Mono and biallelic variants in HCN2 cause severe neurodevelopmental disorders.

Hyperpolarization activated Cyclic Nucleotide (HCN) gated channels are crucial for various neurophysiological functions, including learning and sensory functions, and their dysfunction are responsible for brain disorders, such as epilepsy. To date, HCN2 variants have only been associated with mild epilepsy and recently, one monoallelic missense variant has been linked to developmental and epileptic encephalopathy. Here, we expand the phenotypic spectrum of HCN2- related disorders by describing twenty-one additional individuals from fifteen unrelated families carrying HCN2 variants. Seventeen individuals had developmental delay/intellectual disability (DD/ID), two had borderline DD/ID, and one had borderline DD. Ten individuals had epilepsy with DD/ID, with median age of onset of 10 months, and one had epilepsy with normal development. Molecular diagnosis identified thirteen different pathogenic HCN2 variants, including eleven missense variants affecting highly conserved amino acids, one frameshift variant, and one in-frame deletion. Seven variants were monoallelic of which five occurred de novo, one was not maternally inherited, one was inherited from a father with mild learning disabilities, and one was of unknown inheritance. The remaining six variants were biallelic, with four homozygous and two compound heterozygous variants. Functional studies using two-electrode voltage-clamp recordings in Xenopus laevis oocytes were performed on three monoallelic variants, p.(Arg324His), p.(Ala363Val), and p.(Met374Leu), and three biallelic variants, p.(Leu377His), p.(Pro493Leu) and p.(Gly587Asp). The p.(Arg324His) variant induced a strong increase of HCN2 conductance, while p.(Ala363Val) and p.(Met374Leu) displayed dominant negative effects, leading to a partial loss of HCN2 channel function. By confocal imaging, we found that the p.(Leu377His), p.(Pro493Leu) and p.(Gly587Asp) pathogenic variants impaired membrane trafficking, resulting in a complete loss of HCN2 elicited currents in Xenopus oocytes. Structural 3D-analysis in depolarized and hyperpolarized states of HCN2 channels, revealed that the pathogenic variants p.(His205Gln), p.(Ser409Leu), p.(Arg324Cys), p.(Asn369Ser) and p.(Gly460Asp) modify molecular interactions altering HCN2 function. Taken together, our data broadens the clinical spectrum associated with HCN2 variants, and disclose that HCN2 is involved in developmental encephalopathy with or without epilepsy.

Expanding the phenotype of PPP1R21-related neurodevelopmental disorder.

PPP1R21 encodes for a conserved protein that is involved in endosomal maturation. Biallelic pathogenic variants in PPP1R21 have been associated with a syndromic neurodevelopmental disorder from studying 13 affected individuals. In this report, we present 11 additional individuals from nine unrelated families and their clinical, radiological, and molecular findings. We identified eight different variants in PPP1R21, of which six were novel variants. Global developmental delay and hypotonia are neurological features that were observed in all individuals. There is also a similar pattern of dysmorphic features with coarse faces as a gestalt observed in several individuals. Common findings in 75% of individuals with available brain imaging include delays in myelination, wavy outline of the bodies of the lateral ventricles, and slight prominence of the bodies of the lateral ventricles. PPP1R21-related neurodevelopmental disorder is associated with a consistent phenotype and should be considered in highly consanguineous individuals presenting with developmental delay/intellectual disability along with coarse facial features.

Biallelic MED27 variants lead to variable ponto-cerebello-lental degeneration with movement disorders.

MED27 is a subunit of the Mediator multiprotein complex, which is involved in transcriptional regulation. Biallelic MED27 variants have recently been suggested to be responsible for an autosomal recessive neurodevelopmental disorder with spasticity, cataracts and cerebellar hypoplasia. We further delineate the clinical phenotype of MED27-related disease by characterizing the clinical and radiological features of 57 affected individuals from 30 unrelated families with biallelic MED27 variants. Using exome sequencing and extensive international genetic data sharing, 39 unpublished affected individuals from 18 independent families with biallelic missense variants in MED27 have been identified (29 females, mean age at last follow-up 17 ± 12.4 years, range 0.1-45). Follow-up and hitherto unreported clinical features were obtained from the published 12 families. Brain MRI scans from 34 cases were reviewed. MED27-related disease manifests as a broad phenotypic continuum ranging from developmental and epileptic-dyskinetic encephalopathy to variable neurodevelopmental disorder with movement abnormalities. It is characterized by mild to profound global developmental delay/intellectual disability (100%), bilateral cataracts (89%), infantile hypotonia (74%), microcephaly (62%), gait ataxia (63%), dystonia (61%), variably combined with epilepsy (50%), limb spasticity (51%), facial dysmorphism (38%) and death before reaching adulthood (16%). Brain MRI revealed cerebellar atrophy (100%), white matter volume loss (76.4%), pontine hypoplasia (47.2%) and basal ganglia atrophy with signal alterations (44.4%). Previously unreported 39 affected individuals had seven homozygous pathogenic missense MED27 variants, five of which were recurrent. An emerging genotype-phenotype correlation was observed. This study provides a comprehensive clinical-radiological description of MED27-related disease, establishes genotype-phenotype and clinical-radiological correlations and suggests a differential diagnosis with syndromes of cerebello-lental neurodegeneration and other subtypes of 'neuro-MEDopathies'.

A reverse genetics and genomics approach to gene paralog function and disease: Myokymia and the juxtaparanode.

The leucine-rich glioma-inactivated (LGI) family consists of four highly conserved paralogous genes, LGI1-4, that are highly expressed in mammalian central and/or peripheral nervous systems. LGI1 antibodies are detected in subjects with autoimmune limbic encephalitis and peripheral nerve hyperexcitability syndromes (PNHSs) such as Isaacs and Morvan syndromes. Pathogenic variations of LGI1 and LGI4 are associated with neurological disorders as disease traits including familial temporal lobe epilepsy and neurogenic arthrogryposis multiplex congenita 1 with myelin defects, respectively. No human disease has been reported associated with either LGI2 or LGI3. We implemented exome sequencing and family-based genomics to identify individuals with deleterious variants in LGI3 and utilized GeneMatcher to connect practitioners and researchers worldwide to investigate the clinical and electrophysiological phenotype in affected subjects. We also generated Lgi3-null mice and performed peripheral nerve dissection and immunohistochemistry to examine the juxtaparanode LGI3 microarchitecture. As a result, we identified 16 individuals from eight unrelated families with loss-of-function (LoF) bi-allelic variants in LGI3. Deep phenotypic characterization showed LGI3 LoF causes a potentially clinically recognizable PNHS trait characterized by global developmental delay, intellectual disability, distal deformities with diminished reflexes, visible facial myokymia, and distinctive electromyographic features suggestive of motor nerve instability. Lgi3-null mice showed reduced and mis-localized Kv1 channel complexes in myelinated peripheral axons. Our data demonstrate bi-allelic LoF variants in LGI3 cause a clinically distinguishable disease trait of PNHS, most likely caused by disturbed Kv1 channel distribution in the absence of LGI3.

Seroprevalence of Anti- N-methyl-D-aspartate receptor antibodies in children with seizures of unknown cause.

BACKGROUND: Anti- N-methyl-D-aspartate receptor (NMDAR) antibodies were found most probably to be accompanied by seizures, particularly in children and sometimes it may be the sole presenting feature. Therefore, testing these antibodies in children with seizure of unexplained cause might be helpful to identify the spectrum of these antibody-mediated disorders. The objective of this study was to determine the frequency of anti-NMDAR antibodies in patients who presented with seizures of unknown cause. METHODS: A case-control study was conducted in two hospitals in Medical City Complex-Baghdad in 2019. Children aged 2-18 years who manifested seizures solely without identified causes were recruited over a period of ten months, with an additional sex- and age-matched control group (forty children in each group). Serum was tested in both groups for anti-NMDAR antibodies. RESULTS: In the study group, males predominated in ages younger than 5 years. The mean age was 6.6 years and the mean duration since their seizures` onset was 2 months. In contrast to male patients, female patients manifested more focal seizures. Only 5 patients (12.5%) were positive for Anti- NMDAR antibodies, in contrast to no one in the control group. Significantly, most of the seropositive patients were females (4, 80%) and showed focal types of seizures (4, 80%). CONCLUSIONS: This is a preliminary epidemiological study about the prevalence of anti-NMDAR antibodies in a sample of pediatric patients with isolated seizures of unknown cause. Anti-NMDAR antibodies were found to be prevalent in a relatively small proportion of children who presented with seizures of unknown causes. Demographic characteristics of the patients with variable testing status were found to be nearly comparable to the results from other related studies.

Pitfalls in the diagnosis of Gaucher disease in Iraq: A diagnostic experience from a developing country.

BACKGROUND AND OBJECTIVES: Gaucher disease (GD) is a rare hereditary disorder caused by deficiency of the lysosomal enzyme ß-glucocerebrosidase. An early and definitive diagnosis minimizes the sequelae of misdiagnoses, and unnecessary and invasive diagnostic procedures. METHODS: A cross-sectional descriptive study was conducted in the period from June to August, 2018, to analysing data of thirteen patients, retrospectively. They presented to the gastrointestinal and metabolic clinics in Children Welfare Teaching Hospital in Iraq, and had wrong and delayed diagnosis of GD. RESULTS: Two groups of patients were identified, based on diagnosis by enzymatic assay (considering the test positive when the enzyme level is below the normal value); those who had false positive (low level of the enzyme) result and received enzyme replacement therapy for long time, and those who had false negative (normal level of the enzyme) results which caused delay in their diagnosis and treatment. Two main factors that misled the diagnosis were identified. CONCLUSION: Each patient with Gaucher disease need to be approached by taking a thorough history, a proper clinical examination, and then by being investigated, accordingly. Biomarkers and molecular genetic studies are more accurate and solid additional tools, to the enzymatic assays on dried blood sample (DBS).

Haploinsufficiency of ARFGEF1 is associated with developmental delay, intellectual disability, and epilepsy with variable expressivity.

PURPOSE: ADP ribosylation factor guanine nucleotide exchange factors (ARFGEFs) are a family of proteins implicated in cellular trafficking between the Golgi apparatus and the plasma membrane through vesicle formation. Among them is ARFGEF1/BIG1, a protein involved in axon elongation, neurite development, and polarization processes. ARFGEF1 has been previously suggested as a candidate gene for different types of epilepsies, although its implication in human disease has not been well characterized. METHODS: International data sharing, in silico predictions, and in vitro assays with minigene study, western blot analyses, and RNA sequencing. RESULTS: We identified 13 individuals with heterozygous likely pathogenic variants in ARFGEF1. These individuals displayed congruent clinical features of developmental delay, behavioral problems, abnormal findings on brain magnetic resonance image (MRI), and epilepsy for almost half of them. While nearly half of the cohort carried de novo variants, at least 40% of variants were inherited from mildly affected parents who were clinically re-evaluated by reverse phenotyping. Our in silico predictions and in vitro assays support the contention that ARFGEF1-related conditions are caused by haploinsufficiency, and are transmitted in an autosomal dominant fashion with variable expressivity. CONCLUSION: We provide evidence that loss-of-function variants in ARFGEF1 are implicated in sporadic and familial cases of developmental delay with or without epilepsy.

A novel homozygous SLC13A5 whole-gene deletion generated by Alu/Alu-mediated rearrangement in an Iraqi family with epileptic encephalopathy.

Biallelic loss-of-function (LoF) of SLC13A5 (solute carrier family 13, member 5) induced deficiency in sodium/citrate transporter (NaCT) causes autosomal recessive developmental epileptic encephalopathy 25 with hypoplastic amelogenesis imperfecta (DEE25; MIM #615905). Many pathogenic SLC13A5 single nucleotide variants (SNVs) and small indels have been described; however, no cases with copy number variants (CNVs) have been sufficiently investigated. We describe a consanguineous Iraqi family harboring an 88.5 kb homozygous deletion including SLC13A5 in Chr17p13.1. The three affected male siblings exhibit neonatal-onset epilepsy with fever-sensitivity, recurrent status epilepticus, global developmental delay/intellectual disability (GDD/ID), and other variable neurological findings as shared phenotypical features of DEE25. Two of the three affected subjects exhibit hypoplastic amelogenesis imperfecta (AI), while the proband shows no evidence of dental abnormalities or AI at 2 years of age with apparently unaffected primary dentition. Characterization of the genomic architecture at this locus revealed evidence for genomic instability generated by an Alu/Alu-mediated rearrangement; confirmed by break-point junction Sanger sequencing. This multiplex family from a distinct population elucidates the phenotypic consequence of complete LoF of SLC13A5 and illustrates the importance of read-depth-based CNV detection in comprehensive exome sequencing analysis to solve cases that otherwise remain molecularly unsolved.

TMX2 Is a Crucial Regulator of Cellular Redox State, and Its Dysfunction Causes Severe Brain Developmental Abnormalities.

The redox state of the neural progenitors regulates physiological processes such as neuronal differentiation and dendritic and axonal growth. The relevance of endoplasmic reticulum (ER)-associated oxidoreductases in these processes is largely unexplored. We describe a severe neurological disorder caused by bi-allelic loss-of-function variants in thioredoxin (TRX)-related transmembrane-2 (TMX2); these variants were detected by exome sequencing in 14 affected individuals from ten unrelated families presenting with congenital microcephaly, cortical polymicrogyria, and other migration disorders. TMX2 encodes one of the five TMX proteins of the protein disulfide isomerase family, hitherto not linked to human developmental brain disease. Our mechanistic studies on protein function show that TMX2 localizes to the ER mitochondria-associated membranes (MAMs), is involved in posttranslational modification and protein folding, and undergoes physical interaction with the MAM-associated and ER folding chaperone calnexin and ER calcium pump SERCA2. These interactions are functionally relevant because TMX2-deficient fibroblasts show decreased mitochondrial respiratory reserve capacity and compensatory increased glycolytic activity. Intriguingly, under basal conditions TMX2 occurs in both reduced and oxidized monomeric form, while it forms a stable dimer under treatment with hydrogen peroxide, recently recognized as a signaling molecule in neural morphogenesis and axonal pathfinding. Exogenous expression of the pathogenic TMX2 variants or of variants with an in vitro mutagenized TRX domain induces a constitutive TMX2 polymerization, mimicking an increased oxidative state. Altogether these data uncover TMX2 as a sensor in the MAM-regulated redox signaling pathway and identify it as a key adaptive regulator of neuronal proliferation, migration, and organization in the developing brain.

HTRA2 Defect: A Recognizable Inborn Error of Metabolism with 3-Methylglutaconic Aciduria as Discriminating Feature Characterized by Neonatal Movement Disorder and Epilepsy-Report of 11 Patients.

Neonatal-onset movement disorders, especially in combination with seizures, are rare and often related to mitochondrial disorders. 3-methylglutaconic aciduria (3-MGA-uria) is a marker for mitochondrial dysfunction. In particular, consistently elevated urinary excretion of 3-methylglutaconic acid is the hallmark of a small but growing group of inborn errors of metabolism (IEM) due to defective phospholipid remodeling or mitochondrial membrane-associated disorders (mutations in TAZ, SERAC1, OPA3, CLPB, DNAJC19, TMEM70, TIMM50). Exome/genome sequencing is a powerful tool for the diagnosis of the clinically and genetically heterogeneous mitochondrial disorders. Here, we report 11 individuals, of whom 2 are previously unpublished, with biallelic variants in high temperature requirement protein A2 (HTRA2) encoding a mitochondria-localized serine protease. All individuals presented a recognizable phenotype with neonatal- or infantile-onset neurodegeneration and death within the first month of life. Hallmark features were central hypopnea/apnea leading to respiratory insufficiency, seizures, neutropenia, 3-MGA-uria, tonus dysregulation, and dysphagia. Tremor, jitteriness, dystonia, and/or clonus were also common. HTRA2 defect should be grouped under the IEM with 3-MGA-uria as discriminating feature. Clinical characteristics overlap with other disorders of this group suggesting a common underlying pathomechanism. Urinary organic acid analysis is a noninvasive and inexpensive test that can guide further genetic testing in children with suggestive clinical findings.