Abstract
Loss of BAP1 induces malignant transformation in human cholangiocyte organoids.
Major finding: Loss of BAP1 induces malignant transformation in human cholangiocyte organoids.
Concept: Restoration of BAP1 expression in the nucleus reverses malignant organoid histology and gene expression.
Impact: Organoid technology enables mechanistic studies of gene alterations that lead to malignant transformation.
The tumor suppressor protein BAP1 is inactivated in a number of tumor types, including cholangiocarcinoma, but the mechanisms by which it promotes tumorigenesis are not fully understood. To study the role of BAP1 loss in cholangiocarcinoma, Artegiani and colleagues introduced BAP1 mutations into human-derived cholangiocyte organoids using CRISPR/Cas9. BAP1 loss induced morphologic changes including impaired cell polarity and epithelium organization, decreased cell-to-cell adhesion, and increased motility. Transcriptomic and mass spectrometry analyses revealed differentially expressed proteins involved in cell junction, adhesion, migration, and cytoskeleton assembly that phenocopied characteristics of cholangiocarcinoma. Restoration of BAP1 expression in the nucleus reversed the malignant organoid morphology. Further investigation of the effect of BAP1 on chromatin accessibility using wild-type organoids, BAP1-mutant organoids, and BAP1-mutant organoids with BAP1 reexpression indicated that BAP1 regulates gene expression depending on epigenetic landscape and allows identification of putative molecular targets of BAP1. BAP1 loss correlated with increased chromatin accessibility in regions marked by repressive histone modifications and with decreased chromatin accessibility in regions marked by histone modifications typically associated with open chromatin, suggesting that the effects of BAP1 loss are context-dependent and influenced by the preexisting genetic landscape. In addition, direct correlation was found between BAP1 loss and altered expression of junction complex proteins including periplakin, claudins, and tensins. The organoids were further mutated using CRISPR/Cas9 to model human cholangiocarcinoma by targeting tumor-suppressor genes often mutated in this type of tumor, including TP53, SMAD4, NF1, and PTEN. Within this genetic background, the tumor-suppressor function of BAP1 was validated in in vivo models of xenografted or orthotopically transplanted mutated organoids, showing similar morphologic changes only when BAP1 was mutated and demonstrating its critical role for the acquisition of malignant features. Collectively, these results provide a mechanistic link between BAP1 loss and malignant transformation and demonstrate the use of CRISPR/Cas9 technology in organoids as a model to study oncogenesis.
Note: Research Watch is written by Cancer Discovery editorial staff. Readers are encouraged to consult the original articles for full details. For more Research Watch, visit Cancer Discovery online at http://cancerdiscovery.aacrjournals.org/CDNews.
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