Mitochondrial DNA mutations and ageing

Mitochondrial Dysfunction and Imeglimin: A New Ray of Hope for the Treatment of Type-2 Diabetes Mellitus

Diabetes is a rapidly growing health challenge and epidemic in many developing countries, including India. India, being the diabetes capital of the world, has the dubious dual distinction of being the leading nations for both undernutrition and overnutrition. Diabetes prevalence has increased in both rural and urban areas, affected the younger population and increased the risk of complications and economic burden. These alarming statistics ring an alarm bell to achieve glycemic targets in the affected population in order to decrease diabetes-related morbidity and mortality. In the recent years, diabetes pathophysiology has been extended from an ominous triad through octet and dirty dozen etc. There is a new scope to target multiple pathways at the molecular level to achieve a better glycemic target and further prevent micro- and macrovascular complications. Mitochondrial dysfunction has a pivotal role in both β-cell failure and insulin resistance. Hence, targeting this molecular pathway may help with both insulin secretion and peripheral tissue sensitization to insulin. Imeglimin is the latest addition to our anti-diabetic armamentarium. As imeglimin targets, this root cause of defective energy metabolism and insulin resistance makes it a new add-on therapy in different diabetic regimes to achieve the proper glycemic targets. Its good tolerability and efficacy profiles in recent studies shows a new ray of hope in the journey to curtail diabetes-related morbidity.

Natural Bioactive Products and Alzheimer’s Disease Pathology: Lessons from Caenorhabditis elegans Transgenic Models

Alzheimer’s disease (AD) is an age-dependent, progressive disorder affecting millions of people. Currently, the therapeutics for AD only treat the symptoms. Although they have been used to discover new products of interest for this disease, mammalian models used to investigate the molecular determinants of this disease are often prohibitively expensive, time-consuming and very complex. On the other hand, cell cultures lack the organism complexity involved in AD. Given the highly conserved neurological pathways between mammals and invertebrates, Caenorhabditis elegans has emerged as a powerful tool for the investigation of the pathophysiology of human AD. Numerous models of both Tau- and Aβ-induced toxicity, the two prime components observed to correlate with AD pathology and the ease of performing RNA interference for any gene in the C. elegans genome, allow for the identification of multiple therapeutic targets. The effects of many natural products in main AD hallmarks using these models suggest promising health-promoting effects. However, the way in which they exert such effects is not entirely clear. One of the reasons is that various possible therapeutic targets have not been evaluated in many studies. The present review aims to explore shared therapeutical targets and the potential of each of them for AD treatment or prevention.

DdCBE mediates efficient and inheritable modifications in mouse mitochondrial genome

Critical mutations of mitochondrial DNA (mtDNA) generally lead to maternally inheritable diseases that affect multiple organs and systems; however, it was difficult to alter mtDNA in mammalian cells to intervene in or cure mitochondrial disorders. Recently, the discovery of DddA-derived cytosine base editor (DdCBE) enabled the precise manipulation of mtDNA. To test its feasibility for in vivo use, we selected several sites in mouse mtDNA as DdCBE targets to resemble the human pathogenic mtDNA G-to-A mutations. The efficiency of DdCBE-mediated mtDNA editing was first screened in mouse Neuro-2A cells and DdCBE pairs with the best performance were chosen for in vivo targeting. Microinjection of the mRNAs of DdCBE halves in the mouse zygotes or 2-cell embryo successfully generated edited founder mice with a base conversion rate ranging from 2.48% to 28.51%. When backcrossed with wild-type male mice, female founders were able to transmit the mutations to their offspring with different mutation loads. Off-target analyses demonstrated a high fidelity for DdCBE-mediated base editing in mouse mtDNA both in vitro and in vivo. Our study demonstrated that the DdCBE is feasible for generation of mtDNA mutation models to facilitate disease study and for potential treatment of mitochondrial disorders.

Sperm mitochondrial DNA biomarkers and couple fecundity

STUDY QUESTION

Do sperm mitochondrial DNA measures predict probability of pregnancy among couples in the general population?

SUMMARY ANSWER

Those with high sperm mitochondrial DNA copy number (mtDNAcn) had as much as 50% lower odds of cycle-specific pregnancy, and 18% lower probability of pregnancy within 12 months.

WHAT IS KNOWN ALREADY

Semen parameters have been found to poorly predict reproductive success yet are the most prevalent diagnostic tool for male infertility. Increased sperm mtDNAcn and mitochondrial DNA deletions (mtDNAdel) have been associated with decreased semen quality and lower odds of fertilization in men seeking fertility treatment.

STUDY DESIGN, SIZE, DURATION

A population-based prospective cohort study of couples discontinuing contraception to become pregnant recruited from 16 US counties from 2005 to 2009 followed for up to 16 months.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Sperm mtDNAcn and mtDNAdel from 384 semen samples were assessed via triplex probe-based quantitative PCR. Probability of pregnancy within 1 year was compared by mitochondrial DNA, and discrete-time proportional hazards models were used to evaluate the relations with time-to-pregnancy (TTP) with adjustment for covariates.

MAIN RESULTS AND THE ROLE OF CHANCE

Higher sperm mtDNAcn was associated with lower pregnancy probability within 12 months and longer TTP. In unadjusted comparisons by quartile (Q), those in Q4 had a pregnancy probability of 63.5% (95% CI: 53.1% to 73.1%) compared to 82.3% (95% CI: 73.2% to 89.9%) for Q1 (P = 0.002). Similar results were observed in survival analyses adjusting for covariates to estimate fecundability odds ratios (FORs) comparing mtDNAcn in quartiles. Relative to those in Q1 of mtDNAcn, FORs (95% CI) were for Q2 of 0.78 (0.52 to 1.16), Q3 of 0.65 (0.44 to 0.96) and Q4 of 0.55 (0.37 to 0.81), and this trend of decreasing fecundability with increasing mtDNAcn quartile was statistically significant (FOR per log mtDNAcn = 0.37; P < 0.001). Sperm mtDNAdel was not associated with TTP.

LIMITATIONS, REASONS FOR CAUTION

This prospective cohort study consisted primarily of Caucasian men and women and thus large diverse cohorts are necessary to confirm the associations between sperm mtDNAcn and couple pregnancy success in other races/ethnicities.

WIDER IMPLICATIONS OF THE FINDINGS

Our results demonstrate that sperm mtDNAcn has utility as a biomarker of male reproductive health and probability of pregnancy success in the general population.

STUDY FUNDING/COMPETING INTEREST(S)

This work was funded in part by the National Institute of Environmental Health Sciences, National Institutes of Health (R01-ES028298; PI: J.R.P.) and the Intramural Research Program of the Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland (Contracts N01-HD-3-3355, N01-HD-3-3356 and N01-HD-3-3358). The authors declare no competing interests.

TRIAL REGISTRATION NUMBER

N/A.

Double sperm cloning (DSC) is a promising strategy in mammalian genetic engineering and stem cell research

Embryonic stem cells (ESCs) derived from somatic cell nuclear transfer (SCNT) and induced pluripotent stem cells (iPSCs) are promising tools for meeting the personalized requirements of regenerative medicine. However, some obstacles need to be overcome before clinical trials can be undertaken. First, donor cells vary, and the reprogramming procedures are diverse, so standardization is a great obstacle regarding SCNT and iPSCs. Second, somatic cells derived from a patient may carry mitochondrial DNA mutations and exhibit telomere instability with aging or disease, and SCNT-ESCs and iPSCs retain the epigenetic memory or epigenetic modification errors. Third, reprogramming efficiency has remained low. Therefore, in addition to improving their success rate, other alternatives for producing ESCs should be explored. Producing androgenetic diploid embryos could be an outstanding strategy; androgenic diploid embryos are produced through double sperm cloning (DSC), in which two capacitated sperms (XY or XX, sorted by flow cytometer) are injected into a denucleated oocyte by intracytoplasmic sperm injection (ICSI) to reconstruct embryo and derive DSC-ESCs. This process could avoid some potential issues, such as mitochondrial interference, telomere shortening, and somatic epigenetic memory, all of which accompany somatic donor cells. Oocytes are naturally activated by sperm, which is unlike the artificial activation that occurs in SCNT. The procedure is simple and practical and can be easily standardized. In addition, DSC-ESCs can overcome ethical concerns and resolve immunological response matching with sperm providers. Certainly, some challenges must be faced regarding imprinted genes, epigenetics, X chromosome inactivation, and dosage compensation. In mice, DSC-ESCs have been produced and have shown excellent differentiation ability. Therefore, the many advantages of DSC make the study of this process worthwhile for regenerative medicine and animal breeding.

Mitochondrial DNA and Neurodegeneration: Any Role for Dietary Antioxidants?

The maintenance of the mitochondrial function is essential in preventing and counteracting neurodegeneration. In particular, mitochondria of neuronal cells play a pivotal role in sustaining the high energetic metabolism of these cells and are especially prone to oxidative damage. Since overproduction of reactive oxygen species (ROS) is involved in the pathogenesis of neurodegeneration, dietary antioxidants have been suggested to counteract the detrimental effects of ROS and to preserve the mitochondrial function, thus slowing the progression and limiting the extent of neuronal cell loss in neurodegenerative disorders. In addition to their role in the redox-system homeostasis, mitochondria are unique organelles in that they contain their own genome (mtDNA), which acts at the interface between environmental exposures and the molecular triggers of neurodegeneration. Indeed, it has been demonstrated that mtDNA (including both genetics and, from recent evidence, epigenetics) might play relevant roles in modulating the risk for neurodegenerative disorders. This mini-review describes the link between the mitochondrial genome and cellular oxidative status, with a particular focus on neurodegeneration; moreover, it provides an overview on potential beneficial effects of antioxidants in preserving mitochondrial functions through the protection of mtDNA.

Muscle Changes During Atrophy

Muscle atrophy typically is a direct effect of protein degradation induced by a diversity of pathophysiologic states such as disuse, immobilization, denervation, aging, sepsis, cachexia, glucocorticoid treatment, hereditary muscular disorders, cancer, diabetes and obesity, kidney and heart failure, and others. Muscle atrophy is defined by changes in the muscles, consisting in shrinkage of myofibers, changes in the types of fiber and myosin isoforms, and a net loss of cytoplasm, organelles and overall a protein loss. Although in the literature there are extensive studies in a range of animal models, the paucity of human data is a reality. This chapter is focused on various aspects of muscle wasting and describes the transitions of myofiber types during the progression of muscle atrophy in several pathological states. Clinical conditions associated with muscle atrophy have been grouped based on the fast-to-slow or slow-to-fast fiber-type shifts. We have also summarized the ultrastructural and histochemical features characteristic for muscle atrophy in clinical and experimental models for aging, cancer, diabetes and obesity, and heart failure and arrhythmia.

Hepatic DNA Damage Induced by Electronic Cigarette Exposure Is Associated With the Modulation of NAD+/PARP1/SIRT1 Axis

The prevalence of electronic cigarette (e-cigarettes) use has rapidly increased worldwide. Use of tobacco products has been associated with DNA damage and metabolic syndrome. Using Apolipoprotein E knockout (ApoE^-/-) mice on a western diet (WD), a mouse model of non-alcoholic fatty liver disease (NAFLD), we recently demonstrated that nicotine in e-cigarettes activates hepatocyte apoptosis, and causes hepatic steatosis. This study examines the harmful effects of e-cigarettes on the liver with a special emphasis on DNA damage and mitochondrial dysfunction. ApoE^-/- mice were exposed to saline, e-cigarettes without nicotine or e-cigarettes with 2.4% nicotine for 12 weeks using our newly developed mouse e-cigarette exposure model system that delivers nicotine to mice leading to equivalent serum cotinine levels found in human cigarette users. Mice exposed to e-cigarette (2.4% nicotine) had increased apurinic/apyrimidinic (AP) sites, a manifestation of DNA damage. Additionally, e-cigarette (2.4% nicotine) produced a decrease in NAD+/NADH ratio and increased oxidative stress in hepatic cells, in comparison with saline and e-cigarette (0%). Western blot analysis showed that mice treated with e-cigarette (2.4% nicotine) had increased poly (ADP ribose) polymerase (PARP1) activity associated with reduced levels of Sirtuin 1 (SIRT1). Furthermore, mitochondrial DNA mutations and PTEN-induced kinase 1 (PINK1) were increased in mice treated with e-cigarette (2.4% nicotine). Transmission electron microscopy revealed that hepatocytes of mice treated with e-cigarette (2.4% nicotine) exhibited increased vacuolization of the mitochondria and a reduction in cellular organelles. These results demonstrate the adverse effects of e-cigarettes exposure leading to NAD+ deficiency which may suggest a mechanistic link between e-cigarette-induced hepatic DNA damage and mitochondrial dysfunction.

Overexpression of PGC-1α in aging muscle enhances a subset of young-like molecular patterns

PGC-1α is a transcriptional co-activator known as the master regulator of mitochondrial biogenesis. Its control of metabolism has been suggested to exert critical influence in the aging process. We have aged mice overexpressing PGC-1α in skeletal muscle to determine whether the transcriptional changes reflected a pattern of expression observed in younger muscle. Analyses of muscle proteins showed that Pax7 and several autophagy markers were increased. In general, the steady-state levels of several muscle proteins resembled that of muscle from young mice. Age-related mtDNA deletion levels were not increased by the PGC-1α-associated increase in mitochondrial biogenesis. Accordingly, age-related changes in the neuromuscular junction were minimized by PGC-1α overexpression. RNA-Seq showed that several genes overexpressed in the aged PGC-1α transgenic are expressed at higher levels in young when compared to aged skeletal muscle. As expected, there was increased expression of genes associated with energy metabolism but also of pathways associated with muscle integrity and regeneration. We also found that PGC-1α overexpression had a mild but significant effect on longevity. Taken together, overexpression of PGC-1α in aged muscle led to molecular changes that resemble the patterns observed in skeletal muscle from younger mice.

Quantifying mitochondrial DNA copy number using robust regression to interpret real time PCR results

BACKGROUND

Real time PCR (rtPCR) is a quantitative assay to determine the relative DNA copy number in a sample versus a reference. The [Formula: see text] method is the standard for the analysis of the output data generated by an rtPCR experiment. We developed an alternative based on fitting a robust regression to the rtPCR signal. This new data analysis tool reduces potential biases and does not require all of the compared DNA fragments to have the same PCR efficiency.

RESULTS

Comparing the two methods when analysing 96 identical PCR preparations showed similar distributions of the estimated copy numbers. Estimating the efficiency with the [Formula: see text] method, however, required a dilution series, which is not necessary for the robust regression method. We used rtPCR to quantify mitochondrial DNA (mtDNA) copy numbers in three different tissues types: breast, colon and prostate. For each type, normal tissue and a tumor from the same three patients were analysed. This gives a total of six samples. The mitochondrial copy number is estimated to lie between 200 and 300 copies per cell. Similar results are obtained when using the robust regression or the [Formula: see text] method. Confidence ratios were slightly narrower for the robust regression. The new data analysis method has been implemented as an R package.

Mitochondrial DNA 4977-base pair common deletion in blood leukocytes and melanoma risk

The 4977-base pair common deletion DmtDNA4977 is the most frequently observed mitochondrial DNA mutation in human tissues. Because mitochondrial DNA mutations are mainly caused by reactive oxygen species (ROS), and given that oxidative stress plays an important role in melanoma carcinogenesis, the investigation of DmtDNA4977 may be particularly relevant to the development of melanoma. In this study, we compared DmtDNA4977 levels in blood leukocytes from 206 melanoma patients and 219 healthy controls. Overall, melanoma cases had significantly higher levels of DmtDNA4977 than healthy controls (median: 0.60 vs 0.20, P = 0.008). The difference was evident among individuals who were older than 47 yrs, women, and had pigmentation risk factors (e.g., blond or red hair, blue eye, fair skin, light, or none tanning ability after prolonged sun exposure, and freckling in the sun as a child). The difference was also evident among those who had at least one lifetime sunburn with blistering and had no reported use of a sunlamp. Interestingly, among controls, DmtDNA4977 levels differed by phenotypic index and reported use of a sunlamp. In the risk assessment, increased levels of DmtDNA4977 were associated with a 1.23-fold increased risk of melanoma (odds ratio (OR): 1.23, 95% confidence interval (90% CI): 1.01, 1.50). A significant dose-response relationship was observed in quartile analysis (P = 0.001). In summary, our study suggests that high levels of DmtDNA4977 in blood leukocytes are associated with increased risk of melanoma and that association is affected by both pigmentation and personal history of sun exposure.

Age-dependent accumulation of mitochondrial DNA deletions in the aortic root of atherosclerosis-prone apolipoprotein E-knockout mice

PURPOSE

To investigate whether mitochondrial DNA (mtDNA) damage, specifically deletion, contributes to the development of atherosclerosis or is simply a secondary effect of the primary factors causing atherosclerosis.

MATERIALS AND METHODS

mtDNA deletion was detected by PCR in the aortic root of atherosclerosis-prone C57BL/6J apolipoprotein (Apo) E gene deficient (-/-) mice and control C57BL/6J mice at different ages. Atherosclerotic plaques in the Apo E-/- mice were assessed using frozen sections of the aortic root. The protein levels of COX III and 8-oxoguanine glycosylase (OGG1) were determined.

RESULTS

while mtDNA deletions accumulated significantly in mice as young as 2- month-old, atherosclerotic plaques were not detected until mice were 6 months old or older, suggesting that mtDNA deletion occurs prior to the formation of atherosclerotic plaques in the aortic root of these mice. Moreover, the expression levels of mtDNA-encoded COX III protein in both 2-month-old and 16-month-old C57BL/6J ApoE-/- mice were significantly lower than those in C57BL/6J mice (p<0.05). Additionally, the protein level of 8-oxoguanine glycosylase (OGG1), a mitochondrial enzyme that functions in DNA excision repair, decreased with age in these mice, indicating that age-related down-regulation of mtDNA excision repair also contributes to atherosclerosis in C57BL/6J ApoE-/- mice.

CONCLUSION

These results reveal that mtDNA deletions occur during the early "initiation" stage of atherosclerosis in C57BL/6J ApoE-/- mice and have the potential to promote atherosclerosis.

Primer removal during mammalian mitochondrial DNA replication

The small circular mitochondrial genome in mammalian cells is replicated by a dedicated replisome, defects in which can cause mitochondrial disease in humans. A fundamental step in mitochondrial DNA (mtDNA) replication and maintenance is the removal of the RNA primers needed for replication initiation. The nucleases RNase H1, FEN1, DNA2, and MGME1 have been implicated in this process. Here we review the role of these nucleases in the light of primer removal pathways in mitochondria, highlight associations with disease, as well as consider the implications for mtDNA replication initiation.

Oxyphil cell metaplasia in the parathyroids is characterized by somatic mitochondrial DNA mutations in NADH dehydrogenase genes and cytochrome c oxidase activity-impairing genes

Oxyphil cell transformation of epithelial cells due to the accumulation of mitochondria occurs often during cellular aging. To understand the pathogenic mechanisms, we studied mitochondrial DNA (mtDNA) alterations in the three cell types of the parathyroids using multiplex real-time PCR and next-generation sequencing. mtDNA was analyzed from cytochrome c oxidase (COX)-positive and COX-negative areas of 19 parathyroids. Mitochondria-rich pre-oxyphil/oxyphil cells were more prone to develop COX defects than the mitochondria-poor clear chief cells (P < 0.001). mtDNA increased approximately 2.5-fold from clear chief to oxyphil cells. In COX deficiency, the increase was even more pronounced, and COX-negative oxyphil cells had approximately two times more mtDNA than COX-positive oxyphil cells (P < 0.001), illustrating the influence of COX deficiency on mtDNA biosynthesis, probably as a consequence of insufficient ATP synthesis. Next-generation sequencing revealed a broad spectrum of putative pathogenic mtDNA point mutations affecting NADH dehydrogenase and COX genes as well as regulatory elements of mtDNA. NADH dehydrogenase gene mutations preferentially accumulated in COX-positive pre-oxyphil/oxyphil cells and, therefore, could be essential for inducing oxyphil cell transformation by increasing mtDNA/mitochondrial biogenesis. In contrast, COX-negative cells predominantly harbored mutations in the MT-CO1 and MT-CO3 genes and in regulatory mtDNA elements, but only rarely NADH dehydrogenase mutations. Thus, multiple hits in NADH dehydrogenase and COX activity-impairing genes represent the molecular basis of oxyphil cell transformation in the parathyroids.

Selfish Mitochondrial DNA Proliferates and Diversifies in Small, but not Large, Experimental Populations of Caenorhabditis briggsae

Evolutionary interactions across levels of biological organization contribute to a variety of fundamental processes including genome evolution, reproductive mode transitions, species diversification, and extinction. Evolutionary theory predicts that so-called “selfish” genetic elements will proliferate when the host effective population size (N_e) is small, but direct tests of this prediction remain few. We analyzed the evolutionary dynamics of deletion-containing mitochondrial DNA (ΔmtDNA) molecules, previously characterized as selfish elements, in six different natural strains of the nematode Caenorhabditis briggsae allowed to undergo experimental evolution in a range of population sizes (N = 1, 10, 100, and 1,000) for a maximum of 50 generations. Mitochondrial DNA (mtDNA) was analyzed for replicate lineages at each five-generation time point. Ten different ΔmtDNA molecule types were observed and characterized across generations in the experimental populations. Consistent with predictions from evolutionary theory, lab lines evolved in small-population sizes (e.g., nematode N = 1) were more susceptible to accumulation of high levels of preexisting ΔmtDNA compared with those evolved in larger populations. New ΔmtDNA elements were observed to increase in frequency and persist across time points, but almost exclusively at small population sizes. In some cases, ΔmtDNA levels decreased across generations when population size was large (nematode N = 1,000). Different natural strains of C. briggsae varied in their susceptibilities to ΔmtDNA accumulation, owing in part to preexisting compensatory mtDNA alleles in some strains that prevent deletion formation. This analysis directly demonstrates that the evolutionary trajectories of ΔmtDNA elements depend upon the population-genetic environments and molecular-genetic features of their hosts.

The exonuclease activity of DNA polymerase γ is required for ligation during mitochondrial DNA replication

Mitochondrial DNA (mtDNA) polymerase γ (POLγ) harbours a 3′–5′ exonuclease proofreading activity. Here we demonstrate that this activity is required for the creation of ligatable ends during mtDNA replication. Exonuclease-deficient POLγ fails to pause on reaching a downstream 5′-end. Instead, the enzyme continues to polymerize into double-stranded DNA, creating an unligatable 5′-flap. Disease-associated mutations can both increase and decrease exonuclease activity and consequently impair DNA ligation. In mice, inactivation of the exonuclease activity causes an increase in mtDNA mutations and premature ageing phenotypes. These mutator mice also contain high levels of truncated, linear fragments of mtDNA. We demonstrate that the formation of these fragments is due to impaired ligation, causing nicks near the origin of heavy-strand DNA replication. In the subsequent round of replication, the nicks lead to double-strand breaks and linear fragment formation.

Mitochondrial DNA (mtDNA) polymerase γ has a 3′–5′ exonuclease proofreading activity. Here, the authors show it is required for creating ligatable ends during mtDNA replication, and inactivation of the activity in mice causes strand-specific nicks in DNA and the formation of linear mtDNA fragments.

Context-Dependent Role of Mitochondrial Fusion-Fission in Clonal Expansion of mtDNA Mutations

The accumulation of mutant mitochondrial DNA (mtDNA) molecules in aged cells has been associated with mitochondrial dysfunction, age-related diseases and the ageing process itself. This accumulation has been shown to often occur clonally, where mutant mtDNA grow in number and overpopulate the wild-type mtDNA. However, the cell possesses quality control (QC) mechanisms that maintain mitochondrial function, in which dysfunctional mitochondria are isolated and removed by selective fusion and mitochondrial autophagy (mitophagy), respectively. The aim of this study is to elucidate the circumstances related to mitochondrial QC that allow the expansion of mutant mtDNA molecules. For the purpose of the study, we have developed a mathematical model of mitochondrial QC process by extending our previous validated model of mitochondrial turnover and fusion-fission. A global sensitivity analysis of the model suggested that the selectivity of mitophagy and fusion is the most critical QC parameter for clearing de novo mutant mtDNA molecules. We further simulated several scenarios involving perturbations of key QC parameters to gain a better understanding of their dynamic and synergistic interactions. Our model simulations showed that a higher frequency of mitochondrial fusion-fission can provide a faster clearance of mutant mtDNA, but only when mutant–rich mitochondria that are transiently created are efficiently prevented from re-fusing with other mitochondria and selectively removed. Otherwise, faster fusion-fission quickens the accumulation of mutant mtDNA. Finally, we used the insights gained from model simulations and analysis to propose a possible circumstance involving deterioration of mitochondrial QC that permits mutant mtDNA to expand with age.

Mitochondria are responsible for most energy generation in human and animal cells. Loss or pathological alteration of mitochondrial function is a hallmark of many age-related diseases. Mitochondrial dysfunction may be a central and conserved feature of the ageing process. As part of quality control (QC), mitochondria are continually replicated and degraded. Furthermore, two mitochondria can fuse to form a single mitochondrion, and a mitochondrion can divide (fission) into two separate organelles. Despite this QC, mutant mitochondrial DNA (mtDNA) molecules have been observed to accumulate in cells with age which may lead to mitochondrial dysfunction. In this study, we created a detailed mathematical model of mitochondrial QC and performed model simulations to investigate circumstances allowing or preventing the accumulation of mutant mtDNA. We found that more frequent fusion-fission could quicken mutant mtDNA clearance, but only when mitochondria harboring a high fraction of mutant molecules were strongly prevented from fusing with other mitochondria and selectively degraded. Otherwise, faster fusion-fission would actually enhance the accumulation of mutant mtDNA. Our results suggested that the expansion of mutant mtDNA likely involves a decline in the selectivity of mitochondrial degradation and fusion. This insight might open new avenues for experiment and possible development of future therapies.

Triplex real-time PCR--an improved method to detect a wide spectrum of mitochondrial DNA deletions in single cells

Mitochondrial DNA (mtDNA) mutations are commonly found in the skeletal muscle of patients with mitochondrial disease, inflammatory myopathies and sarcopenia. The majority of these mutations are mtDNA deletions, which accumulate to high levels in individual muscle fibres causing a respiratory defect. Most mtDNA deletions are major arc deletions with breakpoints located between the origin of light strand (OL) and heavy strand (OH) replication within the major arc. However, under certain disease conditions, rarer, minor arc deletions are detected. Currently, there are few techniques which would allow the detection and quantification of both types of mtDNA deletions in single muscle fibres. We have designed a novel triplex real-time PCR assay which simultaneously amplifies the MT-ND4 gene in the major arc, the MT-ND1 gene in the minor arc, and the non-coding D-Loop region. We demonstrate that this assay is a highly sensitive and reliable tool for the detection and quantification of a broad range of major and minor arc mtDNA deletions with the potential to investigate the molecular pathogenesis in both research and diagnostic settings.

Clonal expansion of early to mid-life mitochondrial DNA point mutations drives mitochondrial dysfunction during human ageing

Age-related decline in the integrity of mitochondria is an important contributor to the human ageing process. In a number of ageing stem cell populations, this decline in mitochondrial function is due to clonal expansion of individual mitochondrial DNA (mtDNA) point mutations within single cells. However the dynamics of this process and when these mtDNA mutations occur initially are poorly understood. Using human colorectal epithelium as an exemplar tissue with a well-defined stem cell population, we analysed samples from 207 healthy participants aged 17-78 years using a combination of techniques (Random Mutation Capture, Next Generation Sequencing and mitochondrial enzyme histochemistry), and show that: 1) non-pathogenic mtDNA mutations are present from early embryogenesis or may be transmitted through the germline, whereas pathogenic mtDNA mutations are detected in the somatic cells, providing evidence for purifying selection in humans, 2) pathogenic mtDNA mutations are present from early adulthood (<20 years of age), at both low levels and as clonal expansions, 3) low level mtDNA mutation frequency does not change significantly with age, suggesting that mtDNA mutation rate does not increase significantly with age, and 4) clonally expanded mtDNA mutations increase dramatically with age. These data confirm that clonal expansion of mtDNA mutations, some of which are generated very early in life, is the major driving force behind the mitochondrial dysfunction associated with ageing of the human colorectal epithelium.

Mitochondrial metabolism and diabetes

The oversupply of calories and sedentary lifestyle has resulted in a rapid increase of diabetes prevalence worldwide. During the past two decades, lines of evidence suggest that mitochondrial dysfunction plays a key role in the pathophysiology of diabetes. Mitochondria are vital to most of the eukaryotic cells as they provide energy in the form of adenosine triphosphate by oxidative phosphorylation. In addition, mitochondrial function is an integral part of glucose-stimulated insulin secretion in pancreatic β-cells. In the present article, we will briefly review the major functions of mitochondria in regard to energy metabolism, and discuss the genetic and environmental factors causing mitochondrial dysfunction in diabetes. In addition, the pathophysiological role of mitochondrial dysfunction in insulin resistance and β-cell dysfunction are discussed. We argue that mitochondrial dysfunction could be the central defect causing the abnormal glucose metabolism in the diabetic state. A deeper understanding of the role of mitochondria in diabetes will provide us with novel insights in the pathophysiology of diabetes. (J Diabetes Invest, doi: 10.1111/j.2040-1124.2010.00047.x, 2010).

The co-occurrence of mtDNA mutations on different oxidative phosphorylation subunits, not detected by haplogroup analysis, affects human longevity and is population specific

To re-examine the correlation between mtDNA variability and longevity, we examined mtDNAs from samples obtained from over 2200 ultranonagenarians (and an equal number of controls) collected within the framework of the GEHA EU project. The samples were categorized by high-resolution classification, while about 1300 mtDNA molecules (650 ultranonagenarians and an equal number of controls) were completely sequenced. Sequences, unlike standard haplogroup analysis, made possible to evaluate for the first time the cumulative effects of specific, concomitant mtDNA mutations, including those that per se have a low, or very low, impact. In particular, the analysis of the mutations occurring in different OXPHOS complex showed a complex scenario with a different mutation burden in 90+ subjects with respect to controls. These findings suggested that mutations in subunits of the OXPHOS complex I had a beneficial effect on longevity, while the simultaneous presence of mutations in complex I and III (which also occurs in J subhaplogroups involved in LHON) and in complex I and V seemed to be detrimental, likely explaining previous contradictory results. On the whole, our study, which goes beyond haplogroup analysis, suggests that mitochondrial DNA variation does affect human longevity, but its effect is heavily influenced by the interaction between mutations concomitantly occurring on different mtDNA genes.

mtDNA mutations in human aging and longevity: controversies and new perspectives opened by high-throughput technologies

The last 30 years of research greatly contributed to shed light on the role of mitochondrial DNA (mtDNA) variability in aging, although contrasting results have been reported, mainly due to bias regarding the population size and stratification, and to the use of analysis methods (haplogroup classification) that resulted to be not sufficiently adequate to grasp the complexity of the phenomenon. A 5-years European study (the GEHA EU project) collected and analyzed data on mtDNA variability on an unprecedented number of long-living subjects (enriched for longevity genes) and a comparable number of controls (matched for gender and ethnicity) in Europe. This very large study allowed a reappraisal of the role of both the inherited and the somatic mtDNA variability in aging, as an association with longevity emerged only when mtDNA variants in OXPHOS complexes co-occurred. Moreover, the availability of data from both nuclear and mitochondrial genomes on a large number of subjects paves the way for an evaluation at a very large scale of the epistatic interactions at a higher level of complexity. This scenario is expected to be even more clarified in the next future with the use of next generation sequencing (NGS) techniques, which are becoming applicable to evaluate mtDNA variability and, then, new mathematical/bioinformatic analysis methods are urgently needed. Recent advances of association studies on age-related diseases and mtDNA variability will also be discussed in this review, taking into account the bias hidden by population stratification. Finally, very recent findings in terms of mtDNA heteroplasmy (i.e. the coexistence of wild type and mutated copies of mtDNA) and aging as well as mitochondrial epigenetic mechanisms will also be discussed.

Respiratory chain deficiency in aged spinal motor neurons

Sarcopenia, muscle wasting, and strength decline with age, is an important cause of loss of mobility in the elderly individuals. The underlying mechanisms are uncertain but likely to involve defects of motor nerve, neuromuscular junction, and muscle. Loss of motor neurons with age and subsequent denervation of skeletal muscle has been recognized as one of the contributing factors. This study investigated aspects of mitochondrial biology in spinal motor neurons from elderly subjects. We found that protein components of complex I of mitochondrial respiratory chain were reduced or absent in a proportion of aged motor neurons–a phenomenon not observed in fetal tissue. Further investigation showed that complex I-deficient cells had reduced mitochondrial DNA content and smaller soma size. We propose that mitochondrial dysfunction in these motor neurons could lead to the cell loss and ultimately denervation of muscle fibers.

Regulation of lifespan by the mitochondrial electron transport chain: reactive oxygen species-dependent and reactive oxygen species-independent mechanisms

SIGNIFICANCE

Aging is a consequence of the accumulation of cellular damage that impairs the capacity of an aging organism to adapt to stress. The Mitochondrial Free Radical Theory of Aging (MFRTA) has been one of the most influential ideas over the past 50 years. The MFRTA is supported by the accumulation of oxidative damage during aging along with comparative studies demonstrating that long-lived species or individuals produce fewer mitochondrial reactive oxygen species and have lower levels of oxidative damage.

RECENT ADVANCES

Recently, however, species that combine high oxidative damage with a longer lifespan (i.e., naked mole rats) have been described. Moreover, most of the interventions based on antioxidant supplementation do not increase longevity, as would be predicted by the MFRTA. Studies to date provide a clear understanding that mitochondrial function regulates the rate of aging, but the underlying mechanisms remain unclear.

CRITICAL ISSUES

Here, we review the reactive oxygen species (ROS)-dependent and ROS-independent mechanisms by which mitochondria can affect longevity. We discuss the role of different ROS (superoxide, hydrogen peroxide, and hydroxyl radical), both as oxidants as well as signaling molecules. We also describe how mitochondria can regulate longevity by ROS-independent mechanisms. We discuss alterations in mitochondrial DNA, accumulation of cellular waste as a consequence of glyco- and lipoxidative damage, and the regulation of DNA maintenance enzymes as mechanisms that can determine longevity without involving ROS.

FUTURE DIRECTIONS

We also show how the regulation of longevity is a complex process whereby ROS-dependent and ROS-independent mechanisms interact to determine the maximum lifespan of species and individuals.

Mathematical modeling of the role of mitochondrial fusion and fission in mitochondrial DNA maintenance

Accumulation of mitochondrial DNA (mtDNA) mutations has been implicated in a wide range of human pathologies, including neurodegenerative diseases, sarcopenia, and the aging process itself. In cells, mtDNA molecules are constantly turned over (i.e. replicated and degraded) and are also exchanged among mitochondria during the fusion and fission of these organelles. While the expansion of a mutant mtDNA population is believed to occur by random segregation of these molecules during turnover, the role of mitochondrial fusion-fission in this context is currently not well understood. In this study, an in silico modeling approach is taken to investigate the effects of mitochondrial fusion and fission dynamics on mutant mtDNA accumulation. Here we report model simulations suggesting that when mitochondrial fusion-fission rate is low, the slow mtDNA mixing can lead to an uneven distribution of mutant mtDNA among mitochondria in between two mitochondrial autophagic events leading to more stochasticity in the outcomes from a single random autophagic event. Consequently, slower mitochondrial fusion-fission results in higher variability in the mtDNA mutation burden among cells in a tissue over time, and mtDNA mutations have a higher propensity to clonally expand due to the increased stochasticity. When these mutations affect cellular energetics, nuclear retrograde signalling can upregulate mtDNA replication, which is expected to slow clonal expansion of these mutant mtDNA. However, our simulations suggest that the protective ability of retrograde signalling depends on the efficiency of fusion-fission process. Our results thus shed light on the interplay between mitochondrial fusion-fission and mtDNA turnover and may explain the mechanism underlying the experimentally observed increase in the accumulation of mtDNA mutations when either mitochondrial fusion or fission is inhibited.

Accumulation of mtDNA variations in human single CD34+ cells from maternally related individuals: effects of aging and family genetic background

Marked sequence variation in the mtDNA control region has been observed in human single CD34(+) cells, which persist in vivo and are present also in differentiated hematopoietic cells. In this study, we analyzed 5071 single CD34(+) cells from 49 individuals (including 31 maternally related members from four families and 18 unrelated donors) in order to determine the mutation spectrum within the mtDNA control region in single cells, as related to aging and family genetic background. Many highly mutated sites among family members were hypervariable sites in the mtDNA control region. Further, CD34(+) cells from members of the same family also shared several unique mtDNA variants, suggesting pedigree-specific occurrence of these variants. Overall age-related accumulation of mtDNA mutations in CD34(+) cells varied in different families, suggesting a specific accumulation pattern, which might be modulated by family genetic background. Our current findings have implications for the occurrence of mtDNA mutations in hematopoietic stem cells and progenitors.

MTERF3 regulates mitochondrial ribosome biogenesis in invertebrates and mammals

Regulation of mitochondrial DNA (mtDNA) expression is critical for the control of oxidative phosphorylation in response to physiological demand, and this regulation is often impaired in disease and aging. We have previously shown that mitochondrial transcription termination factor 3 (MTERF3) is a key regulator that represses mtDNA transcription in the mouse, but its molecular mode of action has remained elusive. Based on the hypothesis that key regulatory mechanisms for mtDNA expression are conserved in metazoans, we analyzed Mterf3 knockout and knockdown flies. We demonstrate here that decreased expression of MTERF3 not only leads to activation of mtDNA transcription, but also impairs assembly of the large mitochondrial ribosomal subunit. This novel function of MTERF3 in mitochondrial ribosomal biogenesis is conserved in the mouse, thus we identify a novel and unexpected role for MTERF3 in coordinating the crosstalk between transcription and translation for the regulation of mammalian mtDNA gene expression.

Non-random mtDNA segregation patterns indicate a metastable heteroplasmic segregation unit in m.3243A>G cybrid cells

Many pathogenic mitochondrial DNA mutations are heteroplasmic, with a mixture of mutated and wild-type mtDNA present within individual cells. The severity and extent of the clinical phenotype is largely due to the distribution of mutated molecules between cells in different tissues, but mechanisms underpinning segregation are not fully understood. To facilitate mtDNA segregation studies we developed assays that measure m.3243A>G point mutation loads directly in hundreds of individual cells to determine the mechanisms of segregation over time. In the first study of this size, we observed a number of discrete shifts in cellular heteroplasmy between periods of stable heteroplasmy. The observed patterns could not be parsimoniously explained by random mitotic drift of individual mtDNAs. Instead, a genetically metastable, heteroplasmic mtDNA segregation unit provides the likely explanation, where stable heteroplasmy is maintained through the faithful replication of segregating units with a fixed wild-type/m.3243A>G mutant ratio, and shifts occur through the temporary disruption and re-organization of the segregation units. While the nature of the physical equivalent of the segregation unit remains uncertain, the factors regulating its organization are of major importance for the pathogenesis of mtDNA diseases.

Selfish little circles: transmission bias and evolution of large deletion-bearing mitochondrial DNA in Caenorhabditis briggsae nematodes

Selfish DNA poses a significant challenge to genome stability and organismal fitness in diverse eukaryotic lineages. Although selfish mitochondrial DNA (mtDNA) has known associations with cytoplasmic male sterility in numerous gynodioecious plant species and is manifested as petite mutants in experimental yeast lab populations, examples of selfish mtDNA in animals are less common. We analyzed the inheritance and evolution of mitochondrial DNA bearing large heteroplasmic deletions including nad5 gene sequences (nad5Δ mtDNA), in the nematode Caenorhabditis briggsae. The deletion is widespread in C. briggsae natural populations and is associated with deleterious organismal effects. We studied the inheritance patterns of nad5Δ mtDNA using eight sets of C. briggsae mutation-accumulation (MA) lines, each initiated from a different natural strain progenitor and bottlenecked as single hermaphrodites across generations. We observed a consistent and strong drive toward higher levels of deletion-bearing molecules in the heteroplasmic pool of mtDNA after ten generations of bottlenecking. Our results demonstrate a uniform transmission bias whereby nad5Δ mtDNA accumulates to higher levels relative to intact mtDNA in multiple genetically diverse natural strains of C. briggsae. We calculated an average 1% per-generation transmission bias for deletion-bearing mtDNA relative to intact genomes. Our study, coupled with known deleterious phenotypes associated with high deletion levels, shows that nad5Δ mtDNA are selfish genetic elements that have evolved in natural populations of C. briggsae, offering a powerful new system to study selfish mtDNA dynamics in metazoans.

Dietary restriction of amino acids other than methionine prevents oxidative damage during aging: involvement of telomerase activity and telomere length

AIMS

It has been suggested that variations in the proportions of some dietary amino acids can slow down aging. In this study, the influence of amino acids other than methionine on aging was investigated.

MAIN METHODS

Rats were fed either with normal (ND) or except methionine, protein restricted diet (PREMD) for 4 months and oxygen radical production, oxidative protein and DNA damage along with telomere length and telomerase activity were evaluated in the liver.

KEY FINDINGS

Except mitochondrial superoxide production rate, feeding with PREMD significantly decreased the oxygen radical production rate and protein carbonyl levels in the homogenate and mitochondria of 16-month-old rats. Feeding with PREMD prevented 8-OHdG formation in mitochondrial DNA but not in the genomic DNA. Although liver telomerase activities of rats receiving either ND or PREMD seemed to have some variations, these did not reach a statistical significance. Feeding with PREMD conserved the telomere length in the liver. The telomere length of 8- and 16-month-old rats fed PREMD was similar, 16-month-old rats fed ND had telomeres shortened by 36% (p<0.05).

SIGNIFICANCE

Long-term restriction of the amino acids other than methionine may decrease oxygen radical generation and oxidative damage of cellular constituents, and may also prevent telomere shortening in rat liver.

Alternative oxidase rescues mitochondria-mediated dopaminergic cell loss in Drosophila

Mitochondrial dysfunction is commonly observed in degenerative disorders, including Alzheimer's and Parkinson's disease that are characterized by the progressive and selective loss of neuronal subpopulations. It is currently unclear, however, whether mitochondrial dysfunction is primary or secondary to other pathogenic processes that eventually lead to age-related neurodegeneration. Here we establish an in vivo Drosophila model of mitochondrial dysfunction by downregulating the catalytic subunit of mitochondrial DNA (mtDNA) polymerase in cholinergic, serotonergic and dopaminergic neurons. The resulting flies are characterized by lowered respiratory chain activity, premature aging, age-related motor deficits as well as adult onset, progressive and cell-type-specific, dopaminergic neurodegeneration. Using this model, we find that associated lethality can be partially rescued by targeting PINK1/parkin signaling or Drp1, both of which have been implicated in mitochondrial dynamics and Parkinson's disease. Bypassing mitochondrial complex III/IV deficiencies with Alternative oxidase (AOX), however, fully restores ATP levels and prevents dopaminergic neurodegeneration. In contrast, ATP levels and neurodegeneration are not rescued when mitochondrial complex I deficiencies are bypassed with NADH-Q oxidoreductase. Our results demonstrate that mtDNA-mediated mitochondrial dysfunction can cause age-related and cell-type-specific neurodegeneration which AOX is able to alleviate and indicate that AOX or its surrogates may prove useful as a therapeutic tool for limiting respiratory chain deficiencies caused by mtDNA decline in healthy aging and neurodegenerative disease.

Mechanisms of mitochondrial diseases

Mitochondria are essential organelles with multiple functions, the most well known being the production of adenosine triphosphate (ATP) through oxidative phosphorylation (OXPHOS). The mitochondrial diseases are defined by impairment of OXPHOS. They are a diverse group of diseases that can present in virtually any tissue in either adults or children. Here we review the main molecular mechanisms of mitochondrial diseases, as presently known. A number of disease-causing genetic defects, either in the nuclear genome or in the mitochondria's own genome, mitochondrial DNA (mtDNA), have been identified. The most classical genetic defect causing mitochondrial disease is a mutation in a gene encoding a structural OXPHOS subunit. However, mitochondrial diseases can also arise through impaired mtDNA maintenance, defects in mitochondrial translation factors, and various more indirect mechanisms. The putative consequences of mitochondrial dysfunction on a cellular level are discussed.

Functional effects of mutations in cytochrome c oxidase related to prostate cancer

A number of missense mutations in subunit I of cytochrome c oxidase (CytcO) have previously been linked to prostate cancer (Petros et al., 2005). To investigate the effects of these mutations at the molecular level, in the present study we prepared four different structural variants of the bacterial Rhodobacter sphaeroides CytcO (cytochrome aa(3)), each carrying one amino-acid residue replacement corresponding to the following substitutions identified in the above-mentioned study: Asn11Ser, Ala122Thr, Ala341Ser and Val380Ile (residues Asn25, Ser168, Ala384 and Val423 in the R. sphaeroides oxidase). This bacterial CytcO displays essentially the same structural and functional characteristics as those of the mitochondrial counterpart. We investigated the overall activity, proton pumping and internal electron- and proton-transfer reactions in the structural variants. The results show that the turnover activities of the mutant CytcOs were reduced by at most a factor of two. All variants pumped protons, but in Ser168Thr, Ala384Ser and Val423Ile we observed slight internal proton leaks. In all structural variants the internal electron equilibrium was slightly shifted away from the catalytic site at high pH (10), resulting in a slower observed ferryl to oxidized transition. Even though the effects of the mutations were relatively modest, the results suggest that they destabilize the proton-gating machinery. Such effects could be manifested in the presence of a transmembrane electrochemical gradient resulting in less efficient energy conservation. This article is part of a Special Issue entitled: Allosteric cooperativity in respiratory proteins.

Caenorhabditis elegans as a model system to study intercompartmental proteostasis: Interrelation of mitochondrial function, longevity, and neurodegenerative diseases

The protein quality control system, composed of molecular chaperones and proteases, is of vital importance for the maintenance and function of the proteome and the health of the cell. To achieve this, the cellular proteostasis network integrates the protein folding machinery across all compartments of the eukaryotic cell to enable efficient communication and coordinate a rapid response of folding capacity. Quality control in the mitochondria, however, differs from its cytosolic counterpart due to its prokaryotic origin, and is entirely encoded by the nuclear genome. The control and regulatory cross-talk of mitochondrial function in cellular proteostasis is essential for cellular metabolism, organismal development, and lifespan. Consequently, mitochondrial dysfunction has dramatic effects on the development and progression of a number of neurodegenerative diseases, such as Friedreich's ataxia and Parkinson's disease. Studies using Caenorhabditis elegans as a model system have greatly contributed to our current knowledge of inter-compartmental proteostasis on the cellular and organismal levels.

Active site mutations in mammalian DNA polymerase delta alter accuracy and replication fork progression

DNA polymerase δ (pol δ) is one of the two main replicative polymerases in eukaryotes; it synthesizes the lagging DNA strand and also functions in DNA repair. In previous work, we demonstrated that heterozygous expression of the pol δ L604G variant in mice results in normal life span and no apparent phenotype, whereas a different substitution at the same position, L604K, is associated with shortened life span and accelerated carcinogenesis. Here, we report in vitro analysis of the homologous mutations at position Leu-606 in human pol δ. Four-subunit human pol δ variants that harbor or lack 3' → 5'-exonucleolytic proofreading activity were purified from Escherichia coli. The pol δ L606G and L606K holoenzymes retain catalytic activity and processivity similar to that of wild type pol δ. pol δ L606G is highly error prone, incorporating single noncomplementary nucleotides at a high frequency during DNA synthesis, whereas pol δ L606K is extremely accurate, with a higher fidelity of single nucleotide incorporation by the active site than that of wild type pol δ. However, pol δ L606K is impaired in the bypass of DNA adducts, and the homologous variant in mouse embryonic fibroblasts results in a decreased rate of replication fork progression in vivo. These results indicate that different substitutions at a single active site residue in a eukaryotic polymerase can either increase or decrease the accuracy of synthesis relative to wild type and suggest that enhanced fidelity of base selection by a polymerase active site can result in impaired lesion bypass and delayed replication fork progression.

Parkin and PINK1 parkinsonism may represent nigral mitochondrial cytopathies distinct from Lewy body Parkinson's disease

Recent authors have concluded that Parkinson's disease (PD) is too heterogeneous to still be considered a single discrete disorder. They advise broadening the concept of PD to include genetic parkinsonisms, and discard Lewy pathology as the confirmatory biomarker. However, PD seen in the clinic is more homogeneous than often recognized if viewed from a long-term perspective. With appropriate diagnostic criteria, it is consistently associated with Lewy neuropathology, which should remain the gold standard for PD diagnostic confirmation. PD seen in the clinic has an inexorable course with eventual development of not only levodopa-refractory motor symptoms, but often cognitive dysfunction and prominent dysautonomia. This contrasts with homozygous parkin, PINK1 or DJ1 parkinsonism, characterized by young-onset (usually <40 years), and a comparatively benign course of predominantly levodopa-responsive symptoms without dementia or prominent dysautonomia. Parkin neuropathology is non-Lewy, with neurodegeneration predominantly confined to substantia nigra (and locus ceruleus), consistent with the limited clinical phenotype. Given the restricted and persistently levodopa-responsive phenotype, these familial cases might be considered "nigropathies". Based on emerging laboratory evidence linking parkin and PINK1 (and perhaps DJ1) to mitochondrial dysfunction, these nigropathies may represent nigral mitochondrial cytopathies. The dopaminergic substantia nigra is uniquely vulnerable to mitochondrial challenges, which might at least be partially attributable to large energy demands consequent to thin, unmyelinated axons with enormous terminal fields. Although sporadic PD is also associated with mitochondrial dysfunction, Lewy neurodegeneration represents a more pervasive disorder with perhaps a second, or different primary mechanism.

Mitochondrial DNA mutations and human disease

Mitochondrial disorders are a group of clinically heterogeneous diseases, commonly defined by a lack of cellular energy due to oxidative phosphorylation (OXPHOS) defects. Since the identification of the first human pathological mitochondrial DNA (mtDNA) mutations in 1988, significant efforts have been spent in cataloguing the vast array of causative genetic defects of these disorders. Currently, more than 250 pathogenic mtDNA mutations have been identified. An ever-increasing number of nuclear DNA mutations are also being reported as the majority of proteins involved in mitochondrial metabolism and maintenance are nuclear-encoded. Understanding the phenotypic diversity and elucidating the molecular mechanisms at the basis of these diseases has however proved challenging. Progress has been hampered by the peculiar features of mitochondrial genetics, an inability to manipulate the mitochondrial genome, and difficulties in obtaining suitable models of disease. In this review, we will first outline the unique features of mitochondrial genetics before detailing the diseases and their genetic causes, focusing specifically on primary mtDNA genetic defects. The functional consequences of mtDNA mutations that have been characterised to date will also be discussed, along with current and potential future diagnostic and therapeutic advances.