When TEAD2 DNA-binding domain was fused with virus protein 16 transcriptional activation domain, it synergized with activated β-catenin to promote HB formation in vivo.
Thus, we demonstrate that β-catenin point mutants can also collaborate with YAP1 in HB development, albeit with a distinct molecular profile from the deletion mutant, which may have implications in both biology and therapy.
The Wnt/beta-catenin pathway is one of the major developmental pathways and is believed to play an important role in the pathogenesis of HB and CSC formation.
Here, we investigated the oncogenic properties of 14 different HB- and non-HB-associated β-catenin mutants encoded by Sleeping Beauty vectors following their delivery into the mouse liver by hydrodynamic tail-vein injection.
The β-catenin mutation is frequently observed in hepatoblastoma (HB), but the underlying mechanism by which Wnt/β-catenin signaling induces HB tumor formation is unknown.
The Wnt/β-catenin pathway plays a central role in the pathogenesis of most hepatoblastomas (HBs), that is, up to 60-80% carry activating CTNNB1 mutations.
Also, niclosamide, a Food and Drug Administration approved antihelminth compound, could effectively inhibit HB cell growth in vitro and in vivo via downregulation of Dvl-2 and β-catenin expression.
Recent studies suggest that activation of Yes-associated protein (YAP) is a major molecular event in HB development, as activated YAP synergizes with mutant β-catenin to promote HB formation in mice (YAP/β-catenin).
We detected nuclear β-catenin immunostaining in nearly all untreated HBs, including in fetal and embryonal epithelial components and in mesenchymal elements.
However, variable survival rates of 60-80% and debilitating chemotherapy sequelae argue for more informed treatment selection, which is not possible by grading the Wnt-β-catenin over activity present in most HBL tumors.
To address their roles in the pathogenesis of HB, we generated mice in which Myc and mutant β-catenin were targeted to immature cells of the developing mouse liver.
By contrast, the expression levels of epithelial-cadherin (E-cadherin) and cytosolic accumulation of β-catenin, the two most prominent markers involved in epithelial-mesenchymal transition (EMT), were reduced in liver specimens from patients with metastatic HB compared with that of healthy adjacent control tissue.
We transfected the parental HuH6 hepatoblastoma cell line with a doxycycline-inducible shRNA against CTNNB1 (gene coding for β-catenin) to obtain an isogenic cell line pair with or without aberrant β-catenin signaling.
These findings suggested the activation of the Wnt pathway in HB, which was confirmed by immunohistochemical staining of the β-catenin in 42 HB tumors.
Whole-exome sequencing identified HB as a genetically very simple tumour (2.9 mutations per tumour) with recurrent mutations in ß-catenin (CTNNB1) (12/15 cases) and the transcription factor NFE2L2 (2/15 cases).
The current observation of a somatic CTNNB1 mutation in a hepatoblastoma from a patient with a germline GPC3 mutation supports the notion that the mutation in GPC3 may influence one of the initial steps in tumorigenesis and the progression to hepatoblastoma.
In conclusion, our data clearly identify protein stabilizing mutations of the β-catenin gene as a common feature of nested stromal epithelial tumors of the liver, similarly as in hepatoblastomas.