Margaret A. Johnson

Mitochondrial enzyme activity in amyotrophic lateral sclerosis: implications for the role of mitochondria in neuronal cell death.

The mechanism of selective loss of motor neurons in amyotrophic lateral sclerosis (ALS) has not been clarified. Mitochondrial pathology is present in central nervous system tissue from ALS cases and occurs as an early event in a mouse model of ALS. We demonstrate that, in sporadic ALS, there is a selective decrease in the activity of the mitochondrial DNA–encoded enzyme cytochrome c oxidase in human spinal cord motor neurons. We propose that this may not only be important in neuronal cell death but could well be caused by oxidative damage to mitochondrial DNA leading to the accumulation of mitochondrial DNA mutations.

Histochemical localisation of mitochondrial enzyme activity in human optic nerve and retina.

AIMS To demonstrate the quantitative distribution of mitochondrial enzymes within the human optic nerve and retina in relation to the pathogenesis of ophthalmic disease. METHODS Enucleations were performed at the time of multiple organ donation and the optic nerve and peripapillary retina immediately excised en bloc and frozen. Reactivities of the mitochondrial enzymes cytochromec oxidase and succinate dehydrogenase were demonstrated in serial cryostat sections using specific histochemical assays. RESULTS In the optic nerve the unmyelinated prelaminar and laminar regions were rich in both cytochrome c oxidase and succinate dehydrogenase. Myelination of fibres as they exited the lamina cribrosa was associated with an abrupt reduction in enzyme activity. Within the retina, high levels of enzyme activity were found localised within the retinal ganglion cells and nerve fibre layer, the outer plexiform layer, inner segments of photoreceptors, and the retinal pigment epithelium. CONCLUSIONS Mitochondrial enzyme activity is preserved in human optic nerve and retina retrieved at the time of multiple organ donation. The distribution of enzyme activity within the eye has implications for the understanding of the pattern of ophthalmic involvement seen in mitochondrial diseases and the site of ganglion cell dysfunction in those patients with optic nerve involvement.

Muscle fibers in inflammatory myopathies and cultured myoblasts express the nonclassical major histocompatibility antigen HLA-G

We demonstrate that HLA‐G, a nonclassical major histocompatibility complex class I antigen, is expressed in muscle fibers in various inflammatory myopathies. Further, interferon‐γ induces surface expression and upregulation of mRNA transcripts corresponding to different isoforms of HLA‐G in myoblasts cultured from control subjects and patients. HLA‐G may have important immunological functions in inflammatory myopathies and other local immune reactions as they occur during vaccination, myoblast transplantation, and gene therapy. Ann Neurol 2000;48:679–684

An mtDNA Mutation in the Initiation Codon of the Cytochrome C Oxidase Subunit II Gene Results in Lower Levels of the Protein and a Mitochondrial Encephalomyopathy

A novel heteroplasmic 7587T-->C mutation in the mitochondrial genome which changes the initiation codon of the gene encoding cytochrome c oxidase subunit II (COX II), was found in a family with mitochondrial disease. This T-->C transition is predicted to change the initiating methionine to threonine. The mutation load was present at 67% in muscle from the index case and at 91% in muscle from the patients clinically affected son. Muscle biopsy samples revealed isolated COX deficiency and mitochondrial proliferation. Single-muscle-fiber analysis revealed that the 7587C copy was at much higher load in COX-negative fibers than in COX-positive fibers. After microphotometric enzyme analysis, the mutation was shown to cause a decrease in COX activity when the mutant load was >55%-65%. In fibroblasts from one family member, which contained >95% mutated mtDNA, there was no detectable synthesis or any steady-state level of COX II. This new mutation constitutes a new mechanism by which mtDNA mutations can cause disease-defective initiation of translation.

Mitochondrial involvement in the ageing process. Facts and controversies.

Mitochondria are believed to be involved in human ageing. Whilst it is clear that various mitochondrial DNA mutations do accumulate in human tissues with age, whether or not they interfere with respiratory chain function is uncertain. We question the results of previous studies which have measured respiratory chain function in human skeletal muscle with age. Whilst cytochrome c oxidase deficient fibres are a real finding in skeletal muscle, the contribution of mitochondrial DNA mutations to human ageing is still controversial. Our results show for mitochondria to be involved in ageing then it must be through a more subtle mechanism than a global decline in respiratory chain function. (Mol Cell Biochem 174: 325–328, 1997)

Intracellular mitochondrial triplasmy in a patient with two heteroplasmic base changes.

We report the clinical, biochemical, and genetic investigation of a patient with a severe mitochondrial encephalomyopathy. Genetic studies identified a novel, heteroplasmic tRNA mutation at nt 10010. This T-->C transition is located in the DHU loop of mitochondrial tRNA(Gly). In skeletal muscle, it was present at lower levels in cytochrome c oxidase (COX)-normal (87.2% +/- 11%) compared with COX-deficient fibers (97.3% +/- 2.6%); it was found in skin fibroblasts and blood cells, but at lower levels of heteroplasmy (15% +/- 6% and 17% +/- 10%, respectively). A second, heteroplasmic transition (A-->G), at nt 5656, showed a different distribution than the tRNA(Gly) mutation, with very low levels in skeletal muscle (< 3%) but higher levels in blood (22.7% +/- 3%) and skin fibroblasts (21% +/- 2%). These transitions were followed both in vivo, by repeat biopsy and blood sampling, and in vitro, by establishing primary cultures of myoblasts and skin fibroblasts. Repeat muscle biopsy showed a dramatic increase in COX-deficient fibers, but not of the tRNAGly mutation. Indeed, no significant change in heteroplasmy was measured for either substitution in muscle or blood. In vitro analysis gave very different results. The T10010C was not found in cultured myoblasts, even at early passage. In uncloned fibroblasts, the T10010C was stable (approximately 10%) for several passages but then gradually was lost. In contrast, the A5656G rose progressively from 27% to 91%. In cloned fibroblasts, different combinations of both base-pair changes and wild type could be identified, confirming the presence of clonal, intracellular triplasmy.

Mitochondrial fatty acid beta-oxidation in the retinal pigment epithelium.

Pigmentary retinopathy is an important feature of long-chain 3-hydroxyacyl-CoA dehydrogenase (LCHAD) deficiency, a disorder of mitochondrial fatty acid β-oxidation. Pathogenesis of this complication remains unknown. The retinal pigment epithelium (RPE) is affected early in this retinopathy. We wanted to determine whether there is evidence of mitochondrial fatty acid β-oxidation in the RPE cells. Fatty acid oxidation was measured from cultured porcine RPE cells by incubating them with [U-13C]-hexadecanoic acid. Acylcarnitine esters were analyzed by tandem mass spectrometry. The activity of LCHAD and carnitine uptake capacity were measured from the cultured cells. Antibodies to the human mitochondrial trifunctional protein (MTP) containing LCHAD activity were used to analyze the expression of the MTP in the cultured RPE cell lysate and in human retinal sections by immunoblotting and immunohistochemistry. Fatty acid oxidation analysis showed normal chain shortening of hexadecanoic acid and production of acetylcarnitine in cultured RPE cells. Immunoblotting revealed expression of the MTP and enzyme assay showed the activity of LCHAD in the RPE cells. RPE cells were also capable of carnitine uptake. Positive labeling to the MTP antibodies was detected in the RPE, photoreceptors, and ganglion cells. The results give strong in vitro evidence for the presence of mitochondrial fatty acid β-oxidation in RPE cells and the expression of the MTP in the RPE and other layers of the retina. Further studies are required to clarify whether this pathway acts also in vivo in the retina.

A novel autosomal dominant distal myopathy with early respiratory failure: clinico-pathologic characteristics and exclusion of linkage to candidate genetic loci.

We describe a novel autosomal dominant myopathy presenting in mid–adult life with tibialis anterior weakness. We carried out a detailed clinical assessment of 24 individuals spanning three generations, documenting pathologic features of the muscles in 7 of the 11 affected individuals, including an autopsy study on one case. The second generation of affected individuals presented at an earlier age, and the disease progressed more rapidly than in the first generation. Lung function tests revealed progressive global respiratory muscle weakness detectable from the time of presentation, with preferential diaphragmatic involvement in some cases. Hip girdle and shoulder girdle weakness appeared later in the disease course. We observed a striking correlation between the clinical and pathological features. Clinically unaffected muscles had minimal pathologic change. Fiber splitting, eosinophilic inclusions, and vacuoles with basophilic rims were seen in moderately affected muscles, and fat and fibrous connective tissue replaced muscle fibers in the severely involved muscles. The inclusions were Congophilic and reacted with antibodies to desmin, β‐amyloid, and phosphorylated tau protein. The disease was not linked to any of the known loci associated with distal myopathies, confirming that the disorder in this family is both genetically and phenotypically distinct. Ann Neurol 2001;49:443–452

A novel point mutation in the mitochondrial tRNA(Trp) gene produces a neurogastrointestinal syndrome.

We report a novel, heteroplasmic point mutation in the mitochondrial tRNA for tryptophan at position 5532. The mutation was present in all the tissues studied and segregated with the biochemical defect, with higher levels of mutation present in cytochrome c oxidase-deficient muscle fibres. The patient manifested a neurogastrointestinal syndrome with features including failure to thrive, psychomotor retardation, ophthalmoplegia, sensorineural deafness and encephalopathy together with vomiting, diarrhoea and colitis.

Lipid storage myopathy: a recognizable clinicopathological entity?

SummaryThere have been several recent descriptions of myopathies associated with disordered oxidative metabolism in muscle fibres and particularly with accumulation of fat within the fibres. Some of these have been associated with abnormal muscle mitochondria but in others there may be an extramitochondrial defect of lipid metabolism. In this paper we describe a 38 year old man suffering from a progressive myopathy which clinically resembled polymyositis and which responded to treatment with steroids. Pathologically the condition was characterised by an apparent increase in the amount of lipid within the muscle fibres, an observation subsequently confirmed by quantitative chemical analysis, although the muscle mitochondria appeared to be normal. This case closely resembles others recently described in this department and elsewhere and it is suggested that they may be examples of a previously unrecognized clinico-pathological entity.