Leber hereditary optic neuropathy: how do mitochondrial DNA mutations cause degeneration of the optic nerve?

Leber's hereditary optic neuropathy: Current approaches and future perspectives on Mesenchymal stem cell-mediated rescue

Leber's Hereditary Optic Neuropathy (LHON) is an inherited optic nerve disorder. It is a mitochondrially inherited disease due to point mutation in the MT-ND1, MT-ND4, and MT-ND6 genes of mitochondrial DNA (mtDNA) coding for complex I subunit proteins. These mutations affect the assembly of the mitochondrial complex I and hence the electron transport chain leading to mitochondrial dysfunction and oxidative damage. Optic nerve cells like retinal ganglion cells (RGCs) are more sensitive to mitochondrial loss and oxidative damage which results in the progressive degeneration of RGCs at the axonal region of the optic nerve leading to bilateral vision loss. Currently, gene therapy using Adeno-associated viral vector (AAV) is widely studied for the therapeutics application in LHON. Our review highlights the application of cell-based therapy for LHON. Mesenchymal stem cells (MSCs) are known to rescue cells from the pre-apoptotic stage by transferring healthy mitochondria through tunneling nanotubes (TNT) for cellular oxidative function. Empowering the transfer of healthy mitochondria using MSCs may replace the mitochondria with pathogenic mutation and possibly benefit the cells from progressive damage. This review discusses the ongoing research in LHON and mitochondrial transfer mechanisms to explore its scope in inherited optic neuropathy.

Intereye Comparison of the Characteristics of the Peripapillary Choroid in Patients with Unilateral Normal-Tension Glaucoma

PURPOSE

To compare the characteristics of the peripapillary choroid (PPC) between the 2 eyes of patients with unilateral treatment-naïve normal-tension glaucoma (NTG).

DESIGN

Observational case series.

PARTICIPANTS

Sixty-nine patients (138 eyes) with treatment-naïve unilateral NTG.

METHODS

The characteristics of PPC vasculature were evaluated by measuring PPC thickness and assessing the presence of parapapillary deep-layer microvasculature dropout (MvD). Peripapillary choroid thickness was measured by enhanced depth imaging OCT. Microvasculature dropout was assessed using OCT angiography. The area and maximum radial width of β-zone parapapillary atrophy (PPA) were measured on infrared images using the built-in caliper tool of the Spectralis OCT (Heidelberg Engineering).

MAIN OUTCOME MEASURES

Between-eye differences in PPC thickness, MvD frequency, and the area and maximum width of β-zone PPA.

RESULTS

Eyes with NTG showed higher intraocular pressure (IOP), longer axial length, thinner PPC, larger area and maximum radial width of the β-zone PPA, and more frequent MvD (P < 0.01 for each) than contralateral healthy eyes. Multivariate conditional logistic regression analysis revealed that higher IOP, thinner PPC, larger maximum radial width of β-zone PPA, and the presence of MvD were associated independently with the risk of NTG (P < 0.03 for each). In eyes with NTG, MvD location and retinal nerve fiber layer defect were correlated topographically.

CONCLUSIONS

Peripapillary choroid vasculature characteristics are significantly more compromised in eyes with NTG than in contralateral healthy eyes of patients with unilateral NTG.

Homoplasmic mitochondrial tRNAPro mutation causing exercise-induced muscle swelling and fatigue

Objective

To demonstrate the causal role in disease of the MT-TP m.15992A>T mutation observed in patients from 5 independent families.

Methods

Lactate measurement, muscle histology, and mitochondrial activities in patients; PCR-based analyses of the size, amount, and sequence of muscle mitochondrial DNA (mtDNA) and proportion of the mutation; respiration, mitochondrial activities, proteins, translation, transfer RNA (tRNA) levels, and base modification state in skin fibroblasts and cybrids; and reactive oxygen species production, proliferation in the absence of glucose, and plasma membrane potential in cybrids.

Results

All patients presented with severe exercise intolerance and hyperlactatemia. They were associated with prominent exercise-induced muscle swelling, conspicuous in masseter muscles (2 families), and/or with congenital cataract (2 families). MRI confirmed exercise-induced muscle edema. Muscle disclosed severe combined respiratory defect. Muscle mtDNA had normal size and amount. Its sequence was almost identical in all patients, defining the haplotype as J1c10, and sharing 31 variants, only 1 of which, MT-TP m.15992A>T, was likely pathogenic. The mutation was homoplasmic in all tissues and family members. Fibroblasts and cybrids with homoplasmic mutation had defective respiration, low complex III activity, and decreased tRNA^Pro amount. Their respiratory complexes amount and tRNA^Pro aminoacylation appeared normal. Low proliferation in the absence of glucose demonstrated the relevance of the defects on cybrid biology while abnormal loss of cell volume when faced to plasma membrane depolarization provided a link to the muscle edema observed in patients.

Conclusions

The homoplasmic MT-TP m.15992A>T mutation in the J1c10 haplotype causes exercise-induced muscle swelling and fatigue.

Folic acid deficiency and vision: a review

Folic acid (FA), also termed folate, is an essential vitamin for health at all ages since it participates in the biosynthesis of nucleotides, amino acids, neurotransmitters, and certain vitamins. It is therefore crucial for rapidly growing tissues such as those of the fetus. It is becoming clear that FA deficiency and impaired folate pathways are implicated in many diseases of both early life and old age. FA can be transported into the cell by the folate receptor, the reduced folate transporter, and proton-coupled folate transporter. Folate transport proteins are present in certain eye tissues, which explains why FA plays an important role in eye development. The purpose of this literature review is to investigate the evidence relating FA deficiency to eye diseases.

Juxtapapillary choroid is thinner in normal-tension glaucoma than in healthy eyes

PURPOSE

To measure the juxtapapillary choroidal thickness in eyes with normal-tension glaucoma (NTG) and to compare it with healthy eyes.

METHODS

Twelve radial B-scan images of the optic nerve head (ONH) were obtained from 96 patients with NTG and 48 healthy subjects matched by age using swept-source (SS) optical coherence tomography (OCT). The juxtapapillary choroidal thickness was defined as the average choroidal thickness within 500 μm from the border tissue of Elschnig. Choroidal thinning in patients with NTG was assessed by calculating the relative choroidal thickness, defined as the ratio of the measured juxtapapillary choroidal thickness in each meridian to the corresponding value in age-matched healthy controls. Retinal nerve fibre layer (RNFL) damage as reflected by circumpapillary RNFL thickness (measured using spectral-domain OCT) was also assessed.

RESULTS

The juxtapapillary choroid was significantly thinner in NTG eyes than in healthy control eyes in the inferotemporal and superotemporal sectors. The relative choroidal thinning was topographically associated with the hemispheric location of dominant RNFL damage. The average juxtapapillary choroidal thickness was not associated with either the global RNFL thickness or the visual field mean deviation. Age and untreated intraocular pressure were significantly associated with the juxtapapillary choroidal thickness in NTG eyes in both univariate and multivariate analyses (all p < 0.05).

CONCLUSIONS

Decreased microvascular circulation in the ONH as a result of juxtapapillary choroidal thinning could be an important part of the pathogenesis of optic nerve damage in NTG.

Correlational study on mitochondrial DNA mutations as potential risk factors in breast cancer

The presented study performed an mtDNA genome-wide association analysis to screen the peripheral blood of breast cancer patients for high-risk germline mutations. Unlike previous studies, which have used breast tissue in analyzing somatic mutations, we looked for germline mutations in our study, since they are better predictors of breast cancer in high-risk groups, facilitate early, non-invasive diagnoses of breast cancer and may provide a broader spectrum of therapeutic options. The data comprised 22 samples of healthy group and 83 samples from breast cancer patients. The sequencing data showed 170 mtDNA mutations in the healthy group and 393 mtDNA mutations in the disease group. Of these, 283 mtDNA mutations (88 in the healthy group and 232 in the disease group) had never been reported in the literature. Moreover, correlation analysis indicated there was a significant difference in 32 mtDNA mutations. According to our relative risk analysis of these 32 mtDNA mutations, 27 of the total had odds ratio values (ORs) of less than 1, meaning that these mutations have a potentially protective role to play in breast cancer. The remaining 5 mtDNA mutations, RNR2-2463 indelA, COX1-6296 C>A, COX1-6298 indelT, ATP6-8860 A>G, and ND5-13327 indelA, whose ORs were 8.050, 4.464, 4.464, 5.254 and 4.853, respectively, were regarded as risk factors of increased breast cancer. The five mutations identified here may serve as novel indicators of breast cancer and may have future therapeutic applications. In addition, the use of peripheral blood samples was procedurally simple and could be applied as a non-invasive diagnostic technique.

LHON: Mitochondrial Mutations and More

Leber’s hereditary optic neuropathy (LHON) is a mitochondrial disorder leading to severe visual impairment or even blindness by death of retinal ganglion cells (RGCs). The primary cause of the disease is usually a mutation of the mitochondrial genome (mtDNA) causing a single amino acid exchange in one of the mtDNA-encoded subunits of NADH:ubiquinone oxidoreductase, the first complex of the electron transport chain. It was thus obvious to accuse neuronal energy depletion as the most probable mediator of neuronal death. The group of Valerio Carelli and other authors have nicely shown that energy depletion shapes the cell fate in a LHON cybrid cell model. However, the cybrids used were osteosarcoma cells, which do not fully model neuronal energy metabolism. Although complex I mutations may cause oxidative stress, a potential pathogenetic role of the latter was less taken into focus. The hypothesis of bioenergetic failure does not provide a simple explanation for the relatively late disease onset and for the incomplete penetrance, which differs remarkably between genders. It is assumed that other genetic and environmental factors are needed in addition to the ‘primary LHON mutations’ to elicit RGC death. Relevant nuclear modifier genes have not been identified so far. The review discusses the unresolved problems of a pathogenetic hypothesis based on ATP decline and/or ROS-induced apoptosis in RGCs.

A biomechanical paradigm for axonal insult within the optic nerve head in aging and glaucoma

This article is dedicated to Rosario Hernandez for her warm support of my own work and her genuine enthusiasm for the work of her colleagues throughout her career. I first met Rosario as a research fellow in Harry Quigley's laboratory between 1991 and 1993. Along with Harry, John Morrison, Elaine Johnson, Abe Clark, Colm O'Brien and many others, Rosario's work has provided lamina cribrosa astrocyte cellular mechanisms that are biomechanically plausible and in so doing provided credibility to early notions of the optic nerve head (ONH) as a biomechanical structure. We owe a large intellectual debt to Rosario for her dogged persistence in the characterization of the ONH astrocyte and lamina cribrosacyte in age and disease. Two questions run through her work and remain of central importance today. First, how do astrocytes respond to and alter the biomechanical environment of the ONH and the physiologic stresses created therein? Second, how do these physiologic demands on the astrocyte influence their ability to deliver the support to retinal ganglion cell axon transport and flow against the translaminar pressure gradient? The purpose of this article is to summarize what is known about the biomechanical determinants of retinal ganglion cell axon physiology within the ONH in the optic neuropathy of aging and Glaucoma. My goal is to provide a biomechanical framework for this discussion. This framework assumes that the ONH astrocytes and glia fundamentally support and influence both the lamina cribrosa extracellular matrix and retinal ganglion cell axon physiology. Rosario Hernandez was one of the first investigators to recognize the implications of this unique circumstance. Many of the ideas contained herein have been initially presented within or derived from her work (Hernandez, M.R., 2000. The optic nerve head in glaucoma: role of astrocytes in tissue remodeling. Prog Retin Eye Res. 19, 297-321.; Hernandez, M.R., Pena, J.D., 1997. The optic nerve head in glaucomatous optic neuropathy. Arch Ophthalmol. 115, 389-395.).

Pathogenic mitochondrial DNA mutations are common in the general population

Mitochondrial DNA (mtDNA) mutations are a major cause of genetic disease, but their prevalence in the general population is not known. We determined the frequency of ten mitochondrial point mutations in 3168 neonatal-cord-blood samples from sequential live births, analyzing matched maternal-blood samples to estimate the de novo mutation rate. mtDNA mutations were detected in 15 offspring (0.54%, 95% CI = 0.30–0.89%). Of these live births, 0.00107% (95% CI = 0.00087–0.0127) harbored a mutation not detected in the mother's blood, providing an estimate of the de novo mutation rate. The most common mutation was m.3243A→G. m.14484T→C was only found on sub-branches of mtDNA haplogroup J. In conclusion, at least one in 200 healthy humans harbors a pathogenic mtDNA mutation that potentially causes disease in the offspring of female carriers. The exclusive detection of m.14484T→C on haplogroup J implicates the background mtDNA haplotype in mutagenesis. These findings emphasize the importance of developing new approaches to prevent transmission.

The NDUFB11 gene is not a modifier in Leber hereditary optic neuropathy

Over 95% of Leber hereditary optic neuropathy (LHON) cases are due to mutations in mitochondrial DNA-encoded subunits of NADH:ubiquinone oxidoreductase (E.C.1.6.5.3., complex I). A recessive X-linked susceptibility gene that acts synergistically with the primary mtDNA mutation to produce visual loss is suggested by the high male-to-female ratio among LHON patients. The ESSS protein is a recently isolated subunit of bovine heart mitochondrial complex I. We revisited the genomic sequence of NDUFB11, the human homolog mapping to chromosome Xp11.23, and identified two mRNA isoforms showing different expression profiles in human tissues. Cultured skin fibroblasts from four LHON patients showed a pattern of expression similar to normal controls. Moreover, NDUFB11 did not seem to influence risk and age at onset of visual loss in a total of 65 individuals from 35 Italian LHON families. Also, the gene was not affected in 11 children with a severe encephalopathy associated with decreased complex I activity in skeletal muscle.

Yeast NDI1 improves oxidative phosphorylation capacity and increases protection against oxidative stress and cell death in cells carrying a Leber's hereditary optic neuropathy mutation

G11778A in the subunit ND4 gene of NADH dehydrogenase complex is the most common primary mutation found in Leber's hereditary optic neuropathy (LHON) patients. The NDI1 gene, which encodes the internal NADH-quinone oxidoreductase in Saccharomyces cerevisiae, was introduced into the nuclear genome of a mitochondrial defective human cell line, Le1.3.1, carrying the G11778A mutation. In transformant cell lines, LeNDI1-1 and -2, total and complex I-dependent respiration were fully restored and largely resistant to complex I inhibitor, rotenone, indicating a dominant role of NDI1 in the transfer of electrons in the host cells. Whereas the original mutant Le1.3.1 cell grows poorly in medium containing galactose, the transformants have a fully restored growth capacity in galactose medium, although the ATP production was not totally recovered. Furthermore, the increased oxidative stress in the cells carrying the G11778A mutation was alleviated in transformants, demonstrated by a decreased reactive oxygen species (ROS) level. Finally, transformants were also shown to be desensitized to induction to apoptosis and also exhibit greater resistance to paraquat-induced cell death. It is concluded that the yeast NDI1 enzyme can improve the oxidative phosphorylation capacity in cells carrying the G11778A mutation and protect the cells from oxidative stress and cell death.

The unique characteristics of Thai Leber hereditary optic neuropathy: analysis of 30 G11778A pedigrees

Leber hereditary optic neuropathy (LHON) is characterized by acute or subacute bilateral visual loss, and affects mostly young males. The most common mitochondrial DNA mutation responsible for LHON worldwide is G11778A. Despite different genetic backgrounds, which are believed to influence the disease expression, most features of LHON are quite common in different populations. However, there seem to be a few ethnic-specific differences. Analyses of our 30 G11778A LHON pedigrees in Thailand showed some characteristics different from those of Caucasians and Japanese. In particular, our pedigrees showed a lower male to female ratio of affected persons (2.6:1) and much higher prevalence of G11778A blood heteroplasmy (37% of the pedigrees contained at least one heteroplasmic G11778A individual). Heteroplasmicity seemed to influence disease manifestation in our patients but did not appear to alter the onset of the disease. The estimated overall penetrance of our G11778A LHON population was 37% for males and 13% for females. When each of our large pedigrees were considered separately, disease penetration varied from 9 to 45% between the pedigrees, and also varied between different branches of the same large pedigree. Survival analysis showed that the secondary LHON mutations G3316A and C3497T had a synergistic deleterious effect with the G11778A mutation, accelerating the onset of the disease in our patients.

Identification of an X-chromosomal locus and haplotype modulating the phenotype of a mitochondrial DNA disorder

Mitochondrial DNA (mtDNA) mutations are a major cause of human disease. A large number of different molecular defects ultimately compromise oxidative phosphorylation, but it is not clear why the same biochemical defect can cause diverse clinical phenotypes. There is emerging evidence that nuclear genes modulate the phenotype of primary mtDNA disorders. Here, we define an X-chromosomal haplotype that interacts with specific MTND mutations to cause visual failure in the most common mtDNA disease, Leber hereditary optic neuropathy. This effect is independent of the mtDNA genetic background and explains the variable penetrance and sex bias that characterizes this disorder.

Optic neuropathies--importance of spatial distribution of mitochondria as well as function

Optic neuropathies such as Leber's hereditary optic neuropathy, dominant optic atrophy and toxic amblyopia are an important cause of irreversible visual failure. Although they are associated with a defect of mitochondrial energy production, their pathogenesis is poorly understood. A common feature to all these disorders is relatively selective degeneration of the papillomacular bundle of retinal ganglion cells resulting central or caecocentral visual field defects. The striking similarity in the pattern of clinical involvement seen with these disparate disorders suggests a common pathway in their aetiology. The existing hypothesis that the optic nerve head has higher energy demands than other tissues making it uniquely dependent on oxidative phosporylation is not satisfactory. First, other ocular tissues such as photoreceptors, which are more dependent on oxidative phosporylation are not affected. Second, other mitochondrial disorders, which have a greater impact on mitochondrial energy function, do not affect the optic nerve. The optic nerve head has certain unique ultra structural features. Ganglion cell axons exit the eye through a perforated collagen plate, the lamina cribrosa. There is a sharp discontinuity in the density of mitochondria at the optic nerve head, with a very high concentration in the prelaminar nerve fibre layer and low concentration behind the lamina. This has previously been attributed to a mechanical hold up of axoplasmic flow, which has itself been proposed as a factor in the pathogenesis of a number of optic neuropathies. More recent evidence shows that mitochondrial distribution reflects the different energy requirements of the unmyelinated prelaminar axons in comparison to the myelinated retrolaminar axons. The heterogeous distribution of mitochondria is actively maintained to support conduction through the optic nerve head. We propose that factors that disrupt the heterogeneous distribution of mitochondria can result in ganglion cell death. Evidence for this comes from studies of cultured cells with the dominant optic atrophy mutation in which mitochondrial distribution is altered and from some forms of hereditary spastic paraparesis which are associated with optic atrophy. The responsible mutations do not affect ATP production until late in the disease but do affect mitochondrial arrangement, again showing that mitochondrial distribution as well as energy production by individual mitochondria may be important in the pathogenesis of ganglion cell death. Greater understanding of the factors localising mitochondria within the ganglion cell axon in particular the interaction with cytoskeleton is required to formulate new treatments. Boosting energy production alone may not be an effective treatment.

Functional dichotomy: glutathione and vitamin E in homeostasis relevant to primary open-angle glaucoma

Primary open-angle glaucoma (POAG) is a complex chronic neurological disease that can result in blindness. The goal of understanding the aetiology of POAG is to be able to target effective treatment to individuals who will eventually go blind without it. Epidemiological studies of POAG have not specifically addressed the possibility that nutrition may play a role in the development of POAG. A handful of papers have considered that nutrition may have an impact on POAG patients. POAG is not believed to be a 'vitamin-deficiency disease'. The concept of 'vitamin-deficiency diseases' and the recommended daily allowances have not kept pace with the growing understanding of the cellular and molecular functions of vitamins and other micronutrients. The aetiology of POAG remains a mystery. Discoveries in cell physiology can be assimilated from the literature and applied to known homeostatic mechanisms of the eye. In this way the possible roles of nutritional components involved in the aetiology of POAG can be described. The mechanisms may be subject to many influences in ways that have yet to be defined. Two distinct changes in the trabecular meshwork can be identified: trabecular meshwork changes that cause intra-ocular pressure to increase and trabecular meshwork changes that are directly correlated to optic nerve atrophy. Compelling evidence suggests that collagen trabecular meshwork extracellular matrix (ECM) remodelling is correlated to increased intraocular pressure in POAG. Elastin trabecular meshwork ECM remodelling is correlated to POAG optic nerve atrophy. There appear to be two different pathways of ECM remodelling and apoptosis induction in POAG. The pathway for collagen remodelling and apoptosis induction seems to be exogenously influenced by water-soluble antioxidants, for example, glutathione. The pathway for elastin remodelling and apoptosis induction seems to be influenced by endogenous lipid-soluble antioxidants, for example, vitamin E. Roles can be defined for antioxidants in the two different pathways of ECM remodelling and apoptosis induction. This suggests that antioxidants are important in maintaining cellular homeostasis relevant to the aetiology of POAG.

Leber's hereditary optic neuropathy: the spectrum of mitochondrial DNA mutations in Iranian patients

We studied 14 patients with Leber's hereditary optic neuropathy (LHON) to investigate the mtDNA haplotypes associated with the primary mutation(s). Eleven patients carried the mitochondrial DNA (mtDNA) G11778A mutation, while one had the T14484C mutation; one patient had the G3460A mutation and one the G14459A mutation. The Iranian G11778A LHON mutation was not associated with two mtDNA haplogroups-M (0.0% compared with 3.2% in healthy controls) and J (7.7% compared with 10% in healthy controls). Our results showed a similarity in the pattern of LHON primary point mutations between Iranian families with LHON and those of Russian, European, and North American origin. Our results also do not support an association between mtDNA haplogroups J and M with LHON primary point mutations.

Mitochondrial production of oxygen radical species and the role of Coenzyme Q as an antioxidant

The mitochondrial respiratory chain is a powerful source of reactive oxygen species (ROS), which is considered as the pathogenic agent of many diseases and of aging. We have investigated the role of complex I in superoxide radical production and found by the combined use of specific inhibitors of complex I that the one-electron donor to oxygen in the complex is a redox center located prior to the sites where three different types of Coenzyme Q (CoQ) competitors bind, to be identified with an Fe-S cluster, most probably N2, or possibly an ubisemiquinone intermediate insensitive to all the above inhibitors. Short-chain Coenzyme Q analogs enhance superoxide formation, presumably by mediating electron transfer from N2 to oxygen. The clinically used CoQ analog, idebenone, is particularly effective, raising doubts on its safety as a drug. Cells counteract oxidative stress by antioxidants. CoQ is the only lipophilic antioxidant to be biosynthesized. Exogenous CoQ, however, protects cells from oxidative stress by conversion into its reduced antioxidant form by cellular reductases. The plasma membrane oxidoreductase and DT-diaphorase are two such systems, likewise, they are overexpressed under oxidative stress conditions.

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.

Mitochondrial function and dysfunction in the cell: its relevance to aging and aging-related disease

Mitochondria plays a complex multi-factorial role in the cell. In addition to their primary role in ATP generation, the organelles sequester calcium and both generate and detoxify reactive oxygen species. All these functions are intimately inter-linked through the central bioenergetic parameter of the proton electrochemical gradient across the inner mitochondrial membrane. Subtle changes in respiratory chain capacity, substrate supply, glutathione levels, cytoplasmic calcium and membrane potential occur in aging and in conditions predisposing towards neurodegenerative disease. These interactions are incompletely understood and in this review I present an overview of some of the current research in this area, and its possible relevance to aging and aging-related disease.

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.

Role of mitochondria in oxidative stress and aging

The mitochondrial respiratory chain is a powerful source of reactive oxygen species (ROS), considered as the pathogenic agent of many diseases and of aging. We have investigated the role of Complex I in superoxide radical production and found by combined use of specific inhibitors of Complex I that the one-electron donor in the Complex to oxygen is a redox center located prior to the sites where three different types of coenzyme Q (CoQ) competitors bind, to be identified with an Fe-S cluster, most probably N2, or possibly an ubisemiquinone intermediate insensitive to all the above inhibitors. Short-chain coenzyme Q analogues enhance superoxide formation, presumably by mediating electron transfer from N2 to oxygen. The clinically used CoQ analogue idebenone is particularly effective, raising doubts about its safety as a drug. The mitochondrial theory of aging considers somatic mutations of mitochondrial DNA induced by ROS as the primary cause of energy decline; in rat liver mitochondria, Complex I appears to be most affected by aging and to become strongly rate limiting for electron transfer. Mitochondrial energetics is also deranged in human platelets upon aging, as demonstrated by the decreased Pasteur effect (enhancement of lactate production by respiratory inhibitors). Cells counteract oxidative stress by antioxidants: CoQ is the only lipophilic antioxidant to be biosynthesized. Exogenous CoQ, however, protects cells from oxidative stress by conversion into its reduced antioxidant form by cellular reductases. The plasma membrane oxidoreductase and DT-diaphorase are two such systems: likewise, they are overexpressed under oxidative stress conditions.

Cells bearing mutations causing Leber's hereditary optic neuropathy are sensitized to Fas-Induced apoptosis

Three prevalent mitochondrial DNA pathogenic mutations at positions 11778, 3460, and 14484, which affect different subunits of Complex I, cause retinal ganglion cell death and optic nerve atrophy in Leber's hereditary optic neuropathy (LHON). The cell death is painless and without inflammation, consistent with an apoptotic mechanism. We have investigated the possibility that the LHON mutation confers a pro-apoptotic stimulus and have tested the sensitivity of osteosarcoma-derived cybrid cells carrying the most common and severe mutations (11778 and 3460) to cell death induced by Fas. We observed that LHON cybrids were sensitized to Fas-dependent death. Control cells that bear the same mitochondrial genetic background (the J haplogroup) without the pathogenic 11778 mutation are no more sensitive than other controls, indicating that increased Fas-dependent death in LHON cybrids was induced by the LHON pathogenic mutations. The type of death was apoptotic by several criteria, including induction by Fas, inhibition by the caspase inhibitor zVAD-fmk (zVal-Ala-Asp-fluoro-methyl ketone), activation of DEVDase activity (Asp-Glu-Val-Asp protease), specific cleavage of caspase-3, DNA fragmentation, and increased Annexin-V labeling. These data indicate that the most common and severe LHON pathogenic mutations 11778 and 3460 predispose cells to apoptosis, which may be relevant for the pathophysiology of cell death in LHON, and potential therapy.

Progressive mitochondrial disease resulting from a novel missense mutation in the mitochondrial DNA ND3 gene

We describe a 42-year-old man who presented with a progressive history of epilepsy, stroke-like episodes, bilateral optic atrophy, and cognitive decline. Investigation of his muscle biopsy revealed a specific defect in complex I activity. Subsequent analysis of the mitochondrial genome identified a novel heteroplasmic T10191C mutation in the ND3 gene. The mutation was present at lower levels in blood from the patient and unaffected maternal relatives and is the first pathogenic mitochondrial DNA mutation in the ND3 gene to be described.

Leber hereditary optic neuropathy: Does heteroplasmy influence the inheritance and expression of the G11778A mitochondrial DNA mutation?

Leber hereditary optic neuropathy (LHON) is a major cause of inherited blindness in young males. Approximately 1 in 7 individuals with LHON harbor a mixture of mutated and wild-type (normal) mtDNA (heteroplasmy), and the risks of developing blindness in heteroplasmic LHON individuals are not well characterized. MtDNA is inherited exclusively down the maternal line, and although the risks of a relative within a homoplasmic LHON pedigree are relatively well established, the risks of transmission in heteroplasmic LHON pedigrees have not been studied in detail. We analyzed 17 independent pedigrees that harbor the most prevalent LHON mutation: G11778A. The pedigrees were influenced by incomplete ascertainment bias, which was reduced by omitting the affected probands from the analysis. We made the following observations: (1) The frequency of blindness in males was related to the mutation load in that individual's blood. (2) Mothers with < or = 80% mutant mtDNA in blood were less likely to have clinically affected sons than mothers with 100% mutant mtDNA in their blood. (3) Within individual lineages, changes in mutation load from one generation to the next were largely determined by random genetic drift in these pedigrees. This study provides insights into the mutation load, or threshold, necessary for expression of the optic neuropathy, the relationship between mutation load in the mother and the risk of blindness in her children, and the complex inheritance of heteroplasmic mtDNA defects.

The mitochondrial ND6 gene is a hot spot for mutations that cause Leber's hereditary optic neuropathy

Leber's hereditary optic neuropathy (LHON) is a common cause of bilateral optic nerve disease. The majority of LHON patients harbour one of three point mutations of the mitochondrial DNA (mtDNA) complex I, or NADH:ubiquinone oxidoreductase (ND) genes (G11778A in ND4, G3460A in ND1, T14484C in ND6). As a consequence, screening for these mutations has become part of the routine clinical investigation of young adults who present with bilateral optic neuropathy, and the absence of these mutations is interpreted as indicating there is a low likelihood that an optic neuropathy is LHON. However, there are many individuals who develop the clinical features of LHON but who do not harbour one of these primary LHON mutations. We describe two LHON pedigrees that harbour the same novel point mutation within the mtDNA ND6 gene (A14495G). This mutation was heteroplasmic in both families, and sequencing of the mitochondrial genome confirmed that the mutation arose on two independent occasions. This is the seventh mutation in the ND6 gene that causes optic neuropathy, indicating that this gene is a hot spot for LHON mutations. Protein modelling studies indicate that all of these pathogenic mutations lie within close proximity to one another in a hydrophobic cleft or pocket. This is the first evidence for a relationship between a specific disease phenotype and a specific structural domain within a mitochondrial respiratory chain subunit. These findings suggest that the mtDNA ND6 gene should be sequenced in all patients with LHON who do not harbour one of the three common LHON mutations.

Nuclear gene OPA1, encoding a mitochondrial dynamin-related protein, is mutated in dominant optic atrophy

Optic atrophy type 1 (OPA1, MIM 165500) is a dominantly inherited optic neuropathy occurring in 1 in 50,000 individuals that features progressive loss in visual acuity leading, in many cases, to legal blindness. Phenotypic variations and loss of retinal ganglion cells, as found in Leber hereditary optic neuropathy (LHON), have suggested possible mitochondrial impairment. The OPA1 gene has been localized to 3q28-q29 (refs 13-19). We describe here a nuclear gene, OPA1, that maps within the candidate region and encodes a dynamin-related protein localized to mitochondria. We found four different OPA1 mutations, including frameshift and missense mutations, to segregate with the disease, demonstrating a role for mitochondria in retinal ganglion cell pathophysiology.

Use of cytoplasmic hybrid cell lines for elucidating the role of mitochondrial dysfunction in Alzheimer's disease and Parkinson's disease

There is substantial evidence of mitochondrial defects in neurodegenerative disorders such as Alzheimer's and Parkinson's diseases (AD and PD). We have probed the molecular implications of mitochondrial dysfunction in these diseases by transferring mitochondria from platelets obtained from disease and control donors into mitochondrial DNA-depleted recipient neuron-based cells (rho 0 cells). This process creates cytoplasmic hybrid (cybrid) cells where the mitochondrial DNA (mtDNA) from the donor is expressed in the nuclear and cellular background of the host rho 0 cell. Differences in phenotype between disease and control groups can thus be attributed to the exogenous mitochondria and mtDNA. Key methodological issues relating to this approach were addressed by demonstrating that recipient rho 0 cells have < 1 mtDNA copy/cell, and that exclusive repopulation with donor mtDNA occurs in cybrid cells. Further, we describe that sampling of heterogeneous cell populations is a valid approach for cybrid analysis. Our studies show that the focal respiratory chain defects reported in platelets of AD and PD cybrids can be recapitulated in AD and PD cybrids. In addition, both AD and PD cybrids display increased oxidative stress and perturbations in calcium homeostasis. These data suggest that the transfer of a mtDNA defect from disease donor platelets is the likely cause of the cybrid biochemical phenotype, and highlight the potential value of these cell lines as cellular disease models.

Mitochondrial DNA variation in human evolution and disease

Analysis of mitochondrial DNA (mtDNA) variation has permitted the reconstruction of the ancient migrations of women. This has provided evidence that our species arose in Africa about 150000 years before present (YBP), migrated out of Africa into Asia about 60000 to 70000 YBP and into Europe about 40000 to 50000 YBP, and migrated from Asia and possibly Europe to the Americas about 20000 to 30000 YBP. Although much of the mtDNA variation that exists in modern populations may be selectively neutral, studies of the mildly deleterious mtDNA mutations causing Leber's hereditary optic neuropathy (LHON) have demonstrated that some continent-specific mtDNA lineages are more prone to manifest the clinical symptoms of LHON than others. Hence, all mtDNA lineages are not equal, which may provide insights into the extreme environments that were encountered by our ancient ancestor, and which may be of great importance in understanding the pathophysiology of mitochondrial disease.

NADH-quinone oxidoreductase: PSST subunit couples electron transfer from iron-sulfur cluster N2 to quinone

The proton-translocating NADH-quinone oxidoreductase (EC 1.6.99.3) is the largest and least understood enzyme complex of the respiratory chain. The mammalian mitochondrial enzyme (also called complex I) contains more than 40 subunits, whereas its structurally simpler bacterial counterpart (NDH-1) in Paracoccus denitrificans and Thermus thermophilus HB-8 consists of 14 subunits. A major unsolved question is the location and mechanism of the terminal electron transfer step from iron-sulfur cluster N2 to quinone. Potent inhibitors acting at this key region are candidate photoaffinity probes to dissect NADH-quinone oxidoreductases. Complex I and NDH-1 are very sensitive to inhibition by a variety of structurally diverse toxicants, including rotenone, piericidin A, bullatacin, and pyridaben. We designed (trifluoromethyl)diazirinyl[3H]pyridaben ([3H]TDP) as our photoaffinity ligand because it combines outstanding inhibitor potency, a suitable photoreactive group, and tritium at high specific activity. Photoaffinity labeling of mitochondrial electron transport particles was specific and saturable. Isolation, protein sequencing, and immunoprecipitation identified the high-affinity specifically labeled 23-kDa subunit as PSST of complex I. Immunoprecipitation of labeled membranes of P. denitrificans and T. thermophilus established photoaffinity labeling of the equivalent bacterial NQO6. Competitive binding and enzyme inhibition studies showed that photoaffinity labeling of the specific high-affinity binding site of PSST is exceptionally sensitive to each of the high-potency inhibitors mentioned above. These findings establish that the homologous PSST of mitochondria and NQO6 of bacteria have a conserved inhibitor-binding site and that this subunit plays a key role in electron transfer by functionally coupling iron-sulfur cluster N2 to quinone.

Histochemical localisation of mitochondrial enzyme activity in human optic nerve and retina

AIMS

To demonstrate the quantitative distribution of mitochondrial enzymes within the human optic nerve and retina in relation to the pathogenesis of ophthalmic disease.

METHODS

Enucleations were performed at the time of multiple organ donation and the optic nerve and peripapillary retina immediately excised en bloc and frozen. Reactivities of the mitochondrial enzymes cytochrome c oxidase and succinate dehydrogenase were demonstrated in serial cryostat sections using specific histochemical assays.

RESULTS

In the optic nerve the unmyelinated prelaminar and laminar regions were rich in both cytochrome c oxidase and succinate dehydrogenase. Myelination of fibres as they exited the lamina cribrosa was associated with an abrupt reduction in enzyme activity. Within the retina, high levels of enzyme activity were found localised within the retinal ganglion cells and nerve fibre layer, the outer plexiform layer, inner segments of photoreceptors, and the retinal pigment epithelium.

CONCLUSIONS

Mitochondrial enzyme activity is preserved in human optic nerve and retina retrieved at the time of multiple organ donation. The distribution of enzyme activity within the eye has implications for the understanding of the pattern of ophthalmic involvement seen in mitochondrial diseases and the site of ganglion cell dysfunction in those patients with optic nerve involvement.

Human mitochondrial diseases: answering questions and questioning answers

Since the first identification in 1988 of pathogenic mitochondrial DNA (mtDNA) mutations, the mitochondrial diseases have emerged as a major clinical entity. The most striking feature of these disorders is their marked heterogeneity, which extends to their clinical, biochemical, and genetic characteristics. The major mitochondrial encephalomyopathies include MELAS (mitochondrial encephalopathy with lactic acidosis and stroke-like episodes), MERRF (myoclonic epilepsy with ragged red fibers), KSS/CPEO (Kearns-Sayre syndrome/chronic progressive external ophthalmoplegia), and NARP/MILS (neuropathy, ataxia, and retinitis pigmentosum/maternally inherited Leigh syndrome) and they typically present highly variable multisystem defects that usually involve abnormalities of skeletal muscle and/or the CNS. The primary emphasis here is to review recent investigations of these mitochondrial diseases from the standpoint of how the complexities of mitochondrial genetics and biogenesis might determine their varied features. In addition, the mitochondrial encephalomyopathies are compared and contrasted to Leber hereditary optic neuropathy, a mitochondrial disease in which the pathogenic mtDNA mutations produce a more uniform and focal neuropathology. All of these disorders involve, at some level, a mitochondrial respiratory chain dysfunction. Because mitochondrial genetics differs so strikingly from the Mendelian inheritance of chromosomes, recent research on the origin and subsequent segregation and transmission of mtDNA mutations is reviewed.

Clinical features, molecular genetics, and pathophysiology of dominant optic atrophy

Inherited optic neuropathies are a significant cause of childhood and adult blindness and dominant optic atrophy (DOA) is the most common form of autosomally inherited (non-glaucomatous) optic neuropathy. Patients with DOA present with an insidious onset of bilateral visual loss and they characteristically have temporal optic nerve pallor, centrocaecal visual field scotoma, and a colour vision deficit, which is frequently blue-yellow. Evidence from histological and electrophysiological studies suggests that the pathology is confined to the retinal ganglion cell. A gene for dominant optic atrophy (OPA1) has been mapped to chromosome 3q28-qter, and studies are under way to refine the genetic interval in which the gene lies, to map the region physically, and hence to clone the gene. A second locus for dominant optic atrophy has recently been shown to map to chromosome 18q12.2-12.3 near the Kidd blood group locus. The cloning of genes for dominant optic atrophy will provide important insights into the pathophysiology of the retinal ganglion cell in health and disease. These insights may prove to be of great value in the understanding of other primary ganglion cell diseases, such as the mitochondrially inherited Leber's hereditary optic neuropathy and other diseases associated with ganglion cell loss, such as glaucoma.