Cerebellar ataxia and severe muscle CoQ10 deficiency in a patient with a novel mutation in ADCK3.

Inherited ataxias are a group of heterogeneous disorders in children or adults but their genetic definition remains still undetermined in almost half of the patients. However, CoQ10 deficiency is a rare cause of cerebellar ataxia and ADCK3 is the most frequent gene associated with this defect. We herein report a 48 year old man, who presented with dysarthria and walking difficulties. Brain magnetic resonance imaging showed a marked cerebellar atrophy. Serum lactate was elevated. Tissues obtained by muscle and skin biopsies were studied for biochemical and genetic characterization. Skeletal muscle biochemistry revealed decreased activities of complexes I+III and II+III and a severe reduction of CoQ10 , while skin fibroblasts showed normal CoQ10 levels. A mild loss of maximal respiration capacity was also found by high-resolution respirometry. Molecular studies identified a novel homozygous deletion (c.504del_CT) in ADCK3, causing a premature stop codon. Western blot analysis revealed marked reduction of ADCK3 protein levels. Treatment with CoQ10 was started and, after 1 year follow-up, patient neurological condition slightly improved. This report suggests the importance of investigating mitochondrial function and, in particular, muscle CoQ10 levels, in patients with adult-onset cerebellar ataxia. Moreover, clinical stabilization by CoQ10 supplementation emphasizes the importance of an early diagnosis.

A mitochondrial tRNA anticodon swap associated with a muscle disease.

We have identified an unusual mitochondrial (mt) tRNA mutation in a seven year-old girl with a pure myopathy. This G to A transition at mtDNA position 15990 changed the anticodon normally found in proline tRNAs (UGG) to the one found in serine tRNAs (UGA), and is the first pathogenic anticodon alteration described in a higher eukaryote. The mutant mtDNA was heteroplasmic (85% mutant) in muscle but was undetectable in white blood cells from the patient and her mother. Analysis of single muscle fibres indicated that mutant mtDNAs severely impaired mitochondrial protein synthesis and respiratory chain activity, but only when present at greater than 90%. The recessive behaviour of this mtDNA alteration may explain the patient's relatively mild clinical phenotype.

Molecular analysis of the muscle pathology associated with mitochondrial DNA deletions.

Large-scale deletions of mitochondrial DNA (mtDNA) are associated with a subgroup of mitochondrial encephalomyopathies. We studied seven patients with Kearns-Sayre syndrome or isolated ocular myopathy who harboured a sub-population of partially-deleted mitochondrial genomes in skeletal muscle. Variable cytochrome c oxidase (COX) deficiencies and reduction of mitochondrially-encoded polypeptides were found in affected muscle fibres, but while many COX-deficient fibres had increased levels of mutant mtDNA, they almost invariably had reduced levels of normal mtDNA. Our results suggest that a specific ratio between mutant and wild-type mitochondrial genomes is the most important determinant of a focal respiratory chain deficiency, even though absolute copy numbers may vary widely.

Fatal infantile cardioencephalomyopathy with COX deficiency and mutations in SCO2, a COX assembly gene.

Mammalian cytochrome c oxidase (COX) catalyses the transfer of reducing equivalents from cytochrome c to molecular oxygen and pumps protons across the inner mitochondrial membrane. Mitochondrial DNA (mtDNA) encodes three COX subunits (I-III) and nuclear DNA (nDNA) encodes ten. In addition, ancillary proteins are required for the correct assembly and function of COX (refs 2, 3, 4, 5, 6). Although pathogenic mutations in mtDNA-encoded COX subunits have been described, no mutations in the nDNA-encoded subunits have been uncovered in any mendelian-inherited COX deficiency disorder. In yeast, two related COX assembly genes, SCO1 and SCO2 (for synthesis of cytochrome c oxidase), enable subunits I and II to be incorporated into the holoprotein. Here we have identified mutations in the human homologue, SCO2, in three unrelated infants with a newly recognized fatal cardioencephalomyopathy and COX deficiency. Immunohistochemical studies implied that the enzymatic deficiency, which was most severe in cardiac and skeletal muscle, was due to the loss of mtDNA-encoded COX subunits. The clinical phenotype caused by mutations in human SCO2 differs from that caused by mutations in SURF1, the only other known COX assembly gene associated with a human disease, Leigh syndrome.

Progress and problems in muscle glycogenoses.

In this selective review, we consider a number of unsolved questions regarding the glycogen storage diseases (GSD). Thus, the pathogenesis of Pompe disease (GSD II) is not simply explained by excessive intralysosomal glycogen storage and may relate to a more general dysfunction of autophagy. It is not clear why debrancher deficiency (GSD III) causes fixed myopathy rather than exercise intolerance, unless this is due to the frequent accompanying neuropathy. The infantile neuromuscular presentation of branching enzyme deficiency (GSD IV) is underdiagnosed and is finally getting the attention it deserves. On the other hand, the late-onset variant of GSD IV (adult polyglucosan body disease APBD) is one of several polyglucosan disorders (including Lafora disease) due to different etiologies. We still do not understand the clinical heterogeneity of McArdle disease (GSD V) or the molecular basis of the rare fatal infantile form. Similarly, the multisystemic infantile presentation of phosphofructokinase deficiency (GSD VII) is a conundrum. We observed an interesting association between phosphoglycerate kinase deficiency (GSD IX) and juvenile Parkinsonism, which is probably causal rather than casual. Also unexplained is the frequent and apparently specific association of phosphoglycerate mutase deficiency (GSD X) and tubular aggregates. By paying more attention to problems than to progress, we aimed to look to the future rather than to the past.

Pathogenesis and treatment of mitochondrial myopathies: recent advances.

In this brief review, I have highlighted recent advances in several areas of mitochondrial medicine, including mtDNA-related diseases, mendelian mitochondrial encephalomyopathies, and therapy. The pathogenic mechanisms of mtDNA mutations, especially those affecting mitochondrial protein synthesis, are still largely unknown. The pathogenicity of homoplasmic mtDNA mutations has become evident but has also called attention to modifying nuclear genes, yet another example of impaired intergenomic signaling. The functional significance of the homoplasmic changes associated with mitochondrial haplogroups has been confirmed. Among the mendelian disorders, a new form of "indirect hit" has been described, in which the ultimate pathogenesis is toxic damage to the respiratory chain. Three therapeutic strategies look promising: (i) allogeneic hematopoietic stem cell transplantation in MNGIE (mitochondrial neurogastrointestinal encephalomyopathy); (ii) bezafibrate, an activator of PGC-1alpha, has proven effective in animal models of mitochondrial myopathy; and (iii) pronucleus transfer into a normal oocyte is effective in eliminating maternal transmission of mtDNA, thus preventing the appearance of mtDNA-related disorders.

Muscle carnitine palmityltransferase deficiency and myoglobinuria.

Muscle carnitine palmityltransferase activity, measured by three different methods, was very low (0 to 20 percent of controls) in a patient with a familial syndrome of recurrent myoglobinuria. Long-chain fatty acyl CoA synthetase activity was normal; acetylcarnitine transferase activity was decreased by 40 percent, and carnitine content was 1.7 times higher than the mean control value. Utilization of palmitate by isolated mitochondria was more impaired than utilization of palmitylcarnitine, suggesting a more severe defect of carnitine palmityltransferase I than transferase II. Thus, myoglobinuria may be due to a genetic defect of lipid metabolism in skeletal muscle.

Human muscle phosphoglycerate mutase deficiency: newly discovered metabolic myopathy.

Muscle phosphoglycerate mutase activity was decreased (5.7 percent of the lowest control value) in a 52-year-old man with intolerance for strenuous exercise and recurrent pigmenturia since adolescence. All of the other enzymes of glycolysis had normal activities, and glycogen concentration was normal. Electrophoretic, heat lability, and mercury inhibition studies showed that the small residual activity in the patient's muscle was represented by the brain (BB) isoenzyme of phosphoglycerate mutase, suggesting a genetic defect of the M subunit which predominates in normal muscle. The prevalence of the BB isoenzyme in other tissues, including muscle culture, may explain why symptoms were confined to muscle.

A direct repeat is a hotspot for large-scale deletion of human mitochondrial DNA.

Kearns-Sayre syndrome (KSS) and progressive external ophthalmoplegia (PEO) are related neuromuscular disorders characterized by ocular myopathy and ophthalmoplegia. Almost all patients with KSS and about half with PEO harbor large deletions in their mitochondrial genomes. The deletions differ in both size and location, except for one, 5 kilobases long, that is found in more than one-third of all patients examined. This common deletion was found to be flanked by a perfect 13-base pair direct repeat in the normal mitochondrial genome. This result suggests that homologous recombination deleting large regions of intervening mitochondrial DNA, which previously had been observed only in lower eukaryotes and plants, operates in mammalian mitochondrial genomes as well, and is at least one cause of the deletions found in these two related mitochondrial myopathies.

Inborn errors of metabolism and motor disturbances in children.

Motor disturbances are very common in paediatric neurology. Often families can be reassured that these are just variants of normal development. However, abnormal movements can also be the hallmark of severe brain dysfunction of different and complex origins. This review concentrates on motor disturbances as frequent and important symptoms of inborn errors of metabolism. A structured diagnostic approach is developed taking into account age-dependent physiological developments and pathophysiological responses of gross and fine motor functions. A series of investigations are presented with the primary aim of early diagnosis of treatable conditions. The correct recognition and differentiation of movement disorders (ataxia, rigid akinetic syndrome (Fparkinsonism_), dystonia, athetosis, tremor,and others), spasticity, and neuromuscular disorders, requires profound neurological expertise. A high level of suspicion and close interaction between paediatric neurologists and specialists in inborn errors of metabolism are indispensable to effectively and timely identify patients in whom motor disturbances are the presenting and/or main symptom of an inborn error.

Further pitfalls in the diagnosis of mtDNA mutations: homoplasmic mt-tRNA mutations.

BACKGROUND: Mitochondrial DNA (mtDNA) mutations are important causes of human genetic disease, with mutations in tRNA genes particularly prevalent. In many patients, mutations are heteroplasmic, affecting a population of mtDNA molecules. Establishing the pathogenicity of homoplasmic mitochondrial tRNA (mt-tRNA) mutations, in which the mutation is present in every mtDNA molecule, is extremely difficult. These mutations must conform to specific pathogenic criteria, documenting unequivocally a functional defect of the mutant mt-tRNA. AIMS: To investigate the pathogenic nature of two homoplasmic mt-tRNA(Thr) deletions, m.15940delT (previously reported as pathogenic) and m.15937delA, by assessing the steady state levels of the mutant mt-tRNA in tissue and cell-line samples from six unrelated families, in which affected individuals were thoroughly investigated for mitochondrial DNA disease on the basis of clinical presentations. Rates of de novo mitochondrial protein synthesis were also examined in control and m.15937delA mutant fibroblasts. RESULTS: Our data strongly suggest that both single nucleotide deletions are neutral polymorphisms; no obvious defects were apparent in either steady state mt-tRNA(Thr) levels or rates of mitochondrial protein synthesis. CONCLUSIONS: These findings have important implications for the investigation of other families with suspected mtDNA disease, in particular the requirement to fulfil strict and established pathogenic criteria in order to avoid misattribution of pathogenicity to mt-tRNA variants.

Placental involvement in glycogen storage disease type IV.

Glycogen storage disease type IV (GSD IV) is a rare autosomal recessive disorder caused by glycogen branching enzyme (GBE) deficiency and resulting in the storage of abnormal glycogen (polyglucosan). Prenatal diagnosis is based on biochemical assay of GBE activity or on mutation analysis, but polyglucosan can also be identified histologically in fetal tissues. We document placental involvement at 25 and 35 weeks of gestation in two cases with genetically confirmed GSD IV. Intracellular inclusions were seen mainly in the extravillous trophoblast. Our findings suggest the possibility of prenatal diagnosis by histological evaluation of placental biopsies.

A novel mutation in NDUFS4 causes Leigh syndrome in an Ashkenazi Jewish family.

Leigh syndrome is a neurodegenerative disorder of infancy or childhood generally due to mutations in nuclear or mitochondrial genes involved in mitochondrial energy metabolism. We performed linkage analysis in an Ashkenazi Jewish (AJ) family without consanguinity with three affected children. Linkage to microsatellite markers D5S1969 and D5S407 led to evaluation of the complex I gene NDUFS4, in which we identified a novel homozygous c.462delA mutation that disrupts the reading frame. The resulting protein lacks a cAMP-dependent protein kinase phosphorylation site required for activation of mitochondrial respiratory chain complex I. In a random sample of 5000 healthy AJ individuals, the carrier frequency of the NDUFS4 mutation c.462delA was 1 in 1000, suggesting that it should be considered in all AJ patients with Leigh syndrome.

Human CoQ10 deficiencies.

Coenzyme Q10 (CoQ10 or ubiquinone) is a lipid-soluble component of virtually all cell membranes and has multiple metabolic functions. A major function of CoQ10 is to transport electrons from complexes I and II to complex III in the respiratory chain which resides in the mitochondrial inner membrane. Deficiencies of CoQ10 (MIM 607426) have been associated with four major clinical phenotypes: 1) encephalomyopathy characterized by a triad of recurrent myoglobinuria, brain involvement, and ragged-red fibers; 2) infantile multisystemic disease typically with prominent nephropathy and encephalopathy; 3) cerebellar ataxia with marked cerebellar atrophy; and 4) pure myopathy. Primary CoQ10 deficiencies due to mutations in ubiquinone biosynthetic genes (COQ2, PDSS1, PDSS2, and ADCK3 [CABC1]) have been identified in patients with the infantile multisystemic and cerebellar ataxic phenotypes. In contrast, secondary CoQ10 deficiencies, due to mutations in genes not directly related to ubiquinone biosynthesis (APTX, ETFDH, and BRAF), have been identified in patients with cerebellar ataxia, pure myopathy, and cardiofaciocutaneous syndrome. In many patients with CoQ10 deficiencies, the causative molecular genetic defects remain unknown; therefore, it is likely that mutations in additional genes will be identified as causes of CoQ10 deficiencies.

Characterization of the enzymatic defect in late-onset muscle phosphofructokinase deficiency. New subtype of glycogen storage disease type VII.

Human phosphofructokinase (PFK) exists in tetrameric isozymic forms, at least in vitro. Muscle and liver contain homotetramers M4 and L4, respectively, whereas red cells contain five isozymes composed of M (muscle) and L (liver) type subunits, i.e., M4, M3L, M2L2, and ML3, and L4. Homozygous deficiency of muscle PFK results in the classic glycogen storage disease type VII characterized by exertional myopathy and hemolytic syndrome beginning in early childhood. The genetic lesion results in a total and partial loss of muscle and red cell PFK, respectively. Characteristically, the residual red cell PFK from the patients consists of isolated L4 isozyme; the M-containing hybrid isozymes are completely absent. In this study, we investigated an 80-yr-old man who presented with a 10-yr history of progressive weakness of the lower limbs as the only symptom. The residual red cell PFK showed the presence of a few M-containing isozymes in addition to the predominant L4 species, indicating that the genetic lesion is a "leaky" mutation of the gene coding for the M subunit. The presence of a small amount of enzyme activity in the muscle may account for the atypical myopathy in this patient.

Muscle phosphofructokinase deficiency. Biochemical and immunological studies of phosphofructokinase isozymes in muscle culture.

Muscle cultures from three unrelated patients with muscle phosphofructokinase (PFK; EC 2.7.1.11) deficiency (Glycogenosis type VII; Tarui disease) had normal PFK activity and normal morphology. Chromatographic and immunological studies showed that normal muscle cultures express all three PFK subunits, M (muscle-type), L (liver-type), and P (platelet-type) and contain multiple homotetrameric and heterotetrameric isozymes. Muscle cultures from patients lack catalytically active M subunit-containing isozymes, but this is compensated for by the presence of P- and L-containing isozymes. Despite the lack of muscle-type PFK activity, presence of immunoreactive M subunit was demonstrable by indirect immunofluorescence, suggesting a mutation of the structural gene coding for the M-subunit of PFK.

Exercise intolerance, lactic acidosis, and abnormal cardiopulmonary regulation in exercise associated with adult skeletal muscle cytochrome c oxidase deficiency.

A 27-yr-old woman with lifelong severe exercise intolerance manifested by muscle fatigue, lactic acidosis, and prominent symptoms of dyspnea and tachycardia induced by trivial exercise was found to have a skeletal muscle respiratory chain defect characterized by low levels of reducible cytochromes a + a3 and b in muscle mitochondria and marked deficiency of cytochrome c oxidase (complex IV) as assessed biochemically and immunologically. Investigation of the pathophysiology of the exercise response in the patient revealed low maximal oxygen uptake (1/3 that of normal sedentary women) in cycle exercise and impaired muscle oxygen extraction as indicated by profoundly low maximal systemic arteriovenous oxygen difference (5.8 ml/dl; controls = 15.4 +/- 1.4, mean +/- SD). The increases in cardiac output and ventilation during exercise, normally closely coupled to muscle metabolic rate, were markedly exaggerated (more than two- to threefold normal) relative to oxygen uptake and carbon dioxide production accounting for prominent tachycardia and dyspnea at low workloads. Symptoms in our patient are similar to those reported in other human skeletal muscle respiratory chain defects involving complexes I and III, and the exaggerated circulatory response resembles that seen during experimental inhibition of the mitochondrial respiratory chain. These results suggest that impaired oxidative phosphorylation in working muscle disrupts the normal regulation of cardiac output and ventilation relative to muscle metabolic rate in exercise.

Two novel pathogenic mitochondrial DNA mutations affecting organelle number and protein synthesis. Is the tRNA(Leu(UUR)) gene an etiologic hot spot?

We identified two patients with pathogenic single nucleotide changes in two different mitochondrial tRNA genes: the first mutation in the tRNA(Asn) gene, and the ninth known mutation in the tRNA(Leu(UUR)) gene. The mutation in tRNA(Asn) was associated with isolated ophthalmoplegia, whereas the mutation in tRNA(Leu(UUR)) caused a neurological syndrome resembling MERRF (myoclonus epilepsy and ragged-red fibers) plus optic neuropathy, retinopathy, and diabetes. Both mutations were heteroplasmic, with higher percentages of mutant mtDNA in affected tissues, and undetectable levels in maternal relatives. Analysis of single muscle fibers indicated that morphological and biochemical alterations appeared only when the proportions of mutant mtDNA exceeded 90% of the total cellular mtDNA pool. The high incidence of mutations in the tRNA(Leu(UUR)) gene suggests that this region is an "etiologic hot spot" in mitochondrial disease.