Abstract 4272: Mutations in RABL3 alter RAS prenylation and are associated with hereditary pancreatic cancer

Pancreatic ductal adenocarcinoma (PDAC) is one of the deadliest solid cancers with limited treatment options despite intensive research efforts. Familial predisposition to PDAC is thought to occur in ~10% of cases, but causative genes have not been identified in most of these families. Uncovering the genetic basis for PDAC susceptibility has immediate prognostic implications for families and can provide precious mechanistic clues to PDAC pathogenesis. Here, we perform whole-genome sequence analysis in a family with high incidence of PDAC and identify a germline nonsense mutation in the member of RAS oncogene family-like 3 (RABL3) gene that has never before been directly associated with hereditary cancer. The truncated mutant allele (RABL3_p.Ser36*) co-segregates with cancer occurrence. To evaluate the contribution of the RABL3 mutant allele in hereditary cancer, we generated rabl3 heterozygous mutant zebrafish and found increased susceptibility to cancer formation in two independent cancer models. Complementary unbiased approaches implicate RABL3 in RAS pathway regulation. RNA-Seq and genome-set enrichment analysis of juvenile rabl3 mutants reveals a KRAS upregulation signature. Furthermore, affinity-purification mass-spectrometry for proteins associated with RABL3 or RABL3_p.Ser36* identifies Rap1 GTPase-GDP Dissociation Stimulator 1 (RAP1GDS1, SmgGDS), a chaperone that regulates prenylation of RAS GTPases. Indeed, in vitro studies demonstrate that RABL3_p.Ser36* accelerates KRAS prenylation, and this impact is lost in the absence of H/N/KRAS proteins. Whereas heterozygous rabl3 mutant zebrafish exhibit cancer predisposition, homozygous rabl3 mutant zebrafish develop severe craniofacial, skeletal, and growth defects consistent with human RASopathies, and these defects are partially rescued with the MEK inhibitor trametinib. Our findings support a gain-of-function rather than a null function typically associated with premature protein truncations. The discovered causative RABL3 germline mutation provides new diagnostic opportunities for genetic testing in other cancer families and uncovers an alternative mechanism for dysregulated RAS signaling in development and cancer. Note: This abstract was not presented at the meeting. Citation Format: Sahar Nissim, Ignaty Leshchiner, Joseph D. Mancias, Matthew B. Greenblatt, Ophélia Maertens, Christopher A. Cassa, Jill A. Rosenfeld, Andrew G. Cox, John Hedgepeth, Julia Wücherpfennig, Andrew J. Kim, Jake E. Henderson,