Somatic mutations of β-catenin play a crucial role in the tumorigenesis of sporadic hepatoblastoma

Hepatoblastoma (HB) is the most common malignant hepatic tumor during early childhood. Its molecular pathogenesis is still poorly understood. Mutations of adenomatous polyposis coli (APC) gene have been identified in sporadic cases and in individuals associated with familial adenomatous polyposis syndrome. β-Catenin is a key element in the cadherin-mediated cell adhesion system and Wnt/wingless pathway, and is controlled by APC. APC affects the degradation of β-catenin by its NH2-terminal phosphorylation on the serine/threonine residues of exon 3. Mutations of these phosphorylation sites are primary targets for activating mutations in several types of human cancer and lead to nuclear accumulation of β-catenin protein. In this study, we examined nine patients with HB using immunohistochemistry and direct DNA sequencing. All nine cases showed predominant nuclear expression of β-catenin. Eight cases (89%) showed mutations involving exon 3 of the β-catenin gene, including five with deletions and three with missense mutations. All five deletions were in-frame deletions without frameshift. The very high frequency of mutations in the β-catenin gene suggests that β-catenin mutations are crucial in the tumorigenesis of HB.