Abstract
BRCA1 plays a critical role in homology-directed repair (HDR) of DNA double strand breaks, and the repair defect of BRCA1-mutant cancer cells is being targeted with platinum drugs and poly (ADP-ribose) polymerase (PARP) inhibitors. We have employed relatively simple and sensitive assays to determine the function of BRCA1 variants or mutants in two HDR mechanisms, homologous recombination (HR) and single strand annealing (SSA), and in conferring resistance to cisplatin and olaparib in human cancer cells. Our results define the functionality of the top 22 patient-derived BRCA1 missense variants and the contribution of different domains of BRCA1 and its E3 ubiquitin ligase activity to HDR and drug resistance. Importantly, our results also demonstrate that the BRCA1-PALB2 interaction dictates the choice between HR and SSA. These studies establish functional and mutational landscapes of BRCA1 for HDR and therapy resistance, while revealing novel insights into BRCA1 regulatory mechanisms and HDR pathway choice.
Highlights
Germline, heterozygous mutations in BRCA1 confer high risk of breast and ovarian cancer development in an autosomal dominant fashion (Couch et al, 2014; Fackenthal and Olopade, 2007)
The RING domain of BRCA1 binds to the RING domain of BARD1, and the BRCA1/BARD1 heterodimer formation is critical for their stability, their E3 ubiquitin ligase activity and the nuclear retention of BRCA1 (Fabbro et al, 2002; Hashizume et al, 2001; Ruffner et al, 2001)
Conditional knock out of either Brca1 or Bard1 in murine mammary epithelial cells led to the development of mammary carcinomas that are indistinguishable from each other (Shakya et al, 2008), and the BRCA1 C61G mutant that abrogates BARD1 binding failed to suppress mammary tumor development in mice (Drost et al, 2011)
Summary
Heterozygous mutations in BRCA1 confer high risk of breast and ovarian cancer development in an autosomal dominant fashion (Couch et al, 2014; Fackenthal and Olopade, 2007). BRCA1 has been implicated in numerous cellular processes including DNA repair, cell cycle checkpoints, centrosome duplication, and transcriptional regulation, etc. Tumors arising from BRCA1 mutation carriers usually show loss of the wild-type (wt) allele, which renders tumor cells biallelically null for the gene. It is generally believed that genome instability resulting from the DNA repair defect following the loss of BRCA1 is a driver of tumor development (Li and Greenberg, 2012; Venkitaraman, 2014). The very DNA repair defect that leads to tumor development is an ‘Achilles’ Heel’ of the resulting tumor cells, which can be selectively killed by suitable DNA-damaging agents that target HR defect, such as platinum drugs and poly (ADP-ribose) polymerase (PARP) inhibitors (Lord and Ashworth, 2016)
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