Evolutionarily conserved epigenetic regulation of transcriptional cell states during vertebrate pancreatic tumorigenesis.
BACKGROUND AND AIMS While the majority of human pancreatic cancers are classified as pancreatic ductal adenocarcinomas (PDAC), tumors are comprised of diverse dedifferentiated tumor cell states that dynamically interact with an equally complex microenvironment, making it imperative to understand, in vivo, how normal epithelial identity is (dys)regulated during tumorigenesis. METHODS Here, we integrate data from mouse and human PDAC with a unique zebrafish model of pancreatic cancer to elucidate evolutionarily conserved mechanisms regulating dedifferentiation during pancreatic tumorigenesis. RESULTS Histological and single-cell transcriptional profiling of heterogeneous tumors developing in dual-transgenic ptf1a:Gal4-VP16; UAS:mKO2- KRASG12D adult zebrafish revealed mixed populations of progenitor-like cells with acinar features, progenitor-like cells with ductal features and undifferentiated progenitor-like cells, providing the opportunity to interrogate specific transcriptional and epigenetic modifiers controlling neoplastic pancreatic cellular differentiation. Screening a panel of chromatin modifiers and chromatin readers for differential expression in these distinct cell clusters revealed hdac9b to be upregulated in the progenitor-like cells enriched for ductal features. RNA velocity trajectories supported an acinar cell-of-origin for these hdac9b-expressing progenitor-like cells. Treatment of fish with the class IIA HDAC inhibitor TMP195 confirmed a functional role of HDAC activity in regulating neoplastic cell differentiation through repression of the ductal-like and progenitor cell states and associated re-activation of acinar gene expression (Graphical Abstract). Cross-species analyses in autochthonous murine tumors confirmed an early, evolutionarily conserved role for Hdac9 in pancreatic tumorigenesis, marking tumor cell subpopulations characterized by loss of epithelial differentiation and a basal subtype identity. In human pancreatic cancer, higher HDAC9 expression is significantly correlated with poorly differentiated tumor histology and predicts shorter overall survival in PDAC patients with classical subtype-enriched tumors. CONCLUSIONS These findings suggest a highly conserved role for HDAC9 and class IIA HDACs in vertebrate pancreatic tumorigenesis and may lead to new strategies for reactivating (normal acinar/epithelial) differentiation programs to intercept and treat PDAC.