Tissue mitochondrial DNA changes. A stochastic system

Increased TFAM binding to mtDNA damage hot spots is associated with mtDNA loss in aged rat heart

The well-known age-related mitochondrial dysfunction deeply affects heart because of the tissue's large dependence on mitochondrial ATP provision. Our study revealed in aged rat heart a significant 25% decrease in mtDNA relative content, a significant 29% increase in the 4.8 Kb mtDNA deletion relative content, and a significant inverse correlation between such contents as well as a significant 38% decrease in TFAM protein amount. The TFAM-binding activity to specific mtDNA regions increased at those encompassing the mtDNA replication origins, D-loop and Ori-L. The same mtDNA regions were screened for different kinds of oxidative damage, namely Single Strand Breaks (SSBs), Double Strand Breaks (DSBs), abasic sites (AP sites) and oxidized bases as 7,8-dihydro-8-oxoguanine (8oxoG). A marked increase in the relative content of mtDNA strand damage (SSBs, DSBs and AP sites) was found in the D-loop and Ori-L regions in the aged animals, unveiling for the first time in vivo an age-related, non-stochastic accumulation of oxidative lesions in these two regions that appear as hot spots of mtDNA damage. The use of Formamidopyrimidine glycosylase (Fpg) demonstrated also a significant age-related accumulation of oxidized purines particularly in the D-loop and Ori-L regions. The detected increased binding of TFAM to the mtDNA damage hot spots in aged heart suggests a link between TFAM binding to mtDNA and loss of mitochondrial genome likely through hindrance of repair processes.

Comparative analysis of Ancylostoma ceylanicum mitochondrial genome with other Ancylostoma species

Ancylostoma ceylanicum may inhabit the small intestine of canids, felids and humans, can pose a potential risk to public health. This study is the first time to amplify complete mitochondrial genome sequence of A. ceylanicum from dog and to compare it with Ancylostoma tubaeforme, Ancylostoma duodenale and Ancylostoma caninum. The results showed that the complete mitochondrial genome of A. ceylanicum was 13,660 bp in length, including 12 protein-coding genes, 2 rRNA genes and 22 tRNA genes and 3 non-coding regions (AT-rich region, SNCR and LNCR). Its mtDNA was the shortest, biased toward A and T at base composition, and higher than other three Ancylostoma species at total AT content. Its nad5 and nad6 genes used TTG and ATT as initiation codons, while other three Ancylostoma species used ATT and GTG or ATG. The 22 tRNA genes were different in length among four Ancylostoma species, but their anticodons were the same. Among 12 protein-coding genes, the cox1 gene was the lowest at AT content and minimum at Ka/Ks while the nad2 gene was the opposite. The phylogenetic tree showed that in the lineage of Ancylostoma, A. ceylanicum occurred on a branch external to other three Ancylostoma species, and A. caninum and A. tubaeforme had closer phylogenetic relationship than A. duodenale. This study not only enhances the mitochondrial genome database of Ancylostomatidae nematodes, but also provides new data for further phylogenetic studies among Ancylostomatidae nematodes.

Evolution of Cell-to-Cell Variability in Stochastic, Controlled, Heteroplasmic mtDNA Populations

Populations of physiologically vital mitochondrial DNA (mtDNA) molecules evolve in cells under control from the nucleus. The evolution of populations of mixed mtDNA types is complicated and poorly understood, and variability of these controlled admixtures plays a central role in the inheritance and onset of genetic disease. Here, we develop a mathematical theory describing the evolution of, and variability in, these stochastic populations for any type of cellular control, showing that cell-to-cell variability in mtDNA and mutant load inevitably increases with time, according to rates that we derive and which are notably independent of the mechanistic details of feedback signaling. We show with a set of experimental case studies that this theory explains disparate quantitative results from classical and modern experimental and computational research on heteroplasmy variance in different species. We demonstrate that our general model provides a host of specific insights, including a modification of the often-used but hard-to-interpret Wright formula to correspond directly to biological observables, the ability to quantify selective and mutational pressure in mtDNA populations, and characterization of the pronounced variability inevitably arising from the action of possible mtDNA quality-control mechanisms. Our general theoretical framework, supported by existing experimental results, thus helps us to understand and predict the evolution of stochastic mtDNA populations in cell biology.

CHRONICITY OF DEPRESSION AND MOLECULAR MARKERS IN A LARGE SAMPLE OF HAN CHINESE WOMEN

BACKGROUND

Major depressive disorder (MDD) has been associated with changes in mean telomere length and mitochondrial DNA (mtDNA) copy number. This study investigates if clinical features of MDD differentially impact these molecular markers.

METHODS

Data from a large, clinically ascertained sample of Han Chinese women with recurrent MDD were used to examine whether symptom presentation, severity, and comorbidity were related to salivary telomere length and/or mtDNA copy number (maximum N = 5,284 for both molecular and phenotypic data).

RESULTS

Structural equation modeling revealed that duration of longest episode was positively associated with mtDNA copy number, while earlier age of onset of most severe episode and a history of dysthymia were associated with shorter telomeres. Other factors, such as symptom presentation, family history of depression, and other comorbid internalizing disorders, were not associated with these molecular markers.

CONCLUSIONS

Chronicity of depressive symptoms is related to more pronounced telomere shortening and increased mtDNA copy number among individuals with a history of recurrent MDD. As these molecular markers have previously been implicated in physiological aging and morbidity, individuals who experience prolonged depressive symptoms are potentially at greater risk of adverse medical outcomes.

Renal involvement in mitochondrial cytopathies

Mitochondrial cytopathies constitute a group of rare diseases that are characterized by their frequent multisystemic involvement, extreme variability of phenotype and complex genetics. In children, renal involvement is frequent and probably underestimated. The most frequent renal symptom is a tubular defect that, in most severe forms, corresponds to a complete De Toni-Debré-Fanconi syndrome. Incomplete proximal tubular defects and other tubular diseases have also been reported. In rare cases, patients present with chronic tubulo-interstitial nephritis or cystic renal diseases. Finally, a group of patients develop primarily a glomerular disease. These patients correspond to sporadic case reports or can be classified into two major defects, namely 3243 A>G tRNA(LEU) mutations and coenzyme Q10 biosynthesis defects. The latter group is particularly important because it represents the only treatable renal mitochondrial defect. In this Educational Review, the principal characteristics of these diseases and the main diagnostic approaches are summarized.

Mitochondrial proteome: toward the detection and profiling of disease associated alterations

Existing at the heart of cellular energy metabolism, the mitochondrion is uniquely positioned to have a major impact on human disease processes. Examples of mitochondrial impact on human pathology abound and include etiologies ranging from inborn errors of metabolism to the site of activity of a variety of toxic compounds. In this review, the unique aspects of the mechanisms related to the mitochondrial proteome are discussed along with an overview of the literature related to mitochondrial proteomic exploration. The review includes discussion of potential areas for exploration and advantages of applying proteomic techniques to the study of mitochondria.

Mitochondrial DNA deletion and sarcopenia

PURPOSE

Accumulation of mitochondrial DNA deletions and the resultant impaired oxidative phosphorylation may play a pathogenic role in the mediation of age-related sarcopenia.

METHODS

Twenty four participants of the New Mexico Aging Process Study were classified as normal lean (n = 15) or sarcopenic (n = 9) based on body composition determined by Dual Energy x-ray Absorptiometry. Complex I and Complex IV activities were measured in the skeletal muscle samples obtained from gastrocnemius muscle. A two-stage nested polymerase chain reaction strategy was used to identify the mitochondrial DNA deletions in the entire mitochondrial genome in the skeletal muscle samples.

RESULTS

Although Complex I activity was not significantly different (5.5 +/- 0.9 vs. 4.6 +/- 0.7 mU/mg protein, P > 0.05), Complex IV activity was higher in sarcopenic subjects (1.4 +/- 0.3 vs. 1.0 +/- 0.1 mU/mg protein, P < 0.05). Mitochondrial DNA deletions were mostly located in the region of Complex I and spanned from nicotinamide adenine dinucleotide dehydrogenase 1 to nicotinamide adenine dinucleotide dehydrogenase 6. Deletions in the 8,577-10,407 bp and 10,233-11,249 bp regions were associated with a significant decrease in Complex I activity (P < 0.05 and P = 0.02, respectively). Total cumulative deletion, defined as the sum of individual length of deletions in a subject, was comparable in subjects with and without sarcopenia (1760 +/- 726 vs. 1782 +/- 888 bp, P > 0.05). The magnitude of mitochondrial DNA deletion, however, correlated positively with lean body mass (r = 0.43, P < 0.05).

CONCLUSION

Thus, mitochondrial DNA deletions are common in elderly subjects and are negatively related to Complex I activity. The positive association between mitochondrial DNA deletions and lean body mass needs to be confirmed by studies in a larger study population.

Mitochondrial DNA deletions of blood lymphocytes as genetic markers of low folate-related mitochondrial genotoxicity in peripheral tissues

BACKGROUND

A low folate status and mitochondrial DNA (mtDNA) mutations are risk factors for various cancers and degenerative diseases. It is not known if lymphocytic mtDNA deletions can be used as genetic "markers" to reflect global mtDNA damage during folate deficiency.

AIM OF THE STUDY

The aim of this study was to characterize folate-related mtDNA deletions in lymphocytes and their associations with mt genotoxicity in peripheral tissues.

METHODS

Weaning Wistar rats were fed folate-deficient and folate-replete (control) diets for 2 and 4 weeks. Folate levels of blood lymphocytes and various tissues were assayed by the Lactobacillus casei method. mtDNA deletions were measured by a real-time polymerase chain reaction analysis of whole DNA extracts.

RESULTS

Compared to the control counterparts, mtDNA deletions of lymphocytes increased by 3.5-fold (P < 0.05) after 4 weeks of folate deficiency. Lymphocytic mtDNA deletions were inversely associated with plasma (r = -0.619, P = 0.018), red blood cell (r = -0.668, P = 0.009), and lymphocytic folate levels (r = -0.536, P = 0.048). Frequencies of lymphatic mtDNA deletions were positively correlated with mtDNA deletions in tissues including the lungs (r = 0.803, P = 0.0005), muscles (r = 0.755, P = 0.001), heart (r = 0.633, P = 0.015), liver (r = 0.722, P = 0.003), kidneys (r = 0.737, P = 0.006), pancreas (r = 0.666, P = 0.009), and brain (r = 0.917, P < 0.0001).

CONCLUSIONS

Our data demonstrate that accumulated mtDNA deletions of lymphocytes depended upon dietary folate deprivation. The accumulated mt deletions in lymphocytes closely reflected the mt genotoxicity in the peripheral tissues during folate deficiency.

Healthy aging: regulation of the metabolome by cellular redox modulation and prooxidant signaling systems: the essential roles of superoxide anion and hydrogen peroxide

The production of reactive oxygen species (ROS) and reactive nitrogen species (RNS) has long been proposed as leading to random deleterious modification of macromolecules with an associated progressive development of age associated systemic disease. ROS and RNS formation has been posited as a major contributor to the aging process. On the contrary, this review presents evidence that superoxide anion (and hydrogen peroxide) and nitric oxide (and peroxynitrite) constitute regulated prooxidant second messenger systems, with specific sub-cellular locales of production and are essential for normal metabolome and physiological function. The role of these second messengers in the regulation of the metabolome is discussed in terms of radical formation as an essential contributor to the physiologically normal regulation of sub-cellular bioenergy systems; proteolysis regulation; transcription activation; enzyme activation; mitochondrial DNA changes; redox regulation of metabolism and cell differentiation; the concept that orally administered small molecule antioxidant therapy is a chimera. The formation of superoxide anion/hydrogen peroxide and nitric oxide do not conditionally lead to random macromolecular damage; under normal physiological conditions their production is actually regulated consistent with their second messenger roles.

Coenzyme Q(10)--its role as a prooxidant in the formation of superoxide anion/hydrogen peroxide and the regulation of the metabolome

Coenzyme Q10 plays a central role in cellular bioenergy generation and its regulation. Closed membrane systems generate a proton motive force to create transient localized bio-capacitors; the captured energy is used for the synthesis of mitochondrial ATP but also for many other processes, such as metabolite translocations, nerve conduction and a host of other bioenergy requiring processes. Coenzyme Q10 plays a key role in many of these sub-cellular membrane energy generating systems. Integral to this phenomenon is the prooxidant role of coenzyme Q10 in generating the major superoxide anion/hydrogen peroxide second messenger system. This messenger system, largely but not exclusively, arises from coenzyme Q10 semiquinone function; it contributes to the regulation of sub-cellular redox potential levels; transcription/gene expression control; is essential for modulated protein turnover and activation; mediates hormone and growth factor extracellular signaling. The regulated prooxidant formation of the superoxide anion/H2O2 second messenger system is essential for the normal physiological function of the metabolome. The normally functioning metabolome is the expression of a finely tuned dynamic equilibrium comprised of thousands of anabolic and catabolic reactions and all cellular signaling systems must be finely regulated. There is still much to be learnt about the up/down regulation of the H2O2 messenger system. The concept that superoxide anion/H2O2 cause random macromolecular damage is rebutted. The administration of antioxidants to quench the inferred toxicity of these compounds as a therapy for age associated diseases is unsupported by extant mammalian clinical trials and should be subject to serious re-evaluation. The role of ascorbic acid as a beneficial hydrogen peroxide prodrug is discussed.

The essential requirement for superoxide radical and nitric oxide formation for normal physiological function and healthy aging

Contrary to the dogma that superoxide anion and hydrogen peroxide formation are highly deleterious to cell function and healthy aging, we suggest this premise is flawed. Superoxide anion and hydrogen peroxide formation are essential to normal cellular function; they constitute a second messenger system absolutely required for the regulation of the metabolome. Embraced within this regulation is the modulation of cellular redox poise, bioenergy output, gene expression and cell differentiation. A key component in the overall process is coenzyme Q10 whose prooxidant function through the formation of superoxide anion and hydrogen peroxide is a major factor in the overall processes. The free radical gas, nitric oxide (similarly to superoxide anion), functions in the regulation of a wide range of cell systems. As part of the normal physiological process, superoxide anion and NO function separately and interactively as second messengers. Superoxide anion and nitric oxide play an intrinsic role in the regulated ordered turnover of proteins, rather than randomly cause protein damage and their inactivation. The proposition that metabolic free radical formation is unequivocally deleterious to cell function is rebutted; their toxicity as primary effectors in the aging process has been overemphasized. The concept that a dietary supplement of high concentrations of small-molecule antioxidants is a prophylactic/amelioration therapy for the aging process and age-associated diseases is questioned as to its clinical validity.

Mitochondrial DNA content increase in response to cigarette smoking

An increase in mitochondrial DNA (mtDNA) content and decline in mitochondrial function occurs with aging and in response to DNA-damaging agents, including tobacco smoke. We did a cross-sectional study and quantified changes in mtDNA content in a population of individuals with varied smoking and alcohol exposure. Age, smoking history, ethanol intake, and other demographic data were characterized for 604 individuals participating in a screening study for smoking-related upper aerodigestive malignancy. Total DNA was extracted from exfoliated cells in saliva. DNA from a nuclear gene, beta-actin, and two mitochondrial genes, cytochrome c oxidase I and II (Cox I and Cox II), were quantified by real-time PCR. mtDNA content was correlated with age, exposure history, and other variables using multivariate regression analyses. A significant increase (P<0.001) in mtDNA content was noted in smokers (31% and 29% increase for Cox I and Cox II, respectively) and former smokers (31% and 34%) when compared with never smokers. This association persisted after adjustment for other significant factors including age, alcohol drinking, and income (P<0.001). Increased mtDNA content was positively associated with pack-years of smoking (P=0.02). Despite an average smoking cessation interval of 21 years in former smokers, tobacco cessation interval was not statistically significantly associated with mtDNA content. Smoking is associated with increased mtDNA content in a dose-dependent fashion. Mitochondrial DNA alterations in response to smoking persist for several decades after smoking cessation, consistent with long-term, smoking-related damage.

Proteomic identification of differentially expressed proteins in the aging murine olfactory system and transcriptional analysis of the associated genes

Decline in olfactory ability has been associated with aging as well as neurodegenerative disorders. The aim of this study was to gain fundamental insight into molecular events associated with the aging olfactory system. We report a comparative proteomic analysis of the olfactory epithelium (OE) and olfactory bulb (OB) of old (80-week old) and young (6-week old) mice with further analysis of age-related differences in differentially expressed proteins at the mRNA level using real-time RT-PCR. Nine proteins in the OE and 20 in the OB were differentially expressed in old and young mice; of these, aldolase 1, peptidyl prolyl isomerase A, mitochondrial aconitase 2, mitochondrial aldehyde dehydrogenase 2 and albumin 1 were identified in the OE; and ATP synthase isoform 1, enolase 1, ferritin heavy chain, malate dehydrogenase 1, tropomyosin alpha 3 chain and dynamin 1 were identified in the OB. At the transcriptional level, aconitase 2 in the OE and ferritin heavy chain 1 in the OB were differentially expressed with aging, in concordance with the proteomic data. Our results demonstrate an altered proteomic profile of the aged murine olfactory system. The identified proteins fall into three broadly defined functional categories: (i) metabolism, (ii) transport/motility and (iii) stress response. Our transcriptional analysis provides insight into possible mechanisms by which protein expression may be regulated in the OE and OB. The results are discussed in relation to the decrement in olfactory sensitivity with aging.

Selective mitochondrial autophagy, or mitophagy, as a targeted defense against oxidative stress, mitochondrial dysfunction, and aging

In autophagy, portions of cytoplasm are sequestered into autophagosomes and delivered to lysosomes for degradation. Long assumed to be a random process, increasing evidence suggests that autophagy of mitochondria, peroxisomes, and possibly other organelles is selective. A recent paper (Kissova et al., J. Biol. Chem. 2004;279:39068-39074) shows in yeast that a specific outer membrane protein, Uth1p, is required for efficient mitochondrial autophagy. For this selective autophagy of mitochondria, we propose the term "mitophagy" to emphasize the non-random nature of the process. Mitophagy may play a key role in retarding accumulation of somatic mutations of mtDNA with aging.

What the papers say: Where is the somatic mutation that causes aging?

It has been proposed that somatic mutations make major contributions to aging. The first paper, based on a gene knock-in mouse, supports a contributory role for mutation in mtDNA in aging, but does not support a damaged-mtDNA-producing-more-damaged-mtDNA hypothesis. The second paper indicates some GC-rich sequences in the nuclear DNA are more sensitive to oxidative damage than mtDNA. As a result, key genes involved in brain function and mitochondrial function are progressively inactivated with age. Failure in these nucleus-encoded mitochondrial genes may be a primary reason for mitochondrial failure in old age.

Oxidative damage to mitochondrial DNA in atrial muscle of patients with atrial fibrillation

Atrial fibrillation (AF) is the most common cause of arrhythmia and is an aging-related disease encountered in clinical practice. The electrophysiological remolding with Ca(2+) overloading and cellular structure changes were found in cardiomyocytes of AF patients. In previous studies, increased oxidative stress and oxidative damage was found in cardiomyocytes during the ischemia/reperfusion injury. Besides, mitochondrial DNA (mtDNA) deletion and mtDNA proliferation occur frequently in affected tissues of patients with certain degenerative diseases and during aging of the human. However, it remains unclear whether high oxidative stress and alteration of mtDNA play a role in the pathophysiology of AF. In this study, we first screened for large-scale deletions of mtDNA in the atrial muscle of AF patients by long-range polymerase chain reaction (PCR). The results showed that large-scale deletions between nucleotide positions 7900 and 16500 of mtDNA occurred at a high frequency. Among them, the 4977 bp deletion was the most frequent and abundant one, and the mean proportion of mtDNA with the 4977 bp deletion in the atrial muscle of the patients with AF was 3.75-fold higher than that of the patients without AF (p <.005). Furthermore, quantitative PCR was performed to evaluate lesions in mtDNA caused by oxidative damage. We found that the degree of mtDNA damage in the patients with AF was greater than that of the patients without AF (3.29 vs.1.60 per 10 kb, p <.0005). The 8-OHdG, which is one of the most common products of oxidative damage to DNA, was also found at a higher frequency in mtDNA of patients with AF as compared with those without AF. In addition, the mtDNA content was found to increase significantly in the patients with AF (p =.0051). The level of mtDNA lesion and the mtDNA content was positively correlated (r = 0.44). These results suggest that oxidative injury and deletion of mtDNA in cardiac muscle are increased in the patients with AF, which may contribute to the impairment of bioenergetic function of mitochondria and induction of the oxidative vicious cycle involved in the pathogenesis of atrial myopathy in AF.

Identification of 30 protein species involved in replicative senescence and stress-induced premature senescence

Exposure of human proliferative cells to subcytotoxic stress triggers stress-induced premature senescence (SIPS) which is characterized by many biomarkers of replicative senescence. Proteomic comparison of replicative senescence and stress-induced premature senescence indicates that, at the level of protein expression, stress-induced premature senescence and replicative senescence are different phenotypes sharing however similarities. In this study, we identified 30 proteins showing changes of expression level specific or common to replicative senescence and/or stress-induced premature senescence. These changes affect different cell functions, including energy metabolism, defense systems, maintenance of the redox potential, cell morphology and transduction pathways.

In memoriam Bernard Strehler--genomic instability in ageing: a persistent challenge

Genomic instability comprises a broad spectrum of mutational alterations in the genome, such as point mutations in DNA, microsatellite expansions or contractions, amplifications and deletions of DNA sequences, gene rearrangements and structural or numerical chromosomal aberrations. A substantial body of data demonstrates an increase of genomic instability during normal ageing. This includes cytogenetic changes; loss of rDNA; formation of extrachromosomal circular DNA species; loss of telomeric repeats; increased microsatellite instability; as well as point mutations and deletions in global nuclear and mitochondrial DNA. Evidence has accumulated supporting a causative role of genomic instability in ageing. Genomic instability can be counteracted by a number of proteins including antioxidant enzymes, the WRN protein (deficient in Werner syndrome), telomerase, poly(ADP-ribose) polymerase-1 and a range other others, as well as by multi-protein systems such as DNA mismatch repair, base-excision repair and nucleotide-excision repair. Important research tasks for the future will be to elucidate how and what extent the various expressions of genomic instability contribute to the ageing process and to understand the molecular mechanisms and regulation of the above factors and pathways involved in limiting the induction of ageing-associated genomic instability.

Human aging and global function of coenzyme Q10

In this paper, we review two parts of our recent work on human skeletal muscle. The first part mainly describes changes occurring during aging, whereas the second part discusses the functions of coenzyme Q10 (CoQ10), particularly in relation to the aging process. During the lifetime of an individual, mtDNA undergoes a variety of mutation events and rearrangements. These mutations and their consequent bioenergenic decline, together with nuclear DNA damage, contribute to the reduced function of cells and organs, especially in postmitotic tissues. In skeletal muscle, this functional decline can be observed by means of changes with age in fiber type profile and the reduction in the number and size of the muscle fibers. In addition to the functions of coenzyme Q10 as an electron carrier in the respiratory chain and as an antioxidant, CoQ10 has been shown to regulate global gene expression in skeletal muscle. We hypothesize that this regulation is achieved via superoxide formation with H2O2 as a second messenger to the nucleus.

Mitochondrial DNA damage in lymphocytes: a role in immunosenescence?

An age-related increase of DNA damage/mutation has been previously reported in human lymphocytes. The high copy number and mutation rate make the mtDNA genome an ideal candidate for assessing damage and to act as a potential biomarker of ageing. In the present study, two assays were developed to evaluate the level of mtDNA(4977) and the accumulation of point mutations with age. A competitive polymerase chain reaction (PCR) methodology incorporating three primers was used to detect and quantify the levels of mtDNA(4977) and a novel heteroduplex reference strand conformational analysis (RSCA) technique was used to analyse the accumulation of point mutations. The assays were applied to an in vitro model of T cell ageing and ex vivo DNA samples from an elderly cohort of subjects and a younger control group. The mtDNA(4977) was detected in all the DNA samples examined but only a very low concentration was observed and no age-related increase or accumulation was observed. No accumulation of point mutations was identified using RSCA within the T cell clones as they were aged or the ex vivo lymphocytes from the elderly cohort. A higher level of variation was observed within the ex vivo DNA samples, verifying the high resolution of RSCA and its ability to identify different mtDNA species, although no correlation with age was observed. The low level of mtDNA damage observed with respect to the ex vivo lymphocyte DNA samples within this study may be due in part to the high turnover of blood cells/mtDNA, which may inhibit the accumulation of genetically abnormal mtDNA that may play a role in immunosenescence. A similar explanation may also apply to the in vitro model of T cell ageing if the vast majority of the cells are replicating rather than entering senescence.

Method of proof and evidences for the concept that mitochondrial genome is a thrifty genome

A hypothesis that mitochondrial genome itself is the thrifty genome has been advanced, as both its mutation, or quality, and amount in the peripheral blood, or quantity, are found to have close relationship to diabetes mellitus, or more specifically insulin resistance. Mere presence of relationship, however, does not establish cause-effect relation. Usually a genetic 'cause' is established, when a knockout of putative gene produces disease phenotype. However, absence of mitochondrial genome is incompatible with life. Here the argument advanced by Bradford Hill was applied to the proof of a hypothesis, as most of the data are from epidemiological observations; it is biologically plausible that both mtDNA quality and quantity are related to insulin resistance. The relationship had enough strength of association, dose-response relation, and consistency of relationship. Temporally association, decrease of mtDNA content preceding onset of diabetes, was also observed. Good hypothesis generates many predictions, which could be verified experimentally and which is biologically plausible.

Mechanisms of ageing

Recent experimental work from a variety of biological systems, ranging from yeast to human beings, lends increasing support to the view that stochastic damage inflicted to biological macromolecules is the driving force for the ageing process. The damage is derived from small reactive molecules, most prominently reactive oxygen intermediates (ROI), that arise during normal cellular metabolism and are associated with important if not essential cellular functions. The major classes of macromolecules at risk are proteins, lipids and DNA, but damage to DNA (both nuclear and mitochondrial) may entail particularly severe consequences. Cellular dysfunction resulting from macromolecular damage can be detected as a variety of expressions, such as genomic instability, inappropriate cell differentiation events or cell death. While for post-mitotic cell types replacement of the dead cell by another cell of the same lineage is not possible, mitotic cell types may initially replace dead cells via cell proliferation. But exhaustion of the self-renewal capacity of the respective lineage, by either replication-associated or damage-associated telomere shortening, will ultimately also lead to loss of parenchymal cell mass and functional impairment of tissues, the latter being a typical feature of ageing of tissues and organs. It has been demonstrated in various experimental systems that the rate ageing of can be retarded by lowering the production of endogenous ROI or by improving cellular anti-oxidative defences. Whether augmentation of cellular DNA repair capacity will have the same effect remains to be seen.

A specific mitochondrial DNA deletion (mtDNA4977) is identified in a pedigree of a family with hearing loss

This paper presents a family pedigree of sensorineural hearing loss in patients with a mitochondrial DNA (mtDNA) deletion. Genomic DNA screenings including myo 15 and connexin 26 were normal. MtDNA deletions are associated with many pathophysiologic conditions, including neurological disorders, sensorineural hearing loss, ischemia, cardiomyopathies and aging. Several mitochondrial disorders secondary to mutations or deletions in mtDNA have been identified in association with deafness. The present study describes a pedigree of five individuals with hearing loss who harbor a 4977 bp common aging deletion, in their mtDNA. Chromosomal analysis was normal in all affected individuals. Audiologic and molecular biologic findings of these patients suggest that the common aging deletion of mtDNA may be a predisposing factor in sensorineural hearing loss in this family.