Impact of homologous recombination deficiency on CDK4/6 inhibitor sensitivity in HR+/HER2-breast cancer

While the combination of endocrine therapy (ET) and CDK4/6 inhibitors (CDK4/6i) improves progression-free survival (PFS) in HR+/HER2- metastatic breast cancer, resistance remains a major challenge. BRCA2 pathogenic variants have been linked to reduced PFS, potentially due to co-deletion of the neighboring RB1 gene on chromosome 13q. As RB1 is a key target of CDK4/6, its loss drives resistance. Using CRISPR/Cas9, we generated cell lines with single and combined BRCA2 and RB1 deletions. Loss of RB1 but not BRCA2 increased proliferation and conferred resistance to the CDK4/6i palbociclib and abemaciclib. Dual loss reduced proliferation but increased resistance to CDK4/6i in vitro. However, sensitivity to the PARP inhibitor olaparib was maintained. Finally, analysis of real-world clinical data revealed that RB1 mutations were more frequent in tumors exhibiting homologous recombination deficiency signatures and 13q loss. These genomic features were associated with shorter treatment duration on CDK4/6i plus ET. In conclusion, our findings suggest that RB1 loss, alone or with BRCA2 deletion, contributes to CDK4/6 inhibitor resistance and may help explain reduced efficacy in patients with BRCA2 mutations. Importantly, despite this resistance, sensitivity to PARP inhibition is retained, highlighting a potential therapeutic vulnerability in this molecular context.