An ND-6 mitochondrial DNA mutation associated with Leber hereditary optic neuropathy.

A mitochondrial DNA mutation at nucleotide position 14,484 was found in 14 independent probands with Leber hereditary optic neuropathy and in 0/250 controls. The 14,484 mutation, which changes methionine-64 to valine in a conserved domain of the ND-6 gene, occurred in association with a mitochondrial DNA haplotype that includes the 13,708 secondary mutation in 10/14 probands. An associated mutation at nucleotide position 3,394, which changes conserved tyrosine-30 to histidine in the ND-1 gene, was observed in 5/14 probands positive for the 14,484 mutation, all of whom harbored the same mitochondrial DNA haplotype. Multiple mitochondrial DNA mutations may interact in the pathogenesis of Leber hereditary optic neuropathy and the 13,708 secondary mutation appears to play a central role in this process.     

Genetic and biochemical impairment of mitochondrial complex I activity in a family with Leber hereditary optic neuropathy and hereditary spastic dystonia.

A rare form of Leber hereditary optic neuropathy (LHON) that is associated with hereditary spastic dystonia has been studied in a large Dutch family. Neuropathy and ophthalmological lesions were present together in some family members, whereas only one type of abnormality was found in others. mtDNA mutations previously reported in LHON were not present. Sequence analysis of the protein-coding mitochondrial genes revealed two previously unreported mtDNA mutations. A heteroplasmic A-->G transition at nucleotide position 11696 in the ND4 gene resulted in the substitution of an isoleucine for valine at amino acid position 312. A second mutation, a homoplasmic T-->A transition at nucleotide position 14596 in the ND6 gene, resulted in the substitution of a methionine for the isoleucine at amino acid residue 26. Biochemical analysis of a muscle biopsy revealed a severe complex I deficiency, providing a link between these unique mtDNA mutations and this rare, complex phenotype including Leber optic neuropathy.     

Leber hereditary optic neuropathy mutations in the ND6 subunit of mitochondrial complex I affect ubiquinone reduction kinetics in a bacterial model of the enzyme

LHON (Leber hereditary optic neuropathy) is a maternally inherited disease that leads to sudden loss of central vision at a young age. There are three common primary LHON mutations, occurring at positions 3460, 11778 and 14484 in the human mtDNA (mitochondrial DNA), leading to amino acid substitutions in mitochondrial complex I subunits ND1, ND4 and ND6 respectively. We have now examined the effects of ND6 mutations on the function of complex I using the homologous NuoJ subunit of Escherichia coli NDH-1 (NADH:quinone oxidoreductase) as a model system. The assembly level of the NDH-1 mutants was assessed using electron transfer from deamino-NADH to the 'shortcut' electron acceptor HAR (hexammine ruthenium), whereas ubiquinone reductase activity was determined using DB (decylubiquinone) as a substrate. Mutant growth in minimal medium with malate as the main carbon source was used for initial screening of the efficiency of energy conservation by NDH-1. The results indicated that NuoJ-M64V, the equivalent of the common LHON mutation in ND6, had a mild effect on E. coli NDH-1 activity, while nearby mutations, particularly NuoJ-Y59F, NuoJ-V65G and NuoJ-M72V, severely impaired the DB reduction rate and cell growth on malate. NuoJ-Met64 and NuoJ-Met72 position mutants lowered the affinity of NDH-1 for DB and explicit C-type inhibitors, whereas NuoJ-Y59C displayed substrate inhibition by oxidized DB. The results are compatible with the notion that the ND6 subunit delineates the binding cavity of ubiquinone substrate, but does not directly take part in the catalytic reaction. How these changes in the enzyme's catalytic properties contribute to LHON pathogenesis is discussed.     

Leber hereditary optic neuropathy: involvement of the mitochondrial ND1 gene and evidence for an intragenic suppressor mutation.

A large Queensland family has an extreme form of Leber hereditary optic neuropathy (LHON) in which several neurological abnormalities and an infantile encephalopathy are present in addition to the characteristic ophthalmological changes. Sequence analysis of the seven mitochondrial genes encoding subunits of respiratory chain complex I (NADH-ubiquinone oxidoreductase) reveals two novel features of the etiology of this mitochondrial genetic disease. The first conclusion from these studies is that the ophthalmological and neurological deficits in this family are produced by a mutation at nucleotide 4160 of the ND1 gene. This nucleotide alteration results in the substitution of proline for the highly conserved leucine residue at position 285 of the ND1 protein. Secondary-structure analysis predicts that the proline replacement disrupts a small alpha helix in a hydrophilic loop. All nine family members analyzed were homoplasmic for this mutation. The second major result from these studies is that the members of one branch of this family carry, at nucleotide 4136 of the same gene, a second mutation, also homoplasmic, which produces a cysteine-for-tyrosine replacement at position 277. The clinical and biochemical phenotypes of the family members indicate that this second nucleotide substitution may function as an intragenic suppressor mutation which ameliorates the neurological abnormalities and complex I deficiency.     

A mitochondrial DNA variant, identified in Leber hereditary optic neuropathy patients, which extends the amino acid sequence of cytochrome c oxidase subunit I.

A G-to-A transition at nucleotide pair (np) 7444 in the mtDNA was found to correlate with Leber hereditary optic neuropathy (LHON). The mutation eliminates the termination codon of the cytochrome c oxidase subunit I (COI) gene, extending the COI polypeptide by three amino acids. The mutation was discovered as an XbaI restriction-endonuclease-site loss present in 2 (9.1%) of 22 LHON patients who lacked the np 11778 LHON mutation and in 6 (1.1%) of 545 unaffected controls. The mutant polypeptide has an altered mobility on SDS-PAGE, suggesting a structural alteration, and the cytochrome c oxidase enzyme activity of patient lymphocytes is reduced approximately 40% relative to that in controls. These data suggest that the np 7444 mutation results in partial respiratory deficiency and thus contributes to the onset of LHON.