Abstract A029: Identification of THOC3 as a key regulator of subtype specification in basal-squamous pancreas cancer

Abstract Pancreas cancer is an extremely deadly malignancy expected to become the second leading cause of cancer-related deaths by 2030. While pancreatic ductal adenocarcinoma (PDAC) has been the main focus of the field, subclones of different subtypes of disease, including basal-squamous, are mixed in tumors. The basal-squamous signature aligns with adenosquamous and is associated with worse prognosis and gemcitabine resistance. This demonstrates a pressing need to understand the fate determinants and regulation of basal-squamous pancreas cancer so that effective therapies may be developed. The Reya lab recently developed a novel mouse model Msi2-Myc, which induces the expression of stabilized MycT58A in Musashi2 (Msi2)+ stem and progenitor cells. Msi2-Myc mice can form multiple subtypes of pancreas cancer and reliably form basal-squamous tumors. This powerful model enables the ability to track subtype specification from a common precancer pool to determine potential mediators of fate. To uncover key regulators of basal-squamous pancreas cancer growth and identity, we identified genes upregulated in basal-squamous pancreas cancer compared to precancers and PDAC by RNA-sequencing, subsequently screening them to find candidates that both impaired growth in vitro and downregulated key basal-squamous marker gene expression. Of the genes screened, the RNA binding protein THO Complex 3 (THOC3) was the only to impact growth and identity. Functional assays demonstrated that THOC3 is required for colony and organoid formation of basal-squamous cells across mouse and human models. Additionally, we determined THOC3 is required for tumor growth in vivo in mouse and patient sample models, demonstrating it as a dependency of basal-squamous growth. We also defined THOC3 as a key regulator of basal-squamous fate, as knockdown of THOC3 results in a reduction of the Moffitt basal gene signature. Furthermore, resultant tumors exhibit a loss of key basal-squamous marker expression by immunofluorescence. Downstream, RNA-seq analyses identify the downregulation of several histones associated with development with THOC3 knockdown, further suggesting its role in mediating tumor subtype specification. Collectively, these data identify THOC3 as a key regulator of basal-squamous pancreas cancer and begin to define its role in oncogenesis. Citation Format: Hannah N. Pettit, Cynthia Quintero, Nirakar Rajbhandari, Michael Hamilton, Tannishtha Reya. Identification of THOC3 as a key regulator of subtype specification in basal-squamous pancreas 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 A029.