Coenzyme Q(10) (CoQ(10)) is essential for electron transport in the mitochondrial respiratory chain and antioxidant defense. The relative importance of respiratory chain defects, ROS production, and apoptosis in the pathogenesis of CoQ(10) deficiency is unknown. We determined previously that severe CoQ(10) deficiency in cultured skin fibroblasts harboring COQ2 and PDSS2 mutations produces divergent alterations of bioenergetics and oxidative stress. Here, to better understand the pathogenesis of CoQ(10) deficiency, we have characterized the effects of varying severities of CoQ(10) deficiency on ROS production and mitochondrial bioenergetics in cells harboring genetic defects of CoQ(10) biosynthesis. Levels of CoQ(10) seem to correlate with ROS production; 10-15% and >60% residual CoQ(10) are not associated with significant ROS production, whereas 30-50% residual CoQ(10) is accompanied by increased ROS production and cell death. Our results confirm that varying degrees of CoQ(10) deficiency cause variable defects of ATP synthesis and oxidative stress. These findings may lead to more rational therapeutic strategies for CoQ(10) deficiency.

Background: The number of molecular causes of MELAS (a syndrome consisting of mitochondrial encephalomyopathy, lactic acidosis, and strokelike episodes) and Leigh syndrome (LS) has steadily increased. Among these, mutations in the ND5 gene (OMIM 516005) of mitochondrial DNA are important, and the A13513A change has emerged as a hotspot. Objective: To describe the clinical features, muscle pathological and biochemical characteristics, and molecular study findings of 12 patients harboring the G13513A mutation in the ND5 gene of mitochondrial DNA compared with 14 previously described patients with the same mutation. Design: Clinical examinations and morphological, biochemical, and molecular analyses. Setting: Tertiary care university hospital and molecular diagnostic laboratory. Patients: Three patients had the typical syndrome features of MELAS; the other 9 had typical clinical and radiological features of LS. Results: Family history suggested maternal inheritance in a few cases; morphological studies of muscle samples rarely showed typical ragged-red fibers and more often exhibited strongly succinate dehydrogenasereactive blood vessels. Biochemically, complex I deficiency was inconsistent and generally mild. The mutation load was relatively high in the muscle and blood specimens. Conclusion: The G13513A mutation is a common cause of MELAS and LS, even in the absence of obvious maternal inheritance, pathological findings in muscle, or severe complex I deficiency.

Childhood-onset mitochondrial encephalomyopathies are usually severe, relentlessly progressive conditions that have a fatal outcome. However, a puzzling infantile disorder, long known as ‘benign cytochrome c oxidase deficiency myopathy’ is an exception because it shows spontaneous recovery if infants survive the first months of life. Current investigations cannot distinguish those with a good prognosis from those with terminal disease, making it very difficult to decide when to continue intensive supportive care. Here we define the principal molecular basis of the disorder by identifying a maternally inherited, homoplasmic m.14674T>C mt-tRNAGlu mutation in 17 patients from 12 families. Our results provide functional evidence for the pathogenicity of the mutation and show that tissue-specific mechanisms downstream of tRNAGlu may explain the spontaneous recovery. This study provides the rationale for a simple genetic test to identify infants with mitochondrial myopathy and good prognosis.

Mitochondrial DNA depletion syndrome (MDS) is characterized by a reduction in mtDNA copy number and has been associated with mutations in eight nuclear genes, including enzymes involved in mitochondrial nucleotide metabolism (POLG, TK2, DGUOK, SUCLA2, SUCLG1, PEO1) and MPV17. Recently, mutations in the RRM2B gene, encoding the p53-controlled ribonucleotide reductase subunit, have been described in seven infants from four families, who presented with various combinations of hypotonia, tubulopathy, seizures, respiratory distress, diarrhea, and lactic acidosis. All children died before 4 months of age. We sequenced the RRM2B gene in three unrelated cases with unexplained severe mtDNA depletion. The first patient developed intractable diarrhea, profound weakness, respiratory distress, and died at 3 months. The other two unrelated patients had a much milder phenotype and are still alive at ages 27 and 36 months. All three patients had lactic acidosis and severe depletion of mtDNA in muscle. Muscle histochemistry showed RRF and COX deficiency. Sequencing the RRM2B gene revealed three missense mutations and two single nucleotide deletions in exons 6, 8, and 9, confirming that RRM2B mutations are important causes of MDS and that the clinical phenotype is heterogeneous and not invariably fatal in infancy.

dult polyglucosan body disease (APBD) is characterized after 50 years of age by the onset of progressive pyramidal paraparesis, distal sensory deficits, neurogenic bladder, ambulation loss, and premature death owing to complications of myelopathy and peripheral neuropathy. 1,2 The disease, which is often included in the differential diagnoses of multiple sclerosis and amyotrophic lateral sclerosis, is distinct from multiple sclerosis by lateonset progressive symmetric course and peripheral neuropathy; from amyotrophic lateral sclerosis by sensory deficits, incontinence, and florid subcortical and spinal cord changes on magnetic resonance imaging; and from both by autosomal recessive inheritance. 2-6 The neuropathological hallmarks of APBD are polyglucosan bodies (PBs), which are accumulations of aggregated, poorly branched, and insoluble glycogen both in the central nervous system and in the peripheral nervous system. In neurons, PBs are principally in axons, often appearing to clog the axonal flow. Other features include central nervous system demyelination and gliosis and loss of peripheral nervous system myelinated fibers. 6-12 Adult polyglucosan body disease is allelic to glycogenosis IV (glycogen storage disease IV [GSD-IV]; OMIM 232500). Patients with classic GSD-IV have profound glycogen branching enzyme (GBE) deficiency and die in childhood of liver failure with massive hepatic and extrahepatic polyglu-IMPORTANCE We describe a deep intronic mutation in adult polyglucosan body disease. Similar mechanisms can also explain manifesting heterozygous cases in other inborn metabolic diseases. OBJECTIVE To explain the genetic change consistently associated with manifesting heterozygous patients with adult polyglucosan body disease. DESIGN, SETTING, AND PARTICIPANTS This retrospective study took place from November 8, 2012, to November 7, 2014. We studied 35 typical patients with adult polyglucosan body disease, of whom 16 were heterozygous for the well-known c.986A>C mutation in the glycogen branching enzyme gene (GBE1) but harbored no other known mutation in 16 exons. MAIN OUTCOMES AND MEASURES All 16 manifesting heterozygous patients had lower glycogen branching activity compared with homozygous patients, which showed inactivation of the apparently normal allele. We studied the messenger ribonucleic acid (mRNA) structure and the genetic change due to the elusive second mutation. RESULTS When we reverse transcribed and sequenced the mRNA of GBE1, we found that all manifesting heterozygous patients had the c.986A>C mutant mRNA and complete lack of mRNA encoded by the second allele. We identified a deep intronic mutation in this allele, GBE1-IVS15 + 5289_5297delGTGTGGTGGinsTGTTTTTTACATGACAGGT, which acts as a gene trap, creating an ectopic last exon. The mRNA transcript from this allele missed the exon 16 and 3′UTR and encoded abnormal GBE causing further decrease of enzyme activity from 18% to 8%. CONCLUSIONS AND RELEVANCE We identified the deep intronic mutation, which acts as a gene trap. This second-most commo...