RAS Internal Tandem Duplication Disrupts GAP-binding to Activate Oncogenic Signaling

Abstract

Molecular testing of oncogenic RAS mutations in colorectal cancer (CRC) have led to increased identification of mutations in patients with CRC. NRAS-mutated CRC has not been well characterized because it is less common (<6%) than KRAS mutations. Here, we report a novel 10 amino acid internal tandem duplication (ITD) in NRAS, which disrupts the switch II domain, in a patient with widely disseminated CRC. Hotspot next generation sequencing of a brain metastasis identified the NRAS ITD and a TP53 missense mutation (p.P275F). Whole exome sequencing of the primary tumor and two metastatic lesions (lung and brain) confirmed that the NRAS ITD and TP53 mutation were conserved between the primary tumor and both metastatic tumors, and identified an additional pathogenic mutation in CSMD1 (a tumor suppressor gene). Structural biology and biochemical analyses demonstrated that the NRAS ITD prevented binding to GAP protein, leading to sustained RAS activation, increased interaction with RAF, and downstream MAPK activation. Additionally, we provide the first crystal structure of the RAS ITD. In conclusion, these studies indicate that the NRAS ITD was the probable primary driver mutation of this aggressive CRC. Identical or biologically similar ITDs in NRAS and KRAS may be rare drivers of CRC and other aggressive malignancies.