PROJECT SUMMARY Cutaneous melanoma is an aggressive form of skin cancer. In the US alone, approximately 90,000 new cases and 9,000 deaths due to melanoma were reported in 2017. Selective inhibitors of BRAFV600E/K work potently against melanomas driven by oncogenic BRAF. Unfortunately, drug resistance is ubiquitous, and most patients will progress within two years of therapy. Large-scale genome- and transcriptome- profiling reveal that resistance to MAPK inhibition (MAPKi) is either MAPK- dependent or redundant. MAPK-dependent (resistance ~50%) resistance is characterized by the reactivation of MAPK typically through mutations that augment MAPK signaling. These MAPKi-resistant melanomas are nearly identical at the transcriptome level to their respective parental cancers, which further support these cancers simply restore MAPK. By contrast, MAPK-redundant resistance (remaining 50%) is characterized by extensive transcriptional reprogramming and is often devoid of recurrent mutations. Mechanistically, MAPK-dependent resistance is centered on the formation of Raf homo- or heter- dimers that are resistant to current generation Raf inhibitors but sensitive to next-generation Raf inhibitors that have dimer activity. However, MAPK-redundant resistance is not uniform, not well-understood, and lacks a common therapeutic target. This knowledge is important as there are no current treatment options for melanomas with MAPK-redundant resistance. To address this critical gap, we performed a forward genetic screen to identify novel mechanisms that drive resistance to MAPKi. This screen was performed using our well-established Sleeping Beauty Transposon system. The outcome of this screen was that SFKs and PDGFRβ were the preferred MAPK-redundant mechanisms of resistance for BRAFV600E melanomas A375 and SKMEL28. However the precise mechanisms by which SFKs and PDGFRβ drive MAPK-redundant resistance are unknown. In Aim 1, I will elucidate the mechanism of SFKs- and PDGFRβ- driven resistance to MAPKi by testing the hypothesis that actin remodeling is central to MAPKi resistance that is driven by both SFKs or PDGFRβ and that integrins are critical upstream of the SFKs resistance program and Akt is a critical downstream meditator of the PDGFR resistance program. In Aim 2 I will test the hypothesis that SFKs and PDGFRβ promotes MAPKi resistance by activating the Yap/Taz/TEAD/AP-1 transcriptional complex. The central hypothesis is that actin remodeling and the YAP/TAZ/AP-1 complex are critical mediators of the SFKs- and PDGFRβ- resistance programs.

PROJECT NARRATIVE Targeted inhibition of mutant Raf works potently against melanoma by inhibiting an oncogenic protein that drives melanoma growth. However, the effects of Raf inhibitors are short-lived due to the acquisition of drug resistance. Thus, understanding the mechanisms of resistance to Raf inhibitors will inform treatment paradigms that will help melanoma patients live longer.