Random genetic drift in the female germline explains the rapid segregation of mammalian mitochondrial DNA

Do mitochondrial mutations cause recurrent miscarriage?

The cause of recurrent miscarriage (RM) can be identified in approximately 50% of cases, whereas in others, unknown genetic factors are actively being sought. As mitochondrial functions, and therefore also the mitochondrial genome [mitochondrial DNA (mtDNA)], have an important role in human development, through ATP production and participation in apoptosis, we aimed to study the role of mtDNA variations in RM. We screened 48 women with RM and 48 age-matched control women for heteroplasmic mitochondrial mutations using denaturing high performance liquid chromatography, a sensitive method that can detect approximately 5% heteroplasmy. As a result, we detected a heteroplasmic mtDNA variation in 13 RM women (27%) and in 9 control women (19%). Seven synonymous and five non-synonymous changes were detected within coding regions. In addition, seven heteroplasmic variations were detected within the non-coding control region. We were also able to show the presence of the variations in eight placental samples from three heteroplasmic women. In three of these cases, the proportion of variant mtDNA was higher in the placenta compared with that in the mother. We conclude that our sensitive methodology revealed a higher frequency of samples with heteroplasmic variations than expected in women with both RM and controls. However, no apparent increased frequency of heteroplasmic mtDNA variations or amounts of aberrant mtDNA was detected in the RM group. In addition, none of the detected variations were previously known to be pathogenic and therefore they are an unlikely cause of miscarriage.

Revealing the hidden complexities of mtDNA inheritance

Mitochondrial DNA (mtDNA) is a pivotal tool in molecular ecology, evolutionary and population genetics. The power of mtDNA analyses derives from a relatively high mutation rate and the apparent simplicity of mitochondrial inheritance (maternal, without recombination), which has simplified modelling population history compared to the analysis of nuclear DNA. However, in biology things are seldom simple, and advances in DNA sequencing and polymorphism detection technology have documented a growing list of exceptions to the central tenets of mitochondrial inheritance, with paternal leakage, heteroplasmy and recombination now all documented in multiple systems. The presence of paternal leakage, recombination and heteroplasmy can have substantial impact on analyses based on mtDNA, affecting phylogenetic and population genetic analyses, estimates of the coalescent and the myriad of other parameters that are dependent on such estimates. Here, we review our understanding of mtDNA inheritance, discuss how recent findings mean that established ideas may need to be re-evaluated, and we assess the implications of these new-found complications for molecular ecologists who have relied for decades on the assumption of a simpler mode of inheritance. We show how it is possible to account for recombination and heteroplasmy in evolutionary and population analyses, but that accurate estimates of the frequencies of biparental inheritance and recombination are needed. We also suggest how nonclonal inheritance of mtDNA could be exploited, to increase the ways in which mtDNA can be used in analyses.

The causes of mutation accumulation in mitochondrial genomes

A fundamental observation across eukaryotic taxa is that mitochondrial genomes have a higher load of deleterious mutations than nuclear genomes. Identifying the evolutionary forces that drive this difference is important to understanding the rates and patterns of sequence evolution, the efficacy of natural selection, the maintenance of sex and recombination and the mechanisms underlying human ageing and many diseases. Recent studies have implicated the presumed asexuality of mitochondrial genomes as responsible for their high mutational load. We review the current body of knowledge on mitochondrial mutation accumulation and recombination, and conclude that asexuality, per se, may not be the primary determinant of the high mutation load in mitochondrial DNA (mtDNA). Very little recombination is required to counter mutation accumulation, and recent evidence suggests that mitochondrial genomes do experience occasional recombination. Instead, a high rate of accumulation of mildly deleterious mutations in mtDNA may result from the small effective population size associated with effectively haploid inheritance. This type of transmission is nearly ubiquitous among mitochondrial genomes. We also describe an experimental framework using variation in mating system between closely related species to disentangle the root causes of mutation accumulation in mitochondrial genomes.

Mouse models of mitochondrial DNA defects and their relevance for human disease

Qualitative and quantitative changes in mitochondrial DNA (mtDNA) have been shown to be common causes of inherited neurodegenerative and muscular diseases, and have also been implicated in ageing. These diseases can be caused by primary mtDNA mutations, or by defects in nuclear-encoded mtDNA maintenance proteins that cause secondary mtDNA mutagenesis or instability. Furthermore, it has been proposed that mtDNA copy number affects cellular tolerance to environmental stress. However, the mechanisms that regulate mtDNA copy number and the tissue-specific consequences of mtDNA mutations are largely unknown. As post-mitotic tissues differ greatly from proliferating cultured cells in their need for mtDNA maintenance, and as most mitochondrial diseases affect post-mitotic cell types, the mouse is an important model in which to study mtDNA defects. Here, we review recently developed mouse models, and their contribution to our knowledge of mtDNA maintenance and its role in disease.

Mouse models of oxidative phosphorylation defects: powerful tools to study the pathobiology of mitochondrial diseases

Defects in the oxidative phosphorylation system (OXPHOS) are responsible for a group of extremely heterogeneous and pleiotropic pathologies commonly known as mitochondrial diseases. Although many mutations have been found to be responsible for OXPHOS defects, their pathogenetic mechanisms are still poorly understood. An important contribution to investigate the in vivo function of several mitochondrial proteins and their role in mitochondrial dysfunction, has been provided by mouse models. Thanks to their genetic and physiologic similarity to humans, mouse models represent a powerful tool to investigate the impact of pathological mutations on metabolic pathways. In this review we discuss the main mouse models of mitochondrial disease developed, focusing on the ones that directly affect the OXPHOS system.

Mitochondrial encephalopathy, lactic acidosis, and strokelike episodes: basic concepts, clinical phenotype, and therapeutic management of MELAS syndrome

Since the initial description almost 25 years ago, the syndrome of mitochondrial encephalopathy, lactic acidosis, and strokelike episodes (MELAS) has been a useful model to study the complex interplay of factors that define mitochondrial disease. This syndrome, most commonly caused by an A-to-G transition mutation at position 3243 of the mitochondrial genome, is typified by characteristic neurological manifestations including seizures, encephalopathy, and strokelike episodes, as well as other frequent secondary manifestations including short stature, cognitive impairment, migraines, depression, cardiomyopathy, cardiac conduction defects, and diabetes mellitus. In this review, we discuss the history, pathogenesis, clinical features, and diagnostic and management strategies of mitochondrial disease in general and of MELAS in particular. We explore features of mitochondrial genetics, including the concepts of heteroplasmy, mitotic segregation, and threshold effect, as a basis for understanding the variability and complicated inheritance patterns seen with this group of diseases. We also describe systemic manifestations of MELAS-associated mutations, including cardiac, renal, endocrine, gastrointestinal, and endothelial abnormalities and pathology, as well as the hypothetical role of derangements to COX enzymatic function in driving the unique pathology and clinical manifestations of MELAS. Although therapeutic options for MELAS and other mitochondrial diseases remain limited, and recent trials have been disappointing, we also consider current and potential therapeutic modalities.

The mitochondrial DNA genetic bottleneck results from replication of a subpopulation of genomes

In mammals, mitochondrial DNA (mtDNA) sequence variants are observed to segregate rapidly between generations despite the high mtDNA copy number in the oocyte. This has led to the concept of a genetic bottleneck for the transmission of mtDNA, but the mechanism remains contentious. Several studies have suggested that the bottleneck occurs during embryonic development, as a result of a marked reduction in germline mtDNA copy number. Mitotic segregation of mtDNAs during preimplantation, or during the expansion of primordial germ cells (PGCs) before they colonize the gonad, is thought to account for the increase in genotypic variance observed among mature oocytes from heteroplasmic mothers. This view has, however, been challenged by studies suggesting that the bottleneck occurs without a reduction in germline mtDNA content. To resolve this controversy, we measured mtDNA heteroplasmy and copy number in single germ cells isolated from heteroplasmic mice. By directly tracking the evolution of mtDNA genotypic variance during oogenesis, we show that the genetic bottleneck occurs during postnatal folliculogenesis and not during embryonic oogenesis.

Mouse models of oxidative phosphorylation dysfunction and disease

Oxidative phosphorylation (OXPHOS) deficiency results in a number of human diseases, affecting at least one in 5000 of the general population. Altering the function of genes by mutations are central to our understanding their function. Prior to the development of gene targeting, this approach was limited to rare spontaneous mutations that resulted in a phenotype. Since its discovery, targeted mutagenesis of the mouse germline has proved to be a powerful approach to understand the in vivo function of genes. Gene targeting has yielded remarkable understanding of the role of several gene products in the OXPHOS system. We provide a "tool box" of mouse models with OXPHOS defects that could be used to answer diverse scientific questions.

The distribution of mitochondrial DNA heteroplasmy due to random genetic drift

Cells containing pathogenic mutations in mitochondrial DNA (mtDNA) generally also contain the wild-type mtDNA, a condition called heteroplasmy. The amount of mutant mtDNA in a cell, called the heteroplasmy level, is an important factor in determining the amount of mitochondrial dysfunction and therefore the disease severity. mtDNA is inherited maternally, and there are large random shifts in heteroplasmy level between mother and offspring. Understanding the distribution in heteroplasmy levels across a group of offspring is an important step in understanding the inheritance of diseases caused by mtDNA mutations. Previously, our understanding of the heteroplasmy distribution has been limited to just the mean and variance of the distribution. Here we give equations, adapted from the work of Kimura on random genetic drift, for the full mtDNA heteroplasmy distribution. We describe how to use the Kimura distribution in mitochondrial genetics, and we test the Kimura distribution against human, mouse, and Drosophila data sets.

Naturally occurring mitochondrial DNA heteroplasmy in the MRL mouse

The MRL/MpJ mouse is an inbred laboratory strain of Mus musculus, known to exhibit enhanced autoimmunity, increased wound healing, and increased regeneration properties. We report the full-length mitochondrial DNA (mtDNA) sequence of the MRL mouse (Accession # EU450583), and characterize the discovery of two naturally occurring heteroplasmic sites. The first is a T3900C substitution in the TPsiC loop of the tRNA methionine gene (tRNA-Met; mt-Tm). The second is a heteroplasmic insertion of 1-6 adenine nucleotides in the A-tract of the tRNA arginine gene (tRNA-Arg; mt-Tr) at positions 9821-9826. The level of heteroplasmy varied independently at these two sites in MRL individuals. The length of the tRNA-Arg A-tract increased with age, but heteroplasmy at the tRNA-Met site did not change with age. The finding of naturally occurring mtDNA heteroplasmy in an inbred strain of mouse makes the MRL mouse a powerful new experimental model for studies designed to explore therapeutic measures to alter the cellular burden of heteroplasmy.

Characterization of a donor mitochondrial DNA transmission bottleneck in nuclear transfer derived cow lineages

In embryos derived by nuclear-transfer (NT), fusion of donor cells with recipient oocytes resulted in varying patterns of mitochondrial DNA (mtDNA) transmission in NT animals. Distribution of donor cell mtDNA (D-mtDNA) found in offspring of NT-derived founders may also vary from donor cell and host embryo heteroplasmy to host embryo homoplasmy. Here we examined the transmission of mtDNA from NT cows to G(1) offspring. Eleven NT founder cows were produced by fusion of enucleated oocytes (Holstein/Japanese Black) with Jersey/ Holstein oviduct epithelial cells, or Holstein/Japanese Black cumulus cells. Transmission of mtDNA was analyzed by PCR mediated single-strand conformation polymorphism of the D-loop region. In six of seven animals sampled postmortem, heteroplasmy were detected in various tissues, while D-mtDNA could not be detected in blood or hair samples from four live animals. The average proportion of D-mtDNA detected in one NT cow was 7.6%, and those in other cows were <5%. Heteroplasmic NT cows (n = 6) generated a total 12 G(1) offspring. Four of 12 G(1) offspring exhibited high percentages of D-mtDNA populations (range 17-51%). The remaining eight G(1) offspring had slightly or undetectable D-mtDNA (<5%). Generally, a genetic bottleneck in the female germ-line should favor a homoplasmic state. However, proportions of some G(1) offspring maintained heteroplasmy with a much higher percentage of D-mtDNA than their NT dams, which may also reflect a segregation distortion caused by the proposed mitochondrial bottleneck. These results demonstrate that D-mtDNA in NT cows is transmitted to G(1) offspring with varying efficiencies.

Mitochondrial nucleoids maintain genetic autonomy but allow for functional complementation

Mitochondrial DNA (mtDNA) is packaged into DNA-protein assemblies called nucleoids, but the mode of mtDNA propagation via the nucleoid remains controversial. Two mechanisms have been proposed: nucleoids may consistently maintain their mtDNA content faithfully, or nucleoids may exchange mtDNAs dynamically. To test these models directly, two cell lines were fused, each homoplasmic for a partially deleted mtDNA in which the deletions were nonoverlapping and each deficient in mitochondrial protein synthesis, thus allowing the first unequivocal visualization of two mtDNAs at the nucleoid level. The two mtDNAs transcomplemented to restore mitochondrial protein synthesis but were consistently maintained in discrete nucleoids that did not intermix stably. These results indicate that mitochondrial nucleoids tightly regulate their genetic content rather than freely exchanging mtDNAs. This genetic autonomy provides a molecular mechanism to explain patterns of mitochondrial genetic inheritance, in addition to facilitating therapeutic methods to eliminate deleterious mtDNA mutations.

Analysis of mitochondrial length heteroplasmy in monozygous and non-monozygous siblings

The segregation of mitochondrial genomes and the inheritance of mitochondrial DNA are constant matters of debate. To obtain more information about this issue and to answer the question whether or not it is possible to distinguish mitochondrial DNA (mtDNA) samples from monozygous individuals by analysing heteroplasmic length variants, 290 monozygous and 121 dizygous twin pairs and 34 sets of multiples were studied by RFLP and partly by direct sequencing. A factor D describing the respective pattern of length variants in a given sample was also calculated. The results show that monozygous individuals exhibit a significantly lower median and closer distribution of D than non-monozygous siblings. Thus, a differentiation of mtDNA samples from monozygous twins by this trait is not possible. The high percentage of heteroplasmic individuals, the low median of the D values and the unexpectedly very similar distribution of length variants in monozygotic individuals support the existence of a relatively wide bottleneck or the assumption of a regeneration of length heteroplasmy following a tight bottleneck and agree with a random segregation of mtDNA genomes in dividing oocytes.

The kinetics of donor cell mtDNA in embryonic and somatic donor cell-derived bovine embryos

The mechanisms controlling the outcome of donor cell-derived mitochondrial DNA (mtDNA) in cloned animals remain largely unknown. This research was designed to investigate the kinetics of somatic and embryonic mtDNA in reconstructed bovine embryos during preimplantation development, as well as in cloned animals. The experiment involved two different procedures of embryo reconstruction and their evaluation at five distinct phases of embryo development to measure the proportion of donor cell mtDNA (Bos indicus), as well as the segregation of this mtDNA during cleavage. The ratio of donor cell (B. indicus) to host oocyte (B. taurus) mtDNA (heteroplasmy) from blastomere(NT-B) and fibroblast(NT-F) reconstructed embryos was estimated using an allele-specific PCR with fluorochrome-stained specific primers in each sampled blastomere, in whole blastocysts, and in the tissues of a fibroblast-derived newborn clone. NT-B zygotes and blastocysts show similar levels of heteroplasmy (11.0% and 14.0%, respectively), despite a significant decrease at the 9-16 cell stage (5.8%; p<0.05). Heteroplasmy levels in NT-F reconstructed zygotes, however, increased from an initial low level (4.7%), to 12.9% (p<0.05) at the 9-16 cell stage. The NT-F blastocysts contained low levels of heteroplasmy (2.2%) and no somatic-derived mtDNA was detected in the gametes or the tissues of the newborn calf cloned. These results suggest that, in contrast to the mtDNA of blastomeres, that of somatic cells either undergoes replication or escapes degradation during cleavage, although it is degraded later after the blastocyst stage or lost during somatic development, as revealed by the lack of donor cell mtDNA at birth.

Mitochondrial heteroplasmy and the evolution of insecticide resistance: non-Mendelian inheritance in action

Genes encoded by mitochondrial DNA (mtDNA) exist in large numbers per cell but can be selected very rapidly as a result of unequal partitioning of mtDNA between germ cells during embryogenesis. However, empirical studies of this "bottlenecking" effect are rare because of the apparent scarcity of heteroplasmic individuals possessing more than one mtDNA haplotype. Here, we report an example of insecticide resistance in an arthropod pest (Tetranychus urticae) being controlled by mtDNA and on its inheritance in a heteroplasmic mite strain. Resistance to the insecticide bifenazate is highly correlated with remarkable mutations in cytochrome b, a mitochondrially encoded protein in the respiratory pathway. Four sites in the Q(o) site that are absolutely conserved across fungi, protozoa, plants, and animals are mutated in resistant mite strains. Despite the unusual nature of these mutations, resistant mites showed no fitness costs in the absence of insecticide. Partially resistant strains, consisting of heteroplasmic individuals, transmit their resistant and susceptible haplotypes to progeny in highly variable ratios consistent with a sampling bottleneck of approximately 180 copies. Insecticide selection on heteroplasmic individuals favors those carrying resistant haplotypes at a frequency of 60% or more. This combination of factors enables very rapid evolution and accounts for mutations being fixed in most field-collected resistant strains. The results provide a rare insight into non-Mendelian mechanisms of mitochondrial inheritance and evolution, relevant to anticipating and understanding the development of other mitochondrially encoded adaptations in arthropods. They also provide strong evidence of cytochrome b being the target site for bifenazate in spider mites.

Strong purifying selection in transmission of mammalian mitochondrial DNA

There is an intense debate concerning whether selection or demographics has been most important in shaping the sequence variation observed in modern human mitochondrial DNA (mtDNA). Purifying selection is thought to be important in shaping mtDNA sequence evolution, but the strength of this selection has been debated, mainly due to the threshold effect of pathogenic mtDNA mutations and an observed excess of new mtDNA mutations in human population data. We experimentally addressed this issue by studying the maternal transmission of random mtDNA mutations in mtDNA mutator mice expressing a proofreading-deficient mitochondrial DNA polymerase. We report a rapid and strong elimination of nonsynonymous changes in protein-coding genes; the hallmark of purifying selection. There are striking similarities between the mutational patterns in our experimental mouse system and human mtDNA polymorphisms. These data show strong purifying selection against mutations within mtDNA protein-coding genes. To our knowledge, our study presents the first direct experimental observations of the fate of random mtDNA mutations in the mammalian germ line and demonstrates the importance of purifying selection in shaping mitochondrial sequence diversity.

Mammalian mitochondrial DNA (mtDNA) is maternally transmitted and does not undergo bi-parental recombination in the germ line. This asexual mode of transmission, together with a high rate of mutation, should eventually lead to the accumulation of numerous deleterious mtDNA mutations and a “mutational meltdown” (a phenomenon know as Muller's Ratchet). In this study, we utilized a genetic mouse model, the mtDNA mutator mouse, to introduce random mtDNA mutations, and followed transmission of these mutations. Maternal transmission of mtDNA is typically subjected to a bottleneck phenomenon whereby only a fraction of the mtDNA copies in the germ-cell precursor are amplified to generate the approximately 10⁵ mtDNA copies present in the mature oocyte. As a consequence of this phenomenon, the established maternal mouse lines carried high levels of a few mtDNA mutations. We sequenced the entire mtDNA to characterize the maternally transmitted mutations in the established mouse lines. Surprisingly, mutations causing amino acid changes were strongly underrepresented in comparison with “silent” changes in the protein-coding genes. These results show that mtDNA is subject to strong purifying selection in the maternal germ line. Such selection of functional mtDNA genomes likely involves a mechanism for functional testing to prevent transmission of mutated genomes to the offspring.

We have used a genetic mouse model with a proofreading-deficient mitochondrial polymerase to mutagenize the mouse mitochondrial genome. The inherited mutations are subject to rapid purifying selection against amino acid substitutions.

A mouse model of mitochondrial disease reveals germline selection against severe mtDNA mutations

The majority of mitochondrial DNA (mtDNA) mutations that cause human disease are mild to moderately deleterious, yet many random mtDNA mutations would be expected to be severe. To determine the fate of the more severe mtDNA mutations, we introduced mtDNAs containing two mutations that affect oxidative phosphorylation into the female mouse germ line. The severe ND6 mutation was selectively eliminated during oogenesis within four generations, whereas the milder COI mutation was retained throughout multiple generations even though the offspring consistently developed mitochondrial myopathy and cardiomyopathy. Thus, severe mtDNA mutations appear to be selectively eliminated from the female germ line, thereby minimizing their impact on population fitness.

Detection of unrecognized low-level mtDNA heteroplasmy may explain the variable phenotypic expressivity of apparently homoplasmic mtDNA mutations

Mitochondrial DNA (mtDNA) mutations are an important cause of human disease. Most mtDNA mutations are found in heteroplasmy, in which the proportion of mutant vs. wild-type species is believed to explain some of the observed high phenotypic heterogeneity. However, homoplasmic mutations also observe phenotypic heterogeneity, which may be in part due to undetected low levels of heteroplasmy. In the present report, we have developed two assays, using DHPLC and Pyrosequencing (Biotage AB, Uppsala, Sweden), for reliably and accurately detecting low-level mtDNA heteroplasmy. Using these assays we have identified a three-generation family segregating two mtDNA mutations in heteroplasmy: the deafness-related m.1555A>G mutation in the 12S rRNA gene (MTRNR1) and a new variant (m.15287T>C) in the cytochrome b gene (MTCYB). Both heteroplasmic mtDNA mutations are transmitted through generations in a random manner, thus showing differences in mutation load between siblings within the family. In addition, the developed assays were also used to screen a group of deaf subjects of unknown etiology for the presence of heteroplasmy for both mtDNA variants. Two additional heteroplasmic m.1555A>G samples, previously considered as homoplasmic, and two deaf subjects carrying m.15287T>C variant were identified, thus confirming the high specificity and reliability of the approach. The development of assays for reliably detecting low-level heteroplasmy, together with the study of heteroplasmic mtDNA transmission, are essential steps for a better knowledge and clinical management of mtDNA diseases.

Selection against pathogenic mtDNA mutations in a stem cell population leads to the loss of the 3243A-->G mutation in blood

The mutation 3243A-->G is the most common heteroplasmic pathogenic mitochondrial DNA (mtDNA) mutation in humans, but it is not understood why the proportion of this mutation decreases in blood during life. Changing levels of mtDNA heteroplasmy are fundamentally related to the pathophysiology of the mitochondrial disease and correlate with clinical progression. To understand this process, we simulated the segregation of mtDNA in hematopoietic stem cells and leukocyte precursors. Our observations show that the percentage of mutant mtDNA in blood decreases exponentially over time. This is consistent with the existence of a selective process acting at the stem cell level and explains why the level of mutant mtDNA in blood is almost invariably lower than in nondividing (postmitotic) tissues such as skeletal muscle. By using this approach, we derived a formula from human data to correct for the change in heteroplasmy over time. A comparison of age-corrected blood heteroplasmy levels with skeletal muscle, an embryologically distinct postmitotic tissue, provides independent confirmation of the model. These findings indicate that selection against pathogenic mtDNA mutations occurs in a stem cell population.

A reduction of mitochondrial DNA molecules during embryogenesis explains the rapid segregation of genotypes

Mammalian mitochondrial DNA (mtDNA) is inherited principally down the maternal line, but the mechanisms involved are not fully understood. Females harboring a mixture of mutant and wild-type mtDNA (heteroplasmy) transmit a varying proportion of mutant mtDNA to their offspring. In humans with mtDNA disorders, the proportion of mutated mtDNA inherited from the mother correlates with disease severity. Rapid changes in allele frequency can occur in a single generation. This could be due to a marked reduction in the number of mtDNA molecules being transmitted from mother to offspring (the mitochondrial genetic bottleneck), to the partitioning of mtDNA into homoplasmic segregating units, or to the selection of a group of mtDNA molecules to re-populate the next generation. Here we show that the partitioning of mtDNA molecules into different cells before and after implantation, followed by the segregation of replicating mtDNA between proliferating primordial germ cells, is responsible for the different levels of heteroplasmy seen in the offspring of heteroplasmic female mice.

Prevalence, segregation, and phenotype of the mitochondrial DNA 3243A>G mutation in children

OBJECTIVE

We studied the prevalence, segregation, and phenotype of the mitochondrial DNA 3243A>G mutation in children in a defined population in Northern Ostrobothnia, Finland.

METHODS

Children with diagnoses commonly associated with mitochondrial diseases were ascertained. Blood DNA from 522 selected children was analyzed for 3243A>G. Children with the mutation were clinically examined. Information on health history before the age of 18 years was collected from previously identified adult patients with 3243A>G. Mutation segregation analysis in buccal epithelial cells was performed in mothers with 3243A>G and their children whose samples were analyzed anonymously.

RESULTS

Eighteen children were found to harbor 3243A>G in a population of 97,609. A minimum estimate for the prevalence of 3243A>G was 18.4 in 100,000 (95% confidence interval, 10.9-29.1/100,000). Information on health in childhood was obtained from 37 adult patients with 3243A>G. The first clinical manifestations appearing in childhood were sensorineural hearing impairment, short stature or delayed maturation, migraine, learning difficulties, and exercise intolerance. Mutation analysis from 13 mothers with 3243A>G and their 41 children gave a segregation rate of 0.80. The mothers with heteroplasmy greater than 50% tended to have offspring with lower or equal heteroplasmy, whereas the opposite was true for mothers with heteroplasmy less than or equal to 50% (p = 0.0016).

INTERPRETATION

The prevalence of 3243A>G is relatively high in the pediatric population, but the morbidity in children is relatively low. The random genetic drift model may be inappropriate for the transmission of the 3243A>G mutation.

Single lymphocytes from two healthy individuals with mitochondrial point heteroplasmy are mainly homoplasmic

The nature of mitochondrial DNA heteroplasmy is still unclear. It could either be caused by two mitochondrial DNA (mtDNA) haplotypes coexisting within a single cell or by an admixture of homoplasmic cells, each of which contains only one type of mtDNA molecule. To address this question, single lymphocytes were separated by flow cytometry assisted cell sorting and analyzed by cycle sequencing or minisequencing. To attain the required PCR sensitivity, the reactions were carried out on the surface of chemically structured glass slides in a reaction volume of 1-2 microl. In this study, blood samples from two healthy donors showing mitochondrial point heteroplasmy in direct sequencing (195Y and 234R, respectively) were analyzed. Nearly 96% of single lymphocytes tested were found to be in a homoplasmic state, but heteroplasmic cells were also detected. These results suggest that mitochondrial point heteroplasmy in blood may well be mainly due to the mixture of homoplasmic cells.

Why do we still have a maternally inherited mitochondrial DNA? Insights from evolutionary medicine

The human cell is a symbiosis of two life forms, the nucleus-cytosol and the mitochondrion. The nucleus-cytosol emphasizes structure and its genes are Mendelian, whereas the mitochondrion specializes in energy and its mitochondrial DNA (mtDNA) genes are maternal. Mitochondria oxidize calories via oxidative phosphorylation (OXPHOS) to generate a mitochondrial inner membrane proton gradient (DeltaP). DeltaP then acts as a source of potential energy to produce ATP, generate heat, regulate reactive oxygen species (ROS), and control apoptosis, etc. Interspecific comparisons of mtDNAs have revealed that the mtDNA retains a core set of electron and proton carrier genes for the proton-translocating OXPHOS complexes I, III, IV, and V. Human mtDNA analysis has revealed these genes frequently contain region-specific adaptive polymorphisms. Therefore, the mtDNA with its energy controlling genes may have been retained to permit rapid adaptation to new environments.

DNA replication and transcription in mammalian mitochondria

The mitochondrion was originally a free-living prokaryotic organism, which explains the presence of a compact mammalian mitochondrial DNA (mtDNA) in contemporary mammalian cells. The genome encodes for key subunits of the electron transport chain and RNA components needed for mitochondrial translation. Nuclear genes encode the enzyme systems responsible for mtDNA replication and transcription. Several of the key components of these systems are related to proteins replicating and transcribing DNA in bacteriophages. This observation has led to the proposition that some genes required for DNA replication and transcription were acquired together from a phage early in the evolution of the eukaryotic cell, already at the time of the mitochondrial endosymbiosis. Recent years have seen a rapid development in our molecular understanding of these machineries, but many aspects still remain unknown.

Neutral Mitochondrial Heteroplasmy Alters Physiological Function in Mice1

Cytoplasmic transfer is an assisted reproductive technique that involves the infusion of ooplasm from a donor oocyte into a recipient oocyte of inferior developmental competence. Although this technique has shown some success for couples with recurrent in vitro fertilization failure, it results in mitochondrial heteroplasmy in the offspring, defined as the presence of two different mitochondrial genomes in the same individual. Because the long-term health consequences of mitochondrial heteroplasmy are unknown, there is a need for appropriate animal models to evaluate any physiological changes of dual mtDNA genotypes. This longitudinal study was designed as a preliminary screen of basic physiological functions for heteroplasmic mice (NZB mtDNA on a BALB/cByJ background). The mice were tested for cardiovascular and metabolic function, hematological parameters, body mass analysis, ovarian reserve, and tissue histologic abnormalities over a period of 15 mo. Heteroplasmic mice developed systemic hypertension that corrected over time and was accompanied by cardiac changes consistent with pulmonary hypertension. In addition, heteroplasmic animals had increased body mass and fat mass compared with controls at all ages. Finally, these animals had abnormalities in electrolytes and hematological parameters. Our findings suggest that there are significant physiological differences between heteroplasmic and control mice. Because ooplasm transfer appears to be consistently associated with mitochondrial heteroplasmy, children conceived through ooplasm transfer should be closely followed to determine if they are at risk for any health problems.

Single-cell A3243G mitochondrial DNA mutation load assays for segregation analysis

Segregation of mitochondrial DNA (mtDNA) is an important underlying pathogenic factor in mtDNA mutation accumulation in mitochondrial diseases and aging, but the molecular mechanisms of mtDNA segregation are elusive. Lack of high-throughput single-cell mutation load assays lies at the root of the paucity of studies in which, at the single-cell level, mitotic mtDNA segregation patterns have been analyzed. Here we describe development of a novel fluorescence-based, non-gel PCR restriction fragment length polymorphism method for single-cell A3243G mtDNA mutation load measurement. Results correlated very well with a quantitative in situ Padlock/rolling circle amplification-based genotyping method. In view of the throughput and accuracy of both methods for single-cell A3243G mtDNA mutation load determination, we conclude that they are well suited for segregation analysis.

The mitochondrial bottleneck occurs without reduction of mtDNA content in female mouse germ cells

Observations of rapid shifts in mitochondrial DNA (mtDNA) variants between generations prompted the creation of the bottleneck theory. A prevalent hypothesis is that a massive reduction in mtDNA content during early oogenesis leads to the bottleneck. To test this, we estimated the mtDNA copy number in single germline cells and in single somatic cells of early embryos in mice. Primordial germ cells (PGCs) show consistent, moderate mtDNA copy numbers across developmental stages, whereas primary oocytes demonstrate substantial mtDNA expansion during early oocyte maturation. Some somatic cells possess a very low mtDNA copy number. We also demonstrated that PGCs have more than 100 mitochondria per cell. We conclude that the mitochondrial bottleneck is not due to a drastic decline in mtDNA copy number in early oogenesis but rather to a small effective number of segregation units for mtDNA in mouse germ cells. These results provide new information for mtDNA segregation models and for understanding the recurrence risks for mtDNA diseases.

Difficulties and possible solutions in the genetic management of mtDNA disease in the preimplantation embryo

Families who have had a child die of a severe, maternally inherited mitochondrial DNA (mtDNA) disease are usually desperate to avoid having further affected children. Here we discuss the problems of applying classical genetic management to mtDNA diseases (Poulton and Turnbull, 2000) and the biology underlying these problems. We explain why these disorders have lagged so far behind the genetics revolution. We then outline the directions in which management is likely to develop, including the use of preimplantation genetic diagnosis (PGD).

Cybrid models of mtDNA disease and transmission, from cells to mice

Oxidative phosphorylation (OXPHOS) is the only mammalian biochemical pathway dependent on the coordinated assembly of protein subunits encoded by both nuclear and mitochondrial DNA (mtDNA) genes. Cytoplasmic hybrid cells, cybrids, are created by introducing mtDNAs of interest into cells depleted of endogenous mtDNAs, and have been a central tool in unraveling effects of disease-linked mtDNA mutations. In this way, the nuclear genetic complement is held constant so that observed effects on OXPHOS can be linked to the introduced mtDNA. Cybrid studies have confirmed such linkage for many defined, disease-associated mutations. In general, a threshold principle is evident where OXPHOS defects are expressed when the proportion of mutant mtDNA in a heteroplasmic cell is high. Cybrids have also been used where mtDNA mutations are not known, but are suspected, and have produced some support for mtDNA involvement in more common neurodegenerative diseases. Mouse modeling of mtDNA transmission and disease has recently taken advantage of cybrid approaches. By using cultured cells as intermediate carriers of mtDNAs, ES cell cybrids have been produced in several laboratories by pretreatment of the cells with rhodamine 6G before cytoplast fusion. Both homoplasmic and heteroplasmic mice have been produced, allowing modeling of mtDNA transmission through the mouse germ line. We also briefly review and compare other transgenic approaches to modeling mtDNA dynamics, including mitochondrial injection into oocytes or zygotes, and embryonic karyoplast transfer. When breakthrough technology for mtDNA transformation arrives, cybrids will remain valuable for allowing exchange of engineered mtDNAs between cells.

Mitochondrial DNA and the Mammalian Oocyte

In mammals, mitochondria and mitochondrial DNA (mtDNA) are transmitted through the female germ line. Mature oocytes contain at least 100,000 copies of mtDNA, organized at 1-2 copies per organelle. Despite the high genome copy number, mtDNA sequence variants are observed to segregate rapidly between generations, and this has led to the concept of a developmental bottleneck for the transmission of mtDNA. Ultrastructural investigations of primordial germ cells show that they contain approximately 10 mitochondria, suggesting that mitochondrial biogenesis is arrested during early embryogenesis, and that the mitochondria contributing to the germ cell precursors are simply apportioned from those present in the zygote. Thus, as few as 0.01% of the mitochondria in the oocyte actually contribute to the offspring of the next generation. Mitochondrial replication restarts in the migrating primordial germ cells, and mitochondrial numbers steadily increase to a few thousand in primordial oocytes. Genetic evidence from both heteroplasmic mice and human pedigrees suggests that segregation of mtDNA sequence variants is largely a stochastic process that occurs during the mitotic divisions of the germ cell precursors. This process is essentially complete by the time the primary oocyte population is differentiated in fetal life. Analysis of the distribution of pathogenic mtDNA mutations in the offspring of carrier mothers shows that risk of inheriting a pathogenic mutation increases with the proportion in the mother, but there is no bias toward transmitting more or less of the mutant mtDNAs. This implies that there is no strong selection against oocytes carrying pathogenic mutations and that atresia is not a filter for oocyte quality based on oxidative phosphorylation capacity. The large number of mitochondria and mtDNAs present in the oocyte may simply represent a genetic mechanism to ensure their distribution to the gametes and somatic cells of the next generation. If true, mtDNA copy number, and by inference mitochondrial number, may be the most important determinant of oocyte quality, not because of the effects on oocyte metabolism, but because too few would result in a maldistribution in the early embryo.

DNA recombination protein-dependent mechanism of homoplasmy and its proposed functions

Homoplasmy is a basic genetic state of mitochondria, in which all of the hundreds to thousands of mitochondrial (mt)DNA copies within a cell or an individual have the same nucleotide-sequence. It was recently found that "vegetative segregation" to generate homoplasmic cells is an active process under genetic control. In the yeast Saccharomyces cerevisiae, the Mhr1 protein which catalyzes a key reaction in mtDNA homologous recombination, plays a pivotal role in vegetative segregation. Conversely, within the nuclear genome, homologous DNA recombination causes genetic diversity. Considering these contradictory roles of this key reaction in DNA recombination, possible functions of homoplasmy are discussed.

Molecular-clinical correlations in a family with variable tissue mitochondrial DNA T8993G mutant load

Unlike many pathogenic mitochondrial DNA mutations, the T8993G mutation associated with Leigh syndrome (LS) and neurogenic muscle weakness, ataxia, retinitis pigmentosa (NARP) typically shows little variation in mutant load between different tissue types. We describe the molecular and clinical findings in a family with variable disease severity and tissue T8993G mutant loads. Real-time ARMS qPCR testing showed that two brothers with features of NARP and LS had high mutant loads (>90%) in all tissues tested, similar to previously reported cases. Their sister, who has mild speech delay but attends normal school, was found to have a relatively high mutant load (mean 93%) in tissues derived from endoderm (buccal mucosa) and mesoderm (blood and skin fibroblasts). However, in tissue derived from ectoderm (hair bulbs), she carried a considerably lower proportion of mutant mtDNA. Because both surface ectoderm, which gives rise to outer epithelia and hair, and neuroectoderm, which gives rise to the central nervous system, are derived from ectoderm, it is tempting to speculate that the mutant load detected in the oligosymptomatic sister's hair bulbs is a reflection of the brain mutant load. We conclude that significant variation in tissue mutant load may occur in at least some individuals that harbor the T8993G mutation. This adds additional complexity to genetic counseling and prenatal diagnosis in such instances. Given the shared embryonic origin of hair bulbs and brain, we recommend performing hair bulb mtDNA analysis in asymptomatic or oligosymptomatic individuals that have high blood mutant loads in order to understand better the genotype-phenotype correlations related to the T8993G mutation.

Transmission of mitochondrial DNA in pigs and progeny derived from nuclear transfer of Meishan pig fibroblast cells

In embryos derived by nuclear transfer (NT), fusion, or injection of donor cells with recipient oocytes caused mitochondrial heteroplasmy. Previous studies have reported varying patterns of mitochondrial DNA (mtDNA) transmission in cloned calves. Here, we examined the transmission of mtDNA from NT pigs to their progeny. NT pigs were created by microinjection of Meishan pig fetal fibroblast nuclei into enucleated oocytes (maternal Landrace background). Transmission of donor cell (Meishan) mtDNA was analyzed using 4 NT pigs and 25 of their progeny by PCR-mediated single-strand conformation polymorphism (PCR-SSCP) analysis, PCR-RFLP, and a specific PCR to detect Meishan mtDNA single nucleotide polymorphisms (SNP-PCR). In the blood and hair root of NT pigs, donor mtDNAs were not detected by PCR-SSCP and PCR-RFLP, but detected by SNP-PCR. These results indicated that donor mtDNAs comprised between 0.1% and 1% of total mtDNA. Only one of the progeny exhibited heteroplasmy with donor cell mtDNA populations, ranging from 0% to 44% in selected tissues. Additionally, other progeny of the same heteroplasmic founder pig were analyzed, and 89% (16/18) harbored donor cell mtDNA populations. The proportion of donor mtDNA was significantly higher in liver (12.9 +/- 8.3%) than in spleen (5.0 +/- 3.9%), ear (6.7 +/- 5.3%), and blood (5.8 +/- 3.7%) (P < 0.01). These results demonstrated that donor mtDNAs in NT pigs could be transmitted to progeny. Moreover, once heteroplasmy was transmitted to progeny of NT-derived pigs, it appears that the introduced mitochondrial populations become fixed and maternally-derived heteroplasmy was more readily maintained in subsequent generations.

Heteroplasmy as a common state of mitochondrial genetic information in plants and animals

Plant and animal mitochondrial genomes, although quite distinct in size, structure, expression and evolutionary dynamics both may exhibit the state of heteroplasmy--the presence of more than one type of mitochondrial genome in an organism. This review is focused on heteroplasmy in plants, but we also highlight the most striking similarities and differences between plant and animal heteroplasmy. First we summarize the information on heteroplasmy generation and methods of its detection. Then we describe examples of quantitative changes in heteroplasmic populations of mitochondrial DNA (mtDNA) and consequences of such events. We also summarize the current knowledge about transmission and somatic segregation of heteroplasmy in plants and animals. Finally, factors which influence the stoichiometry of heteroplasmic mtDNA variants are discussed. Despite the apparent differences between the plant and animal heteroplasmy, the observed similarities allow one to conclude that this condition must play an important role in the mitochondrial biology of living organisms.

Mosaicism for mitochondrial DNA polymorphic variants in placenta has implications for the feasibility of prenatal diagnosis in mtDNA diseases

Women who have had a child with mitochondrial DNA (mtDNA) disease need to know the risk of recurrence, but this risk is difficult to estimate because mutant and wild-type (normal) mtDNA coexist in the same person (heteroplasmy). The possibility that a single sample may not reflect the whole organism both impedes prenatal diagnosis of most mtDNA diseases, and suggests radical alternative strategies such as nuclear transfer. We used naturally occurring mtDNA variants to investigate mtDNA segregation in placenta. Using large samples of control placenta, we demonstrated that the level of polymorphic heteroplasmic mtDNA variants is very similar in mother, cord blood and placenta. However, where placental samples were very small (< 10 mg) there was clear evidence of variation in the distribution of mtDNA polymorphic variants. We present the first evidence for variation in mutant load, that is, mosaicism for mtDNA polymorphic variants in placenta. This suggests that mtDNA mutants may segregate in placenta and that a single chorionic villous sample (CVS) may be unrepresentative of the whole placenta. Duplicates may be necessary where CVS are small. However, the close correlation of mutant load in maternal, fetal blood and placental mtDNA suggests that the average load in placenta does reflect the load of mutant mtDNA in the baby. Provided that segregation of neutral and pathogenic mtDNA mutants is similar in utero, our results are generally encouraging for developing prenatal diagnosis for mtDNA diseases. Identifying mtDNA segregation in human placenta suggests studies of relevance to placental evolution and to developmental biology.

Assaying the probabilities of obtaining maternally inherited heteroplasmy as the basis for modeling OXPHOS diseases in animals

Gross alterations in cell energy metabolism underlie manifestations of hereditary OXPHOS (oxidative phosphorylation) diseases, many of which depend on proportion of mutant mitochondrial DNA (mtDNA) in tissues. An animal model of OXPHOS disease with maternal inheritance of mitochondrial heteroplasmy might help understanding the peculiarities of abnormal mtDNA distribution and its effect on pre- and postnatal development. Previously we obtained mice that carry human mtDNA in some tissues. It co-existed with murine mtDNA (heteroplasmy) and was transmitted maternally to the progeny of animals developed from zygotes injected with human mitochondria. To analyze the probability of obtaining heteroplasmic mice we increased the number of experiments with early embryos and obtained more specimens from F1. About 33% of zygotes injected with human mtDNA developed into post-implantation embryos (7th-13th days). Lower amount of such developed into neonate mice (ca. 21%). Among post-implantation embryos and in generations F0 and F1 percentages of human mtDNA-carriers were ca. 14-16%. Such percentages are sufficient for modeling maternally inherited heteroplasmy in small animal groups. More data are needed to understand the regularities of anomalous mtDNA distribution among cells and tissues and whether heart and muscles frequently carrying human mtDNA in our experiments are particularly susceptible to heteroplasmy.

Organization, dynamics and transmission of mitochondrial DNA: focus on vertebrate nucleoids

Eukaryotic cells contain numerous copies of the mitochondrial genome (from 50 to 100 copies in the budding yeast to some thousands in humans) that localize to numerous intramitochondrial nucleoprotein complexes called nucleoids. The transmission of mitochondrial DNA differs significantly from that of nuclear genomes and depends on the number, molecular composition and dynamic properties of nucleoids and on the organization and dynamics of the mitochondrial compartment. While the localization, dynamics and protein composition of mitochondrial DNA nucleoids begin to be described, we are far from knowing all mechanisms and molecules mediating and/or regulating these processes. Here, we review our current knowledge on vertebrate nucleoids and discuss similarities and differences to nucleoids of other eukaryots.

Role of the mitochondrial genome in preimplantation development and assisted reproductive technologies

Our fascination for mitochondria relates to their origin as symbiotic, semi-independent organisms on which we, as eukaryotic beings, rely nearly exclusively to produce energy for every cell function. Therefore, it is not surprising that these organelles play an essential role in many events during early development and in artificial reproductive technologies (ARTs) applied to humans and domestic animals. However, much needs to be learned about the interactions between the nucleus and the mitochondrial genome (mtDNA), particularly with respect to the control of transcription, replication and segregation during preimplantation. Nuclear-encoded factors that control transcription and replication are expressed during preimplantation development in mice and are followed by mtDNA transcription, but these result in no change in mtDNA copy number. However, in cattle, mtDNA copy number increases during blastocyst expansion and hatching. Nuclear genes influence the mtDNA segregation patterns in heteroplasmic animals. Because many ARTs markedly modify the mtDNA content in embryos, it is essential that their application is preceded by careful experimental scrutiny, using suitable animal models.

Aberrant nucleo-cytoplasmic cross-talk results in donor cell mtDNA persistence in cloned embryos

Mitochondrial DNA is an extranuclear genome normally maternally inherited through the oocyte. However, the use of nuclear transfer can result in both donor cell and recipient oocyte mitochondrial DNA persisting through to blastocyst and being transmitted to the offspring. The degree of donor mitochondrial DNA transmission appears to be random and currently no evidence exists to explain this phenomenon. To determine whether this is a dilution factor or directly related to the transcriptional status of the donor cell in respect of mitochondrial DNA transcription factors, we have generated sheep nuclear transfer embryos using donor cells: (1) possessing their full mitochondrial DNA complement, (2) those partially depleted, and (3) those depleted but containing residual levels. For each donor type, donor mitochondrial DNA persisted in some blastocysts. It is evident from the donor cells used that nuclear-encoded mitochondrial DNA transcription and replication factors persist even after mitochondrial DNA depletion, as do transcripts for some of the mitochondrial-encoded genes. These cells are therefore still programmed to drive mitochondrial DNA replication and transcription. In nuclear transfer-derived embryos, we have observed the persistence of these nuclear-encoded mitochondrial DNA transcription and replication factors but not in those embryos generated through in vitro fertilization. Consequently, nucleo-mitochondrial interaction following nuclear transfer is out of sequence as the onset of mitochondrial replication is a postimplantation event.

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.

Mechanisms of Disordered Granulopoiesis in Congenital Neutropenia

Neutrophils are critical components of the innate immune response, and persistent neutropenia is associated with a marked susceptibility to infection. There are a number of inherited clinical syndromes in which neutropenia is a prominent feature. A study of these rare disorders has provided insight into the mechanisms regulating normal neutrophil homeostasis. Tremendous progress has been made at defining the genetic basis of these disorders. Herein, progress in understanding the genetic basis and molecular mechanisms of these disorders is discussed. We have focused our discussion on inherited disorders in which neutropenia is the sole or major hematopoietic defect.

Aberrant heteroplasmic transmission of mtDNA in cloned pigs arising from double nuclear transfer

Double nuclear transfer begins with the transfer of nuclear DNA from a donor cell into an enucleated recipient oocyte. This reconstructed oocyte is allowed to develop to the pronuclear stage, where the pronuclei are transferred into an enucleated zygote. This reconstructed zygote is then transferred to a surrogate sow. The genetic integrity of cloned offspring can be compromised by the transmission of mitochondrial DNA from the donor cell, the recipient oocyte and the recipient zygote. We have verified through the use of sequence analysis, restriction fragment length polymorphism analysis, allele specific PCR and primer extension polymorphism analysis that following double nuclear transfer the donor cell mtDNA is eliminated. However, it is likely that the recipient oocyte and zygote mitochondrial DNA are transmitted to the offspring, indicating bimaternal mitochondrial DNA transmission. This pattern of mtDNA inheritance is similar to that observed following cytoplasmic transfer and violates the strict unimaternal inheritance of mitochondrial DNA to offspring. This form of transmission raises concerns regarding the genetic integrity of cloned offspring and their uses in studies that require metabolic analysis or a stable genetic environment where only one variable is under analysis, such as in knockout technology.

Exercise and Training in Mitochondrial Myopathies

The intriguing concept of exercise training as therapy for mitochondrial disease is currently unsettled: in the unique setting of mitochondrial heteroplasmy, what are the effects of chronic exercise on skeletal muscle containing a mixture of mutated and wild-type mitochondrial DNA (mtDNA)? Furthermore, what are the consequences of habitual physical inactivity on mitochondrial heteroplasmy? In patients with mtDNA defects, deleterious effects of limited physical activity likely magnify the mitochondrial oxidative impairment contributing to varying degrees of exercise intolerance. Normal adaptive responses to endurance training offer the potential to increase levels of functional mitochondria, improving exercise tolerance. The few clinical studies assessing such training effects in patients with mtDNA defects have unequivocally demonstrated physiologic and biochemical adaptations that improve exercise tolerance and quality of life. Uncertain, however, is the training effect on mitochondrial heteroplasmy. To determine therapeutic advisability of endurance training, it remains imperative to establish whether: reported increases in mutant mtDNA levels can be offset by increases in absolute wild-type mtDNA levels; and chronic physical inactivity leads to a selective down-regulation of wild-type mtDNA. Resistance exercise training offers an alternate, innovative therapeutic approach in patients with sporadic mtDNA mutations; exercise-induced transfer of normal mtDNA templates from muscle satellite cells to mature myofibers, thereby lowering mutation load (increasing functional mitochondrial load). Efficacy and safety of this approach needs to be replicated in a larger group of patients. Currently, appropriate recommendation (either in support or against) exercise training in mitochondrial disease is lacking, which is frustrating for physicians and disheartening for patients. Although considerable progress has been made, an immediate urgency exists to resolve the effects of chronic exercise on skeletal muscle in patients with heteroplasmic mtDNA mutations.