Understanding the phenotypic spectrum of ASXL-related disease: Ten cases and a review of the literature

ASXL3-related disorder: Molecular phenotyping and comprehensive review providing insights into disease mechanism

ASXL3-related disorder, sometimes referred to as Bainbridge-Ropers syndrome, was first identified as a distinct neurodevelopmental disorder by Bainbridge et al. in 2013. Since then, there have been a number of case series and single case reports published worldwide. A comprehensive review of the literature was carried out. Abstracts were screened, relevant literature was analysed, and descriptions of common phenotypic features were quantified. ASXL3 variants were collated and categorised. Common phenotypic features comprised global developmental delay or intellectual disability (97%), feeding problems (76%), hypotonia (88%) and characteristic facial features (93%). The majority of genetic variants were de novo truncating variants in exon 11 or 12 of the ASXL3 gene. Several gaps in our knowledge of this disorder were identified, namely, underlying pathophysiology and disease mechanism, disease contribution of missense variants, relevance of variant location, prevalence and penetrance data. Clinical information is currently limited by patient numbers and lack of longitudinal data, which this review aims to address.

Identification of a de novo variant in the ASXL2 gene related to Shashi-Pena syndrome

BACKGROUND

ASXL2 encodes proteins involved in epigenetic regulation and the assembly of transcription factors at specific genomic loci. Germline de novo truncating variants in ASXL2 have been implicated in Shashi-Pena syndrome, which results in features of developmental delay (DD), glabellar nevus flammeus, hypotonia, and cardiac disorders. However, the variants are rare, and the clinical spectrum may be incomplete.

METHODS

The clinical data such as brain MRI were collect. The whole exome sequencing was performed for genetic etiology analysis.

RESULTS

Here, we report a patient with DD, hypotonia, early atrial septal defect, and abnormal white matter signal. She presented with Shashi-Pena syndrome with a truncated variant in ASXL2 (NM_018263.6, c.2142_2152del, p.Ser714Argfs*5). She died of a digestive tract infection when she was 1 year and 6 months old.

CONCLUSIONS

Our study further expanded the spectrum of phenotypes and genetic variations of the syndrome, and we believe that it is necessary to screen the ASXL2 gene in patients with DD and cardiac and bone disorders.

A newborn with a pathogenic variant in ASXL2 expanding the phenotype of SHAPNS: a case report and literature review

BACKGROUND

Shashi-Pena syndrome (SHAPNS) is a developmental disorder caused by mutations in additional sex combs-like Protein 2 (ASXL2). Since 2016, only 12 cases from 10 families have been reported. However, neonatal period characteristics remain largely unknown. Herein, we report a case with a pathogenic variant in ASXL2 in a newborn.

CASE DESCRIPTION

A newborn was diagnosed with a previously unreported de novo truncating mutation in ASXL2 (NM_018263.6) at 21 days and the clinical characteristics of all probands with ASXL2-related SHAPNS was reported in the literature. He had persistent hypoglycemia caused by inappropriate insulin levels and achieved stable glucose levels after octreotide treatment. Magnetic resonance imaging (MRI) revealed a small cerebellum, and fundoscopy showed bilateral retinal paving-stone-like white lesions. The results of trio-based whole exome sequencing (WES) were returned on the 21st day of life, and a heterozygous de novo truncating pathogenic c.1792C>T (p.Gln598*) variant in exon 11 of the ASXL2 gene was identified. The clinical features of our patient and another 10 probands with ASXL2-related SHAPNS reported in the literature were included in this review. More than half shared recognizable clinical features, including hypertelorism (11/11), broad nasal tip (10/11), arched eyebrows (9/11), a large V-shaped glabellar nevus flammeus on the forehead (9/11), low-set ears (8/11), posteriorly rotated ears (7/11), proptosis (6/11) and deep palm creases (6/11). Major clinical issues included feeding difficulties (10/11), developmental delay (10/11), skeletal and/or extremity abnormalities (8/11), progressive macrocephaly (8/11), hypotonia (8/11), hypoglycemia (6/11) and seizures (6/11). Neurodevelopmental regression was possible in patients (2/11) with normal MRI findings who later developed nonfebrile seizures.

CONCLUSIONS

We present a newborn diagnosing the SHAPNS by trio-WES, which is the earliest age of diagnosis. The application of octreotide for hypoglycemia, the small cerebellum and bilateral paving-stone-like white lesions of the retinas are described for the first time in an individual with ASXL2-related SHAPNS. Additional clinical reports of neonates with damaging ASXL2 variants are necessary to verify the mechanism and optimal treatment of ASXL2-related hypoglycemia, neurological damage and optic impairment. Neurological, endocrinological, ophthalmological, and rehabilitative follow-ups of these patients are necessary and important.

RINGs, DUBs and Abnormal Brain Growth-Histone H2A Ubiquitination in Brain Development and Disease

During mammalian neurodevelopment, signaling pathways converge upon transcription factors (TFs) to establish appropriate gene expression programmes leading to the production of distinct neural and glial cell types. This process is partially regulated by the dynamic modulation of chromatin states by epigenetic systems, including the polycomb group (PcG) family of co-repressors. PcG proteins form multi-subunit assemblies that sub-divide into distinct, yet functionally related families. Polycomb repressive complexes 1 and 2 (PRC1 and 2) modify the chemical properties of chromatin by covalently modifying histone tails via H2A ubiquitination (H2AK119ub1) and H3 methylation, respectively. In contrast to the PRCs, the Polycomb repressive deubiquitinase (PR-DUB) complex removes H2AK119ub1 from chromatin through the action of the C-terminal hydrolase BAP1. Genetic screening has identified several PcG mutations that are causally associated with a range of congenital neuropathologies associated with both localised and/or systemic growth abnormalities. As PRC1 and PR-DUB hold opposing functions to control H2AK119ub1 levels across the genome, it is plausible that such neurodevelopmental disorders arise through a common mechanism. In this review, we will focus on advancements regarding the composition and opposing molecular functions of mammalian PRC1 and PR-DUB, and explore how their dysfunction contributes to the emergence of neurodevelopmental disorders.

ASXL3 De Novo Variant-Related Neurodevelopmental Disorder Presenting as Dystonic Cerebral Palsy

ASXL3 loss-of-function variants represent a well-established cause of Bainbridge-Ropers syndrome, a syndromic neurodevelopmental disorder with intellectual and motor disabilities. Although a recent large-scale genomics-based study has suggested an association between ASXL3 variation and cerebral palsy, there have been no detailed case descriptions. We report, here, a female individual with a de novo pathogenic c.1210C > T, p.Gln404* nonsense variant in ASXL3, identified within the frame of an ongoing research project applying trio whole-exome sequencing to the diagnosis of dystonic cerebral palsy. The patient presented with a mixture of infantile-onset limb/trunk dystonic postures and secondarily evolving distal spastic contractures, in addition to more typical features of ASXL3-related diseases such as severe feeding issues, intellectual disability, speech impairment, and facial dysmorphic abnormalities. Our case study confirms a role for ASXL3 pathogenic variants in the etiology of cerebral-palsy phenotypes and indicates that dystonic features can be part of the clinical spectrum in Bainbridge-Ropers syndrome. ASXL3 should be added to target-gene lists used for molecular evaluation of cerebral palsy.

ASXL1/2 mutations and myeloid malignancies

Myeloid malignancies develop through the accumulation of genetic and epigenetic alterations that dysregulate hematopoietic stem cell (HSC) self-renewal, stimulate HSC proliferation and result in differentiation defects. The polycomb group (PcG) and trithorax group (TrxG) of epigenetic regulators act antagonistically to regulate the expression of genes key to stem cell functions. The genes encoding these proteins, and the proteins that interact with them or affect their occupancy at chromatin, are frequently mutated in myeloid malignancies. PcG and TrxG proteins are regulated by Enhancers of Trithorax and Polycomb (ETP) proteins. ASXL1 and ASXL2 are ETP proteins that assemble chromatin modification complexes and transcription factors. ASXL1 mutations frequently occur in myeloid malignancies and are associated with a poor prognosis, whereas ASXL2 mutations frequently occur in AML with t(8;21)/RUNX1-RUNX1T1 and less frequently in other subtypes of myeloid malignancies. Herein, we review the role of ASXL1 and ASXL2 in normal and malignant hematopoiesis by summarizing the findings of mouse model systems and discussing their underlying molecular mechanisms.

Prepubertal onset of type 2 diabetes in Shashi-Pena syndrome due to ASXL2 mutation

Type 2 diabetes remains rare in the pediatric population and the majority of cases occur during puberty. A combination of genetic and environmental factors leads to the development of insulin resistance and β-cell failure. An increased prevalence is recognized in a number of rare genetic disorders such as Alström and Bardet-Biedl syndromes. Recently, a rare neurodevelopmental disorder, Shashi-Pena syndrome due to the dominant negative effect of heterozygous mutations in additional Sex-Combs-Like Genes 2 (ASXL2) has been reported. ASXL2 null mice exhibit glucose intolerance, insulin resistance and lipodystrophy. The regulatory role of ASXL2 in glucose and lipid homeostasis occurs through its interaction with peroxisome proliferator-activated receptor gamma (PPARγ), a gene implicated in the pathogenesis of type 2 diabetes on genome-wide association studies. Thiazolidinediones, used for the treatment of type 2 diabetes, exert their effects as direct agonists of PPARγ. We report the first case of type 2 diabetes in Shashi-Pena syndrome, occurring in an 8-year-old prepubertal boy with no family history. In addition, the proband had dyslipidemia, and fatty infiltration of the liver with elevated transaminases. Mutation of ASXL2 in humans, through its interaction with PPARγ appears to cause a phenotype of insulin resistance, type 2 diabetes, and dyslipidemia. Further reported cases will assist in confirming this association.

DNA methylation signature associated with Bohring-Opitz syndrome: a new tool for functional classification of variants in ASXL genes

The additional sex combs-like (ASXL) gene family-encoded by ASXL1, ASXL2, and ASXL3-is crucial for mammalian development. Pathogenic variants in the ASXL gene family are associated with three phenotypically distinct neurodevelopmental syndromes. Our previous work has shown that syndromic conditions caused by pathogenic variants in epigenetic regulatory genes show consistent patterns of genome-wide DNA methylation (DNAm) alterations, i.e., DNAm signatures in peripheral blood. Given the role of ASXL1 in chromatin modification, we hypothesized that pathogenic ASXL1 variants underlying Bohring-Opitz syndrome (BOS) have a unique DNAm signature. We profiled whole-blood DNAm for 17 ASXL1 variants, and 35 sex- and age-matched typically developing individuals, using Illumina's Infinium EPIC array. We identified 763 differentially methylated CpG sites in individuals with BOS. Differentially methylated sites overlapped 323 unique genes, including HOXA5 and HOXB4, supporting the functional relevance of DNAm signatures. We used a machine-learning classification model based on the BOS DNAm signature to classify variants of uncertain significance in ASXL1, as well as pathogenic ASXL2 and ASXL3 variants. The DNAm profile of one individual with the ASXL2 variant was BOS-like, whereas the DNAm profiles of three individuals with ASXL3 variants were control-like. We also used Horvath's epigenetic clock, which showed acceleration in DNAm age in individuals with pathogenic ASXL1 variants, and the individual with the pathogenic ASXL2 variant, but not in individuals with ASXL3 variants. These studies enhance our understanding of the epigenetic dysregulation underpinning ASXL gene family-associated syndromes.

Identification of differential microRNAs and messenger RNAs resulting from ASXL transcriptional regulator 3 knockdown during during heart development

Congenital heart disease (CHD) is the most common birth defect. Although ASXL transcriptional regulator 3 (ASXL3) has been reported to cause hereditary CHD, ASXL3-mediated mechanisms in heart development remain unclear. In this study, we used dimethyl sulfoxide (DMSO) to induce differentiation in P19 cells, observed cell morphology using light microscopy after ASXL3 knockdown, and determined the levels of associated myocardial cell markers using reverse transcription-quantitative polymerase chain reaction and western blotting. Subsequently, we used microRNA sequencing, messenger RNA (mRNA) sequencing, and bioinformatics to initially identify the possible mechanisms through which ASXL3-related microRNAs and mRNAs affect heart development. The results indicated that DMSO induced P19 cell differentiation, which could be inhibited by ASXL3 knockdown. We screened 1214 and 1652 differentially expressed microRNAs and mRNAs, respectively, through ASXL3 knockdown and sequencing; these differentially expressed miRNAs were largely enriched in PI3K-Akt, mitogen-activated protein kinase, and Rap1 signaling pathways. Additionally, 11 miRNAs associated with heart development were selected through a literature review. Our analysis indicated the involvement of mmu-miR-323-3p in P19 cell differentiation through the PI3K-Akt pathway. In conclusion, ASXL3 may be involved in the regulation of heart development. This comprehensive study of differentially expressed microRNAs and mRNAs through ASXL3 knockdown in P19 cells provides new insights that may aid the prevention and treatment of CHD.

A de novo and novel nonsense variants in ASXL2 gene is associated with Shashi-Pena syndrome

This ASXL2 gene encodes a member of a family of epigenetic regulators that bind various histone-modifying enzymes and are involved in the assembly of transcription factors at specific genomic loci. Recent research has found that pathogenic variants in ASXL2 gene can lead to Shashi-Pena syndrome. However, clinical reports of individuals with damaging ASXL2 variants were limited and clinical phenotypic information may also be incomplete at present. Here, we reported a patient from Chinese family presenting with Shashi-Pena syndrome duo to a nonsense variant c.2485C  >  T; p. (Gln829*) in ASXL2 and analyzed the clinical phenotypes of the patient. In addition to the typical facial appearance, feeding difficulty, cardiac dysfunction and developmental delay, the patient also demonstrated multiple clinical problems not reported in other published cases, including granulocytopenia, thrombocytopenia and "simian line". Additionally, this is also the first case of premature death associated to Shashi-Pena syndrome induced by ASXL2 variants in a Chinese population. Our results provided important information for genetic counseling of the family and broaden the spectrum of phenotypes and genetic variations of the syndrome.

Oral findings and healthcare management in Shashi-Pena syndrome

Shashi Pena syndrome (SHAPNS) is a newly recognized and rare neurodevelopmental disorder with unique phenotypic features. The syndrome is caused by truncating variants in ASXL2 gene. To date, there are only seven documented cases in scientific literature, but oral and dental findings are sparsely reported. Here, we present a new case of SHAPNS and describe the oro-dental findings and oral healthcare management of affected patients.

A de novo Variant of ASXL1 Is Associated With an Atypical Phenotype of Bohring-Opitz Syndrome: Case Report and Literature Review

Bohring-Opitz syndrome (BOS) is a rare genetic disease first reported by Bohring et al. in 1999. With the recent development of exome sequencing (ES), de novo truncating mutations in the additional sex-combs-like 1 (ASXL1) gene have been causally implicated in BOS. Herein, we describe a 7-month-old girl with intrauterine growth restriction, severe pulmonary infection, seizures, and craniofacial abnormalities (microcephaly, micro/retrognathia, hypertelorism, depressed nasal bridge, low-set ears and hypertrichosis) at birth. At a later stage, the patient developed global developmental delay. We performed ES and identified a de novo heterozygous mutation in ASXL1, namely, c.1210C>T/p.R404^*. However, this case did not have trigonocephaly, facial hemangioma, prominent eyes, myopia, BOS posture, or brain abnormalities (enlarged subarachnoid spaces, agenesis of the corpus callosum, moderately enlarged cerebral ventricles, or prominent frontal subarachnoid spaces), which are common characteristics in most patients with BOS-harboring ASXL1 mutations. These new data expand the phenotype of BOS driven by ASXL1 and may assist in more accurately delineating the phenotypes caused by variants of this gene.