The histone methyltransferase KMT2D, mutated in Kabuki syndrome patients, is required for neural crest cell formation and migration

KMT2D regulates activation, localization, and integrin expression by T-cells

Individuals with Kabuki syndrome present with immunodeficiency; however, how pathogenic variants in the gene encoding the histone-modifying enzyme lysine methyltransferase 2D (KMT2D) lead to immune alterations remain poorly understood. Following up on our prior report of KMT2D-altered integrin expression in B-cells, we performed targeted analyses of KMT2D's influence on integrin expression in T-cells throughout development (thymocytes through peripheral T-cells) in murine cells with constitutive- and conditional-targeted Kmt2d deletion. Using high-throughput RNA-sequencing and flow cytometry, we reveal decreased expression (both at the transcriptional and translational levels) of a cluster of leukocyte-specific integrins, which perturb aspects of T-cell activation, maturation, adhesion/localization, and effector function. H3K4me3 ChIP-PCR suggests that these evolutionary similar integrins are under direct control of KMT2D. KMT2D loss also alters multiple downstream programming/signaling pathways, including integrin-based localization, which can influence T-cell populations. We further demonstrated that KMT2D deficiency is associated with the accumulation of murine CD8+ single-positive (SP) thymocytes and shifts in both human and murine peripheral T-cell populations, including the reduction of the CD4+ recent thymic emigrant (RTE) population. Together, these data show that the targeted loss of Kmt2d in the T-cell lineage recapitulates several distinct features of Kabuki syndrome-associated immune deficiency and implicates epigenetic mechanisms in the regulation of integrin signaling.

The Mendelian disorders of chromatin machinery: Harnessing metabolic pathways and therapies for treatment

The Mendelian disorders of chromatin machinery (MDCMs) represent a distinct subgroup of disorders that present with neurodevelopmental disability. The chromatin machinery regulates gene expression by a range of mechanisms, including by post-translational modification of histones, responding to histone marks, and remodelling nucleosomes. Some of the MDCMs that impact on histone modification may have potential therapeutic interventions. Two potential treatment strategies are to enhance the intracellular pool of metabolites that can act as substrates for histone modifiers and the use of medications that may inhibit or promote the modification of histone residues to influence gene expression. In this article we discuss the influence and potential treatments of histone modifications involving histone acetylation and histone methylation. Genomic technologies are facilitating earlier diagnosis of many Mendelian disorders, providing potential opportunities for early treatment from infancy. This has parallels with how inborn errors of metabolism have been afforded early treatment with newborn screening. Before this promise can be fulfilled, we require greater understanding of the biochemical fingerprint of these conditions, which may provide opportunities to supplement metabolites that can act as substrates for chromatin modifying enzymes. Importantly, understanding the metabolomic profile of affected individuals may also provide disorder-specific biomarkers that will be critical for demonstrating efficacy of treatment, as treatment response may not be able to be accurately assessed by clinical measures.

Small molecule-mediated reprogramming of Xenopus blastula stem cells to a neural crest state

Neural crest cells are a stem cell population unique to vertebrates that give rise to a diverse array of derivatives, including much of the peripheral nervous system, pigment cells, cartilage, mesenchyme, and bone. Acquisition of these cells drove the evolution of vertebrates and defects in their development underlies a broad set of neurocristopathies. Moreover, studies of neural crest can inform differentiation protocols for pluripotent stem cells and regenerative medicine applications. Xenopus embryos are an important system for studies of the neural crest and have provided numerous insights into the signals and transcription factors that control the formation and later lineage diversification of these stem cells. Pluripotent animal pole explants are a particularly powerful tool in this system as they can be cultured in simple salt solution and instructed to give rise to any cell type including the neural crest. Here we report a protocol for small molecule-mediated induction of the neural crest state from blastula stem cells and validate it using transcriptome analysis and grafting experiments. This is an powerful new tool for generating this important cell type that will facilitate future studies of neural crest development and mutations and variants linked to neurocristopathies.

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.

SETting up the genome: KMT2D and KDM6A genomic function in the Kabuki syndrome craniofacial developmental disorder

BACKGROUND

Kabuki syndrome is a congenital developmental disorder that is characterized by distinctive facial gestalt and skeletal abnormalities. Although rare, the disorder shares clinical features with several related craniofacial syndromes that manifest from mutations in chromatin-modifying enzymes. Collectively, these clinical studies underscore the crucial, concerted functions of chromatin factors in shaping developmental genome structure and driving cellular transcriptional states. Kabuki syndrome predominantly results from mutations in KMT2D, a histone H3 lysine 4 methylase, or KDM6A, a histone H3 lysine 27 demethylase.

AIMS

In this review, we summarize the research efforts to model Kabuki syndrome in vivo to understand the cellular and molecular mechanisms that lead to the craniofacial and skeletal pathogenesis that defines the disorder.

DISCUSSION

As several studies have indicated the importance of KMT2D and KDM6A function through catalytic-independent mechanisms, we highlight noncanonical roles for these enzymes as recruitment centers for alternative chromatin and transcriptional machinery.

The H2A.Z and NuRD associated protein HMG20A controls early head and heart developmental transcription programs

Specialized chromatin-binding proteins are required for DNA-based processes during development. We recently established PWWP2A as a direct histone variant H2A.Z interactor involved in mitosis and craniofacial development. Here, we identify the H2A.Z/PWWP2A-associated protein HMG20A as part of several chromatin-modifying complexes, including NuRD, and show that it localizes to distinct genomic regulatory regions. Hmg20a depletion causes severe head and heart developmental defects in Xenopus laevis. Our data indicate that craniofacial malformations are caused by defects in neural crest cell (NCC) migration and cartilage formation. These developmental failures are phenocopied in Hmg20a-depleted mESCs, which show inefficient differentiation into NCCs and cardiomyocytes (CM). Consequently, loss of HMG20A, which marks open promoters and enhancers, results in chromatin accessibility changes and a striking deregulation of transcription programs involved in epithelial-mesenchymal transition (EMT) and differentiation processes. Collectively, our findings implicate HMG20A as part of the H2A.Z/PWWP2A/NuRD-axis and reveal it as a key modulator of intricate developmental transcription programs that guide the differentiation of NCCs and CMs.

Distinct Roles of Histone Lysine Demethylases and Methyltransferases in Developmental Eye Disease

Histone lysine methyltransferase and demethylase enzymes play a central role in chromatin organization and gene expression through the dynamic regulation of histone lysine methylation. Consistent with this, genes encoding for histone lysine methyltransferases (KMTs) and demethylases (KDMs) are involved in complex human syndromes, termed congenital regulopathies. In this report, we present several lines of evidence for the involvement of these genes in developmental ocular phenotypes, suggesting that individuals with structural eye defects, especially when accompanied by craniofacial, neurodevelopmental and growth abnormalities, should be examined for possible variants in these genes. We identified nine heterozygous damaging genetic variants in KMT2D (5) and four other histone lysine methyltransferases/demethylases (KMT2C, SETD1A/KMT2F, KDM6A and KDM5C) in unrelated families affected with developmental eye disease, such as Peters anomaly, sclerocornea, Axenfeld-Rieger spectrum, microphthalmia and coloboma. Two families were clinically diagnosed with Axenfeld-Rieger syndrome and two were diagnosed with Peters plus-like syndrome; others received no specific diagnosis prior to genetic testing. All nine alleles were novel and five of them occurred de novo; five variants resulted in premature truncation, three were missense changes and one was an in-frame deletion/insertion; and seven variants were categorized as pathogenic or likely pathogenic and two were variants of uncertain significance. This study expands the phenotypic spectra associated with KMT and KDM factors and highlights the importance of genetic testing for correct clinical diagnosis.

Ash2l, an obligatory component of H3K4 methylation complexes, regulates neural crest development

The vertebrate nervous system develops from embryonic neural plate and neural crest. Although genetic mechanisms governing vertebrate neural development have been investigated in depth, epigenetic regulation of this process remains less understood. Redundancy of epigenetic factors and early lethality of animals deficient in critical epigenetic components pose major challenges in characterization of epigenetic factors in vertebrate neural development. In this study, we use the amphibian model Xenopus laevis to investigate the roles of non-redundant, obligatory components of all histone H3K4 activating methylation complexes (COMPASS, also known as SET1/MLL complexes) in early neural development. The two genes that we focus on, Ash2l and Dpy30, regulate mesendodermal differentiation in mouse embryonic stem cells and cause early embryonic lethality when removed from mouse embryos. Using targeted knockdown of the genes in dorsal ectoderm of Xenopus that gives rise to future nervous system, we show here that ash2l and dpy30 are required for neural and neural crest marker expression in Xenopus late neurula embryos but are dispensable for early neural and neural plate border gene expression. Co-immunoprecipitation assays reveal that Dpy30 and Ash2L associate with the neural plate border transcription factors, such as Msx1 and Tfap2a. Chromatin immunoprecipitation (ChIP) assay further demonstrates that Ash2L and the H3K4me3 active histone mark accumulate at the promoter regions of the neural crest gene sox10 in a Tfap2a-dependent manner. Collectively, our data suggest that Ash2l and Dpy30 interact with specific transcription factors to recruit COMPASS complexes to the regulatory regions of neural crest specification genes to control their expression and influence development of the nervous system during vertebrate embryogenesis.

Single-Cell Transcriptome Analysis Defines Expression of Kabuki Syndrome-Associated KMT2D Targets and Interacting Partners

Objectives. Kabuki syndrome (KS) is a rare genetic disorder characterized by developmental delay, retarded growth, and cardiac, gastrointestinal, neurocognitive, renal, craniofacial, dental, and skeletal defects. KS is caused by mutations in the genes encoding histone H3 lysine 4 methyltransferase (KMT2D) and histone H3 lysine 27 demethylase (KDM6A), which are core components of the complex of proteins associated with histone H3 lysine 4 methyltransferase SET1 (SET1/COMPASS). Using single-cell RNA data, we examined the expression profiles of Kmt2d and Kdm6a in the mouse dental pulp. In the incisor pulp, Kmt2d and Kdm6a colocalize with other genes of the SET1/COMPASS complex comprised of the WD-repeat protein 5 gene (Wdr5), the retinoblastoma-binding protein 5 gene (Rbbp5), absent, small, and homeotic 2-like protein-encoding gene (Ash2l), nuclear receptor cofactor 6 gene (Ncoa6), and Pax-interacting protein 1 gene (Ptip1). In addition, we found that Kmt2d and Kdm6a coexpress with the downstream target genes of the Wingless and Integrated (WNT) and sonic hedgehog signaling pathways in mesenchymal stem/stromal cells (MSCs) at different stages of osteogenic differentiation. Taken together, our results suggest an essential role of KMT2D and KDK6A in directing lineage-specific gene expression during differentiation of MSCs.

Histone-lysine N-methyltransferase 2 (KMT2) complexes - a new perspective

Histone H3 Lys4 (H3K4) methylation is catalyzed by the Histone-Lysine N-Methyltransferase 2 (KMT2) protein family, and its members are required for gene expression control. In vertebrates, the KMT2s function in large multisubunit complexes known as COMPASS or COMPASS-like complexes (COMplex of Proteins ASsociated with Set1). The activity of these complexes is critical for proper development, and mutation-induced defects in their functioning have frequently been found in human cancers. Moreover, inherited or de novo mutations in KMT2 genes are among the etiological factors in neurodevelopmental disorders such as Kabuki and Kleefstra syndromes. The canonical role of KMT2s is to catalyze H3K4 methylation, which results in a permissive chromatin environment that drives gene expression. However, current findings described in this review demonstrate that these enzymes can regulate processes that are not dependent on methylation: noncatalytic functions of KMT2s include DNA damage response, cell division, and metabolic activities. Moreover, these enzymes may also methylate non-histone substrates and play a methylation-dependent function in the DNA damage response. In this review, we present an overview of the new, noncanonical activities of KMT2 complexes in a variety of cellular processes. These discoveries may have crucial implications for understanding the functions of these methyltransferases in developmental processes, disease, and epigenome-targeting therapeutic strategies in the future.

Diabetes mellitus and insulin resistance associated with Kabuki syndrome—A case report and literature review

Kabuki syndrome (KS) is a genetic disorder characterized by distinctive facies, intellectual disability, and multi‐organ anomalies. This case report highlights the importance of clinical recognizable phenotype in patients with diabetes. The development of diabetes should be considered an endocrine complication in KS patients.

High prevalence of deleterious mutations in concomitant nonsyndromic cleft and outflow tract heart defects

Our previous work demonstrating enrichment of outflow tract (OFT) congenital heart disease (CHD) in children with cleft lip and/or palate (CL/P) suggests derangements in common underlying developmental pathways. The current pilot study examines the underlying genetics of concomitant nonsyndromic CL/P and OFT CHD phenotype. Of 575 patients who underwent CL/P surgery at Children's Hospital Los Angeles, seven with OFT CHD, negative chromosomal microarray analysis, and no recognizable syndromic association were recruited with their parents (as available). Whole genome sequencing of blood samples paired with whole‐blood‐based RNA sequencing for probands was performed. A pathogenic or potentially pathogenic variant was identified in 6/7 (85.7%) probands. A total of seven candidate genes were mutated (CHD7, SMARCA4, MED12, APOB, RNF213, SETX, and JAG1). Gene ontology analysis of variants predicted involvement in binding (100%), regulation of transcription (42.9%), and helicase activity (42.9%). Four patients (57.1%) expressed gene variants (CHD7, SMARCA4, MED12, and RNF213) previously involved in the Wnt signaling pathway. Our pilot analysis of a small cohort of patients with combined CL/P and OFT CHD phenotype suggests a potentially significant prevalence of deleterious mutations. In our cohort, an overrepresentation of mutations in molecules associated with Wnt‐signaling was found. These variants may represent an expanded phenotypic heterogeneity within known monogenic disease genes or provide novel evidence of shared developmental pathways. The mechanistic implications of these mutations and subsequent developmental derangements resulting in the CL/P and OFT CHD phenotype require further analysis in a larger cohort of patients.

The role of KMT2 gene in human tumors

Histone methylation plays a crucial role in the regulation of gene transcriptional expression, and aberration of methylation-modifying enzyme genes can lead to a variety of genetic diseases, including human cancers. The histone modified protein KMT2 (lysin methyltransferase) family are involved in cell proliferation, growth, development and differentiation through regulating gene expression, and are closely related with many blood cancers and solid tumors. In recent years, several studies have shown that mutations in the KMT2 gene occur frequently in a variety of human cancers and the mutation status of the KMT2 gene may be correlated with the occurrence, development and prognosis of some tumors. Research uncovering the clinical characteristics and molecular mechanisms of KMT2 mutation in human tumors will be helpful for early diagnosis and prognosis of tumors as well as drug development for targeted therapies.

16p12.1 Deletion Orthologs are Expressed in Motile Neural Crest Cells and are Important for Regulating Craniofacial Development in Xenopus laevis

Copy number variants (CNVs) associated with neurodevelopmental disorders are characterized by extensive phenotypic heterogeneity. In particular, one CNV was identified in a subset of children clinically diagnosed with intellectual disabilities (ID) that results in a hemizygous deletion of multiple genes at chromosome 16p12.1. In addition to ID, individuals with this deletion display a variety of symptoms including microcephaly, seizures, cardiac defects, and growth retardation. Moreover, patients also manifest severe craniofacial abnormalities, such as micrognathia, cartilage malformation of the ears and nose, and facial asymmetries; however, the function of the genes within the 16p12.1 region have not been studied in the context of vertebrate craniofacial development. The craniofacial tissues affected in patients with this deletion all derive from the same embryonic precursor, the cranial neural crest, leading to the hypothesis that one or more of the 16p12.1 genes may be involved in regulating neural crest cell (NCC)-related processes. To examine this, we characterized the developmental role of the 16p12.1-affected gene orthologs, polr3e, mosmo, uqcrc2, and cdr2, during craniofacial morphogenesis in the vertebrate model system, Xenopus laevis. While the currently-known cellular functions of these genes are diverse, we find that they share similar expression patterns along the neural tube, pharyngeal arches, and later craniofacial structures. As these genes show co-expression in the pharyngeal arches where NCCs reside, we sought to elucidate the effect of individual gene depletion on craniofacial development and NCC migration. We find that reduction of several 16p12.1 genes significantly disrupts craniofacial and cartilage formation, pharyngeal arch migration, as well as NCC specification and motility. Thus, we have determined that some of these genes play an essential role during vertebrate craniofacial patterning by regulating specific processes during NCC development, which may be an underlying mechanism contributing to the craniofacial defects associated with the 16p12.1 deletion.

Whole-exome sequencing in syndromic craniosynostosis increases diagnostic yield and identifies candidate genes in osteogenic signaling pathways

Craniosynostosis (CS) is a common congenital anomaly defined by premature fusion of one or more cranial sutures. Syndromic CS involves additional organ anomalies or neurocognitive deficits and accounts for 25%-30% of the cases. In a recent population-based study by our group, 84% of the syndromic CS cases had a genetically verified diagnosis after targeted analyses. A number of different genetic causes were detected, confirming that syndromic CS is highly heterogeneous. In this study, we performed whole-exome sequencing of 10 children and parents from the same cohort where previous genetic results were negative. We detected pathogenic, or likely pathogenic, variants in four additional genes (NFIA, EXTL3, POLR2A, and FOXP2) associated with rare conditions. In two of these (POLR2A and FOXP2), CS has not previously been reported. We further detected a rare predicted damaging variant in SH3BP4, which has not previously been related to human disease. All findings were clustered in genes involved in the pathways of osteogenesis and suture patency. We conclude that whole-exome sequencing expands the list of genes associated with syndromic CS, and provides new candidate genes in osteogenic signaling pathways.

Case report: A study on the de novo KMT2D variant of Kabuki syndrome with Goodpasture's syndrome by whole exome sequencing

Kabuki syndrome (KS) is a rare genetic disorder characterized by dysmorphic facial features, skeletal abnormalities, and intellectual disability. KMT2D and KDM6A were identified as the main causative genes. To our knowledge, there exist no cases of KS, which were reported with pneumorrhagia. In this study, a 10-month-old male was diagnosed to have KS with typical facial features, skeletal anomalies, and serious postnatal growth retardation. Whole exome sequencing of the trio family revealed the presence of a de novo KMT2D missense variant (c.15143G > A, p. R5048H). The child was presented to the pediatric emergency department several times because of cough, hypoxemia, and anemia. After performing chest CT and fiberoptic bronchoscopy, we found that the child had a pulmonary hemorrhage. During research on the cause of pulmonary hemorrhage, the patient's anti-GBM antibodies gradually became positive, and the urine microalbumin level was elevated at the age of 12-month-old. After glucocorticoids and immunosuppressant therapy, the patient became much better. But he had recurrent pulmonary hemorrhage at the age of 16 months. Therefore, the patient underwent digital subtraction angiography (DSA). However, the DSA showed three abnormal bronchial arteries. This single case expands the phenotypes of patients with KS and Goodpasture's syndrome, which were found to have a de novo KMT2D missense variant.

A Novel Intronic KMT2D Variant as a Cause of Kabuki Syndrome: A Case Report

Kabuki syndrome (KS) is an autosomal dominant genetic disorder in which most cases are caused by de novo mutations. KS type 1 is caused by mutations in KMT2D (OMIM: #147920) and is more common. KS type 2 is caused by mutations in KDM6A (OMIM: #300867). Both genes encode proteins that modify histones and are involved in epigenetic regulation. The enzyme histone-lysine N-methyltransferase 2D, the product of KMT2D, is expressed in most adult tissues and is essential for early embryonic development. The main clinical manifestations of KS include dysmorphic facial features, such as elongated palpebral fissures, eversion of the lateral third of the lower eyelids, and short nasal columella with a broad and depressed nasal tip. Additionally, patients also present with skeletal abnormalities, dermatoglyphic features, mild-to-moderate intellectual disability, hearing loss, and postnatal growth deficiency. We describe an 11-year-old girl from Colombia, who presented with characteristic clinical signs of KS. Genetic studies showed a KMT2D intronic variant (KMT2D NM_003482.3: c.511‐2A> T) as a cause of KS.

Epigenetic Regulation of Cardiac Neural Crest Cells

The cardiac neural crest cells (cNCCs) is a transient, migratory cell population that contribute to the formation of major arteries and the septa and valves of the heart. Abnormal development of cNCCs leads to a spectrum of congenital heart defects that mainly affect the outflow region of the hearts. Signaling molecules and transcription factors are the best studied regulatory events controlling cNCC development. In recent years, however, accumulated evidence supports that epigenetic regulation also plays an important role in cNCC development. Here, we summarize the functions of epigenetic regulators during cNCC development as well as cNCC related cardiovascular defects. These factors include ATP-dependent chromatin remodeling factors, histone modifiers and DNA methylation modulators. In many cases, mutations in the genes encoding these factors are known to cause inborn heart diseases. A better understanding of epigenetic regulators, their activities and their roles during heart development will ultimately contribute to the development of new clinical applications for patients with congenital heart disease.

Kabuki Syndrome-Clinical Review with Molecular Aspects

Kabuki syndrome (KS) is a rare developmental disorder principally comprised of developmental delay, hypotonia and a clearly defined dysmorphism: elongation of the structures surrounding the eyes, a shortened and depressed nose, thinning of the upper lip and thickening of the lower lip, large and prominent ears, hypertrichosis and scoliosis. Other characteristics include poor physical growth, cardiac, gastrointestinal and renal anomalies as well as variable behavioral issues, including autistic features. De novo or inherited pathogenic/likely pathogenic variants in the KMT2D gene are the most common cause of KS and account for up to 75% of patients. Variants in KDM6A cause up to 5% of cases (X-linked dominant inheritance), while the etiology of about 20% of cases remains unknown. Current KS diagnostic criteria include hypotonia during infancy, developmental delay and/or intellectual disability, typical dysmorphism and confirmed pathogenic/likely pathogenic variant in KMT2D or KDM6A. Care for KS patients includes the control of physical and psychomotor development during childhood, rehabilitation and multi-specialist care. This paper reviews the current clinical knowledge, provides molecular and scientific links and sheds light on the treatment of Kabuki syndrome individuals.

Genetic and phenotypic analysis of 101 patients with developmental delay or intellectual disability using whole-exome sequencing

Whole-exome sequencing (WES) enables identification of pathogenic variants, including copy number variants (CNVs). In this study, we performed WES in 101 Japanese patients with unexplained developmental delay (DD) or intellectual disability (ID) (63 males and 38 females), 98 of them with trio-WES. Pathogenic variants were identified in 54 cases (53.5%), including four cases with pathogenic CNVs. In one case, a pathogenic variant was identified by reanalysis of exome data; and in two cases, two molecular diagnoses were identified. Among 58 pathogenic variants, 49 variants occurred de novo in 48 patients, including two somatic variants. The accompanying autism spectrum disorder and external ear anomalies were associated with detection of pathogenic variants with odds ratios of 11.88 (95% confidence interval [CI] 2.52-56.00) and 3.46 (95% CI 1.23-9.73), respectively. These findings revealed the importance of reanalysis of WES data and detection of CNVs and somatic variants in increasing the diagnostic yield for unexplained DD/ID. In addition, genetic testing is recommended when patients suffer from the autism spectrum disorder or external ear anomalies, which potentially suggests the involvement of genetic factors associated with gene expression regulation.

Neurobiology of puberty and its disorders

Puberty, which in humans is considered to include both gonadarche and adrenarche, is the period of becoming capable of reproducing sexually and is recognized by maturation of the gonads and development of secondary sex characteristics. Gonadarche referring to growth and maturation of the gonads is fundamental to puberty since it encompasses increased gonadal steroid secretion and initiation of gametogenesis resulting from enhanced pituitary gonadotropin secretion, triggered in turn by robust pulsatile GnRH release from the hypothalamus. This chapter reviews the development of GnRH pulsatility from before birth until the onset of puberty. In humans, GnRH pulse generation is restrained during childhood and juvenile development. This prepubertal hiatus in hypothalamic activity is considered to result from a neurobiological brake imposed upon the GnRH pulse generator resident in the infundibular nucleus. Reactivation of the GnRH pulse generator initiates pubertal development. Current understanding of the genetics and physiology of the brake will be discussed, as will hypotheses proposed to account for timing the resurgence in pulsatile GnRH and initiation of puberty. The chapter ends with a discussion of disorders associated with precocious or delayed puberty with a focus on those with etiologies attributed to aberrant GnRH neuron anatomy or function. A pediatric approach to patients with pubertal disorders is provided and contemporary treatments for both precocious and delayed puberty outlined.

Using Xenopus to analyze neurocristopathies like Kabuki syndrome

Neurocristopathies are human congenital syndromes that arise from defects in neural crest (NC) development and are typically associated with malformations of the craniofacial skeleton. Genetic analyses have been very successful in identifying pathogenic mutations, however, model organisms are required to characterize how these mutations affect embryonic development thereby leading to complex clinical conditions. The African clawed frog Xenopus laevis provides a broad range of in vivo and in vitro tools allowing for a detailed characterization of NC development. Due to the conserved nature of craniofacial morphogenesis in vertebrates, Xenopus is an efficient and versatile system to dissect the morphological and cellular phenotypes as well as the signaling events leading to NC defects. Here, we review a set of techniques and resources how Xenopus can be used as a disease model to investigate the pathogenesis of Kabuki syndrome and neurocristopathies in a wider sense.

Genetics Underlying the Interactions between Neural Crest Cells and Eye Development

The neural crest is a unique, transient stem cell population that is critical for craniofacial and ocular development. Understanding the genetics underlying the steps of neural crest development is essential for gaining insight into the pathogenesis of congenital eye diseases. The neural crest cells play an under-appreciated key role in patterning the neural epithelial-derived optic cup. These interactions between neural crest cells within the periocular mesenchyme and the optic cup, while not well-studied, are critical for optic cup morphogenesis and ocular fissure closure. As a result, microphthalmia and coloboma are common phenotypes in human disease and animal models in which neural crest cell specification and early migration are disrupted. In addition, neural crest cells directly contribute to numerous ocular structures including the cornea, iris, sclera, ciliary body, trabecular meshwork, and aqueous outflow tracts. Defects in later neural crest cell migration and differentiation cause a constellation of well-recognized ocular anterior segment anomalies such as Axenfeld–Rieger Syndrome and Peters Anomaly. This review will focus on the genetics of the neural crest cells within the context of how these complex processes specifically affect overall ocular development and can lead to congenital eye diseases.

Neonatal hyperinsulinemic hypoglycemia: case report of kabuki syndrome due to a novel KMT2D splicing-site mutation

Background

Persistent neonatal hypoglycemia, owing to the possibility of severe neurodevelopmental consequences, is a leading cause of neonatal care admission. Hyperinsulinemic hypoglycemia is often resistant to dextrose infusion and needs rapid diagnosis and treatment. Several congenital conditions, from single gene defects to genetic syndromes should be considered in the diagnostic approach. Kabuki syndrome type 1 (MIM# 147920) and Kabuki syndrome type 2 (MIM# 300867), can be associated with neonatal hyperinsulinemic hypoglycemia.

Patient presentation

We report a female Italian (Sicilian) child, born preterm at 35 weeks gestation, with persistent hypoglycemia. Peculiar facial dysmorphisms, neonatal hypotonia, and cerebellar vermis hypoplasia raised suspicion of Kabuki syndrome. Hyperinsulinemic hypoglycemia was confirmed with glucagon test and whole-exome sequencing (WES) found a novel heterozygous splicing-site mutation (c.674-1G > A) in KMT2D gene. Hyperinsulinemic hypoglycemia was successfully treated with diazoxide. At 3 months corrected age for prematurity, a mild global neurodevelopmental delay, postnatal weight and occipitofrontal circumference growth failure were reported.

Conclusions

Kabuki syndrome should be considered when facing neonatal persistent hypoglycemia. Diazoxide may help to improve hyperinsulinemic hypoglycemia. A multidisciplinary and individualized follow-up should be carried out for early diagnosis and treatment of severe pathological associated conditions.

Role of epigenetics and miRNAs in orofacial clefts

Orofacial clefts (OFCs) have multiple etiologies and likely result from an interplay between genetic and environmental factors. Within the last decade, studies have implicated specific epigenetic modifications and noncoding RNAs as additional facets of OFC etiology. Altered gene expression through DNA methylation and histone modification offer novel insights into how specific genes contribute to distinct OFC subtypes. Epigenetics research has also provided further evidence that cleft lip only (CLO) is a cleft subtype with distinct etiology. Polymorphisms or misexpression of genes encoding microRNAs, as well as their targets, contribute to OFC risk. The ability to experimentally manipulate epigenetic changes and noncoding RNAs in animal models, such as zebrafish, Xenopus, mice, and rats, has offered novel insights into the mechanisms of various OFC subtypes. Although much remains to be understood, recent advancements in our understanding of OFC etiology may advise future strategies of research and preventive care.

The KMT2D Kabuki syndrome histone methylase controls neural crest cell differentiation and facial morphology

Kabuki syndrome (KS) is a congenital craniofacial disorder resulting from mutations in the KMT2D histone methylase (KS1) or the UTX histone demethylase (KS2). With small cohorts of KS2 patients, it is not clear whether differences exist in clinical manifestations relative to KS1. We mutated KMT2D in neural crest cells (NCCs) to study cellular and molecular functions in craniofacial development with respect to UTX. Similar to UTX, KMT2D NCC knockout mice demonstrate hypoplasia with reductions in frontonasal bone lengths. We have traced the onset of KMT2D and UTX mutant NCC frontal dysfunction to a stage of altered osteochondral progenitor differentiation. KMT2D NCC loss-of-function does exhibit unique phenotypes distinct from UTX mutation, including fully penetrant cleft palate, mandible hypoplasia and deficits in cranial base ossification. KMT2D mutant NCCs lead to defective secondary palatal shelf elevation with reduced expression of extracellular matrix components. KMT2D mutant chondrocytes in the cranial base fail to properly differentiate, leading to defective endochondral ossification. We conclude that KMT2D is required for appropriate cranial NCC differentiation and KMT2D-specific phenotypes may underlie differences between Kabuki syndrome subtypes.

Functions of histone modifications and histone modifiers in Schwann cells

Schwann cells (SCs) are the main glial cells present in the peripheral nervous system (PNS). Their primary functions are to insulate peripheral axons to protect them from the environment and to enable fast conduction of electric signals along big caliber axons by enwrapping them in a thick myelin sheath rich in lipids. In addition, SCs have the peculiar ability to foster axonal regrowth after a lesion by demyelinating and converting into repair cells that secrete neurotrophic factors and guide axons back to their former target to finally remyelinate regenerated axons. The different steps of SC development and their role in the maintenance of PNS integrity and regeneration after lesion are controlled by various factors among which transcription factors and chromatin-remodeling enzymes hold major functions. In this review, we discussed how histone modifications and histone-modifying enzymes control SC development, maintenance of PNS integrity and response to injury. The functions of histone modifiers as part of chromatin-remodeling complexes are discussed in another review published in the same issue of Glia.