Hepatocellular carcinoma-associated AXIN1 mutations drive low levels of Wnt/β-catenin pathway activity that allow for niche-independent growth and YAP/TAZ signaling

In healthy cells, AXIN1 organizes assembly of a large destruction complex that mediates proteolysis of the transcriptional co-activator β-catenin to prevent inappropriate Wnt/β-catenin pathway activation. In hepatocellular carcinoma (HCC), AXIN1 mutations (11%) associate with a poor-prognosis subtype that is molecularly distinct from β-catenin-mutant HCC (28-40%). How AXIN1 deficiency drives HCC formation has remained highly debated. Here, we address this issue by introducing HCC-associated AXIN1 and CTNNB1 mutations in human liver cancer cells and liver-derived organoids. We show that different mutant AXIN1 classes activate varying degrees of Wnt signaling, although at lower overall levels than CTNNB1 mutations. Strikingly, premature stop codons in 5’ coding regions do not classify as knock-out mutations but drive alternative translation of an N-terminally truncated AXIN1 variant with partially retained suppressor activity. All AXIN1 variants endow liver progenitor organoids with the capacity to grow in the absence of R-spondin and Wnt, indicative of aberrant Wnt/β-catenin pathway activation. Additionally, induced Wnt/β-catenin pathway activation inversely correlates with YAP/TAZ-mediated signaling, thus leaving higher residual YAP/TAZ activity in AXIN1 -mutant versus CTNNB1 -mutant cells. We conclude that AXIN1 mutations drive physiologically relevant Wnt/β-catenin signaling in HCC, while providing a permissive environment for YAP/YAZ signaling, thereby distinguishing AXIN1 mutations from those in CTNNB1 .