Weaver Syndrome-Associated EZH2 Protein Variants Show Impaired Histone Methyltransferase Function In Vitro

Polycomb repressive complex 2 is critical for mouse cortical glutamatergic neuron development

The Polycomb Repressive Complex 2 (PRC2) regulates corticogenesis, yet the consequences of mutations to this epigenetic modifier in the mature brain are poorly defined. Importantly, PRC2 core genes are haploinsufficient and causative of several human neurodevelopmental disorders. To address the role of PRC2 in mature cortical structure and function, we conditionally deleted the PRC2 gene Eed from the developing mouse dorsal telencephalon. Adult homozygotes displayed smaller forebrain structures. Single-nucleus transcriptomics revealed that glutamatergic neurons were particularly affected, exhibiting dysregulated gene expression profiles, accompanied by aberrations in neuronal morphology and connectivity. Remarkably, homozygous mice performed well on challenging cognitive tasks. In contrast, while heterozygous mice did not exhibit clear anatomical or behavioral differences, they displayed dysregulation of neuronal genes and altered neuronal morphology that was strikingly different from homozygous phenotypes. Collectively, these data reveal how alterations to PRC2 function shape the mature brain and reveal a dose-specific role for PRC2 in determining glutamatergic neuron identity.

Intrauterine growth in chromatinopathies: A long road for better understanding and for improving clinical management

BACKGROUND

Chromatinopathies are a heterogeneous group of genetic disorders caused by pathogenic variants in genes coding for chromatin state balance proteins. Remarkably, many of these syndromes present unbalanced postnatal growth, both under- and over-, although little has been described in the literature. Fetal growth measurements are common practice in pregnancy management and values within normal ranges indicate proper intrauterine growth progression; on the contrary, abnormalities in intrauterine fetal growth open the discussion of possible pathogenesis affecting growth even in the postnatal period.

METHODS

Among the numerous chromatinopathies, we have selected six of the most documented in the literature offering evidence about two fetal overgrowth (Sotos and Weaver syndrome) and four fetal undergrowth syndromes (Bohring Opitz, Cornelia de Lange, Floating-Harbor, and Meier Gorlin syndrome), describing their molecular characteristics, maternal biochemical results and early pregnancy findings, prenatal ultrasound findings, and postnatal characteristics.

RESULTS/CONCLUSION

To date, the scarce data in the literature on prenatal findings are few and inconclusive, even though these parameters may contribute to a more rapid and accurate diagnosis, calling for a better and more detailed description of pregnancy findings.

Fetal growth delay caused by loss of non-canonical imprinting is resolved late in pregnancy and culminates in offspring overgrowth

Germline epigenetic programming, including genomic imprinting, substantially influences offspring development. Polycomb Repressive Complex 2 (PRC2) plays an important role in Histone 3 Lysine 27 trimethylation (H3K27me3)-dependent imprinting, loss of which leads to growth and developmental changes in mouse offspring. In this study, we show that offspring from mouse oocytes lacking the PRC2 protein Embryonic Ectoderm Development (EED) were initially developmentally delayed, characterised by low blastocyst cell counts and substantial growth delay in mid-gestation embryos. This initial developmental delay was resolved as offspring underwent accelerated fetal development and growth in late gestation resulting in offspring that were similar stage and weight to controls at birth. The accelerated development and growth in offspring from Eed-null oocytes was associated with remodelling of the placenta, which involved an increase in fetal and maternal tissue size, conspicuous expansion of the glycogen-enriched cell population, and delayed parturition. Despite placental remodelling and accelerated offspring fetal growth and development, placental efficiency, and fetal blood glucose levels were low, and the fetal blood metabolome was unchanged. Moreover, while expression of the H3K27me3-imprinted gene and amino acid transporter Slc38a4 was increased, fetal blood levels of individual amino acids were similar to controls, indicating that placental amino acid transport was not enhanced. Genome-wide analyses identified extensive transcriptional dysregulation and DNA methylation changes in affected placentas, including a range of imprinted and non-imprinted genes. Together, while deletion of Eed in growing oocytes resulted in fetal growth and developmental delay and placental hyperplasia, our data indicate a remarkable capacity for offspring fetal growth to be normalised despite inefficient placental function and the loss of H3K27me3-dependent genomic imprinting.

Biallelic loss-of-function variants of EZH1 cause a novel developmental disorder with central precocious puberty

Pathogenic variants of polycomb repressive complex-2 (PRC2) subunits are associated with overgrowth syndromes and neurological diseases. EZH2 is a major component of PRC2 and mediates the methylation of H3K27 trimethylation (H3K27me3). Germline variants of EZH2 have been identified as a cause of Weaver syndrome (WS), an overgrowth/intellectual disability (OGID) syndrome characterized by overgrowth, macrocephaly, accelerated bone age, intellectual disability (ID), and characteristic facial features. Germline variants of SUZ12 and EED, other components of PRC2, have also been reported in the WS or Weaver-like syndrome. EZH1 is a homolog of EZH2 that interchangeably associates with SUZ12 and EED. Recently, pathogenic variants of EZH1 have been reported in individuals with dominant and recessive neurodevelopmental disorders. We herein present sisters with biallelic loss-of-function variants of EZH1. They showed developmental delay, ID, and central precocious puberty, but not the features of WS or other OGID syndromes.

Epigenetic regulators controlling osteogenic lineage commitment and bone formation

Bone formation and homeostasis are controlled by environmental factors and endocrine regulatory cues that initiate intracellular signaling pathways capable of modulating gene expression in the nucleus. Bone-related gene expression is controlled by nucleosome-based chromatin architecture that limits the accessibility of lineage-specific gene regulatory DNA sequences and sequence-specific transcription factors. From a developmental perspective, bone-specific gene expression must be suppressed during the early stages of embryogenesis to prevent the premature mineralization of skeletal elements during fetal growth in utero. Hence, bone formation is initially inhibited by gene suppressive epigenetic regulators, while other epigenetic regulators actively support osteoblast differentiation. Prominent epigenetic regulators that stimulate or attenuate osteogenesis include lysine methyl transferases (e.g., EZH2, SMYD2, SUV420H2), lysine deacetylases (e.g., HDAC1, HDAC3, HDAC4, HDAC7, SIRT1, SIRT3), arginine methyl transferases (e.g., PRMT1, PRMT4/CARM1, PRMT5), dioxygenases (e.g., TET2), bromodomain proteins (e.g., BRD2, BRD4) and chromodomain proteins (e.g., CBX1, CBX2, CBX5). This narrative review provides a broad overview of the covalent modifications of DNA and histone proteins that involve hundreds of enzymes that add, read, or delete these epigenetic modifications that are relevant for self-renewal and differentiation of mesenchymal stem cells, skeletal stem cells and osteoblasts during osteogenesis.

Polycomb-mediated silencing of miR-8 is required for maintenance of intestinal stemness in Drosophila melanogaster

Balancing maintenance of self-renewal and differentiation is a key property of adult stem cells. The epigenetic mechanisms controlling this balance remain largely unknown. Herein, we report that the Polycomb Repressive Complex 2 (PRC2) is required for maintenance of the intestinal stem cell (ISC) pool in the adult female Drosophila melanogaster. We show that loss of PRC2 activity in ISCs by RNAi-mediated knockdown or genetic ablation of the enzymatic subunit Enhancer of zeste, E(z), results in loss of stemness and precocious differentiation of enteroblasts to enterocytes. Mechanistically, we have identified the microRNA miR-8 as a critical target of E(z)/PRC2-mediated tri-methylation of histone H3 at Lys27 (H3K27me3) and uncovered a dynamic relationship between E(z), miR-8 and Notch signaling in controlling stemness versus differentiation of ISCs. Collectively, these findings uncover a hitherto unrecognized epigenetic layer in the regulation of stem cell specification that safeguards intestinal homeostasis.

Loss of Ezh2 in the medial ganglionic eminence alters interneuron fate, cell morphology and gene expression profiles

Introduction

Enhancer of zeste homolog 2 (Ezh2) is responsible for trimethylation of histone 3 at lysine 27 (H3K27me3), resulting in repression of gene expression. Here, we explore the role of Ezh2 in forebrain GABAergic interneuron development.

Methods

We removed Ezh2 in the MGE by generating Nkx2-1Cre;Ezh2 conditional knockout mice. We then characterized changes in MGE-derived interneuron fate and electrophysiological properties in juvenile mice, as well as alterations in gene expression, chromatin accessibility and histone modifications in the MGE.

Results

Loss of Ezh2 increases somatostatin-expressing (SST+) and decreases parvalbumin-expressing (PV+) interneurons in the forebrain. We observe fewer MGE-derived interneurons in the first postnatal week, indicating reduced interneuron production. Intrinsic electrophysiological properties in SST+ and PV+ interneurons are normal, but PV+ interneurons display increased axonal complexity in Ezh2 mutant mice. Single nuclei multiome analysis revealed differential gene expression patterns in the embryonic MGE that are predictive of these cell fate changes. Lastly, CUT&Tag analysis revealed that some genomic loci are particularly resistant or susceptible to shifts in H3K27me3 levels in the absence of Ezh2, indicating differential selectivity to epigenetic perturbation.

Discussion

Thus, loss of Ezh2 in the MGE alters interneuron fate, morphology, and gene expression and regulation. These findings have important implications for both normal development and potentially in disease etiologies.

Epigenetic Causes of Overgrowth Syndromes

Human overgrowth disorders are characterized by excessive prenatal and/or postnatal growth of various tissues. These disorders often present with tall stature, macrocephaly, and/or abdominal organomegaly and are sometimes associated with additional phenotypic abnormalities such as intellectual disability and increased cancer risk. As the genetic etiology of these disorders have been elucidated, a surprising pattern has emerged. Multiple monogenic overgrowth syndromes result from variants in epigenetic regulators: variants in histone methyltransferases NSD1 and EZH2 cause Sotos syndrome and Weaver syndrome, respectively, variants in DNA methyltransferase DNMT3A cause Tatton-Brown-Rahman syndrome, and variants in chromatin remodeler CHD8 cause an autism spectrum disorder with overgrowth. In addition, very recently, a variant in histone reader protein SPIN4 was identified in a new X-linked overgrowth disorder. In this review, we discuss the genetics of these overgrowth disorders and explore possible common underlying mechanisms by which epigenetic pathways regulate human body size.

A mouse model of Weaver syndrome displays overgrowth and excess osteogenesis reversible with KDM6A/6B inhibition

Weaver syndrome is a Mendelian disorder of the epigenetic machinery (MDEM) caused by germline pathogenic variants in EZH2, which encodes the predominant H3K27 methyltransferase and key enzymatic component of Polycomb repressive complex 2 (PRC2). Weaver syndrome is characterized by striking overgrowth and advanced bone age, intellectual disability, and distinctive facies. We generated a mouse model for the most common Weaver syndrome missense variant, EZH2 p.R684C. Ezh2R684C/R684C mouse embryonic fibroblasts (MEFs) showed global depletion of H3K27me3. Ezh2R684C/+ mice had abnormal bone parameters, indicative of skeletal overgrowth, and Ezh2R684C/+ osteoblasts showed increased osteogenic activity. RNA-Seq comparing osteoblasts differentiated from Ezh2R684C/+, and Ezh2+/+ BM-mesenchymal stem cells (BM-MSCs) indicated collective dysregulation of the BMP pathway and osteoblast differentiation. Inhibition of the opposing H3K27 demethylases KDM6A and KDM6B substantially reversed the excessive osteogenesis in Ezh2R684C/+ cells both at the transcriptional and phenotypic levels. This supports both the ideas that writers and erasers of histone marks exist in a fine balance to maintain epigenome state and that epigenetic modulating agents have therapeutic potential for the treatment of MDEMs.

A. Epigenetic regulation of craniofacial development and disease

BACKGROUND

The formation of the craniofacial complex relies on proper neural crest development. The gene regulatory networks (GRNs) and signaling pathways orchestrating this process have been extensively studied. These GRNs and signaling cascades are tightly regulated as alterations to any stage of neural crest development can lead to common congenital birth defects, including multiple syndromes affecting facial morphology as well as nonsyndromic facial defects, such as cleft lip with or without cleft palate. Epigenetic factors add a hierarchy to the regulation of transcriptional networks and influence the spatiotemporal activation or repression of specific gene regulatory cascades; however less is known about their exact mechanisms in controlling precise gene regulation.

AIMS

In this review, we discuss the role of epigenetic factors during neural crest development, specifically during craniofacial development and how compromised activities of these regulators contribute to congenital defects that affect the craniofacial complex.

H3K27me3 spreading organizes canonical PRC1 chromatin architecture to regulate developmental programs

Polycomb Repressive Complex 2 (PRC2)-mediated histone H3K27 tri-methylation (H3K27me3) recruits canonical PRC1 (cPRC1) to maintain heterochromatin. In early development, polycomb-regulated genes are connected through long-range 3D interactions which resolve upon differentiation. Here, we report that polycomb looping is controlled by H3K27me3 spreading and regulates target gene silencing and cell fate specification. Using glioma-derived H3 Lys-27-Met (H3K27M) mutations as tools to restrict H3K27me3 deposition, we show that H3K27me3 confinement concentrates the chromatin pool of cPRC1, resulting in heightened 3D interactions mirroring chromatin architecture of pluripotency, and stringent gene repression that maintains cells in progenitor states to facilitate tumor development. Conversely, H3K27me3 spread in pluripotent stem cells, following neural differentiation or loss of the H3K36 methyltransferase NSD1, dilutes cPRC1 concentration and dissolves polycomb loops. These results identify the regulatory principles and disease implications of polycomb looping and nominate histone modification-guided distribution of reader complexes as an important mechanism for nuclear compartment organization.

Highlights

The confinement of H3K27me3 at PRC2 nucleation sites without its spreading correlates with increased 3D chromatin interactions.The H3K27M oncohistone concentrates canonical PRC1 that anchors chromatin loop interactions in gliomas, silencing developmental programs.Stem and progenitor cells require factors promoting H3K27me3 confinement, including H3K36me2, to maintain cPRC1 loop architecture.The cPRC1-H3K27me3 interaction is a targetable driver of aberrant self-renewal in tumor cells.

Non-canonical functions of EZH2 in cancer

Mutations in chromatin modifying genes frequently occur in many kinds of cancer. Most mechanistic studies focus on their canonical functions, while therapeutic approaches target their enzymatic activity. Recent studies, however, demonstrate that non-canonical functions of chromatin modifiers may be equally important and therapeutically actionable in different types of cancer. One epigenetic regulator that demonstrates such a dual role in cancer is the histone methyltransferase EZH2. EZH2 is a core component of the polycomb repressive complex 2 (PRC2), which plays a crucial role in cell identity, differentiation, proliferation, stemness and plasticity. While much of the regulatory functions and oncogenic activity of EZH2 have been attributed to its canonical, enzymatic activity of methylating lysine 27 on histone 3 (H3K27me3), a repressive chromatin mark, recent studies suggest that non-canonical functions that are independent of H3K27me3 also contribute towards the oncogenic activity of EZH2. Contrary to PRC2's canonical repressive activity, mediated by H3K27me3, outside of the complex EZH2 can directly interact with transcription factors and oncogenes to activate gene expression. A more focused investigation into these non-canonical interactions of EZH2 and other epigenetic/chromatin regulators may uncover new and more effective therapeutic strategies. Here, we summarize major findings on the non-canonical functions of EZH2 and how they are related to different aspects of carcinogenesis.

A de novo missense variant in EZH1 associated with developmental delay exhibits functional deficits in Drosophila melanogaster

EZH1, a polycomb repressive complex-2 component, is involved in a myriad of cellular processes. EZH1 represses transcription of downstream target genes through histone 3 lysine27 (H3K27) trimethylation (H3K27me3). Genetic variants in histone modifiers have been associated with developmental disorders, while EZH1 has not yet been linked to any human disease. However, the paralog EZH2 is associated with Weaver syndrome. Here we report a previously undiagnosed individual with a novel neurodevelopmental phenotype identified to have a de novo missense variant in EZH1 through exome sequencing. The individual presented in infancy with neurodevelopmental delay and hypotonia and was later noted to have proximal muscle weakness. The variant, p.A678G, is in the SET domain, known for its methyltransferase activity, and an analogous somatic or germline mutation in EZH2 has been reported in patients with B-cell lymphoma or Weaver syndrome, respectively. Human EZH1/2 are homologous to fly Enhancer of zeste (E(z)), an essential gene in Drosophila, and the affected residue (p.A678 in humans, p.A691 in flies) is conserved. To further study this variant, we obtained null alleles and generated transgenic flies expressing wildtype [E(z)WT] and the variant [E(z)A691G]. When expressed ubiquitously the variant rescues null-lethality similar to the wildtype. Overexpression of E(z)WT induces homeotic patterning defects but notably the E(z)A691G variant leads to dramatically stronger morphological phenotypes. We also note a dramatic loss of H3K27me2 and a corresponding increase in H3K27me3 in flies expressing E(z)A691G, suggesting this acts as a gain-of-function allele. In conclusion, here we present a novel EZH1 de novo variant associated with a neurodevelopmental disorder. Furthermore, we found that this variant has a functional impact in Drosophila.

M, Katona G. Survivin prevents the polycomb repressor complex 2 from methylating histone 3 lysine 27

This study investigates the role of survivin in epigenetic control of gene transcription through interaction with the polycomb repressive complex 2 (PRC2). PRC2 is responsible for silencing gene expression by trimethylating lysine 27 on histone 3. We observed differential expression of PRC2 subunits in CD4+ T cells with varying levels of survivin expression, and ChIP-seq results indicated that survivin colocalizes with PRC2 along DNA. Inhibition of survivin resulted in a significant increase in H3K27 trimethylation, implying that survivin prevents PRC2 from functioning. Peptide microarray showed that survivin interacts with peptides from PRC2 subunits, and machine learning revealed that amino acid composition contains relevant information for predicting survivin interaction. NMR and BLI experiments supported the interaction of survivin with PRC2 subunit EZH2. Finally, protein-protein docking revealed that the survivin-EZH2 interaction interface overlaps with catalytic residues of EZH2, potentially inhibiting its H3K27 methylation activity. These findings suggest that survivin inhibits PRC2 function.

Novel mouse model of Weaver syndrome displays overgrowth and excess osteogenesis reversible with KDM6A/6B inhibition

Weaver syndrome is a Mendelian disorder of the epigenetic machinery (MDEM) caused by germline pathogenic variants in EZH2, which encodes the predominant H3K27 methyltransferase and key enzymatic component of Polycomb repressive complex 2 (PRC2). Weaver syndrome is characterized by striking overgrowth and advanced bone age, intellectual disability, and distinctive facies. We generated a mouse model for the most common Weaver syndrome missense variant, EZH2 p.R684C. Ezh2R684C/R684C mouse embryonic fibroblasts (MEFs) showed global depletion of H3K27me3. Ezh2R684C/+ mice had abnormal bone parameters indicative of skeletal overgrowth, and Ezh2R684C/+ osteoblasts showed increased osteogenic activity. RNA-seq comparing osteoblasts differentiated from Ezh2R684C/+ and Ezh2+/+ bone marrow mesenchymal stem cells (BM-MSCs) indicated collective dysregulation of the BMP pathway and osteoblast differentiation. Inhibition of the opposing H3K27 demethylases Kdm6a/6b substantially reversed the excessive osteogenesis in Ezh2R684C/+ cells both at the transcriptional and phenotypic levels. This supports both the ideas that writers and erasers of histone marks exist in a fine balance to maintain epigenome state, and that epigenetic modulating agents have therapeutic potential for the treatment of MDEMs.

Imagawa-Matsumoto syndrome: SUZ12-related overgrowth disorder

The SUZ12 gene encodes a subunit of polycomb repressive complex 2 (PRC2) that is essential for development by silencing the expression of multiple genes. Germline heterozygous variants in SUZ12 have been found in Imagawa-Matsumoto syndrome (IMMAS) characterized by overgrowth and multiple dysmorphic features. Similarly, both EZH2 and EED also encode a subunit of PRC2 each and their pathogenic variants cause Weaver syndrome and Cohen-Gibson syndrome, respectively. Clinical manifestations of these syndromes significantly overlap, although their different prevalence rates have recently been noted: generalized overgrowth, intellectual disability, scoliosis, and excessive loose skin appear to be less prevalent in IMMAS than in the other two syndromes. We could not determine any apparent genotype-phenotype correlation in IMMAS. The phenotype of neurofibromatosis type 1 arising from NF1 deletion was also shown to be modified by the deletion of SUZ12, 560 kb away. This review deepens our understanding of the clinical and genetic characteristics of IMMAS together with other overgrowth syndromes related to PRC2. We also report on a novel IMMAS patient carrying a splicing variant (c.1023+1G>C) in SUZ12. This patient had a milder phenotype than other previously reported IMMAS cases, with no macrocephaly or overgrowth phenotypes, highlighting the clinical variation in IMMAS.

Transient Polycomb activity represses developmental genes in growing oocytes

BACKGROUND

Non-genetic disease inheritance and offspring phenotype are substantially influenced by germline epigenetic programming, including genomic imprinting. Loss of Polycomb Repressive Complex 2 (PRC2) function in oocytes causes non-genetically inherited effects on offspring, including embryonic growth restriction followed by post-natal offspring overgrowth. While PRC2-dependent non-canonical imprinting is likely to contribute, less is known about germline epigenetic programming of non-imprinted genes during oocyte growth. In addition, de novo germline mutations in genes encoding PRC2 lead to overgrowth syndromes in human patients, but the extent to which PRC2 activity is conserved in human oocytes is poorly understood.

RESULTS

In this study, we identify a discrete period of early oocyte growth during which PRC2 is expressed in mouse growing oocytes. Deletion of Eed during this window led to the de-repression of 343 genes. A high proportion of these were developmental regulators, and the vast majority were not imprinted genes. Many of the de-repressed genes were also marked by the PRC2-dependent epigenetic modification histone 3 lysine 27 trimethylation (H3K27me3) in primary-secondary mouse oocytes, at a time concurrent with PRC2 expression. In addition, we found H3K27me3 was also enriched on many of these genes by the germinal vesicle (GV) stage in human oocytes, strongly indicating that this PRC2 function is conserved in the human germline. However, while the 343 genes were de-repressed in mouse oocytes lacking EED, they were not de-repressed in pre-implantation embryos and lost H3K27me3 during pre-implantation development. This implies that H3K27me3 is a transient feature that represses a wide range of genes in oocytes.

CONCLUSIONS

Together, these data indicate that EED has spatially and temporally distinct functions in the female germline to repress a wide range of developmentally important genes and that this activity is conserved in the mouse and human germlines.

Germline Abnormalities in DNA Methylation and Histone Modification and Associated Cancer Risk

PURPOSE OF REVIEW

Somatic mutations in DNA methyltransferases and other DNA methylation associated genes have been found in a wide variety of cancers. Germline mutations in these genes have been associated with several rare hereditary disorders. Among the described germline/congenital disorders, neurological dysfunction and/or growth abnormalities appear to be a common phenotype. Here, we outline known germline abnormalities and examine the cancer risks associated with these mutations.

RECENT FINDINGS

The increased use and availability of sequencing techniques in the clinical setting has expanded the identification of germline abnormalities involving DNA methylation machinery. This has provided additional cases to study these rare hereditary disorders and their predisposition to cancer. Studying these syndromes may offer an opportunity to better understand the contribution of these genes in cancer development.

PRC2, Chromatin Regulation, and Human Disease: Insights From Molecular Structure and Function

Polycomb repressive complex 2 (PRC2) is a multisubunit histone-modifying enzyme complex that mediates methylation of histone H3 lysine 27 (H3K27). Trimethylated H3K27 (H3K27me3) is an epigenetic hallmark of gene silencing. PRC2 plays a crucial role in a plethora of fundamental biological processes, and PRC2 dysregulation has been repeatedly implicated in cancers and developmental disorders. Here, we review the current knowledge on mechanisms of cellular regulation of PRC2 function, particularly regarding H3K27 methylation and chromatin targeting. PRC2-related disease mechanisms are also discussed. The mode of action of PRC2 in gene regulation is summarized, which includes competition between H3K27 methylation and acetylation, crosstalk with transcription machinery, and formation of high-order chromatin structure. Recent progress in the structural biology of PRC2 is highlighted from the aspects of complex assembly, enzyme catalysis, and chromatin recruitment, which together provide valuable insights into PRC2 function in close-to-atomic detail. Future studies on the molecular function and structure of PRC2 in the context of native chromatin and in the presence of other regulators like RNAs will continue to deepen our understanding of the stability and plasticity of developmental transcriptional programs broadly impacted by PRC2.

Histological and immunohistochemical features and genetic alterations in the malignant progression of giant cell tumor of bone: a possible association with TP53 mutation and loss of H3K27 trimethylation

In rare cases, giant cell tumor of bone (GCTB) can undergo primary or secondary malignant transformation to malignant giant cell tumor of bone (MGCTB), but the details of the molecular alterations are still unclear. The present study aimed to elucidate the clinicopathologic and molecular features of MGCTBs based on immunohistochemistry, fluorescence in situ hybridization (FISH) and next generation sequencing (NGS) of nine MGCTBs (five primary and four secondary). Seven (78%) of 9 MGCTBs were immunohistochemically positive for H3.3 G34W. In two (22%) patients, although GCTB components were focally or diffusely positive for H3.3 G34W, their malignant components were entirely negative for H3.3 G34W, which was associated with heterozygous loss of H3F3A by FISH. NGS on four MGCTBs revealed pathogenic mutations in TP53 (n = 3), EZH2 (n = 1) and several other genes. Immunohistochemical analysis of the nine MGCTBs confirmed the p53 nuclear accumulation (n = 5) and loss of H3K27me3 expression (n = 3) and showed that they were mutually exclusive. In addition, four (80%) of five cases of pleomorphic or epithelioid cell-predominant MGCTBs were positive for p53, while three (75%) of four cases of spindle cell-predominant MGCTBs were negative for trimethylation at lysine 27 of histone 3 (H3K27me3). The results suggested that p53 alteration and dysfunction of histone methylation as evidenced by H3K27me3 loss may play an important role in the malignant progression of GCTB, and might contribute to the phenotype-genotype correlation in MGCTB. The combined histologic, immunohistochemical and molecular information may be helpful in part for the diagnosis of challenging cases.

Intellectual disability-associated disruption of O-GlcNAc cycling impairs habituation learning in Drosophila

O-GlcNAcylation is a reversible co-/post-translational modification involved in a multitude of cellular processes. The addition and removal of the O-GlcNAc modification is controlled by two conserved enzymes, O-GlcNAc transferase (OGT) and O-GlcNAc hydrolase (OGA). Mutations in OGT have recently been discovered to cause a novel Congenital Disorder of Glycosylation (OGT-CDG) that is characterized by intellectual disability. The mechanisms by which OGT-CDG mutations affect cognition remain unclear. We manipulated O-GlcNAc transferase and O-GlcNAc hydrolase activity in Drosophila and demonstrate an important role of O-GlcNAcylation in habituation learning and synaptic development at the larval neuromuscular junction. Introduction of patient-specific missense mutations into Drosophila O-GlcNAc transferase using CRISPR/Cas9 gene editing leads to deficits in locomotor function and habituation learning. The habituation deficit can be corrected by blocking O-GlcNAc hydrolysis, indicating that OGT-CDG mutations affect cognition-relevant habituation via reduced protein O-GlcNAcylation. This study establishes a critical role for O-GlcNAc cycling and disrupted O-GlcNAc transferase activity in cognitive dysfunction, and suggests that blocking O-GlcNAc hydrolysis is a potential strategy to treat OGT-CDG.

Polycomb-mediated gene regulation in human brain development and neurodevelopmental disorders

The neocortex is considered the seat of higher cognitive function in humans. It develops from a sheet of neural progenitor cells, most of which eventually give rise to neurons. This process of cell fate determination is controlled by precise temporal and spatial gene expression patterns that in turn are affected by epigenetic mechanisms including Polycomb group (PcG) regulation. PcG proteins assemble in multiprotein complexes and catalyze repressive posttranslational histone modifications. Their association with neurodevelopmental disease and various types of cancer of the central nervous system, as well as observations in mouse models, has implicated these epigenetic modifiers in controlling various stages of cortex development. The precise mechanisms conveying PcG-associated transcriptional repression remain incompletely understood and are an active field of research. PcG activity appears to be highly context-specific, raising the question of species-specific differences in the regulation of neural stem and progenitor regulation. In this review, we will discuss our growing understanding of how PcG regulation affects human cortex development, based on studies in murine model systems, but focusing mostly on findings obtained from examining impaired PcG activity in the context of human neurodevelopmental disorders and cancer. Furthermore, we will highlight relevant experimental approaches for functional investigations of PcG regulation in human cortex development.

Interplay between chromatin marks in development and disease

DNA methylation (DNAme) and histone post-translational modifications (PTMs) have important roles in transcriptional regulation. Although many reports have characterized the functions of such chromatin marks in isolation, recent genome-wide studies reveal surprisingly complex interactions between them. Here, we focus on the interplay between DNAme and methylation of specific lysine residues on the histone H3 tail. We describe the impact of genetic perturbation of the relevant methyltransferases in the mouse on the landscape of chromatin marks as well as the transcriptome. In addition, we discuss the specific neurodevelopmental growth syndromes and cancers resulting from pathogenic mutations in the human orthologues of these genes. Integrating these observations underscores the fundamental importance of crosstalk between DNA and histone H3 methylation in development and disease.

Reprogramming of the epigenome in neurodevelopmental disorders

The etiology of neurodevelopmental disorders (NDDs) remains a challenge for researchers. Human brain development is tightly regulated and sensitive to cellular alterations caused by endogenous or exogenous factors. Intriguingly, the surge of clinical sequencing studies has revealed that many of these disorders are monogenic and monoallelic. Notably, chromatin regulation has emerged as highly dysregulated in NDDs, with many syndromes demonstrating phenotypic overlap, such as intellectual disabilities, with one another. Here we discuss epigenetic writers, erasers, readers, remodelers, and even histones mutated in NDD patients, predicted to affect gene regulation. Moreover, this review focuses on disorders associated with mutations in enzymes involved in histone acetylation and methylation, and it highlights syndromes involving chromatin remodeling complexes. Finally, we explore recently discovered histone germline mutations and their pathogenic outcome on neurological function. Epigenetic regulators are mutated at every level of chromatin organization. Throughout this review, we discuss mechanistic investigations, as well as various animal and iPSC models of these disorders and their usefulness in determining pathomechanism and potential therapeutics. Understanding the mechanism of these mutations will illuminate common pathways between disorders. Ultimately, classifying these disorders based on their effects on the epigenome will not only aid in prognosis in patients but will aid in understanding the role of epigenetic machinery throughout neurodevelopment.

Proximal variants in CCND2 associated with microcephaly, short stature, and developmental delay: A case series and review of inverse brain growth phenotypes

Cyclin D2 (CCND2) is a critical cell cycle regulator and key member of the cyclin D2-CDK4 (DC) complex. De novo variants of CCND2 clustering in the distal part of the protein have been identified as pathogenic causes of brain overgrowth (megalencephaly, MEG) and severe cortical malformations in children including the megalencephaly-polymicrogyria-polydactyly-hydrocephalus (MPPH) syndrome. Megalencephaly-associated CCND2 variants are localized to the terminal exon and result in accumulation of degradation-resistant protein. We identified five individuals from three unrelated families with novel variants in the proximal region of CCND2 associated with microcephaly, mildly simplified cortical gyral pattern, symmetric short stature, and mild developmental delay. Identified variants include de novo frameshift variants and a dominantly inherited stop-gain variant segregating with the phenotype. This is the first reported association between proximal CCND2 variants and microcephaly, to our knowledge. This series expands the phenotypic spectrum of CCND2-related disorders and suggests that distinct classes of CCND2 variants are associated with reciprocal effects on human brain growth (microcephaly and megalencephaly due to possible loss or gain of protein function, respectively), adding to the growing paradigm of inverse phenotypes due to dysregulation of key brain growth genes.

Telomerase Dysfunction in the Tumorigenesis of Genetic Disorders

Telomeres are nucleoprotein complexes present at the ends of chromosome to maintain its integrity. Telomere length is maintained by an enzyme called "telomerase". Thus, telomerase activity and telomere length are crucial for the initiation of cancer and tumors survival. Also, oxidative stress will cause DNA, protein, and/or lipid damage, which end with changes in chromosome instability, genetic mutation, and may affect cell growth and lead to cancer. Some genetic diseases such as chromosomal instability syndrome, overgrowth syndrome, and neurofibromatosis make the patients at higher risk for developing different types of cancers. Therefore, we aimed to estimate telomerase activity and oxidative stress in these patients. Blood samples were collected from 31 patients (10 with neurofibromatosis, 11 with chromosomal breakage, and 10 with overgrowth syndrome) and 12 healthy subjects. Blood hTERT mRNA was detected by real time quantitative reverse-transcription PCR (RT-qPCR). All patients were subjected to chromosomal examination and chromosome breakage study using diepoxybutane method. Moreover, serum glutathione (GSH), glutathione-s-transferase (GST) activity and nitric oxide (NO) levels were measured among the control and patients groups. Receiver operating characteristic (ROC) curve was drawn to evaluate the efficiency of telomerase activity as a biomarker for the prediction of cancer occurrence. The relative telomerase activity in neurofibromatosis patients was significantly higher than controls (P = 0.014), while it was non-significantly higher in chromosomal breakage and overgrowth patients (P = 0.424 and 0.129, respectively). NO levels in neurofibromatosis, chromosomal breakage and overgrowth patients significantly increased with respect to control (P = 0.021, 0.002, 0.050, respectively). GSH levels were non-significantly lower in neurofibromatosis and chromosomal breakage patients in comparison with the control group, while it remained unchanged in overgrowth patients. The GST activity was significantly upregulated in neurofibromatosis, chromosomal breakage and overgrowth groups in comparison with the control group (P = 0.001, 0.009, and 0.025, respectively). Chromosomal examination revealed normal karyotype in all four chromosomal breakage patients with positive diepoxybutane test. The results of the present study revealed altered telomerase activity and oxidative stress in the studied genetic disorders. More research studies with a larger number of patients are required to confirm whether this alteration is related to cancer occurrence risk or not.

The interplay between DNA and histone methylation: molecular mechanisms and disease implications

Methylation of cytosine in CpG dinucleotides and histone lysine and arginine residues is a chromatin modification that critically contributes to the regulation of genome integrity, replication, and accessibility. A strong correlation exists between the genome-wide distribution of DNA and histone methylation, suggesting an intimate relationship between these epigenetic marks. Indeed, accumulating literature reveals complex mechanisms underlying the molecular crosstalk between DNA and histone methylation. These in vitro and in vivo discoveries are further supported by the finding that genes encoding DNA- and histone-modifying enzymes are often mutated in overlapping human diseases. Here, we summarize recent advances in understanding how DNA and histone methylation cooperate to maintain the cellular epigenomic landscape. We will also discuss the potential implication of these insights for understanding the etiology of, and developing biomarkers and therapies for, human congenital disorders and cancers that are driven by chromatin abnormalities.

A Structural Perspective on Gene Repression by Polycomb Repressive Complex 2

Polycomb Repressive Complex 2 (PRC2) is a major repressive chromatin complex formed by the Polycomb Group (PcG) proteins. PRC2 mediates trimethylation of histone H3 lysine 27 (H3K27me3), a hallmark of gene silencing. PRC2 is a key regulator of development, impacting many fundamental biological processes, like stem cell differentiation in mammals and vernalization in plants. Misregulation of PRC2 function is linked to a variety of human cancers and developmental disorders. In correlation with its diverse roles in development, PRC2 displays a high degree of compositional complexity and plasticity. Structural biology research over the past decade has shed light on the molecular mechanisms of the assembly, catalysis, allosteric activation, autoinhibition, chemical inhibition, dimerization and chromatin targeting of various developmentally regulated PRC2 complexes. In addition to these aspects, structure-function analysis is also discussed in connection with disease data in this chapter.

Epigenetic principles underlying epileptogenesis and epilepsy syndromes

Epilepsy is a network disorder driven by fundamental changes in the function of the cells which compose these networks. Driving this aberrant cellular function are large scale changes in gene expression and gene expression regulation. Recent studies have revealed rapid and persistent changes in epigenetic control of gene expression as a critical regulator of the epileptic transcriptome. Epigenetic-mediated gene output regulates many aspects of cellular physiology including neuronal structure, neurotransmitter assembly and abundance, protein abundance of ion channels and other critical neuronal processes. Thus, understanding the contribution of epigenetic-mediated gene regulation could illuminate novel regulatory mechanisms which may form the basis of novel therapeutic approaches to treat epilepsy. In this review we discuss the effects of epileptogenic brain insults on epigenetic regulation of gene expression, recent efforts to target epigenetic processes to block epileptogenesis and the prospects of an epigenetic-based therapy for epilepsy, and finally we discuss technological advancements which have facilitated the interrogation of the epigenome.

Reciprocal skeletal phenotypes of PRC2-related overgrowth and Rubinstein-Taybi syndromes: potential role of H3K27 modifications

Within histone H3, lysine 27 (H3K27) is one of the residues that functions as a molecular switch, by virtue of being subject to mutually exclusive post-translational modifications that have reciprocal effects on gene expression. Whereas acetylation of H3K27 is associated with transcriptional activation, methylation at this residue causes transcriptional silencing; these two modifications are mutually exclusive. Establishment of these epigenetic marks is important in defining cellular identity and for maintaining normal cell function, as evidenced by rare genetic disorders of epigenetic writers involved in H3K27 post-translational modification. Polycomb repressive complex (PRC2)-related overgrowth and Rubinstein-Taybi syndrome (RSTS) are respectively associated with impaired H3K27 methylation and acetylation. Whereas these syndromes share commonalities like intellectual disability and susceptibility to cancers, they are generally divergent in their skeletal growth phenotypes, potentially through dysregulation of their opposing H3K27 writer functions. In this review, we discuss the requirement of H3K27 modifications for successful embryogenesis, highlighting data from relevant mouse knockout studies. Although such gene ablation studies are integral for defining fundamental biological roles of methyl- and acetyltransferase function in vivo, studies of partial loss-of-function models are likely to yield more meaningful translational insight into progression of PRC2-related overgrowth or RSTS. Thus, modeling of rare human PRC2-related overgrowth and RSTS variants in mice is needed to fully understand the causative role of aberrant H3K27 modification in the pathophysiology of these syndromes.

A systems approach identifies Enhancer of Zeste Homolog 2 (EZH2) as a protective factor in epilepsy

Complex neurological conditions can give rise to large scale transcriptomic changes that drive disease progression. It is likely that alterations in one or a few transcription factors or cofactors underlie these transcriptomic alterations. Identifying the driving transcription factors/cofactors is a non-trivial problem and a limiting step in the understanding of neurological disorders. Epilepsy has a prevalence of 1% and is the fourth most common neurological disorder. While a number of anti-seizure drugs exist to treat seizures symptomatically, none is curative or preventive. This reflects a lack of understanding of disease progression. We used a novel systems approach to mine transcriptome profiles of rodent and human epileptic brain samples to identify regulators of transcriptional networks in the epileptic brain. We find that Enhancer of Zeste Homolog 2 (EZH2) regulates differentially expressed genes in epilepsy across multiple rodent models of acquired epilepsy. EZH2 undergoes a prolonged upregulation in the epileptic brain. A transient inhibition of EZH2 immediately after status epilepticus (SE) robustly increases spontaneous seizure burden weeks later. This suggests that EZH2 upregulation is a protective. These findings are the first to characterize a role for EZH2 in opposing epileptogenesis and debut a bioinformatic approach to identify nuclear drivers of complex transcriptional changes in disease.

Rare SUZ12 variants commonly cause an overgrowth phenotype

The Polycomb repressive complex 2 is an epigenetic writer and recruiter with a role in transcriptional silencing. Constitutional pathogenic variants in its component proteins have been found to cause two established overgrowth syndromes: Weaver syndrome (EZH2-related overgrowth) and Cohen-Gibson syndrome (EED-related overgrowth). Imagawa et al. (2017) initially reported a singleton female with a Weaver-like phenotype with a rare coding SUZ12 variant-the same group subsequently reported two additional affected patients. Here we describe a further 10 patients (from nine families) with rare heterozygous SUZ12 variants who present with a Weaver-like phenotype. We report four frameshift, two missense, one nonsense, and two splice site variants. The affected patients demonstrate variable pre- and postnatal overgrowth, dysmorphic features, musculoskeletal abnormalities and developmental delay/intellectual disability. Some patients have genitourinary and structural brain abnormalities, and there may be an association with respiratory issues. The addition of these 10 patients makes a compelling argument that rare pathogenic SUZ12 variants frequently cause overgrowth, physical abnormalities, and abnormal neurodevelopmental outcomes in the heterozygous state. Pathogenic SUZ12 variants may be de novo or inherited, and are sometimes inherited from a mildly-affected parent. Larger samples sizes will be needed to elucidate whether one or more clinically-recognizable syndromes emerge from different variant subtypes.

PRC2-complex related dysfunction in overgrowth syndromes: A review of EZH2, EED, and SUZ12 and their syndromic phenotypes

The EZH2, EED, and SUZ12 genes encode proteins that comprise core components of the polycomb repressive complex 2 (PRC2), an epigenetic "writer" with H3K27 methyltransferase activity, catalyzing the addition of up to three methyl groups on histone 3 at lysine residue 27 (H3K27). Partial loss-of-function variants in genes encoding the EZH2 and EED subunits of the complex lead to overgrowth, macrocephaly, advanced bone age, variable intellectual disability, and distinctive facial features. EZH2-associated overgrowth, caused by constitutional heterozygous mutations within Enhancer of Zeste homologue 2 (EZH2), has a phenotypic spectrum ranging from tall stature without obvious intellectual disability or dysmorphic features to classical Weaver syndrome (OMIM #277590). EED-associated overgrowth (Cohen-Gibson syndrome; OMIM #617561) is caused by germline heterozygous mutations in Embryonic Ectoderm Development (EED), and manifests overgrowth and intellectual disability (OGID), along with other features similar to Weaver syndrome. Most recently, rare coding variants in SUZ12 have also been described that present with clinical characteristics similar to the previous two syndromes. Here we review the PRC2 complex and clinical syndromes of OGID associated with core components EZH2, EED, and SUZ12.

PRC2 functions in development and congenital disorders

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.

A genetic-phenotypic classification for syndromic micrognathia

Micrognathia is a common craniofacial deformity which represents hypoplastic development of the mandible, accompanied by retrognathia and consequent airway problems. Usually, micrognathia is accompanied by multiple systematic defects, known as syndromic micrognathia, and is in close association with genetic factors. Now, large quantities of pathogenic genes of syndromic micrognathia have been revealed. However, how these different pathogenic genes could lead to similar phenotypes, and whether there are some common characteristics among these pathogenic genes are still unknown. In this study, we proposed a genetic-phenotypic classification of syndromic micrognathia based on pathogenic genes information obtained from Phenolyzer, DAVID, OMIM, and PubMed database. Pathogenic genes of syndromic micrognathia could be divided into four groups based on gene function, including cellular processes and structures, cell metabolism, cartilage and bone development, and neuromuscular function. In addition, these four groups exhibited various clinical characteristics, and the affected systems, such as central nervous system, skeletal system, cardiovascular system, oral and dental system, respiratory system and muscle, were different in these four groups. This classification could provide meaningful insights into the pathogenesis of syndromic micrognathia, and offer some clues for understanding the molecular mechanism, as well as guiding precise clinical diagnosis and treatment for syndromic micrognathia.

Epigenetics as a New Frontier in Orthopedic Regenerative Medicine and Oncology

Skeletal regenerative medicine aims to repair or regenerate skeletal tissues using pharmacotherapies, cell-based treatments, and/or surgical interventions. The field is guided by biological principles active during development, wound healing, aging, and carcinogenesis. Skeletal development and tissue maintenance in adults represent highly intricate biological processes that require continuous adjustments in the expression of cell type-specific genes that generate, remodel, and repair the skeletal extracellular matrix. Errors in these processes can facilitate musculoskeletal disease including cancers or injury. The fundamental molecular mechanisms by which cell type-specific patterns in gene expression are established and retained during successive mitotic divisions require epigenetic control, which we review here. We focus on epigenetic regulatory proteins that control the mammalian epigenome at the level of chromatin with emphasis on proteins that are amenable to drug intervention to mitigate skeletal tissue degeneration (e.g., osteoarthritis and osteoporosis). We highlight recent findings on a number of druggable epigenetic regulators, including DNA methyltransferases (e.g., DNMT1, DNMT3A, and DNMT3B) and hydroxylases (e.g., TET1, TET2, and TET3), histone methyltransferases (e.g., EZH1, EZH2, and DOT1L) as well as histone deacetylases (e.g., HDAC3, HDAC4, and HDAC7) and histone acetyl readers (e.g., BRD4) in relation to the development of bone or cartilage regenerative drug therapies. We also review how histone mutations lead to epigenomic catastrophe and cause musculoskeletal tumors. The combined body of molecular and genetic studies focusing on epigenetic regulators indicates that these proteins are critical for normal skeletogenesis and viable candidate drug targets for short-term local pharmacological strategies to mitigate musculoskeletal tissue degeneration. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:1465-1474, 2019.

Brain expansion promoted by polycomb-mediated anterior enhancement of a neural stem cell proliferation program

During central nervous system (CNS) development, genetic programs establish neural stem cells and drive both stem and daughter cell proliferation. However, the prominent anterior expansion of the CNS implies anterior–posterior (A–P) modulation of these programs. In Drosophila, a set of neural stem cell factors acts along the entire A–P axis to establish neural stem cells. Brain expansion results from enhanced stem and daughter cell proliferation, promoted by a Polycomb Group (PcG)->Homeobox (Hox) homeotic network. But how does PcG->Hox modulate neural-stem-cell–factor activity along the A–P axis? We find that the PcG->Hox network creates an A–P expression gradient of neural stem cell factors, thereby driving a gradient of proliferation. PcG mutants can be rescued by misexpression of the neural stem cell factors or by mutation of one single Hox gene. Hence, brain expansion results from anterior enhancement of core neural-stem-cell–factor expression, mediated by PcG repression of brain Hox expression.

A study in fruit flies shows that the anterior expansion of the central nervous system, to form the brain, is driven by Polycomb-mediated repression of Hox genes, resulting in anterior enhancement of a neural stem cell program.

The central nervous system displays a pronounced anterior expansion that forms the brain. In the fruit fly Drosophila melanogaster, this expansion is driven by enhanced anterior cell proliferation. Recent studies reveal that cell proliferation in the brain is promoted by the Polycomb Group Complex, a key epigenetic complex. During development of the central nervous system, the Polycomb Group Complex acts to exclude Hox homeotic gene expression from the brain, thereby rendering the brain a Hox-free zone. Hox genes act in an antiproliferative manner, which explains the hyperproliferation observed in the brain, as well as the gradient of proliferation along the anterior–posterior axis of the central nervous system. Here, we find that Hox genes act by repressing a common neural stem cell proliferation program in more posterior regions, resulting in an anterior–posterior gradient of “stemness.” Hence, elevated anterior proliferation is promoted by the Polycomb Group Complex acting to keep the brain free of negative Hox input, thereby ensuring elevated expression of neural stem cell factors in the brain. Strikingly, mutants of the Polycomb Group Complex can be rescued by mutation of one single Hox gene, demonstrating that the primary role of the Polycomb Group Complex is indeed Hox repression. This study advances our understanding of how neural stem cell programs operate at different axial levels of the central nervous system and may have implications also for stem cell and organoid biology.

EED and EZH2 constitutive variants: A study to expand the Cohen-Gibson syndrome phenotype and contrast it with Weaver syndrome

Overgrowth-intellectual disability (OGID) syndromes are characterized by increased growth (height and/or head circumference ≥+2 SD) in association with an intellectual disability. Constitutive EED variants have previously been reported in five individuals with an OGID syndrome, eponymously designated Cohen-Gibson syndrome and resembling Weaver syndrome. Here, we report three additional individuals with constitutive EED variants, identified through exome sequencing of an OGID patient series. We compare the EED phenotype with that of Weaver syndrome (56 individuals), caused by constitutive EZH2 variants. We conclude that while there is considerable overlap between the EED and EZH2 phenotypes with both characteristically associated with increased growth and an intellectual disability, individuals with EED variants more frequently have cardiac problems and cervical spine abnormalities, boys have cryptorchidism and the facial gestalts can usually be distinguished.

Novel EZH2 mutation in a patient with secondary B-cell acute lymphocytic leukemia after deletion 5q myelodysplastic syndrome treated with lenalidomide: A case report

RATIONALE

The gene deletion (5)(q22q35) is reported in 10-20% of myelodysplastic syndrome (MDS) cases and is associated with response to lenalidomide and favorable prognosis. The authors report here a clinical case of MDS transformation to B-cell acute lymphocytic leukemia (B-ALL) with an associated accrual of an additional mutation following treatment with lenalidomide.

PATIENT CONCERNS

A 69-year-old man presented with progressive anemia, normal white blood cell count, and thrombocytopenia consistent with MDS. He was administered lenalidomide for 27 months, then developed acute B-cell lymphocytic leukemia and acquired a previously unreported mutation in the gene enhancer of zeste homolog 2 (EZH2).

DIAGNOSES

After 27 months of therapy with lenalidomide, a surveillance bone marrow aspiration (BMA) revealed 90% cellularity with persistent multilineage dysplasia and a population of blasts comprising 54% of all bone marrow elements by morphology, consistent with B-ALL, even though the patient was asymptomatic. Conventional karyotype showed no signs of del(5)(q22q35) MDS, however bone marrow next-generation sequencing (NGS) demonstrated the accrual of a nonsense mutation (c.211del pL71*) in exon 3 of EZH2. A confirmatory BMA yielded 70% blasts and clinical features indicative of B-ALL.

INTERVENTIONS

Mini-hyper-CVD (cyclophosphamide and dexamethasone at 50% dose reduction, no anthracycline, methotrexate at 75% dose reduction, cytarabine at 0.5 g/m × 4 doses) was administered for 21 days.

OUTCOMES

A follow-up BMA was performed 2 months after mini-hyper-CVD therapy, showing dysplastic features with 25% ring sideroblasts, but no evidence of B-ALL. The patient is currently receiving monthly-low dose decitabine, ofatumumab, and dexamethasone, and is transfusion independent and asymptomatic after 7 cycles.

LESSONS

The present study shows an extremely rare progression of del(5)(q22q35) MDS to B-ALL with accompanying NGS data and a newly described acquisition of an EZH2 frameshift mutation. This case highlights the importance of NGS as a diagnostic and surveillance tool for MDS.

Pseudoacromegaly

Individuals with acromegaloid physical appearance or tall stature may be referred to endocrinologists to exclude growth hormone (GH) excess. While some of these subjects could be healthy individuals with normal variants of growth or physical traits, others will have acromegaly or pituitary gigantism, which are, in general, straightforward diagnoses upon assessment of the GH/IGF-1 axis. However, some patients with physical features resembling acromegaly - usually affecting the face and extremities -, or gigantism - accelerated growth/tall stature - will have no abnormalities in the GH axis. This scenario is termed pseudoacromegaly, and its correct diagnosis can be challenging due to the rarity and variability of these conditions, as well as due to significant overlap in their characteristics. In this review we aim to provide a comprehensive overview of pseudoacromegaly conditions, highlighting their similarities and differences with acromegaly and pituitary gigantism, to aid physicians with the diagnosis of patients with pseudoacromegaly.

The Dynamic Partnership of Polycomb and Trithorax in Brain Development and Diseases

Epigenetic mechanisms, including DNA and histone modifications, are pivotal for normal brain development and functions by modulating spatial and temporal gene expression. Dysregulation of the epigenetic machinery can serve as a causal role in numerous brain disorders. Proper mammalian brain development and functions depend on the precise expression of neuronal-specific genes, transcription factors and epigenetic modifications. Antagonistic polycomb and trithorax proteins form multimeric complexes and play important roles in these processes by epigenetically controlling gene repression or activation through various molecular mechanisms. Aberrant expression or disruption of either protein group can contribute to neurodegenerative diseases. This review focus on the current progress of Polycomb and Trithorax complexes in brain development and disease, and provides a future outlook of the field.

Loss of histone methyltransferase Ezh2 stimulates an osteogenic transcriptional program in chondrocytes but does not affect cartilage development

Ezh2 is a histone methyltransferase that suppresses osteoblast maturation and skeletal development. We evaluated the role of Ezh2 in chondrocyte lineage differentiation and endochondral ossification. Ezh2 was genetically inactivated in the mesenchymal, osteoblastic, and chondrocytic lineages in mice using the Prrx1-Cre, Osx1-Cre, and Col2a1-Cre drivers, respectively. WT and conditional knockout mice were phenotypically assessed by gross morphology, histology, and micro-CT imaging. Ezh2-deficient chondrocytes in micromass culture models were evaluated using RNA-Seq, histologic evaluation, and Western blotting. Aged mice with Ezh2 deficiency were also evaluated for premature development of osteoarthritis using radiographic analysis. Ezh2 deficiency in murine chondrocytes reduced bone density at 4 weeks of age but caused no other gross developmental effects. Knockdown of Ezh2 in chondrocyte micromass cultures resulted in a global reduction in trimethylation of histone 3 lysine 27 (H3K27me3) and altered differentiation in vitro RNA-Seq analysis revealed enrichment of an osteogenic gene expression profile in Ezh2-deficient chondrocytes. Joint development proceeded normally in the absence of Ezh2 in chondrocytes without inducing excessive hypertrophy or premature osteoarthritis in vivo In summary, loss of Ezh2 reduced H3K27me3 levels, increased the expression of osteogenic genes in chondrocytes, and resulted in a transient post-natal bone phenotype. Remarkably, Ezh2 activity is dispensable for normal chondrocyte maturation and endochondral ossification in vivo, even though it appears to have a critical role during early stages of mesenchymal lineage commitment.

RARA and RARG gene downregulation associated with EZH2 mutation in acute promyelocytic-like morphology leukemia

Most acute promyelocytic leukemia (APL) patients express PML-RARA fusion; in rare cases, RARA is rearranged with partner genes other than PML. To date, only 2 patients presenting features similar to APL showing the RARG gene rearrangement have been described. We report an acute myeloid leukemia patient with morphology resembling APL without involvement of the RARA gene. Molecular and fluorescent in situ hybridization analyses excluded PML-RARA fusion and variant rearrangements involving RARA and RARG loci. Targeted next-generation sequencing showed EZH2- D185H mutation. As this mutation involved the region of interaction with DNA methyltransferases, we speculate an epigenetic alteration of genes involved in the APL-like phenotype. Expression analysis by droplet digital polymerase chain reaction revealed downregulation of the RARA and RARG genes. We hypothesize a novel mechanism of EZH2 function alteration, which may be responsible for an acute myeloid leukemia with APL-like phenotype featuring dysregulation of the RARA and RARG genes.

Novel SUZ12 mutations in Weaver-like syndrome

SUZ12 is a core component of polycomb repressive complex 2 (PRC2) along with EZH2 and EED. Recently, germline mutations in the SUZ12, EZH2 and EED genes have been reported in Weaver syndrome (WS) or Weaver-like syndrome, suggesting a functional link between PRC2 deficits and WS. However, only one case of a SUZ12 mutation presenting with Weaver-like syndrome has been reported. Here, we report a missense and a frameshift mutation in SUZ12 (c.1797A>C; p.Gln599His and c.844_845del; p.Ala282Glnfs*7), both of which are novel, in two individuals. Their clinical features included postnatal overgrowth, increased bifrontal diameter, large ears, round face, horizontal chin crease and skeletal anomalies, but did not fulfill the WS diagnostic criteria. These data provide strong evidence that SUZ12 mutations cause Weaver-like syndrome.

Co-occurrence of a maternally inherited DNMT3A duplication and a paternally inherited pathogenic variant in EZH2 in a child with growth retardation and severe short stature: atypical Weaver syndrome or evidence of a DNMT3A dosage effect?

Overgrowth syndromes are a clinically heterogeneous group of disorders characterized by localized or generalized tissue overgrowth and varying degrees of developmental and intellectual disability. An expanding list of genes associated with overgrowth syndromes include the histone methyltransferase genes EZH2 and NSD1, which cause Weaver and Sotos syndrome, respectively, and the DNA methyltransferase (DNMT3A) gene that results in Tatton-Brown–Rahman syndrome (TBRS). Here, we describe a 5-year-old female with a paternally inherited pathogenic mutation in EZH2 (c.2050C>T, p.Arg684Cys) and a maternally inherited 505-kb duplication of uncertain significance at 2p23.3 (encompassing five genes, including DNMT3A) who presented with intrauterine growth restriction, slow postnatal growth, short stature, hypotonia, developmental delay, and neuroblastoma diagnosed at the age of 8 mo. Her father had tall stature, dysmorphic facial features, and intellectual disability consistent with Weaver syndrome, whereas her mother had short stature, cognitive delays, and chronic nonprogressive leukocytosis. It has been previously shown that EZH2 directly controls DNA methylation through physical association with DNMTs, including DNMT3A, with concomitant H3K27 methylation and CpG promoter methylation leading to repression of EZH2 target genes. Interestingly, NSD1 is involved in H3K36 methylation, a mark associated with transcriptional activation, and exhibits exquisite dosage sensitivity leading to overgrowth when deleted and severe undergrowth when duplicated in vivo. Although there is currently no evidence of dosage effects for DNMT3A, the co-occurrence of a duplication involving this gene and a pathogenic alteration in EZH2 in a patient with severe undergrowth is suggestive of a similar paradigm and further study is warranted.