De novo truncating mutations in AHDC1 in individuals with syndromic expressive language delay, hypotonia, and sleep apnea

Novel Insights from Clinical Practice: Xia-Gibbs Syndrome with Pes Cavus, Conjunctival Melanosis, and Eye Asymmetry due to a de novo AHDC1 Gene Variant - A Case Report and a Brief Review of the Literature

Introduction

Xia-Gibbs syndrome (OMIM 615829) is a rare developmental disorder, caused by heterozygous de novo variants in the AHDC1 gene. Hallmark features include global developmental delay, facial dysmorphisms, and behavioral problems. To date, more than 250 individuals have been diagnosed worldwide.

Case Report

We report a 13-year-old female who, in association with typical features of Xia-Gibbs syndrome, presented with macrocrania, pes cavus, and conjunctival melanosis. Whole-exome sequencing identified a de novo frameshift variant, which had not been reported in the literature before.

Conclusion

We summarized the main clinical and phenotypic features of patients described in the literature, and in addition, we discuss another feature found in our patient and observed in other cases described, eye asymmetry, which has never been highlighted, and suggest that it could be part of the typical clinical presentation of this condition.

Ahdc1 is a potent regulator of obesity and energy metabolism

AT-hook DNA-binding motif-containing protein 1 (AHDC1) is a causal gene of intellectual disability/developmental delay in humans. The biological role of AHDC1 is unclear. Recently, some clues from AHDC1 mutation carriers hinted that AHDC1 may participate in body-weight regulation. In this first metabolic phenotype study of Ahdc1 deficiency, we generated a Ahdc1-deficienct mouse line and found that Ahdc1 deficiency in both male and female mice led to adiposity from weaning and obesity characterized by reduced energy expenditure and respiratory quotient, with progressive development of hyperleptinemia, insulin resistance, abnormal glycolipid metabolism, and fatty liver. Our findings show that Ahdc1 is a novel key regulator of obesity and energy metabolism, which provides new insight into the physiological mechanisms of obesity.NEW & NOTEWORTHY In this first metabolic phenotype study of Ahdc1 deficiency, we generated a survivable Ahdc1-deficient mouse line. We found that Ahdc1 deficiency in both male and female mice resulted in adiposity from weaning and obesity characterized by reduced energy expenditure and respiratory quotient. Additionally, there was a progressive development of hyperleptinemia, insulin resistance, abnormal glycolipid metabolism, and fatty liver. These findings demonstrate that Ahdc1 is a novel key regulator of obesity and energy metabolism.

Latex-Free Anesthesia for Craniosynostosis Surgery Associated With Xia-Gibbs Syndrome: A Case Report

This case report describes the anesthetic management of a one-year-old patient with Xia-Gibbs syndrome, which is a rare genetic condition caused by a mutation in the AHDC1 gene. The procedure involved calvarial vault remodeling and fronto-orbital advancement to correct a left coronal craniosynostosis. In addition, the patient had a history of seizures and latex-fruit syndrome, which necessitated careful preoperative planning and management. Despite the difficulties provided by the patient's cranial abnormalities and the paucity of literature on anesthetic experiences with the condition, the treatment was completed successfully and without complications. Insights are offered about the anesthetic approach for this syndromic pediatric patient undergoing neurosurgery with a high risk of bleeding. It is important to understand and prepare for the perioperative implications of this disease in order to achieve a safe outcome.

Generation of a human induced pluripotent stem cell line (FDCHi010-A) from a patient with Xia-Gibbs syndrome carrying AHDC1 mutation (c.2062C > T)

A human induced pluripotent stem cell line (iPSC), FDCHi010-A, was derived from the peripheral blood of a 3-year-old patient with the c.2062C > T (p.R688*) mutation in the AHDC1 gene. The established cell line displayed a typical human embryonic stem cell-like morphology, exhibited a normal euploid karyotype, and fully expressed pluripotency markers. In addition, it retained the ability to differentiate to three germ layers. This cell line with a specific mutation may provide a useful tool for studying the pathogenesis and drug therapy screening of Xia-Gibbs syndrome caused by the AHDC1 gene.

Establishment of iPSC lines and zebrafish with loss-of-function AHDC1 variants: models for Xia-Gibbs syndrome

Xia-Gibbs syndrome (XGS) is a syndromic form of intellectual disability caused by heterozygous AHDC1 variants, but the pathophysiological mechanisms underlying this syndrome are still unclear. In this manuscript, we describe the development of two different functional models: three induced pluripotent stem cell (iPSC) lines with different loss-of-function (LoF) AHDC1 variants, derived by reprogramming peripheral blood mononuclear cells from XGS patients, and a zebrafish strain with a LoF variant in the ortholog gene (ahdc1) obtained through CRISPR/Cas9-mediated editing. The three iPSC lines showed expression of pluripotency factors (SOX2, SSEA-4, OCT3/4, and NANOG). To verify the capacity of iPSC to differentiate into the three germ layers, we obtained embryoid bodies (EBs), induced their differentiation, and confirmed the mRNA expression of ectodermal, mesodermal, and endodermal markers using the TaqMan hPSC Scorecard. The iPSC lines were also approved for the following quality tests: chromosomal microarray analysis (CMA), mycoplasma testing, and short tandem repeat (STR) DNA profiling. The zebrafish model has an insertion of four base pairs in the ahdc1 gene, is fertile, and breeding between heterozygous and wild-type (WT) animals generated offspring in a genotypic proportion in agreement with Mendelian law. The established iPSC and zebrafish lines were deposited on the hpscreg.eu and zfin.org platforms, respectively. These biological models are the first for XGS and will be used in future studies that investigate the pathophysiology of this syndrome, unraveling its underlying molecular mechanisms.

Long read sequencing and expression studies of AHDC1 deletions in Xia-Gibbs syndrome reveal a novel genetic regulatory mechanism

Xia-Gibbs syndrome (XGS; MIM# 615829) is a rare mendelian disorder characterized by Development Delay (DD), intellectual disability (ID), and hypotonia. Individuals with XGS typically harbor de novo protein-truncating mutations in the AT-Hook DNA binding motif containing 1 (AHDC1) gene, although some missense mutations can also cause XGS. Large de novo heterozygous deletions that encompass the AHDC1 gene have also been ascribed as diagnostic for the disorder, without substantial evidence to support their pathogenicity. We analyzed 19 individuals with large contiguous deletions involving AHDC1, along with other genes. One individual bore the smallest known contiguous AHDC1 deletion (∼350 Kb), encompassing eight other genes within chr1p36.11 (Feline Gardner-Rasheed, IFI6, FAM76A, STX12, PPP1R8, THEMIS2, RPA2, SMPDL3B) and terminating within the first intron of AHDC1. The breakpoint junctions and phase of the deletion were identified using both short and long read sequencing (Oxford Nanopore). Quantification of RNA expression patterns in whole blood revealed that AHDC1 exhibited a mono-allelic expression pattern with no deficiency in overall AHDC1 expression levels, in contrast to the other deleted genes, which exhibited a 50% reduction in mRNA expression. These results suggest that AHDC1 expression in this individual is compensated by a novel regulatory mechanism and advances understanding of mutational and regulatory mechanisms in neurodevelopmental disorders.

A rigorous in silico genomic interrogation at 1p13.3 reveals 16 autosomal dominant candidate genes in syndromic neurodevelopmental disorders

Genome-wide chromosomal microarray is extensively used to detect copy number variations (CNVs), which can diagnose microdeletion and microduplication syndromes. These small unbalanced chromosomal structural rearrangements ranging from 1 kb to 10 Mb comprise up to 15% of human mutations leading to monogenic or contiguous genomic disorders. Albeit rare, CNVs at 1p13.3 cause a variety of neurodevelopmental disorders (NDDs) including development delay (DD), intellectual disability (ID), autism, epilepsy, and craniofacial anomalies (CFA). Most of the 1p13.3 CNV cases reported in the pre-microarray era encompassed a large number of genes and lacked the demarcating genomic coordinates, hampering the discovery of positional candidate genes within the boundaries. In this study, we present four subjects with 1p13.3 microdeletions displaying DD, ID, autism, epilepsy, and CFA. In silico comparative genomic mapping with three previously reported subjects with CNVs and 22 unreported DECIPHER CNV cases has resulted in the identification of four different sub-genomic loci harboring five positional candidate genes for DD, ID, and CFA at 1p13.3. Most of these genes have pathogenic variants reported, and their interacting genes are involved in NDDs. RT-qPCR in various human tissues revealed a high expression pattern in the brain and fetal brain, supporting their functional roles in NDDs. Interrogation of variant databases and interacting protein partners led to the identification of another set of 11 potential candidate genes, which might have been dysregulated by the position effect of these CNVs at 1p13.3. Our studies define 1p13.3 as a genomic region harboring 16 NDD candidate genes and underscore the critical roles of small CNVs in in silico comparative genomic mapping for disease gene discovery. Our candidate genes will help accelerate the isolation of pathogenic heterozygous variants from exome/genome sequencing (ES/GS) databases.

AT-hook DNA-binding motif-containing protein one knockdown downregulates EWS-FLI1 transcriptional activity in Ewing's sarcoma cells

Ewing's sarcoma is the second most common bone malignancy in children or young adults and is caused by an oncogenic transcription factor by a chromosomal translocation between the EWSR1 gene and the ETS transcription factor family. However, the transcriptional mechanism of EWS-ETS fusion proteins is still unclear. To identify the transcriptional complexes of EWS-ETS fusion transcription factors, we applied a proximal labeling system called BioID in Ewing's sarcoma cells. We identified AHDC1 as a proximal protein of EWS-ETS fusion proteins. AHDC1 knockdown showed a reduced cell growth and transcriptional activity of EWS-FLI1. AHDC1 knockdown also reduced BRD4 and BRG1 protein levels, both known as interacting proteins of EWS-FLI1. Our results suggest that AHDC1 supports cell growth through EWS-FLI1.

Novel frameshift mutation in the AHDC1 gene in a Chinese global developmental delay patient: A case report

BACKGROUND

Xia–Gibbs syndrome (XGS, OMIM: 615829), caused by mutations within the AT-Hook DNA-binding motif-containing protein 1 (AHDC1) gene (OMIM: 615790), located on the short arm of chromosome 1 within the cytogenetic band 1p36.11, contains five noncoding 5 exons, a single 4.9-kb coding exon, and a noncoding 3 exon.

CASE SUMMARY

In this case report, we diagnosed and treated a 6-mo-old girl with XGS. The primary clinical symptoms included global developmental delay, hypotonia, and mild dysmorphic features. Using high-throughput whole-exosome sequencing to sequence the patient and her parents, and the results showed a novel frameshift mutation of c.1155dupG (p.Arg386Alafs*3) in the AHDC1 gene. The paternal gene was wild type.

CONCLUSION

This report extends the mutation spectrum of the AHDC1 gene to provide the diagnostic basis for genetic counseling in families with XGS.

Complex Diagnostics of Non-Specific Intellectual Developmental Disorder

Intellectual development disorder (IDD) is characterized by a general deficit in intellectual and adaptive functioning. In recent years, there has been a growing interest in studying the genetic structure of IDD. Of particular difficulty are patients with non-specific IDD, for whom it is impossible to establish a clinical diagnosis without complex genetic diagnostics. We examined 198 patients with non-specific IDD from 171 families using whole-exome sequencing and chromosome microarray analysis. Hereditary forms of IDD account for at least 35.7% of non-specific IDD, of which 26.9% are monogenic forms. Variants in the genes associated with the BAF (SWI/SNF) complex were the most frequently identified. We were unable to identify phenotypic features that would allow differential diagnosis of monogenic and microstructural chromosomal rearrangements in non-specific IDD at the stage of clinical examination, but due to its higher efficiency, exome sequencing should be the diagnostic method of the highest priority study after the standard examination of patients with NIDD in Russia.

Genotype-phenotype spectrum and correlations in Xia-Gibbs syndrome: Report of five novel cases and literature review

Background

Xia‐Gibbs syndrome (XGS) is a rare neurodevelopmental disorder caused by pathogenic variants in the AT‐hook DNA‐binding motif‐containing 1 gene (AHDC1), encoding a protein with a crucial role in transcription and epigenetic regulation, axonogenesis, brain function, and neurodevelopment. AHDC1 variants possibly act through a dominant‐negative mechanism and may interfere with DNA repair processes, leading to genome instability and impaired DNA translesion repair. Variants affecting residues closer to the N‐terminal are thought to determine a milder phenotype with better cognitive performances. However, clean‐cut genotype–phenotype correlations are still lacking.

Cases

In this study, we investigated five subjects with XGS in whom exome sequencing led to the identification of five novel de novo pathogenic variants in AHDC1. All variants were extremely rare and predicted to cause a loss of protein function. The phenotype of the reported patients included developmental delay, hypotonia, and distinctive facial dysmorphisms. Additionally, uncommon clinical features were observed, including congenital hypothyroidism and peculiar skeletal abnormalities.

Conclusions

In this study, we report uncommon XGS features associated with five novel truncating variants in AHDC, thus expanding the genotype and phenotypic spectrum of this complex condition. We also compared our cases to previously reported cases, discussing the current status of genotype–phenotype correlations in XGS.

A Case of Ophthalmoplegia, Hypotonia, and Developmental Delay in the Setting of Corpus Callosum Hypoplasia

Anomalies of the corpus callosum, including complete agenesis, partial agenesis, and hypoplasia, are some of the most common brain malformations. Corpus callosum abnormalities are potentially syndromic, many of which have identifiable genetic etiologies. Patients affected with either syndromic or non-syndromic corpus callosum anomalies may also have associated ophthalmologic abnormalities. Some of the syndromes with corpus callosum malformations that also involve ophthalmologic findings include Aicardi syndrome, Mowat-Wilson syndrome, and Xia-Gibbs syndrome. This case report describes a patient with hypoplasia and possible dysgenesis of the corpus callosum noted on magnetic resonance imaging (MRI) who had several ophthalmologic findings, including ophthalmoplegia, strabismus, and nystagmus, associated with microcephaly, dysmorphic facial features, global developmental delay, hypotonia, and cryptorchidism. While several previously identified syndromes share similar clinical features with this patient, these findings may also represent an unidentified genetic syndrome, and the patient remains under evaluation for a genetic diagnosis. This report explores the differential for ophthalmologic abnormalities in the setting of corpus callosum hypoplasia.

Gibbin mesodermal regulation patterns epithelial development

Proper ectodermal patterning during human development requires previously identified transcription factors such as GATA3 and p63, as well as positional signalling from regional mesoderm1-6. However, the mechanism by which ectoderm and mesoderm factors act to stably pattern gene expression and lineage commitment remains unclear. Here we identify the protein Gibbin, encoded by the Xia-Gibbs AT-hook DNA-binding-motif-containing 1 (AHDC1) disease gene7-9, as a key regulator of early epithelial morphogenesis. We find that enhancer- or promoter-bound Gibbin interacts with dozens of sequence-specific zinc-finger transcription factors and methyl-CpG-binding proteins to regulate the expression of mesoderm genes. The loss of Gibbin causes an increase in DNA methylation at GATA3-dependent mesodermal genes, resulting in a loss of signalling between developing dermal and epidermal cell types. Notably, Gibbin-mutant human embryonic stem-cell-derived skin organoids lack dermal maturation, resulting in p63-expressing basal cells that possess defective keratinocyte stratification. In vivo chimeric CRISPR mouse mutants reveal a spectrum of Gibbin-dependent developmental patterning defects affecting craniofacial structure, abdominal wall closure and epidermal stratification that mirror patient phenotypes. Our results indicate that the patterning phenotypes seen in Xia-Gibbs and related syndromes derive from abnormal mesoderm maturation as a result of gene-specific DNA methylation decisions.

First reported cases with Xia-Gibbs syndrome from India harboring novel variants in AHDC1

We report here two girls from different Indian families identified with novel variants in the AT Hook DNA Binding Motif Containing 1 gene (AHDC1) causing Xia-Gibbs syndrome. The diagnosis was made by clinical exome in both cases. Inconsistent dysmorphic features such as dolichocephaly in the first patient and brachycephaly in the second were observed. Prominent jaw and gelastic seizures were other features of patient 1. Thus, this syndrome, with developmental delay, poor expressive language and overlapping clinical phenotype requires the utility of next generation sequencing for diagnostic confirmation.

Genetic investigation of syndromic forms of obesity

BACKGROUND

Syndromic obesity (SO) refers to obesity with additional phenotypes, including intellectual disability (ID)/developmental delay (DD), dysmorphic features, or organ-specific abnormalities. SO is rare, has high phenotypic variability, and frequently follows a monogenic pattern of inheritance. However, the genetic etiology of most cases of SO has not been elucidated.

SUBJECTS AND METHODS

In this study, we investigated 20 SO patients by whole-exome sequencing (WES) trios to identify causal genetic variants.

RESULTS

4/20 patients had negative results for array comparative genomic hybridization (aCGH) analyses. In the remaining 15 patients, in addition to SNVs and indels, CNVs were also evaluated. Pathogenic/likely pathogenic (P/LP) SNVs/indels were detected in 6/20 patients (involving MED13L, AHDC1, EHMT1, MYT1L, GRIA3, and SETD1A), while two patients carried an inherited VUS. In addition, P/LP CNVs were observed in 3/15 patients (involving SATG2, KIAA0442, and MEIS2).

CONCLUSIONS

All nine detected P/LP variants involved genes already known to lead to syndromic ID/DD; however, for only two genes (EHMT1 and MYT1L) is the link with obesity well established. This is the first study applying a comprehensive genomic investigation of an SO cohort, showing a high diagnostic yield (~47%). Additionally, our findings suggested that several known ID/DD genes may also predispose individuals to SO.

AHDC1 missense mutations in Xia-Gibbs syndrome

Xia-Gibbs syndrome (XGS; MIM: 615829) is a phenotypically heterogeneous neurodevelopmental disorder (NDD) caused by newly arising mutations in the AT-Hook DNA-Binding Motif-Containing 1 (AHDC1) gene that are predicted to lead to truncated AHDC1 protein synthesis. More than 270 individuals have been diagnosed with XGS worldwide. Despite the absence of an independent assay for AHDC1 protein function to corroborate potential functional consequences of rare variant genetic findings, there are also reports of individuals with XGS-like trait manifestations who have de novo missense AHDC1 mutations and who have been provided a molecular diagnosis of the disorder. To investigate a potential contribution of missense mutations to XGS, we mapped the missense mutations from 10 such individuals to the AHDC1 conserved protein domain structure and detailed the observed phenotypes. Five newly identified individuals were ascertained from a local XGS Registry, and an additional five were taken from external reports or databases, including one publication. Where clinical data were available, individuals with missense mutations all displayed phenotypes consistent with those observed in individuals with AHDC1 truncating mutations, including delayed motor milestones, intellectual disability (ID), hypotonia, and speech delay. A subset of the 10 reported missense mutations cluster in two regions of the AHDC1 protein with known conserved domains, likely representing functional motifs. Variants outside the clustered regions score lower for computational prediction of their likely damaging effects. Overall, de novo missense variants in AHDC1 are likely diagnostic of XGS when in silico analysis of their position relative to conserved regions is considered together with disease trait manifestations.

Phenotypic heterogeneity and mosaicism in Xia-Gibbs syndrome: Five Danish patients with novel variants in AHDC1

Xia-Gibbs syndrome (XGS) is a neurodevelopmental disorder characterized by intellectual disability, developmental delay, seizures, hypotonia, obstructive sleep apnoea and mild facial dysmorphism. Heterozygosity for loss-of-function variants in AHDC1, encoding the AT-hook DNA binding motif containing protein 1, were discovered in 2014 as the likely genetic cause of Xia-Gibbs syndrome. We present five patients with Xia-Gibbs syndrome caused by previously unreported variants in AHDC1. Two of the patients share a frameshift variant: c.2849del (p.(Pro950Argfs*192)) in AHDC1. Despite sharing this variant, the two patients show remarkable phenotypic differences underscoring the clinical heterogeneity of Xia-Gibbs syndrome. In addition, we present a case of Xia-Gibbs syndrome caused by mosaicism for an AHDC1 variant.

Focusing on Autism Spectrum Disorder in Xia-Gibbs Syndrome: Description of a Female with High Functioning Autism and Literature Review

Background: Xia–Gibbs syndrome (XGS) is a rare disorder caused by de novo mutations in the AT-Hook DNA binding motif Containing 1 (AHDC1) gene, which is characterised by a wide spectrum of clinical manifestations, including global developmental delay, intellectual disability, structural abnormalities of the brain, global hypotonia, feeding problems, sleep difficulties and apnoea, facial dysmorphisms, and short stature. Methods: Here, we report on a girl patient who shows a peculiar cognitive and behavioural profile including high-functioning autism spectrum disorder (ASD) without intellectual disability and provide information on her developmental trajectory with the aim of expanding knowledge of the XGS clinical spectrum. On the basis of the current clinical case and the literature review, we also attempt to deepen understanding of behavioural and psychiatric manifestations associated with XGS. Results: In addition to the patient we described, a considerable rate of individuals with XGS display autistic symptoms or have been diagnosed with an autistic spectrum disorder. Moreover, the analysis of the few psychopathological profiles of patients with XGS described in the literature shows a frequent presence of aggressive and self-injurious behaviours that could be either an expression of autistic functioning or an additional symptom of the ASD evolution. A careful investigation of the abovementioned symptoms is therefore required, since they could represent a “red flag” for ASD.

Phenotypic and protein localization heterogeneity associated with AHDC1 pathogenic protein-truncating alleles in Xia-Gibbs syndrome

Xia-Gibbs syndrome (XGS) is a rare Mendelian disease typically caused by de novo stop-gain or frameshift mutations in the AT-hook DNA binding motif containing 1 (AHDC1) gene. Patients usually present in early infancy with hypotonia and developmental delay and later exhibit intellectual disability (ID). The overall presentation is variable, however, and the emerging clinical picture is still evolving. A detailed phenotypic analysis of 34 XGS individuals revealed five core phenotypes (delayed motor milestones, speech delay, low muscle tone, ID, and hypotonia) in more than 80% of individuals and an additional 12 features that occurred more variably. Seizures and scoliosis were more frequently associated with truncations that arise before the midpoint of the protein although the occurrence of most features could not be predicted by the mutation position. Transient expression of wild type and different patient truncated AHDC1 protein forms in human cell lines revealed abnormal patterns of nuclear localization including a diffuse distribution of a short truncated form and nucleolar aggregation in mid-protein truncated forms. Overall, both the occurrence of variable phenotypes and the different distribution of the expressed protein reflect the heterogeneity of this syndrome.

Atypical aplasia cutis in association with Xia Gibbs syndrome

Xia Gibbs syndrome is a genetic disorder first defined in 2014 characterized by hypotonia, intellectual disability, global developmental delay, and dysmorphic facial features. While many additional features may be present, there are few reports of dermatologic findings. We report a case of atypical aplasia cutis in a female infant who was found to have Xia Gibbs syndrome. This case highlights consideration of cutaneous manifestations of Xia Gibbs syndrome which may aid in diagnosis.

Clinical presentation and evolution of Xia-Gibbs syndrome due to p.Gly375ArgfsTer3 variant in a patient from DR Congo (Central Africa)

Xia-Gibbs syndrome (XGS) is a very rare genetic condition. The clinical spectrum is very broad and variable. The phenotype and evolution in a Congolese boy with XGS have been reported. At 6 years he had speech delay, drooling, marked hyperactivity, attention deficit, aggressive behavior, and intellectual disability. Dysmorphological evaluation revealed strabismus, mild unilateral ptosis, uplifted ear lobes, flat philtrum, thin upper lip vermillion, high arched palate, and flat feet. Patient-only whole exome sequencing identified a known pathogenic frameshift variant in the AHDC1 gene [NM_001029882.3(AHDC1):c.1122dupC;(p.Gly375ArgfsTer3)]. The clinical follow-up revealed the deterioration of his fine motor skills and significant cerebellar phenotype including tremor, pes cavus, and gait instability at the age of 12 years. This patient was compared with three previously reported patients with the same variant but did not identify a consistent pattern in the evolution of symptoms with age.

Xia-Gibbs Syndrome: A Review of Literature

Xia-Gibbs syndrome (XGS) is a rare genetic disorder that has been discovered as a distinct clinical entity in the recent past. The occurrence has been attributed to the mutation of AT Hook DNA binding motif Containing 1 (AHDC1) gene that is carried on chromosome 1p36. The concerned gene participates in deoxyribonucleic acid (DNA) repair apart from other crucial functions. The mutation results in dysfunction that leads to neurodevelopmental delay. The spectrum of manifestations constitutes intellectual disabilities, hypotonia, expressive language delay, sleep difficulties, and short stature. Dysmorphic facial features include depressed nasal bridge, hypertelorism, down-slanting or up-slanting palpebral fissures, horizontal eyebrows, dysplastic dentition, thin upper lip vermilion, and micrognathia. The phenotype is still expanding. The condition may range from mild to severe dysfunction depending on the area and site of genetic aberration but variation is evident. Thus, the correlation between genotype and phenotype is largely unclear. XGS should be considered as a differential diagnosis for patients presenting with intellectual as well as developmental disabilities. Whole-exome sequencing (WES) is the genetic test that is largely used for the confirmation of diagnosis. Less is known about the natural history as only a few adults with XGS have been documented in the literature. Age-appropriate cancer screening is recommended for patients with XGS as the gene mutation alters DNA repair mechanisms that may trigger tumour formation. The management of patients diagnosed with XGS is an area that needs investigation. Though use of growth hormone replacement therapy and physiotherapy intervention have been reported as effective in previous studies, research on effective means of care of these patients is warranted on a larger number of patients. We present a review of current literature on what is known about XGS that would facilitate to identify knowledge gaps for paving a way for further studies. This, in turn, will help in provision of early and effective rehabilitation services for patients with XGS.

Xia-Gibbs Syndrome: A Rare Case Report of a Male Child and Insight into Physiotherapy Management

Xia-Gibbs syndrome (XGS) is a recently discovered genetic disorder. It is characterized by global developmental delay, intellectual impairment, hypotonia, and sleep abnormalities. While the current literature emphasizes on the genotype and phenotype of this rare condition, it does not provide any description of the physiotherapy management of patients with XGS. We report a case of a 27-month-old Indian male diagnosed with XGS, who presented with difficulty in sitting without support. He had dysmorphic facies, hypotonia, hyperextensible joints, mild kyphoscoliosis, and global developmental delay. His parents and an elder female sibling were clinically asymptomatic. The physiotherapy intervention was based on the principles of neurodevelopmental treatment (NDT) and sensory integration (SI). The management included facilitation of transitions, weight-bearing exercises, wheelbarrow walking, joint compressions, rib cage mobilization, multidirectional reaching, and pushing-pulling activities along with the use of equipment like Swiss ball, balance board, stability disc, trampoline, swing system, walker (rollator), and walking harness. Also, stabilizing pressure input orthosis (SPIO) for the trunk and ankle-foot orthosis (AFO) followed by supramalleolar orthosis (SMO) were used for support. Thereafter, the child was able to stand and walk without support at the age of 36 months, and walk on uneven terrain at the age of 42 months. In addition, he could negotiate stairs using handrails with mild assistance. His gross motor function measure-88 (GMFM-88) total score improved from 21% at the presentation to 66.6% following the treatment. It was observed that the NDT and SI approaches along with the use of appropriate orthoses accelerated the achievement of motor milestones in this case. To the best of our knowledge, this is the first case report of a child with XGS that emphasizes on the course of physiotherapy management for the associated motor delay.

Novel AHDC1 Gene Mutation in a Brazilian Individual: Implications of Xia-Gibbs Syndrome

Xia-Gibbs syndrome (XGS) is a rare neurological disorder characterized by global developmental delay, hypotonia, intellectual disability, seizures, and sleep apnea. XGS is defined by monoallelic pathogenic variants in AHDC1. In this study, we identified a Brazilian patient carrying a likely de novo AHDC1 nonsense mutation (c.451C>T; p.Arg151*) which was absent in both parents. All disease-causative variants already associated with XGS have been reviewed and the mutation described here corresponds to the closest one to the N-terminal region. Our findings were discussed based on the suggested genotype-phenotype correlation of the disease.

Rare Mutations in AHDC1 in Patients with Obstructive Sleep Apnea

Objectives

Obstructive sleep apnea (OSA) is a common disorder influenced by genetic and environmental factors. Mutations of AT-hook DNA-binding motif containing 1 (AHDC1) gene have been implicated which could cause rare syndromes presenting OSA. This study aims to investigate some rare mutations of AHDC1 in Chinese Han individuals with OSA.

Patients and Methods

Three hundred and seventy-five patients with OSA and one hundred and nine control individuals underwent polysomnography. A targeted sequencing experiment was taken in 100 patients with moderate-to-severe OSA, and genotyping was taken in 157 moderate-to-severe OSA and 100 control individuals. The effect of mutations was validated by the luciferase reporter assay.

Results

One rare missense mutation (AHDC1: p.G1484D) and two mutations (c.-88C>T; c.-781C>G) in 5′-untranslated region (UTR) of AHDC1 were identified. The rare mutation (c.-781C>G) in 5′-UTR that was identified in several patients presenting more severe clinical manifestations affects expression of AHDC1. Conclusions. Our results revealed three rare mutations of AHDC1 in patients with OSA in Chinese Hanindividuals.

Xia-Gibbs syndrome in adulthood: a case report with insight into the natural history of the condition

A 55-yr-old male with severe intellectual disability, behavioral problems, kyphoscoliosis, and dysmorphic features was referred for a genetic evaluation. Chromosomal microarray, RASopathy gene panel, mitochondrial sequencing, and fragile X testing were all negative. Subsequent whole-exome sequencing revealed a heterozygous, truncating variant in the AHDC1 gene, consistent with a diagnosis of Xia-Gibbs syndrome (XGS). Review of his clinical history showed many classic dysmorphic and clinical features of XGS, but no major health issues in adulthood other than intellectual disability. This individual is the oldest published XGS case to date, demonstrates the wide phenotypic spectrum of the disorder, and provides information on the condition's natural history. As more adults undergo genomic studies, we will continue to learn about the adult phenotypes of genetic conditions typically diagnosed in the pediatric setting.

Exome-Based Rare-Variant Analyses in CKD

BACKGROUND

Studies have identified many common genetic associations that influence renal function and all-cause CKD, but these explain only a small fraction of variance in these traits. The contribution of rare variants has not been systematically examined.

METHODS

We performed exome sequencing of 3150 individuals, who collectively encompassed diverse CKD subtypes, and 9563 controls. To detect causal genes and evaluate the contribution of rare variants we used collapsing analysis, in which we compared the proportion of cases and controls carrying rare variants per gene.

RESULTS

The analyses captured five established monogenic causes of CKD: variants in PKD1, PKD2, and COL4A5 achieved study-wide significance, and we observed suggestive case enrichment for COL4A4 and COL4A3. Beyond known disease-associated genes, collapsing analyses incorporating regional variant intolerance identified suggestive dominant signals in CPT2 and several other candidate genes. Biallelic mutations in CPT2 cause carnitine palmitoyltransferase II deficiency, sometimes associated with rhabdomyolysis and acute renal injury. Genetic modifier analysis among cases with APOL1 risk genotypes identified a suggestive signal in AHDC1, implicated in Xia-Gibbs syndrome, which involves intellectual disability and other features. On the basis of the observed distribution of rare variants, we estimate that a two- to three-fold larger cohort would provide 80% power to implicate new genes for all-cause CKD.

CONCLUSIONS

This study demonstrates that rare-variant collapsing analyses can validate known genes and identify candidate genes and modifiers for kidney disease. In so doing, these findings provide a motivation for larger-scale investigation of rare-variant risk contributions across major clinical CKD categories.

Syndromic Intellectual Disability Caused by a Novel Truncating Variant in AHDC1: A Case Report

Mutations in the AHDC1 gene are associated with the Xia-Gibbs syndrome (XGS), a sporadic genetic disorder characterised by developmental delay, intellectual disability, hypotonia, obstructive sleep apnoea, dysmorphic facial features, and cerebral malformations with plagiocephaly. Here we report the case of a 13-year-old Colombian female patient with a history of developmental delay, speech delay, sleep disturbances, and dysmorphic craniofacial features. The whole exome sequencing (WES) test revealed a novel de novo heterozygous frameshift mutation in AHDC1. The present case report describes the second case of mutations in AHDC1 in a Latin American patient. A literature review showed that the clinical features were similar in all reported patients. The WES test enabled the identification of the causality of this disorder characterised by high clinical and genetic heterogeneity.

Two Chinese Xia-Gibbs syndrome patients with partial growth hormone deficiency

Background

Heterozygous mutations in the AT‐hook DNA‐binding motif containing one (AHDC1, OMIM * 615790) gene cause an autosomal dominant multisystem developmental disorder known as Xia‐Gibbs syndrome (OMIM #615829). Xia‐Gibbs syndrome typically presented with global developmental delay, hypotonia, obstructive sleep apnea, seizures, delayed myelination, micrognathia, and other mild dysmorphic features.

Methods

Description of the clinical materials of two Chinese boys who were diagnosed with Xia‐Gibbs syndrome based on clinical presentations and next generation sequencing. Review of clinical features and AHDC1 mutations in previously reported Xia‐Gibbs syndrome patients together with our two new patients.

Results

The Xia‐Gibbs syndrome patients exhibited short stature, hypotonia, global developmental delay, speech delay, simian crease, and mild dysmorphic features. Next generation sequencing revealed de novo heterozygous variants in AHDC1 gene. In addition, laboratory test revealed partial growth hormone deficiency. Both patients underwent growth hormone replacement therapy for 24 and 9 months, respectively, and exhibited good response to the treatment.

Conclusion

This is the first report of Xia‐Gibbs syndrome patients to be treated with growth hormone. Review of previously reported Xia‐Gibbs syndrome patient indicated that short stature is a frequent feature of this condition, but its underlying cause needs to be further investigated.

Heterozygous Truncating Variants in POMP Escape Nonsense-Mediated Decay and Cause a Unique Immune Dysregulatory Syndrome

The proteasome processes proteins to facilitate immune recognition and host defense. When inherently defective, it can lead to aberrant immunity resulting in a dysregulated response that can cause autoimmunity and/or autoinflammation. Biallelic or digenic loss-of-function variants in some of the proteasome subunits have been described as causing a primary immunodeficiency disease that manifests as a severe dysregulatory syndrome: chronic atypical neutrophilic dermatosis with lipodystrophy and elevated temperature (CANDLE). Proteasome maturation protein (POMP) is a chaperone for proteasome assembly and is critical for the incorporation of catalytic subunits into the proteasome. Here, we characterize and describe POMP-related autoinflammation and immune dysregulation disease (PRAID) discovered in two unrelated individuals with a unique constellation of early-onset combined immunodeficiency, inflammatory neutrophilic dermatosis, and autoimmunity. We also begin to delineate a complex genetic mechanism whereby de novo heterozygous frameshift variants in the penultimate exon of POMP escape nonsense-mediated mRNA decay (NMD) and result in a truncated protein that perturbs proteasome assembly by a dominant-negative mechanism. To our knowledge, this mechanism has not been reported in any primary immunodeficiencies, autoinflammatory syndromes, or autoimmune diseases. Here, we define a unique hypo- and hyper-immune phenotype and report an immune dysregulation syndrome caused by frameshift mutations that escape NMD.

Heterozygous Truncating Variants in POMP Escape Nonsense-Mediated Decay and Cause a Unique Immune Dysregulatory Syndrome

The proteasome processes proteins to facilitate immune recognition and host defense. When inherently defective, it can lead to aberrant immunity resulting in a dysregulated response that can cause autoimmunity and/or autoinflammation. Biallelic or digenic loss-of-function variants in some of the proteasome subunits have been described as causing a primary immunodeficiency disease that manifests as a severe dysregulatory syndrome: chronic atypical neutrophilic dermatosis with lipodystrophy and elevated temperature (CANDLE). Proteasome maturation protein (POMP) is a chaperone for proteasome assembly and is critical for the incorporation of catalytic subunits into the proteasome. Here, we characterize and describe POMP-related autoinflammation and immune dysregulation disease (PRAID) discovered in two unrelated individuals with a unique constellation of early-onset combined immunodeficiency, inflammatory neutrophilic dermatosis, and autoimmunity. We also begin to delineate a complex genetic mechanism whereby de novo heterozygous frameshift variants in the penultimate exon of POMP escape nonsense-mediated mRNA decay (NMD) and result in a truncated protein that perturbs proteasome assembly by a dominant-negative mechanism. To our knowledge, this mechanism has not been reported in any primary immunodeficiencies, autoinflammatory syndromes, or autoimmune diseases. Here, we define a unique hypo- and hyper-immune phenotype and report an immune dysregulation syndrome caused by frameshift mutations that escape NMD.

The phenotypic spectrum of Xia-Gibbs syndrome

Xia-Gibbs syndrome (XGS: OMIM # 615829) results from de novo truncating mutations within the AT-Hook DNA Binding Motif Containing 1 gene (AHDC1). To further define the phenotypic and molecular spectrum of this disorder, we established an XGS Registry and recruited patients from a worldwide pool of approximately 60 probands. Additional de novo truncating mutations were observed among 25 individuals, extending both the known number of mutation sites and the range of positions within the coding region that were sensitive to alteration. Detailed phenotypic examination of 20 of these patients via clinical records review and data collection from additional surveys showed a wider age range than previously described. Data from developmental milestones showed evidence for delayed speech and that males were more severely affected. Neuroimaging from six available patients showed an associated thinning of the corpus callosum and posterior fossa cysts. An increased risk of both scoliosis and seizures relative to the population burden was also observed. Data from a modified autism screening tool revealed that XGS shares significant overlap with autism spectrum disorders. These details of the phenotypic heterogeneity of XGS implicate specific genotype/phenotype correlations and suggest potential clinical management guidelines.

Clinical Application of Genome and Exome Sequencing as a Diagnostic Tool for Pediatric Patients: a Scoping Review of the Literature

PURPOSE

Availability of clinical genomic sequencing (CGS) has generated questions about the value of genome and exome sequencing as a diagnostic tool. Analysis of reported CGS application can inform uptake and direct further research. This scoping literature review aims to synthesize evidence on the clinical and economic impact of CGS.

METHODS

PubMed, Embase, and Cochrane were searched for peer-reviewed articles published between 2009 and 2017 on diagnostic CGS for infant and pediatric patients. Articles were classified according to sample size and whether economic evaluation was a primary research objective. Data on patient characteristics, clinical setting, and outcomes were extracted and narratively synthesized.

RESULTS

Of 171 included articles, 131 were case reports, 40 were aggregate analyses, and 4 had a primary economic evaluation aim. Diagnostic yield was the only consistently reported outcome. Median diagnostic yield in aggregate analyses was 33.2% but varied by broad clinical categories and test type.

CONCLUSION

Reported CGS use has rapidly increased and spans diverse clinical settings and patient phenotypes. Economic evaluations support the cost-saving potential of diagnostic CGS. Multidisciplinary implementation research, including more robust outcome measurement and economic evaluation, is needed to demonstrate clinical utility and cost-effectiveness of CGS.

Phenotypic expansion illuminates multilocus pathogenic variation

Purpose:

Multilocus variation, pathogenic variants in two or more disease genes, can potentially explain the underlying genetic basis for apparent phenotypic expansion in cases for which the observed clinical features extend beyond those reported in association with a “known” disease gene.

Methods:

Analyses focused on 106 patients, 19 for which apparent phenotypic expansion was previously attributed to variation at known disease genes. We performed a retrospective computational re-analysis of whole exome sequencing data using stringent Variant Call File filtering criteria to determine whether molecular diagnoses involving additional disease loci might explain the observed expanded phenotypes.

Results:

Multilocus variation was identified in 31.6% (6/19) of families with phenotypic expansion and 2.3% (2/87) without phenotypic expansion. Intrafamilial clinical variability within 2 families was explained by multilocus variation identified in the more severely affected sibling.

Conclusions:

Our findings underscore the role of multiple rare variants at different loci in the etiology of genetically and clinically heterogeneous cohorts. Intrafamilial phenotypic and genotypic variability allowed a dissection of genotype-phenotype relationships in 2 families. Our data emphasize the critical role of the clinician in diagnostic genomic analyses and demonstrate that apparent phenotypic expansion may represent blended phenotypes resulting from pathogenic variation at more than one locus.

Historical and Clinical Perspectives on Chromosomal Translocations

Chromosomal translocations, rearrangements involving the exchange of segments between chromosomes, were documented in humans in 1959. The first accurately reported clinical phenotype resulting from a translocation was that of Down syndrome. In a small percentage of Down syndrome cases, an extra 21q is provided by a Robertsonian translocation chromosome, either occurring de novo or inherited from a phenotypically normal parent with the translocation chromosome and a balanced genome of 45 chromosomes. Balanced translocations, including both Robertsonian and reciprocal translocations, are typically benign, but meiosis in germ cells with balanced translocations may result in meiotic arrest and subsequent infertility, or in unbalanced gametes, with attendant risks of miscarriage and unbalanced progeny. Most reciprocal translocations are unique. A few to several percent of translocations disrupt haploinsufficient genes or their regulatory regions and result in clinical phenotypes. Balanced translocations from patients with clinical phenotypes have been valuable in mapping disease genes and in illuminating cis-regulatory regions. Mapping of discordant mate pairs from long-insert, low-pass genome sequencing now permits efficient and cost-effective discovery and nucleotide-level resolution of rearrangement breakpoints, information that is absolutely necessary for interpreting the etiology of clinical phenotypes in patients with rearrangements. Pathogenic translocations and other balanced chromosomal rearrangements constitute a class of typically highly penetrant mutation that is cryptic to both clinical microarray and exome sequencing. A significant proportion of rearrangements include additional complexity that is not visible by conventional karyotype analysis. Some proportion of patients with negative findings on exome/genome sequencing and clinical microarray will be found to have etiologic balanced rearrangements only discoverable by genome sequencing with analysis pipelines optimized to recover rearrangement breakpoints.

Whole-Exome Sequencing Identifies a de novo AHDC1 Mutation in a Colombian Patient with Xia-Gibbs Syndrome

Xia-Gibbs syndrome is an autosomal dominant multisystem developmental disorder characterized by global developmental delay, hypotonia, obstructive sleep apnea, seizures, retrocerebellar cysts, delayed myelination, micrognathia, and mild dysmorphic features. Using whole-exome sequencing, we identified a de novo AHDC1 frameshift mutation c.2030_2030delG (p.G677Afs*52) in a Colombian patient, which was absent in both parents. Furthermore, we summarized the phenotypes of patients reported in the literature.

Diagnostic value of exome and whole genome sequencing in craniosynostosis

Background

Craniosynostosis, the premature fusion of one or more cranial sutures, occurs in ∼1 in 2250 births, either in isolation or as part of a syndrome. Mutations in at least 57 genes have been associated with craniosynostosis, but only a minority of these are included in routine laboratory genetic testing.

Methods

We used exome or whole genome sequencing to seek a genetic cause in a cohort of 40 subjects with craniosynostosis, selected by clinical or molecular geneticists as being high-priority cases, and in whom prior clinically driven genetic testing had been negative.

Results

We identified likely associated mutations in 15 patients (37.5%), involving 14 different genes. All genes were mutated in single families, except for IL11RA (two families). We classified the other positive diagnoses as follows: commonly mutated craniosynostosis genes with atypical presentation (EFNB1, TWIST1); other core craniosynostosis genes (CDC45, MSX2, ZIC1); genes for which mutations are only rarely associated with craniosynostosis (FBN1, HUWE1, KRAS, STAT3); and known disease genes for which a causal relationship with craniosynostosis is currently unknown (AHDC1, NTRK2). In two further families, likely novel disease genes are currently undergoing functional validation. In 5 of the 15 positive cases, the (previously unanticipated) molecular diagnosis had immediate, actionable consequences for either genetic or medical management (mutations in EFNB1, FBN1, KRAS, NTRK2, STAT3).

Conclusions

This substantial genetic heterogeneity, and the multiple actionable mutations identified, emphasises the benefits of exome/whole genome sequencing to identify causal mutations in craniosynostosis cases for which routine clinical testing has yielded negative results.

The genomic landscape of balanced cytogenetic abnormalities associated with human congenital anomalies

Despite the clinical significance of balanced chromosomal abnormalities (BCAs), their characterization has largely been restricted to cytogenetic resolution. We explored the landscape of BCAs at nucleotide resolution in 273 subjects with a spectrum of congenital anomalies. Whole-genome sequencing revised 93% of karyotypes and demonstrated complexity that was cryptic to karyotyping in 21% of BCAs, highlighting the limitations of conventional cytogenetic approaches. At least 33.9% of BCAs resulted in gene disruption that likely contributed to the developmental phenotype, 5.2% were associated with pathogenic genomic imbalances, and 7.3% disrupted topologically associated domains (TADs) encompassing known syndromic loci. Remarkably, BCA breakpoints in eight subjects altered a single TAD encompassing MEF2C, a known driver of 5q14.3 microdeletion syndrome, resulting in decreased MEF2C expression. We propose that sequence-level resolution dramatically improves prediction of clinical outcomes for balanced rearrangements and provides insight into new pathogenic mechanisms, such as altered regulation due to changes in chromosome topology.

The Spectrum of Clinical Utilities in Molecular Pathology Testing Procedures for Inherited Conditions and Cancer: A Report of the Association for Molecular Pathology

Clinical utility describes the benefits of each laboratory test for that patient. Many stakeholders have adopted narrow definitions for the clinical utility of molecular testing as applied to targeted pharmacotherapy in oncology, regardless of the population tested or the purpose of the testing. This definition does not address all of the important applications of molecular diagnostic testing. Definitions consistent with a patient-centered approach emphasize and recognize that a clinical test result's utility depends on the context in which it is used and are particularly relevant to molecular diagnostic testing because of the nature of the information they provide. Debates surrounding levels and types of evidence needed to properly evaluate the clinical value of molecular diagnostics are increasingly important because the growing body of knowledge, stemming from the increase of genomic medicine, provides many new opportunities for molecular testing to improve health care. We address the challenges in defining the clinical utility of molecular diagnostics for inherited diseases or cancer and provide assessment recommendations. Starting with a modified analytic validity, clinical validity, clinical utility, and ethical, legal, and social implications model for addressing clinical utility of molecular diagnostics with a variety of testing purposes, we recommend promotion of patient-centered definitions of clinical utility that appropriately recognize the valuable contribution of molecular diagnostic testing to improve patient care.

De novo truncating variants in the AHDC1 gene encoding the AT-hook DNA-binding motif-containing protein 1 are associated with intellectual disability and developmental delay

Whole-exome sequencing (WES) represents a significant breakthrough in clinical genetics, and identifies a genetic etiology in up to 30% of cases of intellectual disability (ID). Using WES, we identified seven unrelated patients with a similar clinical phenotype of severe intellectual disability or neurodevelopmental delay who were all heterozygous for de novo truncating variants in the AT-hook DNA-binding motif–containing protein 1 (AHDC1). The patients were all minimally verbal or nonverbal and had variable neurological problems including spastic quadriplegia, ataxia, nystagmus, seizures, autism, and self-injurious behaviors. Additional common clinical features include dysmorphic facial features and feeding difficulties associated with failure to thrive and short stature. The AHDC1 gene has only one coding exon, and the protein contains conserved regions including AT-hook motifs and a PDZ binding domain. We postulate that all seven variants detected in these patients result in a truncated protein missing critical functional domains, disrupting interactions with other proteins important for brain development. Our study demonstrates that truncating variants in AHDC1 are associated with ID and are primarily associated with a neurodevelopmental phenotype.

Molecular subtyping and improved treatment of neurodevelopmental disease

The next-generation sequencing revolution has substantially increased our understanding of the mutated genes that underlie complex neurodevelopmental disease. Exome sequencing has enabled us to estimate the number of genes involved in the etiology of neurodevelopmental disease, whereas targeted sequencing approaches have provided the means for quick and cost-effective sequencing of thousands of patient samples to assess the significance of individual genes. By leveraging such technologies and clinical exome sequencing, a genotype-first approach has emerged in which patients with a common genotype are first identified and then clinically reassessed as a group. This approach has proven a powerful methodology for refining disease subtypes. We propose that the molecular characterization of these genetic subtypes has important implications for diagnostics and also for future drug development. Classifying patients into subgroups with a common genetic etiology and applying treatments tailored to the specific molecular defect they carry is likely to improve management of neurodevelopmental disease in the future.

Novel genetic causes for cerebral visual impairment

Cerebral visual impairment (CVI) is a major cause of low vision in children due to impairment in projection and/or interpretation of the visual input in the brain. Although acquired causes for CVI are well known, genetic causes underlying CVI are largely unidentified. DNAs of 25 patients with CVI and intellectual disability, but without acquired (eg, perinatal) damage, were investigated by whole-exome sequencing. The data were analyzed for de novo, autosomal-recessive, and X-linked variants, and subsequently classified into known, candidate, or unlikely to be associated with CVI. This classification was based on the Online Mendelian Inheritance in Man database, literature reports, variant characteristics, and functional relevance of the gene. After classification, variants in four genes known to be associated with CVI (AHDC1, NGLY1, NR2F1, PGAP1) in 5 patients (20%) were identified, establishing a conclusive genetic diagnosis for CVI. In addition, in 11 patients (44%) with CVI, variants in one or more candidate genes were identified (ACP6, AMOT, ARHGEF10L, ATP6V1A, DCAF6, DLG4, GABRB2, GRIN1, GRIN2B, KCNQ3, KCTD19, RERE, SLC1A1, SLC25A16, SLC35A2, SOX5, UFSP2, UHMK1, ZFP30). Our findings show that diverse genetic causes underlie CVI, some of which will provide insight into the biology underlying this disease process.

DVL1 frameshift mutations clustering in the penultimate exon cause autosomal-dominant Robinow syndrome

Robinow syndrome is a genetically heterogeneous disorder characterized by mesomelic limb shortening, genital hypoplasia, and distinctive facial features and for which both autosomal-recessive and autosomal-dominant inheritance patterns have been described. Causative variants in the non-canonical signaling gene WNT5A underlie a subset of autosomal-dominant Robinow syndrome (DRS) cases, but most individuals with DRS remain without a molecular diagnosis. We performed whole-exome sequencing in four unrelated DRS-affected individuals without coding mutations in WNT5A and found heterozygous DVL1 exon 14 mutations in three of them. Targeted Sanger sequencing in additional subjects with DRS uncovered DVL1 exon 14 mutations in five individuals, including a pair of monozygotic twins. In total, six distinct frameshift mutations were found in eight subjects, and all were heterozygous truncating variants within the penultimate exon of DVL1. In five families in which samples from unaffected parents were available, the variants were demonstrated to represent de novo mutations. All variant alleles are predicted to result in a premature termination codon within the last exon, escape nonsense-mediated decay (NMD), and most likely generate a C-terminally truncated protein with a distinct -1 reading-frame terminus. Study of the transcripts extracted from affected subjects' leukocytes confirmed expression of both wild-type and variant alleles, supporting the hypothesis that mutant mRNA escapes NMD. Genomic variants identified in our study suggest that truncation of the C-terminal domain of DVL1, a protein hypothesized to have a downstream role in the Wnt-5a non-canonical pathway, is a common cause of DRS.