Overexpression of the BRC repeat 8 of BRCA2 hyperstabilizes RAD51 and alters DNA repair dynamics.

RAD51 plays an essential role in maintaining genomic stability via homologous recombination (HR). Aberrant RAD51 expression compromises genomic integrity and influences cellular responses to DNA-damaging agents. RAD51 expression is tightly regulated in normal cells, and its increased expression is associated with therapeutic resistance and poor prognosis in various cancers. BRCA2, a tumor suppressor gene product, promotes HR by directly interacting with RAD51 via eight evolutionarily conserved BRC repeats (BRC1-8). Individual BRC repeats share relatively low sequence similarity and possess distinct biochemical properties. We previously reported that the expression of certain BRC repeat peptides alters RAD51 protein levels, suggesting that individual BRC repeats distinctly affect RAD51 expression. We aimed to identify the specific BRC repeat regions affecting RAD51 protein levels. Notably, BRC8 expression significantly elevated RAD51 protein levels and foci formation, possibly by inhibiting its ubiquitin-mediated degradation. Paradoxically, despite increased RAD51 foci formation, BRC8 expression significantly reduced HR repair efficiency and increased sensitivity to DNA-damaging agents. Our findings suggest that BRC8 overexpression stabilizes RAD51 by inhibiting ubiquitin-dependent degradation of RAD51, leading to increased persistence of RAD51 foci, indicating impaired timely removal of RAD51 from DNA damage sites and reduced HR efficiency. Moreover, altered cell cycle distribution may contribute to reduced non-homologous end-joining efficiency, further enhancing cellular sensitivity to DNA-damaging agents. Collectively, these findings provide a basis to further explore the potential of BRC8-based peptides as tools for modulating RAD51 stability and sensitizing cells to DNA damage.