A novel class of tests for the detection of mitochondrial DNA-mutation involvement in diseases

Mitochondrial impairment but not peripheral inflammation predicts greater Gulf War illness severity

Gulf War illness (GWI) is an important exemplar of environmentally-triggered chronic multisymptom illness, and a potential model for accelerated aging. Inflammation is the main hypothesized mechanism for GWI, with mitochondrial impairment also proposed. No study has directly assessed mitochondrial respiratory chain function (MRCF) on muscle biopsy in veterans with GWI (VGWI). We recruited 42 participants, half VGWI, with biopsy material successfully secured in 36. Impaired MRCF indexed by complex I and II oxidative phosphorylation with glucose as a fuel source (CI&CIIOXPHOS) related significantly or borderline significantly in the predicted direction to 17 of 20 symptoms in the combined sample. Lower CI&CIIOXPHOS significantly predicted GWI severity in the combined sample and in VGWI separately, with or without adjustment for hsCRP. Higher-hsCRP (peripheral inflammation) related strongly to lower-MRCF (particularly fatty acid oxidation (FAO) indices) in VGWI, but not in controls. Despite this, whereas greater MRCF-impairment predicted greater GWI symptoms and severity, greater inflammation did not. Surprisingly, adjusted for MRCF, higher hsCRP significantly predicted lesser symptom severity in VGWI selectively. Findings comport with a hypothesis in which the increased inflammation observed in GWI is driven by FAO-defect-induced mitochondrial apoptosis. In conclusion, impaired mitochondrial function-but not peripheral inflammation-predicts greater GWI symptoms and severity.

The mitochondrial tRNAAsp T7561C, tRNAHis C12153T and A12172G mutations may be associated with essential hypertension in a Han Chinese pedigree

OBJECTIVES

Mutations in mitochondrial tRNA (mt-tRNA) are the important causes for maternally inherited hypertension, however, the pathophysiology of mt-tRNA mutations in clinical expression of hypertension remains poorly understood.

MATERIAL AND METHODS

In this study, we report the molecular features of a Han Chinese pedigree with maternally transmitted essential hypertension. The entire mitochondrial genomes are PCR amplified and sequenced, Moreover, phylogenetic analysis, haplogroup analysis, as well as pathogenicity scoring system are used to assess the potential roles for mtDNA mutations.

RESULTS

Strikingly, among ten matrilineal relatives, three of them suffer from variable degree of hypertension at different age at onset. Sequence analysis of the complete mitochondrial genomes suggests the presence of three possible pathogenic mtDNA mutations: tRNAAsp T7561C, tRNAHis C12153T and A12172G, together with a set of variants belonging to East Asian mitochondrial haplogroup M7a. Interestingly, the T7561C mutation occurs at position 44 in the variable region of tRNAAsp, while the C12153T and A12172G mutations are localized at extremely conserved nucleotides in the D-arm and anticodon stem of tRNAHis gene, respectively, which are critical for tRNA steady-state level and function.

CONCLUSIONS

Mitochondrial T7561C, C12153T and A12172G mutations may lead to the failure in tRNAs metabolism, and cause mitochondrial dysfunction that is responsible for hypertension. However, the homoplasmy form of mt-tRNA mutations, incomplete penetrance of hypertension suggest that T7561C, C12153T and A12172G mutations are insufficient to produce the clinical phenotype, hence, other risk factors such as environmental factors, nuclear genes and epigenetic modifications may contribute to the phenotypic manifestation of maternally inherited hypertension in this Chinese pedigree.

Conplastic strains for identification of retrograde effects of mitochondrial dna variation on cardiometabolic traits in the spontaneously hypertensive rat

Mitochondrial retrograde signaling is a pathway of communication from mitochondria to the nucleus. Recently, natural mitochondrial genome (mtDNA) polymorphisms (haplogroups) received increasing attention in the pathophysiology of human common diseases. However, retrograde effects of mtDNA variants on such traits are difficult to study in humans. The conplastic strains represent key animal models to elucidate regulatory roles of mtDNA haplogroups on defined nuclear genome background. To analyze the relationship between mtDNA variants and cardiometabolic traits, we derived a set of rat conplastic strains (SHR-mtBN, SHR-mtF344 and SHR-mtLEW), harboring all major mtDNA haplotypes present in common inbred strains on the nuclear background of the spontaneously hypertensive rat (SHR). The BN, F344 and LEW mtDNA differ from the SHR in multiple amino acid substitutions in protein coding genes and also in variants of tRNA and rRNA genes. Different mtDNA haplotypes were found to predispose to various sets of cardiometabolic phenotypes which provided evidence for significant retrograde effects of mtDNA in the SHR. In the future, these animals could be used to decipher individual biochemical components involved in the retrograde signaling.

Mitochondrial genome mutations in 13 subunits of respiratory chain complexes in Chinese Han and Mongolian hypertensive individuals

Mitochondrial DNA (mtDNA) mutations are associated with cardiovascular disease, including hypertension (HTN). Here we performed a genetic and molecular analysis of 13 mtDNA-encoded subunits of respiratory chain complexes in 100 Chinese Han and 80 Mongolian HTN cases, and 100 Han and 42 Mongolian normotension subjects. The total cholesterol of the Mongolian normotensive subjects was higher than that of the Han normotensive group (p < .05). Sequence analysis identified 636 point mutations in the 13 mtDNA-encoded subunits in the Han and Mongolian hypertensive individuals, including 66 in NADH dehydrogenase subunit 1(ND1), 62 in ND2, 71 in COI, 29 in COII, 17 in ATP8, one in ATP6/8, 49 in ATP6, 27 in COIII, 27 in ND3, 14 in ND4L, 74 in ND4, 97 in ND5, 24 in ND6, and 78 in CYTB. Eight of these point mutations were present at significantly different frequencies in Han and Mongolian hypertensive individuals. Thirty-one point mutations were present only in Mongolian hypertensive individuals, while 73 were present only in Han hypertensive individuals. The relation between point mutations in 13 mtDNA-encoded subunits of respiratory chain complexes and HTN is worth to further research in future; however, the functional effects of these mutations require elucidation.

Mitochondrial tRNA mutations in Chinese hypertensive individuals

PURPOSE

Hypertension is a very important risk factor for cardiac vascular disease. The previous studies showed that mitochondrial DNA mutations are associated with cardiovascular disease, including hypertension.

METHODS

In this study we did systematical analysis on the total 22 mitochondrial tRNAs and the clinical, genetic and molecular changes of 140 Chinese hypertension and 124 controls.

RESULTS

This analysis identified 22 nucleotide changes among 15 different tRNA genes. There are 15 mutations with CI (Conservation index) larger than 75%. Of these, there are 26 patients with CI larger than 75% in the HTN group, higher than the 6 subjects in the control group (P=0.00). The tRNA(Phe) G586A, tRNA(Lys) G8313A and tRNA(His) G12147A mutations create highly conservative base-pairings on the D-stem, tRNA(Lys) G8342A on the T-stem, tRNA(Phe) T616C, tRNA(Ala) T5628C, tRNA(Tyr) G5856A and tRNA(Thr) A15924G on the AC stem, tRNA(Leu(CUN)) G12300A on the AC loop, tRNA(Met) C4467T, tRNA(Trp) T5578C, tRNA(Lys) A8296G, tRNA(Arg) T10463C and tRNA(Thr) C15891T on ACC stem, and tRNA(Ser(UCN)) C7492T on D-A junction, while the other tRNA variants were polymorphisms. The pedigrees of PLAH78 carrying the T5578C, PLAH84 carrying the C4467T, PLAH60 carrying the T5628C and PLAH118 carrying the C7492T mutation exhibited maternal transmission of essential hypertension. Sequence analysis of their mitochondrial genomes revealed the presence of T5578C, C4467T, T5628C or C7492T mutations but the absence of other functionally significant mutations in all matrilineal relatives of these families.

CONCLUSIONS

These tRNAs mutations, associated with altered structures of tRNAs and mitochondrial dysfunction, may contribute to the hypertension in Chinese population. A lot of work still should be done for the mechanism and functional effect of the mtDNA mutation on hypertension.

Revisiting heritability accounting for shared environmental effects and maternal inheritance

Heritability measures the proportion of phenotypic variation attributable to genetic factors. In addition to a shared nuclear genetic component, a number of additional variance components, such as spousal correlation, sibship, household and maternal effects, may have strong contributions to inter-individual phenotype variation. In humans, the confounding effects of these components on heritability have not been studied thoroughly. We sought to obtain unbiased heritability estimates for complex traits in the presence of multiple variance components and also to estimate the contributions of these variance components to complex traits. We compared regression and variance component methods to estimate heritability in simulations when additional variance components existed. We then revisited heritability for several traits in Framingham Heart Study (FHS) participants. Using simulations, we found that failure to account for or misclassification of necessary variance components yielded biased heritability estimates. The direction and magnitude of the bias varied depending on a variance structure and an estimation method. Using the best fitted models to account for necessary variance components, we found that heritability estimates for most FHS traits were overestimated, ranging from 4 to 47 %, when we compared models that considered necessary variance components to models that only considered familial relationships. Spousal correlation explained 14-36 % of phenotypic variation in several anthropometric and lifestyle traits. Maternal and sibling effects also contributed to phenotypic variation, ranging from 3 to 5 % and 4 to 7 %, respectively, in several anthropometric and metabolic traits. Our findings may explain, in part, the missing heritability for some traits.

Increased Prevalence of Hypertension in Young Adults with High Heteroplasmy Levels of the MELAS m.3243A>G Mutation

BACKGROUND

The pathophysiology of hypertension in patients with mitochondrial diseases is different from that of the general population. Growing evidence exists linking mtDNA, its mutations, and mitochondrial dysfunction to the pathogenesis of hypertension. No reports on the prevalence of hypertension in late-onset mtDNA diseases have been described.

METHODS

We performed a retrospective chart review of adult patients with late-onset mtDNA diseases between January 1999 and January 2012 at our center. We grouped them into age categories to allow comparison with previously reported Canadian Health Measures Survey (CHMS) prevalence data.

RESULTS

Twenty-three subjects with hypertension were identified for a crude prevalence of 39.7 % (95 % CI 27-53 %) as compared to the CHMS age-predicted prevalence of 30.5 %. When analyzed by individual age group, there were no significant differences between the observed and the CHMS predicted prevalence rates in the 40 years and older cohorts (age category 40-59, p = 0.63; age category 60-79, p = 0.85). However, hypertension rates were significantly higher than predicted in the under 40 years cohort (55.6 vs. 2.8 %, p < 0.001, CI 21-86 %), in which hypertensive patients with the MELAS m.3243A>G mutation were significantly clustered (p < 0.01). This younger MELAS cohort (n = 4, mean age = 24 years) with hypertension had heteroplasmy levels (mean = 68 %) that were significantly higher than the levels found in the older non-hypertensive MELAS cohort (n = 8, mean age = 52 years, mean = 33 %) (p = 0.04).

CONCLUSION

Relative to age, gender, and mtDNA disease subtype, young adults with high heteroplasmy levels of the MELAS m.3243A>G mutation demonstrate an increased prevalence of hypertension. Further prospective data are needed to confirm this initial finding, which has potentially important treatment implications.

Integration of cellular bioenergetics with mitochondrial quality control and autophagy

Bioenergetic dysfunction is emerging as a cornerstone for establishing a framework for understanding the pathophysiology of cardiovascular disease, diabetes,cancer and neurodegeneration. Recent advances in cellular bioenergetics have shown that many cells maintain a substantial bioenergetic reserve capacity, which is a prospective index of ‘ healthy ’ mitochondrial populations.The bioenergetics of the cell are likely regulated by energy requirements and substrate availability. Additionally,the overall quality of the mitochondrial population and the relative abundance of mitochondria in cells and tissues also impinge on overall bioenergetic capacity and resistance to stress. Because mitochondria are susceptible to damage mediated by reactive oxygen/nitrogen and lipid species, maintaining a ‘ healthy ’ population of mitochondria through quality control mechanisms appears to be essential for cell survival under conditions of pathological stress. Accumulating evidence suggest that mitophagy is particularly important for preventing amplification of initial oxidative insults, which otherwise would further impair the respiratory chain or promote mutations in mitochondrial DNA (mtDNA). The processes underlying the regulation of mitophagy depend on several factors, including the integrity of mtDNA, electron transport chain activity, and the interaction and regulation of the autophagic machinery. The integration and interpretation of cellular bioenergetics in the context of mitochondrial quality control and genetics is the theme of this review.

Association of genetic variation in the mitochondrial genome with blood pressure and metabolic traits

Elevated blood pressure (BP) is a major risk factor for cardiovascular disease. Several studies have noted a consistent maternal effect on BP; consequently, mitochondrial DNA variation has become an additional target of investigation of the missing BP heritability. Analyses of common mitochondrial DNA polymorphisms, however, have not found evidence of association with hypertension. To explore associations of uncommon (frequency>5%) mitochon drial DNA variants with BP, we identified uncommon/rare variants through sequencing the entire mitochondrial genome in 32 unrelated individuals with extreme-high BP in the Framingham Heart Study and genotyped 40 mitochondrial single nucleotide polymorphisms in 7219 Framingham Heart Study participants. The nonsynonymous mitochondrial single nucleotide polymorphism 5913G>A (Asp4Asn) in the cytochrome c oxidase subunit 1 of respiratory complex IV demonstrated significant associations with BP and fasting blood glucose (FBG) levels. Individuals with the rare 5913A allele had, on average, 7-mm Hg higher systolic BP at baseline (Pempirical=0.05) and 17-mg/dL higher mean FBG over 25 years of follow-up (Pempirical=0.009). Significant associations with FBG levels were also detected for nonsynonymous mitochondrial single nucleotide polymorphism 3316G>A (Ala4Thr) in the NADH dehydrogenase subunit 1 of complex I. On average, individuals with rare allele 3316A had 17- and 25-mg/dL higher FBG at baseline (Pempirical=0.01) and over 25 years of follow-up (Pempirical=0.007). Our findings provide the first evidence of putative association of variants in the mitochondrial genome with systolic BP and FBG in the general population. Replication in independent samples, however, is needed to confirm these putative associations.

Nonsynonymous variants in mt-Nd2, mt-Nd4, and mt-Nd5 are linked to effects on oxidative phosphorylation and insulin sensitivity in rat conplastic strains

Common inbred strains of the laboratory rat can be divided into four different mitochondrial DNA haplotype groups represented by the SHR, BN, LEW, and F344 strains. In the current study, we investigated the metabolic and hemodynamic effects of the SHR vs. LEW mitochondrial genomes by comparing the SHR to a new SHR conplastic strain, SHR-mt(LEW); these strains are genetically identical except for their mitochondrial genomes. Complete mitochondrial DNA (mtDNA) sequence analysis comparing the SHR and LEW strains revealed gene variants encoding amino acid substitutions limited to a single mitochondrial enzyme complex, NADH dehydrogenase (complex I), affecting subunits 2, 4, and 5. Two of the variants in the mt-Nd4 subunit gene are located close to variants known to be associated with exercise intolerance and diabetes mellitus in humans. No variants were found in tRNA or rRNA genes. These variants in mt-Nd2, mt-Nd4, and mt-Nd5 in the SHR-mt(LEW) conplastic strain were linked to reductions in oxidative and nonoxidative glucose metabolism in skeletal muscle. In addition, SHR-mt(LEW) conplastic rats showed increased serum nonesterified fatty acid levels and resistance to insulin stimulated incorporation of glucose into adipose tissue lipids. These results provide evidence that inherited variation in mitochondrial genes encoding respiratory chain complex I subunits, in the absence of variation in the nuclear genome and other confounding factors, can influence glucose and lipid metabolism when expressed on the nuclear genetic background of the SHR strain.

The role of mitochondria in the pathophysiology of skeletal muscle insulin resistance

Multiple organs contribute to the development of peripheral insulin resistance, with the major contributors being skeletal muscle, liver, and adipose tissue. Because insulin resistance usually precedes the development of type 2 diabetes mellitus (T2DM) by many years, understanding the pathophysiology of insulin resistance should enable development of therapeutic strategies to prevent disease progression. Some subjects with mitochondrial genomic variants/defects and a subset of lean individuals with hereditary predisposition to T2DM exhibit skeletal muscle mitochondrial dysfunction early in the course of insulin resistance. In contrast, in the majority of subjects with T2DM the plurality of evidence implicates skeletal muscle mitochondrial dysfunction as a consequence of perturbations associated with T2DM, and these mitochondrial deficits then contribute to subsequent disease progression. We review the affirmative and contrarian data regarding skeletal muscle mitochondrial biology in the pathogenesis of insulin resistance and explore potential therapeutic options to intrinsically modulate mitochondria as a strategy to combat insulin resistance. Furthermore, an overview of restricted molecular manipulations of skeletal muscle metabolic and mitochondrial biology offers insight into the mitochondrial role in metabolic substrate partitioning and in promoting innate adaptive and maladaptive responses that collectively regulate peripheral insulin sensitivity. We conclude that skeletal muscle mitochondrial dysfunction is not generally a major initiator of the pathophysiology of insulin resistance, although its dysfunction is integral to this pathophysiology and it remains an intriguing target to reverse/delay the progressive perturbations synonymous with T2DM.

Voltage-dependent anion channel (VDAC) is involved in apoptosis of cell lines carrying the mitochondrial DNA mutation

BACKGROUND

The mitochondrial voltage-dependent anion channel (VDAC) is increasingly implicated in the control of apoptosis. We have studied the effects the mitochondrial DNA (mtDNA) tRNAIle mutation on VDAC expression, localization, and apoptosis.

METHODS

Lymphoblastoid cell lines were derived from 3 symptomatic and 1 asymptomatic members of a family with hypertension associated with the A4263G tRNAIle mutation as well as from control subjects. Mitochondrial potential (DeltaPsim) and apoptosis were measured by flow cytometry; co-localization of VDAC and Bax was evaluated by confocal microscopy.

RESULTS

Expression of VDAC and Bax in mtDNA cell lines was found to be increased compared to controls, while expression of the small conductance calcium-dependant potassium channel (sKCa) was unchanged. Confocal imaging revealed co-localization of VDAC/Bax on the outer mitochondrial membrane of A4263G cell lines but not from controls. Flow cytometry indicated that the mitochondrial potential was decreased by 32% in mutated cells versus controls while rates of apoptosis were increased (P < 0.05). The difference was attenuated by Cyclosporin A (CsA, 2 muM), a blocker of VDAC.

CONCLUSION

We conclude that increased expression of mitochondrial VDAC and subcellular co-localization of VDAC/Bax increases mitochondrial permeability and apoptosis in cell lines carrying the mtDNA tRNAIle A4263G mutation.

Genetic variants in mitochondrial tRNA genes are associated with essential hypertension in a Chinese Han population

BACKGROUND

Of multiple factors contributing to essential hypertension, mitochondrial variants exhibited the trends for serving as molecular and genetic markers for the disease in last five years. However, previous studies focused on African-American or Caucasian pedigrees, knowledge of mitochondrial tRNA genes and population-based Chinese hypertensives were limited.

METHODS

We performed sequence analysis in tRNA genes, hot spots for cardiovascular diseases, in 270 Chinese Han essential hypertensives and 270 controls. Lymphoblastoid cell lines were immortalized by transformation with the Epstein-Barr virus. Rates of oxygen consumption in intact cells were determined with a YSI 5300 oxygraph (Yellow Springs Instruments) on samples, harboring variants in tRNA genes.

RESULTS

There were 26 variants in tRNA genes that were found in hypertensives and these variants were not in controls. Functional analysis found that these variants may lead to deficiencies in tRNA 3' end metabolism and/or impairment of critical subunits of the respiratory chain. Most importantly, the oxygen consumption rate in cells harboring variants T4454C (P=0.0010) and A4263G (P=0.0001) decreased as compared to the average level of control cell lines.

CONCLUSIONS

Variants located in mitochondrial tRNA genes may have biologic plausibility to implicate in the pathogenesis of Chinese essential hypertension.

Maternal influence on blood pressure suggests involvement of mitochondrial DNA in the pathogenesis of hypertension: the Framingham Heart Study

OBJECTIVE

To investigate the contribution of the mitochondrial genome to hypertension and quantitative blood pressure (BP) phenotypes in the Framingham Heart Study cohort, a randomly ascertained, community-based sample.

METHODS

Longitudinal BP values of 6421 participants (mean age, 53 years; 46% men) from 1593 extended families were used for analyses. In analyses of BP as a continuous trait, a variance components model with a variance component for maternal effects was used to estimate the mitochondrial heritability of the long-term average BP adjusted for age, sex, body mass index, and hypertension treatment. For analyses of BP as a categorical trait, a nonparametric test sensitive to excessive maternal inheritance was used to test for mitochondrial effect on long-term hypertension, defined as systolic BP of at least 140 mmHg or diastolic BP of at least 90 mmHg or use of antihypertensive medication in one-half or more of qualifying examinations. This test was based on 353 pedigrees comprised of 403 individuals informative for mitochondrial DNA contribution.

RESULTS

The estimated fraction of hypertensive pedigrees potentially due to mitochondrial effects was 35.2% (95% confidence interval, 27-43%, P < 10). The mitochondrial heritabilities for multivariable-adjusted long-term average systolic BP and diastolic BP were, respectively, 5% (P < 0.02) and 4% (P = 0.11).

CONCLUSION

Our data provide support for a maternal effect on hypertension status and quantitative systolic BP, consistent with mitochondrial influence. Additional studies are warranted to identify mitochondrial DNA variant(s) affecting BP.

Direct linkage of mitochondrial genome variation to risk factors for type 2 diabetes in conplastic strains

Recently, the relationship of mitochondrial DNA (mtDNA) variants to metabolic risk factors for diabetes and other common diseases has begun to attract increasing attention. However, progress in this area has been limited because (1) the phenotypic effects of variation in the mitochondrial genome are difficult to isolate owing to confounding variation in the nuclear genome, imprinting phenomena, and environmental factors; and (2) few animal models have been available for directly investigating the effects of mtDNA variants on complex metabolic phenotypes in vivo. Substitution of different mitochondrial genomes on the same nuclear genetic background in conplastic strains provides a way to unambiguously isolate effects of the mitochondrial genome on complex traits. Here we show that conplastic strains of rats with identical nuclear genomes but divergent mitochondrial genomes that encode amino acid differences in proteins of oxidative phosphorylation exhibit differences in major metabolic risk factors for type 2 diabetes. These results (1) provide the first direct evidence linking naturally occurring variation in the mitochondrial genome, independent of variation in the nuclear genome and other confounding factors, to inherited variation in known risk factors for type 2 diabetes; and (2) establish that spontaneous variation in the mitochondrial genome per se can promote systemic metabolic disturbances relevant to the pathogenesis of common diseases.

Mitochondrial disease: maintenance of mitochondrial genome and molecular diagnostics

Mitochondrial DNA (mtDNA) is essential for the aerobic ATP synthesis system that is responsible for about 80% of normal cellular energy demands. In addition to rare genetic disorders causing neuromyopathy, alterations of mtDNA have been found also in so-called common diseases such as heart failure, diabetes, and cancer. Although some of these alterations are inherited, some are considered to be generated and/or accumulated in somatic cells with age. One reason for the somatic mutations is that mtDNA is more vulnerable than is nuclear DNA. For example, mitochondrial respiratory chain produces a large amount of reactive oxygen species as inevitable byproducts of oxidative phosphorylation. However, the molecular mechanisms for maintenance of mitochondrial genome are much less elucidated than those for nuclear genome. In spite of its increasing importance, the molecular diagnosis of mitochondrial DNA-related diseases is well done only in very limited expert laboratories. In this chapter, we focus on maintenance of mtDNA in somatic cells, its clinical importance, and recent developments of molecular tests.

Molecular genetics of human hypertension

EH (essential hypertension) is a major public health problem in many countries due to its high prevalence and its association with coronary heart disease, stroke, renal disease, peripheral vascular disease and other disorders. Epidemiological studies have demonstrated that EH is heritable. Owing to the fact that blood pressure is controlled by cardiac output and total peripheral resistance, many molecular pathways are believed to be involved in the disease. In this review, recent genetic studies investigating the molecular basis of EH, including different molecular pathways, will be highlighted.

Mitochondrial microheteroplasmy and a theory of aging and age-related disease

We implicate a recently described form of mitochondrial mutation, mitochondrial microheteroplasmy, as a candidate for the principal component of aging. Microheteroplasmy is the presence of hundreds of independent mutations in one organism, with each mutation usually found in 1-2% of all mitochondrial genomes. Despite the low abundance of single mutations, the vast majority of mitochondrial genomes in all adults are mutated. This mutational burden includes inherited mutations, de novo germline mutations, as well as somatic mutations acquired either during early embryonic development or later in adult life. We postulate that microheteroplasmy is sufficient to explain the pathomechanism of several age-associated diseases, especially in conditions with known mitochondrial involvement, such as diabetes (DM), cardiovascular disease, Parkinson's disease (PD), and Alzheimer's disease (AD) and cancer. The genetic properties of microheteroplasmy reconcile the results of disease models (cybrids, hypermutable PolG variants and mitochondrial toxins), with the relatively low levels of maternal inheritance in the aforementioned diseases, and provide an explanation of their delayed, progressive course.

Mitochondrial DNA in somatic cells: A promising target of routine clinical tests

Alterations of mitochondrial DNA have long been considered only from a point of view of rare genetic disorders causing neuromyopathy. Recently, alterations of mitochondrial DNA have been found in so-called common diseases such as heart failure, diabetes, and cancer; some of these alterations are inherited, and some are generated and/or accumulated in somatic cells with age. Mitochondrial DNA is more vulnerable to alteration than is nuclear DNA. For example, mitochondria produce a large amount of reactive oxygen species as an inevitable byproduct of oxidative phosphorylation. Therefore, mitochondrial DNA is under much stronger oxidative stress than is nuclear DNA. In spite of the importance, it is much less elucidated in the mitochondrial genome than in the nuclear genome how the genome is maintained. In this review, we focus on maintenance of mitochondrial DNA in somatic cells and its clinical importance.