Accumulation of somatic mutation in mitochondrial DNA and atherosclerosis in diabetic patients

Mitochondrial and metabolic dysfunction in ageing and age-related diseases

Organismal ageing is accompanied by progressive loss of cellular function and systemic deterioration of multiple tissues, leading to impaired function and increased vulnerability to death. Mitochondria have become recognized not merely as being energy suppliers but also as having an essential role in the development of diseases associated with ageing, such as neurodegenerative and cardiovascular diseases. A growing body of evidence suggests that ageing and age-related diseases are tightly related to an energy supply and demand imbalance, which might be alleviated by a variety of interventions, including physical activity and calorie restriction, as well as naturally occurring molecules targeting conserved longevity pathways. Here, we review key historical advances and progress from the past few years in our understanding of the role of mitochondria in ageing and age-related metabolic diseases. We also highlight emerging scientific innovations using mitochondria-targeted therapeutic approaches.

Do Mitochondrial DNA Mutations Play a Key Role in the Chronification of Sterile Inflammation? Special Focus on Atherosclerosis

BACKGROUND

The aim of the elucidation of mechanisms implicated in the chronification of inflammation is to shed light on the pathogenesis of disorders that are responsible for the majority of the incidences of diseases and deaths, and also causes of ageing. Atherosclerosis is an example of the most significant inflammatory pathology. The inflammatory response of innate immunity is implicated in the development of atherosclerosis arising locally or focally. Modified low-density lipoprotein (LDL) was regarded as the trigger for this response. No atherosclerotic changes in the arterial wall occur due to the quick decrease in inflammation rate. Nonetheless, the atherosclerotic lesion formation can be a result of the chronification of local inflammation, which, in turn, is caused by alteration of the response of innate immunity.

OBJECTIVE

In this review, we discussed potential mechanisms of the altered response of the immunity in atherosclerosis with a particular emphasis on mitochondrial dysfunctions.

CONCLUSION

A few mitochondrial dysfunctions can be caused by the mitochondrial DNA (mtDNA) mutations. Moreover, mtDNA mutations were found to affect the development of defective mitophagy. Modern investigations have demonstrated the controlling mitophagy function in response to the immune system. Therefore, we hypothesized that impaired mitophagy, as a consequence of mutations in mtDNA, can raise a disturbed innate immunity response, resulting in the chronification of inflammation in atherosclerosis.

Possible Role of Mitochondrial DNA Mutations in Chronification of Inflammation: Focus on Atherosclerosis

Chronification of inflammation is the process that lies at the basis of several human diseases that make up to 80% of morbidity and mortality worldwide. It can also explain a great deal of processes related to aging. Atherosclerosis is an example of the most important chronic inflammatory pathology in terms of public health impact. Atherogenesis is based on the inflammatory response of the innate immunity arising locally or focally. The main trigger for this response appears to be modified low-density lipoprotein (LDL), although other factors may also play a role. With the quick resolution of inflammation, atherosclerotic changes in the arterial wall do not occur. However, a violation of the innate immunity response can lead to chronification of local inflammation and, as a result, to atherosclerotic lesion formation. In this review, we discuss possible mechanisms of the impaired immune response with a special focus on mitochondrial dysfunction. Some mitochondrial dysfunctions may be due to mutations in mitochondrial DNA. Several mitochondrial DNA mutations leading to defective mitophagy have been identified. The regulatory role of mitophagy in the immune response has been shown in recent studies. We suggest that defective mitophagy promoted by mutations in mitochondrial DNA can cause innate immunity disorders leading to chronification of inflammation.

The alterations of mitochondrial DNA in coronary heart disease

Coronary heart disease (CHD) is the major cause of death in modern society. CHD is characterized by atherosclerosis, which could lead to vascular cavity stenosis or obstruction, resulting in ischemic cardiac conditions such as angina and myocardial infarction. In terms of the mitochondrion, the main function is to produce adenosine triphosphate (ATP) for cells. And the alterations (including mutations, altered copy number and haplogroups) of mitochondrial DNA (mtDNA) are associated with the abnormal expression of oxidative phosphorylation (OXPHOS) system, resulting in mitochondrial dysfunction, then leading to perturbation on the electron transport chain and increased ROS generation and reduction in ATP level, contributing to ATP-producing disorders and oxidative stress, which may further accelerate development or vulnerability of atherosclerosis and myocardial ischemic injury. Therefore, the mtDNA defects may play an important role in making an early diagnosis, identifying disease-specific biomarkers and therapeutic targets, and predicting outcomes for patients with atherosclerosis and CHD. In this review, we aim to summarize the contribution of mtDNA mutations, altered mtDNA copy number and mtDNA haplogroups on the occurrence and development of CHD.

Changes in Mitochondrial Genome Associated with Predisposition to Atherosclerosis and Related Disease

Atherosclerosis-related cardiovascular diseases remain the leading cause of morbidity and mortality, and the search for novel diagnostic and therapeutic methods is ongoing. Mitochondrial DNA (mtDNA) mutations associated with atherosclerosis represent one of the less explored aspects of the disease pathogenesis that may bring some interesting opportunities for establishing novel molecular markers and, possibly, new points of therapeutic intervention. Recent studies have identified a number of mtDNA mutations, for which the heteroplasmy level was positively or negatively associated with atherosclerosis, including the disease at its early, subclinical stages. In this review, we summarize the results of these studies, providing a list of human mtDNA mutations potentially involved in atherosclerosis. The molecular mechanisms underlying such involvement remain to be elucidated, although it is likely that some of them may be responsible for the increased oxidative stress, which plays an important role in atherosclerosis.

Mitochondrial DNA damage and vascular function in patients with diabetes mellitus and atherosclerotic cardiovascular disease

OBJECTIVE

Prior studies demonstrate mitochondrial dysfunction with increased reactive oxygen species generation in peripheral blood mononuclear cells in diabetes mellitus. Oxidative stress-mediated damage to mitochondrial DNA promotes atherosclerosis in animal models. Thus, we evaluated the relation of mitochondrial DNA damage in peripheral blood mononuclear cells s with vascular function in patients with diabetes mellitus and with atherosclerotic cardiovascular disease.

APPROACH AND RESULTS

We assessed non-invasive vascular function and mitochondrial DNA damage in 275 patients (age 57 ± 9 years, 60 % women) with atherosclerotic cardiovascular disease alone (N = 55), diabetes mellitus alone (N = 74), combined atherosclerotic cardiovascular disease and diabetes mellitus (N = 48), and controls age >45 without diabetes mellitus or atherosclerotic cardiovascular disease (N = 98). Mitochondrial DNA damage measured by quantitative PCR in peripheral blood mononuclear cells was higher with clinical atherosclerosis alone (0.55 ± 0.65), diabetes mellitus alone (0.65 ± 1.0), and combined clinical atherosclerosis and diabetes mellitus (0.89 ± 1.32) as compared to control subjects (0.23 ± 0.64, P < 0.0001). In multivariable models adjusting for age, sex, and relevant cardiovascular risk factors, clinical atherosclerosis and diabetes mellitus remained associated with higher mitochondrial DNA damage levels (β = 0.14 ± 0.13, P = 0.04 and β = 0.21 ± 0.13, P = 0.002, respectively). Higher mitochondrial DNA damage was associated with higher baseline pulse amplitude, a measure of arterial pulsatility, but not with flow-mediated dilation or hyperemic response, measures of vasodilator function.

CONCLUSIONS

We found greater mitochondrial DNA damage in patients with diabetes mellitus and clinical atherosclerosis. The association of mitochondrial DNA damage and baseline pulse amplitude may suggest a link between mitochondrial dysfunction and excessive small artery pulsatility with potentially adverse microvascular impact.

Association of mutations in the mitochondrial genome with the subclinical carotid atherosclerosis in women

The importance of the study of an association of mitochondrial DNA mutations with asymptomatic atherosclerosis in women is undeniable. In the present study, a series of PCR with primers for mutation region and further amplificate pyrosequencing were carried out to identify point substitutions or microdeletions of the mitochondrial genome. The results obtained were processed using the original method of estimating the level of heteroplasmy. Five mutations in the mitochondrial genome, namely C3256T, G14709A, G12315A, G13513A and G14846A, in which the heteroplasmy level was associated with the degree of preclinical atherosclerosis in women, were identified. The data obtained in the study showed that C3256T, G14709A and G12315A mutations have a positive correlation with atherosclerosis while G13513A and G14846A mutations have a negative correlation with atherosclerotic lesions. Total mutational load of the mitochondrial genome for C3256T, G14709A, G12315A, G13513A and G14846A mutations explains 68% of the variability of thickness of the carotid intima-medial layer, while the complex of traditional risk factors for cardiovascular diseases explains only 8% of the IMT variability. Data on the correlation between heteroplasmy levels of C3256T, G14709A, G12315A, G13513A and G14846A mutations prompt a suggestion that these mutations may be present on the same haplotypes of mitochondrial genome, associated with atherosclerosis.

Dipeptidyl peptidase-4 inhibitor linagliptin attenuates neointima formation after vascular injury

BACKGROUND

Recently, glucagon-like peptide-1 (GLP-1)-based therapy, including dipeptidyl peptidase-4 (DPP-4) inhibitors and GLP-1 receptor agonists, has emerged as one of the most popular anti-diabetic therapies. Furthermore, GLP-1-based therapy has attracted increased attention not only for its glucose-lowering ability, but also for its potential as a tissue-protective therapy. In this study, we investigated the vascular-protective effect of the DPP-4 inhibitor, linagliptin, using vascular smooth muscle cells (VSMCs).

METHODS

Six-week-old male C57BL/6 mice were divided into control (n =19) and linagliptin (3 mg/kg/day, n =20) treated groups. Endothelial denudation injuries were induced in the femoral artery at 8 weeks of age, followed by evaluation of neointima formation at 12 weeks. To evaluate cell proliferation of rat aortic smooth muscle cells, a bromodeoxyuridine (BrdU) incorporation assay was performed.

RESULTS

Linagliptin treatment reduced vascular injury-induced neointima formation, compared with controls (p <0.05). In these non-diabetic mice, the body weight and blood glucose levels did not change after treatment with linagliptin. Linagliptin caused an approximately 1.5-fold increase in serum active GLP-1 concentration, compared with controls. In addition, the vascular injury-induced increase in the oxidative stress marker, urinary 8-OHdG, was attenuated by linagliptin treatment, though this attenuation was not statistically significant (p =0.064). Moreover, linagliptin did not change the serum stromal cell-derived factor-1α (SDF-1α) or the serum platelet-derived growth factor (PDGF) concentration. However, linagliptin significantly reduced in vitro VSMC proliferation.

CONCLUSION

Linagliptin attenuates neointima formation after vascular injury and VSMC proliferation beyond the glucose-lowering effect.

Mitochondria, endothelial cell function, and vascular diseases

Mitochondria are perhaps the most sophisticated and dynamic responsive sensing systems in eukaryotic cells. The role of mitochondria goes beyond their capacity to create molecular fuel and includes the generation of reactive oxygen species, the regulation of calcium, and the activation of cell death. In endothelial cells, mitochondria have a profound impact on cellular function under both healthy and diseased conditions. In this review, we summarize the basic functions of mitochondria in endothelial cells and discuss the roles of mitochondria in endothelial dysfunction and vascular diseases, including atherosclerosis, diabetic vascular dysfunction, pulmonary artery hypertension, and hypertension. Finally, the potential therapeutic strategies to improve mitochondrial function in endothelial cells and vascular diseases are also discussed, with a focus on mitochondrial-targeted antioxidants and calorie restriction.

Quantitative assessment of heteroplasmy of mitochondrial genome: perspectives in diagnostics and methodological pitfalls

The role of alterations of mitochondrial DNA (mtDNA) in the development of human pathologies is not understood well. Most of mitochondrial mutations are characterized by the phenomenon of heteroplasmy which is defined as the presence of a mixture of more than one type of an organellar genome within a cell or tissue. The level of heteroplasmy varies in wide range, and the expression of disease is dependent on the percent of alleles bearing mutations, thus allowing consumption that an upper threshold level may exist beyond which the mitochondrial function collapses. Recent findings have demonstrated that some mtDNA heteroplasmic mutations are associated with widely spread chronic diseases, including atherosclerosis and cancer. Actually, each etiological mtDNA mutation has its own heteroplasmy threshold that needs to be measured. Therefore, quantitative evaluation of a mutant allele of mitochondrial genome is an obvious methodological challenge, since it may be a keystone for diagnostics of individual genetic predisposition to the disease. This review provides a comprehensive comparison of methods applicable to the measurement of heteroplasmy level of mitochondrial mutations associated with the development of pathology, in particular, in atherosclerosis and its clinical manifestations.

Type II diabetes increases mitochondrial DNA mutations in the left ventricle of the Goto-Kakizaki diabetic rat

Mitochondrial dysfunction has a significant role in the development of diabetic cardiomyopathy. Mitochondrial oxidant stress has been accepted as the singular cause of mitochondrial DNA (mtDNA) damage as an underlying cause of mitochondrial dysfunction. However, separate from a direct effect on mtDNA integrity, diabetic-induced increases in oxidant stress alter mitochondrial topoisomerase function to propagate mtDNA mutations as a contributor to mitochondrial dysfunction. Both glucose-challenged neonatal cardiomyocytes and the diabetic Goto-Kakizaki (GK) rat were studied. In both the GK left ventricle (LV) and in cardiomyocytes, chronically elevated glucose presentation induced a significant increase in mtDNA damage that was accompanied by decreased mitochondrial function. TTGE analysis revealed a number of base pair substitutions in the 3' end of COX3 from GK LV mtDNA that significantly altered the protein sequence. Mitochondrial topoisomerase DNA cleavage activity in isolated mitochondria was significantly increased in the GK LV compared with Wistar controls. Both hydroxycamptothecin, a topoisomerase type 1 inhibitor, and doxorubicin, a topoisomerase type 2 inhibitor, significantly exacerbated the DNA cleavage activity of isolated mitochondrial extracts indicating the presence of multiple functional topoisomerases in the mitochondria. Mitochondrial topoisomerase function was significantly altered in the presence of H2O2 suggesting that separate from a direct effect on mtDNA, oxidant stress mediated type II diabetes-induced alterations of mitochondrial topoisomerase function. These findings are significant in that the activation/inhibition state of the mitochondrial topoisomerases will have important consequences for mtDNA integrity and the well being of the diabetic myocardium.

Heteroplasmic m.1624C>T mutation of the mitochondrial tRNA(Val) gene in a proband and his mother with repeated consciousness disturbances

Homoplasmic m.1624C>T mutation of the mitochondrial tRNA(Val) gene was previously demonstrated to cause fatal neonatal Leigh syndrome. Here, we report the clinical phenotypes of a Japanese male and his mother with heteroplasmic m.1624C>T mutation. The 36-year-old male presented with repeated episodes of consciousness disturbance since the age of 25, cognitive decline, and personality change. Cerebrospinal fluid levels of lactate and pyruvate were elevated. His mother showed similar symptoms and course. The mutation m.1624C>T was identified heteroplasmically in the proband's muscle and leukocytes and in the mother's leukocytes. The heteroplasmy load decreased with age.

Mitochondrial dysfunction and mitochondrial DNA mutations in atherosclerotic complications in diabetes

Mitochondrial DNA (mtDNA) is particularly prone to oxidation due to the lack of histones and a deficient mismatch repair system. This explains an increased mutation rate of mtDNA that results in heteroplasmy, e.g., the coexistence of the mutant and wild-type mtDNA molecules within the same mitochondrion. In diabetes mellitus, glycotoxicity, advanced oxidative stress, collagen cross-linking, and accumulation of lipid peroxides in foam macrophage cells and arterial wall cells may significantly decrease the mutation threshold required for mitochondrial dysfunction, which in turn further contributes to the oxidative damage of the diabetic vascular wall, endothelial dysfunction, and atherosclerosis.

Stability of mitochondrial DNA against reactive oxygen species (ROS) generated in diabetes

BACKGROUND

Increased production of reactive oxygen species (ROS) in mitochondria has been proposed as the pathogenic mechanism for chronic complications of diabetes. Mitochondrial DNA (mtDNA) is more vulnerable to reactive oxygen species. However, there are few data on the mitochondrial DNA damage in diabetes and these are available only from patients with different duration of the disease and tissues not relevant to the chronic complications of diabetes. We therefore proposed to study the stability of mitochondrial DNA under controlled experimental conditions, to understand its contribution to chronic complications of diabetes.

METHODS

The mitochondrial DNA damage was evaluated by long-fragment polymerase chain reaction in human dermal fibroblasts exposed to high glucose level and hypoxia (an additional source of reactive oxygen species) or in organs from diabetic animals (db/db mice) at different ages. Reactive oxygen species production was assessed in vitro by fluorescence and in vivo by nitrosylation of the proteins. The antioxidant enzymes were assessed by enzyme activity and by quantitative real-time polymerase chain reaction while the mitochondrial repair activity (base excision repair) was determined by using abasic site-containing oligonucleotides as substrates.

RESULTS

Hyperglycaemia, when combined with hypoxia, is able to induce mitochondrial DNA damage in human dermal fibroblasts. The deleterious effect is mediated by mitochondrial reactive oxygen species, being abolished when the mitochondria electron transport is blocked. The accumulation of mitochondrial DNA damage in vivo is, however, decreased in 'old' diabetic animals (db/db) despite higher reactive oxygen species levels. This mitochondrial DNA protection might be conferred by an increased base excision repair activity.

CONCLUSION

Increased base excision repair activity in tissues affected by the chronic complications of diabetes is a potential mechanism that can overcome mitochondrial DNA damage induced by hyperglycaemia-related reactive oxygen species overproduction.

Chronically elevated glucose compromises myocardial mitochondrial DNA integrity by alteration of mitochondrial topoisomerase function

Mitochondrial dysfunction has a significant role in the development and complications of diabetic cardiomyopathy. Mitochondrial dysfunction and mitochondrial DNA (mtDNA) mutations are also associated with different types of cancer and neurodegenerative diseases. The goal of this study was to determine if chronically elevated glucose increase in mtDNA damage contributed to mitochondrial dysfunction and identify the underlying basis for mtDNA damage. H9c2 myotubes (a cardiac-derived cell line) were studied in the presence of 5.5, 16.5, or 33.0 mM glucose for up to 13 days. Tests of mitochondria function (Complex I and IV activity and ATP generation) were all significantly depressed by elevated media glucose. Intramitochondrial superoxide and intracellular superoxide levels were transiently increased during the experimental period. AnnexinV binding (a marker of apoptosis) was significantly increased after 7 and 13 days of high glucose. Thirteen days of elevated glucose significantly increased mtDNA damage globally and across the region encoding for the three subunits of cytochrome oxidase. Using mitochondria isolated from cells chronically exposed to elevated glucose, we observed significant increases in topoisomerase-linked DNA cleavage. Mitochondria-dependent DNA cleavage was significantly exacerbated by H(2)O(2) and that immunoprecipitation of mitochondrial extracts with a mtTOP1 antibody significantly decreased DNA cleavage, indicating that at least part of this activity could be attributed to mtTOP1. We conclude that even mild increases in glucose presentation compromised mitochondrial function as a result of a decline in mtDNA integrity. Separate from a direct impact of oxidative stress on mtDNA, ROS-induced alteration of mitochondrial topoisomerase activity exacerbated and propagated increases in mtDNA damage. These findings are significant in that the activation/inhibition state of the mitochondrial topoisomerases will have important consequences for mitochondrial DNA integrity and the well being of the myocardium.

Role of somatic mutations in vascular disease formation

Coronary artery disease, cerebrovascular disease, pulmonary artery hypertension and Alzheimer's disease all lead to substantial morbidity and mortality, and we currently lack effective treatments for these vascular diseases. Since the discovery, decades ago, that atherosclerotic lesions display clonal growth, atherosclerosis and other vascular diseases have been postulated to be neoplastic processes, arising through a series of critical somatic mutations. There is conflicting evidence supporting this but studies of DNA damage and mutagenesis, both genomic and mitochondrial, in atherosclerotic and vascular lesions, have yielded evidence that somatic mutations are involved in atherogenesis and vascular disease development. The roles of mitochondrial DNA damage, oxidative stress and signaling by members of the TGF-beta receptor family are implicated. With the increasing convenience and cost-effectiveness of genome sequencing, it is feasible to continue to seek specific genetic targets in the pathogenesis of these devastating diseases, with the hope of developing personalized genomic medicine in the future.

Clinical features, diagnosis and management of maternally inherited diabetes and deafness (MIDD) associated with the 3243A>G mitochondrial point mutation

Maternally inherited diabetes and deafness (MIDD) affects up to 1% of patients with diabetes but is often unrecognized by physicians. It is important to make an accurate genetic diagnosis, as there are implications for clinical investigation, diagnosis, management and genetic counselling. This review summarizes the range of clinical phenotypes associated with MIDD; outlines the advances in genetic diagnosis and pathogenesis of MIDD; summarizes the published prevalence data and provides guidance on the clinical management of these patients and their families.

Genetic, pathogenetic, and phenotypic implications of the mitochondrial A3243G tRNALeu(UUR) mutation

Mitochondrial disorders are frequently caused by mutations in mitochondrial genes and usually present as multisystem disease. One of the most frequent mitochondrial mutations is the A3,243G transition in the tRNALeu(UUR) gene. The phenotypic expression of the mutation is variable and comprises syndromic or non-syndromic mitochondrial disorders. Among the syndromic manifestations the mitochondrial encephalopathy, lactacidosis, and stroke-like episode (MELAS) syndrome is the most frequent. In single cases the A3,243G mutation may be associated with maternally inherited diabetes and deafness syndrome, myoclonic epilepsy and ragged-red fibers (MERRF) syndrome, MELAS/MERRF overlap syndrome, maternally inherited Leigh syndrome, chronic external ophthalmoplegia, or Kearns-Sayre syndrome. The wide phenotypic variability of the mutation is explained by the peculiarities of the mitochondrial DNA, such as heteroplasmy and mitotic segregation, resulting in different mutation loads in different tissues and family members. Moreover, there is some evidence that additional mtDNA sequence variations (polymorphisms, haplotypes) influence the phenotype of the A3,243G mutation. This review aims to give an overview on the actual knowledge about the genetic, pathogenetic, and phenotypic implications of the A3,243G mtDNA mutation.

Rapid and sensitive real-time polymerase chain reaction method for detection and quantification of 3243A>G mitochondrial point mutation

Maternally inherited diabetes and deafness and mitochondrial encephalomyopathy, lactic acidosis with stroke-like episodes result from the 3243A>G mitochondrial point mutation. Current methods to detect the presence of the mutation have limited sensitivity and may lead to potential misclassification of patients with low levels of heteroplasmy. Here, we describe development and validation of a rapid real-time polymerase chain reaction (PCR) method for detection and quantification of levels of heteroplasmy in a single assay. Standard curve analysis indicated that the sensitivity of detection was less than 0.1%. Time from sample loading to data analysis was 110 minutes. We tested 293 samples including 23 known positives, 40 known negatives, and 230 samples from patients clinically classified as having type 2 diabetes. All positive samples were correctly detected, and of those samples previously quantified, heteroplasmy levels determined using the real-time assay correlated well (r(2) = 0.88 and 0.93) with results from fluorescently labeled PCR-restriction fragment length polymorphism and pyrosequencing methods. Screening of 230 patients classified as having type 2 diabetes revealed one patient with 0.6% heteroplasmy who had previously tested negative by PCR-restriction fragment length polymorphism. Real-time PCR provides rapid simultaneous detection and quantification of the 3243A>G mutation to a detection limit of less than 0.1%, without post-PCR manipulation.

Genetic determinants of carotid ultrasound traits

Atherosclerosis is a complex disease with various intermediate phenotypes that are themselves complex and influenced by many factors. Through the use of carotid ultrasound techniques, the intermediate stages of vascular disease can be imaged and studied for association with potential genetic determinants. In this article we review the most recent available data (reports published since 2004) on the genetic determinants of atherosclerosis, as measured by one-, two-, and three-dimensional ultrasonography of the carotid arteries. In general, associations are disparate and modest. For intima-media thickness, promising associations have been found for both TNFRSF1A R92Q and PPARG P12A, but associations also differed in the same individuals depending on the specific ultrasound trait studied (eg, linear intima-media thickness versus total plaque volume in carotid arteries). Some of the challenging issues for future studies include accounting for gene-environment interactions, sex-specific associations, and the distinctiveness of different carotid ultrasound measures.