Molecular etiology of arthrogryposis in multiple families of mostly Turkish origin

Structure of the endosomal CORVET tethering complex

Cells depend on their endolysosomal system for nutrient uptake and downregulation of plasma membrane proteins. These processes rely on endosomal maturation, which requires multiple membrane fusion steps. Early endosome fusion is promoted by the Rab5 GTPase and its effector, the hexameric CORVET tethering complex, which is homologous to the lysosomal HOPS. How these related complexes recognize their specific target membranes remains entirely elusive. Here, we solve the structure of CORVET by cryo-electron microscopy and revealed its minimal requirements for membrane tethering. As expected, the core of CORVET and HOPS resembles each other. However, the function-defining subunits show marked structural differences. Notably, we discover that unlike HOPS, CORVET depends not only on Rab5 but also on phosphatidylinositol-3-phosphate (PI3P) and membrane lipid packing defects for tethering, implying that an organelle-specific membrane code enables fusion. Our data suggest that both shape and membrane interactions of CORVET and HOPS are conserved in metazoans, thus providing a paradigm how tethering complexes function.

Bi-allelic variants in MYH3 cause recessively-inherited arthrogryposis

Arthrogryposis is a clinical feature defined by congenital joint contractures in two or more different body areas which occurs in between 1/3000 and 1/5000 live births. Variants in multiple genes have been associated with distal arthrogryposis syndromes. Heterozygous variants in MYH3 have been identified to cause the dominantly-inherited distal arthrogryposis conditions, Freeman-Sheldon syndrome, Sheldon-Hall syndrome, and multiple pterygium syndrome. In contrast, MYH3 variants underlie both dominantly and recessively inherited Contractures, Pterygia, and Spondylocarpotarsal Fusion syndromes (CPSFS) which are characterized by extensive bony abnormalities in addition to congenital contractures. Here we report two affected sibs with distal arthrogryposis born to unaffected, distantly related parents. Sequencing revealed that both sibs were homozygous for two ultra-rare MYH3 variants, c.3445G>A (p.Glu1149Lys) and c.4760T>C (p.Leu1587Pro). Sequencing and deletion/duplication analysis of 169 other arthrogryposis genes yielded no other compelling candidate variants. This is the first report of biallelic variants in MYH3 being implicated in a distal arthrogryposis phenotype without the additional features of CPSFS. Thus, akin to CPSFS, both dominant and recessively inherited distal arthrogryposis can be caused by variants in MYH3.

Clinical and molecular characteristics of 26 fetuses with lethal multiple congenital contractures

Multiple congenital contractures (MCC) due to fetal akinesia manifest across a broad spectrum of diseases, ranging from mild distal arthrogryposis to lethal fetal akinesia deformation sequence. We hereby present a series of 26 fetuses displaying severe MCC phenotypes from 18 families and describe detailed prenatal ultrasound findings, postmortem clinical evaluations, and genetic investigations. Most common prenatal findings were abnormal facial profile (65%), central nervous system abnormalities (62%), polyhydramnios (50%), increased nuchal translucency (50%), and fetal hydrops (35%). Postmortem examinations unveiled additional anomalies including facial dysmorphisms, dysplastic skeletal changes, ichthyosis, multiple pterygia, and myopathy, allowing preliminary diagnosis of particular Mendelian disorders in multiple patients. Evaluation of the parents revealed maternal grip myotonia in one family. By exome sequencing and targeted testing, we identified causative variants in ACTC1, CHST14, COG6, DMPK, DOK7, HSPG2, KLHL7, KLHL40, KIAA1109, NEB, PSAT1, RAPSN, USP14, and WASHC5 in 15 families, and one patient with a plausible diagnosis associated with biallelic NEB variants. Three patients received a dual diagnosis. Pathogenic alterations in newly discovered genes or in previously known genes recently linked to new MCC phenotypes were observed in 44% of the cohort. Our results provide new insights into the clinical and molecular landscape of lethal MCC phenotypes.

Functional role of myosin-binding protein H in thick filaments of developing vertebrate fast-twitch skeletal muscle

Myosin-binding protein H (MyBP-H) is a component of the vertebrate skeletal muscle sarcomere with sequence and domain homology to myosin-binding protein C (MyBP-C). Whereas skeletal muscle isoforms of MyBP-C (fMyBP-C, sMyBP-C) modulate muscle contractility via interactions with actin thin filaments and myosin motors within the muscle sarcomere "C-zone," MyBP-H has no known function. This is in part due to MyBP-H having limited expression in adult fast-twitch muscle and no known involvement in muscle disease. Quantitative proteomics reported here reveal MyBP-H is highly expressed in prenatal rat fast-twitch muscles and larval zebrafish, suggesting a conserved role in muscle development, and promoting studies to define its function. We take advantage of the genetic control of the zebrafish model and a combination of structural, functional, and biophysical techniques to interrogate the role of MyBP-H. Transgenic, FLAG-tagged MyBP-H or fMyBP-C both localize to the C-zones in larval myofibers, whereas genetic depletion of endogenous MyBP-H or fMyBP-C leads to increased accumulation of the other, suggesting competition for C-zone binding sites. Does MyBP-H modulate contractility from the C-zone? Globular domains critical to MyBP-C's modulatory functions are absent from MyBP-H, suggesting MyBP-H may be functionally silent. However, our results suggest an active role. Small angle x-ray diffraction of intact larval tails revealed MyBP-H contributes to the compression of the myofilament lattice accompanying stretch or contraction, while in vitro motility experiments indicate MyBP-H shares MyBP-C's capacity as a molecular "brake". These results provide new insights and raise questions about the role of the C-zone during muscle development.

A novel compound heterozygous variant of ECEL1 induced joint dysfunction and cartilage degradation: a case report and literature review

Background

Distal arthrogryposis type 5D (DA5D) represents a subtype of distal arthrogryposis (DA) characterized by congenital joint contractures in the distal extremities. DA5D is inherited in a rare autosomal recessive manner and is associated with the ECEL1 gene. In this report, we describe a case of an infant with bilateral knee contractures and ptosis, caused by a novel compound heterozygous mutation of ECEL1.

Case presentation

We conducted DNA extraction, whole-exome sequencing analysis, and mutation analysis of ECEL1 to obtain genetic data on the patient. We subsequently analyzed the patient's clinical and genetic data. The proband was a 6 months-old male infant who presented with significant bilateral knee contracture disorders and bilateral ptosis. MRI demonstrated cartilage degradation in knee joint. Whole-exome sequencing of the patient's DNA revealed a compound heterozygous mutation of c.2152-15C>A and c.110_155del in ECEL1. Analysis with the MutationTaster application indicated that c.110_155del was pathogenic (probability = 1), causing frameshift mutations affecting 151 amino acids (p.F37Cfs*151). The truncated protein lost the substructure of a transmembranous site based on the predicted protein crystal structure AF-O95672-F1. The variant of c.2152-15C>A of ECEL1 was also predicted to be disease-causing (probability = 0.98) as it impaired the methylation of ECEL1 serving as an H3K27me3 modification site, which led to the dysfunction of the second topological domain. Therefore, we concluded that the compound heterozygous mutation caused the pathogenic phenotype of this proband.

Conclusion

The present case highlights the usefulness of molecular genetic screening in diagnosing unexpected joint disorder. Identification of novel mutations in the ECEL1 gene broadens the mutation spectrum of this gene and adds to the genotype-phenotype map of DA5D. Furthermore, rapid whole-exome sequencing analysis enabled timely diagnosis of this rare disease, facilitating appropriate treatment and scheduled follow-up to improve clinical outcomes.

De novo variants in PLCG1 are associated with hearing impairment, ocular pathology, and cardiac defects

Phospholipase C isozymes (PLCs) hydrolyze phosphatidylinositol 4,5-bisphosphate into inositol 1,4,5-trisphosphate and diacylglycerol, important signaling molecules involved in many cellular processes. PLCG1 encodes the PLCγ1 isozyme that is broadly expressed. Hyperactive somatic mutations of PLCG1 are observed in multiple cancers, but only one germline variant has been reported. Here we describe three unrelated individuals with de novo heterozygous missense variants in PLCG1 (p.Asp1019Gly, p.His380Arg, and p.Asp1165Gly) who exhibit variable phenotypes including hearing loss, ocular pathology and cardiac septal defects. To model these variants in vivo, we generated the analogous variants in the Drosophila ortholog, small wing (sl). We created a null allele slT2A and assessed the expression pattern. sl is broadly expressed, including in wing discs, eye discs, and a subset of neurons and glia. Loss of sl causes wing size reductions, ectopic wing veins and supernumerary photoreceptors. We document that mutant flies exhibit a reduced lifespan and age-dependent locomotor defects. Expressing wild-type sl in slT2A mutant rescues the loss-of-function phenotypes whereas expressing the variants causes lethality. Ubiquitous overexpression of the variants also reduces viability, suggesting that the variants are toxic. Ectopic expression of an established hyperactive PLCG1 variant (p.Asp1165His) in the wing pouch causes severe wing phenotypes, resembling those observed with overexpression of the p.Asp1019Gly or p.Asp1165Gly variants, further arguing that these two are gain-of-function variants. However, the wing phenotypes associated with p.His380Arg overexpression are mild. Our data suggest that the PLCG1 de novo heterozygous missense variants are pathogenic and contribute to the features observed in the probands.

Unlocking the Role of sMyBP-C: A Key Player in Skeletal Muscle Development and Growth

Skeletal muscle is the largest organ in the body, responsible for gross movement and metabolic regulation. Recently, variants in the MYBPC1 gene have been implicated in a variety of developmental muscle diseases, such as distal arthrogryposis. How MYBPC1 variants cause disease is not well understood. Here, through a collection of novel gene-edited mouse models, we define a critical role for slow myosin binding protein-C (sMyBP-C), encoded by MYBPC1, across muscle development, growth, and maintenance during prenatal, perinatal, postnatal and adult stages. Specifically, Mybpc1 knockout mice exhibited early postnatal lethality and impaired skeletal muscle formation and structure, skeletal deformity, and respiratory failure. Moreover, a conditional knockout of Mybpc1 in perinatal, postnatal and adult stages demonstrates impaired postnatal muscle growth and function secondary to disrupted actomyosin interaction and sarcomere structural integrity. These findings confirm the essential role of sMyBP-C in skeletal muscle and reveal specific functions in both prenatal embryonic musculoskeletal development and postnatal muscle growth and function.

HOPS, CORVET and newly-identified Hybrid tethering complexes contribute differentially towards multiple modes of endocytosis

Vesicular transport driven by membrane trafficking systems conserved in eukaryotes is critical to cellular functionality and homeostasis. It is known that homotypic fusion and vacuole protein sorting (HOPS) and class C core endosomal vacuole tethering (CORVET) interact with Rab-GTPases and SNARE proteins to regulate vesicle transport, fusion, and maturation in autophagy and endocytosis pathways. In this study, we identified two novel "Hybrid" tethering complexes in mammalian cells in which one of the subunits of HOPS or CORVET is replaced with the subunit from the other. Substrates taken up by receptor-mediated endocytosis or pinocytosis were transported by distinctive pathways, and the newly identified hybrid complexes contributed to pinocytosis in the presence of HOPS, whereas receptor-mediated endocytosis was exclusively dependent on HOPS. Our study provides new insights into the molecular mechanisms of the endocytic pathway and the function of the vacuolar protein sorting-associated (VPS) protein family.

The Relationship between Changes in MYBPC3 Single-Nucleotide Polymorphism-Associated Metabolites and Elite Athletes' Adaptive Cardiac Function

Athletic performance is a multifactorial trait influenced by a complex interaction of environmental and genetic factors. Over the last decades, understanding and improving elite athletes' endurance and performance has become a real challenge for scientists. Significant tools include but are not limited to the development of molecular methods for talent identification, personalized exercise training, dietary requirements, prevention of exercise-related diseases, as well as the recognition of the structure and function of the genome in elite athletes. Investigating the genetic markers and phenotypes has become critical for elite endurance surveillance. The identification of genetic variants contributing to a predisposition for excellence in certain types of athletic activities has been difficult despite the relatively high genetic inheritance of athlete status. Metabolomics can potentially represent a useful approach for gaining a thorough understanding of various physiological states and for clarifying disorders caused by strength-endurance physical exercise. Based on a previous GWAS study, this manuscript aims to discuss the association of specific single-nucleotide polymorphisms (SNPs) located in the MYBPC3 gene encoding for cardiac MyBP-C protein with endurance athlete status. MYBPC3 is linked to elite athlete heart remodeling during or after exercise, but it could also be linked to the phenotype of cardiac hypertrophy (HCM). To make the distinction between both phenotypes, specific metabolites that are influenced by variants in the MYBPC3 gene are analyzed in relation to elite athletic performance and HCM. These include theophylline, ursodeoxycholate, quinate, and decanoyl-carnitine. According to the analysis of effect size, theophylline, quinate, and decanoyl carnitine increase with endurance while decreasing with cardiovascular disease, whereas ursodeoxycholate increases with cardiovascular disease. In conclusion, and based on our metabolomics data, the specific effects on athletic performance for each MYBPC3 SNP-associated metabolite are discussed.

Impaired gating of γ- and ε-AChR respectively causes Escobar syndrome and fast-channel myasthenia

OBJECTIVE

To dissect the kinetic defects of acetylcholine receptor (AChR) γ subunit variant in an incomplete form of the Escobar syndrome without pterygium and compare it with those of a variant of corresponding residue in the AChR ε subunit in a congenital myasthenic syndrome (CMS).

METHODS

Whole exome sequencing, α-bungarotoxin binding assay, single channel patch-clamp recordings, and maximum likelihood analysis of channel kinetics.

RESULTS

We identified compound heterozygous variants in AChR γ and ε subunits in three Escobar syndrome (1-3) and three CMS patients (4-6), respectively. Each Escobar syndrome patient carries γP121R along with γV221Afs*44 in patients 1 and 2, and γY63* in patient 3. Three CMS patients share εP121T along with εR20W, εG-8R, and εY15H in patients 4, 5, and 6, respectively. Surface expressions of γP121R- and εP121T-AChR were 80% and 138% of the corresponding wild-type AChR, whereas εR20W, εG-8R, and εY15H reduced receptor expression to 27%, 35%, and 30% of wild-type εAChR, respectively. γV221Afs*44 and γY63* are null variants. Thus, γP121R and εP121T determine the phenotype. γP121R and εP121T shorten channel opening burst duration to 28% and 18% of corresponding wild-type AChR by reducing the channel gating equilibrium constant 44- and 63-fold, respectively.

INTERPRETATION

Similar impairment of channel gating efficiency of a corresponding P121 residue in the acetylcholine-binding site of the AChR γ and ε subunits causes Escobar syndrome without pterygium and fast-channel CMS, respectively, suggesting that therapy for the fast-channel CMS will benefit Escobar syndrome.

Etiology of genetic muscle disorders induced by mutations in fast and slow skeletal MyBP-C paralogs

Skeletal muscle, a highly complex muscle type in the eukaryotic system, is characterized by different muscle subtypes and functions associated with specific myosin isoforms. As a result, skeletal muscle is the target of numerous diseases, including distal arthrogryposes (DAs). Clinically, DAs are a distinct disorder characterized by variation in the presence of contractures in two or more distal limb joints without neurological issues. DAs are inherited, and up to 40% of patients with this condition have mutations in genes that encode sarcomeric protein, including myosin heavy chains, troponins, and tropomyosin, as well as myosin binding protein-C (MYBPC). Our research group and others are actively studying the specific role of MYBPC in skeletal muscles. The MYBPC family of proteins plays a critical role in the contraction of striated muscles. More specifically, three paralogs of the MYBPC gene exist, and these are named after their predominant expression in slow-skeletal, fast-skeletal, and cardiac muscle as sMyBP-C, fMyBP-C, and cMyBP-C, respectively, and encoded by the MYBPC1, MYBPC2, and MYBPC3 genes, respectively. Although the physiology of various types of skeletal muscle diseases is well defined, the molecular mechanism underlying the pathological regulation of DAs remains to be elucidated. In this review article, we aim to highlight recent discoveries involving the role of skeletal muscle-specific sMyBP-C and fMyBP-C as well as their expression profile, localization in the sarcomere, and potential role(s) in regulating muscle contractility. Thus, this review provides an overall summary of MYBPC skeletal paralogs, their potential roles in skeletal muscle function, and future research directions.

Clinical and Pathologic Features of Congenital Myasthenic Syndromes Caused by 35 Genes-A Comprehensive Review

Congenital myasthenic syndromes (CMS) are a heterogeneous group of disorders characterized by impaired neuromuscular signal transmission due to germline pathogenic variants in genes expressed at the neuromuscular junction (NMJ). A total of 35 genes have been reported in CMS (AGRN, ALG14, ALG2, CHAT, CHD8, CHRNA1, CHRNB1, CHRND, CHRNE, CHRNG, COL13A1, COLQ, DOK7, DPAGT1, GFPT1, GMPPB, LAMA5, LAMB2, LRP4, MUSK, MYO9A, PLEC, PREPL, PURA, RAPSN, RPH3A, SCN4A, SLC18A3, SLC25A1, SLC5A7, SNAP25, SYT2, TOR1AIP1, UNC13A, VAMP1). The 35 genes can be classified into 14 groups according to the pathomechanical, clinical, and therapeutic features of CMS patients. Measurement of compound muscle action potentials elicited by repetitive nerve stimulation is required to diagnose CMS. Clinical and electrophysiological features are not sufficient to identify a defective molecule, and genetic studies are always required for accurate diagnosis. From a pharmacological point of view, cholinesterase inhibitors are effective in most groups of CMS, but are contraindicated in some groups of CMS. Similarly, ephedrine, salbutamol (albuterol), amifampridine are effective in most but not all groups of CMS. This review extensively covers pathomechanical and clinical features of CMS by citing 442 relevant articles.

Long term ophthalmic complications of distal arthrogryposis type 5D

BACKGROUND

Distal Arthrogryposis type 5D (DA5D) is a rare genetic disease, expressed phenotypically by skeletal and ocular abnormalities.

MATERIALS AND METHODS

Two sisters, ages 42 and 46 years old, were ascertained, both diagnosed with arthrogryposis and unusual ophthalmic late expressions of the disease. They were examined and followed up by both ophthalmologists and medical geneticists. Molecular analysis was performed and population screening followed among healthy individuals of the same ethnic background who reside in the same village.

RESULTS

The two sisters expressed myogenic ptosis with poor levator palpebrae function, limitation in up gaze, lagophthalmos, refractive errors, corneal scarring and vascularization along with severe distal arthrogryposis. The newly reported features were: significant lower lid retraction, causing inferior scleral show. Sanger sequencing of the coding regions of ECEL1 gene revealed a homozygous deletion of 46 bps. The carrier frequency is 1:24 (4.2% carriers) in the probands' village.

CONCLUSIONS

We diagnosed two patients with DA5D carrying a homozygous pathogenic genetic variant previously reported only once. We report the late ophthalmologic manifestations of this rare disorder and emphasize the importance to recognize possible long-term ophthalmic complications. Measures are needed to diagnose this rare disorder at a younger age and to address ophthalmic and orthopedic complications that might be prevented. We revealed the causative genetic variant and a carrier frequency of 1:24 for DA5D, in the probands' village, thus enabling accurate genetic counselling and justifying genetic testing to the residents of this village as a diagnostic and preventive measure.

An Unusual Combination of Arthrogryposis, Gastroschisis, Cecal Volvulus, and Malignant Hyperthermia in a Young Woman: A Case Report

BACKGROUND The purpose of this study is to discuss a patient with a history of conditions, including arthrogryposis, gastroschisis, and malignant hyperthermia, who presented with cecal volvulus requiring urgent surgical intervention. CASE REPORT A 29-year-old woman with a history of arthrogryposis, gastroschisis, malignant hyperthermia, and multiple childhood abdominal surgeries presents to the Emergency Department (ED) with 2 days of abdominal pain and bloody diarrhea. A CT abdomen/pelvis revealed findings concerning for a cecal volvulus. The patient was premedicated and monitored closely by the anesthesia team due to her history of malignant hyperthermia. She underwent an exploratory laparotomy, where a dilated cecum and proximal ascending colon were found to be completely volvulized around the mesentery. Manual bowel detorsion was performed, which resulted in reperfusion of the ischemic-appearing bowel, which then appeared viable. She recovered well after the procedure and was discharged on postoperative day 5. CONCLUSIONS This case highlights a patient who presented with a combination of 4 findings: arthrogryposis, gastroschisis, malignant hyperthermia, and cecal volvulus. With arthrogryposis reported to be associated with gastroschisis and malignant hyperthermia, this report not only corroborates this association, but also aims to draw attention to the fact that these conditions have potential to occur jointly with cecal volvulus. Given the patient's history of gastroschisis requiring extensive abdominal surgeries that contribute as risk factors for cecal volvulus, it is possible there may be other arthrogryposis patients who present with cecal volvulus similar to that seen in this patient.

Diagnostic workup in children with arthrogryposis: description of practices from a single reference centre, comparison with literature and suggestion of recommendations

INTRODUCTION

Arthrogryposis multiplex congenita (AMC) refers to a clinical presentation of congenital contractures involving two or more body areas. More than 400 distinct conditions may lead to AMC, making the aetiological diagnosis challenging. The objective of this work was to set up evidence-based recommendations for the diagnosis of AMC by taking advantage of both data from our nation-wide cohort of children with AMC and from the literature.

MATERIAL AND METHODS

We conducted a retrospective single-centre observational study. Patients had been evaluated at least once at a paediatric age in the AMC clinic of Grenoble University Hospital between 2007 and 2019. After gathering data about their diagnostic procedure, a literature review was performed for each paraclinical investigation to discuss their relevance.

RESULTS

One hundred and twenty-five patients were included, 43% had Amyoplasia, 27% had distal arthrogryposis and 30% had other forms. A definitive aetiological diagnosis was available for 66% of cases. We recommend a two-time diagnostic process: first, non-invasive investigations that aim at classifying patients into one of the three groups, and second, selected investigations targeting a subset of patients.

CONCLUSION

The aetiological management for patients with AMC remains arduous. This process will be facilitated by the increasing use of next-generation sequencing combined with detailed phenotyping. Invasive investigations should be avoided because of their limited yield.

Fetuses and infants with Amyoplasia congenita in congenital Zika syndrome: The evidence of a viral cause. A narrative review of 144 cases

OBJECTIVES

Amyoplasia congenita is the most frequent type of arthrogryposis causing fetal hypokinesia, leading to congenital contractures at birth. The pathogenesis is thought to be impaired blood circulation to the fetus early in pregnancy, with hypotension and hypoxia damaging the anterior horn cells. In animal studies however a prenatal infection with a poliomyelitis-like viral agent was demonstrated. Congenital Zika virus syndrome (CZVS) has recently been described in infants with severe microcephaly, and in 10-25% of cases arthrogryposis.

METHODS

A search in PubMed for CZVS yielded 124 studies. After a selection for arthrogryposis, 35 papers were included, describing 144 cases. The studies were divided into two categories. 1) Those (87 cases) focussing on imaging or histological data of congenital brain defects, contained insufficient information to link arthrogryposis specifically to lesions of the brain or spinal motor neuron. 2) In the other 57 cases detailed clinical data could be linked to neurophysiological, imaging or histological data.

RESULTS

In category 1 the most frequent brain abnormalities in imaging studies were ventriculomegaly, calcifications (subcortical, basal ganglia, cerebellum), hypoplasia of the brainstem and cerebellum, atrophy of the cerebral cortex, migration disorders and corpus callosum anomalies. In category 2, in 38 of 57 cases clinical data were indicative of Amyoplasia congenita. This diagnosis was confirmed by electromyographic findings (13 cases), by MRI (37 cases) or histology (12 cases) of the spinal cord. The latter showed small or absent lateral corticospinal tracts, and cell loss and degeneration of motor neuron cells. Zika virus-proteins and flavivirus-like particles were detected in cytoplasm of spinal neurons.

CONCLUSION

The phenotype of arthrogryposis in CZVS is consistent with Amyoplasia congenita. These findings warrant search for an intrauterine infection with any neurotropic viral agent with affinity to spinal motor neurons in neonates with Amyoplasia.

Combining Gene Mutation with Expression of Candidate Genes to Improve Diagnosis of Escobar Syndrome

Escobar syndrome is a rare, autosomal recessive disorder that affects the musculoskeletal system and the skin. Mutations in the CHRNG and TPM2 genes are associated with this pathology. In this study, we conducted a clinical and genetic investigation of five patients and also explored via in silico and gene expression analysis their phenotypic variability. In detail, we identified a patient with a novel composite heterozygous variant of the CHRNG gene and two recurrent mutations in both CHRNG and TPM2 in the rest of the patients. As for the clinical particularities, we reported a list of modifier genes in a patient suffering from myopathy. Moreover, we identified decreased expression of IGF-1, which could be related to the short stature of Escobar patients, and increased expression of POLG1 specific to patients with TPM2 mutation. Through this study, we identified the genetic spectrum of Escobar syndrome in the Tunisian population, which will allow setting up genetic counseling and prenatal diagnosis for families at risk. In addition, we highlighted relevant biomarkers that could differentiate between patients with different genetic defects.

Cataract, abnormal electroretinogram and visual evoked potentials in a child with SMA-LED2 - extending the phenotype

This case report describes a girl who presented antenatal arthrogryposis and postnatal hypotonia, generalized and respiratory weakness, joint deformities particularly affecting the lower limbs and poor swallow. By 5 months, cataracts, abnormal electroretinograms, visual evoked potentials and global developmental impairments were recognized. No causative variants were identified on targeted gene panels. After her unexpected death at 11 months, gene-agnostic trio whole exome sequencing revealed a likely pathogenic de novo BICD2 missense variant, NM_001003800.1, c.593T>C, p.(Leu198Pro), confirming the diagnosis of spinal muscular atrophy lower extremity predominant type 2 (SMA-LED2). We propose that cataracts and abnormal electroretinograms are novel features of SMA-LED2.

Heterogeneous Distribution of Genetic Mutations in Myosin Binding Protein-C Paralogs

Myosin binding protein-C (MyBP-C) is a sarcomeric protein which regulates the force of contraction in striated muscles. Mutations in the MYBPC family of genes, including slow skeletal (MYBPC1), fast skeletal (MYBPC2) and cardiac (MYBPC3), can result in cardiac and skeletal myopathies. Nonetheless, their evolutionary pattern, pathogenicity and impact on MyBP-C protein structure remain to be elucidated. Therefore, the present study aimed to systematically assess the evolutionarily conserved and epigenetic patterns of MYBPC family mutations. Leveraging a machine learning (ML) approach, the Genome Aggregation Database (gnomAD) provided variants in MYBPC1, MYBPC2, and MYBPC3 genes. This was followed by an analysis with Ensembl’s variant effect predictor (VEP), resulting in the identification of 8,618, 3,871, and 3,071 variants in MYBPC1, MYBPC2, and MYBPC3, respectively. Missense variants comprised 61%–66% of total variants in which the third nucleotide positions in the codons were highly altered. Arginine was the most mutated amino acid, important because most disease-causing mutations in MyBP-C proteins are arginine in origin. Domains C5 and C6 of MyBP-C were found to be hotspots for most mutations in the MyBP-C family of proteins. A high percentage of truncated mutations in cMyBP-C cause cardiomyopathies. Arginine and glutamate were the top hits in fMyBP-C and cMyBP-C, respectively, and tryptophan and tyrosine were the most common among the three paralogs changing to premature stop codons and causing protein truncations at the carboxyl terminus. A heterogeneous epigenetic pattern was identified among the three MYBP-C paralogs. Overall, it was shown that databases using computational approaches can facilitate diagnosis and drug discovery to treat muscle disorders caused by MYBPC mutations.

Centers for Mendelian Genomics: A decade of facilitating gene discovery

PURPOSE

Mendelian disease genomic research has undergone a massive transformation over the past decade. With increasing availability of exome and genome sequencing, the role of Mendelian research has expanded beyond data collection, sequencing, and analysis to worldwide data sharing and collaboration.

METHODS

Over the past 10 years, the National Institutes of Health-supported Centers for Mendelian Genomics (CMGs) have played a major role in this research and clinical evolution.

RESULTS

We highlight the cumulative gene discoveries facilitated by the program, biomedical research leveraged by the approach, and the larger impact on the research community. Beyond generating a list of gene-phenotype relationships and participating in widespread data sharing, the CMGs have created resources, tools, and training for the larger community to foster understanding of genes and genome variation. The CMGs have participated in a wide range of data sharing activities, including deposition of all eligible CMG data into the Analysis, Visualization, and Informatics Lab-space (AnVIL), sharing candidate genes through the Matchmaker Exchange and the CMG website, and sharing variants in Genotypes to Mendelian Phenotypes (Geno2MP) and VariantMatcher.

CONCLUSION

The work is far from complete; strengthening communication between research and clinical realms, continued development and sharing of knowledge and tools, and improving access to richly characterized data sets are all required to diagnose the remaining molecularly undiagnosed patients.

Clinical and Genetic Findings in a Series of Eight Families with Arthrogryposis

The term “arthrogryposis” is used to indicate multiple congenital contractures affecting two or more areas of the body. Arthrogryposis is the consequence of an impairment of embryofetal neuromuscular function and development. The causes of arthrogryposis are multiple, and in newborns, it is difficult to predict the molecular defect as well as the clinical evolution just based on clinical findings. We studied a consecutive series of 13 participants who had amyoplasia, distal arthrogryposis (DA), or syndromic forms of arthrogryposis with normal intellectual development and other motor abilities. The underlying pathogenic variants were identified in 11 out of 13 participants. Correlating the genotype with the clinical features indicated that prenatal findings were specific for DA; this was helpful to identify familial cases, but features were non-specific for the involved gene. Perinatal clinical findings were similar among the participants, except for amyoplasia. Dilatation of the aortic root led to the diagnosis of Loeys–Dietz syndrome (LDS) in one case. The phenotype of DA type 5D (DA5D) and Escobar syndrome became more characteristic at later ages due to more pronounced pterygia. Follow-up indicated that DA type 1 (DA1)/DA type 2B (DA2B) spectrum and LDS had a more favorable course than the other forms. Hand clenching and talipes equinovarus/rocker bottom foot showed an improvement in all participants, and adducted thumb resolved in all forms except in amyoplasia. The combination of clinical evaluation with Next Generation Sequencing (NGS) analysis in the newborn may allow for an early diagnosis and, particularly in the DAs, suggests a favorable prognosis.

Clan genomics: From OMIM phenotypic traits to genes and biology

Clinical characterization of a patient phenotype has been the quintessential approach for elucidating a differential diagnosis and a hypothesis to explore a potential clinical diagnosis. This has resulted in a language of medicine and a semantic ontology, with both specialty- and subspecialty-specific lexicons, that can be challenging to translate and interpret. There is no 'Rosetta Stone' of clinical medicine such as the genetic code that can assist translation and interpretation of the language of genetics. Nevertheless, the information content embodied within a clinical diagnosis can guide management, therapeutic intervention, and potentially prognostic outlook of disease enabling anticipatory guidance for patients and families. Clinical genomics is now established firmly in medical practice. The granularity and informative content of a personal genome is immense. Yet, we are limited in our utility of much of that personal genome information by the lack of functional characterization of the overwhelming majority of computationally annotated genes in the haploid human reference genome sequence. Whereas DNA and the genetic code have provided a 'Rosetta Stone' to translate genetic variant information, clinical medicine, and clinical genomics provide the context to understand human biology and disease. A path forward will integrate deep phenotyping, such as available in a clinical synopsis in the Online Mendelian Inheritance in Man (OMIM) entries, with personal genome analyses.

Expanding the Phenotypic Spectrum of ECEL1-Associated Distal Arthrogryposis

Distal arthrogryposis type 5D (DA5D), a rare autosomal recessive disorder, is caused by mutations in ECEL1. We describe two consanguineous families (three patients) with novel ECEL1 gene mutations detected by next-generation sequencing (NGS). A 12-year-old boy (patient 1) presented with birth asphyxia, motor developmental delay, multiple joint contractures, pes planus, kyphoscoliosis, undescended testis, hypophonic speech with a nasal twang, asymmetric ptosis, facial weakness, absent abductor pollicis brevis, bifacial, and distal lower limb weakness. Muscle MRI revealed asymmetric fatty infiltration of tensor fascia lata, hamstring, lateral compartment of the leg, and gastrocnemius. In addition, 17-year-old monozygotic twins (patients 2 and 3) presented with motor development delay, white hairlock, hypertelorism, tented upper lip, bulbous nose, tongue furrowing, small low set ears, multiple contractures, pes cavus, prominent hyperextensibility at the knee, hypotonia of lower limbs, wasting and weakness of all limbs (distal > proximal), areflexia, and high steppage gait. One had perinatal insult, seizures, mild intellectual disability, unconjugated eye movements, and primary optic atrophy. In the twins, MRI revealed extensive fatty infiltration of the gluteus maximus, quadriceps, hamstrings, and anterior and posterior compartment of the leg. Electrophysiology showed prominent motor axonopathy. NGS revealed rare homozygous missense variants c.602T > C (p.Met201Thr) in patient 1 and c.83C > T (p.Ala28Val) in patients 2 and 3, both localized in exon 2 of ECEL1 gene. Our three cases expand the clinical, imaging, and molecular spectrum of the ECEL1-mutation-related DA5D.

Sarcomeric deficits underlie MYBPC1-associated myopathy with myogenic tremor

Myosin binding protein-C slow (sMyBP-C) comprises a subfamily of cytoskeletal proteins encoded by MYBPC1 that is expressed in skeletal muscles where it contributes to myosin thick filament stabilization and actomyosin cross-bridge regulation. Recently, our group described the causal association of dominant missense pathogenic variants in MYBPC1 with an early-onset myopathy characterized by generalized muscle weakness, hypotonia, dysmorphia, skeletal deformities, and myogenic tremor, occurring in the absence of neuropathy. To mechanistically interrogate the etiologies of this MYBPC1-associated myopathy in vivo, we generated a knock-in mouse model carrying the E248K pathogenic variant. Using a battery of phenotypic, behavioral, and physiological measurements spanning neonatal to young adult life, we found that heterozygous E248K mice faithfully recapitulated the onset and progression of generalized myopathy, tremor occurrence, and skeletal deformities seen in human carriers. Moreover, using a combination of biochemical, ultrastructural, and contractile assessments at the level of the tissue, cell, and myofilaments, we show that the loss-of-function phenotype observed in mutant muscles is primarily driven by disordered and misaligned sarcomeres containing fragmented and out-of-register internal membranes that result in reduced force production and tremor initiation. Collectively, our findings provide mechanistic insights underscoring the E248K-disease pathogenesis and offer a relevant preclinical model for therapeutic discovery.

Postnatal Diagnostic Workup in Children With Arthrogryposis: A Series of 82 Patients

OBJECTIVE

To describe a postnatal series of patients with arthrogryposis multiplex congenita by the causal mechanisms involved.

METHODS

In this single-center study, the local data warehouse was used to identify patients with arthrogryposis multiplex congenita. Patients were classified into different etiologic groups.

RESULTS

Of 82 patients included, the most frequent cause of arthrogryposis multiplex congenita was a neuromuscular disorder (39%), including skeletal muscle (n = 19), neuromuscular junction (n = 3), and peripheral nerve (n = 11) involvement. In other subgroups, 19 patients (23%) were classified by disorders in the central nervous system, 5 (6%) in connective tissue, 7 (8.5%) had mixed mechanisms, and 18 (22%) could not be classified. Contractures topography was not associated with a causal mechanism. Cerebral magnetic resonance imaging (MRI), electroneuromyography, and muscle biopsy were the most conclusive investigations. Metabolic investigations were normal in all the patients tested. Targeted or whole exome sequencing diagnostic rates were 51% and 71%, respectively. Thirty-three percent of patients died (early death occurred in patients with polyhydramnios, prematurity, and ventilatory dependency).

DISCUSSION

The benefits of a precise diagnosis in the neonatal period include more tailored management of arthrogryposis multiplex congenita and better genetic information.

Deep clinicopathological phenotyping identifies a previously unrecognized pathogenic EMD splice variant

Exome sequencing (ES) has revolutionized rare disease management, yet only ~25%-30% of patients receive a molecular diagnosis. A limiting factor is the quality of available phenotypic data. Here, we describe how deep clinicopathological phenotyping yielded a molecular diagnosis for a 19-year-old proband with muscular dystrophy and negative clinical ES. Deep phenotypic analysis identified two critical data points: (1) the absence of emerin protein in muscle biopsy and (2) clinical features consistent with Emery-Dreifuss muscular dystrophy. Sequencing data analysis uncovered an ultra-rare, intronic variant in EMD, the gene encoding emerin. The variant, NM_000117.3: c.188-6A > G, is predicted to impact splicing by in silico tools. This case thus illustrates how better integration of clinicopathologic data into ES analysis can enhance diagnostic yield with implications for clinical practice.

Neuromuscular and Neuroendocrinological Features Associated With ZC4H2-Related Arthrogryposis Multiplex Congenita in a Sicilian Family: A Case Report

Wieacker-Wolff syndrome (WWS) is an X-linked Arthrogryposis Multiplex Congenita (AMC) disorder associated with broad neurodevelopmental impairment. The genetic basis of WWS lies in hemizygous pathogenic variants in ZC4H2, encoding a C4H2 type zinc-finger nuclear factor abundantly expressed in the developing human brain. The main clinical features described in WWS families carrying ZC4H2 pathogenic variants encompass having a short stature, microcephaly, birth respiratory distress, arthrogryposis, hypotonia, distal muscle weakness, and broad neurodevelopmental delay. We hereby report a Sicilian family with a boy clinically diagnosed with WWS and genetically investigated with exome sequencing (ES), leading to the identification of a c.593G>A (p. R198Q) hemizygous pathogenic variant in the ZC4H2 gene. During the first year of life, the onset of central hypoadrenalism led to recurrent hypoglycemic events, which likely contributed to seizure susceptibility. Also, muscle biopsy studies confirmed a pathology of the muscle tissue and revealed peculiar abnormalities of the neuromuscular junction. In conclusion, we expand the phenotypic spectrum of the WWS-related neurodevelopmental disorders and discuss the role of ZC4H2 in the context of the potential neuroendocrinological and neuromuscular features associated with this condition.

Models of Distal Arthrogryposis and Lethal Congenital Contracture Syndrome

Distal arthrogryposis and lethal congenital contracture syndromes describe a broad group of disorders that share congenital limb contractures in common. While skeletal muscle sarcomeric genes comprise many of the first genes identified for Distal Arthrogyposis, other mechanisms of disease have been demonstrated, including key effects on peripheral nerve function. While Distal Arthrogryposis and Lethal Congenital Contracture Syndromes display superficial similarities in phenotype, the underlying mechanisms for these conditions are diverse but overlapping. In this review, we discuss the important insights gained into these human genetic diseases resulting from in vitro molecular studies and in vivo models in fruit fly, zebrafish, and mice.

My, oh, MYO9A! Just how complex can regulation of the podocyte actin cytoskeleton get?

Genetics contributes significantly to the development of kidney diseases. In the case of glomerular diseases such as focal segmental glomerulosclerosis, over a dozen genes involved in maintaining and regulating the actin cytoskeleton of podocytes have been implicated. A new study adds the atypical myosin, MYO9A, to that list using a combination of human and mouse genetics, suggesting a link to enhanced RhoA activity. Unraveling the growing web of actin regulators remains a key challenge to understanding podocytopathies.

Phenotypic spectrum and genomics of undiagnosed arthrogryposis multiplex congenita

BACKGROUND

Arthrogryposis multiplex congenita (AMC) is characterised by congenital joint contractures in two or more body areas. AMC exhibits wide phenotypic and genetic heterogeneity. Our goals were to improve the genetic diagnosis rates of AMC, to evaluate the added value of whole exome sequencing (WES) compared with targeted exome sequencing (TES) and to identify new genes in 315 unrelated undiagnosed AMC families.

METHODS

Several genomic approaches were used including genetic mapping of disease loci in multiplex or consanguineous families, TES then WES. Sanger sequencing was performed to identify or validate variants.

RESULTS

We achieved disease gene identification in 52.7% of AMC index patients including nine recently identified genes (CNTNAP1, MAGEL2, ADGRG6, ADCY6, GLDN, LGI4, LMOD3, UNC50 and SCN1A). Moreover, we identified pathogenic variants in ASXL3 and STAC3 expanding the phenotypes associated with these genes. The most frequent cause of AMC was a primary involvement of skeletal muscle (40%) followed by brain (22%). The most frequent mode of inheritance is autosomal recessive (66.3% of patients). In sporadic patients born to non-consanguineous parents (n=60), de novo dominant autosomal or X linked variants were observed in 30 of them (50%).

CONCLUSION

New genes recently identified in AMC represent 21% of causing genes in our cohort. A high proportion of de novo variants were observed indicating that this mechanism plays a prominent part in this developmental disease. Our data showed the added value of WES when compared with TES due to the larger clinical spectrum of some disease genes than initially described and the identification of novel genes.

A New Intronic Variant in ECEL1 in Two Patients with Distal Arthrogryposis Type 5D

Distal Arthrogryposis type 5D (DA5D) is characterized by congenital contractures involving the distal joints, short stature, scoliosis, ptosis, astigmatism, and dysmorphic features. It is inherited in an autosomal recessive manner, and it is a result of homozygous or compound heterozygous variants in the ECEL1 gene. Here, we report two patients of Sardinian origin harboring a new intronic homozygous variant in ECEL1 (c.1507-9G>A), which was predicted to affect mRNA splicing by activating a cryptic acceptor site. The frequency of the variant is very low in the general human population, and its presence in our families can be attributed to a founder effect. This study provides an updated review of the known causative mutations of the ECEL1 gene, enriching the allelic spectrum to include the noncoding sequence.

MYBPC2 and MYL1 as Significant Gene Markers for Rhabdomyosarcoma

BACKGROUND

Rhabdomyosarcoma is the most common soft tissue tumor in children. Rhabdomyosarcoma commonly results in pain and bleeding caused by tumor compression and is prone to early metastasis and recurrence, which can seriously affect the therapeutic outcomes and long-term prognosis. Up to 37.7% of rhabdomyosarcomas may metastasize. Therefore, the molecular mechanisms underlying rhabdomyosarcoma must be explored to identify an effective target for its early diagnosis and specific treatment.

METHODS

A dataset of 18 rhabdomyosarcoma tissue samples and 6 healthy skeletal muscle samples was downloaded. Differentially expressed genes between rhabdomyosarcoma and healthy tissue samples were identified by GEO2R. Kyoto Encyclopedia of Genes and Genomes and gene ontology pathway enrichment analyses were performed. A protein-protein interaction network was constructed, and hub genes were identified. Expression and survival analyses of hub genes were performed. Additionally, 30 patients with rhabdomyosarcoma were recruited, and overall survival information and samples were collected. Reverse transcription quantitative real-time polymerase chain reaction assays were performed to verify the expression of MYBPC2 and MYL1 in rhabdomyosarcoma tumor tissues. The Kaplan-Meier method was used to explore overall survival based on our clinical data.

RESULTS

In total, 164 genes were up-regulated and 394 were down-regulated in rhabdomyosarcoma tumor tissues. Gene ontology analysis revealed that variations were predominantly enriched in the cell cycle, muscle contraction, muscle system processes, cytoskeleton, nucleotide binding, and cytoskeletal protein binding. The protein-protein interaction network revealed 3274 edges, and 441 nodes were constructed. Ten hub genes were identified; of these, MYBPC2 and MYL1 were significantly up-regulated in rhabdomyosarcoma. Compared with the healthy group, patients with rhabdomyosarcoma exhibiting high expression of MYBPC2 and MYL1 exhibited significantly worse overall survival.

CONCLUSIONS

We found differentially expressed genes between rhabdomyosarcoma and healthy tissue samples. MYBPC2 and MYL1 may be involved in the pathogenesis of rhabdomyosarcoma and therefore deserve further exploration.

Rho-GTPase Activating Protein myosin MYO9A identified as a novel candidate gene for monogenic focal segmental glomerulosclerosis

Focal segmental glomerulosclerosis (FSGS) is a podocytopathy leading to kidney failure, whose molecular cause frequently remains unresolved. Here, we describe a rare MYO9A loss of function nonsense heterozygous mutation (p.Arg701^∗) as a possible contributor to disease in a sibling pair with familial FSGS/proteinuria. MYO9A variants of uncertain significance were identified by whole exome sequencing in a cohort of 94 biopsy proven patients with FSGS. MYO9A is an unconventional myosin with a Rho-GAP domain that controls epithelial cell junction assembly, crosslinks and bundles actin and deactivates the small GTPase protein encoded by the RHOA gene. RhoA activity is associated with cytoskeleton regulation of actin stress fiber formation and actomyosin contractility. Myo9A was detected in mouse and human podocytes in vitro and in vivo. Knockin mice carrying the p.Arg701^∗MYO9A (Myo9A^R701X) generated by gene editing developed proteinuria, podocyte effacement and FSGS. Kidneys and podocytes from Myo9A^R701X/+ mutant mice revealed Myo9A haploinsufficiency, increased RhoA activity, decreased Myo9A-actin-calmodulin interaction, impaired podocyte attachment and migration. Our results indicate that Myo9A is a novel component of the podocyte cytoskeletal apparatus that regulates RhoA activity and podocyte function. Thus, Myo9A^R701X/+ knock-in mice recapitulate the proband FSGS phenotype, demonstrate that p.R701X Myo9A is an FSGS-causing mutation in mice and suggest that heterozygous loss-of-function MYO9A mutations may cause a novel form of human autosomal dominant FSGS. Hence, identification of MYO9A pathogenic variants in additional individuals with familial or sporadic FSGS is needed to ascertain the gene contribution to disease.

A case of severe autosomal dominant spinal muscular atrophy with lower extremity predominance caused by a de novo BICD2 mutation

BACKGROUND

Heterozygous variants in BICD2 cause autosomal dominant spinal muscular atrophy with lower extremity predominance. These variants are also identified in individuals with severe forms of congenital muscle atrophy representing arthrogryposis multiplex.

CASE REPORT

A girl was born with severe muscle weakness and respiratory distress. A fetal ultrasound had detected polyhydramnios and multiple joint contractures in utero. She was born with severe muscle weakness and respiratory distress. Bilateral hip joint dislocation and multiple bone fractures were also present at birth. Although she depends on medical care, including assisted ventilation and tube feeding, she has reached eight years of age. Her motor developmental skills were reduced owing to muscle weakness and deformity of her lower extremities, whereas her cognitive development slowly but steadily grew. Whole exome sequencing revealed a novel de novo missense BICD2 variant (c.1625G > A, p.(Cys542Tyr)), which was evaluated as likely pathogenic.

CONCLUSION

This is the first case report of a severe form of spinal muscular atrophy with lower extremity predominance caused by a de novo BICD2 variant in Japan.

Clinical exome sequencing in the diagnosis of pediatric neuromuscular disease

BACKGROUND

The diagnosis of uncommon pediatric neuromuscular disease (NMD) is challenging due to genetic and phenotypic heterogeneity, yet is important to guide treatment, prognosis, and recurrence risk. Patients with diagnostically challenging presentations typically undergo extensive testing with variable molecular diagnostic yield. Given the advancement in next generation sequencing (NGS), we investigated the value of clinical whole exome sequencing (ES) in uncommon pediatric NMD.

METHODS

A retrospective cohort study of 106 pediatric NMD patients with a combination of ES, chromosomal microarray (CMA), and candidate gene testing was completed at a large tertiary referral center.

RESULTS

A molecular diagnosis was achieved in 37/79 (46%) patients with ES, 4/44 (9%) patients with CMA, and 15/74 (20%) patients with candidate gene testing. In 2/79 (3%) patients, a dual molecular diagnosis explaining the neuromuscular disease process was identified. A total of 42 patients (53%) who received ES remained without a molecular diagnosis at the conclusion of the study.

CONCLUSIONS

Due to NGS, molecular diagnostic yield of rare neurological diseases is at an all-time high. We show that ES has a higher diagnostic rate compared to other genetic tests in a complex pediatric neuromuscular disease cohort and should be considered early in the diagnostic journey for select NMD patients with challenging presentations in which a clinical diagnosis is not evident.

Distal Arthrogryposis and Lethal Congenital Contracture Syndrome - An Overview

Distal arthrogryposis (DA) is a skeletal muscle disorder which can be classified under a broader term as Arthrogryposis multiplex contractures. DA is characterized by the presence of joint contractures at various parts of the body, particularly in distal extremities. It is identified as an autosomal dominant and a rare X-linked recessive disorder associated with increased connective tissue formation around joints in such way that immobilizes muscle movement causing deformities. DA is again classified into various types since it manifests as a range of conditions representing different etiologies. Myopathy is one of the most commonly listed etiologies of DA. The mutations in sarcomeric protein-encoding genes lead to decreased sarcomere integrity, which is often associated with this disorder. Also, skeletal disorders are often associated with cardiac disorders. Some studies mention the presence of cardiomyopathy in patients with skeletal dysfunction. Therefore, it is hypothesized that the congenitally mutated protein that causes DA can also lead to cardiomyopathy. In this review, we will summarize the different forms of DA and their clinical features, along with gene mutations responsible for causing DA in its different forms. We will also examine reports that list mutations also known to cause heart disorders in the presence of DA.

Unusual context of CENPJ variants and primary microcephaly: compound heterozygosity and nonconsanguinity in an Argentinian patient

Primary microcephaly (MCPH) is a genetically heterogeneous disorder showing an autosomal recessive mode of inheritance. Patients with MCPH present head circumference values two or three standard deviations (SDs) significantly below the mean for age- and sex-matched populations. MCPH is associated with a nonprogressive mild to severe intellectual disability, with normal brain structure in most patients, or with a small brain and gyri without visceral malformations. We present the case of an adult patient born from Argentinian nonconsanguineous healthy parents. He had a head circumference >5 SD below the mean, cerebral neuroimaging showing hypoplasia of the corpus callosum, bilateral migration disorder with heterotopia of the sylvian fissure and colpocephaly. The patient was compound heterozygous for pathogenic variants in the CENPJ gene (c.289dupA inherited from his mother and c.1132 C > T inherited from his father). Our patient represents an uncommon situation for the usual known context of CENPJ and MCPH, including family origin (Argentinian), pedigree (nonconsanguineous), and genotype (a compound heterozygous case with two variants predicting a truncated protein). Next-generation sequencing studies applied in a broader spectrum of clinical presentations of MCPH syndromes may discover additional similar patients and families.

Mucopolysaccharidosis-Plus Syndrome

Previously, we reported a novel disease of impaired glycosaminoglycans (GAGs) metabolism without deficiency of known lysosomal enzymes—mucopolysaccharidosis-plus syndrome (MPSPS). MPSPS, whose pathophysiology is not elucidated, is an autosomal recessive multisystem disorder caused by a specific mutation p.R498W in the VPS33A gene. VPS33A functions in endocytic and autophagic pathways, but p.R498W mutation did not affect both of these pathways in the patient’s skin fibroblast. Nineteen patients with MPSPS have been identified: seventeen patients were found among the Yakut population (Russia) and two patients from Turkey. Clinical features of MPSPS patients are similar to conventional mucopolysaccharidoses (MPS). In addition to typical symptoms for conventional MPS, MPSPS patients developed other features such as congenital heart defects, renal and hematopoietic disorders. Diagnosis generally requires evidence of clinical picture similar to MPS and molecular genetic testing. Disease is very severe, prognosis is unfavorable and most of patients died at age of 10–20 months. Currently there is no specific therapy for this disease and clinical management is limited to supportive and symptomatic treatment.

A novel de novo nonsense mutation in ZC4H2 causes Wieacker-Wolff Syndrome

Background

Wieacker‐Wolff syndrome (WWS) is a congenital X‐linked neuromuscular disorder, which was firstly reported in 1985. Zinc finger C4H2‐type containing (ZC4H2) gene has been found to be associated with the disease pathogenesis. However, the underlying mechanism remains elusive.

Methods

Whole‐exome sequencing was performed to identify the mutations. Expression plasmids were constructed and cell culture and immune‐biochemical assays were used to examine the effects of the mutation.

Results

We reported a female patient with classical symptoms of WWS and discovered a novel nonsense heterozygous mutation (p.R67X; c.199C>T) in ZC4H2 gene in the patient but not in her parents. The mutation resulted in a 66 amino‐acid truncated ZC4H2 protein. The mutation is located in the key helix domain and it altered the subcellular locations of the mutant ZC4H2 protein. X‐chromosome inactivation (XCI) pattern analysis revealed that the XCI ratio of the proband was 22:78.

Conclusion

Female heterozygous carriers with nonsense mutation with a truncated ZC4H2 protein could lead to the pathogenesis of Wieacker‐Wolff syndrome and our study provides a potential new target for the disease treatment.

Skeletal MyBP-C isoforms tune the molecular contractility of divergent skeletal muscle systems

Skeletal muscle myosin-binding protein C (MyBP-C) is a myosin thick filament-associated protein, localized through its C terminus to distinct regions (C-zones) of the sarcomere. MyBP-C modulates muscle contractility, presumably through its N terminus extending from the thick filament and interacting with either the myosin head region and/or the actin thin filament. Two isoforms of MyBP-C (fast- and slow-type) are expressed in a muscle type-specific manner. Are the expression, localization, and Ca²⁺-dependent modulatory capacities of these isoforms different in fast-twitch extensor digitorum longus (EDL) and slow-twitch soleus (SOL) muscles derived from Sprague-Dawley rats? By mass spectrometry, 4 MyBP-C isoforms (1 fast-type MyBP-C and 3 N-terminally spliced slow-type MyBP-C) were expressed in EDL, but only the 3 slow-type MyBP-C isoforms in SOL. Using EDL and SOL native thick filaments in which the MyBP-C stoichiometry and localization are preserved, native thin filament sliding over these thick filaments showed that, only in the C-zone, MyBP-C Ca²⁺ sensitizes the thin filament and slows thin filament velocity. These modulatory properties depended on MyBP-C's N terminus as N-terminal proteolysis attenuated MyBP-C's functional capacities. To determine each MyBP-C isoform's contribution to thin filament Ca²⁺ sensitization and slowing in the C-zone, we used a combination of in vitro motility assays using expressed recombinant N-terminal fragments and in silico mechanistic modeling. Our results suggest that each skeletal MyBP-C isoform's N terminus is functionally distinct and has modulatory capacities that depend on the muscle type in which they are expressed, providing the potential for molecular tuning of skeletal muscle performance through differential MyBP-C expression.

Central nervous system involvement in arthrogryposis multiplex congenita: Overview of causes, diagnosis, and care

Arthrogryposis or AMC, arthrogryposis multiplex congenita, is defined as multiple congenital joint contractures in more than two joints and in different body areas. The common cause of all AMC is lack of movement in utero, which in turn can have different causes, one of which is CNS involvement. Intellectual disability/CNS involvement is found in approximately 25% of all AMC. AMC with CNS involvement includes a large number of genetic syndromes. So far, more than 400 genes have been identified as linked to AMC, with and without CNS involvement. A number of neonatally lethal syndromes and syndromes resulting in severe disability due to CNS malfunction belong to this group of syndromes. There are several X-linked disorders with AMC, which are primarily related to intellectual disability. A number of neuromuscular disorders may include AMC and CNS/brain involvement. Careful clinical evaluation by a geneticist and a pediatrician/pediatric neurologist is the first step in making a specific diagnosis. Further investigations may include MRI of the brain and spinal cord, electroencephalogram, blood chemistry for muscle enzymes, other organ investigations (ophtalmology, cardiology, gastrointestinal, and genitourinary systems). Nerve conduction studies, electromyogram, and muscle pathology may be of help when there is associated peripheral nervous system involvement. But most importantly, genetic investigations with targeted or rather whole exome or genome sequencing should be performed. A correct diagnosis is important in planning adequate treatment, in genetic counselling and also for future understanding of pathogenic mechanisms and possible new treatments. A multidiciplinary team is needed both in investigation and treatment.

The diagnostic workup in a patient with AMC: Overview of the clinical evaluation and paraclinical analyses with review of the literature

Arthrogryposis multiplex congenita, or AMC, is a clinical sign defined as congenital contractures of at least two joint levels. These joint contractures are always secondary to diminished fetal movement which can have numerous causes that affect any part of the anatomical structures implicated in movement: the central nervous system, the anterior horn cell, the nerve, the neuromuscular junction, the muscle, or the joint itself. Make a precise diagnosis of the cause in a patient with multiple joint contractures is therefore challenging. The aim of this article is to summarize the use and diagnostic value of common examinations and analyses performed postnatally in patients affected by AMC from a literature review. We also compare this data with results from our clinical practice. Even though it is difficult to give precise guidelines today, it appears that genetic studies, such as whole exome or genome analysis in all patients and chromosomal microarray analysis in patients with intellectual disability and AMC should be preferred as first tier investigations over EMG and muscle biopsy.

Gene ontology analysis of arthrogryposis (multiple congenital contractures)

In 2016, we published an article applying Gene Ontology Analysis to the genes that had been reported to be associated with arthrogryposis (multiple congenital contractures) (Hall & Kiefer, 2016). At that time, 320 genes had been reported to have mutations associated with arthrogryposis. All were associated with decreased fetal movement. These 320 genes were analyzed by biological process and cellular component categories, and yielded 22 distinct groupings. Since that time, another 82 additional genes have been reported, now totaling 402 genes, which when mutated, are associated with arthrogryposis (arthrogryposis multiplex congenita). So, we decided to update the analysis in order to stimulate further research and possible treatment. Now, 29 groupings can be identified, but only 19 groups have more than one gene.

Research platform for children with arthrogryposis multiplex congenita: Findings from the pilot registry

A pediatric registry for arthrogryposis multiplex congenita (AMC) proposes to advance research by providing the platform to inform the distribution, etiology, and natural history of AMC. The registry was piloted on 40 families of children (mean = 8.25 years, 48% males) presenting with AMC across two hospitals in North America. Data on the child's demographic and newborn variables, mothers' and fathers' demographic variables, lifestyle habits, and medical history were collected using a telephone interview with the primary caregiver and review of medical charts. Mean gestational age was 38 weeks, 97% of children presented with lower extremity deformities, and 74% of neonatal interventions targeted the lower extremity. Newborns spent an average of 14 days in the hospital (range 2-56 days) mostly for diagnostic workup and feeding difficulties. Half (49%) of the sample had internal organ involvement. Genetic testing was done on 48% of the children, including chromosome studies, single gene, whole-exome/genome sequencing, and/or microarray studies. Genetic findings were inconclusive in most. Two-thirds of mothers (67%) reported inconsistently feeling fetal movements. This pilot study contributed to the refinement of participant selection, identification of data source, expansion of data sets, and areas for future exploration prior to the implementation of a multisite AMC pediatric registry.

Fetal arthrogryposis multiplex congenita/fetal akinesia deformation sequence (FADS)-Aetiology, diagnosis, and management

Arthrogryposis multiplex congenita (AMC) refers to an aetiologically heterogenous condition, which consists of joint contractures affecting two or more joints starting prenatally. The incidence is approximately one in 3000 live births; however, the prenatal incidence is higher, indicating a high intrauterine mortality. Over 320 genes have been implicated showing the genetic heterogeneity of the condition. AMC can be of extrinsic aetiology resulting from intrauterine crowding secondary to congenital structural uterine abnormalities (eg, bicornuate or septate uterus), uterine tumors (eg, fibroid), or multifetal pregnancy or intrinsic/primary/fetal aetiology, due to functional abnormalities in the brain, spinal cord, peripheral nerves, neuromuscular junction, muscles, bones, restrictive dermopathies, tendons and joints. Unlike many of the intrinsic/primary/fetal causes which are difficult to treat, secondary AMC can be treated by physiotherapy with good response. Primary cases may present prenatally with fetal akinesia associated with joint contractures and occasionally brain abnormalities, decreased muscle bulk, polyhydramnios, and nonvertex presentation while the secondary cases usually present with isolated contractures. Complete prenatal and postnatal investigations are needed to identify an underlying aetiology and provide information regarding its prognosis and inheritance, which is critical for the obstetrical care providers and families to optimize the pregnancy management and address future reproductive plans.

The Genomics of Arthrogryposis, a Complex Trait: Candidate Genes and Further Evidence for Oligogenic Inheritance

Arthrogryposis is a clinical finding that is present either as a feature of a neuromuscular condition or as part of a systemic disease in over 400 Mendelian conditions. The underlying molecular etiology remains largely unknown because of genetic and phenotypic heterogeneity. We applied exome sequencing (ES) in a cohort of 89 families with the clinical sign of arthrogryposis. Additional molecular techniques including array comparative genomic hybridization (aCGH) and Droplet Digital PCR (ddPCR) were performed on individuals who were found to have pathogenic copy number variants (CNVs) and mosaicism, respectively. A molecular diagnosis was established in 65.2% (58/89) of families. Eleven out of 58 families (19.0%) showed evidence for potential involvement of pathogenic variation at more than one locus, probably driven by absence of heterozygosity (AOH) burden due to identity-by-descent (IBD). RYR3, MYOM2, ERGIC1, SPTBN4, and ABCA7 represent genes, identified in two or more families, for which mutations are probably causative for arthrogryposis. We also provide evidence for the involvement of CNVs in the etiology of arthrogryposis and for the idea that both mono-allelic and bi-allelic variants in the same gene cause either similar or distinct syndromes. We were able to identify the molecular etiology in nine out of 20 families who underwent reanalysis. In summary, our data from family-based ES further delineate the molecular etiology of arthrogryposis, yielded several candidate disease-associated genes, and provide evidence for mutational burden in a biological pathway or network. Our study also highlights the importance of reanalysis of individuals with unsolved diagnoses in conjunction with sequencing extended family members.

CORVET, CHEVI and HOPS – multisubunit tethers of the endo-lysosomal system in health and disease

Multisubunit tethering complexes (MTCs) are multitasking hubs that form a link between membrane fusion, organelle motility and signaling. CORVET, CHEVI and HOPS are MTCs of the endo-lysosomal system. They regulate the major membrane flows required for endocytosis, lysosome biogenesis, autophagy and phagocytosis. In addition, individual subunits control complex-independent transport of specific cargoes and exert functions beyond tethering, such as attachment to microtubules and SNARE activation. Mutations in CHEVI subunits lead to arthrogryposis, renal dysfunction and cholestasis (ARC) syndrome, while defects in CORVET and, particularly, HOPS are associated with neurodegeneration, pigmentation disorders, liver malfunction and various forms of cancer. Diseases and phenotypes, however, vary per affected subunit and a concise overview of MTC protein function and associated human pathologies is currently lacking. Here, we provide an integrated overview on the cellular functions and pathological defects associated with CORVET, CHEVI or HOPS proteins, both with regard to their complexes and as individual subunits. The combination of these data provides novel insights into how mutations in endo-lysosomal proteins lead to human pathologies.

A retrospective review of multiple findings in diagnostic exome sequencing: half are distinct and half are overlapping diagnoses

PURPOSE

We evaluated clinical and genetic features enriched in patients with multiple Mendelian conditions to determine which patients are more likely to have multiple potentially relevant genetic findings (MPRF).

METHODS

Results of the first 7698 patients who underwent exome sequencing at Ambry Genetics were reviewed. Clinical and genetic features were examined and degree of phenotypic overlap between the genetic diagnoses was evaluated.

RESULTS

Among patients referred for exome sequencing, 2% had MPRF. MPRF were more common in patients from consanguineous families and patients with greater clinical complexity. The difference in average number of organ systems affected is small: 4.3 (multiple findings) vs. 3.9 (single finding) and may not be distinguished in clinic.

CONCLUSION

Patients with multiple genetic diagnoses had a slightly higher number of organ systems affected than patients with single genetic diagnoses, largely because the comorbid conditions affected overlapping organ systems. Exome testing may be beneficial for all cases with multiple organ systems affected. The identification of multiple relevant genetic findings in 2% of exome patients highlights the utility of a comprehensive molecular workup and updated interpretation of existing genomic data; a single definitive molecular diagnosis from analysis of a limited number of genes may not be the end of a diagnostic odyssey.

CHRNG-related nonlethal multiple pterygium syndrome: Muscle imaging pattern and clinical, histopathological, and molecular genetic findings

Mutations in the CHRNG gene cause autosomal recessive multiple pterygium syndrome (MPS). Herein we present a long-term follow-up of seven patients with CHRNG-related nonlethal MPS and we compare them with the 57 previously published patients. The objective is defining not only the clinical, histopathological, and molecular genetic characteristics, but also the type and degree of muscle involvement on whole-body magnetic resonance imaging (WBMRI). CHRNG mutations lead to a distinctive phenotype characterized by multiple congenital contractures, pterygium, and facial dysmorphism, with a stable clinical course over the years. Postnatal abnormalities at the neuromuscular junction were observed in the muscle biopsy of these patients. WBMRI showed distinctive features different from other arthrogryposis multiple congenita. A marked muscle bulk reduction is the predominant finding, mostly affecting the spinal erector muscles and gluteus maximus. Fatty infiltration was only observed in deep paravertebral muscles and distal lower limbs. Mutations in CHRNG are mainly located at the extracellular domain of the protein. Our study contributes to further define the phenotypic spectrum of CHRNG-related nonlethal MPS, including muscle imaging features, which may be useful in distinguishing it from other diffuse arthrogryposis entities.