Abstract B26: Receptor tyrosine kinase signaling mediates resistance in NRAS mutant melanoma
Authors: Robinson James , Rebecca Vito , Chen Guo , Davies Michael , VanBrocklin Matthew , Holmen Sheri 
Published: July 2015

Abstract
Targeted therapies have revolutionized cancer care but it is clear that for most solid tumors suppression of a single target is not sufficient to prevent the development of resistance. Analysis of resistance mechanisms can provide insight into rational targets for combination therapy to prevent or treat resistant disease. RAS mutations occur in approximately 30% of all cancers and NRAS is mutated in 15-20% of melanomas; however, efforts to pharmacologically inhibit RAS have proven unsuccessful to date. Highly specific inhibitors of MEK have been developed and investigated clinically in advanced melanoma, including those that are NRAS mutant. In a recent phase II clinical trial, the MEK inhibitor MEK162 had a response rate of 20% in NRAS mutant melanoma making it the first targeted agent to demonstrate any activity in this melanoma subgroup. Despite this activity, the progression-free survival with MEK162 treatment was short (median: 3.6 months) supporting the rationale for future development of combination strategies. It is not yet clear which combination strategy will be most optimal in NRAS mutant melanoma. A powerful way to identify rational targets and combination strategies is to use an in vivo genetic approach. We have developed a novel mouse model of melanoma based on the RCAS/TVA retroviral vector system, which allows for tissue- and cell-specific targeted infection of mammalian cells through ectopic expression of the viral receptor. This system utilizes a viral vector, RCASBP(A), derived from the avian leukosis virus. The receptor for RCASBP(A) is encoded by the TVA gene and is normally expressed in avian cells. Transgenic mice that express TVA under the control of the dopachrome tautomerase (DCT) promoter were generated to allow targeting of the virus specifically to melanocytes. To model human melanoma, we generated DCT-TVA;Ink4a/Arf lox/lox mice and delivered viruses containing NRASQ61R and Cre-recombinase postnatally by subcutaneous injection of viral producing cells to somatic melanocytes expressing TVA. Tumors developed in two-thirds of the mice following a mean latency of only seven weeks. In this model, tumors evolve from gene mutations in developmentally normal somatic cells in the context of an unaltered microenvironment and therefore closely mimic the human disease. We have further expanded the utility of this model by engineering the viral vectors to permit regulation of gene expression post-delivery using the tetracycline (tet)-regulated system. To identify rational targets for combination therapy in mutant NRAS-driven melanoma, we expressed NRASQ61R under the control of a tet-responsive element (TRE) and linked its expression to Cre using an internal ribosomal entry site. Co-delivery of Tet-off and TRE-NRASQ61R-IRES-Cre yielded tumors in 72% of the mice. Inhibition of mutant NRAS expression with doxycycline administration caused complete tumor regression of established melanomas but forty percent of tumors became resistant after a mean latency of more than two months. Analysis of resistant tumors revealed that the most common mechanism of resistance was overexpression and activation of receptor tyrosine kinases (RTKs). Interestingly, the most commonly overexpressed RTK was Met and genetic and pharmacologic inhibition of Met overcame NRAS resistance in this context. In this study, we establish for the first time the importance of adaptive RTK signaling in the escape of NRAS mutant melanoma from inhibition of RAS and provide the first rationale for combined blockade of RAS and RTK signaling in this context.
Citation Format: James Robinson, Vito Rebecca, Guo Chen, Michael Davies, Matthew VanBrocklin, Sheri Holmen. Receptor tyrosine kinase signaling mediates resistance in NRAS mutant melanoma. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Melanoma: From Biology to Therapy; Sep 20-23, 2014; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(14 Suppl):Abstract nr B26.