Epigenetic regulation of histone three lysine twenty seven tri methylation dictates mesenchymal stem cell lineage commitment, lifespan and murine skeletal development

Abstract

Epigenetic modifiers are increasingly being implicated as playing major roles in many cellular and biological processes, such as cell growth, differentiation, lifespan, selfrenewal, cancer, and metastasis. Epigenetic modifying proteins such as Enhancer of Zeste homology 2 (EZH2), Lysine demethylase 6A (KDM6A) regulates chromatin structure through the addition or removal histone three lysine twenty seven (H3K27) tri methylation (me3) modification. The presence of H3K27me3 on the promoter of genes leads to the recruitment of chromatin condensation complexes, chromatin compaction and repression of genes transcription. H3K27 demethylases remove the H3K27me3 modification allowing the recruitment of activating transcriptional complexes, opening up of chromatin and gene expression. The Project is based on our initial profiling of histone methylation patterns of genes associated with differentiation and the expression of epigenetic modifying enzymes in MSC clonal populations by cDNA microarray analysis. Our initial studies on the function of EZH2 lineage commitment of human BMSC, suggests that EZH2 is a negative regulator of osteogenesis and a positive regulator of adipogenesis. However, the direct role of the H3K7me3 epigenetic modifiers EZH2 and KDM6A and or KDM6B in BMSC differentiation is unclear, illustrating the importance of determining the epigenetic signatures associated with differentiation and maintenance of MSC. Additionally, EZH2 mutations in one allele of EZH2 methyltransferase SET domain have been identified in patents with Weaver Syndrome. These patients exhibit excess bone growth, aging and mental retardation suggesting the importance of EZH2 in human bone development. Furthermore, with the current use of MSC for Phase II/III clinical trials it’s important to understanding of the molecular pathways and epigenetic changes that regulate maintenance and differentiation of MSC aiding in treatment of skeletal tissue disorders/diseases. Therefore this PhD project identifies that EZH2 and KDM6A acts as a switch regulating MSC lineage commitment. Presence of EZH2 and its H3K27me3 on osteogenic genes such as RUNX2 prevent MSC osteogenic differentiation and intern allows the progression of adipogenic differentiation of MSC. During osteogenesis we believe KDM6A play a role in removing the H3K27me3 off genes critical for osteogenic differentiation. Furthermore during osteogenic differentiation, EZH2 and its H3K27 modifications must be removed from genes such as RUNX2, ZBTB16, MX1 and FHL1 allowing the activation of these genes which are important for osteogenic differentiation. EZH2 conditional deletion in early limb bud mesenchyme reveals EZH2 plays a critical role in skeletal patterning, bone microarchitecture and remodelling.