Background and aims: Hepatoblastoma (HB) is the most common primary liver malignancy in childhood and lacks targeted therapeutic options. We previously engineered, to our knowledge, the first yes-associated protein 1 (YAP1) S127A -inducible mouse model of HB, demonstrating tumor regression and redifferentiation after YAP1 withdrawal through genome-wide enhancer modulation. Probing accessibility, transcription, and YAP1 binding at regulatory elements in HB tumors may provide more insight into YAP1-driven tumorigenesis and expose exploitable vulnerabilities in HB.
Approach and results: Using a multiomics approach, we integrated high-throughput transcriptome and chromatin profiling of our murine HB model to identify dynamic activity at candidate cis -regulatory elements (cCREs). We observed that 1301 of 305,596 cCREs exhibit "tumor-modified" (TM) accessibility in HB. We mapped 241 TM enhancers to corresponding genes using accessibility and histone H3K27Ac profiles. Anti-YAP1 cleavage under targets and tagmentation in tumors revealed 66 YAP1-bound TM cCRE/gene pairs, 31 of which decrease expression after YAP1 withdrawal. We validated the YAP1-dependent expression of a putative YAP1 target, Jun dimerization protein 2 (JDP2), in human HB cell lines using YAP1 and LATS1/2 small interfering RNA knockdown. We also confirmed YAP1-induced activity of the Jdp2 TM enhancer in vitro and discovered an analogous human enhancer in silico. Finally, we used transcription factor (TF) footprinting to identify putative YAP1 cofactors and characterize HB-specific TF activity genome wide.
Conclusions: Our chromatin-profiling techniques define the regulatory frameworks underlying HB and identify YAP1-regulated gene/enhancer pairs. JDP2 is an extensively validated target with YAP1-dependent expression in human HB cell lines and hepatic malignancies.
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