Molecular and functional effects of the T14709C point mutation in the mitochondrial DNA of a patient with maternally inherited diabetes and deafness.

A heteroplasmic T to C transition at nucleotide position 14709 in the mitochondrial tRNA glutamic acid (tRNA(Glu)) gene has previously been associated with maternally inherited diabetes and deafness (MIDD). To investigate the pathogenic mechanism of the T14709C mutation, we have constructed transmitochondrial cell lines by transferring fibroblasts mitochondria from a patient with the mutation into human cells lacking mitochondrial DNA (mtDNA) (rho degrees cells). Clonal cybrid cell lines were obtained containing various levels of the heteroplasmic mutation, or exclusively mutated or wild-type mtDNA. Measurement of respiratory chain enzymatic activities failed to detect a difference between the homoplasmic mutant and homoplasmic wild-type cybrid cell lines. However, a subtle decrease in the steady-state levels of tRNA(Glu) transcripts in some mutant clones. Our studies suggest that the T14709C mutation is insufficient to lead impairment of mitochondrial function in homoplasmic osteosarcoma cybrid clones, and that we cannot exclude that the T14709C mutation affects mitochondrial function by a yet unidentified mechanism.

Induction of ANT2 gene expression in liver of patients with mitochondrial DNA depletion.

We have previously described two cases of children with a liver mitochondrial DNA (mtDNA) depletion syndrome, characterised by a low ratio of mtDNA to nuclear DNA. Light microscopy performed on liver biopsy showed abnormal hepatocytes with a characteristic 'oncocytic' appearance, indicative of perturbed oxidative phosphorylation. The adenine nucleotide translocator (ANT), the last step in oxidative phosphorylation catalyses the exchange of adenosine diphosphate (ADP) to adenosine triphosphate (ATP) between the cytosol and mitochondria. The ANT2 gene, which is not normally expressed in human tissues, encodes an isoform preferentially expressed under conditions of glycolytic metabolism. ANT2 gene expression is regulated by a transcription factor involved in a molecular mechanism selecting for the import of glycolytic ATP into the mitochondrial matrix. This ATP import is required in highly proliferative cells, such as tumour cells, which are highly dependent on glycolysis for ATP synthesis. We postulated that, as a result of the defective oxidative phosphorylation observed in these patient biopsies, the ANT2 expression would be induced. We simultaneously quantified the mtDNA depletion and the ANT2 gene expression in liver biopsies from the two patients and six controls. ANT2 mRNA levels were significantly increased in the two patient liver biopsies. Moreover, in one patient, the liver mtDNA depletion was found to be partially reversed after less than 4 years and this reversion was coupled to a concomitant decrease of the ANT2 expression. These results suggest that dysfunction of oxidative phosphorylation could lead to a switch from mitochondrial to glycolytic ATP production, to restore tissue-specific energy requirements.

[Mitochondrial encephalomyopathy, lactic acidosis and stroke-like episodes (Melas) associated with a Fahr disease and cerebellar calcifications]. / Encéphalomyopathie mitochondriale de type Melas associée à un syndrome de Fahr avec calcifications cérébelleuses.

INTRODUCTION: Melas syndrome is a mitochondrial disease which corresponds to the association of mitochondrial encephalopathy, lactic acidosis and stroke-like espisodes. CASE REPORT: The authors report the case of a 39 year-old woman presenting with hearing loss, seizures, visual field deficit, three stroke-like episodes and calcifications of the basal ganglia and cerebellar dentate nuclei. Melas syndrome was suspected and confirmed by muscle biopsy, showing ragged red fibers and the presence of an A3243G mutation of mitochondrial DNA. CONCLUSION: This clinical, pathological and radiological observation shows that intracerebral calcifications may involve the dentate nuclei of the cerebellum in the Melas syndrome.

[Mitochondrial cytopathy: an unusual infantile cause of total villous atrophy]. / Cytopathie mitochondriale: une cause inhabituelle d'atrophie villositaire totale chez le nourrisson.

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.

Development and implementation of standardized respiratory chain spectrophotometric assays for clinical diagnosis.

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.