RESUMO
Advances in the understanding of the epigenetic events underlying the regulation of developmental genes expression and cell lineage commitment are revealing novel regulatory networks. These also involve distinct components of the epigenetic pathways, including chromatin histone modification, DNA methylation, repression by polycomb complexes and microRNAs. Changes in chromatin structure, DNA methylation status and microRNA expression levels represent flexible, reversible and heritable mechanisms for the maintenance of stem cell states and cell fate decisions. We recently provided novel evidence showing that microRNAs, besides determining the post-transcriptional gene silencing of their targets, also bind to evolutionarily conserved complementary genomic seed-matches present on target gene promoters. At these sites, microRNAs can function as a critical interface between chromatin remodeling complexes and the genome for transcriptional gene silencing. Here, we discuss our novel findings supporting a role of the transcriptional chromatin targeting by polycomb-microRNA complexes in lineage fate determination of human hematopoietic cells.
Assuntos
Linhagem da Célula/genética , MicroRNAs/metabolismo , Proteínas do Grupo Polycomb/metabolismo , Transcrição Gênica , Sequência de Bases , Cromatina/metabolismo , Epigênese Genética , Evolução Molecular , Hematopoese/genética , Humanos , Modelos Genéticos , Regiões Promotoras Genéticas/genéticaRESUMO
Epigenetic modifications regulate developmental genes involved in stem cell identity and lineage choice. NFI-A is a posttranscriptional microRNA-223 (miR-223) target directing human hematopoietic progenitor lineage decision: NFI-A induction or silencing boosts erythropoiesis or granulopoiesis, respectively. Here we show that NFI-A promoter silencing, which allows granulopoiesis, is guaranteed by epigenetic events, including the resolution of opposing chromatin "bivalent domains," hypermethylation, recruitment of polycomb (PcG)-RNAi complexes, and miR-223 promoter targeting activity. During granulopoiesis, miR-223 localizes inside the nucleus and targets the NFI-A promoter region containing PcGs binding sites and miR-223 complementary DNA sequences, evolutionarily conserved in mammalians. Remarkably, both the integrity of the PcGs-RNAi complex and DNA sequences matching the seed region of miR-223 are required to induce NFI-A transcriptional silencing. Moreover, ectopic miR-223 expression in human myeloid progenitors causes heterochromatic repression of NFI-A gene and channels granulopoiesis, whereas its stable knockdown produces the opposite effects. Our findings indicate that, besides the regulation of translation of mRNA targets, endogenous miRs can affect gene expression at the transcriptional level, functioning in a critical interface between chromatin remodeling complexes and the genome to direct fate lineage determination of hematopoietic progenitors.
Assuntos
Regulação da Expressão Gênica , Granulócitos/citologia , MicroRNAs/genética , Fatores de Transcrição NFI/genética , Regiões Promotoras Genéticas/genética , Proteínas Repressoras/metabolismo , Transcrição Gênica , Sequência de Bases , Western Blotting , Imunoprecipitação da Cromatina , Epigenômica , Citometria de Fluxo , Inativação Gênica , Hematopoese/fisiologia , Heterocromatina/genética , Humanos , Imunoprecipitação , Leucemia/genética , Leucemia/metabolismo , Leucemia/patologia , Luciferases/metabolismo , MicroRNAs/metabolismo , Dados de Sequência Molecular , Mutagênese Sítio-Dirigida , Mielopoese/fisiologia , Fatores de Transcrição NFI/antagonistas & inibidores , Fatores de Transcrição NFI/metabolismo , Proteínas do Grupo Polycomb , RNA Mensageiro/genética , RNA Interferente Pequeno/genética , Reação em Cadeia da Polimerase em Tempo Real , Homologia de Sequência do Ácido NucleicoRESUMO
Alteration of lineage-specific transcriptional programs for hematopoiesis causes differentiation block and promotes leukemia development. Here, we show that AML1/ETO, the most common translocation fusion product in acute myeloid leukemia (AML), counteracts the activity of retinoic acid (RA), a transcriptional regulator of myelopoiesis. AML1/ETO participates in a protein complex with the RA receptor alpha (RARalpha) at RA regulatory regions on RARbeta2, which is a key RA target gene mediating RA activity/resistance in cells. At these sites, AML1/ETO recruits histone deacetylase, DNA methyltransferase, and DNA-methyl-CpG binding activities that promote a repressed chromatin conformation. The link among AML1/ETO, heterochromatic RARbeta2 repression, RA resistance, and myeloid differentiation block is indicated by the ability of either siRNA-AML1/ETO or the DNA methylation inhibitor 5-azacytidine to revert these epigenetic alterations and to restore RA differentiation response in AML1/ETO blasts. Finally, RARbeta2 is commonly silenced by hypermethylation in primary AML blasts but not in normal hematopoietic precursors, thus suggesting a role for the epigenetic repression of the RA signaling pathway in myeloid leukemogenesis.