Mitochondrial myopathy

Familial myopathy: new insights into the T14709C mitochondrial tRNA mutation

We have defined the genetic defect in a large family first described in one of the earliest reports of suspected mitochondrial myopathy, as the mutation T14709C in the mitochondrial transfer RNA(Glu) (mt-tRNA(Glu)) gene. Extraordinarily, this mutation has attained homoplasmy (100% mutated mt-tRNA(Glu)) on at least three independent occasions in this family and has done so in one individual who remains asymptomatic with no clinical evidence of disease. Heteroplasmy (dual populations of mutated and wild-type mtDNA) usually is regarded as one of the primary diagnostic criteria for pathogenicity and previous reports of the T14709C mutation detail heteroplasmy in a variety of tissues. In contrast, homoplasmy of mt-tRNA mutations generally has been regarded as evidence of a benign nature, with rare exceptions that result in organ-specific phenotypes. Discovering that T14709C, a common and severe mt-tRNA mutation, can attain homoplasmy without symptoms or clinical signs of disease has profound implications for the identification and prevalence of other pathogenic mt-tRNA mutations. Furthermore, variation in phenotype between homoplasmic individuals implies a crucial contribution from the nuclear genetic environment in determining the clinical outcome of mt-tRNA mutations.

Investigation of muscle disease

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Pancreatic beta-cell secretory defect associated with mitochondrial point mutation of the tRNA(LEU(UUR)) gene: a study in seven families with mitochondrial encephalomyopathy, lactic acidosis and stroke-like episodes (MELAS)

Recent evidence suggests possible linkage between diabetes mellitus and mitochondrial gene mutation. We surveyed mitochondrial tRNA(LEU(UUR)) (3243) mutation in 7 mitochondrial encephalomyopathy, lactic acidosis and stroke-like episode (MELAS) families and identified 24 mutated subjects (7 MELAS probands and 17 non-MELAS relatives) as well as 11 non-mutant family members. An OGTT in the 24 mutant relatives revealed 14 diabetic subjects, 3 with impaired glucose tolerance and 7 with normal glucose tolerance and all non-mutant family members as having normal glucose tolerance. Insulinogenic index was significantly reduced in the mutant diabetic subjects and those with impaired and normal glucose tolerance in comparison with the normal control subjects and the non-mutant members. Urinary 24-h C-peptide immunoreactivity excretion was markedly reduced in the mutant diabetic subjects and those with normal and impaired glucose tolerance, compared with the control subjects and the non-mutant family members. Plasma C-peptide immunoreactivity 6 min after glucagon injection was markedly reduced in the mutant diabetic subjects and those with normal and impaired glucose tolerance compared with the control subjects and the non-mutant family members. Si, an index of insulin sensitivity of the four mutant subjects was within normal range. Islet cell antibodies were negative in sera of eight mutated diabetic subjects, 2 and 6 with impaired and normal glucose tolerance, respectively. Diabetic retinopathy and nephropathy were demonstrated in 7 (50%) and 12 (85.7%) of 14 mutant diabetic subjects, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Mitochondrial gene defects in patients with NIDDM

Non-insulin-dependent diabetes mellitus (NIDDM) has a strong genetic component and maternal factors have recently been implicated in disease inheritance. The mitochondrial myopathies are a group of diseases which often show maternal inheritance as a result of mtDNA defects; some patients have impaired glucose tolerance. Occasional families with maternally inherited diabetes and deafness associated with a deletion or point mutation of mtDNA have been reported. To assess the importance of mitochondrial gene defects in NIDDM, 150 unrelated diabetic subjects from Wales, UK and 68 unrelated patients with diabetes and at least one affected sibling from England, UK were studied. Southern blot analysis did not show any large mtDNA deletions or duplications. One patient had a mutation in the mitochondrial tRNAleu(UUR) gene at bp 3243. This mutation is commonly associated with the syndrome of mitochondrial encephalomyopathy, lactic acidosis and stroke like episodes (MELAS). Study of this patient and his siblings showed a distinct form of late-onset diabetes associated with nerve deafness but no clinical features of the MELAS syndrome. No diabetic subject was shown to have the mtDNA mutation at position 8344 (tRNA(lys)) which has previously been described in the syndrome of mitochondrial encephalomyopathy and red-ragged fibres (MERRF). The role of other mitochondrial gene defects in diabetes and the pathophysiological basis of glucose intolerance in patients with the MELAS mutation requires further elucidation.

Mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes (MELAS) and diabetes mellitus: molecular genetic analysis and family study

Two MELAS (mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes) patients with diabetes mellitus (DM), and their family members are described clinically and genetically. The probands have the following features in common; normal early development, short stature, deterioration of intellectual ability, convulsions, cardiac conduction defect, sensorineural hearing loss, cortical blindness, and hemiparesis. Biochemical tests showed high levels of lactate and pyruvate in the blood and cerebrospinal fluid. Muscle biopsy showed ragged-red fibers. Molecular genetic analysis of both patients revealed that they had an A-to-G substitution at nucleotide position 3243 of the mitochondrial DNA in a heteroplasmic fashion. From these clinical and molecular genetic data they were diagnosed as having MELAS. In addition, fasting blood glucose levels were also high and they were diagnosed as having insulin-dependent DM. Some of the maternal family members in both cases also had insulin-dependent DM and several clinical symptoms of MELAS. DM and clinical features of MELAS were transmitted exclusively in the maternal line. In these cases, DM and MELAS might be a clinical manifestation of the same metabolic defect.

Normal insulin receptors in mitochondrial myopathies with ophthalmoplegia

Seven patients with histologically proven mitochondrial myopathy with ophthalmoplegia (OMM), 6 of them nondiabetic, 1 affected by diabetes mellitus (DM), were submitted to a study of glucose tolerance and of insulin receptors on peripheral mononuclear cells and cultured skin fibroblasts. The diabetic patient, who had the typical features of the Kearns-Sayre syndrome (KSS) and deleted muscle mitochondrial DNA (mtDNA) presented a low insulin secretion rate under physiological stimuli (intravenous glucose and glucagon) whereas the insulin receptor parameters were found normal. The other patients showed a normal glucose tolerance and normal insulin receptors. Our data support the hypothesis that insulin receptors are not involved in the pathogenesis of DM associated with mitochondrial encephalomyopathies, in contrast to other neuromuscular inherited disorders. The clinical and biological features of DM presented by our KSS patient show normal insulin receptor parameters in spite of a defective insulin secretion, possibly depending on mitochondrial dysfunction.

Transient postictal cortical blindness

An 8-year-old boy with insulin-dependent diabetes mellitus and a seizure disorder demonstrated transient visual loss after severe seizure activity. The role of hypoglycemia in relation to his transient cortical blindness remains indeterminate. The nature of the cortical involvement, the rate of visual recovery, and prior reports of postictal phenomena emphasize the relatively benign nature of this condition in children.

Mitochondrial myopathies. A clinico-pathological study of cases with and without extra-ocular muscle involvement

The clinical and pathological features of 28 patients with mitochondrial myopathy were reviewed. The cases were divided into a group with involvement of the extra-ocular muscles alone or with limb muscle involvement, and a group with a facioscapulohumeral syndrome or generalised weakness without extra-ocular muscle involvement. Cardiac and central nervous system manifestations occurred particularly in the first group which included six patients with multisystemic features and two with the complete Kearns-Sayre syndrome. Diabetes mellitus occurred in the second group only. Quantitative histology on limb muscle biopsies showed a higher proportion of fibres with abnormal mitochondrial aggregates in the second group. No one type of mitochondrial inclusion or other ultrastructural change was specific for either group of cases. The findings illustrate the clinical heterogeneity of cases of mitochondrial myopathy and the lack of specificity of any of the myopathological changes for different subgroups of patients.

Maternally inherited mitochondrial myopathy and myoclonic epilepsy

A family is described with familial myoclonic epilepsy associated with mitochondrial myopathy. The disorder follows a maternal inheritance pattern consistent with a mitochondrial DNA (mtDNA) mutation. The large kindred permitted exclusion of autosomal dominant, recessive, and X-linked patterns of transmission. Several characteristics of the inheritance and variability of expression within the pedigree are consistent with recently acquired knowledge about the genetics of human mtDNA. The clinical spectrum of disease is compatible with a proportionality model of mutant and wild-type mtDNAs. Muscle biopsies of affected patients showed an increased number of abnormal muscle mitochondria. Serum levels of pyruvate or pyruvate and lactate were elevated. The most severely affected patient had constant myoclonic jerking, dementia, ataxia, spasticity, hearing loss, and hypoventilation. Cerebral dysfunction in patients with mild involvement was marked by prominent photic driving seen on electroencephalograms and high-amplitude visual and somatosensory evoked responses but no myoclonus, ataxia, or dementia. The individual clinical features of the disease worsen over time for all patients; however, mildly affected patients have not become moderately affected and moderately affected patients have not become severely affected.

Abnormal mitochondria on a renal biopsy from a case of mitochondrial myopathy

Mitochondrial myopathy (MM) is reported in a 5-year-old girl with short stature, hypotonia, ptosis, retinal pigmentation, and Fanconi's syndrome. A muscle biopsy showed the characteristic features of MM, and a renal biopsy revealed abnormal mitochondria in tubular cells that were similar to those seen in the muscle. This is the first report of such findings in association with MM.

Electrophysiological findings including single fibre EMG in a family with mitochondrial myopathy

Nerve conduction studies, conventional and quantitative concentric needle EMG and single fibre ENG were performed on 5 clinically affected and 7 clinically asymptomatic members of a family with a mitochondrial myopathy manifesting as a facioscapulohumeral syndrome. Abnormalities of nerve conduction present in 3 clinically affected cases were attributed to co-existent diabetes mellitus. Quantitative CNEMG showed a reduction of the mean motor unit potential duration and increased incidence of polyphasic potentials in all 5 clinically manifest cases consistent with a primary myopathic disorder. Similar but less marked changes were found in 6 of the clinically asymptomatic individuals revealing the presence of a subclinical myopathy. Abnormalities on SFEMG consisting of increases in fibre density and/or jitter were present in all the clinically affected and in 5 clinically normal cases. These changes indicate local reorganization of the spatial arrangement of muscle fibres of the motor unit and a disturbance of neuromuscular transmission. The CNEMG and SFEMG findings are discussed in relation to the histopathological changes in 4 cases.