The altered activity of complex III may contribute to the high penetrance of Leber's hereditary optic neuropathy in a Chinese family carrying the ND4 G11778A mutation

Abnormal morphology and function in retinal ganglion cells derived from patients-specific iPSCs generated from individuals with Leber's hereditary optic neuropathy

Leber's hereditary optic neuropathy (LHON) is a maternally inherited eye disease that results from degeneration of retinal ganglion cells (RGC). Mitochondrial ND4 11778G > A mutation, which affects structural components of complex I, is the most prevalent LHON-associated mitochondrial DNA (mtDNA) mutation worldwide. The m.11778G > A mutation is the primary contributor underlying the development of LHON and X-linked PRICKLE3 allele (c.157C > T, p.Arg53Trp) linked to biogenesis of ATPase interacts with m.11778G > A mutation to cause LHON. However, the lack of appropriate cell and animal models of LHON has been significant obstacles for deep elucidation of disease pathophysiology, specifically the tissue-specific effects. Using RGC-like cells differentiated from induced pluripotent stem cells (iPSCs) from members of one Chinese family (asymptomatic subjects carrying only m.11778G > A mutation or PRICKLE3 p.Arg53Trp mutation, symptomatic individuals bearing both m.11778G > A and PRICKLE3 p.Arg53Trp mutations and control lacking these mutations), we demonstrated the deleterious effects of mitochondrial dysfunctions on the morphology and functions of RGCs. Notably, iPSCs bearing only m.11778G > A or p.Arg53Trp mutation exhibited mild defects in differentiation to RGC-like cells. The RGC-like cells carrying only m.11778G > A or p.Arg53Trp mutation displayed mild defects in RGC morphology, including the area of soma and numbers of neurites, electrophysiological properties, ATP contents and apoptosis. Strikingly, those RGC-like cells derived from symptomatic individuals harboring both m.11778G > A and p.Arg53Trp mutations displayed greater defects in the development, morphology and functions than those in cells bearing single mutation. These findings provide new insights into pathophysiology of LHON arising from RGC deficiencies caused by synergy between m.11778G > A and PRICKLE3 p.Arg53Trp mutation.

Oxidative Stress in Optic Neuropathies

Increasing evidence indicates that changes in the redox system may contribute to the pathogenesis of multiple optic neuropathies. Optic neuropathies are characterized by the neurodegeneration of the inner-most retinal neurons, the retinal ganglion cells (RGCs), and their axons, which form the optic nerve. Often, optic neuropathies are asymptomatic until advanced stages, when visual impairment or blindness is unavoidable despite existing treatments. In this review, we describe systemic and, whenever possible, ocular redox dysregulations observed in patients with glaucoma, ischemic optic neuropathy, optic neuritis, hereditary optic neuropathies (i.e., Leber's hereditary optic neuropathy and autosomal dominant optic atrophy), nutritional and toxic optic neuropathies, and optic disc drusen. We discuss aspects related to anti/oxidative stress biomarkers that need further investigation and features related to study design that should be optimized to generate more valuable and comparable results. Understanding the role of oxidative stress in optic neuropathies can serve to develop therapeutic strategies directed at the redox system to arrest the neurodegenerative processes in the retina and RGCs and ultimately prevent vision loss.

PRICKLE3 linked to ATPase biogenesis manifested Leber's hereditary optic neuropathy

Leber's hereditary optic neuropathy (LHON) is a maternally inherited eye disease. X-linked nuclear modifiers were proposed to modify the phenotypic manifestation of LHON-associated mitochondrial DNA (mtDNA) mutations. By whole-exome sequencing, we identified the X-linked LHON modifier (c.157C>T, p.Arg53Trp) in PRICKLE3 encoding a mitochondrial protein linked to biogenesis of ATPase in 3 Chinese families. All affected individuals carried both ND4 11778G>A and p.Arg53Trp mutations, while subjects bearing only a single mutation exhibited normal vision. The cells carrying the p.Arg53Trp mutation exhibited defective assembly, stability, and function of ATP synthase, verified by PRICKLE3-knockdown cells. Coimmunoprecipitation indicated the direct interaction of PRICKLE3 with ATP synthase via ATP8. Strikingly, cells bearing both p.Arg53Trp and m.11778G>A mutations displayed greater mitochondrial dysfunction than those carrying only a single mutation. This finding indicated that the p.Arg53Trp mutation acted in synergy with the m.11778G>A mutation and deteriorated mitochondrial dysfunctions necessary for the expression of LHON. Furthermore, we demonstrated that Prickle3-deficient mice exhibited pronounced ATPase deficiencies. Prickle3-knockout mice recapitulated LHON phenotypes with retinal deficiencies, including degeneration of retinal ganglion cells and abnormal vasculature. Our findings provided new insights into the pathophysiology of LHON that were manifested by interaction between mtDNA mutations and X-linked nuclear modifiers.

Complex I mutations synergize to worsen the phenotypic expression of Leber's hereditary optic neuropathy

Leber's hereditary optic neuropathy (LHON) is a maternal inheritance of eye disease because of the mitochondrial DNA (mtDNA) mutations. We previously discovered a 3866T>C mutation within the gene for the ND1 subunit of complex I as possibly amplifying disease progression for patients bearing the disease-causing 11778G>A mutation within the gene for the ND4 subunit of complex I. However, whether and how the ND1 mutation exacerbates the ND4 mutation were unknown. In this report, we showed that four Chinese families bearing both m.3866T>C and m.11778G>A mutations exhibited higher penetrances of LHON than 6 Chinese pedigrees carrying only the m.3866T>C mutation or families harboring only the m.11778G>A mutation. The protein structure analysis revealed that the m.3866T>C (I187T) and m.11778G>A (R340H) mutations destabilized the specific interactions with other residues of ND1 and ND4, thereby altering the structure and function of complex I. Cellular data obtained using cybrids, constructed by transferring mitochondria from the Chinese families into mtDNA-less (ρ°) cells, demonstrated that the mutations perturbed the stability, assembly, and activity of complex I, leading to changes in mitochondrial ATP levels and membrane potential and increasing the production of reactive oxygen species. These mitochondrial dysfunctions promoted the apoptotic sensitivity of cells and decreased mitophagy. Cybrids bearing only the m.3866T>C mutation displayed mild mitochondrial dysfunctions, whereas those harboring both m.3866T>C and m.11778G>A mutations exhibited greater mitochondrial dysfunctions. These suggested that the m.3866T>C mutation acted in synergy with the m.11778G>A mutation, aggravating mitochondrial dysfunctions and contributing to higher penetrance of LHON in these families carrying both mtDNA mutations.

Contribution of mitochondrial ND1 3394T>C mutation to the phenotypic manifestation of Leber's hereditary optic neuropathy

Mitochondrial DNA (mtDNA) mutations have been associated with Leber's hereditary optic neuropathy (LHON) and their pathophysiology remains poorly understood. In this study, we investigated the pathophysiology of a LHON susceptibility allele (m.3394T>C, p.30Y>H) in the Mitochondrial (MT)-ND1 gene. The incidence of m.3394T>C mutation was 2.7% in the cohort of 1741 probands with LHON. Extremely low penetrances of LHON were observed in 26 pedigrees carrying only m.3394T>C mutation, while 21 families bearing m.3394T>C, together with m.11778G>A or m.14484T>C mutation, exhibited higher penetrance of LHON than those in families carrying single mtDNA mutation(s). The m.3394T>C mutation disrupted the specific electrostatic interactions between Y30 of p.MT-ND1 with the sidechain of E4 and backbone carbonyl group of M1 of NDUFA1 (NADH dehydrogenase [ubiquinone] 1 alpha subcomplex subunit 1) of complex I, thereby altering the structure and function of complex I. We demonstrated that these cybrids bearing only m.3394T>C mutation caused mild mitochondrial dysfunctions and those harboring both m.3394T>C and m.11778G>A mutations exhibited greater mitochondrial dysfunctions than cybrids carrying only m.11778G>A mutation. In particular, the m.3394T>C mutation altered the stability of p.MT-ND1 and complex I assembly. Furthermore, the m.3394T>C mutation decreased the activities of mitochondrial complexes I, diminished mitochondrial ATP levels and membrane potential and increased the production of reactive oxygen species in the cybrids. These m.3394T>C mutation-induced alterations aggravated mitochondrial dysfunctions associated with the m.11778G>A mutation. These resultant biochemical defects contributed to higher penetrance of LHON in these families carrying both mtDNA mutations. Our findings provide new insights into the pathophysiology of LHON arising from the synergy between mitochondrial ND1 and ND4 mutations.

Leber's hereditary optic neuropathy (LHON)-associated ND5 12338T > C mutation altered the assembly and function of complex I, apoptosis and mitophagy

Mutations in mitochondrial DNA (mtDNA) have been associated with Leber's hereditary optic neuropathy (LHON) and their pathophysiology remains poorly understood. In this study, we demonstrated that a missense mutation (m.12338T>C, p.1M>T) in the ND5 gene contributed to the pathogenesis of LHON. The m.12338T>C mutation affected the first methionine (Met1) with a threonine and shortened two amino acids of ND5. We therefore hypothesized that the mutated ND5 perturbed the structure and function of complex I. Using the cybrid cell models, generated by fusing mtDNA-less (ρ°) cells with enucleated cells from LHON patients carrying the m.12338T>C mutation and a control subject belonging to the same mtDNA haplogroup, we demonstrated that the m.12338T>C mutation caused the reduction of ND5 polypeptide, perturbed assemble and activity of complex I. Furthermore, the m.12338T>C mutation caused respiratory deficiency, diminished mitochondrial adenosine triphosphate levels and membrane potential and increased the production of reactive oxygen species. The m.12338T>C mutation promoted apoptosis, evidenced by elevated release of cytochrome c into cytosol and increased levels of apoptosis-activated proteins: caspases 9, 3, 7 and Poly ADP ribose polymerase in the cybrids carrying the m.12338T>C mutation, as compared with control cybrids. Moreover, we also document the involvement of m.12338T>C mutation in decreased mitophagy, as showed by reduced levels of autophagy protein light chain 3 and accumulation of autophagic substrate p62 in the in mutant cybrids as compared with control cybrids. These data demonstrated the direct link between mitochondrial dysfunction caused by complex I mutation and apoptosis or mitophagy. Our findings may provide new insights into the pathophysiology of LHON.

Leber's hereditary optic neuropathy caused by a mutation in mitochondrial tRNAThr in eight Chinese pedigrees

PURPOSE

The purpose of this study was to investigate the pathophysiology underlying Leber's hereditary optic neuropathy (LHON)-associated mitochondrial tRNA mutation.

METHODS

Severn hundred ninety-seven Han Chinese subjects underwent clinical and genetic evaluation and analysis of mitochondrial DNA (mtDNA). The cybrid cell lines were constructed by transferring mitochondria from lymphoblastoid cell lines derived from a Chinese family into mtDNA-less (ρ^o) cells. These cell lines were assayed by tRNA Northern blot and Western blot analyses, respiratory enzymatic activities, the rate of ATP production and the generation of reactive oxygen species.

RESULTS

The tRNA^Thr 15927G>A mutation was identified in eight probands with suggestively maternal inheritance among 352 Han Chinese probands lacking these known LHON-associated mtDNA mutations. The m.15927G>A mutation affected a highly conserved guanine at position 42 at the anticodon-stem of tRNA^Thr, destabilizing the conservative base pairing (28C-42G). We therefore hypothesized that the m.15927G>A mutation, and altered the structure and function of tRNA^Thr. Northern blot analysis revealed 60% decrease in the steady-state level of tRNA^Thr in the mutant cell lines. Western blot analysis showed the variable reductions of 4 mtDNA encoding proteins, especially for marked decrease of ND1 and CYTB observed in mutant cell lines. Furthermore, we demonstrated that the m.15927G>A mutation decreased the activities of mitochondrial complexes I and III, markedly diminished mitochondrial ATP levels, and increased the production of reactive oxygen species in the mutant cells.

CONCLUSIONS

Our data demonstrated the first mitochondrial tRNA mutation leading to LHON. Our findings may provide new insights into the understanding of pathophysiology of LHON.

Mitochondrial tRNAThr 15909A>G mutation associated with hypertension in a Chinese Han pedigree

Mitochondrial DNA mutations are one of the molecular genetic bases of hypertension. Here, we performed clinical, genetic and mutational evaluation, molecular characterization as well as biochemical analysis of a Chinese Han family with maternally inherited hypertension. The m.15909A > G variant in tRNA^Thr was identified. This mutation abolished a highly conserved base pairing (11U-24A) in the D-stem of tRNA^Thr and affected the structure and function of mitochondrial tRNA^Thr. As a result, the overall levels of mitochondrial translation products was decreased. The reduced mitochondrial protein synthesis resulted in the decrease in the activity of complex, and in turn, the production of ATP decreased and the generation of ROS increased. The m.15909A > G mutation maybe an inherited factor leading to the development of hypertension in this Chinese Han pedigree.

A novel ADOA-associated OPA1 mutation alters the mitochondrial function, membrane potential, ROS production and apoptosis

Autosomal dominant optic atrophy (ADOA) is a dominantly inherited optic neuropathy, affecting the specific loss of retinal ganglion cells (RGCs). The majority of affected cases of ADOA are associated with mutations in OPA1 gene. Our previous investigation identified the c.1198C > G (p.P400A) mutation in the OPA1 in a large Han Chinese family with ADOA. In this report, we performed a functional characterization using lymphoblostoid cell lines derived from affected members of this family and control subjects. Mutant cell lines exhibited the aberrant mitochondrial morphology. A ~24.6% decrease in the mitochondrial DNA (mtDNA) copy number was observed in mutant cell lines, as compared with controls. Western blotting analysis revealed the variable reductions (~45.7%) in four mtDNA-encoded polypeptides in mutant cell lines. The impaired mitochondrial translation caused defects in respiratory capacity. Furthermore, defects in mitochondrial ATP synthesis and mitochondrial membrane potential (ΔΨm) were observed in mutant cell lines. These abnormalities resulted in the accumulation of oxidative damage and increasing of apoptosis in the mutant cell lines, as compared with controls. All those alterations may cause the primary degeneration of RGCs and subsequent visual loss. These data provided the direct evidence for c.1198C > G mutation leading to ADOA. Our findings may provide new insights into the understanding of pathophysiology of ADOA.

Late onset nonsyndromic hearing loss in a Dongxiang Chinese family is associated with the 593T>C variant in the mitochondrial tRNAPhe gene

We report here the clinical, genetic, molecular and biochemical characterization of a four-generation Dongxiang Chinese pedigree with suggestively maternally transmitted non-syndromic hearing loss. Five of 10 matrilineal relatives exhibited variable severity and age at onset of sensorineural hearing loss. The average ages at onset of hearing loss in matrilineal relatives of this family were 29years. Molecular analysis of their mitochondrial genomes identified the tRNA^Phe 593T>C variant belonging to Asian haplogroup G2a2a. The m.593T>C variant resided at the position 17 of DHU-loop, where the position is important for the structure and function of tRNA. It was anticipated that the m.593T>C variant altered the structure and function of tRNA^Phe. By using lymphoblastoid cell lines derived from the Chinese family, we showed a 46% decreases in the steady-state level of tRNA^Phe in mutant cell lines. Western blotting analysis showed ∼35% reduction in the levels of mitochondrial translation in mutant cell lines carrying the m.593T>C variant. Impaired mitochondrial translation is apparently a primary contributor to the marked reduction in the rate of respiratory capacity. The respiratory deficiency lowed mitochondrial ATP production in the mutant cell lines. These data provide the evidence that mitochondrial dysfunctions caused by the m.593T>C variant lead to late-onset nonsyndromic hearing loss. Thus, our findings may provide the new insights into the understanding of pathophysiology and valuable information for management and treatment of maternally inherited hearing loss.

Biochemical Evidence for a Nuclear Modifier Allele (A10S) in TRMU (Methylaminomethyl-2-thiouridylate-methyltransferase) Related to Mitochondrial tRNA Modification in the Phenotypic Manifestation of Deafness-associated 12S rRNA Mutation

Nuclear modifier gene(s) was proposed to modulate the phenotypic expression of mitochondrial DNA mutation(s). Our previous investigations revealed that a nuclear modifier allele (A10S) in TRMU (methylaminomethyl-2-thiouridylate-methyltransferase) related to tRNA modification interacts with 12S rRNA 1555A→G mutation to cause deafness. The A10S mutation resided at a highly conserved residue of the N-terminal sequence. It was hypothesized that the A10S mutation altered the structure and function of TRMU, thereby causing mitochondrial dysfunction. Using molecular dynamics simulations, we showed that the A10S mutation introduced the Ser¹⁰ dynamic electrostatic interaction with the Lys¹⁰⁶ residue of helix 4 within the catalytic domain of TRMU. The Western blotting analysis displayed the reduced levels of TRMU in mutant cells carrying the A10S mutation. The thermal shift assay revealed the T_m value of mutant TRMU protein, lower than that of the wild-type counterpart. The A10S mutation caused marked decreases in 2-thiouridine modification of U34 of tRNA^Lys, tRNA^Glu and tRNA^Gln However, the A10S mutation mildly increased the aminoacylated efficiency of tRNAs. The altered 2-thiouridine modification worsened the impairment of mitochondrial translation associated with the m.1555A→G mutation. The defective translation resulted in the reduced activities of mitochondrial respiration chains. The respiratory deficiency caused the reduction of mitochondrial ATP production and elevated the production of reactive oxidative species. As a result, mutated TRMU worsened mitochondrial dysfunctions associated with m.1555A→G mutation, exceeding the threshold for expressing a deafness phenotype. Our findings provided new insights into the pathophysiology of maternally inherited deafness that was manifested by interaction between mtDNA mutation and nuclear modifier gene.

Biochemical evidence for a mitochondrial genetic modifier in the phenotypic manifestation of Leber's hereditary optic neuropathy-associated mitochondrial DNA mutation

Leber's hereditary optic neuropathy (LHON) is the most common mitochondrial disease. Mitochondrial modifiers are proposed to modify the phenotypic expression of primary LHON-associated mitochondrial DNA (mtDNA) mutations. In this study, we demonstrated that the LHON susceptibility allele (m.14502T > C, p. 58I > V) in the ND6 gene modulated the phenotypic expression of primary LHON-associated m.11778G > A mutation. Twenty-two Han Chinese pedigrees carrying m.14502T > C and m.11778G > A mutations exhibited significantly higher penetrance of optic neuropathy than those carrying only m.11778G > A mutation. We performed functional assays using the cybrid cell models, generated by fusing mtDNA-less ρ^o cells with enucleated cells from LHON patients carrying both m.11778G > A and m.14502T > C mutations, only m.14502T > C or m.11778G > A mutation and a control belonging to the same mtDNA haplogroup. These cybrids cell lines bearing m.14502T > C mutation exhibited mild effects on mitochondrial functions compared with those carrying only m.11778G > A mutation. However, more severe mitochondrial dysfunctions were observed in cell lines bearing both m.14502T > C and m.11778G > A mutations than those carrying only m.11778G > A or m.14502T > C mutation. In particular, the m.14502T > C mutation altered assemble of complex I, thereby aggravating the respiratory phenotypes associated with m.11778G > A mutation, resulted in a more defective complex I. Furthermore, more reductions in the levels of mitochondrial ATP and increasing production of reactive oxygen species were also observed in mutant cells bearing both m.14502T > C and m.11778G > A mutation than those carrying only 11778G > A mutation. Our findings provided new insights into the pathophysiology of LHON that were manifested by interaction between primary and secondary mtDNA mutations.

The exome sequencing identified the mutation in YARS2 encoding the mitochondrial tyrosyl-tRNA synthetase as a nuclear modifier for the phenotypic manifestation of Leber's hereditary optic neuropathy-associated mitochondrial DNA mutation

Leber's hereditary optic neuropathy (LHON) is the most common mitochondrial disorder. Nuclear modifier genes are proposed to modify the phenotypic expression of LHON-associated mitochondrial DNA (mtDNA) mutations. By using an exome sequencing approach, we identified a LHON susceptibility allele (c.572G>T, p.191Gly>Val) in YARS2 gene encoding mitochondrial tyrosyl-tRNA synthetase, which interacts with m.11778G>A mutation to cause visual failure. We performed functional assays by using lymphoblastoid cell lines derived from members of Chinese families (asymptomatic individuals carrying m.11778G>A mutation, or both m.11778G>A and heterozygous p.191Gly>Val mutations and symptomatic subjects harboring m.11778G>A and homozygous p.191Gly>Val mutations) and controls lacking these mutations. The 191Gly>Val mutation reduced the YARS2 protein level in the mutant cells. The aminoacylated efficiency and steady-state level of tRNA(Tyr) were markedly decreased in the cell lines derived from patients both carrying homozygous YARS2 p.191Gly>Val and m.11778G>A mutations. The failure in tRNA(Tyr) metabolism impaired mitochondrial translation, especially for polypeptides with high content of tyrosine codon such as ND4, ND5, ND6 and COX2 in cells lines carrying homozygous YARS2 p.191Gly>Val and m.11778G>A mutations. The YARS2 p.191Gly>Val mutation worsened the respiratory phenotypes associated with m.11778G>A mutation, especially reducing activities of complexes I and IV. The respiratory deficiency altered the efficiency of mitochondrial ATP synthesis and increased the production of reactive oxygen species. Thus, mutated YARS2 aggravates mitochondrial dysfunctions associated with the m.11778G>A mutation, exceeding the threshold for the expression of blindness phenotype. Our findings provided new insights into the pathophysiology of LHON that were manifested by interaction between mtDNA mutation and mutated nuclear-modifier YARS2.

A method for mutagenesis of mouse mtDNA and a resource of mouse mtDNA mutations for modeling human pathological conditions

Mutations in human mitochondrial DNA (mtDNA) can cause mitochondrial disease and have been associated with neurodegenerative disorders, cancer, diabetes and aging. Yet our progress toward delineating the precise contributions of mtDNA mutations to these conditions is impeded by the limited availability of faithful transmitochondrial animal models. Here, we report a method for the isolation of mutations in mouse mtDNA and its implementation for the generation of a collection of over 150 cell lines suitable for the production of transmitochondrial mice. This method is based on the limited mutagenesis of mtDNA by proofreading-deficient DNA-polymerase γ followed by segregation of the resulting highly heteroplasmic mtDNA population by means of intracellular cloning. Among generated cell lines, we identify nine which carry mutations affecting the same amino acid or nucleotide positions as in human disease, including a mutation in the ND4 gene responsible for 70% of Leber Hereditary Optic Neuropathies (LHON). Similar to their human counterparts, cybrids carrying the homoplasmic mouse LHON mutation demonstrated reduced respiration, reduced ATP content and elevated production of mitochondrial reactive oxygen species (ROS). The generated resource of mouse mtDNA mutants will be useful both in modeling human mitochondrial disease and in understanding the mechanisms of ROS production mediated by mutations in mtDNA.

Maternally inherited diabetes is associated with a homoplasmic T10003C mutation in the mitochondrial tRNA(Gly) gene

In this report, we investigate molecular pathogenic mechanism of a diabetes-associated homoplasmic mitochondrial tRNA mutation in a Han Chinese family with maternally transmitted diabetes mellitus. Of 10 adult matrilineal relatives, 5 individuals suffered from diabetes (4 subjects with only diabetes, one subject with both diabetes and hearing impairment), while other five matrilineal relatives (one with hearing loss) had glucose intolerance. The average age at onset of diabetes in matrilineal relatives was 50 years. Molecular analysis of their mitochondrial genomes identified the novel homoplasmic T10003C mutation in the tRNA(Gly) gene belonging to haplogroup M11b. The T10003C mutation is expected to form a base-pairing (13C-22G) at the highly conserved D-stem of tRNA(Gly), thereby affecting secondary structure and function of this tRNA. A tRNA Northern analysis revealed that the T10003C mutation caused ~70% reduction in the steady-state level of tRNA(Gly). An in vivo translation analysis showed ~33% reduction in the rate of mitochondrial translation in mutant cells. Oxygen consumption analysis showed the defects in overall respiratory capacity or the ATP-linked, proton leak, and maximal respiration in mutant cells. As a result, the cellular energy deficiency contributes to the development of diabetes in subjects carrying the T10003C mutation. These data provide the first direct evidence that the tRNA(Gly) mutation might be associated with diabetes. Thus, our findings may provide new insights into the understanding of pathophysiology of maternally inherited diabetes.

Profiling the mitochondrial proteome of Leber's Hereditary Optic Neuropathy (LHON) in Thailand: down-regulation of bioenergetics and mitochondrial protein quality control pathways in fibroblasts with the 11778G>A mutation

Leber's Hereditary Optic Neuropathy (LHON) is one of the commonest mitochondrial diseases. It causes total blindness, and predominantly affects young males. For the disease to develop, it is necessary for an individual to carry one of the primary mtDNA mutations 11778G>A, 14484T>C or 3460G>A. However these mutations are not sufficient to cause disease, and they do not explain the characteristic features of LHON such as the higher prevalence in males, incomplete penetrance, and relatively later age of onset. In order to explore the roles of nuclear encoded mitochondrial proteins in development of LHON, we applied a proteomic approach to samples from affected and unaffected individuals from 3 pedigrees and from 5 unrelated controls. Two-dimensional electrophoresis followed by MS/MS analysis in the mitochondrial lysate identified 17 proteins which were differentially expressed between LHON cases and unrelated controls, and 24 proteins which were differentially expressed between unaffected relatives and unrelated controls. The proteomic data were successfully validated by western blot analysis of 3 selected proteins. All of the proteins identified in the study were mitochondrial proteins and most of them were down regulated in 11778G>A mutant fibroblasts. These proteins included: subunits of OXPHOS enzyme complexes, proteins involved in intermediary metabolic processes, nucleoid related proteins, chaperones, cristae remodelling proteins and an anti-oxidant enzyme. The protein profiles of both the affected and unaffected 11778G>A carriers shared many features which differed from those of unrelated control group, revealing similar proteomic responses to 11778G>A mutation in both affected and unaffected individuals. Differentially expressed proteins revealed two broad groups: a cluster of bioenergetic pathway proteins and a cluster involved in protein quality control system. Defects in these systems are likely to impede the function of retinal ganglion cells, and may lead to the development of LHON in synergy with the primary mtDNA mutation.

Leber's hereditary optic neuropathy caused by the homoplasmic ND1 m.3635G>A mutation in nine Han Chinese families

In this report, we investigated the molecular mechanism underlying Leber's hereditary optic neuropathy (LHON)-associated mitochondrial m.3635G>A (p.S110N, ND1) mutation. A mutational screening of ND1 gene in a cohort of 1070 Han Chinese subjects LHON identified the m.3635G>A mutation in nine Chinese families with suggestively maternally transmitted LHON. Thirty-eight (22 males/16 females) of 162 matrilineal relatives in these families exhibited the variable severity and age-at-onset of optic neuropathy. Molecular analysis of their mitochondrial genomes identified the homoplasmic m.3635G>A mutation and distinct sets of polymorphisms belonging to the Asian haplogroups G2a1, R11a, D4, R11a, M7b2, G1a, F1a1, B4, and N9a3, respectively. Using cybrids constructed by transferring mitochondria from lymphoblastoid cell lines derived from one Chinese family into mtDNA-less (ρ(0)) cells, we showed ~27% decrease in the activity of NADH:ubiquinone oxidoreductase (complex I) in mutant cybrids carrying the m.3635G>A mutation, compared with control cybrids. The respiratory deficiency caused by the m.3635G>A mutation results in decreased efficiency of mitochondrial ATP synthesis. These mitochondrial dysfunctions caused an increase in the production of reactive oxygen species in the mutant cybrids. The data provide the direct evidence for the m.3635G>A mutation leading to LHON. Our findings may provide new insights into the understanding of pathophysiology of LHON.

Coronary heart disease is associated with a mutation in mitochondrial tRNA

Coronary heart disease (CHD) is the leading cause of death worldwide. Mitochondrial genetic determinant for the development of CHD remains poorly explored. We report there the clinical, genetic, molecular and biochemical characterization of a four-generation Chinese family with maternally inherited CHD. Thirteen of 32 adult members in this family exhibited variable severity and age-at-onset of CHD. Mutational analysis of their mitochondrial genomes identified the tRNA(Thr) 15927G>A mutation belonging to the Eastern Asian haplogroup B5. The anticipated destabilization of a highly conserved base-pairing (28C-42G) by the 15927G>A mutation affects structure and function of tRNA(Thr). Northern analysis revealed ≈80% decrease in the steady-state level of tRNA(Thr) in the mutant cell lines carrying the 15927G>A mutation. The 15927G>A mutation changed the conformation of tRNA(Thr), as suggested by slower electrophoretic mobility of mutated tRNA with respect to the wild-type molecule. In addition, ∼39% reduction in aminoacylated efficiency of tRNA(Thr) was observed in mutant cells derived from this Chinese family. An in vivo mitochondrial protein labeling analysis showed ∼53% reduction in the rate of mitochondrial translation in mutant cells. The impaired mitochondrial protein synthesis leads to defects in overall respiratory capacity or malate/glutamate-promoted respiration or succinate/glycerol-3-phosphate-promoted respiration, or N,N,N',N'-tetramethyl-pphenylenediamine/ascorbate-promoted respiration in mutant cells. An increasing production of reactive oxygen species was observed in the mutant cells carrying the 15927G>A mutation. These results provide the direct evidence that the tRNA(Thr) 15927G>A mutation is associated with CHD. Our findings may provide new insights into pathophysiology and intervention targets of this disorder.

Leber's hereditary optic neuropathy is associated with the T3866C mutation in mitochondrial ND1 gene in three Han Chinese Families

PURPOSE

To investigate the pathophysiology of Leber's hereditary optic neuropathy (LHON).

METHODS

Seventy-one subjects from three Chinese families with LHON underwent clinical, genetic, molecular, and biochemical evaluations. Biochemical characterizations included the measurements of the rates of endogenous, substrate-dependent respirations, the adenosine triphosphate (ATP) production and generation of reactive oxygen species using lymphoblastoid cell lines derived from five affected matrilineal relatives of these families and three control subjects.

RESULTS

Ten of 41 matrilineal relatives exhibited variable severity and age at onset of optic neuropathy. The average age at onset of optic neuropathy in matrilineal relatives of the three families was 5, 11, and 24 years, respectively. Molecular analysis identified the ND1 T3866C (I187T) mutation and distinct sets of polymorphisms belonging to the Eastern Asian haplogroups D4a, M10a, and R, respectively. The I187T mutation is localized at the highly conserved isoleucine at a transmembrane domain of the ND1 polypeptide. The marked reductions in the rate of endogenous, malate/glutamate-promoted and succinate/glycerol-3-phosphate-promoted respiration were observed in mutant cell lines carrying the T3866C mutation. The deficient respiration is responsible for the reduced ATP synthesis and increased generation of reactive oxygen species.

CONCLUSIONS

Our data convincingly show that the ND1 T3866C mutation leads to LHON. This mutation may be insufficient to produce a clinical phenotype. Other modifier factors may contribute to the phenotypic manifestation of the T3866C mutation. The T3866C mutation should be added to the list of inherited factors for molecular diagnosis of LHON. Thus, our findings may provide new insights into the understanding of pathophysiology and valuable information on the management of LHON.