080 Mutational landscape of MCPyV-positive and MCPyV-negative Merkel cell carcinomas with implications for immunotherapy

080 Mutational landscape of MCPyV-positive and MCPyV-negative Merkel cell carcinomas with implications for immunotherapy T Walradt, V Makarov, R Doumani, N Riaz, K Stafstrom, A Moshiri, L Yelistratova, J Levinsohn, T Chan, P Nghiem, R Lifton and J Choi 1 Dermatology, Northwestern University, Chicago, IL, 2 Dermatology, Yale University, New Haven, CT, 3 Human Oncology and Pathogenesis Program, Memorial Sloan Kettering Cancer Center, New York, NY, 4 Dermatology, University of Washington, Seattle, WA, 5 Genetics, Yale University, New Haven, CT and 6 Biochemistry and Molecular Genetics, Northwestern University, Chicago, IL Merkel cell carcinoma (MCC) is a rare but highly aggressive cutaneous neuroendocrine carcinoma, associated with the merkel cell polyomavirus (MCPyV) in 80% of cases. To define the genetic basis of MCCs, we performed exome sequencing of 49 MCCs. We show that MCPyV-negative MCCs have a high mutation burden (median of 1121 Somatic Single Nucleotide Variants (SSNVs) per-exome with frequent mutations in RB1 and TP53 and additional damaging mutations in genes in the chromatin modification (ASXL1, MLL2, and MLL3), JNK (MAP3K1 and TRAF7), and DNA-damage pathways (ATM, MSH2, and BRCA1). In contrast, MCPyV-positive MCCs harbor few SSNVs (median of 12.5 SSNVs/tumor) with none in the genes listed above. In both subgroups, there are rare cancer-promoting mutations predicted to activate the PI3K pathway (HRAS, KRAS, PIK3CA, PTEN, and TSC1) and to inactivate the NOTCH pathway (NOTCH1 and NOTCH2). Virus-negative MCCs harbor potentially targetable gain-of-function mutations in TP53 at p.R248 and p.P278. Moreover, TP53 mutational status predicts death in early stage MCC (5-year survival in TP53 mutant vs wild-type stage I and II MCCs is 20% vs. 92%, respectively; P1⁄40.0036). Lastly, we identified the tumor neoantigens in MCPyV-negative and MCPyV-positive MCCs. We found that virusnegative MCCs harbor more tumor neoantigens than melanomas or non-small cell lung cancers (median of 173, 65, and 111 neoantigens/sample, respectively), two cancers for which immune checkpoint blockade can produce durable clinical responses. Collectively, these data support the use of immunotherapies for virus-negative MCCs. 081 Resistant basal cell carcinomas require SRF/MRTF to maintain hedgehog pathway activation and tumor growth R Whitson, A Lee, N Urman, J Li, A Mirza, A Brown, C Yao, G Shankar, M Fry, SX Atwood, E Epstein, J Tang and AE Oro 1 Dermatology, Stanford, Stanford, CA, 2 Childrens Hospital Oakland Research Institute, Oakland, CA and 3 UC Irvine, Irvine, CA Basal cell carcinoma (BCC) and other hedgehog-dependent cancers commonly acquire drug resistance to the Smoothened (Smo) inhibitor vismodegib (Atwood et al., 2014; Sekulic et al., 2015). Resistant BCCs maintain activation of the hedgehog pathway suggesting the presence of mutations at or downstream of Smo. Variants in Smo are found in w50% of resistance cases (Atwood et al., 2015b), however, many variants are non-functional (Atwood et al., 2015a) leaving tumor escape mechanisms for a large proportion of patient tumors unexplored. We recently uncovered a novel resistance mechanism using multi-dimensional genomic analysis in human and mouse resistant BCCs to identify a non-canonical hedgehog activation pathway driven by the transcription factor, serum response factor (SRF). The majority of resistant human and mouse BCCs measured acquire the active nuclear form of SRF, which forms a novel protein complex and shares chromosomal occupancy with the hedgehog transcription factor Gli1. SRF is necessary for hedgehog pathway output, and activation by the SRF coactivator myocardin-related transcription factor (MRTF) is sufficient to potentiate hedgehog activation. Remarkably, pharmacological inhibitors targeting MRTF suppress tumor growth in mice harboring resistant BCCs. Our data uncovers a novel non-canonical mechanism for hedgehog transcriptional activation through SRF/MRTF which is activated in a majority of resistant BCCs. Combined with our in vivo data, we highlight SRF/MRTF-driven transcription as a promising therapeutic target to combat resistant BCC growth.