Differential Roles of BRCA1 and BRCA2 in the DNA Damage Response Revealed by isogenic V79 Mutant Cell lines

This study aimed to establish a novel BRCA1 mutant cell line from Chinese hamster V79 cells and clarify the role of BRCA1 in the DNA damage response (DDR) by comparison with a BRCA2 mutant line, V-C8. Using CRISPR/Cas9 editing, we generated a hypomorphic BRCA1 mutant, designated B1-21, carrying a 27-bp in-frame deletion in exon 4. This mutation deletes nine amino acids within the RING domain. B1-21, V79, and V-C8 cells were analyzed for DDR phenotypes. Both mutants showed impaired RAD51 foci formation, defective homologous recombination repair, and increased sensitivity to DNA-damaging agents. B1-21 cells were particularly sensitive to camptothecin and the PARP inhibitor NU1025, whereas V-C8 cells showed higher sensitivity to etoposide, cisplatin, mitomycin C, and bleomycin. Although γH2AX and FANCD2 focus responses were similar between V79 and B1-21, RAD51 recruitment was only partially reduced in B1-21 and completely absent in V-C8. B1-21 also displayed chromosomal instability (19-20 chromosomes), while V79 and V-C8 maintained a stable karyotype. After gamma irradiation, V-C8 cells accumulated substantially more chromatid-type aberrations and retained unrepaired chromatin longer than B1-21. Neither mutant showed normal RAD51 foci formation, radiation-induced sister chromatid exchange or an effective G2/M checkpoint arrest, unlike wild-type cells. Mitotic index measurements further confirmed checkpoint failure: V79 cells suppressed mitotic entry after irradiation, while B1-21 and V-C8 continued to enter mitosis, with V-C8 showing the most complete checkpoint breakdown. These findings indicate that partial disruption of the BRCA1 RING domain results in a hypomorphic phenotype with impaired homologous recombination, defective checkpoint control, and enhanced genotoxic sensitivity. The isogenic BRCA1 and BRCA2 mutant V79 lines offer a valuable model for dissecting DDR pathway differences and developing mutation-specific therapeutic strategies targeting BRCA-mutant cancers.