Morphological heterogeneity in beta-catenin-mutated hepatocellular carcinomas: implications for tumor molecular classification.

Beta-catenin (CTNNB1) is commonly mutated in hepatocellular carcinoma (HCC). CTNNB1-mutated HCC has important clinical correlates, such as being immune cold and less likely to respond to immune checkpoint inhibitor therapies. It remains unclear, however, if they are a morphologically homogenous group of tumors. To better understand the association between the morphology, CTNNB1 mutations, and other molecular features, a detailed study of 338 The Cancer Genome Atlas cases was performed. A characteristic histological morphology was strongly associated with CTNNB1 mutations but was present in only 58% of CTNNB1-mutated HCCs. Tumors with APC mutations tended to have the classic morphology; those with AXIN mutations did not. Pseudoglands are a key feature of the classic morphology, and they were associated with CTNNB1 mutations, male gender, specific CTNNB1 mutation site, and lack of TP53 mutations. Differential gene expression analysis stratified by the presence/absence of pseudoglands identified 60 differentially expressed genes (FDR <5%); clustering according to these differentially expressed genes revealed three groups of tumors, one with pseudoglands and a strong association with genes regulated by Wnt signaling; within this group, TP53 mutations were associated with a loss of the typical morphology of CTNNB1-mutated HCCs. When stratified by gender, further differential gene expression showed Wnt-regulated genes were associated with pseudoglands in men but not women. These findings indicate HCC with CTNNB1 mutations are morphologically heterogeneous, with gene penetrance for morphology dependent in part on gender, specific CTNNB1 mutations, and co-occurring TP53 mutations. This heterogeneity has important implications for the classification of HCC.