The spectrum of mitochondrial DNA mutations in families with Leber hereditary optic neuroretinopathy

The mitochondrial complex I genes were sequenced in seven Leber hereditary optic neuroretinopathy (LHON) families without the ND4/11778 and ND1/3460 mutations. Four replacement mutations restricted only to LHON families were found, one in the ND1 gene at nt 4025, and three in the ND5 gene at nt 12811, 13637, and 13967. The mutations did not change evolutionarily conserved amino acids suggesting that they are not primary LHON mutations in these families. They may be considered as secondary LHON mutations serving as exacerbating factors in an appropriate genetic background. A complex III mutation, cyt b/15257, has been suggested to be one of the primary mutations causing LHON. Its presence was determined for 23 Finnish LHON families, and it was detected in two families harboring the ND4/11778 mutation. Similarly, complex IV mutation COI/7444 was screened in Finnish LHON families, and it was found in one family carrying the ND1/3460 mutation.     

A variant of Leber hereditary optic neuropathy characterized by recovery of vision and by an unusual mitochondrial genetic etiology.

The Tas2 and Vic2 Australian families are affected with a variant of Leber hereditary optic neuropathy (LHON). The risk of developing the optic neuropathy shows strict maternal inheritance, and the ophthalmological changes in affected family members are characteristic of LHON. However, in contrast to the common form of the disease, members of these two families show a high frequency of vision recovery. To ascertain the mitochondrial genetic etiology of the LHON in these families, both (a) the the nucleotide sequences of the seven mitochondrial genes encoding subunits of respiratory-chain complex I and (b) the mitochondrial cytochrome b gene were determined for representatives of both families. Neither family carries any of the previously identified primary mitochondrial LHON mutations: ND4/11778, ND1/3460, or ND1/4160. Instead, both LHON families carry multiple nucleotide changes in the mitochondrial complex I genes, which produce conservative amino acid changes. From the available sequence data, it is inferred that the Vic2 and Tas2 LHON families are phylogenetically related to each other and to a cluster of LHON families in which mutations in the mitochondrial cytochrome b gene have been hypothesized to play a primary etiological role. However, sequencing analysis establishes that the Vic2 and Tas2 LHON families do not carry these cytochrome b mutations. There are two hypotheses to account for the unusual mitochondrial genetic etiology of the LHON in the Tas2 and Vic2 LHON families. One possibility is that there is a primary LHON mutation within the mitochondrial genome but that it is at a site that was not included in the sequencing analyses. Alternatively, the disease in these families may result from the cumulative effects of multiple secondary LHON mutations that have less severe phenotypic consequences.     

An example of Leber hereditary optic neuropathy not involving a mutation in the mitochondrial ND4 gene.

A large Australian family afflicted with Leber's Hereditary Optic Neuropathy (LHON) is analyzed at the nucleotide sequence level in this report. Biochemical assays of platelet mitochondria isolated from members of this family have demonstrated a significant decrease in the specific activity of Complex I (NADH-ubiquinol oxidoreductase) of the electron transport chain. It is shown here, however, that neither this biochemical lesion nor the optic neuropathy are due to the mutation at nucleotide position 11,778 of the mitochondrial ND4 gene first identified by Wallace et al. in several LHON pedigrees. Furthermore, extensive DNA sequencing studies reveal no candidate mutations within the mitochondrial ND3 gene, the ND4L/ND4 genes, or the contiguous tRNA genes. These studies provide the first direct evidence that not all LHON lineages--even those associated with a biochemical defect in mitochondrial respiratory chain Complex I--carry a mutation in the ND4 gene. Members of the Australian LHON family exhibit neurological abnormalities in addition to the well-characterized ophthalmological changes. It is hypothesized that LHON may be a syndrome or set of related diseases in which the clinical abnormalities are a function, at least in part, of the mitochondrial Complex I gene in which the proximate mutation occurs.     

A new mtDNA mutation associated with Leber hereditary optic neuroretinopathy.

A single base mutation at nucleotide position 3460 (nt 3460) in the ND1 gene in human mtDNA was found to be associated with Leber hereditary optic neuroretinopathy (LHON). The G-to-A mutation converts an alanine to a threonine at the 52d codon of the gene. The mutation also abolishes an AhaII restriction site and thus can be detected easily by RFLP analysis. The mutation was found in three independent Finnish LHON families but in none of the 60 controls. None of the families with the nt 3460 mutation in ND1 had the previously reported nt 11778 mutation in the ND4 gene. The G-to-A change at nt 3460 is the second mutation so far detected in LHON.     

The mitochondrial tRNA(Glu) A14693G mutation may influence the phenotypic manifestation of ND1 G3460A mutation in a Chinese family with Leber's hereditary optic neuropathy

We report here the clinical, genetic, and molecular characterization of one Han Chinese family with maternally transmitted Leber's hereditary optic neuropathy (LHON). Three of seven matrilineal relatives in this family exhibited the variable degree of central vision loss at the age of 12, 14, and 16 years old, respectively. Sequence analysis of the complete mitochondrial DNA in this pedigree revealed the presence of the ND1 G3460A mutation and 47 other variants, belonging to the Asian haplogroup M7b2. The G3460A mutation is present at homoplasmy in matrilineal relatives of this Chinese family. Of other variants, the homoplasmic A14693G mutation is of special interest as it was implicated to be associated with other mitochondrial disorders. This mutation is located at the TpsiC-loop, at conventional position 54 of tRNA(Glu). The uridine at this position (U54), which is highly conserved from bacteria to human mitochondria, has been implicated to be important for tRNA structure and function. Thus, the A14693G mutation may alter the tertiary structure of this tRNA, cause a failure in this tRNA metabolism, thereby worsening the mitochondrial dysfunction associated with the primary G3460A mutation. Therefore, the tRNA(Glu) A14693G mutation may have a potential modifier role in the phenotypic manifestation of the primary LHON-associated G3460A mutation in this Chinese family.     

Cosegregation of the ND4 G11696A mutation with the LHON-associated ND4 G11778A mutation in a four generation Chinese family

We report here the characterization of a four-generation Han Chinese family with Leber's hereditary optic neuropathy (LHON). This Chinese family exhibited a variable severity and age-at-onset of visual loss. Notably, the average age-at-onset of vision impairment changed from 26 years (generation III) to 14 years (generation IV), with the average of 18 years in this family. In addition, 30% and 50% of matrilineal relatives in generation III and IV of this family developed visual loss with a variability of severity, ranging from blindness to normal vision. Sequence analysis of the complete mitochondrial DNA in this pedigree revealed the presence of the homoplasmic ND4 G11778A mutation and 33 other variants, belonging to the Asian haplogroup D4. Of other variants, the homoplasmic G11696A mutation in the ND4 gene is of special interest as it was implicated to be associated with LHON in a large Dutch family and five Chinese pedigrees with extremely penetrance of visual loss. In fact, the G11696A mutation caused the substitution of an isoleucine for valine at amino acid position 313, located in a predicted transmembrane region of ND4. These imply that the G11696A mutation may act in synergy with the primary LHON-associated G11778A mutation in this Chinese pedigree.     

线粒体ND1基因T3866C突变可能是Leber’s遗传性视神经病和四肢畸形跛行相关的突变

线粒体DNA(Mitochondrial DNA,mtDNA)突变与人类许多疾病的发病机制相关。现报道1个具有典型母系遗传特征的中国人Leber’s遗传性视神经病和四肢畸形跛行的家系。该家系共5代60人,共27名母系成员,其中4人只有Leber’s遗传性视神经病症状,1人呈现四肢畸形跛行症状,4人同时具有上述两种临床症状,而其他成员无临床症状。对先证者的mtDNA全序列进行分析,发现ND1基因T3866C突变位点和43个多态位点,经系统进化树分析属于东亚单体型D4a3。MtDNAND13866位点T-C碱基的改变使ND1亚基第187位进化高度保守的异亮氨酸转变为苏氨酸,从而改变该蛋白的结构,进而影响其功能。在135名正常对照中未发现该突变。因此,线粒体ND1T3866C可能是与Leber’s遗传性视神经病和四肢畸形跛行相关的线粒体基因突变。     

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: 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.     

The role of mtDNA background in disease expression: a new primary LHON mutation associated with Western Eurasian haplogroup J

Leber's hereditary optic neuropathy (LHON) is a maternally transmitted form of blindness caused by mitochondrial DNA (mtDNA) mutations. Approximately 90% of LHON cases are caused by 3460A, 11778A, or 14484C mtDNA mutations. These are designated "primary" mutations because they impart a high risk for LHON expression. Although the 11778A and 14484C mutations unequivocally predispose carriers to LHON, they are preferentially associated with mtDNA haplogroup J, one of nine Western Eurasian mtDNA lineages, suggesting a synergistic and deleterious interaction between these LHON mutations and haplogroup J polymorphism(s). We report here the characterization of a new primary LHON mutation in the mtDNA ND4L gene at nucleotide pair 10663. The homoplasmic 10663C mutation has been found in three independent LHON patients who lack a known primary mutation and all of which belong to haplogroup J. This mutation has not been found in a large number of haplotype-matched or non-haplogroup-J control mtDNAs. Phylogenetic analysis with primarily complete mtDNA sequence data demonstrates that the 10663C mutation has arisen at least three independent times in haplogroup J, indicating that it is not a rare lineage-specific polymorphism. Analysis of complex I function in patient lymphoblasts and transmitochondrial cybrids has revealed a partial complex I defect similar in magnitude to the 14484C mutation. Thus, the 10663C mutation appears to be a new primary LHON mutation that is pathogenic when co-occurring with haplogroup J. These results strongly support a role for haplogroup J in the expression of certain LHON mutations.     

Leber's hereditary optic neuropathy: no significant evidence for primary or secondary pathogenicity of the 15257 mutation

Leber's hereditary optic neuropathy (LHON) is a maternally inherited disease of the optic nerves associated with various mitochondrial DNA (mtDNA) mutations. Four of these mutations, at nucleotide positions (np) 3460, 11778, 14484 and 15257, have been postulated to be of primary pathogenetical importance. Previously, we described the molecular and clinical findings in patients with the 11778 and 14484 mutations. Here we describe the molecular and clinical findings of patients in eight pedigrees with the 3460 mutation and in three pedigrees with the 15 257 mutation. In all three 15257 positive pedigrees the 3460, the 11778 or the 14484 mutation was also found. The first combination has not been reported before. We compared the clinical findings in these pedigrees with those of the 3460, 11778 and 14484 positive pedigrees that lack the 15257 mutation. No significant differences were found with respect to the age of onset, visual outcome or the probability of developing LHON. We conclude that there is no evidence that the 15257 mutation, which has been reported in normal controls, has primary causal significance, because it may coincide with the 3460, 11778 and 14484 mutations. We presume that the 15257 mutation has no secondary pathogenic importance, since it has no clear contribution to the degree or the probability of phenotypic expression.     

Functional analysis of lymphoblast and cybrid mitochondria containing the 3460, 11778, or 14484 Leber's hereditary optic neuropathy mitochondrial DNA mutation

Leber's hereditary optic neuropathy (LHON) is a form of blindness caused by mitochondrial DNA (mtDNA) mutations in complex I genes. We report an extensive biochemical analysis of the mitochondrial defects in lymphoblasts and transmitochondrial cybrids harboring the three most common LHON mutations: 3460A, 11778A, and 14484C. Respiration studies revealed that the 3460A mutation reduced the maximal respiration rate 20-28%, the 11778A mutation 30-36%, and the 14484C mutation 10-15%. The respiration defects of the 3460A and 11778A mutations transferred in cybrid experiments linking these defects to the mtDNA. Complex I enzymatic assays revealed that the 3460A mutation resulted in a 79% reduction in specific activity and the 11778A mutation resulted in a 20% reduction, while the 14484C mutation did not affect the complex I activity. The enzyme defect of the 3460A mutation transferred with the mtDNA in cybrids. Overall, these data support the conclusion that the 3460A and 11778A mutants result in complex I defects and that the 14484C mutation causes a much milder biochemical defect. These studies represent the first direct comparison of oxidative phosphorylation defects among all of the primary LHON mtDNA mutations, thus permitting insight into the underlying pathophysiological mechanism of the disease.     

Clinical evaluation and mitochondrial DNA sequence analysis in three Chinese families with Leber's hereditary optic neuropathy

We report here the clinical, genetic, and molecular characterization of three Chinese families (WZ4, WZ5, and WZ6) with Leber's hereditary optic neuropathy (LHON). Clinical and genetic evaluations revealed the variable severity and age-of-onset in visual impairment in these families. Penetrances of visual impairment in these Chinese families were 33.3%, 35.7%, and 35.5%, respectively, with an average 34.8%. Furthermore, the average age-at-onset in those Chinese families was 17, 20, and 18 years. In addition, the ratios between affected male and female matrilineal relatives in these Chinese families were 3:0, 1:1, and 1.2:1, respectively. Sequence analysis of the complete mitochondrial genomes in these pedigrees showed the distinct sets of mtDNA polymorphism, in addition to the identical G11778A mutation associated with LHON in many families. The fact that mtDNA of those pedigrees belonged to different haplogroups F1, D4, and M10 suggested that the G11778A mutation occurred sporadically and multiplied through evolution of the mtDNA in China. However, there was the absence of functionally significant mutations in tRNA and rRNAs or secondary LHON mutations in these Chinese families. The I187T mutation in the ND1, the S99A mutation in the A6, the V254I in CO3, and I58V in ND6 mutation, showing high evolutional conservation, may contribute to the phenotypic expression of the G11778A mutation in the WZ6 pedigree. By contrast, none of mtDNA variants are evolutionarily conserved and implicated to have significantly functional consequence in WZ4 and WZ5 pedigrees. Apparently, these variants do not have a potential modifying role in the development of visual impairment associated with G11778A mutation in those two families. Thus, nuclear modifier gene(s) or environmental factor(s) seem to account for the penetrance and expressivity of LHON in these three Chinese families carrying the G11778A mutation.     

Leber's hereditary optic neuropathy (LHON) pathogenic mutations induce mitochondrial-dependent apoptotic death in transmitochondrial cells incubated with galactose medium

Leber's hereditary optic neuropathy (LHON), a maternally inherited form of central vision loss, is associated with mitochondrial DNA pathogenic point mutations affecting different subunits of complex I. We here report that osteosarcoma-derived cytoplasmic hybrids (cybrid) cell lines harboring one of the three most frequent LHON pathogenic mutations, at positions 11778/ND4, 3460/ND1, and 14484/ND6, undergo cell death when galactose replaces glucose in the medium, contrary to control cybrids that maintain some growth capabilities. This is a well known way to produce a metabolic stress, forcing the cells to rely on the mitochondrial respiratory chain to produce ATP. We demonstrate that LHON cybrid cell death is apoptotic, showing chromatin condensation and nuclear DNA laddering. Moreover, we also document the mitochondrial involvement in the activation of the apoptotic cascade, as shown by the increased release of cytochrome c into the cytosol in LHON cybrid cells as compared with controls. Cybrids bearing the 3460/ND1 and 14484/ND6 mutations seemed more readily prone to undergo apoptosis as compared with the 11778/ND4 mutation. In conclusion, LHON cybrid cells forced by the reduced rate of glycolytic flux to utilize oxidative metabolism are sensitized to an apoptotic death through a mechanism involving mitochondria.     

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's hereditary optic neuropathy is associated with the mitochondrial ND4 G11696A mutation in five Chinese families

We report here the clinical, genetic, and molecular characterization of five Chinese families with Leber's hereditary optic neuropathy (LHON). Clinical and genetic evaluations revealed the variable severity and age-of-onset in visual impairment in these families. Strikingly, there were extremely low penetrances of visual impairment in these Chinese families. Sequence analysis of the complete mitochondrial genomes in these pedigrees showed the distinct sets of mtDNA polymorphism, in addition to the identical ND4 G11696A mutation associated with LHON. Indeed, this mutation is present in homoplasmy only in the maternal lineage of those pedigrees but not other members of these families. In fact, the occurrence of the G11696A mutation in these several genetically unrelated subjects affected by visual impairment strongly indicates that this mutation is involved in the pathogenesis of visual impairment. Furthermore, the N405D in the ND5 and G5820A in the tRNA(Cys), showing high evolutional conservation, may contribute to the phenotypic expression of G11696A mutation in the WZ10 pedigree. However, there was the absence of functionally significant mtDNA mutations in other four Chinese pedigrees carrying the G11696A mutation. Therefore, nuclear modifier gene(s) or environmental factor(s) may play a role in the phenotypic expression of the LHON-associated G11696A mutation in these Chinese pedigrees.     

Electron transfer properties of NADH:ubiquinone reductase in the ND1/3460 and the ND4/11778 mutations of the Leber hereditary optic neuroretinopathy (LHON)

We report the electron transfer properties of the NADH:ubiquinone oxidoreductase complex of the respiratory chain (Complex I) in mitochondria of cells derived from LHON patients with two different mutations in mitochondrial DNA (mtDNA). The mutations occur in the mtDNA genes coding for the ND1 and ND4 subunits of Complex I. The ND1/3460 mutation exhibits 80% reduction in rotenone-sensitive and ubiquinone-dependent electron transfer activity, whereas the proximal NADH dehydrogenase activity of the Complex is unaffected. This is in accordance with the proposal that the ND1 subunit interacts with rotenone and ubiquinone. In contrast, the ND4/11778 mutation had no effect on electron transfer activity of the Complex in inner mitochondrial membrane preparations; also Km for NADH and NADH dehydrogenase activity were unaffected. However, in isolated mitochondria with the ND4 mutation, the rate of oxidation of NAD-linked substrates, but not of succinate, was significantly decreased. This suggests that the ND4 subunit might be involved in specific aggregation of NADH-dependent dehydrogenases and Complex I, which may result in fast ('solid state') electron transfer from the former to the latter.     

Quantification of point mutations associated with Leber hereditary optic neuroretinopathy by solid-phase minisequencing

About two-thirds of patients with Leber hereditary optic neuroretinopathy (LHON) harbor mutations in mitochondrial DNA at positions 11778 (ND4) or 3460 (ND1). Thus, the clinical diagnosis of LHON can often be confirmed with mutation analysis. Detection of pathogenic mutations and quantification of heteroplasmy has mainly relied on PCR and restriction site analysis and densitometric scanning. We applied the recently developed solid-phase minisequencing method, based on primerguided nucleotide incorporation, to the simultaneous detection and quantitation of the ND4/11778 and ND1/3460 mutations. The method was highly sensitive, heteroplasmy as low as 1.5% being easily detected. Rapid, reproducible, and accurate results prove solid-phase minisequencing to be the method of choice for quantitative analysis of LHON mutations.     

Sequence analysis of the complete mitochondrial genome in patients with Leber's hereditary optic neuropathy lacking the three most common pathogenic DNA mutations

Leber's hereditary optic neuropathy (LHON) is a maternally inherited disorder characterized by central vision loss in young adults. The majority of LHON cases around the world are associated with mutations in the mitochondrial genome at nucleotide positions (np) 3460, 11,778, and 14,484. Usually, these three mutations are screened in suspected LHON patients. The result is important not only in respect to the diagnosis but also as different LHON mutations lead to variations in expression, severity, and recovery of the disease. There are, however, a significant number of patients without any of these primary mutations. In these situations, genetic counselling of a patient and his family can be difficult. We sequenced the complete mitochondrial DNA (mtDNA) in 14 LHON patients with the typical clinical features but without a primary mtDNA mutation to evaluate the potential of extensive mutation screening for clinical purposes. Our results suggest to include the mutation at np 15,257 in a routine screening as well as the ND6 gene, a hot spot for LHON mutations. Screening for the secondary LHON mutations at np 4216 and np 13,708 may also help in making the diagnosis of LHON as these seem to modify the expression of LHON mutations. Although they do not allow to prove the clinical diagnosis, their presence increases the probability of LHON. Sequencing the complete mitochondrial genome can reveal novel and known rare disease causing mutations. However, considering the effort it adds little value for routine screening.     

The novel G10680A mutation is associated with complete penetrance of the LHON/T14484C family

We report the clinical and genetic characterization of a Chinese Leber′s hereditary optic neuropathy (LHON) family with complete penetrance and high percentage of recovery. Sequence analysis of the complete mitochondrial DNA revealed the presence of heteroplasmic ND6/T14484C mutation and 27 other variants, belonging to the East-Asian haplogroup B4b′d. Of those variants, a novel homoplasmic G10680A mutation substituted a threonine for a highly conserved alanine at ND4L amino acid 71, which was not found in unaffected family members and 100 normal controls. It indicated that G10680A may play a synergistic role with the primary mutation T14484C, leading to the complete penetrance of LHON in the presenting family. In addition, the other modifier factors including nuclear background, mitochondrial haplotypes and other environmental factors should account for the phenotypic variability of visual impairment in this family.