Abstract 137: Increased activity of protein kinase A is sufficient to cause fibrolamellar carcinoma

Abstract Tumor cells of almost all patients with fibrolamellar carcinoma (FLC) have a somatic mutation, a ~400 kB deletion on one copy of chromosome 19 that results in a fusion transcript of DNAJB1 to PRKACA, the catalytic subunit of protein kinase A. The resulting fusion oncoprotein, DNAJB1::PRKACA, has kinase activity, and is required and sufficient for transformation. To understand the oncogenesis of this tumor and to help identify potential therapeutic targets, we explored the pathways of pathogenesis mediated by the aberrant kinase. We explored three mechanisms that might explain transformation by the oncogenic kinase: - A difference in the substrate specificity of DNAJB1::PRKACA kinase versus native PRKACA kinase. - A difference in the interactome of DNAJB1::PRKACA kinase versus native PRKACA kinase. - An increase of total catalytic kinase activity (DNAJB1::PRKACA + PRKACA) versus the native PRKACA condition. We found, comparing cytosolic extracts of tumor and normal liver cells, that there are differences in the substrate specificity of the DNAJB1::PRKACA kinase. Additionally, such comparisons revealed differences in the interactions between the catalytic subunit of the kinase and cytosolic components. We also demonstrated a consistent increase in total PRKACA catalytic subunit in the FLC tumor, relative to the adjacent nontransformed tissue. Further, there is an increase in free, unregulated catalytic activity in the tumor cells and a change in the localization of the catalytic subunit in the tumor cells. Additionally, we have found that this increase in free catalytic activity is sufficient to account for the changes in the transcriptome in tumor cells of FLC patients. Importantly, enhanced expression in hepatocytes of either DNAJB1::PRKACA kinase or native PRKACA kinase caused very similar changes in the transcriptome, and the resultant transcriptomes closely resembled that of tumor cells from FLC patients. This suggests that the DNAJB1 domain is dispensable and not critical for transformation. In our investigation of patient tumors we found two rare and different subclasses of FLC that are indistinguishable from “classic” FLC, based on their transcriptome and based on their drug response profiles. Importantly, both subclasses are associated with changes of PRKACA, but neither involves the DNAJB1 domain. Thus, the DNAJB1 domain is not a critical component of transformation, and the significant driver is an increase of the cellular catalytic activity. Citation Format: Mahsa Shirani, Michael Tomasini, Solomon Levin, soeren heissel, Henrik Molina, Ype De Jong, charles Rice, Philip Coffino, Barbara Lyons, Sanford Simon. Increased activity of protein kinase A is sufficient to cause fibrolamellar carcinoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 137.