Late-onset mitochondrial DNA depletion: DNA copy number, multiple deletions, and compensation.

Through a report of 4 late-onset cases with mitochondrial DNA (mtDNA) depletion, we address the specificity of the clinical entities associated with a very low residual amount of mtDNA. Three of the patients met the diagnostic criteria of Kearns Sayre syndrome, which has never been associated with mtDNA depletion. The fourth patient had an isolated skeletal myopathy. Deleted mtDNA molecules were found by long-range polymerase chain reaction (PCR) only in the Kearns Sayre syndromes, which strengthens the clinical differences between the two types of patients. All patients had extremely low residual amounts of mtDNA as shown by Southern blot analysis. Using an original method based on competitive PCR, we were able to measure the number of mtDNA copies per diploid genome. These results demonstrated the severity of the depletion in the patients by comparison not only to normal controls but also to patients with mtDNA disorders. Despite the severity of the depletion, in situ hybridization using two mtDNA transcripts revealed a normal steady-state level of transcription. Such compensation provides clues to the striking contrast between the severity of mtDNA depletion and the late onset and slowly progressive disease.

A point mutation in the glycerol kinase gene associated with a deletion in the dystrophin gene in a familial X-linked muscular dystrophy: non-contiguous gene syndrome involving Becker muscular dystrophy and glycerol kinase loci.

We report a family with an X-linked recessive muscular dystrophy characterised by exercise-induced myalgia, recurrent pigmenturia and mild proximal muscle involvement. Immunocytochemical and immunoblotting analysis in muscle, using the antibody directed against the rod domain of dystrophin, revealed a loss of immunoreactivity, but the immunolabelling using the antibodies directed against the COOH and NH2 domains of dystrophin were almost normal. The immunoreactions for alpha-sarcoglycan, gamma-sarcoglycan and beta-dystroglycan were normal. In the five male patients of this family with increased serum creatine kinase levels (from x8 to x50), mass spectrometry screening of the urine revealed a large increase in glycerol elimination which was quantified by enzymatic assay (from x14 to x39). An in-frame deletion of the dystrophin gene (exons 13-29) was found in the same five males and in three carrier females. All the deleted chromosomes also carried a missense mutation at nucleotide 947 of the Xp glycerol kinase (GK) gene resulting in a Thr to Met substitution at codon 278. These findings indicate that the two mutations cosegregate on the same chromosome in this family. This is the first reported case of two physically independent mutations, within the DMD and GK genes, which are contiguous but several hundred kilobases apart.

Clinical and molecular heterogeneity of cytochrome c oxidase deficiency in the newborn.

We report 8 cases of severe cytochrome c oxidase deficiency with onset in the neonatal period. Clinical symptoms were heterogeneous: antenatal cerebral malformations, neurological distress with ketoacidosis, severe myopathy, or isolated respiratory control failure. Lactic acid was elevated in blood and/or CSF in 7 cases. Muscle biopsy (7 patients), liver biopsy (4 patients), and cultured skin fibroblasts (7 patients) were used to assess the cytochrome c oxidase deficiency. Among the patients, the enzymatic defect differed in the level of residual activity, expression in different tissues and subunit composition in muscle (as analysed by immunohistochemistry). Southern blot analysis of the mitochondrial DNA was normal in 7 patients. The heterogeneity of cytochrome c oxidase deficiency was therefore demonstrated by these clinical presentations and by the biochemical assessment of the enzyme defect. This reflects, most probably, the diverse nature of the causal mutations.

Morphological studies of skeletal muscle in lactic acidosis.

This paper underscores the contribution of routine morphological examination of skeletal muscle in patients with lactic acidosis. Mitochondrial disorders are by far the most common causes of primary lactic acidosis, in which muscle biopsy analysis helps in diagnosis and in the search for the molecular anomalies. Thus, we focus our attention on one particular point: the contribution of the morphological study of muscle biopsy in primary lactic acidosis due to mitochondrial disorders, especially mitochondrial respiratory-chain diseases.

Immunohistochemical analysis of muscle cytochrome c oxidase deficiency in children.

Despite the demonstration of a clear biochemical defect, the genetic alterations causing childhood forms of cytochrome c oxidase (COX) deficiency remain unknown. The double genetic origin (nuclear and mitochondrial DNA), and the complexity of COX enzyme structure and regulation, indicate the need for genetic investigations of the molecular structure of individual COX subunits. In the present study a new monoclonal antibody, which reacts exclusively with heart-type human COX subunit VIIa (VIIa-H), and other monoclonal antibodies against human COX subunits, were used in the immunohistochemical analysis of skeletal muscle from children with different forms of mitochondrial myopathy with COX deficiency. By immunohistochemical investigation a normal reaction was seen with antibodies to COX subunits IV, Va+Vb, and VIa+VIc in all four cases, and in two cases with antibodies to COX VIIa-H and VIIa+VIIb. In muscle from a fatal infantile case with cardiac and skeletal muscle involvement, no immunohistochemical reaction was seen with the monoclonal antibody against the tissue-specific subunit VIIa-H. In muscle from an 11-year-old boy with exclusive muscular symptoms and signs, immunohistological reactions were absent with COX subunit VIIa-H and COX subunits VIIa+VIIb, and slightly decreased with COX subunit II, thus demonstrating a different molecular mechanism in each case. It is concluded that the molecular basis of COX deficiency in childhood may vary greatly between patients.

Malignant hyperthermia and central core disease: analysis of two families with heterogeneous clinical expression.

We report two families both presenting with malignant hyperthermia susceptibility and "core" or "core-like" changes in the muscle tissue. Combined analysis of the malignant hyperthermia phenotype and the histochemical findings demonstrates the complexity of their association and highly suggests genetic heterogeneity of malignant hyperthermia and central core diseases.