Genetic dissection of DNA damage response pathways reveals divergent modulation of PD-1 blockade response in colorectal cancer

Although MSI-H/dMMR colorectal cancers and the rarer subset with pathogenic POLE exonuclease domain mutations can benefit from immune checkpoint blockade, MSS colorectal cancer remains largely refractory, raising the question of whether other DNA damage response defects may also confer sensitivity to immunotherapy. Here, we performed a systematic in vivo screen of recurrently altered DNA damage response (DDR) genes to determine whether distinct repair defects modulate response to PD-1 blockade in MSS colorectal cancer. Using a genome-edited murine model in immunocompetent mice, we found that DDR perturbations produced heterogeneous outcomes, indicating that the immunologic consequences of DNA repair deficiency may be pathway-specific. Loss of Brca1, Brca2, Atm, and Fanca enhanced response to anti-PD-1 therapy, whereas Atr and Msh6 deficiency were associated with poor benefit. Brca1/2-deficient tumors showed the strongest regressions, supporting a link between core homologous recombination deficiency and increased immune sensitivity. In an independent cohort of colorectal cancer patients treated with ICB from the MSK-IMPACT Clinical Sequencing Cohort, BRCA1 alterations were associated with improved survival status, while FANCA alterations showed a similar trend. Because ATM is therapeutically targetable, we evaluated pharmacologic ATM inhibition in MSS colorectal cancer. Sustained ATM inhibition induced immune-related transcriptional programs in vitro and, when administered as a pre-treatment in vivo, enhanced the antitumor activity of PD-1 blockade. Together, these findings suggest that selected DDR alterations may represent candidate biomarkers of immunotherapy sensitivity, while ATM inhibition may provide a rational combination strategy to enhance PD-1 blockade in MSS colorectal cancer.