RED CELLS, IRON, AND ERYTHROPOIESIS Identification of a PRMT5-dependent repressor complex linked to silencing of human fetal globin gene expression

Defining the molecular mechanisms under-pinning fetal () globin gene silencing may provide strategies for reactivation of -gene expression, a major therapeutic objective in patients with -thalassemia and sickle cell disease (SCD). We have previously demonstrated that symmetric methylation of histone H4 Arginine 3 (H4R3me2s) by the protein arginine methyl-transferase PRMT5 is required for recruit-ment of the DNA methyltransferase DNMT3A to the -promoter, and subse-quent DNA methylation and gene silenc-ing. Here we show in an erythroid cell line, and in primary adult erythroid pro-genitors that PRMT5 induces additional repressive epigenetic marks at the -promoter through the assembly of a multi-protein repressor complex containing the histone modifying enzymes SUV4-20h1, casein kinase 2 (CK2), and compo-nents of the nucleosome remodeling and histone deacetylation complex. Expres-sion of a mutant form of PRMT5 lacking methyltransferase activity or shRNA-mediated knockdown of SUV4-20h1 re-sulted in loss of complex binding to the -promoter, reversal of both histone and DNA repressive epigenetic marks, and in-creased -gene expression. The repressive H4K20me3 mark induced by SUV4-20h1 is enriched on the-promoter in erythroid pro-genitors from adult bone marrow compared with cord blood, suggesting developmental specificity. These studies define coordi-nated epigenetic events linked to fetal glo-bin gene silencing, and provide potential therapeutic targets for the treatment o