BRCA1 Mutation-Specific Responses to 53BP1 Loss-Induced Homologous Recombination and PARP Inhibitor Resistance

Abstract

SUMMARY BRCA1 functions in homologous recombination (HR) both up- and downstream of DNA end resection. However, in cells with 53BP1 gene knockout (KO), BRCA1 is dispensable for the initiation of resection, but whether BRCA1 activity is entirely redundant after end resection is unclear. Here, we found that 53bp1 KO rescued the embryonic viability of a Brea1ΔC/ΔC mouse model that harbors a stop codon in the coiled-coil domain. However, Brca1ΔC/ΔC;53bp1−/− mice were susceptible to tumor formation, lacked Rad51 foci, and were sensitive to PARP inhibitor (PARPi) treatment, indicative of suboptimal HR. Furthermore, BRCA1 mutant cancer cell lines were dependent on truncated BRCA1 proteins that retained the ability to interact with PALB2 for 53BP1 KO induced RAD51 foci and PARPi resistance. Our data suggest that the overall efficiency of 53BP1 loss of function induced HR may be BRCA1 mutation dependent. In the setting of 53BP1 KO, hypomorphic BRCA1 proteins are active downstream of end resection, promoting RAD51 loading and PARPi resistance.

Context Matters: RNF168 Connects with PALB2 to Rewire Homologous Recombination in BRCA1 Haploinsufficiency

“…Moreover, partial loss of the BRCA1-PALB2 interaction in PALB2 +/CC6 RNF168 À/À cells phenocopied the HR defects of BRCA1 +/D11 RNF168 À/À cells. These findings were largely consistent with that of Johnson and colleagues demonstrating that efficient RAD51 loading and HR requires BRCA1 hypomorphic alleles that retain the ability to associate with PALB2 (Nacson et al, 2018). In fact, a very important but still poorly understood aspect in the etiology of BRCA1-mediated breast or ovarian carcinogenesis relates to the different nature of germline BRCA1 mutations.…”

BRCA1 and homologous recombination: implications from mouse embryonic development

“…In Brca1 5382stop/− mouse tumors, no truncated BRCA1ΔBRCT proteins can be detected either [50]. Similarly, no truncated BRCA1ΔC proteins can be found in Brca1 ΔC/ΔC cells [56]. In agreement with these observations in mouse cells, truncated BRCA1ΔBRCT proteins cannot be detected either in several human cancer cell lines with truncating mutations at BRCA1 BRCT domains [56,57].…”

“…Unlike the lethality of Brca1 Δ5-6/Δ5-6 null ES cells, viable Brca1 Δ22-24/Δ22-24 ES cells can be obtained by Cre-mediated incision in Brca1 F22-24/Δ22-24 ES cells [55]. An allele that disrupt both CC and BRCT domains, Brca1 ΔC , is recently generated [56]. Mice homozygous for this allele is also lethal, but embryonic development is not analyzed.…”

PALB2 chromatin recruitment restores homologous recombination in BRCA1-deficient cells depleted of 53BP1

“…Taken together with previous findings 18,19 , our data thus indicated that BRCA1 is critical for RAD51 filament formation and ensuing HR. In light of this, we concluded that the pronounced effects of BRCA1 loss on RAD51 filament formation and HR might largely reflect BRCA1's crucial role(s) associated with recruitment of the PALB2-BRCA2-RAD51 complex to RPA-coated ssDNA 6,7,20 .…”