Consensus guidelines for the diagnosis and management of succinic semialdehyde dehydrogenase deficiency

Succinic semialdehyde dehydrogenase deficiency (SSADHD) (OMIM #271980) is a rare autosomal recessive metabolic disorder caused by pathogenic variants of ALDH5A1. Deficiency of SSADH results in accumulation of γ-aminobutyric acid (GABA) and other GABA-related metabolites. The clinical phenotype of SSADHD includes a broad spectrum of non-pathognomonic symptoms such as cognitive disabilities, communication and language deficits, movement disorders, epilepsy, sleep disturbances, attention problems, anxiety, and obsessive-compulsive traits. Current treatment options for SSADHD remain supportive, but there are ongoing attempts to develop targeted genetic therapies. This study aimed to create consensus guidelines for the diagnosis and management of SSADHD. Thirty relevant statements were initially addressed by a systematic literature review, resulting in different evidence levels of strength according to the Grading of Recommendations Assessment, Development, and Evaluation (GRADE) criteria. The highest level of evidence (level A), based on randomized controlled trials, was unavailable for any of the statements. Based on cohort studies, Level B evidence was available for 12 (40%) of the statements. Thereupon, through a process following the Delphi Method and directed by the Appraisal of Guidelines for Research and Evaluation (AGREE II) criteria, expert opinion was sought, and members of an SSADHD Consensus Group evaluated all the statements. The group consisted of neurologists, epileptologists, neuropsychologists, neurophysiologists, metabolic disease specialists, clinical and biochemical geneticists, and laboratory scientists affiliated with 19 institutions from 11 countries who have clinical experience with SSADHD patients and have studied the disorder. Representatives from parent groups were also included in the Consensus Group. An analysis of the survey's results yielded 25 (83%) strong and 5 (17%) weak agreement strengths. These first-of-their-kind consensus guidelines intend to consolidate and unify the optimal care that can be provided to individuals with SSADHD.

Clinical and molecular outcomes from the 5-Year natural history study of SSADH Deficiency, a model metabolic neurodevelopmental disorder

BACKGROUND

Succinic semialdehyde dehydrogenase deficiency (SSADHD) represents a model neurometabolic disease at the fulcrum of translational research within the Boston Children's Hospital Intellectual and Developmental Disabilities Research Centers (IDDRC), including the NIH-sponsored natural history study of clinical, neurophysiological, neuroimaging, and molecular markers, patient-derived induced pluripotent stem cells (iPSC) characterization, and development of a murine model for tightly regulated, cell-specific gene therapy.

METHODS

SSADHD subjects underwent clinical evaluations, neuropsychological assessments, biochemical quantification of γ-aminobutyrate (GABA) and related metabolites, electroencephalography (standard and high density), magnetoencephalography, transcranial magnetic stimulation, magnetic resonance imaging and spectroscopy, and genetic tests. This was parallel to laboratory molecular investigations of in vitro GABAergic neurons derived from induced human pluripotent stem cells (hiPSCs) of SSADHD subjects and biochemical analyses performed on a versatile murine model that uses an inducible and reversible rescue strategy allowing on-demand and cell-specific gene therapy.

RESULTS

The 62 SSADHD subjects [53% females, median (IQR) age of 9.6 (5.4-14.5) years] included in the study had a reported symptom onset at ∼ 6 months and were diagnosed at a median age of 4 years. Language developmental delays were more prominent than motor. Autism, epilepsy, movement disorders, sleep disturbances, and various psychiatric behaviors constituted the core of the disorder's clinical phenotype. Lower clinical severity scores, indicating worst severity, coincided with older age (R= -0.302, p = 0.03), as well as age-adjusted lower values of plasma γ-aminobutyrate (GABA) (R = 0.337, p = 0.02) and γ-hydroxybutyrate (GHB) (R = 0.360, p = 0.05). While epilepsy and psychiatric behaviors increase in severity with age, communication abilities and motor function tend to improve. iPSCs, which were differentiated into GABAergic neurons, represent the first in vitro neuronal model of SSADHD and express the neuronal marker microtubule-associated protein 2 (MAP2), as well as GABA. GABA-metabolism in induced GABAergic neurons could be reversed using CRISPR correction of the pathogenic variants or mRNA transfection and SSADHD iPSCs were associated with excessive glutamatergic activity and related synaptic excitation.

CONCLUSIONS

Findings from the SSADHD Natural History Study converge with iPSC and animal model work focused on a common disorder within our IDDRC, deepening our knowledge of the pathophysiology and longitudinal clinical course of a complex neurodevelopmental disorder. This further enables the identification of biomarkers and changes throughout development that will be essential for upcoming targeted trials of enzyme replacement and gene therapy.

The continuously evolving phenotype of succinic semialdehyde dehydrogenase deficiency

The objective of the study is to evaluate the evolving phenotype and genetic spectrum of patients with succinic semialdehyde dehydrogenase deficiency (SSADHD) in long-term follow-up. Longitudinal clinical and biochemical data of 22 pediatric and 9 adult individuals with SSADHD from the patient registry of the International Working Group on Neurotransmitter related Disorders (iNTD) were studied with in silico analyses, pathogenicity scores and molecular modeling of ALDH5A1 variants. Leading initial symptoms, with onset in infancy, were developmental delay and hypotonia. Year of birth and specific initial symptoms influenced the diagnostic delay. Clinical phenotype of 26 individuals (median 12 years, range 1.8-33.4 years) showed a diversifying course in follow-up: 77% behavioral problems, 76% coordination problems, 73% speech disorders, 58% epileptic seizures and 40% movement disorders. After ataxia, dystonia (19%), chorea (11%) and hypokinesia (15%) were the most frequent movement disorders. Involvement of the dentate nucleus in brain imaging was observed together with movement disorders or coordination problems. Short attention span (78.6%) and distractibility (71.4%) were the most frequently behavior traits mentioned by parents while impulsiveness, problems communicating wishes or needs and compulsive behavior were addressed as strongly interfering with family life. Treatment was mainly aimed to control epileptic seizures and psychiatric symptoms. Four new pathogenic variants were identified. In silico scoring system, protein activity and pathogenicity score revealed a high correlation. A genotype/phenotype correlation was not observed, even in siblings. This study presents the diversifying characteristics of disease phenotype during the disease course, highlighting movement disorders, widens the knowledge on the genotypic spectrum of SSADHD and emphasizes a reliable application of in silico approaches.

Phenotypic correlates of structural and functional protein impairments resultant from ALDH5A1 variants

To investigate the genotype-to-protein-to-phenotype correlations of succinic semialdehyde dehydrogenase deficiency (SSADHD), an inherited metabolic disorder of γ-aminobutyric acid catabolism. Bioinformatics and in silico mutagenesis analyses of ALDH5A1 variants were performed to evaluate their impact on protein stability, active site and co-factor binding domains, splicing, and homotetramer formation. Protein abnormalities were then correlated with a validated disease-specific clinical severity score and neurological, neuropsychological, biochemical, neuroimaging, and neurophysiological metrics. A total of 58 individuals (1:1 male/female ratio) were affected by 32 ALDH5A1 pathogenic variants, eight of which were novel. Compared to individuals with single homotetrameric or multiple homo and heterotetrameric proteins, those predicted not to synthesize any functional enzyme protein had significantly lower expression of ALDH5A1 (p = 0.001), worse overall clinical outcomes (p = 0.008) and specifically more severe cognitive deficits (p = 0.01), epilepsy (p = 0.04) and psychiatric morbidity (p = 0.04). Compared to individuals with predictions of having no protein or a protein impaired in catalytic functions, subjects whose proteins were predicted to be impaired in stability, folding, or oligomerization had a better overall clinical outcome (p = 0.02) and adaptive skills (p = 0.04). The quantity and type of enzyme proteins (no protein, single homotetramers, or multiple homo and heterotetramers), as well as their structural and functional impairments (catalytic or stability, folding, or oligomerization), contribute to phenotype severity in SSADHD. These findings are valuable for assessment of disease prognosis and management, including patient selection for gene replacement therapy. Furthermore, they provide a roadmap to determine genotype-to-protein-to-phenotype relationships in other autosomal recessive disorders.

Metabolic characterization of neurogenetic disorders involving glutamatergic neurotransmission

The study of inborn errors of neurotransmission has been mostly focused on monoamine disorders, GABAergic and glycinergic defects. The study of the glutamatergic synapse using the same approach than classic neurotransmitter disorders is challenging due to the lack of biomarkers in the CSF. A metabolomic approach can provide both insight into their molecular basis and outline novel therapeutic alternatives. We have performed a semi-targeted metabolomic analysis on CSF samples from 25 patients with neurogenetic disorders with an important expression in the glutamatergic synapse and 5 controls. Samples from patients diagnosed with MCP2, CDKL5-, GRINpathies and STXBP1-related encephalopathies were included. We have performed univariate (UVA) and multivariate statistical analysis (MVA), using Wilcoxon rank-sum test, principal component analysis (PCA), and OPLS-DA. By using the results of both analyses, we have identified the metabolites that were significantly altered and that were important in clustering the respective groups. On these, we performed pathway- and network-based analyses to define which metabolic pathways were possibly altered in each pathology. We have observed alterations in the tryptophan and branched-chain amino acid metabolism pathways, which interestingly converge on LAT1 transporter-dependency to cross the blood-brain barrier (BBB). Analysis of the expression of LAT1 transporter in brain samples from a mouse model of Rett syndrome (MECP2) revealed a decrease in the transporter expression, that was already noticeable at pre-symptomatic stages. The study of the glutamatergic synapse from this perspective advances the understanding of their pathophysiology, shining light on an understudied feature as is their metabolic signature.

Expanding the phenotypic spectrum of TRAPPC11-related muscular dystrophy: 25 Roma individuals carrying a founder variant

BACKGROUND

Limb-girdle muscular dystrophies (LGMD) are a heterogeneous group of genetically determined muscle disorders. TRAPPC11-related LGMD is an autosomal-recessive condition characterised by muscle weakness and intellectual disability.

METHODS

A clinical and histopathological characterisation of 25 Roma individuals with LGMD R18 caused by the homozygous TRAPPC11 c.1287+5G>A variant is reported. Functional effects of the variant on mitochondrial function were investigated.

RESULTS

The c.1287+5G>A variant leads to a phenotype characterised by early onset muscle weakness, movement disorder, intellectual disability and elevated serum creatine kinase, which is similar to other series. As novel clinical findings, we found that microcephaly is almost universal and that infections in the first years of life seem to act as triggers for a psychomotor regression and onset of seizures in several individuals with TRAPPC11 variants, who showed pseudometabolic crises triggered by infections. Our functional studies expanded the role of TRAPPC11 deficiency in mitochondrial function, as a decreased mitochondrial ATP production capacity and alterations in the mitochondrial network architecture were detected.

CONCLUSION

We provide a comprehensive phenotypic characterisation of the pathogenic variant TRAPPC11 c.1287+5G>A, which is founder in the Roma population. Our observations indicate that some typical features of golgipathies, such as microcephaly and clinical decompensation associated with infections, are prevalent in individuals with LGMD R18.

Phenotypic Correlates of Structural and Functional Protein Impairments Resultant from ALDH5A1 Variants

Objective

To investigate the genotype-to-protein-to-phenotype correlations of succinic semialdehyde dehydrogenase deficiency (SSADHD), an inherited metabolic disorder of γ-aminobutyric acid catabolism.

Methods

Bioinformatics and in silico mutagenesis analyses of ALDH5A1 variants were performed to evaluate their impact on protein stability, active site and co-factor binding domains, splicing, and homotetramer formation. Protein abnormalities were then correlated with a validated disease-specific clinical severity score and neurological, neuropsychological, biochemical, neuroimaging, and neurophysiological metrics.

Results

A total of 58 individuals (1:1 male/female ratio) were affected by 32 ALDH5A1 pathogenic variants, eight of which were novel. Compared to individuals with single homotetrameric or multiple homo and heterotetrameric proteins, those predicted not to synthesize any functional enzyme protein had significantly lower expression of ALDH5A1 (p = 0.001), worse overall clinical outcomes (p = 0.008) and specifically more severe cognitive deficits (p = 0.01), epilepsy (p = 0.04) and psychiatric morbidity (p = 0.04). Compared to individuals with predictions of having no protein or a protein impaired in catalytic functions, subjects whose proteins were predicted to be impaired in stability, folding, or oligomerization had a better overall clinical outcome (p = 0.02) and adaptive skills (p = 0.04).

Conclusions

The quantity and type of enzyme proteins (no protein, single homotetramers, or multiple homo and heterotetramers), as well as their structural and functional impairments (catalytic or stability, folding, or oligomerization), contribute to phenotype severity in SSADHD. These findings are valuable for assessment of disease prognosis and management, including patient selection for gene replacement therapy. Furthermore, they provide a roadmap to determine genotype-to-protein-to-phenotype relationships in other autosomal recessive disorders.

Prevalence of DDC genotypes in patients with aromatic L-amino acid decarboxylase (AADC) deficiency and in silico prediction of structural protein changes

Aromatic L-amino acid decarboxylase (AADC) deficiency is a rare autosomal recessive genetic disorder affecting the biosynthesis of dopamine, a precursor of both norepinephrine and epinephrine, and serotonin. Diagnosis is based on the analysis of CSF or plasma metabolites, AADC activity in plasma and genetic testing for variants in the DDC gene. The exact prevalence of AADC deficiency, the number of patients, and the variant and genotype prevalence are not known. Here, we present the DDC variant (n = 143) and genotype (n = 151) prevalence of 348 patients with AADC deficiency, 121 of whom were previously not reported. In addition, we report 26 new DDC variants, classify them according to the ACMG/AMP/ACGS recommendations for pathogenicity and score them based on the predicted structural effect. The splice variant c.714+4A>T, with a founder effect in Taiwan and China, was the most common variant (allele frequency = 32.4%), and c.[714+4A>T];[714+4A>T] was the most common genotype (genotype frequency = 21.3%). Approximately 90% of genotypes had variants classified as pathogenic or likely pathogenic, while 7% had one VUS allele and 3% had two VUS alleles. Only one benign variant was reported. Homozygous and compound heterozygous genotypes were interpreted in terms of AADC protein and categorized as: i) devoid of full-length AADC, ii) bearing one type of AADC homodimeric variant or iii) producing an AADC protein population composed of two homodimeric and one heterodimeric variant. Based on structural features, a score was attributed for all homodimers, and a tentative prediction was advanced for the heterodimer. Almost all AADC protein variants were pathogenic or likely pathogenic.

The presence and severity of epilepsy coincide with reduced γ-aminobutyrate and cortical excitatory markers in succinic semialdehyde dehydrogenase deficiency

OBJECTIVE

Succinic semialdehyde dehydrogenase deficiency (SSADHD) is a rare inherited metabolic disorder caused by a defect of γ-aminobutyrate (GABA) catabolism. Despite the resultant hyper-GABAergic environment facilitated by the metabolic defect, individuals with this disorder have a paradoxically high prevalence of epilepsy. We aimed to study the characteristics of epilepsy in SSADHD and its concordance with GABA-related metabolites and neurophysiologic markers of cortical excitation.

METHODS

Subjects in an international natural history study of SSADHD underwent clinical assessments, electroencephalography, transcranial magnetic stimulation (TMS), magnetic resonance spectroscopy for GABA/N-acetyl aspartate quantification, and plasma GABA-related metabolite measurements.

RESULTS

A total of 61 subjects with SSADHD and 42 healthy controls were included in the study. Epilepsy was present in 49% of the SSADHD cohort. Over time, there was an increase in severity in 33% of the subjects with seizures. The presence of seizures was associated with increasing age (p = .001) and lower levels of GABA (p = .002), γ-hydroxybutyrate (GHB; p = .004), and γ-guanidinobutyrate (GBA; p = .003). Seizure severity was associated with increasing age and lower levels of GABA-related metabolites as well as lower TMS-derived resting motor thresholds (p = .04). The cutoff values with the highest discriminative ability to predict seizures were age > 9.2 years (p = .001), GABA < 2.57 μmol·L-1 (p = .002), GHB < 143.6 μmol·L-1 (p = .004), and GBA < .075 μmol·L-1 (p = .007). A prediction model for seizures in SSADHD was comprised of the additive effect of older age and lower plasma GABA, GHB, and GBA (area under the receiver operating characteristic curve of .798, p = .008).

SIGNIFICANCE

Epilepsy is highly prevalent in SSADHD, and its onset and severity correlate with an age-related decline in GABA and GABA-related metabolite levels as well as TMS markers of reduced cortical inhibition. The reduction of GABAergic activity in this otherwise hyper-GABAergic disorder demonstrates a concordance between epileptogenesis and compensatory responses. These findings may furthermore inform the timing of molecular interventions for SSADHD.

Sphingolipid desaturase DEGS1 is essential for mitochondria-associated membrane integrity

Sphingolipids function as membrane constituents and signaling molecules, with crucial roles in human diseases, from neurodevelopmental disorders to cancer, best exemplified in the inborn errors of sphingolipid metabolism in lysosomes. The dihydroceramide desaturase Δ4-dihydroceramide desaturase 1 (DEGS1) acts in the last step of a sector of the sphingolipid pathway, de novo ceramide biosynthesis. Defects in DEGS1 cause the recently described hypomyelinating leukodystrophy-18 (HLD18) (OMIM #618404). Here, we reveal that DEGS1 is a mitochondria-associated endoplasmic reticulum membrane-resident (MAM-resident) enzyme, refining previous reports locating DEGS1 at the endoplasmic reticulum only. Using patient fibroblasts, multiomics, and enzymatic assays, we show that DEGS1 deficiency disrupts the main core functions of the MAM: (a) mitochondrial dynamics, with a hyperfused mitochondrial network associated with decreased activation of dynamin-related protein 1; (b) cholesterol metabolism, with impaired sterol O-acyltransferase activity and decreased cholesteryl esters; (c) phospholipid metabolism, with increased phosphatidic acid and phosphatidylserine and decreased phosphatidylethanolamine; and (d) biogenesis of lipid droplets, with increased size and numbers. Moreover, we detected increased mitochondrial superoxide species production in fibroblasts and mitochondrial respiration impairment in patient muscle biopsy tissues. Our findings shed light on the pathophysiology of HLD18 and broaden our understanding of the role of sphingolipid metabolism in MAM function.

Monoamine neurotransmitters in early epileptic encephalopathies: New insights into pathophysiology and therapy

AIM

To study neurotransmitter status in children with early epileptic and developmental and epileptic encephalopathy (DEE) and to explore the clinical response to dopaminergic and serotoninergic therapies in a group of patients.

METHOD

Two hundred and five patients (111 males [54.1.%] and 94 females [45.9%], mean age 10 months at the onset of epilepsy [SD 1 year 1 month], range 0-3 year) with epileptic encephalopathy/DEE were recruited, including those with West syndrome, Ohtahara syndrome, early myoclonic encephalopathy, epilepsy of infancy with migrating focal seizures, myoclonic encephalopathy in non-progressive disorders, infantile spasms, Doose syndrome, Lennox-Gastaut syndrome, Landau-Kleffner syndrome, and those unclassified. Cerebrospinal fluid (CSF) neurotransmitter studies and patients' medical records were reviewed. Additionally, we present clinical data of 10 patients with low CSF neurotransmitter levels who received dopaminergic/serotoninergic treatments.

RESULTS

Abnormal neurotransmitter values were identified in 68 (33%) patients. 5-Hydroxyindoleacetic acid (5-HIAA) deficit was the most prevalent alteration (91%). Low CSF 5-HIAA levels were significantly higher in 1- to 3-year-old children. A negative significant correlation was found between 5-HIAA levels and epilepsy duration before CSF study (Spearman's ρ=-0.191, p=0.007). Abnormalities in deep grey matter were associated with low levels of CSF homovanillic acid and 5-HIAA. Ten patients with low CSF neurotransmitter levels received dopamine and/or serotonin therapies. Six of them showed initial decrease of seizure frequency and severity and maintained improvement in some neurodevelopmental skills.

INTERPRETATION

A considerable number of patients showed neurotransmitter abnormalities. Age at seizure onset and duration of epilepsy before CSF study were the principal factors related to neurotransmitter depletion. Early monoamine supplementation would seem advisable as a neuroprotective strategy.

WHAT THIS PAPER ADDS

5-Hydroxyindoleacetic acid homeostasis is especially vulnerable in patients with epileptic encephalopathy/developmental and epileptic encephalopathy. Age of seizure onset and duration of epilepsy are determinants of neurotransmitter depletion.

The clinical and biochemical hallmarks generally associated with GLUT1DS may be caused by defects in genes other than SLC2A1

Glucose transporter 1 deficiency syndrome (GLUT1DS) is a neurometabolic disorder caused by haploinsufficiency of the GLUT1 glucose transporter (encoded by SLC2A1) leading to defective glucose transport across the blood–brain barrier. This work describes the genetic analysis of 56 patients with clinical or biochemical GLUT1DS hallmarks. 55.4% of these patients had a pathogenic variant of SLC2A1, and 23.2% had a variant in one of 13 different genes. No pathogenic variant was identified for the remaining patients. Expression analysis of SLC2A1 indicated a reduction in SLC2A1 mRNA in patients with pathogenic variants of this gene, as well as in one patient with a pathogenic variant in SLC9A6, and in three for whom no candidate variant was identified. Thus, the clinical and biochemical hallmarks generally associated with GLUT1DS may be caused by defects in genes other than SLC2A1.

Pediatric Gaucher disease with intermediate type 2-3 phenotype associated with parkinsonian features and levodopa responsiveness

INTRODUCTION

Gaucher disease (GD) is an autosomal recessive lysosomal storage disorder caused by a deficiency of acid β-glucosidase encoded by the GBA gene. In patients with GD, childhood onset parkinsonian features have been rarely described.

METHODS

Twin siblings with GD are described, including clinical follow-up and treatment response. Bone marrow, enzyme activity studies and genotyping were performed.

RESULTS

By age 9 months, symptoms at onset were thrombocytopenia and splenomegaly. By age 2, hypokinesia, bradykinesia and oculomotor apraxia were observed. By age 5 a complete rigid hypokinetic syndrome was stablished in both patients, including bradykinesia, tremor and rigidity. Treatment with imiglucerase, miglustat, ambroxol and levodopa were performed. Levodopa showed a good response with improvement in motor and non-motor skills. Foamy cells were found in the bone marrow study. Glucocerebrosidase activity was 28% and 26%. Sanger sequencing analysis identified a missense mutation and a complex allele (NP_000148: p.[(Asp448His)]; [(Leu422Profs*4)]) in compound heterozygosity in GBA gene.

CONCLUSIONS

Two siblings with neuronopathic GD with an intermediate form between type 2 and 3, with a systemic and neurological phenotype are described. The complex neurological picture included a hypokinetic-rigid and tremor syndrome that improved with levodopa treatment. These conditions together have not been previously described in pediatric GD. We suggest that in children with parkinsonian features, lysosomal storage disorders must be considered, and a levodopa trial must be performed. Moreover, this report give support to the finding that GBA and parkinsonian features share biological pathways and highlight the importance of lysosomal mechanisms in parkinsonism pathogenesis, what might have therapeutic implications.

GRIN database: A unified and manually curated repertoire of GRIN variants

Glutamatergic neurotransmission is crucial for brain development, wiring neuronal function, and synaptic plasticity mechanisms. Recent genetic studies showed the existence of autosomal dominant de novo GRIN gene variants associated with GRIN-related disorders (GRDs), a rare pediatric neurological disorder caused by N-methyl- d-aspartate receptor (NMDAR) dysfunction. Notwithstanding, GRIN variants identification is exponentially growing and their clinical, genetic, and functional annotations remain highly fragmented, representing a bottleneck in GRD patient's stratification. To shorten the gap between GRIN variant identification and patient stratification, we present the GRIN database (GRINdb), a publicly available, nonredundant, updated, and curated database gathering all available genetic, functional, and clinical data from more than 4000 GRIN variants. The manually curated GRINdb outputs on a web server, allowing query and retrieval of reported GRIN variants, and thus representing a fast and reliable bioinformatics resource for molecular clinical advice. Furthermore, the comprehensive mapping of GRIN variants' genetic and clinical information along NMDAR structure revealed important differences in GRIN variants' pathogenicity and clinical phenotypes, shedding light on GRIN-specific fingerprints. Overall, the GRINdb and web server is a resource for molecular stratification of GRIN variants, delivering clinical and investigational insights into GRDs. GRINdb is accessible at http://lmc.uab.es/grindb.

Novel Protein Biomarkers of Monoamine Metabolism Defects Correlate with Disease Severity

BACKGROUND

Genetic defects of monoamine neurotransmitters are rare neurological diseases amenable to treatment with variable response. They are major causes of early parkinsonism and other spectrum of movement disorders including dopa-responsive dystonia.

OBJECTIVES

The objective of this study was to conduct proteomic studies in cerebrospinal fluid (CSF) samples of patients with monoamine defects to detect biomarkers involved in pathophysiology, clinical phenotypes, and treatment response.

METHODS

A total of 90 patients from diverse centers of the International Working Group on Neurotransmitter Related Disorders were included in the study (37 untreated before CSF collection, 48 treated and 5 unknown at the collection time). Clinical and molecular metadata were related to the protein abundances in the CSF.

RESULTS

Concentrations of 4 proteins were significantly altered, detected by mass spectrometry, and confirmed by immunoassays. First, decreased levels of apolipoprotein D were found in severe cases of aromatic L-amino acid decarboxylase deficiency. Second, low levels of apolipoprotein H were observed in patients with the severe phenotype of tyrosine hydroxylase deficiency, whereas increased concentrations of oligodendrocyte myelin glycoprotein were found in the same subset of patients with tyrosine hydroxylase deficiency. Third, decreased levels of collagen6A3 were observed in treated patients with tetrahydrobiopterin deficiency.

CONCLUSION

This study with the largest cohort of patients with monoamine defects studied so far reports the proteomic characterization of CSF and identifies 4 novel biomarkers that bring new insights into the consequences of early dopaminergic deprivation in the developing brain. They open new possibilities to understand their role in the pathophysiology of these disorders, and they may serve as potential predictors of disease severity and therapies. © 2020 International Parkinson and Movement Disorder Society.

Disease-associated GRIN protein truncating variants trigger NMDA receptor loss-of-function

De novo GRIN variants, encoding for the ionotropic glutamate NMDA receptor subunits, have been recently associated with GRIN-related disorders, a group of rare paediatric encephalopathies. Current investigational and clinical efforts are focused to functionally stratify GRIN variants, towards precision therapies of this primary disturbance of glutamatergic transmission that affects neuronal function and brain. In the present study, we aimed to comprehensively delineate the functional outcomes and clinical phenotypes of GRIN protein truncating variants (PTVs)-accounting for ~20% of disease-associated GRIN variants-hypothetically provoking NMDAR hypofunctionality. To tackle this question, we created a comprehensive GRIN PTVs variants database compiling a cohort of nine individuals harbouring GRIN PTVs, together with previously identified variants, to build-up an extensive GRIN PTVs repertoire composed of 293 unique variants. Genotype-phenotype correlation studies were conducted, followed by cell-based assays of selected paradigmatic GRIN PTVs and their functional annotation. Genetic and clinical phenotypes meta-analysis revealed that heterozygous GRIN1, GRIN2C, GRIN2D, GRIN3A and GRIN3B PTVs are non-pathogenic. In contrast, heterozygous GRIN2A and GRIN2B PTVs are associated with specific neurological clinical phenotypes in a subunit- and domain-dependent manner. Mechanistically, cell-based assays showed that paradigmatic pathogenic GRIN2A and GRIN2B PTVs result on a decrease of NMDAR surface expression and NMDAR-mediated currents, ultimately leading to NMDAR functional haploinsufficiency. Overall, these findings contribute to delineate GRIN PTVs genotype-phenotype association and GRIN variants stratification. Functional studies showed that GRIN2A and GRIN2B pathogenic PTVs trigger NMDAR hypofunctionality, and thus accelerate therapeutic decisions for this neurodevelopmental condition.

Cataract in You-Hoover-Fong syndrome: TELO2 deficiency

INTRODUCTION

Recently, You, Hoover-Fong, and colleagues described a disease caused by a deficiency of the telomere maintenance 2 gene (TELO2) function. The clinical spectrum includes early-onset global delay, dysmorphic facial features, auditory disorder, and reduced vision.

MATERIALS AND METHODS

We report two siblings, diagnosed with You-Hoover-Fong syndrome at the age of 28 and 14 months. Both were genetically studied to find the cause of their developmental delay and microcephaly.

RESULTS

The identical compound heterozygous missense mutations in the TELO2gene were found in each. Ophthalmologically, both siblings were diagnosed with progressive congenital bilateral nuclear-lamellar cataracts.

CONCLUSIONS

We report nuclear-lamellar cataracts in two siblings diagnosed with You-Hoover-Fong syndrome.

Impairment of adenosinergic system in Rett syndrome: Novel therapeutic target to boost BDNF signalling

Rett syndrome (RTT; OMIM#312750) is mainly caused by mutations in the X-linked MECP2 gene (methyl-CpG-binding protein 2 gene; OMIM*300005), which leads to impairments in the brain-derived neurotrophic factor (BDNF) signalling. The boost of BDNF mediated effects would be a significant breakthrough but it has been hampered by the difficulty to administer BDNF to the central nervous system. Adenosine, an endogenous neuromodulator, may accomplish that role since through A_2AR it potentiates BDNF synaptic actions in healthy animals. We thus characterized several hallmarks of the adenosinergic and BDNF signalling in RTT and explored whether A_2AR activation could boost BDNF actions. For this study, the RTT animal model, the Mecp2 knockout (Mecp2^-/y) (B6.129P2 (C)-Mecp2tm1.1Bird/J) mouse was used. Whenever possible, parallel data was also obtained from post-mortem brain samples from one RTT patient. Ex vivo extracellular recordings of field excitatory post-synaptic potentials in CA1 hippocampal area were performed to evaluate synaptic transmission and long-term potentiation (LTP). RT-PCR was used to assess mRNA levels and Western Blot or radioligand binding assays were performed to evaluate protein levels. Changes in cortical and hippocampal adenosine content were assessed by liquid chromatography with diode array detection (LC/DAD). Hippocampal ex vivo experiments revealed that the facilitatory actions of BDNF upon LTP is absent in Mecp2^-/y mice and that TrkB full-length (TrkB-FL) receptor levels are significantly decreased. Extracts of the hippocampus and cortex of Mecp2^-/y mice revealed less adenosine amount as well as less A_2AR protein levels when compared to WT littermates, which may partially explain the deficits in adenosinergic tonus in these animals. Remarkably, the lack of BDNF effect on hippocampal LTP in Mecp2^-/y mice was overcome by selective activation of A_2AR with CGS21680. Overall, in Mecp2^-/y mice there is an impairment on adenosinergic system and BDNF signalling. These findings set the stage for adenosine-based pharmacological therapeutic strategies for RTT, highlighting A_2AR as a therapeutic target in this devastating pathology.

l-Serine dietary supplementation is associated with clinical improvement of loss-of-function GRIN2B-related pediatric encephalopathy

Autosomal dominant mutations in GRIN2B are associated with severe encephalopathy, but little is known about the pathophysiological outcomes and any potential therapeutic interventions. Genetic studies have described the association between de novo mutations of genes encoding the subunits of the N-methyl-d-aspartate receptor (NMDAR) and severe neurological conditions. Here, we evaluated a missense mutation in GRIN2B, causing a proline-to-threonine switch (P553T) in the GluN2B subunit of NMDAR, which was found in a 5-year-old patient with Rett-like syndrome with severe encephalopathy. Structural molecular modeling predicted a reduced pore size of the mutant GluN2B-containing NMDARs. Electrophysiological recordings in a HEK-293T cell line expressing the mutated subunit confirmed this prediction and showed an associated reduced glutamate affinity. Moreover, GluN2B(P553T)-expressing primary murine hippocampal neurons showed decreased spine density, concomitant with reduced NMDA-evoked currents and impaired NMDAR-dependent insertion of the AMPA receptor subunit GluA1 at stimulated synapses. Furthermore, the naturally occurring coagonist d-serine restored function to GluN2B(P553T)-containing NMDARs. l-Serine dietary supplementation of the patient was hence initiated, resulting in the increased abundance of d-serine in the plasma and brain. The patient has shown notable improvements in motor and cognitive performance and communication after 11 and 17 months of l-serine dietary supplementation. Our data suggest that l-serine supplementation might ameliorate GRIN2B-related severe encephalopathy and other neurological conditions caused by glutamatergic signaling deficiency.

Presynaptic disorders: a clinical and pathophysiological approach focused on the synaptic vesicle

The aim of this report is to present a tentative clinical and pathophysiological approach to diseases affecting the neuronal presynaptic terminal, with a major focus on synaptic vesicles (SVs). Diseases are classified depending on which step of the neurobiology of the SV is predominantly affected: (1) biogenesis of vesicle precursors in the neuronal soma; (2) transport along the axon; (3) vesicle cycle at the presynaptic terminal (exocytosis-endocytosis cycle, with the main purpose of neurotransmitter release). Given that SVs have been defined as individual organelles, we highlight the link between the biological processes disturbed by genetic mutations and the clinical presentation of these disorders. The great majority of diseases may present as epileptic encephalopathies, intellectual disability (syndromic or nonsyndromic) with/without autism spectrum disorder (and other neuropsychiatric symptoms), and movement disorders. These symptoms may overlap and present in patients as a combination of clinical signs that results in the spectrum of the synaptopathies. A small number of diseases may also exhibit neuromuscular signs. In general, SV disorders tend to be severe, early encephalopathies that interfere with neurodevelopment. As a consequence, developmental delay and intellectual disability are constant in almost all the defects described. Considering that some of these diseases might mimic other neurometabolic conditions (and in particular treatable disorders), an initial extensive metabolic workup should always be considered. Further knowledge into pathophysiological mechanisms and biomarkers, as well as descriptions of new presynaptic disorders, will probably take place in the near future.

Cellular neurometabolism: a tentative to connect cell biology and metabolism in neurology

It has become increasingly evident that inborn errors of metabolism (IEMs) are particularly prevalent as diseases of the nervous system and that a broader, more inclusive definition of IEM is necessary. In fact, as long as biochemistry is involved, any kind of monogenic disease can become an IEM. This new, extended definition includes new categories and mechanisms, and as a general trend will go beyond a single biochemical pathway and/or organelle, and will appear as a connection of multiple crossroads in a system biology approach.From one side, a simplified and updated classification of IEM is presented that mixes elements from the diagnostic approach with pathophysiological considerations into three large categories based on the size of molecules ("small and simple" or "large and complex") and their implication in energy metabolism. But from another side, whatever their size, metabolites involved in IEM may behave in the brain as signalling molecules, structural components and fuels, and many metabolites have more than one role. Neurometabolism is becoming more relevant, not only in relation to these new categories of diseases but also as a necessary way to explain the mechanisms of brain damage in classically defined categories of IEM. Brain metabolism, which has been largely disregarded in the traditional approach to investigating and treating neurological diseases, is a major clue and probably the next imminent "revolution" in neurology and neuroscience. Biochemistry (metabolism) and cell neurobiology need to meet. Additionally, the brain should be studied as a system (connecting different levels of complexity).

Synaptic metabolism: a new approach to inborn errors of neurotransmission

To date, inborn errors of neurotransmitters have been defined based on the classic concept of inborn error of metabolism (IEM), and they include defects in synthesis, catabolism, and transport pathways. However, the omics era is bringing insights into new diseases and is leading to an extended definition of IEM including new categories and mechanisms. Neurotransmission takes place at the synapse, the most specialized tight junction in the brain. The concept of "synaptic metabolism" would point to the specific chemical composition and metabolic functions of the synapse. Based on these specialized functions, we aim to provide a tentative overview about the major categories of IEM susceptible to affect neurotransmission. Small molecule defects (biogenic amines and amino acids) and energy defects are amongst the most prevalent diseases reported to disturb the concentration of CSF neurotransmitters. In these IEM, the neurological phenotypes have been largely described. Disorders of complex molecules are not typically considered as diseases affecting neurotransmission. However, most of them have been recently discovered and are involved in intracellular vesiculation, trafficking, processing, and quality control mechanisms. In this large group, neurotransmission is affected in disorders of chaperones and autophagy, disorders of the synaptic vesicle, and diseases affecting pre-synaptic membranes (synthesis and remodeling of complex lipids, defects of glycosylation). Disorders of the vesicle pools, receptor trafficking, and the chronobiology of neurotransmission are potentially emerging new categories. Finally, although not considered as IEM, channelopathies are a large group of diseases disturbing neurotransmitter homeostasis. New CSF biomarkers will probably contribute to improve the diagnosis of these disorders and find new therapeutic targets.

Gamma-aminobutyric acid levels in cerebrospinal fluid in neuropaediatric disorders

AIM

Gamma-aminobutyric acid (GABA) is a major modulator in brain maturation and its role in many different neurodevelopmental disorders has been widely reported. Although the involvement of GABA in different disorders has been related to its regulatory function as an inhibitory neurotransmitter in the mature brain, co-transmitter, and signalling molecule, little is known about its role as a clinical biomarker in neuropaediatric disorders. The aim of this study is to report the cerebrospinal fluid (CSF) free-GABA concentrations in a large cohort of patients (n=85) with different neurological disorders.

METHOD

GABA was measured in the CSF of neuropaediatric patients using capillary electrophoresis with laser-induced fluorescence detection. Other neurotransmitters (amino acids and monoamines) were also analysed.

RESULTS

GABA concentrations in CSF were abnormal, with a greater frequency (44%) than monoamines (20%) in neuropaediatric patients compared with our reference values. Although we included a few patients with inborn errors of metabolism, GABA levels in CSF were more frequently abnormal in metabolic disorders than in other nosological groups.

INTERPRETATION

Our work suggests further research into brain GABAergic status in neuropaediatric disorders, which could also lead to new therapeutic strategies.

WHAT THIS PAPER ADDS

Homeostasis of GABA seems more vulnerable than that of monoamines in the developing brain. The highest GABA levels are found in the primary GABA neurotransmitter disorder SSADH deficiency. GABA alterations are not specific for any clinical or neuroimaging presentation.

Rett-like Severe Encephalopathy Caused by a De Novo GRIN2B Mutation Is Attenuated by D-serine Dietary Supplement

BACKGROUND

N-Methyl-D-aspartate receptors (NMDARs) play pivotal roles in synaptic development, plasticity, neural survival, and cognition. Despite recent reports describing the genetic association between de novo mutations of NMDAR subunits and severe psychiatric diseases, little is known about their pathogenic mechanisms and potential therapeutic interventions. Here we report a case study of a 4-year-old Rett-like patient with severe encephalopathy carrying a missense de novo mutation in GRIN2B(p.P553T) coding for the GluN2B subunit of NMDAR.

METHODS

We generated a dynamic molecular model of mutant GluN2B-containing NMDARs. We expressed the mutation in cell lines and primary cultures, and we evaluated the putative morphological, electrophysiological, and synaptic plasticity alterations. Finally, we evaluated D-serine administration as a therapeutic strategy and translated it to the clinical practice.

RESULTS

Structural molecular modeling predicted a reduced pore size of mutant NMDARs. Electrophysiological recordings confirmed this prediction and also showed gating alterations, a reduced glutamate affinity associated with a strong decrease of NMDA-evoked currents. Moreover, GluN2B(P553T)-expressing neurons showed decreased spine density, concomitant with reduced NMDA-evoked currents and impaired NMDAR-dependent insertion of GluA1 at stimulated synapses. Notably, the naturally occurring coagonist D-serine was able to attenuate hypofunction of GluN2B(p.P553T)-containing NMDARs. Hence, D-serine dietary supplementation was initiated. Importantly, the patient has shown remarkable motor, cognitive, and communication improvements after 17 months of D-serine dietary supplementation.

CONCLUSIONS

Our data suggest that hypofunctional NMDARs containing GluN2B(p.P553T) can contribute to Rett-like encephalopathy and that their potentiation by D-serine treatment may underlie the associated clinical improvement.

[Neuronal communication and synaptic metabolism in childhood epilepsy]

INTRODUCTION

Basic neuroscience and neurometabolism are providing a rapidly increasing amount of knowledge on paediatric epilepsy and, more specifically, on the mechanisms involved in synaptic communication. There is, however, a mismatch between these advances and a vision that integrates them in a global way, in clinical and therapeutic practice.

AIMS

To offer an integrative view of the different molecular and metabolic mechanisms that are known and postulated in paediatric epilepsy, and to suggest concepts such as 'synaptic metabolism' and 'synaptic phenotypes' as useful tools for developing this approach.

DEVELOPMENT

We also review the most notable studies that attempt to explain the essential characteristics of synaptic communication in the developing brain by means of different molecules, essentially synaptic proteins, ion channels (chlorine, sodium and potassium co-transporters), and pre- and post-synaptic compartmentalisation, as well as the main players in metabolism (neurotransmitters, energy metabolism, growth factors and lipids). This combination of biological mechanisms has led to examples of 'synaptic phenotypes' being suggested in two specific cases of genetic (SCN1A) and metabolic epilepsy (epilepsy with response to pyridoxine).

CONCLUSIONS

A holistic perspective, which takes into account the diversity of elements that are related and which take place at certain times in neurodevelopment, can help to define phenotypes, channels for synaptic metabolism and brain connectivity, which facilitate not only the understanding of the pathophysiology, but also new therapeutic approaches in paediatric epilepsy.

Mutations in the lipoyltransferase LIPT1 gene cause a fatal disease associated with a specific lipoylation defect of the 2-ketoacid dehydrogenase complexes

Cofactor disorders of mitochondrial energy metabolism are a heterogeneous group of diseases with a wide variety of clinical symptoms, particular metabolic profiles and variable enzymatic defects. Mutations in NFU1, BOLA3, LIAS and IBA57 have been identified in patients with deficient lipoic acid-dependent enzymatic activities and defects in the assembly and activity of the mitochondrial respiratory chain complexes. Here, we report a patient with an early onset fatal lactic acidosis presenting a biochemical phenotype compatible with a combined defect of pyruvate dehydrogenase (PDHC) and 2-ketoglutarate dehydrogenase (2-KGDH) activities, which suggested a deficiency in lipoic acid metabolism. Immunostaining analysis showed that lipoylated E2-PDH and E2-KGDH were extremely reduced in this patient. However, the absence of glycine elevation, the normal activity of the glycine cleavage system and the normal lipoylation of the H protein suggested a defect of lipoic acid transfer to particular proteins rather than a general impairment of lipoic acid biosynthesis as the potential cause of the disease. By analogy with yeast metabolism, we postulated LIPT1 as the altered candidate gene causing the disease. Sequence analysis of the human LIPT1 identified two heterozygous missense mutations (c.212C>T and c.292C>G), segregating in different alleles. Functional complementation experiments in patient's fibroblasts demonstrated that these mutations are disease-causing and that LIPT1 protein is required for lipoylation and activation of 2-ketoacid dehydrogenases in humans. These findings expand the spectrum of genetic defects associated with lipoic acid metabolism and provide the first evidence of a lipoic acid transfer defect in humans.

[Hypotonia in the neonatal period: 12 years' experience]

INTRODUCTION

Hypotonia is one of the most frequent signs of neurological pathology in newborn infants.

AIMS

To determine, in a 12-year retrospective study, the relative frequency of neurological pathologies that can be accompanied by hypotonia during the neonatal period and to describe the neurological development after two years' follow-up.

PATIENTS AND METHODS

We conducted a systematic review of the newborn infants with hypotonia due to an identifiable cause who were admitted to the neonatal unit of the Hospital Sant Joan de Deu between January 1996 and December 2008. Information collected referred to family history, data related with the pregnancy and childbirth, clinical features and complementary tests. RESULTS. A total of 73 hypotonic newborn infants were identified, 21 (28.7%) of whom met eligibility criteria. The majority, 81% (n = 17), were classified as central hypotonias and the remaining 19% (n = 4) were graded as peripheral hypotonias. In the first group, 47% (n = 8) presented chromosomal disorders, 29.4% (n = 5) had metabolic diseases and 23.5% (n = 4) displayed malformations of the central nervous system.

CONCLUSIONS

The central causes of hypotonia continue to be the most prevalent in the neonatal period and, within these, chromosomal disorders are the most frequent. Peripheral causes are less common and have a more severe prognosis. Advances in molecular genetics have allowed many neuromuscular diseases to be diagnosed both genetically and at the molecular level. The diversity and complexity of the pathologies justifies a multidisciplinary approach being taken to treat these newborn infants.