RESUMO
The polycomb repressive complex 2 (PRC2) consists of core subunits SUZ12, EED, RBBP4/7, and EZH1/2 and is responsible for mono-, di-, and tri-methylation of lysine 27 on histone H3. Whereas two distinct forms exist, PRC2.1 (containing one polycomb-like protein) and PRC2.2 (containing AEBP2 and JARID2), little is known about their differential functions. Here, we report the discovery of a family of vertebrate-specific PRC2.1 proteins, "PRC2 associated LCOR isoform 1" (PALI1) and PALI2, encoded by the LCOR and LCORL gene loci, respectively. PALI1 promotes PRC2 methyltransferase activity in vitro and in vivo and is essential for mouse development. Pali1 and Aebp2 define mutually exclusive, antagonistic PRC2 subtypes that exhibit divergent H3K27-tri-methylation activities. The balance of these PRC2.1/PRC2.2 activities is required for the appropriate regulation of polycomb target genes during differentiation. PALI1/2 potentially link polycombs with transcriptional co-repressors in the regulation of cellular identity during development and in cancer.
Assuntos
Complexo Repressor Polycomb 2/genética , Proteínas Repressoras/genética , Vertebrados/genética , Sequência de Aminoácidos , Animais , Diferenciação Celular/genética , Linhagem Celular , Células HEK293 , Histonas/genética , Humanos , Metilação , Metiltransferases/genética , Camundongos , Neoplasias/genética , Alinhamento de SequênciaRESUMO
Polycomb repressive complex 2 (PRC2) is a conserved chromatin regulator that is responsible for the methylation of histone H3 lysine 27 (H3K27). PRC2 is essential for normal development and its loss of function thus results in a range of developmental phenotypes. Here, we review the latest advances in our understanding of mammalian PRC2 activity and present an updated summary of the phenotypes associated with its loss of function in mice. We then discuss recent studies that have highlighted regulatory interplay between the modifications laid down by PRC2 and other chromatin modifiers, including NSD1 and DNMT3A. Finally, we propose a model in which the dysregulation of these modifications at intergenic regions is a shared molecular feature of genetically distinct but highly phenotypically similar overgrowth syndromes in humans.
Assuntos
Anormalidades Congênitas/genética , Desenvolvimento Embrionário/genética , Complexo Repressor Polycomb 2/metabolismo , Animais , Humanos , Mamíferos/embriologia , Mamíferos/genética , Modelos Biológicos , Transdução de Sinais/genéticaRESUMO
Driver mutations in genes encoding histone H3 proteins resulting in p.Lys27Met substitutions (H3-K27M) are frequent in pediatric midline brain tumors. However, the precise mechanisms by which H3-K27M causes tumor initiation remain unclear. Here, we use human hindbrain neural stem cells to model the consequences of H3.3-K27M on the epigenomic landscape in a relevant developmental context. Genome-wide mapping of epitope-tagged histone H3.3 revealed that both the wild type and the K27M mutant incorporate abundantly at pre-existing active enhancers and promoters, and to a lesser extent at Polycomb repressive complex 2 (PRC2)-bound regions. At active enhancers, H3.3-K27M leads to focal H3K27ac loss, decreased chromatin accessibility and reduced transcriptional expression of nearby neurodevelopmental genes. In addition, H3.3-K27M deposition at a subset of PRC2 target genes leads to increased PRC2 and PRC1 binding and augmented transcriptional repression that can be partially reversed by PRC2 inhibitors. Our work suggests that, rather than imposing de novo transcriptional circuits, H3.3-K27M drives tumorigenesis by locking initiating cells in their pre-existing, immature epigenomic state, via disruption of PRC2 and enhancer functions.
Assuntos
Elementos Facilitadores Genéticos , Histonas/metabolismo , Células-Tronco Neurais/fisiologia , Complexo Repressor Polycomb 2/genética , Rombencéfalo/citologia , Animais , Neoplasias Encefálicas/genética , Diferenciação Celular/genética , Linhagem Celular , Proteína Potenciadora do Homólogo 2 de Zeste/genética , Epigenoma , Regulação da Expressão Gênica no Desenvolvimento , Glioma/genética , Histonas/genética , Humanos , Lisina/metabolismo , Masculino , Camundongos Endogâmicos , Mutação , Células-Tronco Neurais/transplante , Oncogenes , Complexo Repressor Polycomb 2/antagonistas & inibidores , Complexo Repressor Polycomb 2/metabolismo , Regiões Promotoras Genéticas , Rombencéfalo/fisiologiaRESUMO
ATRX alterations occur at high frequency in neuroblastoma of adolescents and young adults. Particularly intriguing are the large N-terminal deletions of ATRX (Alpha Thalassemia/Mental Retardation, X-linked) that generate in-frame fusion (IFF) proteins devoid of key chromatin interaction domains, while retaining the SWI/SNF-like helicase region. We demonstrate that ATRX IFF proteins are redistributed from H3K9me3-enriched chromatin to promoters of active genes and identify REST as an ATRX IFF target whose activation promotes silencing of neuronal differentiation genes. We further show that ATRX IFF cells display sensitivity to EZH2 inhibitors, due to derepression of neurogenesis genes, including a subset of REST targets. Taken together, we demonstrate that ATRX structural alterations are not loss-of-function and put forward EZH2 inhibitors as a potential therapy for ATRX IFF neuroblastoma.