C9orf50 is a Targetable Spliceosome Regulator for Cancer Immunotherapy

Cancer cells harbor cell-intrinsic programs capable of sculpting immunogenic landscapes, yet molecular brakes that enforce immune evasion remain poorly defined. Here, we identify C9ORF50, a previously uncharacterized intrinsically disordered protein, as a liquid-liquid phase separation-driven regulator of RNA processing and actionable immunotherapy target. Genetic ablation of C9ORF50 induces selective intron retention in spliceosome component transcripts, leading to cytoplasmic accumulation of immunogenic double-stranded RNA (dsRNA). This dsRNA activates type I interferon pathway that in turn orchestrates a chemokine secretory program critical for T cell recruitment. The resultant enhancement of tumor immunogenicity and reprogramming of tumor immune microenvironment drives robust antitumor immunity, effectively converting immunologically "cold" tumors to "hot" phenotypes. Therapeutic targeting of C9ORF50 via RNA interference achieved robust tumor suppression in syngeneic models, suggesting its translational relevance. Notably, C9ORF50 exhibits tumor-predominant expression, with minimal baseline levels detected in healthy tissues or immune cell populations, underscoring its potential safety as a therapeutic target. Our study not only identifies C9ORF50 as a novel LLPS-dependent regulator of RNA splicing but also establishes it as a promising and potentially safe therapeutic target for cancer immunotherapy.