Background:Tumor mutation burden (TMB) correlates with immunotherapy response, but outcomes still vary in high TMB cancers, and many high TMB tumors lack T cell infiltration. Here, we assessed gene expression signatures of high TMB, including both inflamed and non-inflamed tumors (defined by interferon-γ [IFN-γ] gene signatures).

Methods:Gene set enrichment analysis (GSEA) was assessed from RNA-sequencing data in tumors in TCGA. TMB and IFN-γ signature score were assessed by previously published methods. We performed GSEA on high vs low TMB tumors, and further stratified by high vs low IFN-γ signature expression within high TMB tumors. We validated results in Tempus RNA-sequencing data from 264 patients at Vanderbilt.

Results:4020 TCGA cases in non-small cell lung, melanoma, bladder, head and neck, kidney, and colorectal cancer were included. Major signatures upregulated in the high TMB group included cell cycle signaling, nuclear protein/mRNA export, cholesterol biosynthetics, and ubiquitin signaling. High TMB/high IFN-γ signature tumors were enriched in immune-related pathways (neutrophil and monocyte chemotaxis, T cell activation and proliferation, and interleukin-1 secretion), and non-immune pathways including cAMP metabolic process and phosphatidylinositol signaling. High TMB/low IFN-γ signature tumors had enriched pathways in nonsense mediated decay, electron transport chain/oxidative phosphorylation, and mitochondrial translational disassembly. 264 tumors from Vanderbilt were used for validation; pathways including cholesterol biosynthetics also correlated with immunotherapy response.

Conclusions:High TMB tumors were enriched in pathways such as ubiquitin signaling, which could regulate immune cell function in antitumor immunity. Within high TMB cancers, tumors with higher IFN-γ signatures have enriched immune-related, and metabolic pathways, which could suggest novel therapeutic targets to make high TMB/ low IFN-γ tumors more immunogenic.

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