Abstract A020: Evaluating oncogenic roles of ABI1 isoforms and metastatic potential through alternative splicing deregulation in pancreatic cancer

Abstract Alternative splicing is a fundamental cellular process that expands the repertoire of mRNA transcripts from a given locus and the functions of the resulting protein isoforms. Alternative splicing is deregulated in cancer, including pancreatic, through a variety of mechanisms. However, the biological impacts of alternative splicing on cancer cell survival and metastatic potential are poorly understood. We previously showed the RNA binding protein RBFOX2, involved in alternative splicing regulation, is downregulated during PDAC progression. RBFOX2 depletion promotes metastatic disease in mice and results in an enrichment of exon skipping events that occur in transcripts encoding cytoskeletal remodeling proteins. We hypothesize that alternatively spliced transcripts resulting from RBFOX2 downregulation encode protein isoforms that promote cell migration and invasion, leading to metastatic disease. In preliminary studies, we showed RBFOX2 loss enhances lamellipodia formation in PDAC cells, with a coincident localization shift of ABI1, a scaffolding protein of the lamellipodia-promoting WAVE complex, from the cytoplasm to the leading edge of invading cells. RBFOX2 loss enhances ABI1 exon 9 skipping, which also occurs in PDAC patients, and we sought to understand whether the resulting ABI1∆ex9 protein isoform is more oncogenic. Using orthotopic models, we showed that inducible ABI1∆ex9 cDNA expression increased pancreatic tumor volume and both the multiplicity and size of liver metastases compared to inducible full-length ABI1 (ABI1-FL). Investigating the potential difference in mechanism of action between the ABI1 isoforms, we performed LC MS/MS to define protein partners. We identified direct interactions with known members of the WAVE complex under 2D conditions, with no differences between the ABI1 isoforms. Surprisingly, we identified potential interactions between ABI1 and RNA binding proteins important for cell migration and our ongoing studies are investigating how ABI1 may participate in mRNA trafficking and stability at the leading edge of migrating cells through interactions with RBPs. Finally, to define the potential cooperation of ABI1 isoforms with other RBFOX2-dependent splicing events to promote PDAC cell motility, we designed splice-switching antisense oligonucleotides (ASOs) to modulate exon skipping of individual RBFOX2 target exons in cells with inducible ABI1∆ex9 or ABI1-FL cDNAs. We found that alternative splicing of Cortactin (CTTN) cooperated with ABI1∆ex9 to promote cell migration, while migration of cells with ABI1-FL was not enhanced with CTTN splicing. Our data show that ABI1∆ex9 is more oncogenic than ABI1-FL in promoting PDAC progression in mouse models and cooperates with other protein isoforms generated by RBFOX2-mediated alternative splicing to enhance cell motility ex vivo. Our ongoing research is investigating the mechanisms by which ABI1∆ex9 promotes PDAC liver metastasis to devise potential therapeutic strategies to restrain metastatic recurrence in PDAC patients. Citation Format: Katharine L. Krueger, Taruna Saini, Lancia Darville-Bowleg, Nayeli Gutierrez Trejo, Dave Keng Boon Wee, John Koomen, Ernesto Guccione, Karen M. Mann. Evaluating oncogenic roles of ABI1 isoforms and metastatic potential through alternative splicing deregulation in pancreatic cancer [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Advances in Pancreatic Cancer Research—Emerging Science Driving Transformative Solutions; Boston, MA; 2025 Sep 28-Oct 1; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2025;85(18_Suppl_3):Abstract nr A020.