Abnormal gut motility is a feature of several mitochondrial encephalomyopathies, and mutations in genes such as TYMP and POLG, have been linked to these rare diseases. The human genome encodes three DNA ligases, of which only one, ligase III (LIG3), has a mitochondrial splice variant and is crucial for mitochondrial health. We investigated the effect of reduced LIG3 activity and resulting mitochondrial dysfunction in seven patients from three independent families, who showed the common occurrence of gut dysmotility and neurological manifestations reminiscent of mitochondrial neurogastrointestinal encephalomyopathy. DNA from these patients was subjected to whole exome sequencing. In all patients, compound heterozygous variants in a new disease gene, LIG3, were identified. All variants were predicted to have a damaging effect on the protein. The LIG3 gene encodes the only mitochondrial DNA (mtDNA) ligase and therefore plays a pivotal role in mtDNA repair and replication. In vitro assays in patient-derived cells showed a decrease in LIG3 protein levels and ligase activity. We demonstrated that the LIG3 gene defects affect mtDNA maintenance, leading to mtDNA depletion without the accumulation of multiple deletions as observed in other mitochondrial disorders. This mitochondrial dysfunction is likely to cause the phenotypes observed in these patients. The most prominent and consistent clinical signs were severe gut dysmotility and neurological abnormalities, including leukoencephalopathy, epilepsy, migraine, stroke-like episodes, and neurogenic bladder. A decrease in the number of myenteric neurons, and increased fibrosis and elastin levels were the most prominent changes in the gut. Cytochrome c oxidase (COX) deficient fibres in skeletal muscle were also observed. Disruption of lig3 in zebrafish reproduced the brain alterations and impaired gut transit in vivo. In conclusion, we identified variants in the LIG3 gene that result in a mitochondrial disease characterized by predominant gut dysmotility, encephalopathy, and neuromuscular abnormalities. Bonora et al. identify a new mitochondrial recessive disorder caused by biallelic variants in the LIG3 gene encoding DNA ligase III, which is responsible for mitochondrial DNA repair. Clinical signs include gut dysmotility and neurological features such as leucoencephalopathy, epilepsy and stroke-like episodes.

Short-chain enoyl-CoA hydratase (ECHS1) is involved in amino acid and fatty acid catabolism in mitochondria and its deficiency causes Leigh syndrome or exercise-induced dystonia. More than 60 patients with this condition have been reported till date. The accumulation of intermediate metabolites of valine is assumed to be responsible for the cytotoxicity. Since protein restriction, including valine reportedly improves neurological symptoms, it is essential to consider the possible incidence of and diagnose ECHS1 syndrome in the earlier stages. This study reported the liquid chromatography with tandem mass spectrometry (LC-MS/MS) urine and plasma metabolite analysis in six cases, including four new cases with ECHS1 deficiency. The values of urine cysteine/cysteamine conjugates from valine metabolites, S-(2-carboxypropyl) cysteine/cysteamine from methacrylyl-CoA, and S-(2-carboxyethyl) cysteine/cysteamine from acryloyl-CoA were separated between six patients and six normal controls. The LC-MS/MS analysis revealed that these metabolites can be used for the early diagnosis and evaluation of diet therapy.

Leigh syndrome (LS) is a heterogeneous neurodegenerative disorder caused by mitochondrial dysfunction. Certain LS cases have mutations in ECHS1 , which encodes a short-chain enoyl-CoA hydratase involved in the metabolism of fatty acids and branched-chain amino acids in mitochondria. Using exome sequencing, we diagnosed a Japanese patient with LS and identified the patient as a compound heterozygote for a novel variant of ECHS1 , consisting of NM_004092.4:c.23T>C (p.Leu8Pro) and NM_004092.4:c.176A>G (p.Asn59Ser).

Objective: Cytochrome c oxidase (COX) deficiency is a major mitochondrial respiratory chain defect that has vast genetic and phenotypic heterogeneity. This study aims to identify novel causative genes of COX deficiency with only striated muscle-specific symptoms. Methods: Whole exome sequencing was performed in 2 unrelated individuals who were diagnosed with congenital myopathy and presented COX deficiency in muscle pathology. We assessed the COX6A2 variants using measurements of enzymatic activities and assembly of mitochondrial respiratory chain complexes in the samples from the patients and knockout mice. Results: Both patients presented muscle weakness and hypotonia in 4 limbs along with facial muscle weakness. One patient had cardiomyopathy. Neither patient exhibited involvement from other organs. Whole exome sequencing identified biallelic missense variants in COX6A2, which is expressed only in the skeletal muscle and heart. The variants detected were homozygous c.117C > A (p.Ser39Arg) and compound heterozygous c.117C > A (p.Ser39Arg) and c.127T > C (p.Cys43Arg). We found specific reductions in complex IV activities in the skeletal muscle of both individuals. Assembly of complex IV and its supercomplex formation were impaired in the muscle. Interpretation: This study indicates that biallelic variants in COX6A2 cause a striated muscle-specific form of COX deficiency.

KMT2B-related dystonia (DYT-KMT2B) is a newly identified earlyonset generalized dystonia. 1 KMT2B encodes a lysine-specific histone methyltransferase 2B, which has a role in epigenetic modification of other genes, including those associated with dystonia (eg, THAP1 and TOR1A). DYT-KMT2B cases can exhibit characteristic facial features, microcephaly, short stature, and intellectual disability.The dystonia typically begins in the lower limbs and spreads to become generalized over a number of years, often with craniocervical involvement. Although medical therapies were reported not to be clinically beneficial, deep brain stimulation of the internal segment of globus pallidus (GPi-DBS) improved dystonia in DYT-KMT2B patients. 1 Herein, we present a case of a 10-year-old boy with DYT-KMT2B manifesting disabling asterixis, which markedly improved following bilateral GPi-DBS.

Objective To elucidate the genetic background and genotype‐phenotype correlations for epilepsy with myoclonic‐atonic seizures, also known as myoclonic‐astatic epilepsy (MAE) or Doose syndrome. Methods We collected clinical information and blood samples from 29 patients with MAE. We performed whole‐exome sequencing for all except one MAE case in whom custom capture sequencing identified a variant. Results We newly identified four variants: SLC6A1 and HNRNPU missense variants and microdeletions at 2q24.2 involving SCN1A and Xp22.31 involving STS. Febrile seizures preceded epileptic or afebrile seizures in four patients, of which two patients had gene variants. Myoclonic‐atonic seizures occurred at onset in four patients, of which two had variants, and during the course of disease in three patients. Variants were more commonly identified in patients with a developmental delay or intellectual disability (DD/ID), but genetic status was not associated with the severity of DD/ID. Attention‐deficit/hyperactivity disorder and autistic spectrum disorder were less frequently observed in patients with variants than in those with unknown etiology. Significance MAE patients had genetic heterogeneity, and HNRNPU and STS emerged as possible candidate causative genes. Febrile seizures prior to epileptic seizures and myoclonic‐atonic seizure at onset indicate a genetic predisposition to MAE. Comorbid conditions were not related to genetic predisposition to MAE.

No abstract

Genetic abnormalities in mitochondrial complex assembling factors are associated with leukoencephalopathy. We present a 1-year-old girl with consciousness disturbance after a respiratory infection. Brain MRI revealed leukoencephalopathy with bilaterally symmetrical hyperintensity in the substantia nigra, medial thalamic nuclei, and basal nuclei, as well as cavities in the cerebral white matter and corpus callosum. Lactate levels in the spinal fluid were high, while magnetic resonance spectroscopy of the cerebral white matter and basal nuclei showed high peak lactate levels, suggesting mitochondrial dysfunction. The respiratory enzyme activity of complex I was reduced to 17% to 21% in skeletal muscle. Whole exome sequencing identified compound heterozygous variations in NDUFAF3, involved in the assembly of mitochondrial complex I (c.342_343insGTG:p.117Valdup, c.505C > A:p.Pro169Thr). Two-dimensional, blue-native polyacrylamide gel electrophoresis (PAGE) and sodium dodecyl sulfate-PAGE revealed reductions in Q-module (NDUFS2, NDUFS3, and NDUFA9) and P-module (NDUFB10 and NDUFB11) subunits, indicating disruption of mitochondrial complex I assembly. Our report expands the spectrum of clinical phenotypes associated with pathogenic variants of NDUFAF3.