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Michio Hirano

Publications and source records attributed to Michio Hirano.

63 records · Page 4Linked to original sources
PubMed2002

Pathogenesis of the deafness-associated A1555G mitochondrial DNA mutation.

The pathogenic mechanisms of the A1555G mitochondrial DNA mutation in the 12S rRNA gene, associated with maternally inherited sensorineural deafness, are largely unknown. Previous studies have suggested an involvement of nuclear factor(s). To address this issue cybrids were generated by fusing osteosarcoma cells devoid of mtDNA with enucleated fibroblasts from two genetically unrelated patients. Furthermore, to determine the contribution, if any, of the mitochondrial and nuclear genomes, separately or in combination, in the expression of the disease phenotype, transmitochondrial fibroblasts were constructed using control and patient's fibroblasts as nuclear donors and homoplasmic mutant or wild-type cybrids as mitochondrial donors. Detailed analysis of mutant and wild-type cybrids from both patients and transmitochondrial fibroblast clones did not reveal any respiratory chain dysfunction suggesting that, if nuclear factors do indeed act as modifier agents, they may be tissue-specific. However, in the presence of high concentrations of neomycin or paromomycin, but not of streptomycin, mutant cells exhibit a decrease in the growth rate, when compared to wild-type cells. The decrease did not correlate with the rate of synthesis or stability of mitochondrial DNA-encoded subunits or respiratory chain activity. Further studies are required to determine the underlying biochemical defect.

Adult↗
PubMed2002

Mitochondrial DNA depletion and dGK gene mutations.

Mitochondrial DNA depletion syndrome is a clinically heterogeneous group of disorders characterized by a reduction in mitochondrial DNA copy number. The recent discovery of mutations in the deoxyguanosine kinase (dGK) gene in patients with the hepatocerebral form of mitochondrial DNA depletion syndrome prompted us to screen 21 patients to determine the frequency of dGK mutations, further characterize the clinical spectrum, and correlate genotypes with phenotypes. We detected mutations in three patients (14%). One patient had a homozygous GATT duplication (nucleotides 763-766), and another had a homozygous GT deletion (nucleotides 609-610); both mutations lead to truncated proteins. The third patient was a compound heterozygote for two missense mutations (R142K and E227K) that affect critical residues of the protein. These mutations were associated with variable phenotypes, and their low frequencies suggests that dGK is not the only gene responsible for mitochondrial DNA depletion in liver. The patient with the missense mutations had isolated liver failure and responded well to liver transplantation, which may be a therapeutic option in selected cases.

Amino Acid Sequence↗
PubMed2002

Mitochondrial diseases.

Since the first reports of disorders associated with mitochondrial DNA (mtDNA) defects more than a decade ago, the small mtDNA circle has been a Pandora's box of pathogenic mutations associated with human diseases. The "morbidity map" of mtDNA has gone from one point mutation and a few deletions in 1988 to more than 110 point mutations as of September, 2001. Nuclear DNA defects affecting mitochondrial function and mtDNA replication and integrity have also been identified in the past few years and more are expected. As a result, human "mitochondrial" diseases have evolved beyond the novelty diagnoses of a decade ago into an important area of medicine, and thus, the diagnostic principles of these disorders ought to be familiar to the clinician. In this article, the authors, we summarize the principles of mitochondrial genetics and discuss the common phenotypes, general diagnostic approach, and possible therapeutic venues for these fascinating disorders.

Adult↗
PubMed2002

Mitochondrial neurogastrointestinal encephalomyopathy and thymidine metabolism: results and hypotheses.

Mitochondrial neurogastrointestinal encephalomyopathy (MNGIE) is an autosomal recessive disease with mitochondrial DNA (mtDNA) alterations and is caused by mutations in the nuclear gene encoding thymidine phosphorylase (TP). The cardinal clinical manifestations are ptosis, ophthalmoparesis, gastrointestinal dysmotility, cachexia, peripheral neuropathy, and leukoencephalopathy. Skeletal muscle shows mitochondrial abnormalities, including ragged-red fibers and cytochrome c oxidase deficiency, together with mtDNA depletion, multiple deletions or both. In MNGIE patients, TP mutations cause a loss-of-function of the cytosolic enzyme, TP. As a direct consequence of the TP defect, thymidine metabolism is altered. High blood levels of this nucleoside are likely to lead to mtDNA defects even in cells that do not express TP, such as skeletal muscle. We hypothesize that high concentrations of thymidine affect dNTP (deoxyribonucleoside triphosphate) metabolism in mitochondria more than in cytosol or nuclei, because mitochondrial dNTPs depend mainly on the thymidine salvage pathway, whereas nuclear dNTPs depend mostly on de novo pathway. The imbalance in the mitochondrial dNTP homeostasis affects mtDNA replication, leading to mitochondrial dysfunction.

Journal Article↗
PubMed2001

Altered thymidine metabolism due to defects of thymidine phosphorylase.

Mitochondrial neurogastrointestinal encephalomyopathy (MNGIE) is an autosomal recessive human disease due to mutations in the thymidine phosphorylase (TP) gene. TP enzyme catalyzes the reversible phosphorolysis of thymidine to thymine and 2-deoxy-D-ribose 1-phosphate. We present evidence that thymidine metabolism is altered in MNGIE. TP activities in buffy coats were reduced drastically in all 27 MNGIE patients compared with 19 controls. All MNGIE patients had much higher plasma levels of thymidine than normal individuals and asymptomatic TP mutation carriers. In two patients, the renal clearance of thymidine was approximately 20% that of creatinine, and because hemodialysis demonstrated that thymidine is ultrafiltratable, most of the filtered thymidine is likely to be reabsorbed by the kidney. In vitro, fibroblasts from controls catabolized thymidine in medium; by contrast, MNGIE fibroblasts released thymidine. In MNGIE, severe impairment of TP enzyme activity leads to increased plasma thymidine. In patients who are suspected of having MNGIE, determination of TP activity in buffy coats and thymidine levels in plasma are diagnostic. We hypothesize that excess thymidine alters mitochondrial nucleoside and nucleotide pools leading to impaired mitochondrial DNA replication, repair, or both. Therapies to reduce thymidine levels may be beneficial to MNGIE patients.

Cells, Cultured↗