Accumulation Of Somatic mtDNA Mutations Research Articles

Mitochondrial Dysfunction as a Signaling Target for Therapeutic Intervention in Major Neurodegenerative Disease.

Neurodegenerative diseases (NDD) are incurable and the most prevalent cognitive and motor disorders of elderly. Mitochondria are essential for a wide range of cellular processes playing a pivotal role in a number of cellular functions like metabolism, intracellular signaling, apoptosis, and immunity. A plethora of evidence indicates the central role of mitochondrial functions in pathogenesis of many aging related NDD. Considering how mitochondria function in neurodegenerative diseases, oxidative stress, and mutations in mtDNA both contribute to aging. Many substantial reports suggested the involvement of numerous contributing factors including, mitochondrial dysfunction, oxidative stress, mitophagy, accumulation of somatic mtDNA mutations, compromised mitochondrial dynamics, and transport within axons in neurodegenerative disorders including Alzheimer's disease, Parkinson's disease, Huntington's disease, and Amyotrophic Lateral Sclerosis. Therapies therefore target fundamental mitochondrial processes such as energy metabolism, free-radical generation, mitochondrial biogenesis, mitochondrial redox state, mitochondrial dynamics, mitochondrial protein synthesis, mitochondrial quality control, and metabolism hold great promise to develop pharmacological based therapies in NDD. By emphasizing the most efficient pharmacological strategies to target dysfunction of mitochondria in the treatment of neurodegenerative diseases, this review serves the scientific community engaged in translational medical science by focusing on the establishment of novel, mitochondria-targeted treatment strategies.

Mitochondrial DNA Variants and Common Diseases: A Mathematical Model for the Diversity of Age-Related mtDNA Mutations.

The mitochondrion is the only organelle in the human cell, besides the nucleus, with its own DNA (mtDNA). Since the mitochondrion is critical to the energy metabolism of the eukaryotic cell, it should be unsurprising, then, that a primary driver of cellular aging and related diseases is mtDNA instability over the life of an individual. The mutation rate of mammalian mtDNA is significantly higher than the mutation rate observed for nuclear DNA, due to the poor fidelity of DNA polymerase and the ROS-saturated environment present within the mitochondrion. In this review, we will discuss the current literature showing that mitochondrial dysfunction can contribute to age-related common diseases such as cancer, diabetes, and other commonly occurring diseases. We will then turn our attention to the likely role that mtDNA mutation plays in aging and senescence. Finally, we will use this context to develop a mathematical formula for estimating for the accumulation of somatic mtDNA mutations with age. This resulting model shows that almost 90% of non-proliferating cells would be expected to have at least 100 mutations per cell by the age of 70, and almost no cells would have fewer than 10 mutations, suggesting that mtDNA mutations may contribute significantly to many adult onset diseases.

Deleterious mitochondrial DNA point mutations are overrepresented in Drosophila expressing a proofreading-defective DNA polymerase γ.

Mitochondrial DNA (mtDNA) mutations cause severe maternally inherited syndromes and the accumulation of somatic mtDNA mutations is implicated in aging and common diseases. However, the mechanisms that influence the frequency and pathogenicity of mtDNA mutations are poorly understood. To address this matter, we created a Drosophila mtDNA mutator strain expressing a proofreading-deficient form of the mitochondrial DNA polymerase. Mutator flies have a dramatically increased somatic mtDNA mutation frequency that correlates with the dosage of the proofreading-deficient polymerase. Mutator flies also exhibit mitochondrial dysfunction, shortened lifespan, a progressive locomotor deficit, and loss of dopaminergic neurons. Surprisingly, the frequency of nonsynonymous, pathogenic, and conserved-site mutations in mutator flies exceeded predictions of a neutral mutational model, indicating the existence of a positive selection mechanism that favors deleterious mtDNA variants. We propose from these findings that deleterious mtDNA mutations are overrepresented because they selectively evade quality control surveillance or because they are amplified through compensatory mitochondrial biogenesis.

Mitochondrial genetics and cancer

The first modern human, the Mitochondrial Eve, was traced back to Africa about 200,000 years ago, based on the variation in the mitochondrial DNA (mtDNA). An eruption of a super volcano, Mount Toba, in Sumatra 70,000 years ago may have led to a 'nuclear winter', followed by a 1,000-year ice age. This cold snap would have made life difficult; genetic evidence indicated a sharp reduction in population size around this time, reaching approximately 10,000 individuals. Once the climate started to improve, our ancestors recovered from this near-extinction event. The population expanded, and some courageous explorers ventured beyond Africa. Around 50,000 years ago some of these brave ancestors had successfully crossed the globe to South East Asia and Australia. Some of them settled in the Indonesian archipelago, forming the first settlement of prehistoric Indonesia. The second migration happened around 10,000 years ago, where a group of hunter-gatherers followed the now-submerged river systems that once ran from mainland Asia between the modern islands of Sumatera, Java, and Borneo. Then, around 4,000 years ago the third group of ancestors arrived. This agricultural community brought along their culture of pottery, plant cultivation, and animal domestication, co-inciding with the vast spread of Austronesian languages. Therefore, it is likely that the Indonesian archipelago hosts a wide range of linguistic, ethnic and genetic diversity. 1 Nowadays, the modern Indonesia is home to around 700 ethnic populations, each with distinct cultural and linguistic characteristics, representing vast genome diversity. Our ancestors’ decision to embark on a sea travel and take on its related lifestyle has influenced the development of susceptibility and resistance to various diseases observed today. During the prolonged travel, our ancestors were subjected to changes in global climate and geographic dynamic, which strongly influenced and shaped the genetic background of modern humans, including the mtDNA genome. Mitochondria, a well-adapted endosymbiotic intracellular organelles, became efficient for energy production through-out the course of evolution. They are critical for survival and proliferation of living organisms under aerobic conditions and produce ATP through oxidative phosphorylation (OXPHOS). Adaptation to new environments that favor beneficial traits may have caused genetic risk differences that influence the crucial function of the mitochondria, consequently affecting many function in the cell. 2 The altered function of the mitochondria might act as an important factor for disease susceptibility across many human populations, i.e. mtDNA variation that grouped together forming a certain type/group (the mtDNA haplogroup) was reported to modulate cancer susceptibility 3-5 and resistance 6 in Chinese population. Cancer cells are characterized in general by a decrease of mitochondrial respiration and OXPHOS, a consequence of disruptive mtDNA mutations commonly found in cancer cells, and thus one could say that the growth of cancer cells is directly limited by energetics. 7 In order to survive, cancer cells must modify their mitochondrial physiology to optimize energy production to their changing environments. There are two types of advantageous mtDNA mutation in cancer cells: mutations that impair OXPHOS and serve to stimulate neoplastic transformation, and those that facilitate cancer cell adaption to changing bioenergetics environments. 8 These mtDNA mutations would eventually lead to an enhanced generation of reactive oxygen species (ROS), which can act both as mutagens and cellular mitogens, and contribute directly to cancer progression. 7 Therefore, it can be concluded that mitochondrial alterations are critical for cancer initiation, promotion, and metastasis (Fig 1). Figure 1. Integrated mitochondrial paradigm to explain genetic and phenotypic complexities of metabolic and degenerative disease, aging, and cancer. Top three arrows : factors that have impact on mitochondrial OXPHOS robustness, risk for developing disease symptoms. Central oval arrows : pathophysiological basis of disease processes and the basis of disease progression. Lower five arrows : summarized disease categories and phenotypic outcomes of disturbed mitochondrial energy transformation. Bottom arrow : effect of the problematic accumulation of somatic mtDNA mutations resulting in delayed onset and a progressive course of diseases and aging. Right arrow : clinical problems that can result from reduced energy production in the most energetic tissues: the brain, heart, muscle, and kidney. Left arrow : indicates the metabolic effects of mitochondrial dysfunction, which result in the perturbation of the body’s energy balance. Lower right arrow : mitochondrial alterations are critical for cancer initiation, promotion, and metastasis . Lower left arrow : the hypothesized inflammatory and autoimmune responses that may result from chronic introduction of mitochondria’s bacteria-like DNA and N-formylmethionine proteins into the bloodstream. 9

Metabolomic analysis of exercise effects in the POLG mitochondrial DNA mutator mouse brain

Mitochondrial DNA (mtDNA) mutator mice express a mutated form of mtDNA polymerase gamma that results an accelerated accumulation of somatic mtDNA mutations in association with a premature aging phenotype. An exploratory metabolomic analysis of cortical metabolites in sedentary and exercised mtDNA mutator mice and wild-type littermate controls at 9–10 months of age was performed. Pathway analysis revealed deficits in the neurotransmitters acetylcholine, glutamate, and aspartate that were ameliorated by exercise. Nicotinamide adenine dinucleotide (NAD) depletion and evidence of increased poly(adenosine diphosphate–ribose) polymerase 1 (PARP1)activity were apparent in sedentary mtDNA mutator mouse cortex, along with deficits in carnitine metabolites and an upregulated antioxidant response that largely normalized with exercise. These data highlight specific pathways that are altered in the brain in association with an accelerated age-related accumulation of somatic mtDNA mutations. These results may have relevance to age-related neurodegenerative diseases associated with mitochondrial dysfunction, such as Alzheimer's disease and Parkinson's disease and provide insights into potential mechanisms of beneficial effects of exercise on brain function.

Oxidative Stress Is Not a Major Contributor to Somatic Mitochondrial DNA Mutations

The accumulation of somatic mitochondrial DNA (mtDNA) mutations is implicated in aging and common diseases of the elderly, including cancer and neurodegenerative disease. However, the mechanisms that influence the frequency of somatic mtDNA mutations are poorly understood. To develop a simple invertebrate model system to address this matter, we used the Random Mutation Capture (RMC) assay to characterize the age-dependent frequency and distribution of mtDNA mutations in the fruit fly Drosophila melanogaster. Because oxidative stress is a major suspect in the age-dependent accumulation of somatic mtDNA mutations, we also used the RMC assay to explore the influence of oxidative stress on the somatic mtDNA mutation frequency. We found that many of the features associated with mtDNA mutations in vertebrates are conserved in Drosophila, including a comparable somatic mtDNA mutation frequency (∼10−5), an increased frequency of mtDNA mutations with age, and a prevalence of transition mutations. Only a small fraction of the mtDNA mutations detected in young or old animals were G∶C to T∶A transversions, a signature of oxidative damage, and loss-of-function mutations in the mitochondrial superoxide dismutase, Sod2, had no detectable influence on the somatic mtDNA mutation frequency. Moreover, a loss-of-function mutation in Ogg1, which encodes a DNA repair enzyme that removes oxidatively damaged deoxyguanosine residues (8-hydroxy-2′-deoxyguanosine), did not significantly influence the somatic mtDNA mutation frequency of Sod2 mutants. Together, these findings indicate that oxidative stress is not a major cause of somatic mtDNA mutations. Our data instead suggests that somatic mtDNA mutations arise primarily from errors that occur during mtDNA replication. Further studies using Drosophila should aid in the identification of factors that influence the frequency of somatic mtDNA mutations.

Do Somatic Mitochondrial DNA Mutations Contribute to Parkinson's Disease?

A great deal of evidence supports a role for mitochondrial dysfunction in the pathogenesis of Parkinson's disease (PD), although the origin of the mitochondrial dysfunction in PD remains unclear. Expression of mitochondrial DNA (mtDNA) from PD patients in “cybrid” cell lines recapitulates the mitochondrial defect, implicating a role for mtDNA mutations, but the specific mutations responsible for the mitochondrial dysfunction in PD have been difficult to identify. Somatic mtDNA point mutations and deletions accumulate with age and reach high levels in substantia nigra (SN) neurons. Mutations in mitochondrial DNA polymerase γ (POLG) that lead to the accumulation of mtDNA mutations are associated with a premature aging phenotype in “mutator” mice, although overt parkinsonism has not been reported in these mice, and with parkinsonism in humans. Together these data support, but do not yet prove, the hypothesis that the accumulation of somatic mtDNA mutations in SN neurons contribute to the pathogenesis of PD.

Mitochondrial DNA: An Up‐and‐coming Actor in White Adipose Tissue Pathophysiology

Mitochondrial DNA: An Up‐and‐coming Actor in White Adipose Tissue Pathophysiology

Mitochondrial DNA Deletion in Human Oocytes

To compare the incidence of mitochondrial DNA (mtDNA) deletion in normal and unfertilized oocytes. Laboratory study. Metaphase II oocytes were obtained during IVF cycles. Normal oocytes were obtained from women undergoing IVF treatments because of non-obstructive azoospemia but no sperm could be retrieved from testicular biopsy. The other group of oocytes came from patients undergoing IVF treatment but the oocytes were unfertilized. The presence of mtDNA in single human oocyte was confirmed by standard PCR with a set of primers (forward primer L467 and reverse primer H725) for amplifying a fragment outside the hotspot of deletion area. The nested PCR reaction was employed to detect the presence of mtDNA deletions, and was carried out on those oocytes with a positive band from the first PCR reaction. The two pairs of primers (outer primers L8282 + H13650 and inner primers L8334 + H13578) flanked the region of the K5S mtDNA deletion. If the K5S deletion was present, the nested PCR product was 256 bp long. In the absence of the deletion, there was no DNA amplication. All experiments included water-only negative controls and a positive control. The PCR products were separated on 2% agarose gel stained with ethidium bromide. PCR products were confirmed by DNA sequencing. Thirty normal and 155 unfertilized MII oocytes were obtained from 4 and 62 donors respectively. The incidence of K5S mtDNA deletion in the normal oocytes was 23.3% which was statistically not different from the 34.6% in the unfertilized oocytes (p = 0.228). When the comparison was confined to donors of unfertilized oocytes < 35 years of age (n = 97), the incidence of K5S mtDNA deletion remained comparable (p = 0.626) between the two groups. When only the unfertilized oocytes were studied, women ≥= 35 years of age had a significantly higher incidence of K5S mtDNA deletion (46.6%) when compared with younger women (27.8%) (p = 0.018). The incidence of mtDNA deletion in unfertilized oocytes was similar to that of the normal oocytes, but was higher in women with advanced age. These findings were in concordance with the observation that the rate of accumulation of somatic mtDNA mutations increased during the aging process and hence the incidence of mtDNA deletion can reflect ovarian aging.

Mitochondrial abnormalities in muscle and other aging cells: classification, causes, and effects.

The involvement of mitochondria and of mitochondrial DNA (mtDNA) in the aging process has generated much interest and even more controversy. The mitochondrial theory of aging considers a vicious circle consisting of: (1) accumulation of somatic mtDNA mutations; (2) impairment of respiratory chain function; (3) increased production of reactive oxygen species (ROS) in mitochondria; and (4) further damage to mtDNA. We review the evidence for and against the belief that these steps occur in aging muscle and brain, considering separately morphological, biochemical, and molecular data. The relationship between mitochondrial aging and late-onset neurodegenerative diseases is briefly reviewed. We conclude that mitochondrial dysfunction does play a crucial role in the aging process of both muscle and brain, but it remains unclear whether mitochondria are the culprits or mere accomplices.

A high frequency of mtDNA polymorphisms in HeLa cell sublines

The complete mtDNA sequences from the uncloned “founder” HeLa cells and from five sublines have been determined. These sequences all carry a common “core” of 38 single basepair alterations relative to the revised Cambridge Reference Sequence (CRS). The HeLa mitochondrial genome is of African descent and it is a member of the African L3 haplogroup. The sequence of the HeLa mtDNA resolves the uncertainty surrounding the mosaic composition of the original CRS for human mtDNA. Most importantly, we detected a total of eight polymorphisms that have arisen in the mtDNA coding region of different HeLa sublines. These observations suggest that HeLa mtDNA has a high rate of sequence divergence, relative to the phylogenetically-derived divergence rate for mtDNAs in the human population, which results from a relaxation of negative selection against the fixation of deleterious mutations. Furthermore, this high frequency of polymorphisms in HeLa mtDNA may reflect a process similar to the accumulation of somatic mtDNA mutations in human cancers. Preliminary analysis of single-cell derived subclone lines revealed the occurrence of another polymorphism and provided evidence for a large number of mtDNA segregation units.