Different copy numbers of apparently identically deleted mitochondrial DNA in tissues from a patient with Kearns-Sayre syndrome detected by PCR

Cellular and Molecular Responses to Mitochondrial DNA Deletions in Kearns-Sayre Syndrome: Some Underlying Mechanisms

Kearns-Sayre syndrome (KSS) is a rare multisystem mitochondrial disorder. It is caused by mitochondrial DNA (mtDNA) rearrangements, mostly large-scale deletions of 1.1-10 kb. These deletions primarily affect energy supply through impaired oxidative phosphorylation and reduced ATP production. This impairment gives rise to dysfunction of several tissues, in particular those with high energy demand like brain and muscles. Over the past decades, changes in respiratory chain complexes and energy metabolism have been emphasized, whereas little attention has been paid to other reports on ROS overproduction, protein synthesis inhibition, myelin vacuolation, demyelination, autophagy, apoptosis, and involvement of lipid raft and oligodendrocytes in KSS. Therefore, this paper draws attention towards these relatively underemphasized findings that might further clarify the pathologic cascades following deletions in the mtDNA.

Cardiac dysfunction in patients with chronic progressive external ophthalmoplegia

BACKGROUND

Chronic progressive external ophthalmoplegia (CPEO), which includes Kearns-Sayre syndrome, is a mitochondrial disorder with large deletions of mitochondrial DNA. Recently, mtDNA deletions in cardiac muscle cells were thought to be a cause of dilated cardiomyopathy. However, the cardiac involvement in patients with CPEO is generally considered to be limited to the cardiac conduction system.

HYPOTHESIS

The purpose of this study was to evaluate left ventricular function in patients with CPEO.

METHODS

We evaluated the cardiac function of five patients with CPEO by means of carotid pulse recording and Doppler echocardiography.

RESULTS

The ratio of the pre-ejection period to ejection time was increased to 0.67 in one patient and to 0.50 in another. Echocardiography showed left ventricular dilatation and diffuse hypokinetic wall motion in both cases. Left ventricular fractional shortening was decreased to 5 and 19%, respectively, and the mean rate of circumferential shortening was decreased to 0.12 and 0.63 circ/s, respectively. One of the two patient died of congestive heart failure 2 months after the study. The Doppler pattern of left ventricular filling in the three remaining patients showed a decrease in the ratio of peak flow velocity in early diastole to that in late diastole, with an increase in deceleration time.

CONCLUSION

Although cardiac involvement in patients with CPEO is generally considered to be limited to the cardiac conduction system, left ventricular dysfunction may be present and should receive more attention in the management of patients with CPEO.

Yeast models of human mitochondrial diseases: from molecular mechanisms to drug screening

Mitochondrial diseases are rare diseases most often linked to energy in the form of ATP-depletion. The high number of nuclear- and mitochondrial-DNA-encoded proteins (>500), required for ATP production and other crucial mitochondrial functions such as NADH re-oxidation, explains the increasing number of reported disorders. In recent years, yeast has revealed to be a powerful model to identify responsible genes, to study primary effects of pathogenic mutations and to determine the molecular mechanisms leading to mitochondrial disorders. However, the clinical management of patients with mitochondrial disorders is still essentially supportive. Here we review some of the most fruitful yeast mitochondrial disorder models and propose to subject these models to highthroughput chemical library screening to prospect new therapeutic drugs against mitochondrial diseases.

Evidence that the mitochondrial genome is the thrifty genome

Although mitochondrial DNA (mtDNA) abnormalities are known to cause insulin deficiency, insulin resistance and diabetes mellitus, it's quantitative aspect was not addressed well. In this review, mitochondrial genome hypothesis of thrifty phenomenon is proposed, based on the data and review of literatures. From a population based epidemiologic study, it was found that mtDNA quantity was decreased in the peripheral blood of diabetic subjects, and also in those subjects who will convert to diabetes mellitus within 2 years. In this population, low mtDNA subjects were found to have higher blood pressure and high waist hip ratio. These findings suggested mtDNA status might be quantitatively linked to the insulin resistance syndrome. As quantitative relationships between peripheral blood mtDNA levels and insulin requirement, and energy utilization pattern (fat and carbohydrate oxidation during hyperinsulinemic clamp studies) were observed in a group of male students; and maternal mtDNA content (peripheral blood) correlated with birth weight and peripheral blood mtDNA content of the offspring in another study, possibility of thrifty phenotype phenomena might be due to the low mitochondrial status arose. As thrifty phenotype phenomenon shows the quantitatively continuous relationship between involved parameters and characteristics of 'imprinting', a possible mechanism is suggested.

Developmental genetics of deleted mtDNA in mitochondrial oculomyopathy

Heteroplasmic populations of mtDNA, consisting of normal mtDNA and mtDNA with large deletions, are found in the skeletal muscle and other tissues of certain patients with mitochondrial respiratory chain deficiencies, particularly in those with the CPEO (chronic progressive external ophthalmoplegia) phenotype. To study the developmental genetics of this mitochondrial disorder, the distribution of the deleted mtDNA in a wide range of tissues of different embryonic origins (total 34 samples from 27 tissues obtained at autopsy) was investigated in a patient with the CPEO syndrome. Three species of partially deleted mtDNA were observed, with deletions of 2.3 kb, 5.0 kb and 6.4 kb. Their tissue distribution suggests that the mtDNA deletions have occurred very early during embryonic development, prior to the differentiation events that lead to the formation of the three primary embryonic germ layers, and that the partially deleted mtDNA species were segregated during development mainly to the skeletal muscle and to tissues of the central nervous system.

Kearns-Sayre syndrome associated with mitochondrial DNA deletion or duplication: a molecular genetic and pathological study

The neuropathological findings in 2 patients with Kearns-Sayre syndrome and mitochondrial DNA (mtDNA) rearrangements, one a predominant deletion and the other a predominant duplication, were remarkably similar, showing diffuse vacuolation of white matter. There were some of the pathological features of Leigh's syndrome in the spinal cord of the patient with a duplication. In the patient with a predominant deletion, rearranged mtDNA was undetectable in blood, spleen, and testis, and present in highest amounts in muscle and the brain, but relatively low in cerebellum, reflecting the ratio seen, albeit in much smaller amounts, in normal aged brains. MtDNA rearrangements in this patient were largely deletions or deletion dimers; duplicated mtDNA was present in only trace amounts in some tissues and there was none in skeletal muscle. The patient with a predominant duplication of mtDNA had higher amounts of rearranged mtDNA in blood (mainly duplicated) than muscle (mainly deleted). Correlation of these data with tissue dysfunction is probably complicated by the replicative behaviour of deleted, duplicated and normal mtDNA.

Cardiac involvement in mitochondrial diseases. A study on 17 patients with documented mitochondrial DNA defects

BACKGROUND

Mutations of mitochondrial DNA have been demonstrated as causes of human mitochondrial diseases. While these disorders typically involve multiple organs, the effect of mitochondrial mutations on the heart has not been systematically studied.

METHODS AND RESULTS

We studied mitochondrial mutations and cardiac changes in 17 patients with Kearns-Sayre syndrome; ocular myopathy; myoclonus epilepsy with ragged red fibers (MERRF); and mitochondrial myopathy, encephalopathy, lactic acidosis, and strokelike episodes (MELAS). Cardiac involvement was evaluated by chest radiograph, ECG, His-bundle electrogram, and echocardiogram. All 3 patients with Kearns-Sayre syndrome had large deletions of mitochondrial DNA and disturbances in cardiac conduction. ECG abnormalities were found in 2 of 6 patients with ocular myopathy who showed large deletions of mitochondrial DNA. All 3 patients with MERRF had an A-to-G mutation at nucleotide position 8344; 2 had cardiomegaly, asymmetrical septal hypertrophy, and diffuse hypokinesis of the left ventricle. One patient with asymmetrical septal hypertrophy developed dilated cardiomyopathy 2 years later. All 5 patients with MELAS had an A-to-G mutation at nucleotide position 3243, and 2 had symmetrical left ventricular hypertrophy with or without abnormal wall motion.

CONCLUSIONS

The clinical features of cardiac involvement in mitochondrial diseases vary in the different subgroups of these disorders. Particular mitochondrial mutations can cause characteristic cardiac abnormalities.

Mitochondrial mutations and human disease

The mitochondrial genome is essential for producing ATP (adenosine 5'-triphosphate) via oxidative phosphorylation. The gradual decline of mitochondrial function with age has long been postulated as a factor in aging. More recently, a variety of diseases have been related to molecular defects in human mitochondrial DNA. In both the cases of aging and disease, symptoms were generally neuromuscular, reflecting the tissues most dependent upon mitochondrial function. Also, in both cases novel features of mitochondrial genetics led to complex relations between genotype and phenotype. Little information is yet available about the role of environmental agents in these interactions.

Evidence for a life span-prolonging effect of a linear plasmid in a longevity mutant of Podospora anserina

The linear mitochondrial plasmid pAL2-1 of the long-lived mutant AL2 of Podospora anserina was demonstrated to be able to integrate into the high molecular weight mitochondrial DNA (mtDNA). Hybridization analysis and densitometric evaluation of the mitochondrial genome isolated from cultures of different ages revealed that the mtDNA is highly stable during the whole life span of the mutant. In addition, and in sharp contrast to the situation in certain senescence-prone Neurospora strains, the mutated P. anserina mtDNA molecules containing integrated plasmid copies are not suppressive to wild-type genomes. As demonstrated by hybridization and polymerase chain reaction (PCR) analysis, the proportion of mtDNA molecules affected by the integration of pAL2-1 fluctuates between 10% and 50%. Comparative sequence analysis of free and integrated plasmid copies revealed four differences within the terminal inverted repeats (TIRs). These point mutations are not caused by the integration event since they occur subsequent to integration and at various ages. Interestingly, both repeats contain identical sequences indicating that the mechanism involved in the maintenance of perfect TIRs is active on both free and integrated plasmid copies. Finally, in reciprocal crosses between AL2 and the wild-type strain A, some abnormal progeny were obtained. One group of strains did not contain detectable amounts of plasmid pAL2-1, although the mtDNA was clearly of the type found in the long-lived mutant AL2. These strains exhibited a short-lived phenotype. In contrast, one strain was selected that was found to contain wild-type A-specific mitochondrial genomes and traces of pAL2-1. This strain was characterized by an increased life span. Altogether these data suggest that the linear plasmid pAL2-1 is involved in the expression of longevity in mutant AL2.

Mitochondrial DNA alterations and genetic diseases: a review

We review the main features of human mitochondrial function and structure, and in particular mitochondrial transcription, translation, and replication cycles. Furthermore, some pecularities such as mitochondria's high polymorphism, the existence of mitochondrial pseudogenes, and the various considerations to take into account when studying mitochondrial diseases will also be mentioned. Mitochondrial syndromes mostly affecting the nervous system have, during the past few years, been associated with mitochondrial DNA (mt DNA) alterations such as deletions, duplications, mutations and depletions. We suggest a possible classification of mitochondrial diseases according to the kind of mt DNA mutations: structural mitochondrial gene mutation as in LHON (Leber's Hereditary Optic Neuropathy) and NARP (Neurogenic muscle weakness, Ataxia and Retinitis Pigmentosa) as well as some cases of Leigh's syndrome; transfer RNA and ribosomal RNA mitochondrial gene mutation as in MELAS (Mitochondrial Encephalomyopathy, Lactic Acidosis and Strokelike Episodes) or MERRF (Myoclonic Epilepsy with Ragged Red Fibers) or deafness with aminoglycoside; structural with transfer RNA mitochondrial gene mutations as observed in large-scale deletions or duplications in Kearns-Sayre syndrome, Pearson's syndrome, diabetes mellitus with deafness, and CPEO (Chronic Progressive External Ophtalmoplegia). Depletions of the mt DNA may also be classified in this category. Even though mutations are generally maternally inherited, most of the deletions are sporadic. However, multiple deletions or depletions may be transmitted in a mendelan trait which suggests that nuclear gene products play a primary role in these processes. The relationship between a mutation and a particular phenotype is far from being fully understood. Gene dosage and energic threshold, which are tissue-specific, appear to be the best indicators. However, the recessive or dominant behavior of both the wild type or the mutated genome appears to play a significant role, which can be verified with in vitro studies.

The point mutation of mitochondrial DNA characteristic for MERRF disease is found also in healthy people of different ages

The A-to-G transition mutation in the tRNA(Lys) gene of mitochondrial DNA (mtDNA), characteristic for the maternally inherited MERRF syndrome (myoclonic epilepsy with ragged red fibers), has been identified by point mutation-specific polymerase chain reaction in extraocular muscle from 11 of 16 healthy people of different ages. No mutation was found in navel-string samples from 5 newborns, in HeLa cells, and in 2 individuals younger than 20 years. On the other hand, the mutation is present in all 5 tested 74-89-year-old individuals and in 6 of 9 20-70-year-old individuals. The amount of mutated from total mtDNA was estimated by 'mispairing PCR' in extraocular muscle of 2 individuals of 74 and 89 years to 2.0 and 2.4%, respectively. In most tissue samples the MERRF mutation occurs together with the 'common deletion' of mtDNA, which was previously shown to accumulate in healthy individuals with increasing age. It is proposed that during aging, deletions and point mutations of mtDNA accumulate, which could impair mitochondrial energetics.

Different in situ hybridization patterns of mitochondrial DNA in cytochrome c oxidase-deficient extraocular muscle fibres in the elderly

Previous studies have revealed an increase of cytochrome c oxidase-deficient fibres/cells in the skeletal and heart muscle of humans during ageing. The enzyme defect is due to a lack of both mitochondrial and nuclear coded enzyme subunits. In the present investigation in situ hybridization of mitochondrial DNA (mtDNA) has been performed on extraocular muscles of humans over 70 years of age to show whether mutated mtDNA with the so called common deletion of 4,977 basepairs at position 8,482-13,460 of mtDNA accumulates in the cytochrome c oxidase-deficient fibres. The cytochrome c oxidase-deficient fibres revealed different hybridization patterns: a normal hybridization signal with three different mtDNA probes, a reduced or lacking signal with all three probes indicating depletion of mtDNA and a selective hybridization defect with the probe recognizing the "common deletion" region of mtDNA as evidence of mtDNA deletion. The results suggest that during ageing defects of cytochrome c oxidase are associated with different molecular alterations of mtDNA. Deletion and depletion of mtDNA are not the only nor probably the leading mechanisms responsible for the loss of respiratory chain capacity during ageing. The normal hybridization signal in most of the cytochrome c oxidase-deficient fibres and the loss of mitochondrial and nuclear protein subunits indicate the involvement of other, especially nuclear factors.

The role of mitochondrial DNA rearrangements in aging and human diseases

Instabilities and point mutations of the high molecular weight mitochondrial DNA (mtDNA) were shown to be correlated with various degenerative processes in both lower eukaryotes as well as in mammals. In filamentous fungi, circular and linear plasmids were demonstrated to be involved in mtDNA rearrangements and in the genetic control of senescence. In addition, in these eukaryotic microorganisms, which have proved to be ideal model systems in experimental gerontology, a number of nuclear genes were identified controlling the stability of the mitochondrial genome. Although the mitochondrial genome of mammals, including humans, appears to be quite stable in comparison to other species, mtDNA instabilities of the type described in fungi were observed in mitochondria of patients with different mitochondrial degenerative disorders (CPEO, KSS, Pearson syndrome, LHON, MERRF, MELAS). It was later demonstrated that such mtDNA rearrangements appear to accumulate progressively during aging in human subjects. These data suggest that instabilities of the mitochondrial genome may play an important role in the control of life span not only in lower eukaryotes, but also in humans.

In situ hybridization of mitochondrial DNA in the heart of a patient with Kearns-Sayre syndrome and dilatative cardiomyopathy

Previous studies have revealed cytochrome-c-oxidase-deficient cardiomyocytes and the 4,977 base pair deletion ("common deletion") of mitochondrial DNA (position 8,482-13,459) in the heart of a patient with dilatative cardiomyopathy and Kearns-Sayre syndrome. In the present investigation the co-localization of the enzymatic and genomic defects was studied. In situ hybridization of mitochondrial DNA (mtDNA) revealed different hybridization patterns in the cytochrome-c-oxidase-deficient cells: (1) a selective reduction of the hybridization signal with an mtDNA probe recognizing the common deletion, indicating predominance of the deleted over the nondeleted mtDNA molecules in the cytochrome-c-oxidase-deficient cells; (2) a reduced hybridization signal with different mtDNA probes, indicating depletion of mtDNA; and (3) normal hybridization signals with different probes in single cytochrome-c-oxidase-deficient cardiomyocytes. These results indicate that different mechanisms may co-exist in Kearns-Sayre syndrome and may lead to defective respiratory chain function. The question of the pathogenetic interrelationship is discussed.

Tissue segregation of a heteroplasmic mtDNA mutation in MERRF (myoclonic epilepsy with ragged red fibers) encephalomyopathy

The distribution of the causal 8344A-->G mtDNA mutation has been examined in six tissues of a patient with myoclonic epilepsy with ragged red fibers (MERRF), to study the developmental genetics of this type of mitochondrial disorder, and to determine the pathophysiological importance of the mtDNA heteroplasmy generally observed in such patients. Heteroplasmy of the mtDNA was observed in all six tissues (cerebellum, cerebrum, pancreas, liver, muscle, and heart) suggesting that, whereas the mtDNA mutation is relatively new, the mutated population must have existed before the formation of the three primary embryonic layers. The tissue distribution reveals significant variations in the ratio between the mutated and the normal mtDNA species, indicating the randomness of mtDNA segregation during developmental cell division and differentiation events. The result suggests the existence of tissue-specific nuclear factor(s) that determines the expression of the 8344A-->G mutation in various tissues; in MERRF syndrome, expression is mainly in the central nervous system.

Mitochondrial DNA mutation and the ageing process: bioenergy and pharmacological intervention

A comprehensive hypothesis concerning the contribution of mitochondrial DNA (mtDNA) mutations to the human ageing process is reviewed and the implications for cellular bioenergy loss and pharmacological therapy are considered. The central idea is that random mutations in the population of mtDNA molecules of each cell occur throughout life, and that this is a major contributor to the gradual loss of cellular bioenergy capacity within tissues and organs, associated with general senescence and diseases of ageing. An elaboration of four major aspects of the general proposition, together with relevant supporting data, is presented. (1) An extensive array of deletions in mtDNA of many tissues of humans and other mammals has been observed to occur in an age-related manner. (2) The preservation and selection of fully functional mtDNA molecules in the female germ line cells is proposed to occur via a human mtDNA cycle, in which selective amplification of a limited number of mtDNA templates occurs during oocyte development. This proposal explains the endowment of normal neonates with a mtDNA complement minimally contaminated by damaged mtDNA molecules. The phenomena of maternal inheritance and rapid fixation of sequence variants of mtDNA in mammals, as well as selection of cells based on mitochondrial function, are taken into account. (3) Tissue bioenergy mosaics result from accumulated mtDNA damage during ageing, representing different rates of cellular bioenergy loss within individual cells of a tissue. The random segregation of mtDNA during cell division will also further contribute to the tissue energy mosaic. Cells unable to meet their particular bioenergy demand will become non-functional, leading to cell death; the bioenergy threshold is different for the various cell types in the tissues of the body. (4) In order to bioenergetically resuscitate cells and tissues suffering from impaired mitochondrial functions as a result of the ageing process, we propose that redox compounds may be used therapeutically in the pharmacological configurations of a by-pass strategy or as a redox sink therapy. The role of these compounds is to maintain at least part of the mitochondrial respiratory chain function (by-pass) as well as to maintain adequate levels of cellular NAD+ (redox sink) for ATP synthesis, predominantly by the cytosolic glycolytic pathway, with some contribution from mitochondrial oxidative phosphorylation.

A pattern of accumulation of a somatic deletion of mitochondrial DNA in aging human tissues

An assay that selectively amplifies a specific deletion of the mitochondrial genome has been used to study the extent of the deletion's accumulation in a variety of human tissues. The deletion occurs at much higher levels in nervous and muscle tissues than in all other tissues studied. The variation in deletion level between the same tissues in different persons of similar age appears to be less than the variation among tissues within an individual. Tests for artifactual explanations of the level differences were each negative. Three cellular parameters that are correlated with the level of the deletion are identified. The preferential accumulation of deleterious mitochondrial mutations in a restricted subset of aging human tissues may compound deficiencies of function in those tissues that accrue with age.

The genetic legacy of Mother Goose– phylogeographic patterns of lesser snow goose Chen caerulescens caerulescens maternal lineages

By using the polymerase chain reaction to amplify and sequence 178 bp of a rapidly evolving region of the mtDNA genome (segment I of the control region) from 81 individuals, approximately 11% of the variation present in the lesser snow goose Chen caerulescens caerulescens L. mitochondrial genome was surveyed. The 26 types of mtDNA detected formed two distinct mitochondrial clades that differ by an average of 6.7% and are distributed across the species range. Restriction analysis of amplified fragments was then used to assign the mtDNA of an additional 29 individuals to either of these clades. Within one major clade, sequence among mtDNAs was concordant with geographic location. Within the other major clade the degree of sequence divergence among haplotypes was lower and no consistent geographic structuring was evident. The two major clades presumably result from vicariant separation of lesser snow geese during the Pleistocene.

Multiple mitochondrial DNA deletions in an elderly human individual

We have used the polymerase chain reaction (PCR) to study deletions in the mitochondrial DNA (mtDNA) of an elderly human individual. An extended set of PCR primers has been utilised to identify 10 mitochondrial DNA deletions in a 69-year-old female subject with no known mitochondrial disease. The particular deletions visualised as PCR products depended on the primer pairs used, such that the more distantly separated PCR primers enabled visualisation of larger deletions. Some deletions were common to the heart, brain and skeletal muscle, whereas others were apparently specific to individual tissues. DNA sequencing analysis of PCR products showed that short direct repeat sequences (5 to 13 bp) flanked all deletion breakpoints; in most cases one copy of the repeat was deleted. It is proposed that the accumulation of such multiple deletions is a general phenomenon during the ageing process.

Mitochondrial DNA mutations in human diseases: a review

Human mitochondrial diseases have been associated recently with mitochondrial DNA mutations, duplications and deletions which impair the protein synthesis of the mitochondrial subunits of the respiratory chain complexes. A constant feature is the coincident presence of the mutated and wild type genomes which provide heteroplasmy. The clinical expression of these diseases depends on the relative expression of each kind of mitochondrial DNA in the various tissues, which in turn affects the production of ATP in these tissues. Research on nuclear gene products interfering with mtDNA or with its gene products is the next step towards understanding the etiology of these diseases.

Deletions of the mitochondrial genome

Single large deletions of mitochondrial DNA are found in the muscle of about 40% of patients with mitochondrial myopathies, and are detectable in both blood and muscle in Pearson syndrome. In mitochondrial myopathies, there is a close association between the presence of deletions and involvement of extra-ocular muscles, together with other features of the Kearns-Sayre syndrome. Deletions appear to arise as fresh mutations in the vast majority of patients and are often flanked by direct repeats up to 13 nucleotides in length. They should affect translation of all mitochondrially encoded components of the respiratory chain, but there is evidence to suggest that intramitochondrial complementation occurs in some cases.

A lifetime of retinal light exposure does not appear to increase mitochondrial mutations

Recently, there have been a number of reports of an accumulation of mutations in the mitochondrial (mt) genome with age. Such mutations may be due in part to the mt oxidative metabolic pathways which provide most of the cell's energy, but also generate free radicals. In addition, the mt genome in some tissues, such as the retina, may also accumulate mutations from the effects of ultraviolet light. To obtain information concerning the possible accumulation of retinal mt mutations with age, we cloned retinal mt DNA from a 71-year-old person. Thirty-two kilobases of sequence from 83 independently isolated clones representing two regions, a coding and a noncoding region, of the mt genome were obtained. Three polymorphisms between these sequences and the standard 'Anderson sequence' were discovered. Only one heteroplasmic mutation was found. These results confirm the low somatic mutation rate found in prior studies utilizing different types of human tissues. In addition, these results suggest that there is little if any accumulated damage to the mt DNA of the retina during normal aging.

Respiratory chain activity in tissues from patients (MELAS) with a point mutation of the mitochondrial genome [tRNA(Leu(UUR))]

A heteroplasmic point mutation (transition A to G at position 3243 in the mitochondrial tRNA(Leu(UUR)) gene is indicative for myo-encephalopathy with lactic acidosis and stroke-like episodes (MELAS). Decreased respiratory chain complex activities measured in different tissues from four patients with MELAS syndrome do not correlate with the proportion of mutated mitochondrial genome.

Different mechanisms inferred from sequences of human mitochondrial DNA deletions in ocular myopathies

We have sequenced the deletion borders of the muscle mitochondrial DNA from 24 patients with heteroplasmic deletions. The length of these deletions varies from 2.310 bp to 8.476 bp and spans from position 5.786 to 15.925 of the human mitochondrial genome preserving the heavy chain and light chain origins of replication. 12 cases are common deletions identical to the mutation already described by other workers and characterized by 13 bp repeats at the deletion boundaries, one of these repeats being retained during the deletion process. The other cases (10 out of 12) have shown deletions which have not been previously described. All these deletions are located in the H strand DNA region which is potentially single stranded during mitochondrial DNA replication. In two cases, the retained Adenosine from repeat closed to the heavy strand origin of replication would indicate slippage mispairing. Furthermore in one patient two mt DNA molecules have been cloned and their sequences showed the difference of four nucleotides in the breakpoint of the deletion, possibly dued to slippage mispairing. Taken together our results suggest that deletions occur either by slippage mispairing or by internal recombination at the direct repeat level. They also suggest that different mechanisms account for the deletions since similarly located deletions may display different motives at the boundaries including the absence of any direct repeat.

Cytochrome c oxidase deficient fibres in the limb muscle and diaphragm of man without muscular disease: an age-related alteration

Cytochrome c oxidase (complex IV of the respiratory chain) was studied histochemically in human limb muscle (n = 109) and diaphragm (n = 115) obtained at autopsy revealing randomly distributed muscle fibres without enzyme activity. The defects were present both in normal type I and type II fibres and in ragged red like fibres with increased content of mitochondria. In both organs an age associated manifestation of the defect was observed. First defects occurred sporadically in the 3rd and 4th decade, but were present from the 6th to 9th decade in 66-83% of the limb muscles and 75-100% of the diaphragms. Also the number of defects/cm2 (defect density) increased with age from approx. 5, and 7 in limb muscle and diaphragm below the 6th decade to 54 and 60 defects in the 8th-9th decade (P = 0.000). Between both muscles no statistically significant difference in defect density (P greater than 0.15) existed. Irrespective of the defect density the defect typically affected isolated fibres showing normal histochemical reactivity for succinate dehydrogenase (complex II). The results indicate that cytochrome c oxidase deficient muscle fibres in normal skeletal muscle represent an age related phenomenon which probably results from cellular ageing and might be involved in the reduction of muscle mass and strength during senescence.