Structural insights into respiratory complex I deficiency and assembly from the mitochondrial disease-related ndufs4-/- mouse.

Respiratory complex I (NADH:ubiquinone oxidoreductase) is essential for cellular energy production and NAD+ homeostasis. Complex I mutations cause neuromuscular, mitochondrial diseases, such as Leigh Syndrome, but their molecular-level consequences remain poorly understood. Here, we use a popular complex I-linked mitochondrial disease model, the ndufs4-/- mouse, to define the structural, biochemical, and functional consequences of the absence of subunit NDUFS4. Cryo-EM analyses of the complex I from ndufs4-/- mouse hearts revealed a loose association of the NADH-dehydrogenase module, and discrete classes containing either assembly factor NDUFAF2 or subunit NDUFS6. Subunit NDUFA12, which replaces its paralogue NDUFAF2 in mature complex I, is absent from all classes, compounding the deletion of NDUFS4 and preventing maturation of an NDUFS4-free enzyme. We propose that NDUFAF2 recruits the NADH-dehydrogenase module during assembly of the complex. Taken together, the findings provide new molecular-level understanding of the ndufs4-/- mouse model and complex I-linked mitochondrial disease.

Multi-omics identifies large mitoribosomal subunit instability caused by pathogenic MRPL39 variants as a cause of pediatric onset mitochondrial disease.

MRPL39 encodes one of 52 proteins comprising the large subunit of the mitochondrial ribosome (mitoribosome). In conjunction with 30 proteins in the small subunit, the mitoribosome synthesizes the 13 subunits of the mitochondrial oxidative phosphorylation (OXPHOS) system encoded by mitochondrial Deoxyribonucleic acid (DNA). We used multi-omics and gene matching to identify three unrelated individuals with biallelic variants in MRPL39 presenting with multisystem diseases with severity ranging from lethal, infantile-onset (Leigh syndrome spectrum) to milder with survival into adulthood. Clinical exome sequencing of known disease genes failed to diagnose these patients; however quantitative proteomics identified a specific decrease in the abundance of large but not small mitoribosomal subunits in fibroblasts from the two patients with severe phenotype. Re-analysis of exome sequencing led to the identification of candidate single heterozygous variants in mitoribosomal genes MRPL39 (both patients) and MRPL15. Genome sequencing identified a shared deep intronic MRPL39 variant predicted to generate a cryptic exon, with transcriptomics and targeted studies providing further functional evidence for causation. The patient with the milder disease was homozygous for a missense variant identified through trio exome sequencing. Our study highlights the utility of quantitative proteomics in detecting protein signatures and in characterizing gene-disease associations in exome-unsolved patients. We describe Relative Complex Abundance analysis of proteomics data, a sensitive method that can identify defects in OXPHOS disorders to a similar or greater sensitivity to the traditional enzymology. Relative Complex Abundance has potential utility for functional validation or prioritization in many hundreds of inherited rare diseases where protein complex assembly is disrupted.

N-acetylcysteine and cysteamine bitartrate prevent azide-induced neuromuscular decompensation by restoring glutathione balance in two novel surf1-/- zebrafish deletion models of Leigh syndrome.

SURF1 deficiency (OMIM # 220110) causes Leigh syndrome (LS, OMIM # 256000), a mitochondrial disorder typified by stress-induced metabolic strokes, neurodevelopmental regression and progressive multisystem dysfunction. Here, we describe two novel surf1-/- zebrafish knockout models generated by CRISPR/Cas9 technology. While gross larval morphology, fertility, and survival into adulthood appeared unaffected, surf1-/- mutants manifested adult-onset ocular anomalies and decreased swimming activity, as well as classical biochemical hallmarks of human SURF1 disease, including reduced complex IV expression and enzymatic activity and increased tissue lactate. surf1-/- larvae also demonstrated oxidative stress and stressor hypersensitivity to the complex IV inhibitor, azide, which exacerbated their complex IV deficiency, reduced supercomplex formation, and induced acute neurodegeneration typical of LS including brain death, impaired neuromuscular responses, reduced swimming activity, and absent heartrate. Remarkably, prophylactic treatment of surf1-/- larvae with either cysteamine bitartrate or N-acetylcysteine, but not other antioxidants, significantly improved animal resiliency to stressor-induced brain death, swimming and neuromuscular dysfunction, and loss of heartbeat. Mechanistic analyses demonstrated cysteamine bitartrate pretreatment did not improve complex IV deficiency, ATP deficiency, or increased tissue lactate but did reduce oxidative stress and restore glutathione balance in surf1-/- animals. Overall, two novel surf1-/- zebrafish models recapitulate the gross neurodegenerative and biochemical hallmarks of LS, including azide stressor hypersensitivity that was associated with glutathione deficiency and ameliorated by cysteamine bitartrate or N-acetylcysteine therapy.

Human iPSC-derived cerebral organoids model features of Leigh syndrome and reveal abnormal corticogenesis.

Leigh syndrome (LS) is a rare, inherited neurometabolic disorder that presents with bilateral brain lesions caused by defects in the mitochondrial respiratory chain and associated nuclear-encoded proteins. We generated human induced pluripotent stem cells (iPSCs) from three LS patient-derived fibroblast lines. Using whole-exome and mitochondrial sequencing, we identified unreported mutations in pyruvate dehydrogenase (GM0372, PDH; GM13411, MT-ATP6/PDH) and dihydrolipoyl dehydrogenase (GM01503, DLD). These LS patient-derived iPSC lines were viable and capable of differentiating into progenitor populations, but we identified several abnormalities in three-dimensional differentiation models of brain development. LS patient-derived cerebral organoids showed defects in neural epithelial bud generation, size and cortical architecture at 100 days. The double mutant MT-ATP6/PDH line produced organoid neural precursor cells with abnormal mitochondrial morphology, characterized by fragmentation and disorganization, and showed an increased generation of astrocytes. These studies aim to provide a comprehensive phenotypic characterization of available patient-derived cell lines that can be used to study Leigh syndrome.

Digenic Leigh syndrome on the background of the m.11778G>A Leber hereditary optic neuropathy variant.

Leigh syndrome spectrum (LSS) is a primary mitochondrial disorder defined neuropathologically by a subacute necrotizing encephalomyelopathy and characterized by bilateral basal ganglia and/or brainstem lesions. LSS is associated with variants in several mitochondrial DNA genes and more than 100 nuclear genes, most often related to mitochondrial complex I (CI) dysfunction. Rarely, LSS has been reported in association with primary Leber hereditary optic neuropathy (LHON) variants of the mitochondrial DNA, coding for CI subunits (m.3460G>A in MT-ND1, m.11778G>A in MT-ND4 and m.14484T>C in MT-ND6). The underlying mechanism by which these variants manifest as LSS, a severe neurodegenerative disease, as opposed to the LHON phenotype of isolated optic neuropathy, remains an open question. Here, we analyse the exome sequencing of six probands with LSS carrying primary LHON variants, and report digenic co-occurrence of the m.11778G > A variant with damaging heterozygous variants in nuclear disease genes encoding CI subunits as a plausible explanation. Our findings suggest a digenic mechanism of disease for m.11778G>A-associated LSS, consistent with recent reports of digenic disease in individuals manifesting with LSS due to biallelic variants in the recessive LHON-associated disease gene DNAJC30 in combination with heterozygous variants in CI subunits.

Alternative splicing expands the clinical spectrum of NDUFS6-related mitochondrial disorders.

PURPOSE: We describe 3 families with Charcot-Marie-Tooth neuropathy (CMT), harboring a homozygous NDUFS6 NM_004553.6:c.309+5G>A variant previously linked to fatal Leigh syndrome. We aimed to characterize clinically and molecularly the newly identified patients and understand the mechanism underlying their milder phenotype. METHODS: The patients underwent extensive clinical examinations. Exome sequencing was done in 4 affected individuals. The functional effect of the c.309+5G>A variant was investigated in patient-derived EBV-transformed lymphoblasts at the complementary DNA, protein, and mitochondrial level. Alternative splicing was evaluated using complementary DNA long-read sequencing. RESULTS: All patients presented with early-onset, slowly progressive axonal CMT, and nystagmus; some exhibited additional central nervous system symptoms. The c.309+5G>A substitution caused the expression of aberrantly spliced transcripts and negligible levels of the canonical transcript. Immunoblotting showed reduced levels of mutant isoforms. No detectable defects in mitochondrial complex stability or bioenergetics were found. CONCLUSION: We expand the clinical spectrum of NDUFS6-related mitochondrial disorders to include axonal CMT, emphasizing the clinical and pathophysiologic overlap between these 2 clinical entities. This work demonstrates the critical role that alternative splicing may play in modulating the severity of a genetic disorder, emphasizing the need for careful consideration when interpreting splice variants and their implications on disease prognosis.

Further delineation of short-chain enoyl-CoA hydratase deficiency in the Pacific population.

Short-chain enoyl-coA hydratase (SCEH) deficiency due to biallelic pathogenic ECHS1 variants was first reported in 2014 in association with Leigh syndrome (LS) and increased S-(2-carboxypropyl)cysteine excretion. It is potentially treatable with a valine-restricted, high-energy diet and emergency regimen. Recently, Simon et al. described four Samoan children harbouring a hypomorphic allele (c.489G > A, p.Pro163=) associated with reduced levels of normally-spliced mRNA. This synonymous variant, missed on standard genomic testing, is prevalent in the Samoan population (allele frequency 0.17). Patients with LS and one ECHS1 variant were identified in NZ and Australian genomic and clinical databases. ECHS1 sequence data were interrogated for the c.489G > A variant and clinical data were reviewed. Thirteen patients from 10 families were identified; all had Pacific ancestry including Samoan, Maori, Cook Island Maori, and Tokelauan. All developed bilateral globus pallidi lesions, excluding one pre-symptomatic infant. Symptom onset was in early childhood, and was triggered by illness or starvation in 9/13. Four of 13 had exercise-induced dyskinesia, 9/13 optic atrophy and 6/13 nystagmus. Urine S-(2-carboxypropyl)cysteine-carnitine and other SCEH-related metabolites were normal or mildly increased. Functional studies demonstrated skipping of exon four and markedly reduced ECHS1 protein. These data provide further support for the pathogenicity of this ECHS1 variant which is also prevalent in Maori, Cook Island Maori, and Tongan populations (allele frequency 0.14-0.24). It highlights the need to search for a second variant in apparent heterozygotes with an appropriate phenotype, and has implications for genetic counselling in family members who are heterozygous for the more severe ECHS1 alleles. SYNOPSIS: Short-chain enoyl-CoA hydratase deficiency is a frequent cause of Leigh-like disease in Maori and wider-Pacific populations, due to the high carrier frequency of a hypomorphic ECHS1 variant c.489G > A, p.[Pro163=, Phe139Valfs*65] that may be overlooked by standard genomic testing.

Leigh syndrome with developmental regression and ataxia due to a novel splicing variant in the PMPCB gene.

Only five children with pathogenic PMPCB gene variants have been described and all carried missense variants. Clinical features included a Leigh-like syndrome of developmental regression, basal ganglia lesions and ataxia with or without dystonia and epilepsy. Three of the five died in childhood and none was older than age six when described. We report the first splice site variant in the PMPCB gene in a 39-year old individual who experienced developmental regression and ataxia following otitis media in childhood. A minigene assay confirms this variant results in aberrant splicing and skipping of exon 12.

Interferon-gamma contributes to disease progression in the Ndufs4(-/-) model of Leigh syndrome.

AIM: Leigh syndrome (LS), the most common paediatric presentation of genetic mitochondrial dysfunction, is a multi-system disorder characterised by severe neurologic and metabolic abnormalities. Symmetric, bilateral, progressive necrotizing lesions in the brainstem are defining features of the disease. Patients are often symptom free in early life but typically develop symptoms by about 2 years of age. The mechanisms underlying disease onset and progression in LS remain obscure. Recent studies have shown that the immune system causally drives disease in the Ndufs4(-/-) mouse model of LS: treatment of Ndufs4(-/-) mice with the macrophage-depleting Csf1r inhibitor pexidartinib prevents disease. While the precise mechanisms leading to immune activation and immune factors involved in disease progression have not yet been determined, interferon-gamma (IFNγ) and interferon gamma-induced protein 10 (IP10) were found to be significantly elevated in Ndufs4(-/-) brainstem, implicating these factors in disease. Here, we aimed to explore the role of IFNγ and IP10 in LS. METHODS: To establish the role of IFNγ and IP10 in LS, we generated IFNγ and IP10 deficient Ndufs4(-/-)/Ifng(-/-) and Ndufs4(-/-)/IP10(-/-) double knockout animals, as well as IFNγ and IP10 heterozygous, Ndufs4(-/-)/Ifng(+/-) and Ndufs4(-/-)/IP10(+/-), animals. We monitored disease onset and progression to define the impact of heterozygous or homozygous loss of IFNγ and IP10 in LS. RESULTS: Loss of IP10 does not significantly impact the onset or progression of disease in the Ndufs4(-/-) model. IFNγ loss significantly extends survival and delays disease progression in a gene dosage-dependent manner, though the benefits are modest compared to Csf1r inhibition. CONCLUSIONS: IFNγ contributes to disease onset and progression in LS. Our findings suggest that IFNγ targeting therapies may provide some benefits in genetic mitochondrial disease, but targeting IFNγ alone would likely yield only modest benefits in LS.

Expert Panel Curation of 113 Primary Mitochondrial Disease Genes for the Leigh Syndrome Spectrum.

OBJECTIVE: Primary mitochondrial diseases (PMDs) are heterogeneous disorders caused by inherited mitochondrial dysfunction. Classically defined neuropathologically as subacute necrotizing encephalomyelopathy, Leigh syndrome spectrum (LSS) is the most frequent manifestation of PMD in children, but may also present in adults. A major challenge for accurate diagnosis of LSS in the genomic medicine era is establishing gene-disease relationships (GDRs) for this syndrome with >100 monogenic causes across both nuclear and mitochondrial genomes. METHODS: The Clinical Genome Resource (ClinGen) Mitochondrial Disease Gene Curation Expert Panel (GCEP), comprising 40 international PMD experts, met monthly for 4 years to review GDRs for LSS. The GCEP standardized gene curation for LSS by refining the phenotypic definition, modifying the ClinGen Gene-Disease Clinical Validity Curation Framework to improve interpretation for LSS, and establishing a scoring rubric for LSS. RESULTS: The GDR with LSS across the nuclear and mitochondrial genomes was classified as definitive for 31 of 114 GDRs curated (27%), moderate for 38 (33%), limited for 43 (38%), and disputed for 2 (2%). Ninety genes were associated with autosomal recessive inheritance, 16 were maternally inherited, 5 were autosomal dominant, and 3 were X-linked. INTERPRETATION: GDRs for LSS were established for genes across both nuclear and mitochondrial genomes. Establishing these GDRs will allow accurate variant interpretation, expedite genetic diagnosis of LSS, and facilitate precision medicine, multisystem organ surveillance, recurrence risk counseling, reproductive choice, natural history studies, and determination of eligibility for interventional clinical trials. ANN NEUROL 2023;94:696-712.

Primary mitochondrial diseases: The intertwined pathophysiology of bioenergetic dysregulation, oxidative stress and neuroinflammation.

OBJECTIVES AND SCOPE: Primary mitochondrial diseases (PMDs) are rare genetic disorders resulting from mutations in genes crucial for effective oxidative phosphorylation (OXPHOS) that can affect mitochondrial function. In this review, we examine the bioenergetic alterations and oxidative stress observed in cellular models of primary mitochondrial diseases (PMDs), shedding light on the intricate complexity between mitochondrial dysfunction and cellular pathology. We explore the diverse cellular models utilized to study PMDs, including patient-derived fibroblasts, induced pluripotent stem cells (iPSCs) and cybrids. Moreover, we also emphasize the connection between oxidative stress and neuroinflammation. INSIGHTS: The central nervous system (CNS) is particularly vulnerable to mitochondrial dysfunction due to its dependence on aerobic metabolism and the correct functioning of OXPHOS. Similar to other neurodegenerative diseases affecting the CNS, individuals with PMDs exhibit several neuroinflammatory hallmarks alongside neurodegeneration, a pattern also extensively observed in mouse models of mitochondrial diseases. Based on histopathological analysis of postmortem human brain tissue and findings in mouse models of PMDs, we posit that neuroinflammation is not merely a consequence of neurodegeneration but a potential pathogenic mechanism for disease progression that deserves further investigation. This recognition may pave the way for novel therapeutic strategies for this group of devastating diseases that currently lack effective treatments. SUMMARY: In summary, this review provides a comprehensive overview of bioenergetic alterations and redox imbalance in cellular models of PMDs while underscoring the significance of neuroinflammation as a potential driver in disease progression.

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.

Applying the Airbrakes: Treating Mitochondrial Disease with Hypoxia.

A paper from Jain et al. (2016) using whole-genome CRISPR knockout libraries in human cells and models of mitochondrial disease suggests chronic hypoxia could be an unexpected treatment for disorders of mitochondrial respiration.

Impact of dietary ketosis on volatile anesthesia toxicity in a model of Leigh syndrome.

BACKGROUND: Genetic mitochondrial diseases impact over 1 in 4000 individuals, most often presenting in infancy or early childhood. Seizures are major clinical sequelae in some mitochondrial diseases including Leigh syndrome, the most common pediatric presentation of mitochondrial disease. Dietary ketosis has been used to manage seizures in mitochondrial disease patients. Mitochondrial disease patients often require surgical interventions, leading to anesthetic exposures. Anesthetics have been shown to be toxic in the setting of mitochondrial disease, but the impact of a ketogenic diet on anesthetic toxicities in this setting has not been studied. AIMS: Our aim in this study was to determine whether dietary ketosis impacts volatile anesthetic toxicities in the setting of genetic mitochondrial disease. METHODS: The impact of dietary ketosis on toxicities of volatile anesthetic exposure in mitochondrial disease was studied by exposing young Ndufs4(-/-) mice fed ketogenic or control diet to isoflurane anesthesia. Blood metabolites were measured before and at the end of exposures, and survival and weight were monitored. RESULTS: Compared to a regular diet, the ketogenic diet exacerbated hyperlactatemia resulting from isoflurane exposure (control vs. ketogenic diet in anesthesia mean difference 1.96 mM, Tukey's multiple comparison adjusted p = .0271) and was associated with a significant increase in mortality during and immediately after exposures (27% vs. 87.5% mortality in the control and ketogenic diet groups, respectively, during the exposure period, Fisher's exact test p = .0121). Our data indicate that dietary ketosis and volatile anesthesia interact negatively in the setting of mitochondrial disease. CONCLUSIONS: Our findings suggest that extra caution should be taken in the anesthetic management of mitochondrial disease patients in dietary ketosis.

The Blood-Brain Barrier Is Unaffected in the Ndufs4-/- Mouse Model of Leigh Syndrome.

Mitochondrial dysfunction plays a major role in physiological aging and in many pathological conditions. Yet, no study has explored the consequence of primary mitochondrial deficiency on the blood-brain barrier (BBB) structure and function. Addressing this question has major implications for pharmacological and genetic strategies aimed at ameliorating the neurological symptoms that are often predominant in patients suffering from these conditions. In this study, we examined the permeability of the BBB in the Ndufs4-/- mouse model of Leigh syndrome (LS). Our results indicated that the structural and functional integrity of the BBB was preserved in this severe model of mitochondrial disease. Our findings suggests that pharmacological or gene therapy strategies targeting the central nervous system in this mouse model and possibly other models of mitochondrial dysfunction require the use of specific tools to bypass the BBB. In addition, they raise the need for testing the integrity of the BBB in complementary in vivo models.

Leigh syndrome: an adult presentation of a paediatric disease.

A previously healthy 27-year-old man was admitted to the acute neurology ward with events involving his face, throat and upper limb, which video telemetry later confirmed were refractory focal seizures. He also had progressive pyramidal features, dysarthria and ataxia. MR scans of the brain identified progressive bilateral basal ganglia abnormalities, consistent with Leigh syndrome. However, extensive laboratory and genetic panels did not give a unifying diagnosis. A skeletal muscle biopsy showed no histopathological abnormalities on routine stains. Sequencing of the entire mitochondrial genome in skeletal muscle identified a well-characterised pathogenic variant (m.10191T>C in MT-ND3; NC_012920.1) at 85% heteroplasmy in skeletal muscle. We discuss the clinical and molecular diagnosis of an adult presenting with Leigh syndrome, which is more commonly a paediatric presentation of mitochondrial disease, and how early recognition of a mitochondrial cause is important to support patient care.

Elevated susceptibility to exogenous seizure triggers and impaired interneuron excitability in a mouse model of Leigh syndrome epilepsy.

Mutations in the NADH dehydrogenase (ubiquinone reductase) iron­sulfur protein 4 (NDUFS4) gene, which encodes for a key structural subunit of the OXFOS complex I (CI), lead to the most common form of mitochondrial disease in children known as Leigh syndrome (LS). As in other mitochondrial diseases, epileptic seizures constitute one of the most significant clinical features of LS. These seizures are often very difficult to treat and are a sign of poor disease prognosis. Mice with whole-body Ndufs4 KO are a well-validated model of LS; they exhibit epilepsy and several other clinical features of LS. We have previously shown that mice with Ndufs4 KO in only GABAergic interneurons (Gad2-Ndufs4-KO) reproduce the severe epilepsy phenotype observed in the global KO mice. This observation indicated that these mice represent an excellent model of LS epilepsy isolated from other clinical manifestations of the disease. To further characterize this epilepsy phenotype, we investigated seizure susceptibility to selected exogenous seizure triggers in Gad2-Ndufs4-KO mice. Then, using electrophysiology, imaging, and immunohistochemistry, we studied the cellular, physiological, and neuroanatomical consequences of Ndufs4 KO in GABAergic interneurons. Homozygous KO of Ndufs4 in GABAergic interneurons leads to a prominent susceptibility to exogenous seizure triggers, impaired interneuron excitability and interneuron loss. Finally, we found that the hippocampus and cortex participate in the generation of seizure activity in Gad2-Ndufs4-KO mice. These findings further define the LS epilepsy phenotype and provide important insights into the cellular mechanisms underlying epilepsy in LS and other mitochondrial diseases.

Novel biallelic mutations in TMEM126B cause splicing defects and lead to Leigh-like syndrome with severe complex I deficiency.

Leigh syndrome (LS)/Leigh-like syndrome (LLS) is one of the most common mitochondrial disease subtypes, caused by mutations in either the nuclear or mitochondrial genomes. Here, we identified a novel intronic mutation (c.82-2 A > G) and a novel exonic insertion mutation (c.290dupT) in TMEM126B from a Chinese patient with clinical manifestations of LLS. In silico predictions, minigene splicing assays and patients' RNA analyses determined that the c.82-2 A > G mutation resulted in complete exon 2 skipping, and the c.290dupT mutation provoked partial and complete exon 3 skipping, leading to translational frameshifts and premature termination. Functional analysis revealed the impaired mitochondrial function in patient-derived lymphocytes due to severe complex I content and assembly defect. Altogether, this is the first report of LLS in a patient carrying mutations in TMEM126B. Our data uncovers the functional effect and the molecular mechanism of the pathogenic variants c.82-2 A > G and c.290dupT, which expands the gene mutation spectrum of LLS and clinical spectrum caused by TMEM126B mutations, and thus help to clinical diagnosis of TMEM126B mutation-related mitochondrial diseases.

Leigh Syndrome: A Study of 209 Patients at the Beijing Children's Hospital.

OBJECTIVE: Leigh syndrome (LS) is a heterogeneous neurodegenerative disease and the most frequent pediatric manifestation of mitochondrial disease. In the largest patient collection to date, this study aimed to provide new insights into the clinical and genetic spectrum of LS, defect-specific associations, and predictors of disease course and survival. METHODS: Clinical, metabolic, neuroimaging, onset, and survival data were collected from the medical records of 209 patients referred to the Beijing Children's Hospital with symmetrical basal ganglia and/or brainstem neuroimaging changes indicative of LS by 30 centers from the Chinese network of mitochondrial disease (mitoC-NET) between January 2013 and July 2021 for exploratory analysis. RESULTS: Pathogenic variants were identified in 52 genes, most frequently MT-ATP6, SURF1, and PDHA1. Maternally inherited variants accounted for 42% (heteroplasmy level ≥90% in 64%). Phenotypes spanned 92 Human Phenotype Ontology terms. Elevated serum lactate (144/195), global developmental delay (142/209), and developmental regression (103/209) were most frequent. Discriminating neuroimaging and/or clinical features were identified for MT-ATP6 (m.9176T>C), MT-ND5, PDHA1, SUCLG1, and SURF1. Poorest survival was associated with MT-ND5, MT-ATP6 (m.8993T>C and m.9176T>C), SURF1, and ALDH5A1 (≤50% 3 year's survival), in contrast to milder defects with specific treatment (ECHS1 and SLC19A3, 100% 3 year's survival). INTERPRETATION: Our data define phenotype, onset, and survival of LS in a defect-specific manner, identifying features discriminating between genetic defects and predictive of disease outcome. These findings are essential to early diagnosis, in optimizing family counseling, and to the design and monitoring of future clinical trials, the next frontier of LS research. ANN NEUROL 2022;91:466-482.

Phenotypic Heterogeneity in Patients with Mutations in the Mitochondrial Complex I Assembly Gene NDUFAF5.

BACKGROUND: Rare mutations in NADH:ubiquinone oxidoreductase complex assembly factor 5 (NDUFAF5) are linked to Leigh syndrome. OBJECTIVE: We aimed to describe clinical characteristics and functional findings in a patient cohort with NDUFAF5 mutations. METHODS: Patients with biallelic NDUFAF5 mutations were recruited from multi-centers in Taiwan. Clinical, laboratory, radiological, and follow-up features were recorded and mitochondrial assays were performed in patients' skin fibroblasts. RESULTS: Nine patients from seven unrelated pedigrees were enrolled, eight homozygous for c.836 T > G (p.Met279Arg) in NDUFAF5 and one compound heterozygous for p.Met279Arg. Onset age had a bimodal distribution. The early-onset group (age <3 years) presented with psychomotor delay, seizure, respiratory failure, and hyponatremia. The late-onset group (age ≥5 years) presented with normal development, but slowly progressive dystonia. Combing 25 previously described patients, the p.Met279Arg variant was exclusively identified in Chinese ancestry. Compared with other groups, patients with late-onset homozygous p.Met279Arg were older at onset (P = 0.008), had less developmental delay (P = 0.01), less hyponatremia (P = 0.01), and better prognosis with preserved ambulatory function into early adulthood (P = 0.01). Bilateral basal ganglia necrosis was a common radiological feature, but brainstem and spinal cord involvement was more common with early-onset patients (P = 0.02). A modifier gene analysis showed higher concomitant mutation burden in early-versus late-onset p.Met279Arg homozygous cases (P = 0.04), consistent with more impaired mitochondrial function in fibroblasts from an early-onset case than a late-onset patient. CONCLUSIONS: The p.Met279Arg variant is a common mutation in our population with phenotypic heterogeneity and divergent prognosis based on age at onset. © 2023 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.