Abstract 1447: Identification of an oncogenic germline KRAS truncating mutation in hereditary cancers

Somatic strongly activating KRAS mutations play an oncogenic role across numerous human cancers, while less activating germline KRAS mutations are associated with developmental disorders. KRAS encodes two splice variant products—KRAS-4A and KRAS-4B—differing in their C-terminus through alternative fourth coding exons. Though KRAS-4A is homologous to the original transforming transcript identified in Kirsten rat sarcoma virus, its role in human cancer is less characterized compared to KRAS-4B. Here, through genetic analyses of three cohorts of patients with hereditary and/or aggressive cancers, we identified a rare KRAS-4A specific C-terminal truncating germline mutation (KRAS-4A C180X; rs373169526) in affected men of three families with hereditary prostate cancer and a patient with hereditary melanoma (minor allele frequency [MAF] of 0.0014 in these combined cancer cohorts assessed vs. 0.000056 in the ExAC population database, odds ratio 24.6 [95% confidence interval 5.1-103.5], two sided Fisher’s exact test p = 9.0E-5). The KRAS-4A C180X mutation truncates the C-terminus, removing the polybasic region and -CAAX motifs previously demonstrated to be necessary for Ras family member membrane association, MAP kinase signaling activation and transformation, suggesting a loss of function phenotype. However, in silico assessment of reported human variation demonstrates truncating germline variants only in KRAS-4A and not KRAS-4B, consistent with tolerance. Expression of KRAS-4A protein in NIH3T3 and MDCK leads to loss of exclusive membrane association and inhibits GTP loading, as expected, but paradoxically resulted in modest but significantly increased proliferation and soft agar colony growth compared to control or wildtype KRAS expressing cells. Pro-oncogenic phenotypes were not dependent on MAPK signaling, but showed sensitivity to AKT inhibition. In summary, we identified a germline truncating KRAS-4A mutation over-represented in hereditary cancers that defines a novel mechanism of KRAS activation not dependent on the C-terminal polybasic and -CAAX motifs. Citation Format: Moloy T. Goswami, Daniel H. Hovelson, Anna Johnson, Scott A. Tomlins, Lucy Wang, Kimberly Zhulke, Bhavneet Singh, Sharath Kumar Anand, Andi Cani, Albert Liu, Steven Kamberov, Yi-Mi Wu, Dan Robinson, Arul Chinnaiyan, Kathleen A. Cooney. Identification of an oncogenic germline KRAS truncating mutation in hereditary cancers [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1447. doi:10.1158/1538-7445.AM2017-1447