iPS Cell Based Models of Silent Chromatin and of Gene Expression in Rett Syndrome Neurons

Induced pluripotent stem (iPS) cell technology is an attractive new avenue for studying the reorganization of chromatin during development and for modeling human disease. I demonstrate that the completion of the gradual reprogramming process in murine somatic cells is temporally associated with epigenetic silencing of reprogramming retroviral vectors and acquisition of decondensed constitutive heterochromatin fibres. By specifically investigating H3K9me3-enriched heterochromatin, we provide structural evidence that compact heterochromatin domains are organized as densely packed 10 nm chromatin fibres. To model the effect of mutations in the methyl CpG-binding protein 2 (MECP2) in human neurons, we established iPS cells from a Rett syndrome patient with an exon 1 MECP2 frameshift mutation. Neuronal differentiation specification was unaffected in MECP2 mutant neurons when evaluated by single-cell Fluidigm analyses. Subtle changes in a subset of MECP2 target genes were found implicating MECP2e1 as a transcriptional modulator rather than a master regulator of gene transcription in neurons. Finally, MECP2e1 deficiency is sufficient to reduce neuronal soma-size even in presence of the alternate MECP2e2 isoform demonstrating that MECP2e1 is the dominant endogenous isoform that is essential for healthy neuron function.Ph.D