Diverse Heterochromatin-Associated Proteins Repress Distinct Classes of Genes and Repetitive Elements (ChIP-seq)
Heterochromatin, typically marked by H3K9me3 or H3K27me3, is key to the repression of genes and repetitive elements. The mechanisms responsible for establishing and maintaining heterochromatin in a locus specific manner are unclear. Heterochromatin restricts cell identity by preventing activation of alternative lineage genes. This barrier can be partially overcome by inducing ectopic transcription factors (TFs) to activate an alternative transcriptional program. However, TFs are impeded by highly restrictive heterochromatin leading to low reprogramming rates and incomplete activation of the target transcriptome. Previously we utilized the heterochromatin property of sonication resistance to identify 172 proteins associated with sonication resistant heterochromatin (srHC) by mass spectrometry (Becker, et al., 2017). We hypothesized that by functionally assessing the srHC proteins and identifying the genes they repress during reprogramming we could improve our understanding of heterochromatin maintenance and learn how to disrupt heterochromatin to enhance reprogramming. Focusing on reprogramming from fibroblasts to induced human hepatocytes (hiHeps) we conducted an siRNA screen of 97 srHC proteins without reprogramming factors (noTF) and with hiHep reprogramming and assessed the impact on the expression of genes located in srHC domains by RNA-seq. We identified enhancer of rudimentary homolog (ERH) as a key regulator of H3K9me3 heterochromatin maintenance. ERH was found to associate broadly with H3K9me3 domains and when depleted caused global changes in H3K9me3 and srHC. Collectively our screen of srHC protein repressive function demonstrates specific functionality of groups of srHC proteins and increases our understanding of H3K9me3 gene repression. Overall design: This study was designed to assess changes to H3K9me3 and H3K27me3 following knockdown of sonication resistant heterochromatin depleted proteins in human BJ fibroblasts. This study focused on knockdown of ERH. This study controlled for global changes to these epigenetic marks with the use of Drosophila Spike-In chromatin, and spike-in Drosophila H2Av antibody.