MacroH2A restricts melanoma progression via inhibition of chemokine expression in cancer-associated fibroblasts [epigenomics]

The histone variant macroH2A has been implicated as a tumor suppressor in melanoma and other cancers, yet its role in the tumor microenvironment remains unappreciated. Here we show that mice constitutively lacking macroH2A variants exhibit increased melanoma tumor burden compared to their wild-type counterparts, which is associated with an altered intratumoral immune cell compartment. MacroH2A-deficient tumors display an accumulation of immunosuppressive monocytes and decreased functional cytotoxic T cells. Consistent with this compromised anti-tumor response, macroH2A-deficient tumors displayed upregulation of chemokines such as Ccl2, Cxcl1 and Il6. Through single cell transcriptomics of the entire melanoma microenvironment, we identified the source of these pro-tumor myeloid chemoattractants as cancer-associated fibroblasts, whose frequency and activation were increased in the absence of macroH2A. Chemokine gene expression was hyper-inducible in the absence of macroH2A and accompanied by an altered epigenetic landscape. In sum, we uncovered a tumor suppressive role for macroH2A through repression of chemokine induction in the tumor stroma. Epigenomic analysis of sorted or cultured CAFs from WT and macroH2A melanomas was carried out to understand how macroH2A regulates inflammatory gene expression in this cell type. iDFs were used as a related cell type, developed individually, to validate these findings.