and APPL2 blocked the rescue by GIPC3 overexpression. We found that the Gipc3 transgene can partially rescue the lethality of Brca2 null embryos and contribute to tumorigenesis in K14-Cre; Brca2cko/cko mice. Besides GIPC3, another GIPC family member, GIPC1 can also rescue BRCA2 loss-induced lethality in mESCs. Oncomine analysis showed Gipc2 mRNA up-regulation in BRCA1 deficient human breast tumors, suggesting the possibility that high expression of all the GIPC family members (GIPC1, 2 and 3) might facilitate BRCA1 or BRCA2-loss induced tumorigenesis. Citation Format: Xia Ding, Shyam K. Sharan. GIPC high expression rescues BRCA2 deficiency and promotes tumorigenesis. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 3019. doi:10.1158/1538-7445.AM2015-3019