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E A Schon

Publications and source records attributed to E A Schon.

At least 19 recordsLinked to original sources
PubMed1999

Oligomycin induces a decrease in the cellular content of a pathogenic mutation in the human mitochondrial ATPase 6 gene.

A T --> G mutation at position 8993 in human mitochondrial DNA is associated with the syndrome neuropathy, ataxia, and retinitis pigmentosa and with a maternally inherited form of Leigh's syndrome. The mutation substitutes an arginine for a leucine at amino acid position 156 in ATPase 6, a component of the F0 portion of the mitochondrial ATP synthase complex. Fibroblasts harboring high levels of the T8993G mutation have decreased ATP synthesis activity, but do not display any growth defect under standard culture conditions. Combining the notions that cells with respiratory chain defects grow poorly in medium containing galactose as the major carbon source, and that resistance to oligomycin, a mitochondrial inhibitor, is associated with mutations in the ATPase 6 gene in the same transmembrane domain where the T8993G amino acid substitution is located, we created selective culture conditions using galactose and oligomycin that elicited a pathological phenotype in T8993G cells and that allowed for the rapid selection of wild-type over T8993G mutant cells. We then generated cytoplasmic hybrid clones containing heteroplasmic levels of the T8993G mutation, and showed that selection in galactose-oligomycin caused a significant increase in the fraction of wild-type molecules (from 16 to 28%) in these cells.

Adenosine Triphosphatases↗
PubMed1999

Mitochondrial involvement in Alzheimer's disease.

The causes of most neurodegenerative diseases, including sporadic Alzheimer's disease (AD), remain enigmatic. There is, however, increasing evidence implicating mitochondrial dysfunction resulting from deafferentiation of disconnected neural circuits in the pathogenesis of energy deficit in AD. The patterns of reduced expression of both mitochondrial DNA (mtDNA) and nuclear DNA (nDNA) encoded genes is consistent with a physiological down-regulation of the mitochondrial respiratory chain in response to reduced neuronal activity. On the other hand, the role(s) of somatic cell or maternally inherited mtDNA mutations in the pathogenesis of mitochondrial dysfunction in AD are still controversial.

Alzheimer Disease↗
PubMed1999

Kearns-Sayre syndrome: unusual pattern of expression of subunits of the respiratory chain in the cerebellar system.

Kearns-Sayre syndrome (KSS) is a sporadic multisystem disorder of oxidative phosphorylation associated with clonally expanded rearrangements of mitochondrial DNA (mtDNA). Mitochondrial dysfunction in the central nervous system of patients with KSS accounts for the neurological manifestations of the disease. To gain further insight into the pathogenesis of neuronal dysfunction in KSS, we used antibodies against mtDNA-encoded and nuclear DNA-encoded subunits of the mitochondrial respiratory chain to study the expression of these proteins in the cerebellar cortex, dentate nucleus, and inferior olivary nucleus from 2 autoptic cases of KSS. Neuropathological examination showed a moderate loss of Purkinje cells and spongiform degeneration of the cerebellar white matter. By using immunohistochemistry, we found a decreased expression of mtDNA-encoded proteins only in neurons of the dentate nucleus. We suggest that mitochondrial abnormalities in the dentate nucleus in conjunction with loss of Purkinje cells and spongiform degeneration of the cerebellar white matter may be important factors in the genesis of the cerebellar dysfunction in KSS.

Cerebellum↗
PubMed1999

A calcium signaling defect in the pathogenesis of a mitochondrial DNA inherited oxidative phosphorylation deficiency.

In recent years, genetic defects of the mitochondrial genome (mtDNA) were shown to be associated with a heterogeneous group of disorders, known as mitochondrial diseases, but the cellular events deriving from the molecular lesions and the mechanistic basis of the specificity of the syndromes are still incompletely understood. Mitochondrial calcium (Ca2+) homeostasis depends on close contacts with the endoplasmic reticulum and is essential in modulating organelle function. Given the strong dependence of mitochondrial Ca2+ uptake on the membrane potential and the intracellular distribution of the organelle, both of which may be altered in mitochondrial diseases, we investigated the occurrence of defects in mitochondrial Ca2+ handling in living cells with either the tRNALys mutation of MERRF (myoclonic epilepsy with ragged-red fibers) or the ATPase mutation of NARP (neurogenic muscle weakness, ataxia and retinitis pigmentosa). There was a derangement of mitochondrial Ca2+ homeostasis in MERRF, but not in NARP cells, whereas cytosolic Ca2+ responses were normal in both cell types. Treatment of MERRF cells with drugs affecting organellar Ca2+ transport mostly restored both the agonist-dependent mitochondrial Ca2+ uptake and the ensuing stimulation of ATP production. These results emphasize the differences in the cellular pathogenesis of the various mtDNA defects and indicate specific pharmacological approaches to the treatment of some mitochondrial diseases.

Adenosine Triphosphatases↗
PubMed1999

Mitochondrial DNA genotypes in nuclear transfer-derived cloned sheep.

Eukaryotic cells contain two distinct genomes. One is located in the nucleus (nDNA) and is transmitted in a mendelian fashion, whereas the other is located in mitochondria (mtDNA) and is transmitted by maternal inheritance. Cloning of mammals typically has been achieved via nuclear transfer, in which a donor somatic cell is fused by electoporation with a recipient enucleated oocyte. During this whole-cell electrofusion, nDNA as well as mtDNA ought to be transferred to the oocyte. Thus, the cloned progeny should harbour mtDNAs from both the donor and recipient cytoplasms, resulting in heteroplasmy. Although the confirmation of nuclear transfer has been established using somatic cell-specific nDNA markers, no similar analysis of the mtDNA genotype has been reported. We report here the origin of the mtDNA in Dolly, the first animal cloned from an established adult somatic cell line, and in nine other nuclear transfer-derived sheep generated from fetal cells. The mtDNA of each of the ten nuclear-transfer sheep was derived exclusively from recipient enucleated oocytes, with no detectable contribution from the respective somatic donor cells. Thus, although these ten sheep are authentic nuclear clones, they are in fact genetic chimaeras, containing somatic cell-derived nuclear DNA but oocyte-derived mtDNA.

Animals↗
PubMed1999

Fatal infantile cardioencephalomyopathy with COX deficiency and mutations in SCO2, a COX assembly gene.

Mammalian cytochrome c oxidase (COX) catalyses the transfer of reducing equivalents from cytochrome c to molecular oxygen and pumps protons across the inner mitochondrial membrane. Mitochondrial DNA (mtDNA) encodes three COX subunits (I-III) and nuclear DNA (nDNA) encodes ten. In addition, ancillary proteins are required for the correct assembly and function of COX (refs 2, 3, 4, 5, 6). Although pathogenic mutations in mtDNA-encoded COX subunits have been described, no mutations in the nDNA-encoded subunits have been uncovered in any mendelian-inherited COX deficiency disorder. In yeast, two related COX assembly genes, SCO1 and SCO2 (for synthesis of cytochrome c oxidase), enable subunits I and II to be incorporated into the holoprotein. Here we have identified mutations in the human homologue, SCO2, in three unrelated infants with a newly recognized fatal cardioencephalomyopathy and COX deficiency. Immunohistochemical studies implied that the enzymatic deficiency, which was most severe in cardiac and skeletal muscle, was due to the loss of mtDNA-encoded COX subunits. The clinical phenotype caused by mutations in human SCO2 differs from that caused by mutations in SURF1, the only other known COX assembly gene associated with a human disease, Leigh syndrome.

Amino Acid Sequence↗
PubMed1998

Mitochondria in neuromuscular disorders.

This review considers primary mitochondrial diseases affecting the respiratory chain. As diseases due to mitochondrial DNA defects defy traditional anatomical classifications, we have not limited our discussion to neuromuscular disorders, but have extended it to include mitochondrial encephalomyopathies. Primary mitochondrial diseases can be due to mutations in either the nuclear or the mitochondrial genome. Nuclear mutations can affect (i) genes encoding enzymatic or structural mitochondrial proteins; (ii) translocases; (iii) mitochondrial protein importation; and (iv) intergenomic signaling. We review briefly recent molecular data and outstanding questions regarding these mendelian disorders, with special emphasis on cytochrome c oxidase deficiency and coenzyme Q10 deficiency. Mitochondrial DNA mutations fall into three main categories: (i) sporadic rearrangements (deletions/duplications); (ii) maternally inherited rearrangements (duplications); and (iii) maternally inherited point mutations. We summarize the most common clinical presentations and discuss pathogenic mechanisms, which remain largely elusive. Uncertainties about pathogenesis extend to the process of cell death, although excitotoxicity in neurons and apoptosis in muscle seem to have important roles.

Animals↗
PubMed1998

Study of mitochondrial DNA depletion in muscle by single-fiber polymerase chain reaction.

We studied muscle biopsies from 3 children with a mitochondrial myopathy characterized histochemically by the presence of ragged-red fibers (RRF) and various numbers of cytochrome c oxidase (COX)-negative fibers. We quantitated the absolute amounts of total mitochondrial DNA (mtDNA) in isolated normal COX-positive muscle fibers and in COX-negative RRF. There was severe mtDNA depletion in all fibers from the two most severe cases. In the third case mtDNA depletion could not be established with conventional diagnostic tools, but it was documented in single COX-negative fibers; COX-positive fibers showed the same amounts of mtDNA as fibers from aged-matched controls. Our observations indicate that mtDNA single-fiber PCR quantitation is a highly sensitive and specific method for diagnosing cases with focal mtDNA depletion. This method also allows one to correlate amounts of mtDNA with histochemical phenotypes in individual fibers from patients and age-matched controls, thereby providing important information about the functional role of residual mtDNA.

Biopsy↗
PubMed1998

Neurotrophin-4 is up-regulated in ragged-red fibers associated with pathogenic mitochondrial DNA mutations.

Ragged-red fibers (RRFs) are found more frequently in highly oxidative type I fibers than in glycolytic type II fibers in the muscle of many patients with mitochondrial myopathies. Neurotrophin-4 (NT-4), a neuronal signaling molecule, is also expressed in skeletal muscle, predominantly in type I fibers. We found that NT-4 protein and mRNA were present in both type I and type II fibers but were up-regulated in RRFs of patients with mitochondrial encephalomyopathies; it is noteworthy that NT-4 was not up-regulated in muscle fibers from healthy aerobically trained athletes. Thus, NT-4 might represent a member of a new class of candidate molecules involved in the compensatory adjustments of muscle fibers to oxidative dysfunction, and may even play a role as a signaling molecule for mitochondrial proliferation.

Biopsy↗
PubMed1998

Evidence for the presence of 5S rRNA in mammalian mitochondria.

Mammalian mitochondrial ribosomes contain two prokaryotic-like rRNAs, 12S and 16S, both encoded by mitochondrial DNA. As opposed to cytosolic ribosomes, however, these ribosomes are not thought to contain 5S rRNA. For this reason, it has been unclear whether 5S rRNA, which can be detected in mitochondrial preparations, is an authentic organellar species imported from the cytosol or is merely a copurifying cytosol-derived contaminant. We now show that 5S rRNA is tightly associated with highly purified mitochondrial fractions of human and rat cells and that 5S rRNA transcripts derived from a synthetic gene transfected transiently into human cells are both expressed in vivo and present in highly purified mitochondria and mitoplasts. We conclude that 5S rRNA is imported into mammalian mitochondria, but its function there still remains to be clarified.

Animals↗
PubMed1998

Multiple mtDNA deletions features in autosomal dominant and recessive diseases suggest distinct pathogeneses.

Multiple mitochondrial DNA (mtDNA) deletions have been described in patients with autosomal dominant progressive external ophthalmoplegia (AD-PEO) and in autosomal recessive disorders including mitochondrial neurogastrointestinal encephalomyopathy (MNGIE) and autosomal recessive cardiomyopathy ophthalmoplegia (ARCO). The pathogenic bases of these disorders are unknown. We studied three patients with AD-PEO and three patients with autosomal recessive (AR)-PEO (two patients with MNGIE and one patient with ARCO). Histochemistry and Southern blot analyses of DNA were performed in skeletal muscle from the patients. Muscle mtDNA was used to characterize the pattern and amounts of the multiple mtDNA rearrangements; PCR analysis was performed to obtain finer maps of the deleted regions in both conditions. The patients with AD-PEO had myopathic features; the patients with AR-PEO had multisystem disorders. The percentage of ragged-red and cytochrome c oxidase-negative fibers tended to be higher in muscle from the patients with AD-PEO (19% +/- 13.9, 29.7 +/- 26.3) than in muscle from the patients with AR-PEO (1.4% +/- 1.4, 3.3% +/- 3.2; p < 0.10). The sizes of the multiple mtDNA deletions ranged from approximately 4.0 to 10.0 kilobases in muscle from both groups of patients, and in both groups, we identified only deleted and no duplicated mtDNA molecules. Patients with AD-PEO harbored a greater proportion of deleted mtDNA species in muscle (31% +/- 5.3) than did patients with AR-PEO (9.7% +/- 9.1; p < 0.05). In the patients with AD-PEO, we identified a deletion that included the mtDNA heavy strand promoter (HSP) region, which had been previously described as the HSP deletion. The HSP deletion was not present in the patients with AR-PEO. Our findings show the clinical, histologic, and molecular genetic heterogeneity of these complex disorders. In particular, the proportions of multiple mtDNA deletions were higher in muscle samples from patients with AD-PEO than in those from patients with AR-PEO.

Adult↗
PubMed1998

Clinical manifestations of mitochondrial DNA depletion.

OBJECTIVE: We studied five new patients with mitochondrial DNA (mtDNA) depletion to better define the clinical spectrum of this disorder. BACKGROUND: mtDNA depletion has been associated with myopathy or hepatopathy, or both, in infants and young children. Involvement of the CNS and peripheral nervous system has not been clearly established. METHODS: We reviewed the clinical course and performed morphologic, biochemical, and genetic analyses of muscle samples from five patients. RESULTS: Age at onset ranged from 3 months to 5 years, and one patient survived until age 10 1/2 years. Two patients had laboratory and clinical features reminiscent of dystrophinopathy, two had evidence of brain involvement, and two had peripheral neuropathy. Muscle biopsy specimens in all patients showed abundant ragged-red fibers. Biochemistry showed cytochrome c oxidase deficiency in all patients tested and decreased activities of other respiratory chain complexes in some. CONCLUSIONS: Inheritance appeared to be autosomal recessive, suggesting that mutations in nuclear DNA are responsible for mtDNA depletion. mtDNA depletion should be considered in children with mitochondrial disorders of uncertain etiology, and criteria for diagnosis are proposed.

Blotting, Southern↗
PubMed1997

Apparent mtDNA heteroplasmy in Alzheimer's disease patients and in normals due to PCR amplification of nucleus-embedded mtDNA pseudogenes.

In an unprecedented finding, Davis et al. [Davis, R. E., Miller, S., Herrnstadt, C., Ghosh, S. S., Fahy, E., Shinobu, L. A., Galasko, D., Thal, L. J., Beal, M. F., Howell, N. & Parker, W. D., Jr. (1997) Proc. Natl. Acad. Sci. USA 94, 4526-4531] used an unusual DNA isolation method to show that healthy adults harbor a specific population of mutated mitochondrial cytochrome c oxidase (COX) genes that coexist with normal mtDNAs. They reported that this heteroplasmic population was present at a level of 10-15% in the blood of normal individuals and at a significantly higher level (20-30%) in patients with sporadic Alzheimer's disease. We provide compelling evidence that the DNA isolation method employed resulted in the coamplification of authentic mtDNA-encoded COX genes together with highly similar COX-like sequences embedded in nuclear DNA ("mtDNA pseudogenes"). We conclude that the observed heteroplasmy is an artifact.

Adult↗