Late-onset encephalopathy associated with a C11777A mutation of mitochondrial DNA

Mitochondrial respiratory complexes: Significance in human mitochondrial disorders and cancers

Mitochondria are pivotal organelles that govern cellular energy production through the oxidative phosphorylation system utilizing five respiratory complexes. In addition, mitochondria also contribute to various critical signaling pathways including apoptosis, damage-associated molecular patterns, calcium homeostasis, lipid, and amino acid biosynthesis. Among these diverse functions, the energy generation program oversee by mitochondria represents an immaculate orchestration and functional coordination between the mitochondria and nuclear encoded molecules. Perturbation in this program through respiratory complexes' alteration results in the manifestation of various mitochondrial disorders and malignancy, which is alarmingly becoming evident in the recent literature. Considering the clinical relevance and importance of this emerging medical problem, this review sheds light on the timing and nature of molecular alterations in various respiratory complexes and their functional consequences observed in various mitochondrial disorders and human cancers. Finally, we discussed how this wealth of information could be exploited and tailored to develop respiratory complex targeted personalized therapeutics and biomarkers for better management of various incurable human mitochondrial disorders and cancers.

A Systematic Review of the Impact of Mitochondrial Variations on Male Infertility

According to current estimates, infertility affects one in four couples trying to conceive. Primary or secondary infertility can be due either to both partners or only to the man or the woman. Up to 15% of infertility cases in men can be attributed to genetic factors that can lead to irreversible partial or complete spermatogenic arrest. The increased use of assisted reproductive technology (ART) has provided not only insights into the causes of male infertility but also afforded a diagnostic tool to detect and manage this condition among couples. Genes control a variety of physiological attributes, such as the hypothalamic–pituitary–gonadal axis, development, and germ cell differentiation. In the era of ART, it is important to understand the genetic basis of infertility so as to provide the most tailored therapy and counseling to couples. Genetic factors involved in male infertility can be chromosome abnormalities or single-gene disorders, mitochondrial DNA (mtDNA) mutations, Y-chromosome deletions, multifactorial disorders, imprinting disorders, or endocrine disorders of genetic origin. In this review, we discuss the role of mitochondria and the mitochondrial genome as an indicator of sperm quality and fertility.

Mitochondrial nicotinamide adenine dinucleotide hydride dehydrogenase (NADH) subunit 4 (MTND4) polymorphisms and their association with male infertility

Purpose

The purpose of the present study was to determine the relationship between infertility and the polymorphisms of mitochondrial NADH dehydrogenase subunit 4 (MTND4) by spermatozoa analysis in fertile and subfertile men.

Methods

Samples were divided into 68 subfertile men (case group) and 44 fertile men (control group). After semen analysis, samples were purified. The whole genome was extracted using a QIAamp DNA Mini Kit and the mitochondrial DNA was amplified by using the REPLI-g Mitochondrial DNA Kit. Polymerase chain reaction (PCR) was used to amplify the MT-ND4 gene. Then, samples were purified and sequenced using the Sanger method.

Results

Twenty-five single-nucleotide polymorphisms (SNPs) were identified in the MTND4 gene. The genotype frequencies of the study population showed a statistically significant association between rs2853495 G>A (Gly320Gly) and male infertility (P = 0.0351). Similarly, the allele frequency test showed that rs2853495 G>A (Gly320Gly) and rs869096886 A>G (Leu164Leu) were significantly associated with male infertility (adjusted OR = 2.616, 95% CI = 1.374–4.983, P = 0.002; adjusted OR = 2.237, 95% CI = 1.245–4.017, P = 0.007, respectively).

Conclusion

In conclusion, our findings suggested that male infertility was correlated with rs2853495 and rs869096886 SNPs in MTND4.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10815-021-02199-w.

Blackout in the powerhouse: clinical phenotypes associated with defects in the assembly of OXPHOS complexes and the mitoribosome

Mitochondria produce the bulk of the energy used by almost all eukaryotic cells through oxidative phosphorylation (OXPHOS) which occurs on the four complexes of the respiratory chain and the F₁–F₀ ATPase. Mitochondrial diseases are a heterogenous group of conditions affecting OXPHOS, either directly through mutation of genes encoding subunits of OXPHOS complexes, or indirectly through mutations in genes encoding proteins supporting this process. These include proteins that promote assembly of the OXPHOS complexes, the post-translational modification of subunits, insertion of cofactors or indeed subunit synthesis. The latter is important for all 13 of the proteins encoded by human mitochondrial DNA, which are synthesised on mitochondrial ribosomes. Together the five OXPHOS complexes and the mitochondrial ribosome are comprised of more than 160 subunits and many more proteins support their biogenesis. Mutations in both nuclear and mitochondrial genes encoding these proteins have been reported to cause mitochondrial disease, many leading to defective complex assembly with the severity of the assembly defect reflecting the severity of the disease. This review aims to act as an interface between the clinical and basic research underpinning our knowledge of OXPHOS complex and ribosome assembly, and the dysfunction of this process in mitochondrial disease.

Mitochondrial Medicine: Genetic Underpinnings and Disease Modeling Using Induced Pluripotent Stem Cell Technology

Mitochondrial medicine is an exciting and rapidly evolving field. While the mitochondrial genome is small and differs from the nuclear genome in that it is circular and free of histones, it has been implicated in neurodegenerative diseases, type 2 diabetes, aging and cardiovascular disorders. Currently, there is a lack of efficient treatments for mitochondrial diseases. This has promoted the need for developing an appropriate platform to investigate and target the mitochondrial genome. However, developing these therapeutics requires a model system that enables rapid and effective studying of potential candidate therapeutics. In the past decade, induced pluripotent stem cells (iPSCs) have become a promising technology for applications in basic science and clinical trials, and have the potential to be transformative for mitochondrial drug development. Engineered iPSC-derived cardiomyocytes (iPSC-CM) offer a unique tool to model mitochondrial disorders. Additionally, these cellular models enable the discovery and testing of novel therapeutics and their impact on pathogenic mtDNA variants and dysfunctional mitochondria. Herein, we review recent advances in iPSC-CM models focused on mitochondrial dysfunction often causing cardiovascular diseases. The importance of mitochondrial disease systems biology coupled with genetically encoded NAD⁺/NADH sensors is addressed toward developing an in vitro translational approach to establish effective therapies.

Mitochondrial disorders of the OXPHOS system

Mitochondrial disorders are among the most frequent inborn errors of metabolism, their primary cause being the dysfunction of the oxidative phosphorylation system (OXPHOS). OXPHOS is composed of the electron transport chain (ETC), formed by four multimeric enzymes and two mobile electron carriers, plus an ATP synthase [also called complex V (cV)]. The ETC performs the redox reactions involved in cellular respiration while generating the proton motive force used by cV to synthesize ATP. OXPHOS biogenesis involves multiple steps, starting from the expression of genes encoded in physically separated genomes, namely the mitochondrial and nuclear DNA, to the coordinated assembly of components and cofactors building each individual complex and eventually the supercomplexes. The genetic cause underlying around half of the diagnosed mitochondrial disease cases is currently known. Many of these cases result from pathogenic variants in genes encoding structural subunits or additional factors directly involved in the assembly of the ETC complexes. Here, we review the historical and most recent findings concerning the clinical phenotypes and the molecular pathological mechanisms underlying this particular group of disorders.

Mitochondria as central regulators of neural stem cell fate and cognitive function

Emerging evidence now indicates that mitochondria are central regulators of neural stem cell (NSC) fate decisions and are crucial for both neurodevelopment and adult neurogenesis, which in turn contribute to cognitive processes in the mature brain. Inherited mutations and accumulated damage to mitochondria over the course of ageing serve as key factors underlying cognitive defects in neurodevelopmental disorders and neurodegenerative diseases, respectively. In this Review, we explore the recent findings that implicate mitochondria as crucial regulators of NSC function and cognition. In this respect, mitochondria may serve as targets for stem-cell-based therapies and interventions for cognitive defects.

Mammalian Mitochondrial Complex I Structure and Disease-Causing Mutations

Complex I has an essential role in ATP production by coupling electron transfer from NADH to quinone with translocation of protons across the inner mitochondrial membrane. Isolated complex I deficiency is a frequent cause of mitochondrial inherited diseases. Complex I has also been implicated in cancer, ageing, and neurodegenerative conditions. Until recently, the understanding of complex I deficiency on the molecular level was limited due to the lack of high-resolution structures of the enzyme. However, due to developments in single particle cryo-electron microscopy (cryo-EM), recent studies have reported nearly atomic resolution maps and models of mitochondrial complex I. These structures significantly add to our understanding of complex I mechanism and assembly. The disease-causing mutations are discussed here in their structural context.

Mitochondrial dysfunction and its role in tissue-specific cellular stress

Mitochondrial bioenergetics require the coordination of two different and independent genomes. Mutations in either genome will affect mitochondrial functionality and produce different sources of cellular stress. Depending on the kind of defect and stress, different tissues and organs will be affected, leading to diverse pathological conditions. There is no curative therapy for mitochondrial diseases, nevertheless, there are strategies described that fight the various stress forms caused by the malfunctioning organelles. Here, we will revise the main kinds of stress generated by mutations in mitochondrial genes and outline several ways of fighting this stress.

Post-Translational Oxidative Modifications of Mitochondrial Complex I (NADH: Ubiquinone Oxidoreductase): Implications for Pathogenesis and Therapeutics in Human Diseases

Mitochondrial complex I (NADH: ubiquinone oxidoreductase; CI) is central to the electron transport chain (ETC), oxidative phosphorylation, and ATP production in eukaryotes. CI is a multi-subunit complex with a complicated yet organized structure that optimally connects electron transfer with proton translocation and forms higher-order supercomplexes with other ETC complexes. Efforts to understand the molecular genetics, expression profile of subunits, and structure-function relationship of CI have increased over the years due to the direct role of the complex in human diseases. Although mutations in the nuclear and mitochondrial genes of CI and altered expression of subunits could potentially lower CI activity leading to mitochondrial dysfunction in many diseases, oxidative post-translational modifications (PTMs) have emerged as an important mechanism contributing to altered CI activity. These mainly include reversible and irreversible cysteine modifications, tyrosine nitration, carbonylation, and tryptophan oxidation that are generated following exposure to reactive oxygen species/reactive nitrogen species. Interestingly, oxidative PTMs could contribute either to CI damage, mitochondrial dysfunction, and ensuing cell death or a response mechanism with potential cytoprotective effects. This has also emerged as a promising field for structural biologists since analysis of PTMs could assist in understanding the structure-function relationship of the complex and correlate electron transfer mechanism with energy production. However, analysis of PTMs of CI and their contribution to CI function are incomplete in many physiological and pathological conditions. This review aims to highlight the role of oxidative PTMs in modulating CI activity with implications toward pathobiology of CNS diseases and novel therapeutics.

Mitochondrial DNA sequencing and large-scale genotyping identifies MT-ND4 gene mutation m.11696G>A associated with idiopathic oligoasthenospermia

Genetic variants of mitochondrial DNA (mtDNA) were implicated to be associated with male infertility. Our previous whole mitochondrial genome sequencing and association study has identified two susceptibility mtDNA variants for oligoasthenospermia in Han Chinese men. In this study, we tested promising associations in an extended validation using 670 idiopathic oligoasthenospermia cases and 793 healthy controls to identify additional risk variants. We found that the genetic variant of m.11696G>A showed significantly higher frequency in the case group than that in the control group (odds ratio (OR) 2.21, 95% CI 1.21-4.04) (P=7.90×10⁻³). To elucidate the exact role of the genetic variants in spermatogenesis, two main sperm parameters (sperm count and motility) were taken into account. We found that m.11696G>A was associated with low sperm motility, with the OR of 2.38 (95 % CI 1.27-4.46) (P =5.22×10⁻³). These results advance our understanding of the genetic susceptibility to oligoasthenospermia and more functional studies are needed to provide insights into its pathogenic mechanism.

Dynamics of Human Mitochondrial Complex I Assembly: Implications for Neurodegenerative Diseases

Neurons are extremely energy demanding cells and highly dependent on the mitochondrial oxidative phosphorylation (OXPHOS) system. Mitochondria generate the energetic potential via the respiratory complexes I to IV, which constitute the electron transport chain (ETC), together with complex V. These redox reactions release energy in the form of ATP and also generate reactive oxygen species (ROS) that are involved in cell signaling but can eventually lead to oxidative stress. Complex I (CI or NADH:ubiquinone oxidoreductase) is the largest ETC enzyme, containing 44 subunits and the main contributor to ROS production. In recent years, the structure of the CI has become available and has provided new insights into CI assembly. A number of chaperones have been identified in the assembly and stability of the mature holo-CI, although they are not part of its final structure. Interestingly, CI dysfunction is the most common OXPHOS disorder in humans and defects in the CI assembly process are often observed. However, the dynamics of the events leading to CI biogenesis remain elusive, which precludes our understanding of how ETC malfunctioning affects neuronal integrity. Here, we review the current knowledge of the structural features of CI and its assembly factors and the potential role of CI misassembly in human disorders such as Complex I Deficiencies or Alzheimer's and Parkinson's diseases.

Cognitive dysfunction in mitochondrial disorders

Among the various central nervous system (CNS) manifestations of mitochondrial disorders (MIDs), cognitive impairment is increasingly recognized and diagnosed (mitochondrial cognitive dysfunction). Aim of the review was to summarize recent findings concerning the aetiology, pathogenesis, diagnosis and treatment of cognitive decline in MIDs. Among syndromic MIDs due to mitochondrial DNA (mtDNA) mutations, cognitive impairment occurs in patients with mitochondrial encephalopathy, lactic acidosis and stroke-like episodes syndrome, myoclonus epilepsy with ragged-red fibres syndrome, mitochondrial chronic progressive external ophthalmoplegia, Kearns-Sayre syndrome, neuropathy, ataxia and retinitis pigmentosa syndrome and maternally inherited diabetes and deafness. Among syndromic MIDs due to nuclear DNA (nDNA) mutations, cognitive decline has been reported in myo-neuro-gastro-intestinal encephalopathy, mitochondrial recessive ataxia syndrome, spinocerebellar ataxia with encephalopathy, Mohr-Tranebjaerg syndrome, leuko-encephalopathy; brain and spinal cord involvement and lactic acidosis, CMT2, Wolfram syndrome, Wolf-Hirschhorn syndrome and Leigh syndrome. In addition to syndromic MIDs, a large number of non-syndromic MIDs due to mtDNA as well as nDNA mutations have been reported, which present with cognitive impairment as the sole or one among several other CNS manifestations of a MID. Delineation of mitochondrial cognitive impairment from other types of cognitive impairment is essential to guide the optimal management of these patients. Treatment of mitochondrial cognitive impairment is largely limited to symptomatic and supportive measures. Cognitive impairment may be a CNS manifestation of syndromic as well as non-syndromic MIDs. Correct diagnosis of mitochondrial cognitive impairment is a prerequisite for the optimal management of these patients.

Persistent impairment of mitochondrial and tissue redox status during lithium-pilocarpine-induced epileptogenesis

Mitochondrial dysfunction and oxidative stress are known to occur following acute seizure activity but their contribution during epileptogenesis is largely unknown. The goal of this study was to determine the extent of mitochondrial oxidative stress, changes to redox status, and mitochondrial DNA (mtDNA) damage during epileptogenesis in the lithium-pilocarpine model of temporal lobe epilepsy. Mitochondrial oxidative stress, changes in tissue and mitochondrial redox status, and mtDNA damage were assessed in the hippocampus and neocortex of Sprague-Dawley rats at time points (24h to 3months) following lithium-pilocarpine administration. A time-dependent increase in mitochondrial hydrogen peroxide (H(2)O(2)) production coincident with increased mtDNA lesion frequency in the hippocampus was observed during epileptogenesis. Acute increases (24-48h) in H(2)O(2) production and mtDNA lesion frequency were dependent on the severity of convulsive seizure activity during initial status epilepticus. Tissue levels of GSH, GSH/GSSG, coenzyme A (CoASH), and CoASH/CoASSG were persistently impaired at all measured time points throughout epileptogenesis, that is, acutely (24-48h), during the 'latent period' (48h to 7days), and chronic epilepsy (21days to 3months). Together with our previous work, these results demonstrate the model independence of mitochondrial oxidative stress, genomic instability, and persistent impairment of mitochondrial specific redox status during epileptogenesis. Lasting impairment of mitochondrial and tissue redox status during the latent period, in addition to the acute and chronic phases of epileptogenesis, suggests that redox-dependent processes may contribute to the progression of epileptogenesis in experimental temporal lobe epilepsy.

Mitochondrial DNA 11777C>A mutation associated Leigh syndrome: case report with a review of the previously described pedigrees

Here, we present a male infant with clinical signs of typical Leigh syndrome. The first symptom, myoclonus was presented already on the 5th day of life; at 7 months of age limb convulsions and cerebral paresis with hypotonia were developed. At the age of 11 months, MRI verified increased signal intensity in the entire mesencephalon and medulla oblongata; while gray matter proton spectroscopy revealed presence of lactate increase in the brain. At age of 17 months, the child died in cardiorespiratory arrest. After autopsy, the diagnosis of Leigh syndrome was established; using DNA isolated from skeletal muscle and liver, heteroplasmic (>50%) mitochondrial 11777C>A was detected in the fourth subunit of NADH dehydrogenase enzyme (MTND4) encoding gene, which causes Arg --> Ser replacement. The mutation was also detected in low copy number in blood of mother. Albeit this mutation type is well recognized as a typical mtDNA mutation, according the reports available on the PubMed, this mutation was described only in four patients with wide phenotypic variations; here, we reviewed the characteristic clinical features of them. Taken together, the earliest onset of symptoms, the nature of the first presentation signs, the most rapid progression, the character of minor additional symptoms, and the early fatal outcome differentiate the phenotypic variant of the proband presented here from cases reported so far by others.

Metabolic myopathies: update 2009

Metabolic myopathies are inborn errors of metabolism that result in impaired energy production due to defects in glycogen, lipid, mitochondrial, and possibly adenine nucleotide metabolism. Fatty acid oxidation defects (FAOD), glycogen storage disease, and mitochondrial myopathies represent the 3 main groups of disorders, and some consider myoadenylate deaminase (AMPD1 deficiency) to be a metabolic myopathy. Clinically, a variety of neuromuscular presentations are seen at different ages of life. Newborns and infants commonly present with hypotonia and multisystem involvement (liver and brain), whereas onset later in life usually presents with exercise intolerance with or without progressive muscle weakness and myoglobinuria. In general, the glycogen storage diseases result in high-intensity exercise intolerance, whereas the FAODs and the mitochondrial myopathies manifest predominately during endurance-type activity or under fasted or other metabolically stressful conditions. The clinical examination is often normal, and testing requires various combinations of exercise stress testing, serum creatine kinase activity and lactate concentration determination, urine organic acids, muscle biopsy, neuroimaging, and specific genetic testing for the diagnosis of a specific metabolic myopathy. Prenatal screening is available in many countries for several of the FAODs through liquid chromatography-tandem mass spectrometry. Early identification of these conditions with lifestyle measures, nutritional intervention, and cofactor treatment is important to prevent or delay the onset of muscle weakness and to avoid potential life-threatening complications such as rhabdomyolysis with resultant renal failure or hepatic failure. This article will review the key clinical features, diagnostic tests, and treatment recommendations for the more common metabolic myopathies, with an emphasis on mitochondrial myopathies.

Mitochondrial DNA damage and impaired base excision repair during epileptogenesis

Oxidative stress and mitochondrial dysfunction are acute consequences of status epilepticus (SE). However, the role of mitochondrial oxidative stress and genomic instability during epileptogenesis remains unknown. Using the kainate animal model of temporal lobe epilepsy, we investigated oxidative mitochondrial DNA (mtDNA) damage and changes in the mitochondrial base excision repair pathway (mtBER) in the rat hippocampus for a period of 3 months after SE. Acute seizure activity caused a time-dependent increase in mitochondrial, but not nuclear 8-hydroxy-2-deoxyguanosine (8-OHdG/2dG) levels and a greater frequency of mtDNA lesions. This was accompanied by increased mitochondrial H2O2 production and a transient decrease in mtDNA repair capacity. The mtBER proteins 8-oxoguanine glycosylase (Ogg1) and DNA polymerase gamma (Pol gamma) demonstrated elevated expression at mRNA and protein levels shortly after SE and this was followed by a gradual improvement in mtDNA repair capacity. Recurrent seizures associated with the chronic phase of epilepsy coincided with the accumulation of mtDNA damage, increased mitochondrial H2O2 levels, decreased expression of Ogg1 and Pol gamma and impaired mtDNA repair capacity. Together, increased oxidative mtDNA damage, mitochondrial H2O2 production and alterations in the mtBER pathway provide evidence for mitochondrial oxidative stress in epilepsy and suggest that mitochondrial injury may contribute to epileptogenesis.

Pathogenic mitochondrial DNA mutations in protein-coding genes

More than 200 disease-related mitochondrial DNA (mtDNA) point mutations have been reported in the Mitomap (http://www.mitomap.org) database. These mutations can be divided into two groups: mutations affecting mitochondrial protein synthesis, including mutations in tRNA and rRNA genes; and mutations in protein-encoding genes (mRNAs). This review focuses on mutations in mitochondrial genes that encode proteins. These mutations are involved in a broad spectrum of human diseases, including a variety of multisystem disorders as well as more tissue-specific diseases such as isolated myopathy and Leber hereditary optic neuropathy (LHON). Because the mitochondrial genome contains a large number of apparently neutral polymorphisms that have little pathogenic significance, along with secondary homoplasmic mutations that do not have primary disease-causing effect, the pathogenic role of all newly discovered mutations must be rigorously established. A scoring system has been applied to evaluate the pathogenicity of the mutations in mtDNA protein-encoding genes and to review the predominant clinical features and the molecular characteristics of mutations in each mtDNA-encoded respiratory chain complex.

Analysis of disease-associated ND4 mutations: How do we know which mutation is pathogenic?

It is not uncommon to identify more than one mtDNA replacement mutations in the specimens from patients. However, we usually do not know if the identified mtDNA mutation is pathogenic or not. Even functional assays are available to use, we would not know which mutation(s) is to be tested. To provide a rapid method for initial evaluation for the pathogenicity of the replacement mutation, we compared three evolutional analyses: primate conservation index (PCI), mammalian conservation index (MCI), and conservation index across a wide spectrum of species (CI). After analyzing 35 so-called diseases-associated replacement mutations of ND4, we found 8 pathogenic mutations, 15 nonpathogenic mutations, and 12 mutations of undetermined significance. The MCI classification appears to be the best one among the three systems. This study demonstrates that evolutional analysis can serve as a rapid evaluation for the pathogenicity of mtDNA replacement mutations.

Biochemical and molecular diagnosis of mitochondrial respiratory chain disorders

Biochemical diagnosis of mitochondrial respiratory chain disorders requires caution to avoid misdiagnosis of secondary enzyme defects, and can be improved by the use of conservative diagnostic criteria. Pathogenic mutations causing mitochondrial disorders have now been identified in more than 30 mitochondrial DNA (mtDNA) genes encoding respiratory chain subunits, ribosomal- and t-RNAs. mtDNA mutations appear to be responsible for most adult patients with mitochondrial disease and approximately a quarter of paediatric patients. A family history suggesting maternal inheritance is the exception rather than the norm for children with mtDNA mutations, many of whom have de novo mutations. Prenatal diagnosis and pre-implantation genetic diagnosis can be offered to some women at risk of transmitting a mtDNA mutation, particularly those at lower recurrence risk. Mutations in more than 30 nuclear genes, including those encoding for respiratory chain subunits and assembly factors, have now been shown to cause mitochondrial disorders, creating difficulties in prioritising which genes should be studied by mutation analysis in individual patients. A number of approaches offer promise to guide the choice of candidate genes, including Blue Native-PAGE immunoblotting and microarray expression analysis.

Clinical and molecular findings in children with complex I deficiency

Isolated complex I deficiency, the most frequent OXPHOS disorder in infants and children, is genetically heterogeneous. Mutations have been found in seven mitochondrial DNA (mtDNA) and eight nuclear DNA encoded subunits, respectively, but in most of the cases the genetic basis of the biochemical defect is unknown. We analyzed the entire mtDNA and 11 nuclear encoded complex I subunits in 23 isolated complex I-deficient children, classified into five clinical groups: Leigh syndrome, progressive leukoencephalopathy, neonatal cardiomyopathy, severe infantile lactic acidosis, and a miscellaneous group of unspecified encephalomyopathies. A genetic definition was reached in eight patients (35%). Mutations in mtDNA were found in six out of eight children with Leigh syndrome, indicating a prevalent association between this phenotype and abnormalities in ND genes. In two patients with leukoencephalopathy, homozygous mutations were detected in two different nuclear-encoded complex I genes, including a novel transition in NDUFS1 subunit. In addition to these, a child affected by mitochondrial encephalomyopathy had heterozygous mutations in NDUFA8 and NDUFS2 genes, while another child with neonatal cardiomyopathy had a complex rearrangement in a single NDUFS7 allele. The latter cases suggest the possibility of unconventional patterns of inheritance in complex I defects.

De novo mutations in the mitochondrial ND3 gene as a cause of infantile mitochondrial encephalopathy and complex I deficiency

Both nuclear and mitochondrial DNA mutations can cause energy generation disorders. Respiratory chain complex I deficiency is the most common energy generation disorder and a frequent cause of infantile mitochondrial encephalopathies such as Leigh's disease and lethal infantile mitochondrial disease. Most such cases have been assumed to be caused by nuclear gene defects, but recently an increasing number have been shown to be caused by mutations in the mitochondrially encoded complex I subunit genes ND4, ND5, and ND6. We report the first four cases of infantile mitochondrial encephalopathies caused by mutations in the ND3 subunit gene. Three unrelated children have the same novel heteroplasmic mutation (T10158C), only the second mutation reported in ND3, and one has the previously identified T10191C mutation. Both mutations cause disproportionately greater reductions in enzyme activity than in the amount of fully assembled complex I, suggesting the ND3 subunit plays an unknown but important role in electron transport, proton pumping, or ubiquinone binding. Three cases appear to have a de novo mutation, with no mutation detected in maternal relatives. Mitochondrial DNA disease may be considerably more prevalent in the pediatric population than currently predicted and should be considered in patients with infantile mitochondrial encephalopathies and complex I deficiency.

SOD2 gene transfer protects against optic neuropathy induced by deficiency of complex I

Mutations in genes encoding the NADH ubiquinone oxidoreductase, complex I of the respiratory chain, cause a diverse group of diseases. They include Leber hereditary optic neuropathy, Leigh syndrome, and mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes. There is no effective treatment for these or any other mitochondrial disorder. Using a unique animal model of severe complex I deficiency induced by ribozymes targeted against a critical complex I subunit gene (NDUFA1), we attempted rescue of the optic nerve degeneration associated with Leber hereditary optic neuropathy. We used adenoassociated virus to deliver the human gene for SOD2 to the visual system of disease-induced mice. Relative to mock infection, SOD2 reduced apoptosis of retinal ganglion cells and degeneration of optic nerve fibers, the hallmarks of this disease. Rescue of this animal model supports a critical role for oxidative injury in disorders with complex I deficiency and shows that a respiratory deficit may be effectively treated in mammals, thus offering hope to patients.