Counteracting epigenetic mechanisms regulate the structural development of neuronal circuitry in human neurons

Genetic advances and translational phenotypes in rodent models for Tourette disorder

Tourette disorder (TD) is a neurodevelopmental condition affecting approximately 0.3%-1% of children and adolescents. It is defined by motor and vocal tics but encompasses wide ranging phenotypes due to its complex genetic origins, involving hundreds of risk genes across various signaling pathways. Traditional animal models of TD have focused on circuit manipulation or neuron ablation strategies to investigate its underlying causes and associated brain changes. However, the recent identification of high-confidence risk genes has opened new possibilities for creating models that express the exact genetic variants associated with TD. This review discusses early attempts to model TD in rodents and highlights advancements in next-generation models with true construct validity through the expression of orthologous human mutations in high-confidence risk genes. Additionally, we examine the translational potential of integrating cognitive and sensorimotor approaches to evaluate TD-related phenotypes in rodents, including changes to reinforcement learning, habitual behavior, and incentive motivation.

Novel heterozygous ASH1L nonsense variant involved in mild intellectual disability

Mutations in ASH1L have been associated with a range of phenotypes, including intellectual disability (ID), autism spectrum disorder (ASD), attention deficit hyperactivity disorder (ADHD), seizures, as well as differences in skeletal, muscular, and sleep functions. In this study, we describe a patient diagnosed with mild ID, and whole-exome sequencing (WES) of the family identified a novel heterozygous nonsense variant, NM_018489.2: c.2479A > T (p.Lys827*), located in exon 3 of ASH1L, which was predicted to be pathogenic. The nonsense variant in the mild ID patient may disrupt ASH1L function by destabilizing its spatial conformation, leading to decreased activity of the catalytic H3K36 methylation, thereby affecting neurological function. A review of reported ASH1L nonsense mutations to explore genotype-phenotype correlations suggested that these variants typically result in a loss of function. Our findings contribute to understanding the neurodevelopmental pathogenesis of mild ID in patients with the ASH1L nonsense variant mutation.

Ash1l loss-of-function results in structural birth defects and altered cortical development

The histone methyltransferase ASH1L plays a crucial role in regulating gene expression across various organ systems during development, yet its role in brain development remains largely unexplored. Over 130 individuals with autism harbour heterozygous loss-of-function ASH1L variants, and population studies confirm it as a high-risk autism gene. Previous studies on Ash1l deficient mice have reported autistic-like behaviours and provided insights into the underlying neuropathophysiology. In this study, we used mice with a cre-inducible deletion of Ash1l exon 4, which results in a frame shift and premature stop codon (p.V1693Afs*2). Our investigation evaluated the impact of Ash1l loss-of-function on survival and craniofacial skeletal development. Using a tamoxifen-inducible cre strain, we targeted Ash1l knockout early in cortical development [Emx1-Cre-ERT2; embryonic Day (e) 10.5]. Immunohistochemistry was utilized to assess cortical lamination, while EdU incorporation aided in birthdating cortical neurons. Additionally, single-cell RNA sequencing was employed to compare cortical cell populations and identify genes with differential expression. At e18.5, the proportion of homozygous Ash1l germline knockout embryos appeared normal; however, no live Ash1l null pups were present at birth (e18.5: n = 77, P = 0.90; p0: n = 41, P = 0.00095). Notably, Ash1l-/- exhibited shortened nasal bones (n = 31, P = 0.017). In the cortical-specific knockout model, SATB2 neurons showed increased numbers (n = 6/genotype, P = 0.0001) and were distributed through the cortical plate. Birthdating revealed generation of ectopically placed deep layer neurons that express SATB2 (e13.5 injection: n = 4/genotype, P = 0.0126). Single cell RNA sequencing revealed significant differences in gene expression between control and mutant upper layer neurons, leading to distinct clustering. Pseudotime analysis indicated that the mutant cluster followed an altered cell differentiation trajectory. This study underscores the essential role of Ash1l in postnatal survival and normal craniofacial development. In the cortex, ASH1L exerts broad effects on gene expression and is indispensable for determining the fate of upper layer cortical neurons. These findings provide valuable insights into the potential mechanisms of ASH1L neuropathology, shedding light on its significance in neurodevelopmental disorders like autism.

Dynamic Regulation OF The Chromatin Environment By Ash1L Modulates Human Neuronal Structure And Function

Precise regulation of the chromatin environment through post-translational histone modification modulates transcription and controls brain development. Not surprisingly, mutations in a large number of histone-modifying enzymes underlie complex brain disorders. In particular, the histone methyltransferase ASH1L modifies histone marks linked to transcriptional activation and has been implicated in multiple neuropsychiatric disorders. However, the mechanisms underlying the pathobiology of ASH1L-asociated disease remain underexplored. We generated human isogenic stem cells with a mutation in ASH1L's catalytic domain. We find that ASH1L dysfunction results in reduced neurite outgrowth, which correlates with alterations in the chromatin profile of activating and repressive histone marks, as well as the dysregulation of gene programs important for neuronal structure and function implicated in neuropsychiatric disease. We also identified a novel regulatory node implicating both the SP and Krüppel -like families of transcription factors and ASH1L relevant to human neuronal development. Finally, we rescue cellular defects linked to ASH1L dysfunction by leveraging two independent epigenetic mechanisms that promote transcriptional activation. In summary, we identified an ASH1L-driven epigenetic and transcriptional axis essential for human brain development and complex brain disorders that provide insights into future therapeutic strategies for ASH1L-related disorders.

Ash1L ameliorates psoriasis via limiting neuronal activity-dependent release of miR-let-7b

BACKGROUND AND PURPOSE

Psoriasis is a common autoimmune skin disease that significantly diminishes patients' quality of life. Interactions between primary afferents of the somatosensory system and the cutaneous immune system mediate the pathogenesis of psoriasis. This study aims to elucidate the molecular mechanisms of how primary sensory neurons regulate psoriasis formation.

EXPERIMENTAL APPROACH

Skin and total RNA were extracted from wild-type (WT) and ASH1-like histone lysine methyltransferase (Ash1l+/- ) mice in both naive and imiquimod (IMQ)-induced psoriasis models. Immunohistochemistry, quantitative real-time polymerase chain reaction (qRT-PCR) and fluorescence-activated cell sorting (FACS) were then performed. Microfluidic chamber coculture was used to investigate the interaction between somatosensory neurons and bone marrow dendritic cells (BMDCs) ex vivo. Whole-cell patch clamp recordings were used to evaluate neuronal excitability after Ash1L haploinsufficiency in primary sensory neurons.

KEY RESULTS

The haploinsufficiency of ASH1L, a histone methyltransferase, in primary sensory neurons causes both neurite hyperinnervation and increased neuronal excitability, which promote miR-let-7b release from primary afferents in the skin in a neuronal activity-dependent manner. With a 'GUUGUGU' core sequence, miR-let-7b functions as an endogenous ligand of toll-like receptor 7 (TLR7) and stimulates the activation of dermal dendritic cells (DCs) and interleukin (IL)-23/IL-17 axis, ultimately exacerbating the symptoms of psoriasis. Thus, by limiting miR-let-7b release from primary afferents, ASH1L prevents dermal DC activation and ameliorates psoriasis.

CONCLUSION AND IMPLICATIONS

Somatosensory neuron ASH1L modulates the cutaneous immune system by limiting neuronal activity-dependent release of miR-let-7b, which can directly activate dermal DCs via TLR7 and ultimately lead to aggravated psoriatic lesion.

A-Z of Epigenetic Readers: Targeting Alternative Splicing and Histone Modification Variants in Cancer

Epigenetic 'reader' proteins, which have evolved to interact with specific chromatin modifications, play pivotal roles in gene regulation. There is growing interest in the alternative splicing mechanisms that affect the functionality of such epigenetic readers in cancer etiology. The current review considers how deregulation of epigenetic processes and alternative splicing events contribute to pathophysiology. An A-Z guide of epigenetic readers is provided, delineating the antagonistic 'yin-yang' roles of full-length versus spliced isoforms, where this is known from the literature. The examples discussed underscore the key contributions of epigenetic readers in transcriptional regulation, early development, and cancer. Clinical implications are considered, offering insights into precision oncology and targeted therapies focused on epigenetic readers that have undergone alternative splicing events during disease pathogenesis. This review underscores the fundamental importance of alternative splicing events in the context of epigenetic readers while emphasizing the critical need for improved understanding of functional diversity, regulatory mechanisms, and future therapeutic potential.

Neuroimmune mechanisms in autism etiology - untangling a complex problem using human cellular models

Autism spectrum disorder (ASD) affects 1 in 36 people and is more often diagnosed in males than in females. Core features of ASD are impaired social interactions, repetitive behaviors and deficits in verbal communication. ASD is a highly heterogeneous and heritable disorder, yet its underlying genetic causes account only for up to 80% of the cases. Hence, a subset of ASD cases could be influenced by environmental risk factors. Maternal immune activation (MIA) is a response to inflammation during pregnancy, which can lead to increased inflammatory signals to the fetus. Inflammatory signals can cross the placenta and blood brain barriers affecting fetal brain development. Epidemiological and animal studies suggest that MIA could contribute to ASD etiology. However, human mechanistic studies have been hindered by a lack of experimental systems that could replicate the impact of MIA during fetal development. Therefore, mechanisms altered by inflammation during human pre-natal brain development, and that could underlie ASD pathogenesis have been largely understudied. The advent of human cellular models with induced pluripotent stem cell (iPSC) and organoid technology is closing this gap in knowledge by providing both access to molecular manipulations and culturing capability of tissue that would be otherwise inaccessible. We present an overview of multiple levels of evidence from clinical, epidemiological, and cellular studies that provide a potential link between higher ASD risk and inflammation. More importantly, we discuss how stem cell-derived models may constitute an ideal experimental system to mechanistically interrogate the effect of inflammation during the early stages of brain development.

Dysregulation of the chromatin environment leads to differential alternative splicing as a mechanism of disease in a human model of autism spectrum disorder

Autism spectrum disorder (ASD) affects 1 in 44 children. Chromatin regulatory proteins are overrepresented among genes that contain high risk variants in ASD. Disruption of the chromatin environment leads to widespread dysregulation of gene expression, which is traditionally thought of as a mechanism of disease pathogenesis associated with ASD. Alternatively, alterations in chromatin dynamics could also lead to dysregulation of alternative splicing, which is understudied as a mechanism of ASD pathogenesis. The anticonvulsant valproic acid (VPA) is a well-known environmental risk factor for ASD that acts as a class I histone deacetylase inhibitor. However, the precise molecular mechanisms underlying defects in human neuronal development associated with exposure to VPA are understudied. To dissect how VPA exposure and subsequent chromatin hyperacetylation influence molecular signatures involved in ASD pathogenesis, we conducted RNA sequencing (RNA-seq) in human cortical neurons that were treated with VPA. We observed that differentially expressed genes (DEGs) were enriched for mRNA splicing, mRNA processing, histone modification and metabolism related gene sets. Furthermore, we observed widespread increases in the number and the type of alternative splicing events. Analysis of differential transcript usage (DTU) showed that exposure to VPA induces extensive alterations in transcript isoform usage across neurodevelopmentally important genes. Finally, we find that DEGs and genes that display DTU overlap with known ASD-risk genes. Altogether, these findings suggest that, in addition to differential gene expression, changes in alternative splicing correlated with alterations in the chromatin environment could act as an additional mechanism of disease in ASD.

The role of histone methyltransferases in neurocognitive disorders associated with brain size abnormalities

Brain size is controlled by several factors during neuronal development, including neural progenitor proliferation, neuronal arborization, gliogenesis, cell death, and synaptogenesis. Multiple neurodevelopmental disorders have co-morbid brain size abnormalities, such as microcephaly and macrocephaly. Mutations in histone methyltransferases that modify histone H3 on Lysine 36 and Lysine 4 (H3K36 and H3K4) have been identified in neurodevelopmental disorders involving both microcephaly and macrocephaly. H3K36 and H3K4 methylation are both associated with transcriptional activation and are proposed to sterically hinder the repressive activity of the Polycomb Repressor Complex 2 (PRC2). During neuronal development, tri-methylation of H3K27 (H3K27me3) by PRC2 leads to genome wide transcriptional repression of genes that regulate cell fate transitions and neuronal arborization. Here we provide a review of neurodevelopmental processes and disorders associated with H3K36 and H3K4 histone methyltransferases, with emphasis on processes that contribute to brain size abnormalities. Additionally, we discuss how the counteracting activities of H3K36 and H3K4 modifying enzymes vs. PRC2 could contribute to brain size abnormalities which is an underexplored mechanism in relation to brain size control.

Epigenetic switch controls social actions

Mutations in epigenetic factors are associated with autism spectrum disorder (ASD). In this issue of Neuron, Yan et al. (2022) show that the antagonism of ASH1L and PRC2 switches the equilibrium of histone methylation at the ephrin receptor A7 locus, causing decreased EphA7 expression, deficits in synaptic pruning, and ASD-like behaviors.