Decitabine reverses innate immune gene suppression in rare melanomas

Rare melanoma subtypes, including acral, mucosal, and uveal melanomas, exhibit limited responses to immune checkpoint inhibitors (ICIs), yet the molecular mechanisms of immune resistance remain poorly defined. Here, we performed transcriptomic profiling of patient-derived xenografts (PDXs) and publicly available tumor datasets to systematically compare intratumoral gene expression across cutaneous and rare melanoma subtypes. We identified a convergent downregulation of innate immune pathogen sensing (IIPS) and type I interferon signaling pathways in rare melanomas compared to cutaneous, with lower expression also observed in anti-PD-1 non-responder tumors. CIBERSORT deconvolution of immune populations revealed that lower IIPS gene-expressing tumors exhibited reduced CD8⁺ T cell and memory CD4⁺ T cell infiltration, and enrichment of M2 macrophages, consistent with a more immunosuppressive tumor microenvironment. In vitro screening of epigenetic and immunomodulatory compounds revealed that the DNA hypomethylating agent decitabine robustly induced IIPS and adaptive immune gene expression in rare melanoma cell lines. In vivo treatment of mucosal and uveal melanoma xenograft models with decitabine resulted in durable upregulation of IIPS and antigen presentation genes, and whole transcriptome analysis confirmed that IIPS gene re-expression was the dominant transcriptional consequence of decitabine treatment. These findings highlight silencing of IIPS genes as a recurrent immune evasion mechanism in rare melanomas and nominate decitabine as a potential immunomodulatory strategy for enhancing immune responsiveness.