Serenella Servidei

Mitochondrial DNA Deletions in Progressive External Ophthalmoplegia and Kearns-Sayre Syndrome

We investigated the correlations of deletions of mitochondrial DNA in skeletal muscle with clinical manifestations of mitochondrial myopathies, a group of disorders defined either by biochemical abnormalities of mitochondria or by morphologic changes causing a ragged red appearance of the muscle fibers histochemically. We performed genomic Southern blot analysis of muscle mitochondrial DNA from 123 patients with different mitochondrial myopathies or encephalomyopathies. Deletions were found in the mitochondrial DNA of 32 patients, all of whom had progressive external ophthalmoplegia. Some patients had only ocular myopathy, whereas others had Kearns-Sayre syndrome, a multisystem disorder characterized by ophthalmoplegia, pigmentary retinopathy, heart block, and cerebellar ataxia. The deletions ranged in size from 1.3 to 7.6 kilobases and were mapped to different sites in the mitochondrial DNA, but an identical 4.9-kilobase deletion was found in the same location in 11 patients. Biochemical analysis showed decreased activities of NADH dehydrogenase, rotenone-sensitive NADH-cytochrome c reductase, succinate-cytochrome c reductase, and cytochrome c oxidase, four enzymes of the mitochondrial respiratory chain containing subunits encoded by mitochondrial DNA. We conclude that deletions of muscle mitochondrial DNA are associated with ophthalmoplegia and may result in impaired mitochondrial function. However, the precise relation between clinical and biochemical phenotypes and deletions remains to be defined.

Coenzyme Q10 reverses pathological phenotype and reduces apoptosis in familial CoQ10 deficiency

Two brothers with myopathic coenzyme Q10 (CoQ10) deficiency responded dramatically to CoQ10 supplementation. Muscle biopsies before therapy showed ragged-red fibers, lipid storage, and complex I + III and II + III deficiency. Approximately 30% of myofibers had multiple features of apoptosis. After 8 months of treatment, excessive lipid storage resolved, CoQ10 level normalized, mitochondrial enzymes increased, and proportion of fibers with TUNEL-positive nuclei decreased to 10%. The authors conclude that muscle CoQ10 deficiency can be corrected by supplementation of CoQ10, which appears to stimulate mitochondrial proliferation and to prevent apoptosis.

Dominantly inherited mitochondrial myopathy with multiple deletions of mitochondrial DNA: clinical, morphologic, and biochemical studies.

We studied a large family with a dominantly inherited mitochondrial myopathy characterized by progressive external ophthalmoplegia, dysphagia, cataract, lactic acidosis, exercise intolerance, and early death. Morphologic studies of muscle biopsies suggested mitochondrial heteroplasmy and revealed ragged-red fibers and decreased histochemical reactions for cytochrome c oxidase and succinate dehydrogenase. Biochemistry showed a partial defect of cytochrome c oxidase and a mild generalized reduction of other mitochondrial enzymes requiring mitochondrial DNA-encoded subunits. Southern blot analysis and PCR amplification showed mitochondrial DNA deletions in muscle of all affected members, but not in lymphocytes or fibroblasts, suggesting a tissue-specific distribution. Deletions were multiple and seemed to increase with time and to correlate with the severity of the disease.

Benign reversible muscle cytochrome c oxidase deficiency A second case

A 6-week-old boy had generalized weakness, requiring assisted ventilation, and lactic acidosis. At 6 months, the lactic acidosis resolved, and the patient started to improve; assisted ventilation was discontinued at 15 months. Muscle biopsies at 4 and 11 months showed accumulation of mitochondria, lipid, and glycogen; cytochrome c oxidase (COX) activity was 11% of the lowest control in the first biopsy and 57% in the second. Immunocytochemistry and immunotitration showed presence of immunologically reactive enzyme protein in both biopsies. This case confirms a previous report of benign infantile myopathy due to reversible COX deficiency. The severe fibrosis in the second biopsy may explain the slower rate of clinical recovery in this child.

Cytochrome oxidase deficiency: clinical and biochemical heterogeneity.

In December 1984, the Biochemical Society organized a colloquium on mitochondrial myopathies, and we reviewed the already then apparent clinical and biochemical heterogeneity of cytochrome c oxidase (COX) deficiency.’ During the past 15 months many new patients with COX deficiency have been reported, immunological and immunocytochernical studies have been performed on some of them, and molecular genetic investigations are under way. The new information confirms and extends the concept that COX deficiency causes diverse phenotypes. This heterogeneity is hardly surprising when one considers the complexity of the enzyme. In mammalian tissues, including human heart (FIG. l), thirteen different subunits can be demonstrated.2 The three larger subunits (1-111) are encoded by mitochondrial DNA (mtDNA), while the other 10 polypeptides are encoded by nuclear DNA, synthesized on cytoplasmic ribosomes, and transported into the mitochondria where the complete enzyme is assembled as an integral component of the inner mitochondrial membrane. There is good evidence that the essential properties of the enzyme (electron transport and proton translocation) are related to the mtDNAencoded subunits that contain the four redox centers (heme a and a3 and two copper ions) and the proton ~ h a n n e l . ~ The functions of the nuclear-encoded subunits have not been fully elucidated, but a regulatory role has been proposed by Kadenbach and his co-workers, who have recently shown that most if not all of these subunits occur in tissue-specific isoforms.2

Cardiomyopathy, mental retardation, and autophagic vacuolar myopathy

Two brothers had nonobstructive hypertrophic cardiomyopathy, mental retardation, and vacuolar myopathy, and their mother died of cardiopathy at age 31. Seven families with this syndrome have been described; heredity appears to be X-linked dominant or autosomal dominant, with different expressivity in males and females. The biochemical cause of this lysosomal storage disease is unknown.

Late onset lipid storage myopathy due to multiple acyl CoA dehydrogenase deficiency triggered by valproate

Late onset Multiple Acyl CoA dehydrogenase (MAD) deficiency myopathy is a rare disorder. Only five cases have been reported. We report one case with MAD deficiency in which the clinical features appeared during valproic acid therapy. A 47-yr-old man taking valproic acid for 4 months presented nocturnal calf cramps, exercise intolerance, difficulty in climbing stairs and shortness of breath. Muscle biopsy revealed ragged red fibres and neutral lipid storage. Electron microscopy showed enlarged abnormal mitochondria with abnormal internal structure. The total and free muscle carnitine was decreased. The activities of all short, medium and long chain acyl CoA dehydrogenases were 40% of the normal. In this case a partial defect of MAD is noted, possibly triggered by valproic acid, causing the clinical manifestations of the pre-existing myopathy. After discontinuation of the drug a clinical improvement was observed while therapy with riboflavin resulted in a total relief of the symptoms.

Apoptosis and Oxidative Stress in Mitochondrial Disorders

Mitochondrial encephalomyopathies are clinically heterogeneous disorders under the genetic control of mitochondrial or nuclear DNAs. Since 1988 many qualitative (about one hundred point mutations of tRNA or structural genes and various mtDNA rearrangements) and quantitative (depletion) defects of mtDNA have been associated with an increasing number of inherited or sporadic mitochondrial encephalomyopathies [1, 2]. In sharp contrast with the rapid progress of our knowledge on the molecular genetic defects, the pathogenic mechanisms that lead to the wide biochemical and clinical variability are still far from been elucitated.