Mitochondrial DNA mutations in disease and aging

The small mammalian mitochondrial DNA (mtDNA) is very gene dense and encodes factors critical for oxidative phosphorylation. Mutations of mtDNA cause a variety of human mitochondrial diseases and are also heavily implicated in age-associated disease and aging. There has been considerable progress in our understanding of the role for mtDNA mutations in human pathology during the last two decades, but important mechanisms in mitochondrial genetics remain to be explained at the molecular level. In addition, mounting evidence suggests that most mtDNA mutations may be generated by replication errors and not by accumulated damage.     

The role of mitochondrial DNA mutations in mammalian aging

Mitochondrial DNA (mtDNA) accumulates both base-substitution mutations and deletions with aging in several tissues in mammals. Here, we examine the evidence supporting a causative role for mtDNA mutations in mammalian aging. We describe and compare human diseases and mouse models associated with mitochondrial genome instability. We also discuss potential mechanisms for the generation of these mutations and the means by which they may mediate their pathological consequences. Strategies for slowing the accumulation and attenuating the effects of mtDNA mutations are discussed.     

Mitochondrial dynamics and aging: Mitochondrial interaction preventing individuals from expression of respiratory deficiency caused by mutant mtDNA

In mammalian cells, there is an extensive and continuous exchange of mitochondrial DNA (mtDNA) and its products between mitochondria. This mitochondrial complementation prevents individuals from expression of respiration deficiency caused by mutant mtDNAs. Thus, the presence of mitochondrial complementation does not support the generally accepted mitochondrial theory of aging, which proposes that accumulation of somatic mutations in mtDNA is responsible for age-associated mitochondrial dysfunction. Moreover, the presence of mitochondrial complementation enables gene therapy for mitochondrial diseases using nuclear transplantation of zygotes.     

Age-Associated Alterations of the Mitochondrial Genome

Age-associated alterations of the mitochondrial genome occur in several different species; however, their physiological relevance remains unclear. The age-associated changes of mitochondrial DNA (mtDNA) include nucleotide point mutations and modifications, as well as deletions. In this review, we summarize the current literature on age-associated mtDNA mutations and deletions and comment on their abundance. A clear need exists for a more thorough evaluation of the total damage to the mitochondrial genome that accumulates in aged tissues. © 1997 Elsevier Science Inc.     

Mitochondrial genome maintenance in health and disease

Human mitochondria harbor an essential, high copy number, 16,569 base pair, circular DNA genome that encodes 13 gene products required for electron transport and oxidative phosphorylation. Mutation of this genome can compromise cellular respiration, ultimately resulting in a variety of progressive metabolic diseases collectively known as ‘mitochondrial diseases’. Mutagenesis of mtDNA and the persistence of mtDNA mutations in cells and tissues is a complex topic, involving the interplay of DNA replication, DNA damage and repair, purifying selection, organelle dynamics, mitophagy, and aging. We briefly review these general elements that affect maintenance of mtDNA, and we focus on nuclear genes encoding the mtDNA replication machinery that can perturb the genetic integrity of the mitochondrial genome.     

Mechanisms linking mtDNA damage and aging

In the past century, considerable efforts were made to understand the role of mitochondrial DNA (mtDNA) mutations and of oxidative stress in aging. The classic mitochondrial free radical theory of aging, in which mtDNA mutations cause genotoxic oxidative stress, which in turn creates more mutations, has been a central hypothesis in the field for decades. In the past few years, however, new elements have discredited this original theory. The major sources of mitochondrial DNA mutations seem to be replication errors and failure of the repair mechanisms, and the accumulation of these mutations as observed in aged organisms seems to occur by clonal expansion and not to be caused by a reactive oxygen species-dependent vicious cycle. New hypotheses of how age-associated mitochondrial dysfunction may lead to aging are based on the role of reactive oxygen species as signaling molecules and on their role in mediating stress responses to age-dependent damage. Here, we review the changes that mtDNA undergoes during aging and the past and most recent hypotheses linking these changes to the tissue failure observed in aging.     

Mitochondria, oxidative stress and aging

In the eighties, Miquel and Fleming suggested that mitochondria play a key role in cellular aging. Mitochondria, and specially mitochondrial DNA (mtDNA), are major targets of free radical attack. At present, it is well established that mitochondrial deficits accumulate upon aging due to oxidative damage. Thus, oxidative lesions to mtDNA accumulate with age in human and rodent tissues. Furthermore, levels of oxidative damage to mtDNA are several times higher than those of nuclear DNA. Mitochondrial size increases whereas mitochondrial membrane potential decreases with age in brain and liver.

Recently, we have shown that treatment with certain antioxidants, such as sulphur-containing antioxidants, vitamins C and E or the Ginkgo biloba extract EGb 761, protects against the age-associated oxidative damage to mtDNA and oxidation of mitochondrial glutathione. Moreover, the extract EGb 761 also prevents changes in mitochondrial morphology and function associated with aging of the brain and liver. Thus, mitochondrial aging may be prevented by antioxidants. Furthermore, late onset administration of certain antioxidants is also able to prevent the impairment in physiological performance, particularly motor co-ordination, that occurs upon aging.     

Mitochondrial Genome Mutation in Cell Death and Aging

This article reviews the concept, molecular genetics, and pathology of cell death and agingin relation to mitochondrial genome mutation. Accumulating evidence emphasizes the role ofgenetic factors in the development of naturally occurring cell death and aging. The ATPrequired for a cell's biological activity is almost exclusively produced by mitochondria. Eachmitochondrion possesses its own DNA (mtDNA) that codes essential subunits of themitochondrial energy-transducing system. Recent studies confirm that mtDNA is unexpectedly fragileto hydroxyl radical damage, hence to the oxygen stress. Cellular mtDNA easily fragmentsinto over a hundred-types of deleted mtDNA during the life of an individual. Cumulativeaccumulation of these oxygen damages and deletions in mtDNA results in a defective energytransducing system and in bioenergetic crisis. The crisis leads cells to the collapse ofmitochondrial trans-membrane potential, to the release of the apoptotic protease activating factors intocytosol, to uncontrolled cell death, to tissue degeneration and atrophy, and to aging. Thetotal base sequencing of mtDNA among individuals revealed that germ-line point mutationstransmitted from ancestors accelerate the somatic oxygen damages and mutations in mtDNAleading to phenotypic expression of premature aging and degenerative diseases. A practicalsurvey of point mutations will be useful for genetic diagnosis in predicting the life-span ofan individual.     

mtDNA拷贝数的生物学意义及其调控

线粒体是细胞能量和自由基代谢中心,并在细胞凋亡、钙调控、细胞周期和信号转导中发挥重要作用,维持线粒体功能正常对于细胞正常行使职能意义重大。线粒体的功能与线粒体DNA（mitochondrial DNA,mtDNA）的数量和质量紧密相关,mtDNA的数量即mtDNA拷贝数又受到mtDNA质量的影响,因此mtDNA拷贝数可作为线粒体功能的重要表征。mtDNA拷贝数变异引起线粒体功能紊乱,进而导致疾病发生。本文综述了mtDNA拷贝数变异与神经退行性疾病、心血管疾病、肿瘤等疾病的发生发展和个体衰老之间的关系,以及mtDNA复制转录相关因子、氧化应激、细胞自噬等因素介导mtDNA拷贝数变异的调控机制。以期为进一步深入探究mtDNA拷贝数调控的分子机制,以及未来治疗神经退行性疾病、肿瘤及延缓衰老等提供一定的理论基础。     

Mitochondria,oxidative stress and aging

In the eighties, Miquel and Fleming suggested that mitochondria play a key role in cellular aging. Mitochondria, and specially mitochondrial DNA (mtDNA), are major targets of free radical attack. At present, it is well established that mitochondrial deficits accumulate upon aging due to oxidative damage. Thus, oxidative lesions to mtDNA accumulate with age in human and rodent tissues. Furthermore, levels of oxidative damage to mtDNA are several times higher than those of nuclear DNA. Mitochondrial size increases whereas mitochondrial membrane potential decreases with age in brain and liver.

Recently, we have shown that treatment with certain antioxidants, such as sulphur-containing antioxidants, vitamins C and E or the Ginkgo biloba extract EGb 761, protects against the age-associated oxidative damage to mtDNA and oxidation of mitochondrial glutathione. Moreover, the extract EGb 761 also prevents changes in mitochondrial morphology and function associated with aging of the brain and liver. Thus, mitochondrial aging may be prevented by antioxidants. Furthermore, late onset administration of certain antioxidants is also able to prevent the impairment in physiological performance, particularly motor co-ordination, that occurs upon aging.     

Variation of the mitochondrial genome in the evolution of Drosophila

The evidence on mitochondrial genome variation and its role in evolution of the genus Drosophila are reviewed. The mitochondrial genome is represented by a circular double-stranded DNA molecule 16 to 19 kb in length. The genome contains no introns involved in recombination. The entire mitochondrial genome can be arbitrarily divided into three parts: (1) protein-coding genes; (2) genes encoding rRNA and tRNA; and (3) the noncoding regulatory region (A + T region). The selective importance of mutations within different mtDNA regions is therefore unequal. In Drosophila, the content of the A + T pairs in mtDNA is extremely low and a pattern of nucleotide substitution is characterized by a low transition/transversion ratio (and a low threshold of mutation saturation). The deletions and duplications are of common occurrence in the mitochondrial genome. However, this genome lacks such characteristic for the nuclear genome aberrations as the inversions and transpositions. The phenomena of introgression and heteroplasmy provide an opportunity to study the adaptive role of the mitochondrial genome and its role in speciation. Analysis of evidence concerning mtDNA variation in different species of the genus Drosophila made it possible to ascertain data on phylogenetic relationships among species obtained by studying nuclear genome variation. In some species, mtDNA variation may serve as a reliable marker for population differentiation within a species, although evidence on the population dynamics of the mtDNA variation is very scarce.     

Number matters: control of mammalian mitochondrial DNA copy number

Regulation of mitochondrial biogenesis is essential for proper cellular functioning. Mitochondrial DNA （mtDNA） depletion and the resulting mitochondrial malfunction have been implicated in cancer, neurodegeneration, diabetes, aging, and many other human diseases. Although it is known that the dynamics of the mammalian mitochondrial genome are not linked with that of the nuclear genome, very little is known about the mechanism of mtDNA propagation. Nevertheless, our understanding of the mode of mtDNA replication has ad- vanced in recent years, though not without some controversies. This review summarizes our current knowledge of mtDNA copy number control in mammalian cells, while focusing on both mtDNA replication and turnover. Although mtDNA copy number is seemingly in excess, we reason that mtDNA copy number control is an important aspect of mitochondrial genetics and biogenesis and is essential for normal cellular function.     

An evolutionary perspective on pathogenic mtDNA mutations: haplogroup associations of clinical disorders

More than 75 human diseases have been associated with mitochondrial dysfunction, and many of these are directly caused by overtly pathogenic mutations in the mitochondrial genome (mtDNA). In addition, there have been a number of reports that posit a different, subtler role for mtDNA substitutions in the disease process. As we review here, mtDNA evolution has resulted in the distribution of sequences into continent-specific haplogroups, which are defined by a relatively small number of polymorphisms. Thus, mtDNA sequences can be assigned to European, African, or Asian/Native American haplogroups. There are numerous reports that various diseases are haplogroup-associated, and it has been suggested that some of these haplogroup-associated polymorphisms act as risk factors in these disorders. It has also been suggested that there are haplogroup-associations for aging. As we note here, however, such associations have usually been observed only in single studies and it is difficult to draw broad conclusions on the basis of the available evidence. At a minimum, we suggest that, a haplogroup-group association must be detected in multiple subpopulations or in a large, carefully controlled population survey.     

Age-associated mitochondrial DNA deletions in mouse skeletal muscle: Comparison of different regions of the mitochondrial genome

The abundance of mitochondrial DNA (mtDNA) deletions has been shown to increase with age in a number of species and may contribute to the aging process. Estimating the total mtDNA deletion load of an individual is essential in evaluating the potential physiological impact. In this study, we compared three 5-kb regions of the mitochondrial genome: one in the major arc, one in the minor arc, and a third containing the light strand origin of replication. Through PCR analysis of mouse skeletal muscle, we have determined that not all regions produce equal numbers of age-associated deletions. There are, on average, twofold more detectable deletions in the major arc region than in the minor arc region. Deletions that result in the loss of the light strand origin of replication are rarely detected. Furthermore, the mechanism of deletion formation seems to be similar in both the major and minor arcs, with direct repeats playing an important, although not essential, role. © 1996 Wiley-Liss, Inc.     

线粒体DNA突变与相关人类疾病

在过去的20年里, 人们发现线粒体DNA(mitochondrial DNA, mtDNA)突变与多种人类疾病相关, 其致病范围从单器官组织损害到多系统受累。文章目的在于探讨mtDNA突变与人类疾病的关系。文章重点论述: (1)线粒体遗传学特征; (2) mtDNA突变与人类遗传性疾病; (3)体细胞mtDNA突变在衰老和肿瘤中的作用; (4)mtDNA疾病的诊断和治疗。     

Variation of the Mitochondral Genome in the Evolution of Drosophila

The evidence on mitochondrial genome variation and its role in evolution of the genus Drosophila are reviewed. The mitochondrial genome is represented by a circular double-stranded DNA molecule 16 to 19 kb in length. Mitochondrial genes lack introns and recombination. The entire mitochondrial genome can be arbitrarily divided into three parts: (1) protein-coding genes; (2) genes encoding rRNA and tRNA; and (3) the noncoding regulatory region (A + T region). The selective importance of mutations within different mtDNA regions is therefore unequal. In Drosophila, the content of the A + T pairs in mtDNA is extremely high and a pattern of nucleotide substitution is characterized by a low transition/transversion ratio (and a low threshold of mutation saturation). The deletions and duplications are of common occurrence in the mitochondrial genome. However, this genome lacks such characteristic for the nuclear genome aberrations as inversions and transpositions. The phenomena of introgression and heteroplasmy provide an opportunity to study the adaptive role of the mitochondrial genome and its role in speciation. Analysis of evidence concerning mtDNA variation in different species of the genus Drosophilamade it possible to ascertain data on phylogenetic relationships among species obtained by studying nuclear genome variation. In some species, mtDNA variation may serve as a reliable marker for population differentiation within a species, although evidence on the population dynamics of the mtDNA variation is very scarce.     

Development of Mitochondrial Gene Replacement Therapy

Many "classic" mitochondrial diseases have been described that arise from single homoplasmic mutations in mitochondrial DNA (mtDNA). These diseases typically affect nonmitotic tissues (brain, retina, muscle), present with variable phenotypes, can appear sporadically, and are untreatable. Evolving evidence implicates mtDNA abnormalities in diseases such as Alzheimer's, Parkinson's, and type II diabetes, but specific causal mutations for these conditions remain to be defined. Understanding the mtDNA genotype-phenotype relationships and developing specific treatment for mtDNA-based diseases is hampered by inability to manipulate the mitochondrial genome. We present a novel protein transduction technology ("protofection") that allows insertion and expression of the human mitochondrial genome into mitochondria of living cells. With protofection, the mitochondrial genotype can be altered, or exogenous genes can be introduced to be expressed and either retained in mitochondria or be directed to other organelles. Protofection also delivers mtDNA in vivo, opening the way to rational development of mitochondrial gene replacement therapy of mtDNA-based diseases.     

The variability of the mitochondrial genome in human aging: a key for life and death?

The impressive performance of the research in mitochondrial genetics and human aging in the last decade outlines a new scenery in which the inherited variation of the mitochondrial genome (mtDNA) may play a role in rate and quality of aging. This variation in humans was initially looked at as nearly neutral, and useful just for the reconstruction of human population history. However, recent data suggest that different mtDNA molecules are qualitatively different from each other. The aim of this paper is to discuss current ideas on the relationships among mitochondrial function, mtDNA inherited variation, and aging. The main processes where the mitochondrion is involved and the importance these processes have on aging and death of individuals will be described. A possible connection between programmed death phenomena (mitoptosis, apoptosis, phenoptosis) and rate and quality of aging will be discussed. Finally, the possible role played in these processes by the mtDNA germline variation will be explored.     

An update on the mitochondrial-DNA mutation hypothesis of cell aging.

Our electron microscopic study of aging insects and mammals suggests that metazoan senescence is linked to a gradual process of mitochondrial breakdown (and lipofuscin accumulation) in fixed postmitotic cells. This led us to propose in the early 1980s an oxyradical-mitochondrial DNA damage hypothesis, according to which metazoan aging may be caused by mutation, inactivation or loss of the mitochondrial genome (mtDNA) in irreversibly differentiated cells. This extranuclear somatic gene mutation concept of aging is in agreement with the fact that mtDNA synthesis takes place at the inner mitochondrial membrane near the sites of formation of highly reactive oxygen species and their products. Mitochondrial DNA may be unable to counteract the damage inflicted by those by-products of respiration because, in contrast to the nuclear genome, it lacks excision and recombination repair. Since mtDNA contains the structural genes for 13 hydrophobic proteins of the respiratory chain and ATP synthase as well as mitochondrial rRNAs and tRNAs, damage to this organellar genome will decrease or prevent the 'rejuvenation' of the mitochondria through the process of macromolecular turnover and organelle fission. Thus deprived of the ability to regenerate their mitochondria, the fixed postmitotic cells will sustain a decrease in the number of functional organelles, with resulting decline in ATP production. At higher levels of biological organization, this will lead to a loss in the bioenergetic capacity of cells, with concomitant decreases in ATP dependent protein synthesis and specialized physiological function, thus paving the way for age related degenerative diseases. The above concept is supported by a wealth of recent observations confirming the genomic instability of mitochondria and suggesting that animal and human aging is accompanied by mtDNA deletions and other types of injury to the mitochondrial genome. Our hypothesis of mtDNA damage is integrated with the classic concepts of Weissman and Minot in order to provide a preliminary explanation of the evolutionary roots of aging and reconcile the programed and stochastic views of metazoan senescence.