Mutation-Specific RAS Oncogenicity Explains NRAS Codon 61 Selection in Melanoma

NRAS mutation at codons 12, 13, or 61 is associated with transformation; yet, in melanoma, such alterations are nearly exclusive to codon 61. Here, we compared the melanoma susceptibility of an Nras(Q61R) knock-in allele to similarly designed Kras(G12D) and Nras(G12D) alleles. With concomitant p16(INK4a) inactivation, Kras(G12D) or Nras(Q61R) expression efficiently promoted melanoma in vivo, whereas Nras(G12D) did not. In addition, Nras(Q61R) mutation potently cooperated with Lkb1/Stk11 loss to induce highly metastatic disease. Functional comparisons of Nras(Q61R) and Nras(G12D) revealed little difference in the ability of these proteins to engage PI3K or RAF. Instead, Nras(Q61R) showed enhanced nucleotide binding, decreased intrinsic GTPase activity, and increased stability when compared with Nras(G12D). This work identifies a faithful model of human NRAS mutant melanoma, and suggests that the increased melanomagenecity of Nras(Q61R) over Nras(G12D) is due to heightened abundance of the active, GTP-bound form rather than differences in the engagement of downstream effector pathways. SIGNIFICANCE: This work explains the curious predominance in human melanoma of mutations of codon 61 of NRAS over other oncogenic NRAS mutations. Using conditional "knock-in" mouse models, we show that physiologic expression of NRAS(Q61R), but not NRAS(G12D), drives melanoma formation. (C) 2014 AACR.