Inhibiting YAP1 in hepatoblastoma drives therapeutic differentiation of tumor cells to functional hepatocyte-like cells

Despite advances in surgical care and chemotherapeutic regimens, the five-year survival rate for Stage IV Hepatoblastoma (HB), the predominant pediatric liver tumor, remains at 27%. YAP1 and ß-Catenin co-activation occur in 80% of children's HB; however, a lack of conditional genetic models precludes exploration of tumor maintenance and therapeutic targets. Thus, the clinical need for a targeted therapy remains unmet. Given the predominance of YAP1 and ß-catenin activation in children's tumors, we sought to evaluate YAP1 as a therapeutic target in HB. We engineered the first conditional murine model of HB using hydrodynamic injection to deliver transposon plasmids encoding inducible YAP1S127A, constitutive ß-CateninDelN90, and a luciferase reporter to murine liver. Tumor regression was evaluated using in vivo bioluminescent imaging, and tumor landscape characterized using RNA sequencing, ATAC sequencing and DNA foot-printing. Here we show that YAP1 inhibition mediates >90% tumor regression with survival for 230+ days in mice. Mechanistically, YAP1 inhibition induces a cell fate switch driving therapeutic differentiation of HB tumors into Ki-67 negative “hbHep cells” with hepatocyte-like morphology and mature hepatocyte gene expression. Overall design: RNA-seq performed on YAP1on tumors, YAP1off D6 liver, hbHeps D114+, and FAH DeltaExon5 mouse liver ATAC-seq performed on YAP1on tumors, YAP1off {D14,D33,D64} hb Heps, and mouse liver. Mouse strains purchased from Jax laboratories include: B6.Cg-Gt(ROSA)26Sortm1.1(CAG-rtTA3)Slowe/LdowJ (Strain#029627), B6N.FVB(Cg)-Tg(CAG-rtTA3)4288Slowe/J (Strain #016532), FVB/NJ (Strain #001800). FAH-/- (deltaExon5) mice were a gift from Dr. Markus Grompe (Oregon Health & Science University).