Congenital Myasthenic Syndromes in Belgium: Genetic and Clinical Characterization of Pediatric and Adult Patients.

BACKGROUND: Congenital myasthenic syndromes (CMS) are a group of genetic disorders characterized by impaired neuromuscular transmission. CMS typically present at a young age with fatigable muscle weakness, often with an abnormal response after repetitive nerve stimulation (RNS). Pharmacologic treatment can improve symptoms, depending on the underlying defect. Prevalence is likely underestimated. This study reports on patients with CMS followed in Belgium in 2022. METHODS: Data were gathered retrospectively from the medical charts. Only likely pathogenic and pathogenic variants were included in the analysis. RESULTS: We identified 37 patients, resulting in an estimated prevalence of 3.19 per 1,000,000. The patients harbored pathogenic variants in CHRNE, RAPSN, DOK7, PREPL, CHRNB1, CHRNG, COLQ, MUSK, CHRND, GFPT1, and GMPPB. CHRNE was the most commonly affected gene. Most patients showed disease onset at birth, during infancy, or during childhood. Symptom onset was at adult age in seven patients, caused by variants in CHRNE, DOK7, MUSK, CHRND, and GMPPB. Severity and distribution of weakness varied, as did the presence of respiratory involvement, feeding problems, and extraneuromuscular manifestations. RNS was performed in 23 patients of whom 18 demonstrated a pathologic decrement. Most treatment responses were predictable based on the genotype. CONCLUSIONS: This is the first pooled characterization of patients with CMS in Belgium. We broaden the phenotypical spectrum of pathogenic variants in CHRNE with adult-onset CMS. Systematically documenting larger cohorts of patients with CMS can aid in better clinical characterization and earlier recognition of this rare disease. We emphasize the importance of establishing a molecular genetic diagnosis to tailor treatment choices.

Comparison of SARS-CoV-2 seroconversion in children with chronic diseases with healthy children and adults during the first waves of the COVID-19 pandemic.

Background: Infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is clinically diverse, and children have a low risk of developing severe coronavirus disease 2019 (COVID-19). However, children with chronic diseases have a potentially increased risk. Methods: We performed a prospective surveillance study with longitudinal serum SARS-CoV-2 anti-nucleocapsid antibody quantification and questionnaires in pediatric tertiary care patients during the first waves of the COVID-19 pandemic (November 2020-September 2021). The results were compared with those of healthy children and adults from the same geographic area. Results: We obtained 525 samples from 362 patients (M/F ratio of 1.3:1; median age of 11.1 years) comprising children with immune-suppressive or immune-modulating drugs (32.9%), inborn errors of immunity (23.5%), type 1 diabetes mellitus (15.2%), and rheumatic diseases (11.9%). A total of 51 (9.7%) samples were seropositive among 37/351 children (10.5%). Seropositivity increased from 5.8% in November-December 2020 to 21.6% in July-September 2021. Compared with adults, a longitudinal analysis revealed reduced seroprevalence but similar kinetics as in children from the same country. Demographic or social variables and disease characteristics did not correlate with seropositivity. Being obese and household contact with COVID-19-infected individuals significantly increased the odds of infection. The majority of seropositive patients had mild symptoms (21/37). One-third were asymptomatic and/or unaware of having COVID-19 (10/37). Four patients (4/37) needed hospitalization, with good clinical outcomes. Conclusions: Although harboring a chronic disease, we observed a low SARS-CoV-2 incidence in a cohort of pediatric tertiary care patients, comparable with healthy children during the first year of the pandemic. Infection was mostly associated with mild symptoms.

Functional and clinical studies reveal pathophysiological complexity of CLCN4-related neurodevelopmental condition.

Missense and truncating variants in the X-chromosome-linked CLCN4 gene, resulting in reduced or complete loss-of-function (LOF) of the encoded chloride/proton exchanger ClC-4, were recently demonstrated to cause a neurocognitive phenotype in both males and females. Through international clinical matchmaking and interrogation of public variant databases we assembled a database of 90 rare CLCN4 missense variants in 90 families: 41 unique and 18 recurrent variants in 49 families. For 43 families, including 22 males and 33 females, we collated detailed clinical and segregation data. To confirm causality of variants and to obtain insight into disease mechanisms, we investigated the effect on electrophysiological properties of 59 of the variants in Xenopus oocytes using extended voltage and pH ranges. Detailed analyses revealed new pathophysiological mechanisms: 25% (15/59) of variants demonstrated LOF, characterized by a "shift" of the voltage-dependent activation to more positive voltages, and nine variants resulted in a toxic gain-of-function, associated with a disrupted gate allowing inward transport at negative voltages. Functional results were not always in line with in silico pathogenicity scores, highlighting the complexity of pathogenicity assessment for accurate genetic counselling. The complex neurocognitive and psychiatric manifestations of this condition, and hitherto under-recognized impacts on growth, gastrointestinal function, and motor control are discussed. Including published cases, we summarize features in 122 individuals from 67 families with CLCN4-related neurodevelopmental condition and suggest future research directions with the aim of improving the integrated care for individuals with this diagnosis.

Shortcutting the diagnostic odyssey: the multidisciplinary Program for Undiagnosed Rare Diseases in adults (UD-PrOZA).

BACKGROUND: In order to facilitate the diagnostic process for adult patients suffering from a rare disease, the Undiagnosed Disease Program (UD-PrOZA) was founded in 2015 at the Ghent University Hospital in Belgium. In this study we report the five-year results of our multidisciplinary approach in rare disease diagnostics. METHODS: Patients referred by a healthcare provider, in which an underlying rare disease is likely, qualify for a UD-PrOZA evaluation. UD-PrOZA uses a multidisciplinary clinical approach combined with state-of-the-art genomic technologies in close collaboration with research facilities to diagnose patients. RESULTS: Between 2015 and 2020, 692 patients (94% adults) were referred of which 329 (48%) were accepted for evaluation. In 18% (60 of 329) of the cases a definite diagnosis was made. 88% (53 of 60) of the established diagnoses had a genetic origin. 65% (39 of 60) of the genetic diagnoses were made through whole exome sequencing (WES). The mean time interval between symptom-onset and diagnosis was 19 years. Key observations included novel genotype-phenotype correlations, new variants in known disease genes and the identification of three new disease genes. In 13% (7 of 53), identifying the molecular cause was associated with therapeutic recommendations and in 88% (53 of 60), gene specific genetic counseling was made possible. Actionable secondary findings were reported in 7% (12 of 177) of the patients in which WES was performed. CONCLUSION: UD-PrOZA offers an innovative interdisciplinary platform to diagnose rare diseases in adults with previously unexplained medical problems and to facilitate translational research.

Clinical implementation of RNA sequencing for Mendelian disease diagnostics.

BACKGROUND: Lack of functional evidence hampers variant interpretation, leaving a large proportion of individuals with a suspected Mendelian disorder without genetic diagnosis after whole genome or whole exome sequencing (WES). Research studies advocate to further sequence transcriptomes to directly and systematically probe gene expression defects. However, collection of additional biopsies and establishment of lab workflows, analytical pipelines, and defined concepts in clinical interpretation of aberrant gene expression are still needed for adopting RNA sequencing (RNA-seq) in routine diagnostics. METHODS: We implemented an automated RNA-seq protocol and a computational workflow with which we analyzed skin fibroblasts of 303 individuals with a suspected mitochondrial disease that previously underwent WES. We also assessed through simulations how aberrant expression and mono-allelic expression tests depend on RNA-seq coverage. RESULTS: We detected on average 12,500 genes per sample including around 60% of all disease genes-a coverage substantially higher than with whole blood, supporting the use of skin biopsies. We prioritized genes demonstrating aberrant expression, aberrant splicing, or mono-allelic expression. The pipeline required less than 1 week from sample preparation to result reporting and provided a median of eight disease-associated genes per patient for inspection. A genetic diagnosis was established for 16% of the 205 WES-inconclusive cases. Detection of aberrant expression was a major contributor to diagnosis including instances of 50% reduction, which, together with mono-allelic expression, allowed for the diagnosis of dominant disorders caused by haploinsufficiency. Moreover, calling aberrant splicing and variants from RNA-seq data enabled detecting and validating splice-disrupting variants, of which the majority fell outside WES-covered regions. CONCLUSION: Together, these results show that streamlined experimental and computational processes can accelerate the implementation of RNA-seq in routine diagnostics.

Clinical Heterogeneity in MT-ATP6 Pathogenic Variants: Same Genotype-Different Onset.

Human mitochondrial disease exhibits large variation of clinical phenotypes, even in patients with the same causative gene defect. We illustrate this heterogeneity by confronting clinical and biochemical data of two patients with the uncommon pathogenic homoplasmic NC_012920.1(MT-ATP6):m.9035T>C variant in MT-ATP6. Patient 1 presented as a toddler with severe motor and speech delay and spastic ataxia without extra-neurologic involvement. Patient 2 presented in adolescence with ataxia and ophthalmoplegia without cognitive or motor impairment. Respiratory chain complex activities were normal in cultured skin fibroblasts from both patients when calculated as ratios over citrate synthase activity. Native gels found presence of subcomplexes of complex V in fibroblast and/or skeletal muscle. Bioenergetic measurements in fibroblasts from both patients detected reduced spare respiratory capacities and altered extracellular acidification rates, revealing a switch from mitochondrial respiration to glycolysis to uphold ATP production. Thus, in contrast to the differing disease presentation, biochemical evidence of mitochondrial deficiency turned out quite similar. We conclude that biochemical analysis remains a valuable tool to confirm the genetic diagnosis of mitochondrial disease, especially in patients with new gene variants or atypical clinical presentation.

Human germline nuclear transfer to overcome mitochondrial disease and failed fertilization after ICSI.

PURPOSE: Providing additional insights on the efficacy of human nuclear transfer (NT). Here, and earlier, NT has been applied to minimize transmission risk of mitochondrial DNA (mtDNA) diseases. NT has also been proposed for treating infertility, but it is still unclear which infertility indications would benefit. In this work, we therefore additionally assess the applicability of NT to overcome failed fertilization. METHODS: Patient 1 carries a homoplasmic mtDNA mutation (m.11778G > A). Seventeen metaphase II (MII) oocytes underwent pre-implantation genetic testing (PGT), while five MII oocytes were used for spindle transfer (ST), and one in vitro matured (IVM) metaphase I oocyte underwent early pronuclear transfer (ePNT). Patients 2-3 experienced multiple failed intracytoplasmic sperm injection (ICSI) and ICSI-assisted oocyte activation (AOA) cycles. For these patients, the obtained MII oocytes underwent an additional ICSI-AOA cycle, while the IVM oocytes were subjected to ST. RESULTS: For patient 1, PGT-M confirmed mutation loads close to 100%. All ST-reconstructed oocytes fertilized and cleaved, of which one progressed to the blastocyst stage. The reconstructed ePNT-zygote reached the morula stage. These samples showed an average mtDNA carry-over rate of 2.9% ± 0.8%, confirming the feasibility of NT to reduce mtDNA transmission. For patient 2-3 displaying fertilization failure, ST resulted in, respectively, 4/5 and 6/6 fertilized oocytes, providing evidence, for the first time, that NT can enable successful fertilization in this patient population. CONCLUSION: Our study showcases the repertoire of disorders for which NT can be beneficial, to overcome either mitochondrial disease transmission or failed fertilization after ICSI-AOA.

Prolyl endopeptidase-like is a (thio)esterase involved in mitochondrial respiratory chain function.

Deficiency of the serine hydrolase prolyl endopeptidase-like (PREPL) causes a recessive metabolic disorder characterized by neonatal hypotonia, feeding difficulties, and growth hormone deficiency. The pathophysiology of PREPL deficiency and the physiological substrates of PREPL remain largely unknown. In this study, we connect PREPL with mitochondrial gene expression and oxidative phosphorylation by analyzing its protein interactors. We demonstrate that the long PREPLL isoform localizes to mitochondria, whereas PREPLS remains cytosolic. Prepl KO mice showed reduced mitochondrial complex activities and disrupted mitochondrial gene expression. Furthermore, mitochondrial ultrastructure was abnormal in a PREPL-deficient patient and Prepl KO mice. In addition, we reveal that PREPL has (thio)esterase activity and inhibition of PREPL by Palmostatin M suggests a depalmitoylating function. We subsequently determined the crystal structure of PREPL, thereby providing insight into the mechanism of action. Taken together, PREPL is a (thio)esterase rather than a peptidase and PREPLL is involved in mitochondrial homeostasis.

Expanding the molecular spectrum and the neurological phenotype related to CAMTA1 variants.

The CAMTA1-associated phenotype was initially defined in patients with intragenic deletions and duplications who showed nonprogressive congenital ataxia, with or without intellectual disability. Here, we describe 10 individuals with CAMTA1 variants: nine previously unreported (likely) pathogenic variants comprising one missense, four frameshift and four nonsense variants, and one missense variant of unknown significance. Six patients were diagnosed following whole exome sequencing and four individuals with exome-based targeted panel analysis. Most of them present with developmental delay, manifesting in speech and motor delay. Other frequent findings are hypotonia, cognitive impairment, cerebellar dysfunction, oculomotor abnormalities, and behavioral problems. Feeding problems occur more frequently than previously observed. In addition, we present a systematic review of 19 previously published individuals with causal variants, including copy number, truncating, and missense variants. We note a tendency of more severe cognitive impairment and recurrent dysmorphic features in individuals with a copy number variant. Pathogenic variants are predominantly observed in and near the N- and C- terminal functional domains. Clinical heterogeneity is observed, but 3'-terminal variants seem to associate with less pronounced cerebellar dysfunction.

Diagnostic utility of small fiber analysis in skin biopsies from children with chronic pain.

INTRODUCTION: Small fiber neuropathies (SFN) are associated with a reduction in quality of life. In adults, epidermal nerve fiber density (END) analysis is recommended for the diagnosis of SFN. In children, END assessment is not often performed. We analyzed small nerve fiber innervation to elucidate the potential diagnostic role of skin biopsies in young patients with pain. METHODS: Epidermal nerve fiber density and sudomotor neurite density (SND) were assessed in skin biopsies from 26 patients aged 7 to 20 years (15 female patients) with unexplained chronic pain. The results were compared with clinical data. RESULTS: Epidermal nerve fiber density was abnormal in 50% and borderline in 35% of patients. An underlying medical condition was found in 42% of patients, including metabolic, autoimmune, and genetic disorders. DISCUSSION: Reduction of epidermal nerve fibers can be associated with treatable conditions. Therefore, the analysis of END in children with pain may help to uncover a possible cause and guide potential treatment options.

TANGO2 deficiency as a cause of neurodevelopmental delay with indirect effects on mitochondrial energy metabolism.

Exome sequencing has recently identified mutations in the gene TANGO2 (transport and Golgi organization 2) as a cause of developmental delay associated with recurrent crises involving rhabdomyolysis, cardiac arrhythmias, and metabolic derangements. The disease is not well understood, in part as the cellular function and subcellular localization of the TANGO2 protein remain unknown. Furthermore, the clinical syndrome with its heterogeneity of symptoms, signs, and laboratory findings is still being defined. Here, we describe 11 new cases of TANGO2-related disease, confirming and further expanding the previously described clinical phenotype. Patients were homozygous or compound heterozygous for previously described exonic deletions or new frameshift, splice site, and missense mutations. All patients showed developmental delay with ataxia, dysarthria, intellectual disability, or signs of spastic diplegia. Of importance, we identify two subjects (aged 12 and 17 years) who have never experienced any overt episode of the catabolism-induced metabolic crises typical for the disease. Mitochondrial complex II activity was mildly reduced in patients investigated in association with crises but normal in other patients. In one deceased patient, post-mortem autopsy revealed heterotopic neurons in the cerebral white matter, indicating a possible role for TANGO2 in neuronal migration. Furthermore, we have addressed the subcellular localization of several alternative isoforms of TANGO2, none of which were mitochondrial but instead appeared to have a primarily cytoplasmic localization. Previously described aberrations in Golgi morphology were not observed in cultured skin fibroblasts.

De novo missense variants in RAC3 cause a novel neurodevelopmental syndrome.

PURPOSE: RAC3 is an underexamined member of the Rho GTPase gene family that is expressed in the developing brain and linked to key cellular functions. De novo missense variants in the homolog RAC1 were recently associated with developmental disorders. In the RAC subfamily, transforming missense changes at certain shared residues have been observed in human cancers and previously characterized in experimental studies. The purpose of this study was to determine whether constitutional dysregulation of RAC3 is associated with human disease. METHODS: We discovered a RAC3 variant in the index case using genome sequencing, and searched for additional variants using international data-sharing initiatives. Functional effects of the variants were assessed using a multifaceted approach generalizable to most clinical laboratory settings. RESULTS: We rapidly identified five individuals with de novo monoallelic missense variants in RAC3, including one recurrent change. Every participant had severe intellectual disability and brain malformations. In silico protein modeling, and prior in vivo and in situ experiments, supported a transforming effect for each of the three different RAC3 variants. All variants were observed in databases of somatic variation in cancer. CONCLUSIONS: Missense variants in RAC3 cause a novel brain disorder, likely through a mechanism of constitutive protein activation.

Leigh syndrome followed by parkinsonism in an adult with homozygous c.626C>T mutation in MTFMT.

OBJECTIVE: To report the clinical, radiologic, biochemical, and molecular characteristics in a 46-year-old participant with adult-onset Leigh syndrome (LS), followed by parkinsonism. METHODS: Case description with diagnostic workup included blood and CSF analysis, skeletal muscle investigations, blue native polyacrylamide gel electrophoresis, whole exome sequencing targeting nuclear genes involved in mitochondrial transcription and translation, cerebral MRI, 123I-FP-CIT brain single-photon emission computed tomography (SPECT), and C-11 raclopride positron emission tomography (PET). RESULTS: The participant was found to have a defect in the oxidative phosphorylation caused by a c.626C>T mutation in the gene coding for mitochondrial methionyl-tRNA formyltransferase (MTFMT), which is a pathogenic mutation affecting intramitochondrial protein translation. The proband had a normal concentration of lactate in blood and no abnormal microscopic findings in skeletal muscle. Cerebral MRI showed bilateral lesions in the striatum, mesencephalon, pons, and medial thalamus. Lactate concentration in CSF was increased. FP-CIT SPECT and C-11 raclopride PET demonstrated a defect in the dopaminergic system. CONCLUSIONS: We report on a case with adult-onset LS related to a MTFMT mutation. Two years after the onset of symptoms of LS, the proband developed a parkinson-like disease. The c.626C>T mutation is the most common pathogenic mutation found in 22 patients reported earlier in the literature with a defect in MTFMT. The age of the previously reported cases varied between 14 months and 24 years. Our report expands the phenotypical spectrum of MTFMT-related neurologic disease and provides clinical evidence for involvement of MTFMT in extrapyramidal syndromes.

Severe hepatopathy and neurological deterioration after start of valproate treatment in a 6-year-old child with mitochondrial tryptophanyl-tRNA synthetase deficiency.

BACKGROUND: The first subjects with deficiency of mitochondrial tryptophanyl-tRNA synthetase (WARS2) were reported in 2017. Their clinical characteristics can be subdivided into three phenotypes (neonatal phenotype, severe infantile onset phenotype, Parkinson-like phenotype). RESULTS: Here, we report on a subject who presented with early developmental delay, motor weakness and intellectual disability and who was considered during several years as having a non-progressive encephalopathy. At the age of six years, she had an epileptic seizure which was treated with sodium valproate. In the months after treatment was started, she developed acute liver failure and severe progressive encephalopathy. Although valproate was discontinued, she died six months later. Spectrophotometric analysis of the oxidative phosphorylation complexes in liver revealed a deficient activity of complex III and low normal activities of the complexes I and IV. Activity staining in the BN-PAGE gel confirmed the low activities of complex I, III and IV and, in addition, showed the presence of a subcomplex of complex V. Histochemically, a mosaic pattern was seen in hepatocytes after cytochrome c oxidase staining. Using Whole Exome Sequencing two known pathogenic variants were detected in WARS2 (c.797delC, p.Pro266ArgfsTer10/ c.938 A > T, p.Lys313Met). CONCLUSION: This is the first report of severe hepatopathy in a subject with WARS2 deficiency. The hepatopathy occurred soon after start of sodium valproate treatment. In the literature, valproate-induced hepatotoxicity was reported in the subjects with pathogenic mutations in POLG and TWNK. This case report illustrates that the course of the disease in the subjects with a mitochondrial defect can be non-progressive during several years. The subject reported here was first diagnosed as having cerebral palsy. Only after a mitochondriotoxic medication was started, the disease became progressive, and the diagnosis of a mitochondrial defect was made.

New insights into the phenotype of FARS2 deficiency.

Mutations in FARS2 are known to cause dysfunction of mitochondrial translation due to deficient aminoacylation of the mitochondrial phenylalanine tRNA. Here, we report three novel mutations in FARS2 found in two patients in a compound heterozygous state. The missense mutation c.1082C>T (p.Pro361Leu) was detected in both patients. The mutations c.461C>T (p.Ala154Val) and c.521_523delTGG (p.Val174del) were each detected in one patient. We report abnormal in vitro aminoacylation assays as a functional validation of the molecular genetic findings. Based on the phenotypic data of previously reported subjects and the two subjects reported here, we conclude that FARS2 deficiency can be associated with two phenotypes: (i) an epileptic phenotype, and (ii) a spastic paraplegia phenotype.

Biallelic Mutations in TMEM126B Cause Severe Complex I Deficiency with a Variable Clinical Phenotype.

Complex I deficiency is the most common biochemical phenotype observed in individuals with mitochondrial disease. With 44 structural subunits and over 10 assembly factors, it is unsurprising that complex I deficiency is associated with clinical and genetic heterogeneity. Massively parallel sequencing (MPS) technologies including custom, targeted gene panels or unbiased whole-exome sequencing (WES) are hugely powerful in identifying the underlying genetic defect in a clinical diagnostic setting, yet many individuals remain without a genetic diagnosis. These individuals might harbor mutations in poorly understood or uncharacterized genes, and their diagnosis relies upon characterization of these orphan genes. Complexome profiling recently identified TMEM126B as a component of the mitochondrial complex I assembly complex alongside proteins ACAD9, ECSIT, NDUFAF1, and TIMMDC1. Here, we describe the clinical, biochemical, and molecular findings in six cases of mitochondrial disease from four unrelated families affected by biallelic (c.635G>T [p.Gly212Val] and/or c.401delA [p.Asn134Ilefs(∗)2]) TMEM126B variants. We provide functional evidence to support the pathogenicity of these TMEM126B variants, including evidence of founder effects for both variants, and establish defects within this gene as a cause of complex I deficiency in association with either pure myopathy in adulthood or, in one individual, a severe multisystem presentation (chronic renal failure and cardiomyopathy) in infancy. Functional experimentation including viral rescue and complexome profiling of subject cell lines has confirmed TMEM126B as the tenth complex I assembly factor associated with human disease and validates the importance of both genome-wide sequencing and proteomic approaches in characterizing disease-associated genes whose physiological roles have been previously undetermined.

Megaconial muscular dystrophy caused by mitochondrial membrane homeostasis defect, new insights from skeletal and heart muscle analyses.

Megaconial congenital muscular dystrophy is a disease caused by pathogenic mutations in the gene encoding choline kinase beta (CHKB). Microscopically, the disease is hallmarked by the presence of enlarged mitochondria at the periphery of skeletal muscle fibres leaving the centre devoid of mitochondria. Clinical characteristics are delayed motor development, intellectual disability and dilated cardiomyopathy in half of reported cases. This study describes a patient presenting with the cardinal clinical features, in whom a homozygous nonsense mutation (c.248_249insT; p.Arg84Profs*209) was identified in CHKB and who was treated by heart transplantation. Microscopic evaluation of skeletal and heart muscles typically showed enlarged mitochondria. Spectrophotometric evaluation in both tissues revealed a mild decrease of all OXPHOS complexes. Using BN-PAGE analysis followed by activity staining subcomplexes of complex V were detected in both tissues, indicating incomplete complex V assembly. Mitochondrial DNA content was not depleted in analysed tissues. This is the first report describing the microscopic and biochemical abnormalities in the heart from an affected patient. A likely hypothesis is that the biochemical findings are caused by an abnormal lipid profile in the inner mitochondrial membrane resulting from a defective choline kinase B activity.