Epigenetic control mechanisms and cell-type-specific responsiveness of Wnt/ß-catenin target genes

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During embryonic development the same signal transduction mechanisms are repeatedly used to control a wide variety of differentiation processes. Depending upon cellular context any given signal transduction pathway acts upon only a subset of its potential targets. How is this selectivity achieved? We are using Wnt/β-catenin signaling and its nuclear effectors - β-catenin and the T-cell factors (TCFs) - as a paradigm to investigate this fundamental problem. Previous studies have shown that responsive and non-responsive states of Wnt/β-catenin-regulated genes coincide with characteristic patterns of DNA methylation, histone modifications, and promoter occupancy by TCFs. Therefore, epigenetic mechanisms may be used to separate functionally distinct groups of genes by regulating promoter accessibility for TCFs. The goal of the proposed project is to examine this hypothesis and to test the importance of epigenetic features for cell-type and stage-specific control of Wnt-regulated genes. To gain insight into causal relationships, we will establish temporal profiles of changes in chromatin modifications, TCF occupancy, and Wnt-inducibility which accompany functional transitions of Wnt/β-catenin targets in a cellular differentiation model. An extended characterization of cell-type-specific epigenetic landscapes will be undertaken in a locus-specific and domain-wide manner at Wnt/β-catenin target genes, and interdependencies between the generation of distinct chromatin structural states and promoter occupancy of TCFs will be further examined by analyzing alterations of structural and functional properties in response to depletion of either epigenetic modifiers or TCFs, and when association of TCFs with the promoter regions of stably transfected reporter genes is prevented by binding site mutation.