Targeted therapeutic strategies for HRAS-mutant HNSCC

Objectives: Activating mutations in the HRAS oncogene are present in 5-11% of head and neck squamous cell carcinomas (HNSCC). My studies will determine if HRAS is a driver of HNSCC growth and whether pharmacologic inhibitors of HRAS membrane association and/or RAS effector signaling may have therapeutic value in HNSCC. Methods: To determine the importance of HRAS, we genetically depleted it from HRAS-mutant HNSCC cell lines by stable shRNA suppression. To determine the potential therapeutic value of blocking HRAS membrane association or signaling through the key RAS effector ERK MAPK, we treated HNSCC cells with inhibitors of farnesyltransferase (FTIs, tipifarnib and lonafarnib) or ERK1/2 (ERKi, SCH772984), respectively. We then monitored target inhibition by aqueous/detergent fractionation, gel shift and immunoblotting; quantitated cell viability by Alamar blue assay; and assessed cell cycle distribution and apoptosis by flow cytometry. Results: Genetic depletion of mutant HRAS inhibited the growth of HRAS-mutant HNSCC cell lines in vitro, caused a modest G1 cell cycle arrest, and robustly induced apoptosis. Since genetic depletion is not clinically feasible, we wished to identify targeted therapies suitable for this patient population. Several cancer types driven by mutant RAS are sensitive to inhibitors of the ERK pathway downstream of RAS, and HRAS is the sole RAS isoform whose activity is blocked by FTIs. We found that FTIs and ERKi each inhibited their respective targets at nanomolar range but only FTIs inhibited the growth of HRAS-mutant HNSCC in a target-dependent manner. In cell lines where FTI treatment upregulated ERK signaling, ERKi enhanced sensitivity to FTI. Conclusions: We established that genetic suppression of mutant HRAS blocks HNSCC growth, validating HRAS as a potential therapeutic target. HRAS-mutant HNSCC cell lines are sensitive to FTI, but surprisingly less sensitive to ERKi. The combination of FTI and ERKi may further sensitize a subset of HRAS-mutant HNSCC.