Multiple genotypes, multiple phenotypes, and partial defects

Cardiac involvement in Friedreich's ataxia: a clinical study of 75 patients

To establish the prevalence and to characterize the types of cardiac involvement in Friedreich's ataxia, 75 consecutive patients (39 male and 36 female), aged 10 to 66 years (mean 24) were prospectively studied. Electrocardiograms were performed in all patients, vectorcardiograms in 34 and echocardiograms in 58. Electrocardiographic and vectorcardiographic abnormalities occurred in 69 (92%) of the 75 patients. Electrocardiograms revealed ST-T wave abnormalities in 79%, right axis deviation in 40%, short PR interval in 24%, abnormal R wave in lead V1 in 20%, abnormal inferolateral Q waves in 14% and left ventricular hypertrophy (voltage and repolarization criteria) in 16%. Echocardiograms revealed concentric left ventricular hypertrophy in 11%, asymmetric septal hypertrophy in 9% and globally decreased left ventricular function in 7%. Progression from a normal echocardiogram to concentric left ventricular hypertrophy, asymmetric septal hypertrophy or globally decreased left ventricular function was identified in one patient in each category, although the study was not designed for longitudinal follow-up. Two patients died, and necropsy revealed in both a minimally dilated but flabby left ventricle. On the basis of electrocardiographic and vectorcardiographic and echocardiographic data, 95% of patients had one or more disorders. The most common abnormality was segmental myocardial "dystrophy" (electrocardiographic QRS initial force abnormalities), but global left ventricular hypokinesia occurred more often than previously recognized.(ABSTRACT TRUNCATED AT 250 WORDS)

Mitochondrial myopathies: disorders of the respiratory chain and oxidative phosphorylation

Mitochondrial myopathies are a clinical condition characterized by muscle weakness and fatigue in which the primary defect is localized at the level of the mitochondria. Microscopic examination shows accumulations of mitochondria at the fibre periphery (ragged red fibres) and in some cases mitochondrial paracrystalline inclusions. The spectrum of different mitochondrial defects so far described is reviewed and data from cases investigated in this laboratory are described. The first case was a 17-year-old boy with a multisystem disorder whose muscle mitochondria showed low respiratory activity with all substrates, which doubled in the presence of uncoupler. Further investigation showed that the mitochondrial ATPase activity was only 6% of normal. The next cases were a mother and daughter who showed a typical lipid storage myopathy. The latter was treated successfully with oral carnitine but the myopathy persisted. Mitochondrial investigations indicated a low respiratory activity with NAD-linked substrates but normal activity with succinate and ascorbate + TMPD. A defect in the NADH-CoQ reductase section of the respiratory chain was pinpointed possibly at an iron-sulphur centre. The fourth and fifth cases were two sisters who exhibited no lipid storage myopathy but whose mitochondrial activity was low with NAD-linked substrates but normal with succinate. Again a defect in the NADH-CoQ reductase (complex I) of the respiratory chain was determined. They were also investigated using 31P-NMR. It was found after exercise that their muscle creatine phosphate levels took seven times longer to return to pre-exercise concentrations than control subjects. These results are discussed with respect to the synthesis of mitochondrial proteins and the influence that both the mitochondrial and nuclear DNA have on this process.

Heterogeneity of tropomyosin and actin in normal and diseased muscle

Actin and tropomyosin in muscle samples from normal humans, from human fetuses between 12 and 17 gestational weeks, and from patients with a variety of neuromuscular disorders were studied with two-dimensional electrophoresis using isoelectric focusing with either a broad pH range (8.6-4.5) or a narrow pH range (5.9-3.8) for the first dimension and either SDS or SDS-urea for the second dimension. With the broad pH range, two brothers with Duchenne muscular dystrophy were noted to have a less acidic variant of alpha-tropomyosin in biceps muscle which was not found in biceps muscle from other patients or controls. Studies of 8 additional biopsy specimens from patients with Duchenne muscular dystrophy and comparison with both fetal and normal human muscle using the narrow pH range revealed multiple forms of actin and tropomyosin which varied from individual to individual. This heterogeneity appeared to be unrelated to the dystrophic state but also obscured the ability to detect a change in actin or tropomyosin which could be related to dystrophy.