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 NrasQ^61R knock-in allele to similarly designed Kras^G12D and Nras^G12D alleles. With concomitant p16 INK4a inactivation, Kras G^12D or Nras Q^61R 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.