Acute effects of Axin loss in the mouse liver and embryonic development
Hepatocellular carcinomas carrying Axin1 mutations belong to a subset of tumours with an especially poor prognosis. Data obtained from an Axin1 mutant mouse line, challenged the traditional idea of Axin function; as simply a component of the β-Catenin-destruction complex. Axin1 deletion led to the development of highly proliferative HCC in the absence of an obvious Wnt/β-Catenin signature. \ud In order to uncover the mechanism(s) leading to Axin dependent tumourigenesis, this study focused on the role of Axin in two systems.\ud Firstly, we generated an allelic series of Axin mutant ES cell lines to analyse the role of Axin1 and 2 in ES cells. We could show, that single Axin mutants had a largely normal ES cell phenotype. In Axin double mutant ES cells, Wnt target gene expression was slightly upregulated, but cell proliferation stayed at normal levels. By contrast, upon differentiation into embryoid bodies, multiple readouts of the Wnt pathway were increased and a G2/M and cell cycle related gene expression profile was activated, accompanied by severe differentiation defects. \ud In the second system, we developed Axin1 mutant 3D liver cultures, which allow fate tracing of Axin mutant and wt cells in real time. We could show tightly regulatable gene deletion in vitro and produced preliminary evidence that Axin1 loss in culture closely mimics the in vivo situation in respect to G2/M gene expression in the absence of Wnt activation. \ud Overall, the effects of Axin loss on Wnt signalling and cell cycle regulation appeared to be tissue and cell cycle specific. Future use of the 3D culture system, together with the data obtained in Axin mutant ES cells and embryoid bodies, will not only advance our understanding of the involvement of Axin1 in hepatocellular carcinogenesis and cell cycle regulation; but may also be the starting point in the development of new therapeutic strategies.
Online Research @ Cardiff (http://orca.cf.ac.uk/84458/1/2015offergeldadphd.pdf)