Although the majority of BRAF -mutant melanomas respond to BRAF/MEK inhibitors, these agents are not typically curative. Moreover, they are largely ineffective in NRAS-and NF1 -mutant tumors. Here we report that genetic and chemical suppression of HDAC3 potently cooperates with MAPK pathway inhibitors in all three RAS pathway-driven tumors. Specifically, we show that entinostat dramatically enhances tumor regression when combined with BRAF/MEK inhibitors, in both models that are sensitive or relatively resistant to these agents. Interestingly, MGMT expression predicts responsiveness and marks tumors with latent defects in DNA repair. BRAF/MEK inhibitors enhance these defects by suppressing homologous recombination genes, inducing a BRCAlike state; however, addition of entinostat triggers the concomitant suppression of nonhomologous endjoining genes, resulting in a chemical synthetic lethality caused by excessive DNA damage. Together, these studies identify melanomas with latent DNA repair defects, describe a promising drug combination that capitalizes on these defects, and reveal a tractable therapeutic biomarker. SIGNIFICANCE: BRAF/MEK inhibitors are not typically curative in BRAF-mutant melanomas and are ineffective in NRAS-and NF1-mutant tumors. We show that HDAC inhibitors dramatically enhance the efficacy of BRAF/MEK inhibitors in sensitive and insensitive RAS pathway-driven melanomas by coordinately suppressing two DNA repair pathways, and identify a clinical biomarker that predicts responsiveness.

Highlights d Combined MEK/BET inhibitors are effective in KRAS-mutant NSCLCs d Sensitivity is dependent on HOXC10, which is overexpressed in 51% of tumors d Aberrant HOXC10 expression is largely caused by defects in PRC2 genes d MEK/BET inhibitors co-suppress Ras output and HOXC10 to cause replication defects

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