A non-classical role for polycomb repressive complex 2 in co-ordinating adherens junctions and canonical Wnt signalling in embryonic stem cells and differentiation

Polycomb repressive complex 2 (PRC2) has a well characterised role in maintaining gene silencing and is essential for normal development, tissue homeostasis and is frequently dysregulated in cancer. The classical role of PRC2 is to catalyse post-translational modification of histone tails, with tri-methylation of Histone 3 lysine 27 strongly associated with gene silencing. Recent studies have highlighted additional non-classical roles for PRC2 including methylation of non-histone substrates, direct transcriptional activation and association with cell signalling cascades. Here I show that loss of core PRC2 components results in reduced Wnt signalling in embryonic stem cells (ESCs). Mouse ESCs that lack Suz12 show virtually undetectable canonical Wnt signalling. ESCs lacking Suz12 fail to differentiate. Intriguingly, efficient downregulation of pluripotency genes, but failure of induction of lineage specific genes is observed. In addition to the contribution to canonical Wnt signalling, beta-catenin also complexes with E-cadherin at cell surface adherens junctions. Importantly, altered E-cadherin localisation is observed in PRC2 null ES cells. E-cadherin localisation is normalised at cell-cell contacts between PRC2 mutant and wild type ES cells when grown together. Normalisation of E-cadherin localisation occurs concomitant to reestablishment of canonical Wnt signalling, and rescue of neuronal differentiation. The dual residency protein Afadin, a constituent of the intracellular adherens junction complex is also shown to have altered subcellular localisation in the absence of Jarid2 or Suz12. Taken together these results suggests a potential non-classical role for PRC2 in regulation of the adherens junctions, which in turn modifies the contribution of cadherin associated beta-catenin to Wnt signalling, and differentiation.Open Acces

Frequent Long-Range Epigenetic Silencing of Protocadherin Gene Clusters on Chromosome 5q31 in Wilms' Tumor

Wilms' tumour (WT) is a pediatric tumor of the kidney that arises via failure of the fetal developmental program. The absence of identifiable mutations in the majority of WTs suggests the frequent involvement of epigenetic aberrations in WT. We therefore conducted a genome-wide analysis of promoter hypermethylation in WTs and identified hypermethylation at chromosome 5q31 spanning 800 kilobases (kb) and more than 50 genes. The methylated genes all belong to α-, β-, and γ-protocadherin (PCDH) gene clusters (Human Genome Organization nomenclature PCDHA@, PCDHB@, and PCDHG@, respectively). This demonstrates that long-range epigenetic silencing (LRES) occurs in developmental tumors as well as in adult tumors. Bisulfite polymerase chain reaction analysis showed that PCDH hypermethylation is a frequent event found in all Wilms' tumor subtypes. Hypermethylation is concordant with reduced PCDH expression in tumors. WT precursor lesions showed no PCDH hypermethylation, suggesting that de novo PCDH hypermethylation occurs during malignant progression. Discrete boundaries of the PCDH domain are delimited by abrupt changes in histone modifications; unmethylated genes flanking the LRES are associated with permissive marks which are absent from methylated genes within the domain. Silenced genes are marked with non-permissive histone 3 lysine 9 dimethylation. Expression analysis of embryonic murine kidney and differentiating rat metanephric mesenchymal cells demonstrates that Pcdh expression is developmentally regulated and that Pcdhg@ genes are expressed in blastemal cells. Importantly, we show that PCDHs negatively regulate canonical Wnt signalling, as short-interfering RNA–induced reduction of PCDHG@ encoded proteins leads to elevated β-catenin protein, increased β-catenin/T-cell factor (TCF) reporter activity, and induction of Wnt target genes. Conversely, over-expression of PCDHs suppresses β-catenin/TCF-reporter activity and also inhibits colony formation and growth of cancer cells in soft agar. Thus PCDHs are candidate tumor suppressors that modulate regulatory pathways critical in development and disease, such as canonical Wnt signaling