Consequences of Beta-Catenin Mutations in Pediatric Hepatoblastoma

Hepatoblastoma (HB) is the most common malignant pediatric liver cancer with nearly all affected individuals being <3 years of age. Over 85% of HBs are associated with mutations in the CTNNB1 gene that encodes Beta-catenin, a critical component of the Wnt signaling pathway that functions as a transcriptional co-regulator (see figure). However, unlike virtually all other oncoproteins, Beta-catenin mutations tend to be quite heterogeneous, ranging from missense mutations to large deletions of as many as 90 amino acids. Several other cancers are also associated with recurrent Beta-catenin mutations, which have not been described in HB. These findings raise several questions. For example, might different Beta-catenin mutations determine each of the several different HB pathologic subtypes? Might they also determine tumor stage, growth rates and/or survival? Finally, are different Beta-catenin mutant proteins responsible for activating distinct transcriptional profiles that ultimately determine the above HB features?

We have recently employed a novel and highly penetrant mouse model of HB to better understand the underlying molecular alterations and metabolic reprogramming that accompanies transformation. Using this model, we are creating and characterizing mutant Beta-catenin expression vectors that will be used to generate tumors. 6-8 mutations associated with HB or non HB cancers and ranging from point mutations to deletions will initially be produced. The biological consequences of these mutations are being studied in the above-described animal model. The studies include how these Beta-catenin mutants influence rates of tumorigenesis and growth and histologic subtype. We are also defining the molecular, biochemical and metabolic differences among HBs generated by different Beta-catenin mutants using state of the art methodologies, including transcriptional profiling, proteomic-based mass spectrometry to assess proteins that associate with Beta-catenin mutants and metabolic assays, including small molecule spectroscopy to define differences in the metabolic re-programming that we have previously shown to accompany tumorigenesis.  Finally, we are correlating clinical features in HB patients with their Beta-catenin mutations. To this end, we are capitalizing on the unique repository of >100 human HB clinical samples at our institution to define in unprecedented depth their associated Beta-catenin mutations and correlate these with various clinical features.  Proof that these mutants truly underlie these features will then be confirmed by generating a second generation of these unique tumor-specific Beta-catenin mutations and characterizing them in the above mouse model. Collectively, these studies allow us for the first time to assess the degree to which Beta-catenin mutants drive tumorigenesis and determine biological behaviors and tumor subtypes. Mechanistically, the studies enable us to link these behaviors with specific changes in tumor transcriptional, biochemical and metabolic function.