RAS mutations are highly prevalent in tumors, especially those of the lung, colon, pancreas and cutaneous melanoma. The tissues in which these tumors arise are enriched for specific RAS isoform and codon preferences. Mutations in KRAS are prevalent in lung, colon and pancreas. While NRAS mutations are predominately found in cutaneous melanoma. Generally, mutations in KRAS occur at codon 12 and NRAS mutations at codon 61. Independent of the RAS isoform, the mutated codon contributes to distinct biochemical differences. Codon 61 mutations are largely viewed to be more biochemically active than codon 12 mutations.

Previous RAS-mutant genetically engineered mouse models (GEMMs) resembled the tissue-specificity observed in patients. In the colon, expression of mutant Kras (G12D, G12C, G12R, G13D, A146T) but not Nras (G12D) induced colonic hyperplasia. The degree of colonic hyperproliferation was dependent on the specific Kras mutation. In terms of Nras, melanocytes expressing NrasQ61R, but not NrasG12D, led to the development of melanoma. These data suggest the specific Ras mutation contributes to the strength in signaling in both isoforms.

Given these data, we aimed to understand the preference for codon 61 mutations in Nras and to determine if increased biochemical activity from a codon 61 mutation led to accelerated tumorigenesis in Kras. To achieve this, we generated four conditional knock-in GEMMs to directly compare Kras and Nras mutations at codons 12 and 61; G12D and Q61R. To robustly determine the effects of RAS-mutant expression across a wide-range of tissues, we utilized the ubiquitous and temporally expressed Cre, Rosa26-CreERT2.

Induction of mutant Kras resulted in rapid tumorigenesis compared to Nras. Moreover, KrasQ61R-mutant mice had reduced survival compared to KrasG12D. Expression of KrasG12D and KrasQ61R drove hyperproliferation in both lung and colon tissues, but to varying degrees. While we observed colonic hyperplasia in both Kras-mutant mice, KrasQ61R expression dramatically reduced the number of goblet cells in the colonic epithelium. This phenotype is distinct from the published Kras mouse models and highlights an unmet understanding of KRASQ61-mutant colorectal cancers (CRC); contributing to roughly 5% of all KRAS-mutant CRCs. In the lung, both Kras-mutant mice developed lung adenomas but KrasQ61Rexpression increased the size and number of the adenomas. Neither Nras-mutant models had an effect on the colon or lung tissues. However, all Ras-mutant GEMMs developed myeloproliferative disorders (MPD). Notably, leukemias and MPDs frequently have mutations in KRAS and NRAS at either codon 12 or 61. Our GEMMs recapitulate the tissue-specificity of RAS mutations observed in patients and highlight the unique oncogenic activity of KrasQ61R.

Citation Format: Amanda R. Moore, Jason Liang, Robert Piskol, Lisa McGinnis, Donald S. Kirkpatrick, Shiva Malek. RAS-mutant mouse models confirm tissue-specificity and reveal unique oncogenic activity of KrasQ61R [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 2923.