Noncanonical role of NCOR1 as a facilitator of DNA mismatch repair and its deficiency sensitizes cancers to immune checkpoint blockade therapy

Tumor genomic heterogeneity significantly influences the efficacy of immune checkpoint blockade (ICB) therapy. To elucidate the underlying mechanisms, we pioneered the development of an ICB response score (IRS) based on machine learning analysis of microenvironmental features. Through a pan-cancer genomic screening, we identified the nuclear receptor corepressor NCOR1 as a key regulator, the tumors with genomic deficiency of which display a tumor infiltrating lymphocyte (TIL)-inflamed microenvironment. Genetic ablation of Ncor1 delays the tumorigenesis and metastasis in syngeneic and spontaneous tumor models in immunocompetent mice. Mechanistically, NCOR1 guards DNA mismatch repair by directly interacting with MSH2 and recruiting MutSa to chromatin for mismatch recognition, independent of its transcriptional regulatory activity coupling with histone deacetylase. Ncor1 depletion in hepatocellular carcinoma and melanoma cells promotes tumor immunogenicity by increasing tumor mutation burden (TMB), neoantigen load, and TIL infiltration and activation, thereby enhancing anti-PD-1 therapy efficacy. Our findings reveal that NCOR1 loss represents a novel mechanism underlying TMB-high tumors with intact MMR components, broadening the application of ICB.