Genome-wide linkage scan and association study of PARL to the expression of LHON families in Thailand

Leber hereditary optic neuropathy-new insights and old challenges

Leber hereditary optic neuropathy (LHON) is the most common primary mitochondrial DNA (mtDNA) disorder with the majority of patients harboring one of three primary mtDNA point mutations, namely, m.3460G>A (MTND1), m.11778G>A (MTND4), and m.14484T>C (MTND6). LHON is characterized by bilateral subacute loss of vision due to the preferential loss of retinal ganglion cells (RGCs) within the inner retina, resulting in optic nerve degeneration. This review describes the clinical features associated with mtDNA LHON mutations and recent insights gained into the disease mechanisms contributing to RGC loss in this mitochondrial disorder. Although treatment options remain limited, LHON research has now entered an active translational phase with ongoing clinical trials, including gene therapy to correct the underlying pathogenic mtDNA mutation.

Mitophagy: An Emerging Target in Ocular Pathology

Mitochondrial function is essential for the viability of aerobic eukaryotic cells, as mitochondria provide energy through the generation of adenosine triphosphate (ATP), regulate cellular metabolism, provide redox balancing, participate in immune signaling, and can initiate apoptosis. Mitochondria are dynamic organelles that participate in a cyclical and ongoing process of regeneration and autophagy (clearance), termed mitophagy specifically for mitochondrial (macro)autophagy. An imbalance in mitochondrial function toward mitochondrial dysfunction can be catastrophic for cells and has been characterized in several common ophthalmic diseases. In this article, we review mitochondrial homeostasis in detail, focusing on the balance of mitochondrial dynamics including the processes of fission and fusion, and provide a description of the mechanisms involved in mitophagy. Furthermore, this article reviews investigations of ocular diseases with impaired mitophagy, including Fuchs endothelial corneal dystrophy, primary open-angle glaucoma, diabetic retinopathy, and age-related macular degeneration, as well as several primary mitochondrial diseases with ocular phenotypes that display impaired mitophagy, including mitochondrial encephalopathy lactic acidosis stroke, Leber hereditary optic neuropathy, and chronic progressive external ophthalmoplegia. The results of various studies using cell culture, animal, and human tissue models are presented and reflect a growing awareness of mitophagy impairment as an important feature of ophthalmic disease pathology. As this review indicates, it is imperative that mitophagy be investigated as a targetable mechanism in developing therapies for ocular diseases characterized by oxidative stress and mitochondrial dysfunction.

Mitochondrial Proteases: Multifaceted Regulators of Mitochondrial Plasticity

Mitochondria are essential metabolic hubs that dynamically adapt to physiological demands. More than 40 proteases residing in different compartments of mitochondria, termed mitoproteases, preserve mitochondrial proteostasis and are emerging as central regulators of mitochondrial plasticity. These multifaceted enzymes limit the accumulation of short-lived, regulatory proteins within mitochondria, modulate the activity of mitochondrial proteins by protein processing, and mediate the degradation of damaged proteins. Various signaling cascades coordinate the activity of mitoproteases to preserve mitochondrial homeostasis and ensure cell survival. Loss of mitoproteases severely impairs the functional integrity of mitochondria, is associated with aging, and causes pleiotropic diseases. Understanding the dual function of mitoproteases as regulatory and quality control enzymes will help unravel the role of mitochondrial plasticity in aging and disease.

Frequency and spectrum of MT-TT variants associated with Leber's hereditary optic neuropathy in a Chinese cohort of subjects

Leber’s hereditary optic neuropathy (LHON) is a maternally inherited eye disease. In our previous investigations, we have reported the spectrum and frequency of mitochondrial MT-ND1, MT-ND4 and MT-ND6 gene in Chinese LHON population. This study aimed to assess the molecular epidemiology of MT-TT mutations in Chinese families with LHON. A cohort of 352 Chinese Han probands lacking the known LHON-associated mtDNA mutations and 376 control subjects underwent molecular analysis of mtDNA. All variants were evaluated for evolutionary conservation, structural and functional consequences. Fifteen variants were identified in the MT-TT gene by mitochondrial genome analysis of LHON pedigrees, which was substantially higher than that of individuals from general Chinese populations. The incidences of the two known LHON-associated mutations, m.15927G > A and m.15951A > G, were 2.27% and 1.14%, respectively. Nine putative LHON-associated variants were identified in 20 probands, translated into 2.1% cases of this cohort. Moreover, mtDNAs in 41 probands carrying the MT-TT mutation(s) were widely dispersed among nine Eastern Asian haplogroups. Our results suggest that the MT-TT gene is a mutational hotspot for these 352 Chinese families lacking the known LHON-associated mutations. These data further showed the molecular epidemiology of MT-TT mutations in Chinese Han LHON pedigrees.

Clinical syndromes associated with mtDNA mutations: where we stand after 30 years

The landmark year 1988 can be considered as the birthdate of mitochondrial medicine, when the first pathogenic mutations affecting mtDNA were associated with human diseases. Three decades later, the field still expands and we are not ‘scraping the bottom of the barrel’ yet. Despite the tremendous progress in terms of molecular characterization and genotype/phenotype correlations, for the vast majority of cases we still lack a deep understanding of the pathogenesis, good models to study, and effective therapeutic options. However, recent technological advances including somatic cell reprogramming to induced pluripotent stem cells (iPSCs), organoid technology, and tailored endonucleases provide unprecedented opportunities to fill these gaps, casting hope to soon cure the major primary mitochondrial phenotypes reviewed here. This group of rare diseases represents a key model for tackling the pathogenic mechanisms involving mitochondrial biology relevant to much more common disorders that affect our currently ageing population, such as diabetes and metabolic syndrome, neurodegenerative and inflammatory disorders, and cancer.

Incomplete penetrance in mitochondrial optic neuropathies

Incomplete penetrance characterizes the two most frequent inherited optic neuropathies, Leber's Hereditary Optic Neuropathy (LHON) and dominant optic atrophy (DOA), due to genetic errors in the mitochondrial DNA (mtDNA) and the nuclear DNA (nDNA), respectively. For LHON, compelling evidence has accumulated on the complex interplay of mtDNA haplogroups and environmental interacting factors, whereas the nDNA remains essentially non informative. However, a compensatory mechanism of activated mitochondrial biogenesis and increased mtDNA copy number, possibly driven by a permissive nDNA background, is documented in LHON; when successful it maintains unaffected the mutation carriers, but in some individuals it might be hampered by tobacco smoking or other environmental factors, resulting in disease onset. In females, mitochondrial biogenesis is promoted and maintained within the compensatory range by estrogens, partially explaining the gender bias in LHON. Concerning DOA, none of the above mechanisms has been fully explored, thus mtDNA haplogroups, environmental factors such as tobacco and alcohol, and further nDNA variants may all participate as protective factors or, on the contrary, favor disease expression and severity. Next generation sequencing, complemented by transcriptomics and proteomics, may provide some answers in the next future, even if the multifactorial model that seems to apply to incomplete penetrance in mitochondrial optic neuropathies remains problematic, and careful stratification of patients will play a key role for data interpretation. The deep understanding of which factors impinge on incomplete penetrance may shed light on the pathogenic mechanisms leading to optic nerve atrophy, on their possible compensation and, thus, on development of therapeutic strategies.

Leber Hereditary Optic Neuropathy: A Mitochondrial Disease Unique in Many Ways

Leber hereditary optic neuropathy (LHON) was the first mitochondrial disease to be identified as being caused by mutations in the mitochondrial DNA (mtDNA). This disease has been studied extensively in the past two decades, particularly in Brazilian, Chinese and European populations; and many primary mutations have been reported. However, the disease is enigmatic with many unique features, and there still are several important questions to be resolved. The incomplete penetrance, the male-biased disease expression and the prevalence in young adults all defy a proper explanation. It has been reported that the development of LHON is affected by the interaction between mtDNA mutations, mtDNA haplogroup background, nuclear genes, environmental factors and epigenetics. Furthermore, with the help of new animal models for LHON that have been created in recent years, we are continuing to learn more about the mechanism of this disease. The stage has now been reached at which there is a good understanding of both the genetic basis of the disease and its epidemiology, but just how the blindness that follows from the death of cells in the optic nerve can be prevented remains to be a pharmacological challenge. In this chapter, we summarize the progress that has been made in various recent studies on LHON, focusing on the molecular pathogenic mechanisms, clinical features, biochemical effects, the pharmacology and its treatment.

Leber Hereditary Optic Neuropathy: Exemplar of an mtDNA Disease

The report in 1988 that Leber Hereditary Optic Neuropathy (LHON) was the product of mitochondrial DNA (mtDNA) mutations provided the first demonstration of the clinical relevance of inherited mtDNA variation. From LHON studies, the medical importance was demonstrated for the mtDNA showing its coding for the most important energy genes, its maternal inheritance, its high mutation rate, its presence in hundreds to thousands of copies per cell, its quantitatively segregation of biallelic genotypes during both mitosis and meiosis, its preferential effect on the most energetic tissues including the eye and brain, its wide range of functional polymorphisms that predispose to common diseases, and its accumulation of mutations within somatic tissues providing the aging clock. These features of mtDNA genetics, in combination with the genetics of the 1-2000 nuclear DNA (nDNA) coded mitochondrial genes, is not only explaining the genetics of LHON but also providing a model for understanding the complexity of many common diseases. With the maturation of LHON biology and genetics, novel animal models for complex disease have been developed and new therapeutic targets and strategies envisioned, both pharmacological and genetic. Multiple somatic gene therapy approaches are being developed for LHON which are applicable to other mtDNA diseases. Moreover, the unique cytoplasmic genetics of the mtDNA has permitted the first successful human germline gene therapy via spindle nDNA transfer from mtDNA mutant oocytes to enucleated normal mtDNA oocytes. Such LHON lessons are actively being applied to common ophthalmological diseases like glaucoma and neurological diseases like Parkinsonism.

Common variants in the PARL and PINK1 genes increase the risk to leprosy in Han Chinese from South China

Leprosy is a chronic infectious and neurological disease caused by Mycobacterium leprae, an unculturable pathogen with massive genomic decay and dependence on host metabolism. We hypothesized that mitochondrial genes PARL and PINK1 would confer risk to leprosy. Thirteen tag SNPs of PARL and PINK1 were analyzed in 3620 individuals with or without leprosy from China. We also sequenced the entire exons of PARL, PINK1 and PARK2 in 80 patients with a family history of leprosy by using the next generation sequencing technology (NGS). We found that PARL SNP rs12631031 conferred a risk to leprosy (P_adjusted = 0.019) and multibacillary leprosy (MB, P_adjusted = 0.020) at the allelic level. rs12631031 and rs7653061 in PARL were associated with leprosy and MB (dominant model, P_adjusted < 0.05) at the genotypic level. PINK1 SNP rs4704 was associated with leprosy at the genotypic level (P_adjusted = 0.004). We confirmed that common variants in PARL and PINK1 were associated with leprosy in patients underwent NGS. Furthermore, PARL and PINK1 could physically interact with each other and were involved in the highly connected network formed by reported leprosy susceptibility genes. Together, our results showed that PARL and PINK1 genetic variants are associated with leprosy.

Mitochondrial haplogroup D4j specific variant m.11696G > a(MT-ND4) may increase the penetrance and expressivity of the LHON-associated m.11778G > a mutation in Chinese pedigrees

Leber's hereditary optic neuropathy (LHON) is one of the most common mitochondrial disorders. We report here the clinical, genetic and molecular analysis of mitochondrial DNA (mtDNA) in eight Han Chinese families carrying the known mitochondrial 11778G > A(MT-ND4) mutation. Thirty-seven (26 males/11 females) of 77 matrilineal relatives in these families exhibited the variable severity and age-at-onset of optic neuropathy. The penetrances were from 25% to 75%, with the average of 42%, and the age-at-onset for visual impairment varied from 10 to 25 years, with the average of 17 in these Chinese pedigrees. Molecular analysis of their mtDNA identified distinct sets of variants belonging to the Eastern Asian haplogroupD4j. Except the known m.11778G > A mutation, the m.11696G > A(MT-ND4) mutation caused the substitution of an isoleucine for valineat amino acid position 313, located in a predicted transmembrane region of ND4. And, it is reported that the m.11696G > A mutation was associated with LHON, and appeared to contribute to higher penetrance in these nine Chinese families than other Chinese families carrying only the m.11778G > A mutation. Therefore, the mitochondrial haplogroup D4j specific m.11696G > A mutation may act in synergy with the primary LHON-associated m.11778G > A mutation, thereby increasing the penetrance and expressivity of visual loss in these Chinese families.

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.

Common variants of the PINK1 and PARL genes do not confer genetic susceptibility to schizophrenia in Han Chinese

Schizophrenia is a prevalent psychiatric disorder with a complex etiology. Mitochondrial dysfunction has been frequently reported in schizophrenia. Phosphatase and tension homologue-induced kinase 1 (PINK1) and presenilin-associated rhomboid-like protease (PARL) are mitochondrial proteins, and genetic variants of these two genes may confer genetic susceptibility to schizophrenia by influencing mitochondrial function. In this study, we conducted a two-stage genetic association study to test this hypothesis. We genotyped 4 PINK1 and 5 PARL genetic variants and evaluated the potential association of the 9 SNPs with schizophrenia in two independent case-control cohorts of 2510 Han Chinese individuals. No positive association of common genetic variants of the PINK1 and PARL genes with schizophrenia was identified in our samples after Bonferroni correction. Re-analysis of the newly updated Psychiatric Genetics Consortium (PGC) data sets confirmed our negative result. Intriguingly, one PINK1 SNP (rs10916832), which showed a marginally significant association in only Hunan samples (P = 0.032), is associated with the expression of a schizophrenia susceptible gene KIF17 according to the expression quantitative trait locus (eQTL) analysis. Our study indicated that common genetic variants of the PINK1 and PARL genes are unlikely to be involved in schizophrenia. Further studies are essential to characterize the role of the PINK1 and PARL genes in schizophrenia.

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.

Efficient mitochondrial biogenesis drives incomplete penetrance in Leber's hereditary optic neuropathy

Leber's hereditary optic neuropathy is a maternally inherited blinding disease caused as a result of homoplasmic point mutations in complex I subunit genes of mitochondrial DNA. It is characterized by incomplete penetrance, as only some mutation carriers become affected. Thus, the mitochondrial DNA mutation is necessary but not sufficient to cause optic neuropathy. Environmental triggers and genetic modifying factors have been considered to explain its variable penetrance. We measured the mitochondrial DNA copy number and mitochondrial mass indicators in blood cells from affected and carrier individuals, screening three large pedigrees and 39 independently collected smaller families with Leber's hereditary optic neuropathy, as well as muscle biopsies and cells isolated by laser capturing from post-mortem specimens of retina and optic nerves, the latter being the disease targets. We show that unaffected mutation carriers have a significantly higher mitochondrial DNA copy number and mitochondrial mass compared with their affected relatives and control individuals. Comparative studies of fibroblasts from affected, carriers and controls, under different paradigms of metabolic demand, show that carriers display the highest capacity for activating mitochondrial biogenesis. Therefore we postulate that the increased mitochondrial biogenesis in carriers may overcome some of the pathogenic effect of mitochondrial DNA mutations. Screening of a few selected genetic variants in candidate genes involved in mitochondrial biogenesis failed to reveal any significant association. Our study provides a valuable mechanism to explain variability of penetrance in Leber's hereditary optic neuropathy and clues for high throughput genetic screening to identify the nuclear modifying gene(s), opening an avenue to develop predictive genetic tests on disease risk and therapeutic strategies.

Mitochondrial genetics

INTRODUCTION

In the last 10 years the field of mitochondrial genetics has widened, shifting the focus from rare sporadic, metabolic disease to the effects of mitochondrial DNA (mtDNA) variation in a growing spectrum of human disease. The aim of this review is to guide the reader through some key concepts regarding mitochondria before introducing both classic and emerging mitochondrial disorders.

SOURCES OF DATA

In this article, a review of the current mitochondrial genetics literature was conducted using PubMed (http://www.ncbi.nlm.nih.gov/pubmed/). In addition, this review makes use of a growing number of publically available databases including MITOMAP, a human mitochondrial genome database (www.mitomap.org), the Human DNA polymerase Gamma Mutation Database (http://tools.niehs.nih.gov/polg/) and PhyloTree.org (www.phylotree.org), a repository of global mtDNA variation.

AREAS OF AGREEMENT

The disruption in cellular energy, resulting from defects in mtDNA or defects in the nuclear-encoded genes responsible for mitochondrial maintenance, manifests in a growing number of human diseases.

AREAS OF CONTROVERSY

The exact mechanisms which govern the inheritance of mtDNA are hotly debated.

GROWING POINTS

Although still in the early stages, the development of in vitro genetic manipulation could see an end to the inheritance of the most severe mtDNA disease.

Mitochondrial dynamics in cancer and neurodegenerative and neuroinflammatory diseases

Mitochondria are key organelles in the cell, hosting essential functions, from biosynthetic and metabolic pathways, to oxidative phosphorylation and ATP production, from calcium buffering to red-ox homeostasis and apoptotic signalling pathways. Mitochondria are also dynamic organelles, continuously fusing and dividing, and their localization, size and trafficking are finely regulated. Moreover, in recent decades, alterations in mitochondrial function and dynamics have been implicated in an increasing number of diseases. In this review, we focus on the relationship clarified hitherto between mitochondrial dynamics and cancer, neurodegenerative and neuroinflammatory diseases.

Mitochondrial DNA mutation m.10680G > A is associated with Leber hereditary optic neuropathy in Chinese patients

Background

Leber hereditary optic neuropathy (LHON) is a mitochondrial disorder with gender biased and incomplete penetrance. The majority of LHON patients are caused by one of the three primary mutations (m.3460G > A, m.11778G > A and m.14484T > C). Rare pathogenic mutations have been occasionally reported in LHON patients.

Methods

We screened mutation m.10680G > A in the MT-ND4L gene in 774 Chinese patients with clinical features of LHON but lacked the three primary mutations by using allele specific PCR (AS-PCR). Patients with m.10680G > A were further determined entire mtDNA genome sequence.

Results

The optimal AS-PCR could detect as low as 10% heteroplasmy of mutation m.10680G > A. Two patients (Le1263 and Le1330) were identified to harbor m.10680G > A. Analysis of the complete mtDNA sequences of the probands suggested that they belonged to haplogroups B4a1 and D6a1. There was no other potentially pathogenic mutation, except for a few private yet reported variants in the MT-ND1 and MT-ND5 genes, in the two lineages. A search in reported mtDNA genome data set (n = 9277; excluding Chinese LHON patients) identified no individual with m.10680G > A. Frequency of m.10680G > A in Chinese LHON patients analyzed in this study and our previous studies (3/784) was significantly higher than that of the general populations (0/9277) (P = 0.0005).

Conclusion

Taken together, we speculated that m.10680G > A may be a rare pathogenic mutation for LHON in Chinese. This mutation should be included in future clinical diagnosis.

Mitochondrial DNA haplogroup background affects LHON, but not suspected LHON, in Chinese patients

Recent studies have shown that mtDNA background could affect the clinical expression of Leber hereditary optic neuropathy (LHON). We analyzed the mitochondrial DNA (mtDNA) variation of 304 Chinese patients with m.11778G>A (sample #1) and of 843 suspected LHON patients who lack the three primary mutations (sample #2) to discern mtDNA haplogroup effect on disease onset. Haplogroup frequencies in the patient group was compared to frequencies in the general Han Chinese population (n = 1,689; sample #3). The overall matrilineal composition of the suspected LHON population resembles that of the general Han Chinese population, suggesting no association with mtDNA haplogroup. In contrast, analysis of these LHON patients confirms mtDNA haplogroup effect on LHON. Specifically, the LHON sample significantly differs from the general Han Chinese and suspected LHON populations by harboring an extremely lower frequency of haplogroup R9, in particular of its main sub-haplogroup F (#1 vs. #3, P-value = 1.46×10⁻¹⁷, OR = 0.051, 95% CI: 0.016–0.162; #1 vs. #2, P-value = 4.44×10⁻¹⁷, OR = 0.049, 95% CI: 0.015–0.154; in both cases, adjusted P-value <10⁻⁵) and higher frequencies of M7b (#1 vs. #3, adjusted P-value = 0.001 and #1 vs. #2, adjusted P-value = 0.004). Our result shows that mtDNA background affects LHON in Chinese patients with m.11778G>A but not suspected LHON. Haplogroup F has a protective effect against LHON, while M7b is a risk factor.

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.

Mitochondrial disorders and the eye

The clinical significance of disturbed mitochondrial function in the eye has emerged since mitochondrial DNA (mtDNA) mutation was described in Leber's hereditary optic neuropathy. The spectrum of mitochondrial dysfunction has become apparent through increased understanding of the contribution of nuclear and somatic mtDNA mutations to mitochondrial dynamics and function. Common ophthalmic manifestations of mitochondrial dysfunction include optic atrophy, pigmentary retinopathy, and ophthalmoplegia. The majority of patients with ocular manifestations of mitochondrial disease also have variable central and peripheral nervous system involvement. Mitochondrial dysfunction has recently been associated with age-related retinal disease including macular degeneration and glaucoma. Therefore, therapeutic targets directed at promoting mitochondrial biogenesis and function offer a potential to both preserve retinal function and attenuate neurodegenerative processes.

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

The ND4 G11778A mutation is the most common mitochondrial DNA mutation leading to Leber's hereditary optic neuropathy (LHON). Despite considerable clinical evidences, the modifier role of nuclear background and mitochondrial haplotypes in phenotypic manifestation of LHON remains poorly understood. We investigated the effect of these modifiers on bioenergetics in lymphoblastoid cell lines derived from five affected subjects of one Chinese family carrying the G11778A mutation and five Chinese controls. Significant reductions in the activities of complexes I and III were observed in mutant cell lines from the Chinese family, whereas the mutant cell lines from other families carrying the same mutation exhibited only reduced activity of complex I. The reduced activities of complexes I and III caused remarkably higher reductions of ATP synthesis in mutant cell lines from the Chinese family than those from other families. The deficient respiration increased generation of reactive oxygen species. The defect in complex III activity, likely resulting from the mitochondrial haplotype or nuclear gene alteration, worsens mitochondrial dysfunction caused by the G11778A mutation, thereby causing extremely high penetrance and expressivity of optic neuropathy in this Chinese family. Our data provide the first experimental evidence that altered activity of complex III modulates the phenotypic manifestation of LHON-associated G11778A mutation. Thus, our findings may provide new insights into the pathophysiology of LHON.

Mutation in the mitochondrial tRNA(Val) causes mitochondrial encephalopathy, lactic acidosis and stroke-like episodes

An m.1630A>G mutation in the mitochondrial tRNA(Val) (MTTV) was identified in a patient with hearing impairment, short stature and new onset of stroke. This mutation has previously been identified in a patient with the mitochondrial neurogastrointestinal encephalopathy syndrome (MNGIE). The mother of the proband also had high levels of the m.1630A>G allele present in blood and other tissues, without symptoms. To confirm the pathogenicity of this mutation, we created cybrid cell lines with various mutation loads. The m.1630A>G mutation impairs oxygen consumption, affects the stability of the MTTV and reduces the levels of subunits of the electron transport chain.

Mitochondrial optic neuropathies - disease mechanisms and therapeutic strategies

Leber hereditary optic neuropathy (LHON) and autosomal-dominant optic atrophy (DOA) are the two most common inherited optic neuropathies in the general population. Both disorders share striking pathological similarities, marked by the selective loss of retinal ganglion cells (RGCs) and the early involvement of the papillomacular bundle. Three mitochondrial DNA (mtDNA) point mutations; m.3460G>A, m.11778G>A, and m.14484T>C account for over 90% of LHON cases, and in DOA, the majority of affected families harbour mutations in the OPA1 gene, which codes for a mitochondrial inner membrane protein. Optic nerve degeneration in LHON and DOA is therefore due to disturbed mitochondrial function and a predominantly complex I respiratory chain defect has been identified using both in vitro and in vivo biochemical assays. However, the trigger for RGC loss is much more complex than a simple bioenergetic crisis and other important disease mechanisms have emerged relating to mitochondrial network dynamics, mtDNA maintenance, axonal transport, and the involvement of the cytoskeleton in maintaining a differential mitochondrial gradient at sites such as the lamina cribosa. The downstream consequences of these mitochondrial disturbances are likely to be influenced by the local cellular milieu. The vulnerability of RGCs in LHON and DOA could derive not only from tissue-specific, genetically-determined biological factors, but also from an increased susceptibility to exogenous influences such as light exposure, smoking, and pharmacological agents with putative mitochondrial toxic effects. Our concept of inherited mitochondrial optic neuropathies has evolved over the past decade, with the observation that patients with LHON and DOA can manifest a much broader phenotypic spectrum than pure optic nerve involvement. Interestingly, these phenotypes are sometimes clinically indistinguishable from other neurodegenerative disorders such as Charcot-Marie-Tooth disease, hereditary spastic paraplegia, and multiple sclerosis, where mitochondrial dysfunction is also thought to be an important pathophysiological player. A number of vertebrate and invertebrate disease models has recently been established to circumvent the lack of human tissues, and these have already provided considerable insight by allowing direct RGC experimentation. The ultimate goal is to translate these research advances into clinical practice and new treatment strategies are currently being investigated to improve the visual prognosis for patients with mitochondrial optic neuropathies.

Leber's hereditary optic neuropathy is associated with the T12338C mutation in mitochondrial ND5 gene in six Han Chinese families

PURPOSE

To investigate the molecular pathogenesis of Leber's hereditary optic neuropathy (LHON) in Chinese families.

DESIGN

Six Han Chinese families who seem to have maternally transmitted LHON were studied by clinical, genetic, and molecular evaluations.

PARTICIPANTS

One hundred twenty-seven subjects from 6 Chinese families with a wide range of age-at-onset and severity of visual impairment.

METHODS

All subjects underwent clinical examination, genetic evaluation, and molecular analysis of mitochondrial DNA (mtDNA).

MAIN OUTCOME MEASURES

The ophthalmologic examinations included visual acuity, visual field examination, visual evoked potentials, and fundus photography. The mtDNA analysis included the polymerase chain reaction (PCR) amplification of entire mtDNA and subsequent sequence determination.

RESULTS

Six families exhibited low penetrance of visual impairment, with an average of 10.8%. In particular, 9 (6 males/3 females) of 86 matrilineal relatives in these families exhibited variable severity and age at onset in visual dysfunction. The average age at onset of visual loss was 20 years. Molecular analysis of mtDNA in these families identified the homoplasmic ND5T12338C mutation and distinct set of variants belonging to the Asian haplogroup F2. The T12338C mutation is only present in the maternal lineage of those pedigrees and not in 178 Chinese controls. This mutation resulted in the replacement of the first amino acid, a translation-initiating methionine with a threonine, shortening 2 amino acids of ND5 polypeptide. The T12338C mutation is also located in 2 nucleotides adjacent to the 3' end of the tRNA(Leu(CUN)). Thus, this mutation may alter ND5 mRNA metabolism and the processing of RNA precursors. As a result, this mutation impairs respiratory function, leading to visual impairment.

CONCLUSIONS

Several lines of evidence suggest that the mitochondrial ND5T12338C mutation is associated with LHON. The tissue specificity of this mutation is likely due to the involvement of nuclear modifier genes. The identification of nuclear modifiers is important for the elucidation of the pathogenic mechanism of LHON and an open avenue for therapeutic interventions. The T12338C mutation should be added to the list of inherited risk factors for future molecular diagnosis. Our findings are helpful for counseling families with LHON.

Oestrogens ameliorate mitochondrial dysfunction in Leber's hereditary optic neuropathy

Leber's hereditary optic neuropathy, the most frequent mitochondrial disease due to mitochondrial DNA point mutations in complex I, is characterized by the selective degeneration of retinal ganglion cells, leading to optic atrophy and loss of central vision prevalently in young males. The current study investigated the reasons for the higher prevalence of Leber's hereditary optic neuropathy in males, exploring the potential compensatory effects of oestrogens on mutant cell metabolism. Control and Leber's hereditary optic neuropathy osteosarcoma-derived cybrids (11778/ND4, 3460/ND1 and 14484/ND6) were grown in glucose or glucose-free, galactose-supplemented medium. After having shown the nuclear and mitochondrial localization of oestrogen receptors in cybrids, experiments were carried out by adding 100 nM of 17β-oestradiol. In a set of experiments, cells were pre-incubated with the oestrogen receptor antagonist ICI 182780. Leber's hereditary optic neuropathy cybrids in galactose medium presented overproduction of reactive oxygen species, which led to decrease in mitochondrial membrane potential, increased apoptotic rate, loss of cell viability and hyper-fragmented mitochondrial morphology compared with control cybrids. Treatment with 17β-oestradiol significantly rescued these pathological features and led to the activation of the antioxidant enzyme superoxide dismutase 2. In addition, 17β-oestradiol induced a general activation of mitochondrial biogenesis and a small although significant improvement in energetic competence. All these effects were oestrogen receptor mediated. Finally, we showed that the oestrogen receptor β localizes to the mitochondrial network of human retinal ganglion cells. Our results strongly support a metabolic basis for the unexplained male prevalence in Leber's hereditary optic neuropathy and hold promises for a therapeutic use for oestrogen-like molecules.

Mitochondrial haplogroup M9a specific variant ND1 T3394C may have a modifying role in the phenotypic expression of the LHON-associated ND4 G11778A mutation

We report here the clinical, genetic and molecular characterization of four Han Chinese families with Leber's hereditary optic neuropathy (LHON). The penetrances of optic neuropathy in these Chinese pedigrees were 38%, 38%, 44% and 56%. This observation is in contrast with the previously identified 14 Chinese families with very low penetrance of LHON. The age-at-onset for visual impairment in matrilineal relatives in these Chinese families varied from 18 to 30years. Furthermore, the ratios between affected male and female matrilineal relatives in these families were 3:0, 3:0, 3:1 and 2:3, respectively. Molecular analysis of mitochondrial genomes identified the known ND4 G11778A mutation and distinct sets of variants belonging to the Asian haplogroups M9a. Of these, the ND1 T3394C mutation caused the substitution of a highly conserved histidine for tyrosine (Y30H) at amino acid position 30. This mutation was associated with LHON in other families with low penetrance of optic neuropathy and other clinical abnormalities. The presence of both G11778A and T3394C mutations appears to contribute to higher penetrance of optic neuropathy in these four Chinese families than other Chinese families carrying only the G11778A mutation. Therefore, the mitochondrial haplogroup M9a specific variant T3394C may modulate the phenotypic manifestation of LHON-associated G11778A mutation in these Chinese pedigrees.

Leber's hereditary optic neuropathy affects only female matrilineal relatives in two Chinese families

PURPOSE

The purpose of this study was to investigate the role of modifier factors in the expression of Leber's hereditary optic neuropathy (LHON).

METHODS

Thirty-five subjects from two Han Chinese families with maternally transmitted LHON underwent a clinical and genetic evaluation and molecular analysis of mitochondrial (mt)DNA.

RESULTS

Matrilineal relatives in the two Chinese families exhibited a wide range of severity in visual impairment, from blindness to nearly normal vision. Very strikingly, all nine affected individuals of 21 matrilineal relatives (13 females/8 males) were female, which translates to 33% and 57% of penetrance for optic neuropathy in the two families. The average age at onset was 22 and 25 years. These observations were in contrast with typical features in many LHON pedigrees that have a predominance of affected males. Molecular analysis of their mtDNAs identified the homoplasmic ND4 G11778A mutation and distinct sets of variants belonging to the Asian haplogroups M1 and M10a. Of other variants, the L175F variant in CO3; the I58V variant in ND6; and the I189V, L292R, and S297A variants in CYTB were located at highly conserved residues of polypeptides.

CONCLUSIONS

Only female matrilineal relatives with a wide range of penetrance, severity, and age at onset of optic neuropathy in these two Chinese pedigrees showed the involvement of X-linked or autosomal recessive modifier genes in the phenotypic manifestation of the G11778A mutation. Furthermore, mitochondrial haplogroup-specific variants, together with epigenetic and environmental factors, may contribute to the phenotypic manifestation of the primary LHON-associated G11778A mutation in these pedigrees.