Maternally inherited myopathy and cardiomyopathy: association with mutation in mitochondrial DNA tRNA(Leu)(UUR)

Different point mutations of the mitochondrial genome, which all affect the ability of mitochondria to translate their own genes and lead to partial defects of mtDNA-dependent respiratory complexes, are related to distinct clinical mitochondrial disorders. A new maternally inherited disorder, characterised by a combination of adult-onset myopathy and cardiomyopathy, with no clinical involvement of the nervous system, was found in members of a single large pedigree. A heteroplasmic new mutation was identified in the mtDNA gene specifying tRNA(Leu)(UUR). This mutation segregated specifically with the disorder, and there were significant correlations between the proportion of the mtDNA that was of the mutant form and the activities (normalised for citrate synthase activity) of the two mtDNA-dependent respiratory enzymes (complex I, r = -0.71, p less than 0.005: complex IV r = -0.77, p less than 0.005) and the maximum oxygen consumption (r = -0.82, p less than 0.005), a physiological index of aerobic metabolism. These findings strongly suggest that the tRNA(Leu)(UUR) mutation is the genetic cause of this disorder, and that lesions of mtDNA should be considered in the differential diagnosis of the hereditary cardiomyopathies.

Rapid detection of the A----G(8344) mutation of mtDNA in Italian families with myoclonus epilepsy and ragged-red fibers (MERRF)

We devised a rapid PCR-based method to screen for an A----G transition at nucleotide 8344 of the human mitochondrial tRNA(Lys) gene, which was recently reported, by Shoffner and co-workers, to be associated with myoclonus epilepsy and ragged-red fibers (MERRF), a maternally transmitted mitochondrial encephalomyopathy (Shoffner et al. 1990). We confirmed this association in five of seven Italian MERRF pedigrees. The mutation was specific for the MERRF trait, because it was never found in mtDNA of non-MERRF individuals, including 14 normal and 110 diseased controls. Our study corroborates the idea that the A----G(8344) mutation is the most frequent and widespread genetic cause of MERRF.

Neurological disorders due to mutations of the mitochondrial genome

The rapidly expanding list of human diseases due to lesions of mitochondrial DNA includes myopathies, encephalopathies, cardiomyopathies, or various combinations of the latter, leading to multisystem disorders, which can also affect visceral organs. Five maternally inherited diseases, mainly affecting muscle and brain, are due to point mutations of mitochondrial genes encoding either respiratory chain polypeptides or transfer RNAs. On the other hand, three sporadic entities, Chronic Progressive External Ophthalmoplegia, Kearns-Sayre syndrome, and Pearson's pancreas-bone marrow syndrome, are due to single large-scale deletions of mitochondrial DNA. In addition, multiple deletions are the molecular hallmark of familial encephalomyopathies, inherited as either autosomal dominant or autosomal recessive traits. Finally, tissue-specific depletion of mitochondrial DNA was found in an autosomal recessive disease affecting either muscle, liver, kidney, or a combination of the three. Point mutations and slipped mispairing during, or impairment of, mitochondrial replication are likely mechanisms involved in the pathogenesis of these lesions.

Nucleus-driven multiple large-scale deletions of the human mitochondrial genome: a new autosomal dominant disease

We studied several affected and one nonaffected individuals belonging to three unrelated pedigrees. The pathological trait was an autosomal dominant mitochondrial myopathy due to large-scale multiple deletions of the mitochondrial genome. Clinically, symptomatic patients had progressive external ophthalmoplegia, muscle weakness and wasting, sensorineural hypoacusia, and, in some cases, vestibular areflexia and tremor. The muscle biopsies of all patients examined showed ragged-red fibers, neurogenic changes, and a partially decreased histochemical reaction to cytochrome c oxidase. Multiple mtDNA heteroplasmy was detected in the patients by both Southern blot analysis and PCR amplification, whereas the unaffected individual had the normal homoplasmic hybridization pattern. These findings confirm and add further details to the existence of a new human disease--defined clinically as a mitochondrial myopathy, genetically as a Mendelian autosomal dominant trait, and molecularly by the accumulation of multiple, large-scale deletions of the mitochondrial genome--that is due to impaired nuclear control during mtDNA replication.

Purification, characterization and partial amino acid sequences of carnitine palmitoyl-transferase from human liver

Carnitine palmitoyl-transferase has been extracted with 0.5% Tween-20 from human liver homogenate and purified to homogeneity. The purified enzyme has a native Mr of 274 kDa. The subunit Mr is of 66 kDa, as shown by SDS-PAGE and immunoblots obtained with antibodies raised against human CPT. Purified CPT shows high affinity for palmitoyl-CoA and palmitoyl-carnitine and is not inhibited by malonyl-CoA. Seven tryptic peptides and the N-terminal of purified human CPT have been sequenced, and found homologous to rat CPT sequence. Both antibodies and peptide sequences are important tools for the investigation of the molecular basis of CPT deficiency in man.

Tissue distribution and transmission of mitochondrial DNA deletions in mitochondrial myopathies

By using a combination of Southern blot hybridization analysis, polymerase-chain reaction amplification, and direct nucleotide sequencing, we studied deletions of mitochondrial DNA (mtDNA) in several nonfamilial patients with progressive external ophthalmoplegia and Kearns-Sayre syndrome, and in some of their direct relatives. Results suggest that the heteroplasmic mtDNA populations are already present at a very early stage of development, and that there is no direct transmission of mtDNA heteroplasmy by maternal inheritance.

Fumarase deficiency is an autosomal recessive encephalopathy affecting both the mitochondrial and the cytosolic enzymes

A 7-month-old boy died in a demented state after a clinical history characterized by generalized seizures, psychomotor deterioration, and fumaric aciduria. We found a marked deficiency of both mitochondrial and cytosolic fumarases in skeletal muscle, brain, cerebellum, heart, kidney, liver, and cultured fibroblasts. Fumarase activities were 30 to 50% compared with controls in both mitochondria and cytosol from cultured fibroblasts of the parents. Antifumarase cross-reacting material was present in negligible amounts in the patient's tissues. Our data indicate that this disease is an autosomal recessive encephalopathy, due to a single mutation affecting the gene encoding both forms of the enzyme.

An autosomal dominant disorder with multiple deletions of mitochondrial DNA starting at the D-loop region

Deletions of muscle mitochondrial DNA (mtDNA) have recently been found in patients with mitochondrial myopathy. However, as most of the described cases were sporadic, and individual deletions involved different portions of mtDNA, the mechanism(s) producing the molecular lesions, as well as their mode of transmission, remain unclear. By studying families with mtDNA heteroplasmy, valuable information can be obtained about the role of inheritable factors in the pathogenesis of these disorders. We have studied four members of a family with autosomal dominant mitochondrial myopathy. Multiple deletions, involving the same portion of muscle mtDNA, were identified in all patients. Sequence analysis of the mutant mtDNAs, performed after DNA amplification by the polymerase-chain reaction showed that all the deletions start within a 12-nucleotide stretch at the 5' end of the D-loop region, a site of active communication between the nucleus and the mtDNA. The data indicate that a mutation of a nuclear-coded protein can destroy the integrity of the mitochondrial genome in a specific, heritable way.

Normalization of short-chain acylcoenzyme A dehydrogenase after riboflavin treatment in a girl with multiple acylcoenzyme A dehydrogenase-deficient myopathy

A 12-year-old girl was shown to have carnitine-deficient lipid storage myopathy and organic aciduria compatible with multiple acylcoenzyme A (acyl-CoA) dehydrogenase deficiency. In muscle mitochondria, activities of both short-chain acyl-CoA dehydrogenase (SCAD) and medium-chain acyl-CoA dehydrogenase (MCAD) were 35% of normal. Antibodies against purified SCAD, MCAD, and electron-transfer flavoprotein were used for detection of cross-reacting material (CRM) in the patient's mitochondria. Western blot analysis showed absence of SCAD-CRM, reduced amounts of MCAD-CRM, and normal amounts of electron-transfer flavoprotein-CRM. The patient, who was unresponsive to treatment with oral carnitine, improved dramatically with daily administration of 100 mg oral riboflavin. Increase in muscle bulk and strength and resolution of the organic aciduria were associated with normalization of SCAD activity and "reappearance" of SCAD-CRM. In contrast, both MCAD activity and MCAD-CRM remained lower than normal. These results suggest that in some patients with multiple acyl-CoA dehydrogenase deficiency riboflavin supplementation may be effective in restoring the activity of SCAD, and possibly of other mitochondrial flavin-dependent enzymes.

Cytochrome c oxidase deficiency in Leigh syndrome

We studied 6 mitochondrial enzymes in crude extracts and isolated mitochondria from 5 children with pathologically proven subacute necrotizing encephalomyelopathy (Leigh syndrome). Samples were taken from brain (5 patients), skeletal muscle (4 patients), liver (4 patients), kidney (4 patients), heart (1 patient), and cultured fibroblasts (3 patients). An isolated defect of cytochrome c oxidase (COX) activity was found in brain (decrease of activity to 15 to 39% of the normal mean), muscle (9 to 20%), kidney (1 to 67%), and in the 1 available heart (4%) from a patient with cardiopathy. COX activity was also decreased in liver of 3 patients (2 to 13% of normal) and in cultured fibroblasts of 2 patients (18 and 27%), but it was normal in both liver and fibroblasts from 1 patient. Immunotitration using polyclonal antibodies against human heart COX showed essentially normal amounts of cross-reacting enzyme protein in various tissues from different patients. Electrophoresis of COX immunoprecipitated from brain mitochondrial extracts showed normal patterns of COX subunits in 2 patients. This study confirms the theory that COX deficiency is an important cause of Leigh syndrome.

Myoadenylate deaminase deficiency in twins with recessive olivopontocerebellar atrophy

Two adult non-identical twins with autosomal recessive olivopontocerebellar degeneration (OPCA) had markedly deficient adenylate deaminase in skeletal muscle homogenates. Ischemic exercise failed to increase the blood ammonia, while lactate increased normally. Glutamate dehydrogenase and NADP-dependent malic enzyme activities in muscle mitochondria of both patients were normal. The significance of adenylate deaminase deficiency in these twins with OPCA is discussed.

Systemic carnitine deficiency with peripheral nerve involvement morphological and biochemical study

The patient, a 31-year-old woman, suffered from a weakness of the proximal muscles which subsequently worsened involving the spinal and neck muscles. During the hospitalization, the patient displayed a "burning feet" syndrome. Lab tests showed a moderate increase in CPK, Aldolase, SGPT, SGOT, lactic and pyruvic acids. Needle EMG was interpreted as "myopathic" but a marked reduction of sensitive action potentials was also detected. Muscle biopsy showed neutral lipid accumulation in muscle fibers, many small angular fibers, probably denervated ones were also evident. A liver biopsy showed fatty accumulation in hepatocytes. Free carnitine was extremely reduced in muscle and liver and just below normal level in plasma while there was a tenfold elevation of long-chain carnitine esters in plasma too. The mechanism by which carnitine deficiency can cause both neural and muscular alterations and in particular the role of carnitine in glucose metabolism is discussed.

Friedreich's ataxia in northern Italy: I. Clinical, neurophysiological and in vivo biochemical studies

Eighteen patients with the presumptive diagnosis of Friedreich's ataxia were studied. Clinical, neurophysiological and biochemical data were concordant in 14 patients and led to the diagnosis of typical Friedreich's ataxia in this group of patients. The remaining 4 patients differed from the typical patients in several respects, but mainly in the cardiological findings. It is concluded that no single clinical or laboratory finding is typical of F.A. Multidisciplinary approaches are essential to the diagnosis of Friedreich's ataxia.

Fatal ataxic encephalopathy and carnitine acetyltransferase deficiency: a functional defect of pyruvate oxidation?

A 3-year 8-month-old girl died after 14 months of illness characterized by episodes of intermittent ataxia associated with oculomotor palsy, hypotonia, mental confusion, and disturbances of consciousness. In the last 4 months of life, there were signs of liver dysfunction. Pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase activities were normal in autopsy brain specimens and in cultured fibroblasts from the patient. Carnitine acetyltransferase was deficient in liver, brain, kidney, and cultured fibroblasts. Medium- and long-chain carnitine acyltransferase activities were normal. It is proposed that a functional defect of acetyl-coenzyme A (acetyl-CoA) utilization in brain mitochondria accompanies the carnitine acetyltransferase deficiency.

Hepatic ketogenesis and muscle carnitine deficiency

The levels of plasma free carnitine and ketone bodies have been found to fluctuate inversely in fasting individuals without muscle disease. Circulating short-chain acyl-carnitines paralleled beta-hydroxybutyrate levels. A patient with lipid storage myopathy and muscle carnitine deficiency, and his two daughters, developed exaggerated ketogenesis on fasting. The content of total carnitines in the patient's liver was normal, but free carnitine was reduced to 50 percent, and total esterified carnitines were four times greater than the mean value for the controls. The decreased muscle carnitine content in this case may have resulted from chronic hepatic ketogenesis, draining muscle carnitine. Alternatively, decreased muscle carnitine content may have initiated hepatic ketogenesis.

Muscle carnitine palmityltransferase deficiency: a case with enzyme deficiency in cultured fibroblasts

A further case of carnitine palmityltransferase (CPT) deficiency in a young man is described, the defect being documented by direct enzyme assays of muscle biopsies. The finding of markedly reduced enzyme activity in the patient's cultured fibroblasts supports the concept that CPT deficiency is a systemic rather than an exclusively muscular condition.