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Mitochondrial complex I (CI) deficiencies are causing debilitating neurological diseases, among which, the Leber Hereditary Optic Neuropathy and Leigh Syndrome are the most frequent. Here, we describe the first germinal pathogenic mutation in the NDUFA13/GRIM19 gene encoding a CI subunit, in two sisters with early onset hypotonia, dyskinesia and sensorial deficiencies, including a severe optic neuropathy. Biochemical analysis revealed a drastic decrease in CI enzymatic activity in patient muscle biopsies, and reduction of CI-driven respiration in fibroblasts, while the activities of complex II, III and IV were hardly affected. Western blots disclosed that the abundances of NDUFA13 protein, CI holoenzyme and super complexes were drastically reduced in mitochondrial fractions, a situation that was reproduced by silencing NDUFA13 in control cells. Thus, we established here a correlation between the first mutation yet identified in the NDUFA13 gene, which induces CI instability and a severe but slowly evolving clinical presentation affecting the central nervous system., (© The Author 2015. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com.)

Background: Mitochondrial DNA (mtDNA) diseases are rare disorders whose prevalence is estimated around 1 in 5000. Patients are usually tested only for deletions and for common mutations of mtDNA which account for 5-40% of cases, depending on the study. However, the prevalence of rare mtDNA mutations is not known., Methods: We analysed the whole mtDNA in a cohort of 743 patients suspected of manifesting a mitochondrial disease, after excluding deletions and common mutations. Both heteroplasmic and homoplasmic variants were identified using two complementary strategies (Surveyor and MitoChip). Multiple correspondence analyses followed by hierarchical ascendant cluster process were used to explore relationships between clinical spectrum, age at onset and localisation of mutations., Results: 7.4% of deleterious mutations and 22.4% of novel putative mutations were identified. Pathogenic heteroplasmic mutations were more frequent than homoplasmic mutations (4.6% vs 2.8%). Patients carrying deleterious mutations showed symptoms before 16 years of age in 67% of cases. Early onset disease (<1 year) was significantly associated with mutations in protein coding genes (mainly in complex I) while late onset disorders (>16 years) were associated with mutations in tRNA genes. MTND5 and MTND6 genes were identified as 'hotspots' of mutations, with Leigh syndrome accounting for the large majority of associated phenotypes., Conclusions: Rare mitochondrial DNA mutations probably account for more than 7.4% of patients with respiratory chain deficiency. This study shows that a comprehensive analysis of mtDNA is essential, and should include young children, for an accurate diagnosis that is now accessible with the development of next generation sequencing technology.

Diversity of respiratory chain spectrophotometric assays may lead to difficult comparison of results between centers. The French network of mitochondrial diseases diagnostic centers undertook comparison of the results obtained with different protocols in the French diagnostic centers. The diversity of protocols was shown to have striking consequences, which prompted the network to undertake standardization and optimization of the protocols with respect to clinical diagnosis, i.e. high velocity while maintaining linear kinetics relative to time and enzyme concentration. Assays were set up on animal tissues and verified on control human muscle and fibroblasts. Influence of homogenization buffer and narrow range of optimal concentration of phosphate, substrate and tissue were shown. Experimental data and proposed protocols have been posted on a free access website. Their subsequent use in several diagnostic centers has improved consistency for all assays.

Villous atrophy in an infant immediately suggests food intolerance. We report a case with an unusual cause. This female infant was first examined at 5 months for anorexia and failure to thrive. Intestinal biopsy showed total villous atrophy. A diet excluding gluten and cow milk proteins failed to improve her condition and the infant was hospitalized at 7 months for further investigations. The infant was hypotonic with a head lag. No other clinical sign was noted. Serum transaminases were increased 5- to 10-fold and CSF proteins concentration was increased. Metabolic investigations revealed hyperlactacidaemia and an increased lactate/pyruvate ratio during fasting and feeding, suggesting a mitochondrial cytopathy. Respiratory chain enzymatic activity measurements confirmed the diagnosis and showed severely decreased activities of complexes I, III and IV in both the liver and muscle. Molecular analysis demonstrated depletion of mitochondrial DNA in the liver (75%) and in muscle (97%). The infant was discharged under continuous enteral nutrition. Improvement was of short duration and the infant died at 1 year of age of massive hepatic failure. This is the first report of a mitochondrial DNA depletion with total villous atrophy and malabsorption as early clinical onset. A mitochondrial cytopathy should be considered in such conditions when food exclusion diets fail.

Background: Many mitochondrial pathologies are quantitative disorders related to tissue-specific deletion, depletion, or overreplication of mitochondrial DNA (mtDNA). We developed an assay for the determination of mtDNA copy number by real-time quantitative PCR for the molecular diagnosis of such alterations., Methods: To determine altered mtDNA copy number in muscle from nine patients with single or multiple mtDNA deletions, we generated calibration curves from serial dilutions of cloned mtDNA probes specific to four different mitochondrial genes encoding either ribosomal (16S) or messenger (ND2, ND5, and ATPase6) RNAs, localized in different regions of the mtDNA sequence. This method was compared with quantification of radioactive signals from Southern-blot analysis. We also determined the mitochondrial-to-nuclear DNA ratio in muscle, liver, and cultured fibroblasts from a patient with mtDNA depletion and in liver from two patients with mtDNA overreplication., Results: Both methods quantified 5-76% of deleted mtDNA in muscle, 59-97% of mtDNA depletion in the tissues, and 1.7- to 4.1-fold mtDNA overreplication in liver. The data obtained were concordant, with a linear correlation coefficient (r(2)) between the two methods of 0.94, and indicated that quantitative PCR has a higher sensitivity than Southern-blot analysis., Conclusions: Real-time quantitative PCR can determine the copy number of either deleted or full-length mtDNA in patients with mitochondrial diseases and has advantages over classic Southern-blot analysis.

Mitochondrial DNA depletion is a well established cause of severe liver failure in infancy. The autosomal inheritance of this quantitative mitochondrial DNA defect supports the involvement of a nuclear gene in the control of mitochondrial DNA level. We previously described a case of a 28-month-old child presenting with a progressive liver fibrosis due to a mitochondrial DNA depletion (85% at 12 months of age). As this syndrome was clinically liver-restricted, a liver transplant was initially discussed. We report the clinical, biochemical and molecular follow-up of this child, now 6 years old. The patient displayed a spontaneous gradual improvement of his liver function with continuous increment of clotting factor values since 32 months of age. A marked reduction of the previous extensive fibrosis was evidenced on a liver biopsy performed at 46 months of age associated with a dramatic decrease of the mitochondrial DNA depletion (35%). Consequently, an almost complete restoration of respiratory chain activities containing mitochondrial DNA-encoded subunits was observed. This is the first report of a revertant phenotype in liver mitochondrial DNA depletion syndrome.