S. Shanske

Depletion of muscle mitochondrial DNA in AIDS patients with zidovudine-induced myopathy

Long-term zidovudine therapy in patients with human immunodeficiency virus (HIV) infection can cause a destructive mitochondrial myopathy with histological features of ragged-red fibres (RRF) and proliferation of abnormal mitochondria. In 9 zidovudine-treated patients with this myopathy we found severely reduced amounts (up to 78% reduction vs normal adult controls) of mitochondrial DNA (mtDNA) in muscle biopsy specimens by means of Southern blotting. In 2 HIV-positive patients who had not received zidovudine, muscle mtDNA content did not differ from that in the 4 controls. Depletion of mtDNA seems to be reversible, since 1 patient showed a substantial reduction in RRF and a concomitant pronounced increase in muscle mtDNA content after zidovudine therapy was discontinued. Depletion of muscle mtDNA is probably due to zidovudine-induced inhibition of mtDNA replication by DNA polymerase gamma and is not a secondary effect of HIV infection.

Familial cerebellar ataxia with muscle coenzyme Q10 deficiency.

Objective: To describe a clinical syndrome of cerebellar ataxia associated with muscle coenzyme Q10 (CoQ10) deficiency. Background: Muscle CoQ10 deficiency has been reported only in a few patients with a mitochondrial encephalomyopathy characterized by 1) recurrent myoglobinuria; 2) brain involvement (seizures, ataxia, mental retardation), and 3) ragged-red fibers and lipid storage in the muscle biopsy. Methods: Having found decreased CoQ10 levels in muscle from a patient with unclassified familial cerebellar ataxia, the authors measured CoQ10 in muscle biopsies from other patients in whom cerebellar ataxia could not be attributed to known genetic causes. Results: The authors found muscle CoQ10 deficiency (26 to 35% of normal) in six patients with cerebellar ataxia, pyramidal signs, and seizures. All six patients responded to CoQ10 supplementation; strength increased, ataxia improved, and seizures became less frequent. Conclusions: Primary CoQ10 deficiency is a potentially important cause of familial ataxia and should be considered in the differential diagnosis of this condition because CoQ10 administration seems to improve the clinical picture.

Mitochondrial myopathy of childhood associated with depletion of mitochondrial DNA

We have studied five children with mitochondrial myopathy manifesting within or soon after the first year of life. Muscle biopsies showed ragged-red fibers and decreased respiratory chain activity. All five patients had a severe decrease (2 to 34% of normal) in the amount of muscle mitochondrial DNA (mtDNA). The depletion of mtDNA correlated with absence of mtDNA-encoded translation products and with loss of cytochrome c oxidase enzyme activity in individual muscle fibers. This mitochondrial myopathy of childhood illustrates one phenotypic expression of a novel pathogenetic mechanism in mitochondrial diseases, the specific depletion of mtDNA in affected tissues.

Mitochondrial encephalomyopathy with coenzyme Q10 deficiency

Coenzyme Q10 (CoQ10) transfers electrons from complexes I and II of the mitochondrial respiratory chain to complex 111. There is one published report of human CoQ10 deficiency describing two sisters with encephalopathy, proximal weakness, myoglobinuria, and lactic acidosis. We report a patient who had delayed motor milestones, proximal weakness, premature exertional fatigue, and episodes of exercise-induced pigmenturia. She also developed partial-complex seizures. Serum creatine kinase was approximately four times the upper limit of normal and venous lactate was mildly elevated. Skeletal muscle biopsy revealed many ragged-red fibers, cytochrome c oxidase-deficient fibers, and excess lipid. In isolated muscle mitochondria, impaired oxygen consumption was corrected by the addition of decylubiquinone. During standardized exercise, ventilatory and circulatory responses were compatible with a defect of oxidation-phosphorylation, which was confirmed by near-infrared spectroscopy analysis. Biochemical analysis of muscle extracts revealed decreased activities of complexes I+II and I+III, while CoQ10 concentration was less than 25% of normal. With a brief course of CoQ10 (150 mg daily), the patient reported subjective improvement. The triad of CNS involvement, recurrent myoglobinuria, and ragged-red fibers should alert clinicians to the possibility of CoQ., deficiency.

Maternally inherited hearing loss in a large kindred with a novel T7511C mutation in the mitochondrial DNA tRNASer(UCN) gene

Article abstract Thirty-six of 43 maternally related members of a large African American family experienced hearing loss. A muscle biopsy specimen from the proband showed cytochrome c oxidase (COX)-deficient fibers but no ragged-red fibers; biochemical analysis showed marked reduction of COX activity. A novel T7511C point mutation in the tRNASer(UCN) gene was present in almost homoplasmic levels (>95%) in the blood of 18 of 20 family members, and was also found in lower abundance in the other two. Single-fiber PCR showed that the mutational load was greater in COX-deficient muscle fibers. The tRNASer(UCN) gene may be a “hot spot” for mutations associated with maternally transmitted hearing loss.

Cerebral lactic acidosis correlates with neurological impairment in MELAS

Objective: To evaluate the role of chronic cerebral lactic acidosis in mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS). Methods: The authors studied 91 individuals from 34 families with MELAS and the A3243G point mutation and 15 individuals from two families with myoclonus epilepsy and ragged red fibers (MERRF) and the A8344G mutation. Subjects were divided into four groups. Paternal relatives were studied as controls (Group 1). The maternally related subjects were divided clinically into three groups: asymptomatic (no clinical evidence of neurologic disease) (Group 2), oligosymptomatic (neurologic symptoms but without the full clinical picture of MELAS or MERRF) (Group 3), and symptomatic (fulfilling MELAS or MERRF criteria) (Group 4). The authors performed a standardized neurologic examination, neuropsychological testing, MRS, and leukocyte DNA analysis in all subjects. Results: The symptomatic and oligosymptomatic MELAS subjects had significantly higher ventricular lactate than the other groups. There was a significant correlation between degree of neuropsychological and neurologic impairment and cerebral lactic acidosis as estimated by ventricular MRS lactate levels. Conclusions: High levels of ventricular lactate, the brain spectroscopic signature of MELAS, are associated with more severe neurologic impairment.

Widespread tissue distribution of mitochondrial DNA deletions in Kearns-Sayre syndrome

We performed Southern analysis of mitochondrial DNA (mtDNA) in 6 tissues from a patient with Kearns-Sayre syndrome and found a single deletion of 4.9 kb in all tissues. The percentage of deleted mtDNAs varied widely between tissues, from only 4% in smooth muscle to approximately 50% in skeletal muscle. Samples of DNA obtained from 3 different skeletal muscles and from separate areas of individual tissues showed little variation in percentage of deleted mtDNA. Biochemical analysis showed no clear correlation between mitochondrial enzyme activity and deleted mtDNAs.

Differential features of patients with mutations in two COX assembly genes, SURF-1 and SCO2

We screened 41 patients with undiagnosed encephalomyopathies and cytochrome c oxidase (COX) deficiency for mutations in two COX assembly genes, SURF‐1 and SCO2; 6 patients had mutations in SURF‐1 and 3 had mutations in SCO2. All of the mutations in SURF‐1 were small‐scale rearrangements (deletions/insertions); 3 patients were homozygotes and the other 3 were compound heterozygotes. All patients with SCO2 mutations were compound heterozygotes for nonsense or missense mutations. All of the patients with mutations in SURF‐1 had Leigh syndrome, whereas the 3 patients with SCO2 mutations had a combination of encephalopathy and hypertrophic cardiomyopathy, and the neuropathology did not show the typical features of Leigh syndrome. In patients with SCO2 mutations, onset was earlier and the clinical course and progression to death more rapid than in patients with SURF‐1 mutations. In addition, biochemical and morphological studies showed that the COX deficiency was more severe in patients with SCO2 mutations. Immunohistochemical studies suggested that SURF‐1 mutations result in similarly reduced levels of mitochondrial‐encoded and nuclear‐encoded COX subunits, whereas SCO2 mutations affected mitochondrial‐encoded subunits to a greater degree. We conclude that patients with mutations in SURF‐1 and SCO2 genes have distinct phenotypes despite the common biochemical defect of COX activity. Ann Neurol 2000;47:589–595

A nonsense mutation (G15059A) in the cytochrome b gene in a patient with exercise intolerance and myoglobinuria

We describe a new mitochondrial DNA mutation in the cytochrome b gene in a patient presenting with progressive exercise intolerance and myoglobinuria associated with complex III deficiency in muscle. The point mutation results in the replacement of a glycine at amino acid position 190 with a stop codon. This change predicts premature termination of translation, leading to a truncated protein missing 244 amino acids at the C‐terminus of cytochrome b. The mutation fulfills all the accepted criteria for pathogenicity, suggesting that this is the primary cause of the myopathy in the patient. Ann Neurol 1999;45:127–130

Surprises of genetic engineering A possible model of polyglucosan body disease

Background: The authors previously reported the generation of a knockout mouse model of Pompe disease caused by the inherited deficiency of lysosomal acid alpha-glucosidase (GAA). The disorder in the knockout mice (GAA−/−) resembles the human disease closely, except that the clinical symptoms develop late relative to the lifespan of the animals. In an attempt to accelerate the course of the disease in the knockouts, the authors increased the level of cytoplasmic glycogen by overexpressing glycogen synthase (GSase) or GlutI glucose transporter. Methods: GAA−/− mice were crossed to transgenic mice overexpressing GSase or GlutI in skeletal muscle. Results: Both transgenics on a GAA knockout background (GS/GAA−/− and GlutI/GAA−/−) developed a severe muscle wasting disorder with an early age at onset. This finding, however, is not the major focus of the study. Unexpectedly, the mice bearing the GSase transgene, but not those bearing the GlutI transgene, accumulated structurally abnormal polysaccharide (polyglucosan) similar to that observed in patients with Lafora disease, glycogenosis type IV, and glycogenosis type VII. Ultrastructurally, the periodic acid-Schiff (PAS)-positive polysaccharide inclusions were composed of short, amorphous, irregular branching filaments indistinguishable from classic polyglucosan bodies. The authors show here that increased level of GSase in the presence of normal glycogen branching enzyme (GBE) activity leads to polyglucosan accumulation. The authors have further shown that inactivation of lysosomal acid alpha-glucosidase in the knockout mice does not contribute to the process of polyglucosan formation. Conclusions: An imbalance between GSase and GBE activities is proposed as the mechanism involved in the production of polyglucosan bodies. The authors may have inadvertently created a “muscle polyglucosan disease” by simulating the mechanism for polyglucosan formation.