Epigenetic Silencing of cGAS–STING Pathway Genes Drives Immune Evasion in Immunologically Cold Melanomas

Abstract

Immunologically cold melanomas are characterized by low T cell infiltration and poor responsiveness to immune checkpoint blockade. The cyclic GMP–AMP synthase (cGAS)–stimulator of interferon genes (STING) pathway is a critical innate immune sensor that promotes type I interferon responses and anti-tumor immunity. However, its functional suppression in cold tumors remains poorly understood. We analyzed 60 melanoma samples, classifying them as hot or cold based on CD8+ T cell infiltration. Expression of cGAS and STING was assessed by qRT-PCR, western blotting, and immunohistochemistry. Epigenetic profiling was performed using pyrosequencing and ChIP-qPCR to assess DNA methylation and histone modification. Functional assays including cGAMP stimulation, ISRE reporter activity, and ELISA were conducted to assess pathway responsiveness. Epigenetic reactivation was tested using 5-azacytidine, vorinostat, or their combination. Cold melanomas exhibited significantly reduced expression of cGAS and STING, associated with promoter hypermethylation and enrichment of repressive histone marks. STING expression correlated strongly with CD8+ T cell infiltration (r = 0.68, p < 0.01). STING-low cell lines failed to activate downstream signaling upon cGAMP stimulation. Treatment with epigenetic drugs restored cGAS–STING expression, increased IFN-β secretion, chemokine induction (CXCL10, CCL5), CD8+ T cell chemotaxis, and tumor cell death. Epigenetic silencing of the cGAS–STING pathway impairs innate immune sensing in melanoma. Reversing this silencing through targeted epigenetic therapy restores tumor microenvironment activation and immune responsiveness, offering a promising approach to enhance immunotherapy efficacy in melanoma.