Biallelic Variants of MRPS36 Cause a New Form of Leigh Syndrome.

BACKGROUND: The MRPS36 gene encodes a recently identified component of the 2-oxoglutarate dehydrogenase complex (OGDHC), a key enzyme of the Krebs cycle catalyzing the oxidative decarboxylation of 2-oxoglutarate to succinyl-CoA. Defective OGDHC activity causes a clinically variable metabolic disorder characterized by global developmental delay, severe neurological impairment, liver failure, and early-onset lactic acidosis. METHODS: We investigated the molecular cause underlying Leigh syndrome with bilateral striatal necrosis in two siblings through exome sequencing. Functional studies included measurement of the OGDHC enzymatic activity and MRPS36 mRNA levels in fibroblasts, assessment of protein stability in transfected cells, and structural analysis. A literature review was performed to define the etiological and phenotypic spectrum of OGDHC deficiency. RESULTS: In the two affected brothers, exome sequencing identified a homozygous nonsense variant (c.283G>T, p.Glu95*) of MRPS36. The variant did not affect transcript processing and stability, nor protein levels, but resulted in a shorter protein lacking nine residues that contribute to the structural and functional organization of the OGDHC complex. OGDHC enzymatic activity was significantly reduced. The review of previously reported cases of OGDHC deficiency supports the association of this enzymatic defect with Leigh phenotypic spectrum and early-onset movement disorder. Slightly elevated plasma levels of glutamate and glutamine were observed in our and literature patients with OGDHC defect. CONCLUSIONS: Our findings point to MRPS36 as a new disease gene implicated in Leigh syndrome. The slight elevation of plasma levels of glutamate and glutamine observed in patients with OGDHC deficiency represents a candidate metabolic signature of this neurometabolic disorder. © 2024 International Parkinson and Movement Disorder Society.

Comparative Clustering (CompaCt) of eukaryote complexomes identifies novel interactions and sheds light on protein complex evolution.

Complexome profiling allows large-scale, untargeted, and comprehensive characterization of protein complexes in a biological sample using a combined approach of separating intact protein complexes e.g., by native gel electrophoresis, followed by mass spectrometric analysis of the proteins in the resulting fractions. Over the last decade, its application has resulted in a large collection of complexome profiling datasets. While computational methods have been developed for the analysis of individual datasets, methods for large-scale comparative analysis of complexomes from multiple species are lacking. Here, we present Comparative Clustering (CompaCt), that performs fully automated integrative analysis of complexome profiling data from multiple species, enabling systematic characterization and comparison of complexomes. CompaCt implements a novel method for leveraging orthology in comparative analysis to allow systematic identification of conserved as well as taxon-specific elements of the analyzed complexomes. We applied this method to a collection of 53 complexome profiles spanning the major branches of the eukaryotes. We demonstrate the ability of CompaCt to robustly identify the composition of protein complexes, and show that integrated analysis of multiple datasets improves characterization of complexes from specific complexome profiles when compared to separate analyses. We identified novel candidate interactors and complexes in a number of species from previously analyzed datasets, like the emp24, the V-ATPase and mitochondrial ATP synthase complexes. Lastly, we demonstrate the utility of CompaCt for the automated large-scale characterization of the complexome of the mosquito Anopheles stephensi shedding light on the evolution of metazoan protein complexes. CompaCt is available from https://github.com/cmbi/compact-bio.

Characterisation of an Adult Zebrafish Model for SDHB-Associated Phaeochromocytomas and Paragangliomas.

Phaeochromocytomas and paragangliomas (PPGLs) are rare neuroendocrine tumours arising from chromaffin cells. Pathogenic variants in the gene succinate dehydrogenase subunit B (SDHB) are associated with malignancy and poor prognosis. When metastases arise, limited treatment options are available. The pathomechanism of SDHB-associated PPGL remains largely unknown, and the lack of suitable models hinders therapy development. Germline heterozygous SDHB pathogenic variants predispose to developing PPGLs with a life-long penetrance of around 50%. To mimic the human disease phenotype, we characterised adult heterozygous sdhb mutant zebrafish as a potential model to study SDHB-related PPGLs. Adult sdhb mutant zebrafish did not develop an obvious tumour phenotype and were anatomically and histologically like their wild-type siblings. However, sdhb mutants showed significantly increased succinate levels, a major hallmark of SDHB-related PPGLs. While basal activity was increased during day periods in mutants, mitochondrial complex activity and catecholamine metabolite levels were not significantly different. In conclusion, we characterised an adult in vivo zebrafish model, genetically resembling human carriers. Adult heterozygous sdhb mutants mimicked their human counterparts, showing systemic elevation of succinate levels despite the absence of a tumour phenotype. This model forms a promising basis for developing a full tumour phenotype and gaining knowledge of the pathomechanism behind SDHB-related PPGLs.

Bi-Allelic UQCRFS1 Variants Are Associated with Mitochondrial Complex III Deficiency, Cardiomyopathy, and Alopecia Totalis.

Isolated complex III (CIII) deficiencies are among the least frequently diagnosed mitochondrial disorders. Clinical symptoms range from isolated myopathy to severe multi-systemic disorders with early death and disability. To date, we know of pathogenic variants in genes encoding five out of 10 subunits and five out of 13 assembly factors of CIII. Here we describe rare bi-allelic variants in the gene of a catalytic subunit of CIII, UQCRFS1, which encodes the Rieske iron-sulfur protein, in two unrelated individuals. Affected children presented with low CIII activity in fibroblasts, lactic acidosis, fetal bradycardia, hypertrophic cardiomyopathy, and alopecia totalis. Studies in proband-derived fibroblasts showed a deleterious effect of the variants on UQCRFS1 protein abundance, mitochondrial import, CIII assembly, and cellular respiration. Complementation studies via lentiviral transduction and overexpression of wild-type UQCRFS1 restored mitochondrial function and rescued the cellular phenotype, confirming UQCRFS1 variants as causative for CIII deficiency. We demonstrate that mutations in UQCRFS1 can cause mitochondrial disease, and our results thereby expand the clinical and mutational spectrum of CIII deficiencies.

De novo missense variants in RRAGC lead to a fatal mTORopathy of early childhood.

PURPOSE: Mechanistic target of rapamycin (mTOR) complex 1 (mTORC1) regulates cell growth in response to nutritional status. Central to the mTORC1 function is the Rag-GTPase heterodimer. One component of the Rag heterodimer is RagC (Ras-related GTP-binding protein C), which is encoded by the RRAGC gene. METHODS: Genetic testing via trio exome sequencing was applied to identify the underlying disease cause in 3 infants with dilated cardiomyopathy, hepatopathy, and brain abnormalities, including pachygyria, polymicrogyria, and septo-optic dysplasia. Studies in patient-derived skin fibroblasts and in a HEK293 cell model were performed to investigate the cellular consequences. RESULTS: We identified 3 de novo missense variants in RRAGC (NM_022157.4: c.269C>A, p.(Thr90Asn), c.353C>T, p.(Pro118Leu), and c.343T>C, p.(Trp115Arg)), which were previously reported as occurring somatically in follicular lymphoma. Studies of patient-derived fibroblasts carrying the p.(Thr90Asn) variant revealed increased cell size, as well as dysregulation of mTOR-related p70S6K (ribosomal protein S6 kinase 1) and transcription factor EB signaling. Moreover, subcellular localization of mTOR was decoupled from metabolic state. We confirmed the key findings for all RRAGC variants described in this study in a HEK293 cell model. CONCLUSION: The above results are in line with a constitutive overactivation of the mTORC1 pathway. Our study establishes de novo missense variants in RRAGC as cause of an early-onset mTORopathy with unfavorable prognosis.

Diagnosing, discarding, or de-VUSsing: A practical guide to (un)targeted metabolomics as variant-transcending functional tests.

PURPOSE: For patients with inherited metabolic disorders (IMDs), any diagnostic delay should be avoided because early initiation of personalized treatment could prevent irreversible health damage. To improve diagnostic interpretation of genetic data, gene function tests can be valuable assets. For IMDs, variant-transcending functional tests are readily available through (un)targeted metabolomics assays. To support the application of metabolomics for this purpose, we developed a gene-based guide to select functional tests to either confirm or exclude an IMD diagnosis. METHODS: Using information from a diagnostic IMD exome panel, Kyoto Encyclopedia of Genes and Genomes, and Inborn Errors of Metabolism Knowledgebase, we compiled a guide for metabolomics-based gene function tests. From our practical experience with this guide, we retrospectively selected illustrative cases for whom combined metabolomic/genomic testing improved diagnostic success and evaluated the effect hereof on clinical management. RESULTS: The guide contains 2047 metabolism-associated genes for which a validated or putative variant-transcending gene function test is available. We present 16 patients for whom metabolomic testing either confirmed or ruled out the presence of a second pathogenic variant, validated or ruled out pathogenicity of variants of uncertain significance, or identified a diagnosis initially missed by genetic analysis. CONCLUSION: Metabolomics-based gene function tests provide additional value in the diagnostic trajectory of patients with suspected IMD by enhancing and accelerating diagnostic success.

Variants in Mitochondrial ATP Synthase Cause Variable Neurologic Phenotypes.

OBJECTIVE: ATP synthase (ATPase) is responsible for the majority of ATP production. Nevertheless, disease phenotypes associated with mutations in ATPase subunits are extremely rare. We aimed at expanding the spectrum of ATPase-related diseases. METHODS: Whole-exome sequencing in cohorts with 2,962 patients diagnosed with mitochondrial disease and/or dystonia and international collaboration were used to identify deleterious variants in ATPase-encoding genes. Findings were complemented by transcriptional and proteomic profiling of patient fibroblasts. ATPase integrity and activity were assayed using cells and tissues from 5 patients. RESULTS: We present 10 total individuals with biallelic or de novo monoallelic variants in nuclear ATPase subunit genes. Three unrelated patients showed the same homozygous missense ATP5F1E mutation (including one published case). An intronic splice-disrupting alteration in compound heterozygosity with a nonsense variant in ATP5PO was found in one patient. Three patients had de novo heterozygous missense variants in ATP5F1A, whereas another 3 were heterozygous for ATP5MC3 de novo missense changes. Bioinformatics methods and populational data supported the variants' pathogenicity. Immunohistochemistry, proteomics, and/or immunoblotting revealed significantly reduced ATPase amounts in association to ATP5F1E and ATP5PO mutations. Diminished activity and/or defective assembly of ATPase was demonstrated by enzymatic assays and/or immunoblotting in patient samples bearing ATP5F1A-p.Arg207His, ATP5MC3-p.Gly79Val, and ATP5MC3-p.Asn106Lys. The associated clinical profiles were heterogeneous, ranging from hypotonia with spontaneous resolution (1/10) to epilepsy with early death (1/10) or variable persistent abnormalities, including movement disorders, developmental delay, intellectual disability, hyperlactatemia, and other neurologic and systemic features. Although potentially reflecting an ascertainment bias, dystonia was common (7/10). INTERPRETATION: Our results establish evidence for a previously unrecognized role of ATPase nuclear-gene defects in phenotypes characterized by neurodevelopmental and neurodegenerative features. ANN NEUROL 2022;91:225-237.

Gitelman-Like Syndrome Caused by Pathogenic Variants in mtDNA.

BACKGROUND: Gitelman syndrome is the most frequent hereditary salt-losing tubulopathy characterized by hypokalemic alkalosis and hypomagnesemia. Gitelman syndrome is caused by biallelic pathogenic variants in SLC12A3, encoding the Na+-Cl- cotransporter (NCC) expressed in the distal convoluted tubule. Pathogenic variants of CLCNKB, HNF1B, FXYD2, or KCNJ10 may result in the same renal phenotype of Gitelman syndrome, as they can lead to reduced NCC activity. For approximately 10 percent of patients with a Gitelman syndrome phenotype, the genotype is unknown. METHODS: We identified mitochondrial DNA (mtDNA) variants in three families with Gitelman-like electrolyte abnormalities, then investigated 156 families for variants in MT-TI and MT-TF, which encode the transfer RNAs for phenylalanine and isoleucine. Mitochondrial respiratory chain function was assessed in patient fibroblasts. Mitochondrial dysfunction was induced in NCC-expressing HEK293 cells to assess the effect on thiazide-sensitive 22Na+ transport. RESULTS: Genetic investigations revealed four mtDNA variants in 13 families: m.591C>T (n=7), m.616T>C (n=1), m.643A>G (n=1) (all in MT-TF), and m.4291T>C (n=4, in MT-TI). Variants were near homoplasmic in affected individuals. All variants were classified as pathogenic, except for m.643A>G, which was classified as a variant of uncertain significance. Importantly, affected members of six families with an MT-TF variant additionally suffered from progressive chronic kidney disease. Dysfunction of oxidative phosphorylation complex IV and reduced maximal mitochondrial respiratory capacity were found in patient fibroblasts. In vitro pharmacological inhibition of complex IV, mimicking the effect of the mtDNA variants, inhibited NCC phosphorylation and NCC-mediated sodium uptake. CONCLUSION: Pathogenic mtDNA variants in MT-TF and MT-TI can cause a Gitelman-like syndrome. Genetic investigation of mtDNA should be considered in patients with unexplained Gitelman syndrome-like tubulopathies.

In Vitro Skeletal Muscle Model of PGM1 Deficiency Reveals Altered Energy Homeostasis.

Phosphoglucomutase 1 (PGM1) is a key enzyme for the regulation of energy metabolism from glycogen and glycolysis, as it catalyzes the interconversion of glucose 1-phosphate and glucose 6-phosphate. PGM1 deficiency is an autosomal recessive disorder characterized by a highly heterogenous clinical spectrum, including hypoglycemia, cleft palate, liver dysfunction, growth delay, exercise intolerance, and dilated cardiomyopathy. Abnormal protein glycosylation has been observed in this disease. Oral supplementation with D-galactose efficiently restores protein glycosylation by replenishing the lacking pool of UDP-galactose, and rescues some symptoms, such as hypoglycemia, hepatopathy, and growth delay. However, D-galactose effects on skeletal muscle and heart symptoms remain unclear. In this study, we established an in vitro muscle model for PGM1 deficiency to investigate the role of PGM1 and the effect of D-galactose on nucleotide sugars and energy metabolism. Genome-editing of C2C12 myoblasts via CRISPR/Cas9 resulted in Pgm1 (mouse homologue of human PGM1, according to updated nomenclature) knockout clones, which showed impaired maturation to myotubes. No difference was found for steady-state levels of nucleotide sugars, while dynamic flux analysis based on 13C6-galactose suggested a block in the use of galactose for energy production in knockout myoblasts. Subsequent analyses revealed a lower basal respiration and mitochondrial ATP production capacity in the knockout myoblasts and myotubes, which were not restored by D-galactose. In conclusion, an in vitro mouse muscle cell model has been established to study the muscle-specific metabolic mechanisms in PGM1 deficiency, which suggested that galactose was unable to restore the reduced energy production capacity.

Bi-allelic GOT2 Mutations Cause a Treatable Malate-Aspartate Shuttle-Related Encephalopathy.

Early-infantile encephalopathies with epilepsy are devastating conditions mandating an accurate diagnosis to guide proper management. Whole-exome sequencing was used to investigate the disease etiology in four children from independent families with intellectual disability and epilepsy, revealing bi-allelic GOT2 mutations. In-depth metabolic studies in individual 1 showed low plasma serine, hypercitrullinemia, hyperlactatemia, and hyperammonemia. The epilepsy was serine and pyridoxine responsive. Functional consequences of observed mutations were tested by measuring enzyme activity and by cell and animal models. Zebrafish and mouse models were used to validate brain developmental and functional defects and to test therapeutic strategies. GOT2 encodes the mitochondrial glutamate oxaloacetate transaminase. GOT2 enzyme activity was deficient in fibroblasts with bi-allelic mutations. GOT2, a member of the malate-aspartate shuttle, plays an essential role in the intracellular NAD(H) redox balance. De novo serine biosynthesis was impaired in fibroblasts with GOT2 mutations and GOT2-knockout HEK293 cells. Correcting the highly oxidized cytosolic NAD-redox state by pyruvate supplementation restored serine biosynthesis in GOT2-deficient cells. Knockdown of got2a in zebrafish resulted in a brain developmental defect associated with seizure-like electroencephalography spikes, which could be rescued by supplying pyridoxine in embryo water. Both pyridoxine and serine synergistically rescued embryonic developmental defects in zebrafish got2a morphants. The two treated individuals reacted favorably to their treatment. Our data provide a mechanistic basis for the biochemical abnormalities in GOT2 deficiency that may also hold for other MAS defects.

Correction: Loss of Bardet-Biedl syndrome proteins causes synaptic aberrations in principal neurons.

[This corrects the article DOI: 10.1371/journal.pbio.3000414.].

Loss of Bardet-Biedl syndrome proteins causes synaptic aberrations in principal neurons.

Bardet-Biedl syndrome (BBS), a ciliopathy, is a rare genetic condition characterised by retinal degeneration, obesity, kidney failure, and cognitive impairment. In spite of progress made in our general understanding of BBS aetiology, the molecular and cellular mechanisms underlying cognitive impairment in BBS remain elusive. Here, we report that the loss of BBS proteins causes synaptic dysfunction in principal neurons, providing a possible explanation for the cognitive impairment phenotype observed in BBS patients. Using synaptosomal proteomics and immunocytochemistry, we demonstrate the presence of Bbs proteins in the postsynaptic density (PSD) of hippocampal neurons. Loss of Bbs results in a significant reduction of dendritic spines in principal neurons of Bbs mouse models. Furthermore, we show that spine deficiency correlates with events that destabilise spine architecture, such as impaired spine membrane receptor signalling, known to be involved in the maintenance of dendritic spines. Our findings suggest a role for BBS proteins in dendritic spine homeostasis that may be linked to the cognitive phenotype observed in BBS.

Mitochondrial RNA processing defect caused by a SUPV3L1 mutation in two siblings with a novel neurodegenerative syndrome.

SUPV3L1 encodes a helicase that is mainly localized in the mitochondria. It has been shown in vitro to possess both double-stranded RNA and DNA unwinding activity that is ATP-dependent. Here we report the first two patients for this gene who presented with a homozygous preliminary stop codon resulting in a C-terminal truncation of the SUPV3L1 protein. They presented with a characteristic phenotype of neurodegenerative nature with progressive spastic paraparesis, growth restriction, hypopigmentation, and predisposition to autoimmune disease. Ophthalmological examination showed severe photophobia with corneal erosions, optic atrophy, and pigmentary retinopathy, while neuroimaging showed atrophy of the optic chiasm and the pons with calcification of putamina, with intermittent and mild elevation of lactate. We show that the amino acids that are eliminated by the preliminary stop codon are highly conserved and are predicted to form an amphipathic helix. To investigate if the mutation causes mitochondrial dysfunction, we examined fibroblasts of the proband. We observed very low expression of the truncated protein, a reduction in the mature ND6 mRNA species as well as the accumulation of double-stranded RNA. Lentiviral complementation with the full-length SUPV3L1 cDNA partly restored the observed RNA phenotypes, supporting that the SUPV3L1 mutation in these patients is pathogenic and the cause of the disease.

A novel variant in COX16 causes cytochrome c oxidase deficiency, severe fatal neonatal lactic acidosis, encephalopathy, cardiomyopathy, and liver dysfunction.

COX16 is involved in the biogenesis of cytochrome-c-oxidase (complex IV), the terminal complex of the mitochondrial respiratory chain. We present the first report of two unrelated patients with the homozygous nonsense variant c.244C>T(p. Arg82*) in COX16 with hypertrophic cardiomyopathy, encephalopathy and severe fatal lactic acidosis, and isolated complex IV deficiency. The absence of COX16 protein expression leads to a complete loss of the holo-complex IV, as detected by Western blot in patient fibroblasts. Lentiviral transduction of patient fibroblasts with wild-type COX16 complementary DNA rescued complex IV biosynthesis. We hypothesize that COX16 could play a role in the copper delivery route of the COX2 module as part of the complex IV assembly. Our data provide clear evidence for the pathogenicity of the COX16 variant as a cause for the observed clinical features and the isolated complex IV deficiency in these two patients and that COX16 deficiency is a cause for mitochondrial disease.

Chronic fluoxetine or ketamine treatment differentially affects brain energy homeostasis which is not exacerbated in mice with trait suboptimal mitochondrial function.

Antidepressants have been shown to influence mitochondrial function directly, and suboptimal mitochondrial function (SMF) has been implicated in complex psychiatric disorders. In the current study, we used a mouse model for trait SMF to test the hypothesis that chronic fluoxetine treatment in mice subjected to chronic stress would negatively impact brain bioenergetics, a response that would be more pronounced in mice with trait SMF. In contrast, we hypothesized that chronic ketamine treatment would positively impact mitochondrial function in both WT and mice with SMF. We used an animal model for trait SMF, the Ndufs4GT/GT mice, which exhibit 25% lower mitochondrial complex I activity. In addition to antidepressant treatment, mice were subjected to chronic unpredictable stress (CUS). This paradigm is widely used to model complex behaviours expressed in various psychiatric disorders. We assayed several physiological indices as proxies for the impact of chronic stress and antidepressant treatment. Furthermore, we measured brain mitochondrial complex activities using clinically validated assays as well as established metabolic signatures using targeted metabolomics. As hypothesized, we found evidence that chronic fluoxetine treatment negatively impacted brain bioenergetics. This phenotype was, however, not further exacerbated in mice with trait SMF. Ketamine did not have a significant influence on brain mitochondrial function in either genotype. Here we report that trait SMF could be a moderator for an individual's response to antidepressant treatment. Based on these results, we propose that in individuals with SMF and comorbid psychopathology, fluoxetine should be avoided, whereas ketamine could be a safer choice of treatment.

Bi-allelic Mutations in the Mitochondrial Ribosomal Protein MRPS2 Cause Sensorineural Hearing Loss, Hypoglycemia, and Multiple OXPHOS Complex Deficiencies.

Biogenesis of the mitochondrial oxidative phosphorylation system, which produces the bulk of ATP for almost all eukaryotic cells, depends on the translation of 13 mtDNA-encoded polypeptides by mitochondria-specific ribosomes in the mitochondrial matrix. These mitoribosomes are dual-origin ribonucleoprotein complexes, which contain mtDNA-encoded rRNAs and tRNAs and ∼80 nucleus-encoded proteins. An increasing number of gene mutations that impair mitoribosomal function and result in multiple OXPHOS deficiencies are being linked to human mitochondrial diseases. Using exome sequencing in two unrelated subjects presenting with sensorineural hearing impairment, mild developmental delay, hypoglycemia, and a combined OXPHOS deficiency, we identified mutations in the gene encoding the mitochondrial ribosomal protein S2, which has not previously been implicated in disease. Characterization of subjects' fibroblasts revealed a decrease in the steady-state amounts of mutant MRPS2, and this decrease was shown by complexome profiling to prevent the assembly of the small mitoribosomal subunit. In turn, mitochondrial translation was inhibited, resulting in a combined OXPHOS deficiency detectable in subjects' muscle and liver biopsies as well as in cultured skin fibroblasts. Reintroduction of wild-type MRPS2 restored mitochondrial translation and OXPHOS assembly. The combination of lactic acidemia, hypoglycemia, and sensorineural hearing loss, especially in the presence of a combined OXPHOS deficiency, should raise suspicion for a ribosomal-subunit-related mitochondrial defect, and clinical recognition could allow for a targeted diagnostic approach. The identification of MRPS2 as an additional gene related to mitochondrial disease further expands the genetic and phenotypic spectra of OXPHOS deficiencies caused by impaired mitochondrial translation.

One mutation, three phenotypes: novel metabolic insights on MELAS, MIDD and myopathy caused by the m.3243A > G mutation.

INTRODUCTION: The m.3243A > G mitochondrial DNA mutation is one of the most common mitochondrial disease-causing mutations, with a carrier rate as high as 1:400. This point mutation affects the MT-TL1 gene, ultimately affecting the oxidative phosphorylation system and the cell's energy production. Strikingly, the m.3243A > G mutation is associated with different phenotypes, including mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS), maternally inherited diabetes and deafness (MIDD) and myopathy. OBJECTIVES: We investigated urine metabolomes of MELAS, MIDD and myopathy patients in order to identify affected metabolic pathways and possible treatment options. METHODS: A multiplatform metabolomics approach was used to comprehensively analyze the metabolome and compare metabolic profiles of different phenotypes caused by the m.3243A > G mutation. Our analytical array consisted of NMR spectroscopy, LC-MS/MS and GC-TOF-MS. RESULTS: The investigation revealed phenotypic specific metabolic perturbations, as well as metabolic similarities between the different phenotypes. We show that glucose metabolism is highly disturbed in the MIDD phenotype, but not in MELAS or myopathy, remodeled fatty acid oxidation is characteristic of the MELAS patients, while one-carbon metabolism is strongly modified in both MELAS and MIDD, but not in the myopathy group. Lastly we identified increased creatine in the urine of the myopathy patients, but not in MELAS or MIDD. CONCLUSION: We conclude by giving novel insight on the phenotypes of the m.3243A > G mutation from a metabolomics point of view. Directives are also given for future investigations that could lead to better treatment options for patients suffering from this debilitating disease.

Delineation of molecular findings by whole-exome sequencing for suspected cases of paediatric-onset mitochondrial diseases in the Southern Chinese population.

BACKGROUND: Mitochondrial diseases (MDs) are a group of clinically and genetically heterogeneous disorders characterized by defects in oxidative phosphorylation. Since clinical phenotypes of MDs may be non-specific, genetic diagnosis is crucial for guiding disease management. In the current study, whole-exome sequencing (WES) was performed for our paediatric-onset MD cohort of a Southern Chinese origin, with the aim of identifying key disease-causing variants in the Chinese patients with MDs. METHODS: We recruited Chinese patients who had paediatric-onset MDs and a minimum mitochondrial disease criteria (MDC) score of 3. Patients with positive target gene or mitochondrial DNA sequencing results were excluded. WES was performed, variants with population frequency ≤ 1% were analysed for pathogenicity on the basis of the American College of Medical Genetics and Genomics guidelines. RESULTS: Sixty-six patients with pre-biopsy MDC scores of 3-8 were recruited. The overall diagnostic yield was 35% (23/66). Eleven patients (17%) were found to have mutations in MD-related genes, with COQ4 having the highest mutation rate owing to the Chinese-specific founder mutation (4/66, 6%). Twelve patients (12/66, 18%) had mutations in non-MD-related genes: ATP1A3 (n = 3, two were siblings), ALDH5A1, ARX, FA2H, KCNT1, LDHD, NEFL, NKX2-2, TBCK, and WAC. CONCLUSIONS: We confirmed that the COQ4:c.370G>A, p.(Gly124Ser) variant, was a founder mutation among the Southern Chinese population. Screening for this mutation should therefore be considered while diagnosing Chinese patients suspected to have MDs. Furthermore, WES has proven to be useful in detecting variants in patients suspected to have MDs because it helps to obtain an unbiased and precise genetic diagnosis for these diseases, which are genetically heterogeneous.

Clinical, morphological and genetic characterization of Brody disease: an international study of 40 patients.

Brody disease is an autosomal recessive myopathy characterized by exercise-induced muscle stiffness due to mutations in the ATP2A1 gene. Almost 50 years after the initial case presentation, only 18 patients have been reported and many questions regarding the clinical phenotype and results of ancillary investigations remain unanswered, likely leading to incomplete recognition and consequently under-diagnosis. Additionally, little is known about the natural history of the disorder, genotype-phenotype correlations, and the effects of symptomatic treatment. We studied the largest cohort of Brody disease patients to date (n = 40), consisting of 22 new patients (19 novel mutations) and all 18 previously published patients. This observational study shows that the main feature of Brody disease is an exercise-induced muscle stiffness of the limbs, and often of the eyelids. Onset begins in childhood and there was no or only mild progression of symptoms over time. Four patients had episodes resembling malignant hyperthermia. The key finding at physical examination was delayed relaxation after repetitive contractions. Additionally, no atrophy was seen, muscle strength was generally preserved, and some patients had a remarkable athletic build. Symptomatic treatment was mostly ineffective or produced unacceptable side effects. EMG showed silent contractures in approximately half of the patients and no myotonia. Creatine kinase was normal or mildly elevated, and muscle biopsy showed mild myopathic changes with selective type II atrophy. Sarcoplasmic/endoplasmic reticulum Ca2+ ATPase (SERCA) activity was reduced and western blot analysis showed decreased or absent SERCA1 protein. Based on this cohort, we conclude that Brody disease should be considered in cases of exercise-induced muscle stiffness. When physical examination shows delayed relaxation, and there are no myotonic discharges at electromyography, we recommend direct sequencing of the ATP2A1 gene or next generation sequencing with a myopathy panel. Aside from clinical features, SERCA activity measurement and SERCA1 western blot can assist in proving the pathogenicity of novel ATP2A1 mutations. Finally, patients with Brody disease may be at risk for malignant hyperthermia-like episodes, and therefore appropriate perioperative measures are recommended. This study will help improve understanding and recognition of Brody disease as a distinct myopathy in the broader field of calcium-related myopathies.

Homozygous damaging SOD2 variant causes lethal neonatal dilated cardiomyopathy.

BACKGROUND: Idiopathic dilated cardiomyopathy (DCM) is recognised to be a heritable disorder, yet clinical genetic testing does not produce a diagnosis in >50% of paediatric patients. Identifying a genetic cause is crucial because this knowledge can affect management options, cardiac surveillance in relatives and reproductive decision-making. In this study, we sought to identify the underlying genetic defect in a patient born to consanguineous parents with rapidly progressive DCM that led to death in early infancy. METHODS AND RESULTS: Exome sequencing revealed a potentially pathogenic, homozygous missense variant, c.542G>T, p.(Gly181Val), in SOD2. This gene encodes superoxide dismutase 2 (SOD2) or manganese-superoxide dismutase, a mitochondrial matrix protein that scavenges oxygen radicals produced by oxidation-reduction and electron transport reactions occurring in mitochondria via conversion of superoxide anion (O2-·) into H2O2. Measurement of hydroethidine oxidation showed a significant increase in O2-· levels in the patient's skin fibroblasts, as compared with controls, and this was paralleled by reduced catalytic activity of SOD2 in patient fibroblasts and muscle. Lentiviral complementation experiments demonstrated that mitochondrial SOD2 activity could be completely restored on transduction with wild type SOD2. CONCLUSION: Our results provide evidence that defective SOD2 may lead to toxic increases in the levels of damaging oxygen radicals in the neonatal heart, which can result in rapidly developing heart failure and death. We propose SOD2 as a novel nuclear-encoded mitochondrial protein involved in severe human neonatal cardiomyopathy, thus expanding the wide range of genetic factors involved in paediatric cardiomyopathies.