Molecular mechanisms of allele-specific NRAS signaling and tumorigenesis

ABSTRACT Cancer-associated RAS mutants were once considered oncogenic equivalents. However, it is increasingly clear that mutants of the same RAS isoform (H-, K- or NRAS) and amino acid (codon 12, 13) can exhibit distinct GTPase, effector binding, signal transduction, and tumorigenic properties. These distinctions may contribute to disparities in therapeutic response as well as the enrichment of specific RAS isoforms and amino acid substitutions in each cancer type. Thus, a deeper understanding of how each amino acid substitution influences RAS structure, biochemistry, and function could yield meaningful biological and clinical advances. KRAS mutants are well-studied, but whether the biochemical consequences of an amino acid change in one RAS isoform can be extended to another is unclear. In particular, research into the mutant-specific functions of NRAS, the dominant RAS oncoprotein in thyroid cancer, acute myeloid leukemia, and melanoma, is limited. Here, we examine the ability of eight different oncogenic NRAS oncoproteins to induce spontaneous melanoma in mouse models. We find that oncogenic substitutions, even in the same amino acid of NRAS, have distinct melanomagenic potential. Our preliminary data link the greater melanomagenic potential of these mutants to structural features that enhance BRAF affinity, activation, and Mitogen-activated protein kinase (MAPK) signaling. These observations are consistent with studies linking increasing MAPK activity to melanoma progression in human nevi. Furthermore, we provide evidence that current knowledge of KRAS mutants cannot be translated to NRAS. Based on these data, we hypothesize that melanoma formation depends on substitution- and isoform-specific structural differences in NRAS that enhance BRAF dimerization and MAPK>ERK signaling. In Aim 1, we propose integrated X-ray, NMR, computational, and biochemical approaches to determine the structural ensembles and biochemical properties of NRAS oncoproteins of varying melanomagenic potential and compare these profiles to similar KRAS mutants. In Aim 2, we describe a combination of structural, biochemical, and live-cell reporter experiments to determine how NRAS mutants differentially recognize and promote BRAF activation. In Aim 3, we will test the concept that a transient pharmacological approach could prevent melanoma by eliminating NRAS-mutant cancer precursors with elevated MAPK>ERK signaling from the skin. If successful, these studies will uncover distinguishing structural and biochemical features of the NRAS oncoproteins that may lead to the identification of new interfaces for drug targeting, determine the molecular basis for NRAS-driven melanomagenesis, elucidate RAS isoform-specific mechanisms of RAF activation, and test a potential prevention strategy for NRAS-mutant melanoma.