New insights into the metabolic consequences of large-scale mtDNA deletions: a quantitative analysis of biochemical, morphological, and genetic findings in human skeletal muscle

Nanopore long-read next-generation sequencing for detection of mitochondrial DNA large-scale deletions

Primary mitochondrial diseases are progressive genetic disorders affecting multiple organs and characterized by mitochondrial dysfunction. These disorders can be caused by mutations in nuclear genes coding proteins with mitochondrial localization or by genetic defects in the mitochondrial genome (mtDNA). The latter include point pathogenic variants and large-scale deletions/rearrangements. MtDNA molecules with the wild type or a variant sequence can exist together in a single cell, a condition known as mtDNA heteroplasmy. MtDNA single point mutations are typically detected by means of Next-Generation Sequencing (NGS) based on short reads which, however, are limited for the identification of structural mtDNA alterations. Recently, new NGS technologies based on long reads have been released, allowing to obtain sequences of several kilobases in length; this approach is suitable for detection of structural alterations affecting the mitochondrial genome. In the present work we illustrate the optimization of two sequencing protocols based on long-read Oxford Nanopore Technology to detect mtDNA structural alterations. This approach presents strong advantages in the analysis of mtDNA compared to both short-read NGS and traditional techniques, potentially becoming the method of choice for genetic studies on mtDNA.

Pathological Targets for Treating Temporal Lobe Epilepsy: Discoveries From Microscale to Macroscale

Temporal lobe epilepsy (TLE) is one of the most common and severe types of epilepsy, characterized by intractable, recurrent, and pharmacoresistant seizures. Histopathology of TLE is mostly investigated through observing hippocampal sclerosis (HS) in adults, which provides a robust means to analyze the related histopathological lesions. However, most pathological processes underlying the formation of these lesions remain elusive, as they are difficult to detect and observe. In recent years, significant efforts have been put in elucidating the pathophysiological pathways contributing to TLE epileptogenesis. In this review, we aimed to address the new and unrecognized neuropathological discoveries within the last 5 years, focusing on gene expression (miRNA and DNA methylation), neuronal peptides (neuropeptide Y), cellular metabolism (mitochondria and ion transport), cellular structure (microtubule and extracellular matrix), and tissue-level abnormalities (enlarged amygdala). Herein, we describe a range of biochemical mechanisms and their implication for epileptogenesis. Furthermore, we discuss their potential role as a target for TLE prevention and treatment. This review article summarizes the latest neuropathological discoveries at the molecular, cellular, and tissue levels involving both animal and patient studies, aiming to explore epileptogenesis and highlight new potential targets in the diagnosis and treatment of TLE.

Pathological mechanisms underlying single large-scale mitochondrial DNA deletions

OBJECTIVE

Single, large-scale deletions in mitochondrial DNA (mtDNA) are a common cause of mitochondrial disease. This study aimed to investigate the relationship between the genetic defect and molecular phenotype to improve understanding of pathogenic mechanisms associated with single, large-scale mtDNA deletions in skeletal muscle.

METHODS

We investigated 23 muscle biopsies taken from adult patients (6 males/17 females with a mean age of 43 years) with characterized single, large-scale mtDNA deletions. Mitochondrial respiratory chain deficiency in skeletal muscle biopsies was quantified by immunoreactivity levels for complex I and complex IV proteins. Single muscle fibers with varying degrees of deficiency were selected from 6 patient biopsies for determination of mtDNA deletion level and copy number by quantitative polymerase chain reaction.

RESULTS

We have defined 3 "classes" of single, large-scale deletion with distinct patterns of mitochondrial deficiency, determined by the size and location of the deletion. Single fiber analyses showed that fibers with greater respiratory chain deficiency harbored higher levels of mtDNA deletion with an increase in total mtDNA copy number. For the first time, we have demonstrated that threshold levels for complex I and complex IV deficiency differ based on deletion class.

INTERPRETATION

Combining genetic and immunofluorescent assays, we conclude that thresholds for complex I and complex IV deficiency are modulated by the deletion of complex-specific protein-encoding genes. Furthermore, removal of mt-tRNA genes impacts specific complexes only at high deletion levels, when complex-specific protein-encoding genes remain. These novel findings provide valuable insight into the pathogenic mechanisms associated with these mutations. Ann Neurol 2018;83:115-130.

NEW OBSERVATIONS REGARDING THE RETINOPATHY OF GENETICALLY CONFIRMED KEARNS-SAYRE SYNDROME

PURPOSE

To report novel retinal findings in Kearns-Sayre syndrome and correlate degree of retinopathy with other clinical findings.

METHODS

Observational case series of patients from Saudi Arabia with retinal and neuroophthalmologic examinations, medical chart review, and mitochondrial genetic evaluation.

RESULTS

The three unrelated patients had progressive external ophthalmoplegia and pigmentary retinopathy bilaterally. Muscle biopsy in two of the cases revealed mitochondrial myopathy. All three had abnormal findings on neuroimaging and modestly reduced visual acuity in both eyes with a variable pigmentary retinopathy. One of the patients had bilateral subretinal fibrosis with a full-thickness macular hole in the right eye. All three patients had single, large-scale mitochondrial DNA (mtDNA) deletions (5.0-7.6 kb in size) with blood mtDNA heteroplasmy levels ranging from below 20% to 57%. Severity of pigmentary retinopathy did not correlate with severity of progressive external ophthalmoplegia, but did correspond grossly with electroretinographic abnormalities, just as the degree of ocular motility restriction and ptosis in each patient correlated with the size of their extraocular muscles on neuroimaging. In addition, the size of the single, large-scale mtDNA deletion and level of mtDNA heteroplasmy corresponded with degree of ocular motility restriction but not with severity of retinopathy.

CONCLUSION

Subretinal fibrosis and macular hole are novel retinal observations which expand clinical profile in Kearns-Sayre syndrome. Genetic testing for mtDNA deletions and heteroplasmy in blood, muscle biopsy, careful ocular and retinal examination including electroretinography, and imaging are indispensable tests for this condition.

Mitochondrial DNA Deletions With Low-Level Heteroplasmy in Adult-Onset Myopathy

We report the cases of 2 patients who presented to our Myositis Center with myalgias and elevated creatine kinase levels. Muscle biopsy showed pathological features consistent with mitochondrial myopathy. In both cases, a single large deletion in mitochondrial DNA at low-level heteroplasmy was identified by next-generation sequencing in muscle tissue. In 1 case, the deletion was identified in muscle tissue but not blood. In both cases, the deletion was only identified on next-generation sequencing of muscle mitochondrial DNA and missed on array comparative genome hybridization testing. These cases demonstrate that next-generation sequencing of mitochondrial DNA in muscle tissue is the most sensitive method of molecular diagnosis for mitochondrial myopathy due to mitochondrial DNA deletions.

Mitochondrial haplogroups and expression studies of commonly used human cell lines

We developed a multiplex fragment length analysis (MFLA) for clearly assigning mitochondrial haplogroups mostly endemic in Europe for future cardiac diagnostics. As a technical proof, 23 commonly used human cell lines were haplotyped as reference standards. The functional analysis on mtDNA copies per cell revealed no correlation to haplogroups but a relatively high rate of mitochondria per cell and at the same time a very low expression of all mitochondrial and some nuclear encoded mitochondrial related genes. Established MFLA is an easy to handle method for analysing European mitochondrial haplogroups to perform epidemic studies and elucidate correlations to distinct diseases.

Mitochondrial DNA mutations: an overview of clinical and molecular aspects

Mutations that arise in mitochondrial DNA (mtDNA) may be sporadic, maternally inherited, or Mendelian in character and include mtDNA rearrangements such as deletions, inversions or duplications, point mutations, or copy number depletion. Primary mtDNA mutations occur sporadically or exhibit maternal inheritance and arise due in large part to the high mutation rate of mtDNA. mtDNA mutations may also occur because of defects in the biogenesis or maintenance of mtDNA, reflecting the contribution of nuclear-encoded genes to these processes, and in this case exhibit Mendelian inheritance. Whether maternally inherited, sporadic, or Mendelian, mtDNA mutations can exhibit a complex and broad spectrum of disease manifestations due to the central role mitochondria play in a variety of cellular functions. In addition, because there exist hundreds to thousands of copies of mtDNA in each cell, the proportion of mutant mtDNA molecules can have a profound effect on the cellular and clinical phenotype. This chapter reviews the classification of mtDNA mutations and the clinical features that determine the diagnosis of a primary mtDNA disorder.

Sequence homology at the breakpoint and clinical phenotype of mitochondrial DNA deletion syndromes

Mitochondrial DNA (mtDNA) deletions are a common cause of mitochondrial disorders. Large mtDNA deletions can lead to a broad spectrum of clinical features with different age of onset, ranging from mild mitochondrial myopathies (MM), progressive external ophthalmoplegia (PEO), and Kearns-Sayre syndrome (KSS), to severe Pearson syndrome. The aim of this study is to investigate the molecular signatures surrounding the deletion breakpoints and their association with the clinical phenotype and age at onset. MtDNA deletions in 67 patients were characterized using array comparative genomic hybridization (aCGH) followed by PCR-sequencing of the deletion junctions. Sequence homology including both perfect and imperfect short repeats flanking the deletion regions were analyzed and correlated with clinical features and patients' age group. In all age groups, there was a significant increase in sequence homology flanking the deletion compared to mtDNA background. The youngest patient group (<6 years old) showed a diffused pattern of deletion distribution in size and locations, with a significantly lower sequence homology flanking the deletion, and the highest percentage of deletion mutant heteroplasmy. The older age groups showed rather discrete pattern of deletions with 44% of all patients over 6 years old carrying the most common 5 kb mtDNA deletion, which was found mostly in muscle specimens (22/41). Only 15% (3/20) of the young patients (<6 years old) carry the 5 kb common deletion, which is usually present in blood rather than muscle. This group of patients predominantly (16 out of 17) exhibit multisystem disorder and/or Pearson syndrome, while older patients had predominantly neuromuscular manifestations including KSS, PEO, and MM. In conclusion, sequence homology at the deletion flanking regions is a consistent feature of mtDNA deletions. Decreased levels of sequence homology and increased levels of deletion mutant heteroplasmy appear to correlate with earlier onset and more severe disease with multisystem involvement.

Limited diagnostic value of enzyme analysis in patients with mitochondrial tRNA mutations

We evaluated the diagnostic value of respiratory chain (RC) enzyme analysis of muscle in adult patients with mitochondrial myopathy (MM). RC enzyme activity was measured in muscle biopsies from 39 patients who carry either the 3243A>G mutation, other tRNA point mutations, or single, large-scale deletions of mtDNA. Findings were compared with those obtained from asymptomatic relatives with the 3243A>G mutation, myotonic dystrophy patients, and healthy subjects. Plasma lactate concentration, maximal oxygen uptake, and ragged-red fibers/cytochrome c-negative fibers in muscle were also determined. Only 10% of patients with the 3243A>G point mutation had decreased enzyme activity of one or more RC complexes, whereas this was the case for 83% of patients with other point mutations and 62% of patients with deletions. Abnormal muscle histochemistry was found in 65%, 100%, and 85% of patients, respectively, in these three groups. The results indicate that RC enzyme analysis in muscle is not a sensitive test for MM in adults. In these patients, abnormal muscle histochemistry appears to be a better predictor ofMM.

Mitochondrial involvement in temporal lobe epilepsy

Mitochondrial dysfunction has been identified as a potential cause of epileptic seizures and therapy-resistant forms of severe epilepsy. Thus, a broad variety of mutation in mitochondrial DNA or nuclear genes leading to the impairment of mitochondrial respiratory chain or of mitochondrial ATP synthesis has been associated with epileptic phenotypes. Additionally, with a variety of different methods impaired mitochondrial function has been reported for the seizure focus of patients with temporal lobe epilepsy and Ammon's horn sclerosis and of animal models of temporal lobe epilepsy. Since mitochondrial oxidative phosphorylation provides the major source of ATP in neurons and mitochondria participate in cellular Ca(2+) homeostasis, their dysfunction strongly affects neuronal excitability and synaptic transmission, which is proposed to be highly relevant for seizure generation. Additionally, mitochondrial dysfunction is known to trigger neuronal cell death, which is a prominent feature of therapy-resistant temporal lobe epilepsy. Therefore, mitochondria have to be considered as promising targets for neuroprotective strategies in epilepsy.

Frequency of dystrophic muscle abnormalities in chronic progressive external ophthalmoplegia: analysis of 86 patients

BACKGROUND

There are few reports describing the coexistence of dystrophic features with those typical of mitochondrial myopathies in muscle biopsy. A recent study suggested that dystrophic features are frequent in patients with chronic progressive external ophthalmoplegia (CPEO) with a high mutation load, but the actual frequency of these abnormalities in CPEO remains undetermined.

OBJECTIVE

To review the occurrence of dystrophic abnormalities in a large series of patients with CPEO to assess the frequency of such abnormalities and to verify whether they are correlated with specific mitochondrial DNA (mtDNA) mutations.

METHODS

Retrospective survey of case series (86 patients with CPEO).

RESULTS

Only three cases with dystrophic abnormalities were found: two with a large scale mtDNA deletion and one with the A3251G mutation. All three patients showed predominantly proximal muscular weakness resembling limb girdle muscular dystrophy.

CONCLUSIONS

Dystrophic abnormalities are rare in CPEO and are not correlated with a specific molecular defect.

Visualizing common deletion of mitochondrial DNA-augmented mitochondrial reactive oxygen species generation and apoptosis upon oxidative stress

Common deletion (CD) 4977 bp of mitochondrial DNA (mtDNA) disrupt specifically mitochondrial complex I, IV and V on the electron transport chain (ETC) and is closely associated with wide spectrums of clinical manifestations. To quantitatively investigate how CD-induced ETC defect alters mitochondrial reactive oxygen species (mROS) generation as well as down stream apoptotic signaling, we employed an established array of human CD cytoplasmic hybrids (cybrids) harboring 0%-80% of CD. Pathological effects of CD on the mitochondria were visualized at single cell level by the application of fluorescent probes coupled with conventional and multiphoton imaging microscopy. Intriguingly, we observed CD-augmented mROS generation omitted "threshold effect". CD-augmented mROS generation was associated with depolarized mitochondrial membrane potential (DeltaPsi(m)). Upon oxidative stress, the amount of CD-augmented mROS generation was greatly enhanced to cause pathological apoptotic deterioration including opening of the mitochondrial permeability transition, cytochrome c release, phosphatidylserine externalization and DNA fragmentation. In addition, heterogeneous mitochondrial dysfunctions were found in cybrids containing 80% of CD (D cybrids), i.e., low sensitive-D (LS-D, roughly 80%) and a super sensitive-D (SS-D, 20%). As compared to LS-D, SS-D had higher resting mROS level but slightly hyperpolarized DeltaPsi(m). Upon H2O2 treatment, much faster generation of mROS was observed which induced a faster depolarization of DeltaPsi(m) and later apoptotic deterioration in SS-D. We proposed a dose-dependent, feed-forward and self-accelerating vicious cycle of mROS production might be initiated in CD-induced ETC defect without threshold effect. As CD-augmented mROS generation is obligated to cause an enhanced pathological apoptosis, precise detection of CD-augmented mROS generation and their degree of heterogeneity in single cells may serve as sensitive pathological indexes for early diagnosis, prognosis and treatment of CD-associated diseases.

Enhanced Generation of Mitochondrial Reactive Oxygen Species in Cybrids Containing 4977-bp Mitochondrial DNA Deletion

The poor bioenergetic state in mitochondria containing mtDNA with the 4977-bp deletion has been well documented. However, information on mitochondrial reactive oxygen species (ROS) generation at rest or under intense oxidative stress in mitochondria lacking the 4977-bp mtDNA fragment inside intact living cells was insufficient. We used cybrids containing truncated mtDNA lacking the 4977-bp fragment and measured ROS levels inside cybrids by fluorescence probe, 2',7'-dichlorodihydrofluorescein (DCF), and confocal microscopy. Mitochondrial ROS at resting state was slightly higher in cybrids containing 4977-bp deletion mtDNA as compared to cybrids without mtDNA defects. For intense oxidative stress treatment, cybrids were treated with 5 mM H2O2 for 10 min. Consecutive DCF images were acquired after H2O2 had been washed away. Progressive increase of DCF signals, especially in the mitochondrial area, was observed in cybrids containing 4977-bp deletion mtDNA, even long after the brief, intense H2O2 treatment. This result suggests that a feed-forward, self-accelerating vicious cycle of mitochondrial ROS production could be initiated in cybrids containing 4977-bp deletion fragment mitochondria after brief, intense H2O2 treatment. This mechanism may play an important role in the pathophysiology of the disease process caused by mitochondria containing mtDNA with the 4977-bp deletion.

Pathology of mitochondrial encephalomyopathies

Muscle biopsy provides the best tissue to confirm a mitochondrial cytopathy. Histochemical features often correlate with specific syndromes and facilitate the selection of biochemical and genetic studies. Ragged-red fibres nearly always indicate a combination defect of respiratory complexes I and IV. Increased punctate lipid within myofibers is a regular feature of Kearns-Sayre and PEO, but not of MELAS and MERRF. Total deficiency of succinate dehydrogenase indicates a severe defect in Complex II; total absence of cytochrome-c-oxidase activity in all myofibres correlates with a severe deficiency of Complex IV or of coenzyme-Q10. The selective loss of cytochrome-c-oxidase activity in scattered myofibers, particularly if accompanied by strong succinate dehydrogenase staining in these same fibres, is good evidence of mitochondrial cytopathy and often of a significant mtDNA mutation, though not specific for Complex IV disorders. Glycogen may be excessive in ragged-red zones. Ultrastructure provides morphological evidence of mitochondrial cytopathy, in axons and endothelial cells as well as myocytes. Abnormal axonal mitochondria may contribute to neurogenic atrophy of muscle, a secondary chronic feature. Quantitative determinations of respiratory chain enzyme complexes, with citrate synthase as an internal control, confirm the histochemical impressions or may be the only evidence of mitochondrial disease. Biological and technical artifacts may yield falsely low enzymatic activities. Genetic studies screen common point mutations in mtDNA. The brain exhibits characteristic histopathological alterations in mitochondrial diseases. Skin biopsy is useful for mitochondrial ultrastructure in smooth erector pili muscles and axons; skin fibroblasts may be grown in culture. Mitochondrial alterations occur in many nonmitochondrial diseases and also may be induced by drugs and toxins.

Creatine has no beneficial effect on skeletal muscle energy metabolism in patients with single mitochondrial DNA deletions: a placebo-controlled, double-blind 31P-MRS crossover study

The purpose of our randomized, double-blind, placebo-controlled crossover study in 15 patients with chronic progressive external ophthalmoplegia (CPEO) or Kearns-Sayre syndrome (KSS) because of single large-scale mitochondrial (mt) DNA deletions was to determine whether oral creatine (Cr) monohydrate can improve skeletal muscle energy metabolism in vivo. Each treatment phase with Cr in a dosage of 150 mg/kg body weight/day or placebo lasted 6 weeks. The effect of Cr was estimated by phosphorus-31 magnetic resonance spectroscopy ((31)P-MRS), clinical and laboratory tests. (31)P-MRS analysis prior to treatment showed clear evidence of severe mitochondrial dysfunction. However, there were no relevant changes in (31)P-MRS parameters under Cr. In particular, phosphocreatine (PCr)/ATP at rest did not increase, and there was no facilitation of post-exercise PCr recovery. Clinical scores and laboratory tests did not alter significantly under Cr, which was tolerated without major side-effects in all patients. Cr supplementation did not improve skeletal muscle oxidative phosphorylation in our series of patients. However, one explanation for our negative findings may be the short study duration or the limited number of patients included.

Sensorineural hearing loss in patients with chronic progressive external ophthalmoplegia or Kearns–Sayre syndrome

In the present study we assessed the prevalence and nature of hearing loss in patients with chronic progressive external ophthalmoplegia (CPEO) or Kearns-Sayre syndrome (KSS) due to single large-scale mitochondrial DNA (mtDNA) deletion or mtDNA tRNA (Leu (UUR)) A3243G point mutation (A3243G PM). 14 patients with mtDNA deletion and three patients with A3243G PM underwent audiological evaluation comprising pure-tone and speech audiometry as well as transient evoked otoacoustic emissions (OAE). Audiological evaluation revealed hearing impairment in 10/17 patients. Hearing loss was mild to moderate predominantly affecting high frequencies in five patients with subjective hearing problems (three patients with mtDNA deletions, two patients with A3243G PM). Subclinical hearing deficits restricted to high frequencies were seen in further five asymptomatic patients (four patients with mtDNA deletions, one patients with A3243G PM). Audiological findings suggested a cochlear origin of hearing loss in all subjects. Our results demonstrate that CPEO or KSS patients due to mtDNA deletion or A3243G PM are at high risk of developing sensorineural hearing deficits.

Energetic depression caused by mitochondrial dysfunction

Mitochondria, providing most of ATP needed for cell work, realizing numerous specific functions as biosyntheses or degradations, contributing to Ca2+ signalling also play a key role in the pathways to cell death. Impairment of mitochondrial functions caused by mutations of mt-genome and by acute processes are responsible for numerous diseases. The relations between changes on the level of molecules and the clinical state are rather complex, and the prediction of thresholds is difficult. Therefore investigations on different levels of an organismus (genome, metabolites, enzymes, mitochondrial function in vivo and in vitro) are necessary (multi level approach). Metabolic control theory is a valuable tool for understanding the different effects of mutations on the level of enzyme activities and mitochondrial function. Decreased concentrations of adenine nucleotides, leaky outer and inner mitochondrial membranes, decreased rates of mitochondrial linked pathways and decreased activities of respiratory chain enzymes contribute to depression of cellular energy metabolism characterized by decreased cytosolic phosphorylation potentials as one of the most important consequences of mitochondrial impairments. This review regards classical bioenergetic mechanisms of mitochondrial impairment which contribute to energetic depression.

Diagnostic value of mitochondrial DNA mutation analysis in juvenile unilateral ptosis

PURPOSE

To highlight the diagnostic relevance of mitochondrial DNA (mtDNA) mutation analysis in acquired juvenile unilateral upper eyelid ptosis.

METHODS

A 13-year-old boy presented with acquired, slowly progressive unilateral ptosis. We performed ophthalmological and neurological examinations, laboratory testing, skeletal muscle biopsy including histological and histochemical investigations, biochemical analysis of respiratory chain enzymes in skeletal muscle homogenate and molecular genetic testing of skeletal muscle DNA.

RESULTS

Though clinical, laboratory, histological and biochemical analyses did not reveal any hints suggesting a mitochondrial cytopathy, molecular genetic testing by Southern blot analysis of total DNA from skeletal muscle tissue showed a 5.8 kb mtDNA deletion thus proving the diagnosis of mitochondrial chronic progressive external ophthalmoplegia (CPEO).

CONCLUSIONS

In patients with unexplained acquired juvenile unilateral ptosis, an underlying mitochondrial cytopathy should be considered even in cases of inconspicuous ancillary examinations comprising skeletal muscle histology and biochemistry. To establish the diagnosis, molecular genetic testing of DNA derived from skeletal muscle tissue is essential in those patients.

Probing striated muscle mitochondrial phenotype in neuromuscular disorders

Multisystemic disorders with predominantly neurologic manifestations often present with mitochondrial abnormalities in striated muscle biopsies. Decreased respiratory complex activities and abnormalities in mitochondrial structure and DNA constitute the spectrum of mitochondrial changes used as diagnostic and prognostic indicators in patients with neuromuscular disorders. This study assessed mitochondrial defects present in a cohort of 154 young patients to determine diagnostic efficiency and probe the relationship of mitochondrial to clinical phenotype. Striated muscle biopsies were analyzed for mitochondrial structure and number, levels of enzyme activities of complex I-V and citrate synthase, mitochondrial DNA and specific mitochondrial DNA deletions, and presence of 15 pathogenic mitochondrial DNA point mutations. Reduced complex I, III, IV, and V activities were the most ubiquitous finding, with complex III most commonly affected. Mitochondrial structural defects (39%) included changes in mitochondria sizes/shapes and number and aberrant cristae formation. Mitochondrial DNA deletions were evident in 15 patients, three displayed mitochondrial DNA depletion, and only two harbored pathogenic point mutations. Reductions in specific enzyme activities may be the most sensitive diagnostic indicator, whereas defects in ultrastructure and mitochondrial DNA integrity were frequently accompanied by the full spectrum of mitochondrial abnormalities. Some phenotypes displayed specific mitochondrial abnormalities; however, most clinical phenotypes displayed little specificity with regard to mitochondrial phenotype.

Oxidative capacity correlates with muscle mutation load in mitochondrial myopathy

The purpose of this study was to investigate the correlation between the level of mutated mitochondrial DNA in muscle and oxidative capacity in 24 patients with mitochondrial myopathy (MM). Maximal oxygen uptake (VO(2max)), workload (W(max)), and venous plasma lactate levels were measured during an incremental cycle test to exhaustion in 17 patients with point mutations of mtDNA and in seven with single, large-scale deletions of mtDNA (chronic progressive external ophthalmoplegia [CPEO]). Results were compared with those in 25 healthy matched subjects. The mutation load in MM patients was 67 +/- 5% (range, 29 - 99%). VO(2max) and W(max) correlated with percentage of heteroplasmy (r > 0.82; p < 0.005) and were lower in patients versus healthy subjects (p < 0.000005). Exercise-induced peak increases in heart rate, ventilation, and resting plasma lactate levels correlated with muscle mutation load (r > 0.71; p < 0.005). Exercise-induced increases in plasma lactate correlated with muscle mutation load in CPEO patients (r = 0.95; p < 0.005). Impaired oxidative capacity and ragged red muscle fibers were found in CPEO and 3243A-->G patients with mutation loads as low as 45 and 57%, respectively. The study indicates that oxidative capacity correlates directly with skeletal muscle mutation load in MM patients, and that the mutation threshold level for impaired oxidative metabolism in MM patients is lower than found in in vitro studies.

Congenital cataract as the first symptom of a neuromuscular disease caused by a novel single large-scale mitochondrial DNA deletion

The male proband reported here was born with appropriate anthropometric parameters at term as the second child of healthy nonconsanguineous parents. His only clinical symptom was bilateral congenital cataracts with strabismus at birth, and both lenses were removed surgically at the age of 8 months. The perinatal and infantile period thereafter was clinically uneventful and his psychomotor development appeared almost normal. At the age of 6 years he was hospitalized for slight muscle weakness, minor ptosis, nystagmus and decreased physical activity. Soon after, his general condition worsened, gait ataxia presented, dysphagia and difficulty of speech followed by rapidly progressive generalized ataxia, and myopathy developed. Typical progressive gray matter degeneration with focal necrosis in the basal ganglia characteristic of the Leigh type of neuropathology could be detected by cranial MRI, the muscle histology showed ragged-red fibers. At the age of 7.5 years, unexpected left side hemiparesis with speech disability resembling that seen in MELAS syndrome developed, from which he recovered within 1.5 days. The mtDNA of the patient showed single 6.7 kb large-scale deletion harboring between 7817 and 14 536 bp. This case represents the first report of a verified mtDNA mutation associated with congenital cataracts as the first clinical sign of a later developing progressive neuromuscular disease presented with a combination of Leigh neuropathology, ragged-red fiber histopathology and stroke-like attack.

Executive and visuospatial deficits in patients with chronic progressive external ophthalmoplegia and Kearns-Sayre syndrome

Although neuropsychological deficits have been reported in mitochondrial cytopathies, patients with chronic progressive external ophthalmoplegia (CPEO) or Kearns-Sayre syndrome (KSS) have not been studied systematically using a comprehensive test battery. The aim of our study was to assess the range and extent of putative cognitive dysfunction in 22 patients with CPEO or KSS, and to compare cognitive performance of patients with healthy control subjects matched for age, sex and years of education. Genetic analysis of skeletal muscle tissue from 22 patients with CPEO or KSS included screening for mitochondrial DNA (mtDNA) point mutations (3243/8344) and mtDNA deletions. All patients were examined by a neuropsychological test battery covering verbal skills, verbal and visual memory, visuo-spatial perception, visual construction, attention, abstraction and flexibility, and Quality of Life. Molecular genetic analysis of mtDNA revealed single large-scale deletions in 15 out of 22 patients and the tRNA (Leu) A3243G point mutation in two out of 22 patients. In five out of 22 patients none of the frequently encountered mtDNA mutations could be detected. Neuropsychological testing did not reveal general intellectual deterioration, but specific cognitive deficits, particularly in visual construction, attention and abstraction/flexibility. Subgroup analysis of 15 patients with mtDNA deletions showed similar results when compared with the full group. In our series of patients with CPEO or KSS neuropsychological testing did not reveal signs that would suggest general intellectual decline or dementia, but provided evidence of specific focal neuropsychological deficits, suggesting particular impairment of visuospatial perception associated to parieto-occipital lobes and executive deficits associated to the prefrontal cortex.

Mitochondrial threshold effects

The study of mitochondrial diseases has revealed dramatic variability in the phenotypic presentation of mitochondrial genetic defects. To attempt to understand this variability, different authors have studied energy metabolism in transmitochondrial cell lines carrying different proportions of various pathogenic mutations in their mitochondrial DNA. The same kinds of experiments have been performed on isolated mitochondria and on tissue biopsies taken from patients with mitochondrial diseases. The results have shown that, in most cases, phenotypic manifestation of the genetic defect occurs only when a threshold level is exceeded, and this phenomenon has been named the 'phenotypic threshold effect'. Subsequently, several authors showed that it was possible to inhibit considerably the activity of a respiratory chain complex, up to a critical value, without affecting the rate of mitochondrial respiration or ATP synthesis. This phenomenon was called the 'biochemical threshold effect'. More recently, quantitative analysis of the effects of various mutations in mitochondrial DNA on the rate of mitochondrial protein synthesis has revealed the existence of a 'translational threshold effect'. In this review these different mitochondrial threshold effects are discussed, along with their molecular bases and the roles that they play in the presentation of mitochondrial diseases.

Mitochondrial dysfunction in myofibrillar myopathy

'Myofibrillar myopathy' defines a myopathic condition with focal myofibrillar destruction and accumulation of degraded myofibrillar elements. Despite the fact that a number of mutations in different genes as well as cytotoxic agents lead to the disease, abnormal accumulation of desmin is a typical, common feature. Pathological changes of mitochondrial morphology and function have been observed in animal models with intermediate filament pathology. Therefore, in the present study we tested for mitochondrial pathology in skeletal muscle of five patients with the pathohistological diagnosis of myofibrillar myopathy. Screening for large-scale mtDNA deletions and the frequent MERRF (myoclonic epilepsy; ragged red fibres) and MELAS (mitochondrial encephalomyopathy; lactic acidosis; stroke) point mutations was negative in all patients. Histologically, all muscle biopsies showed nonspecific abnormalities of the oxidative/mitochondrial enzyme stainings (histochemistry for reduced nicotinamide adenine dinucleotide, succinic dehydrogenase, cytochrome c oxidase), only one of them had ragged red fibres and a significant number of cytochrome c oxidase-negative fibres. Upon biochemical investigation, four of our patients showed pathologically low respiratory chain complex I activities. Only one of our patients had a pathologically low complex IV activity, while the measurements of the others were within low normal range. The single patient with pathological values for both complex I and IV was the one with the clear histological hallmarks (ragged red and cytochrome c oxidase-negative fibres) of mitochondrial pathology. She also was the only patient with clinical signs hinting at a mitochondrial disorder. Together with data from observations in desmin- and plectin-deficient mice, our results support the view that desmin intermediate filament pathology in these cases is closely linked to mitochondrial dysfunction in skeletal muscle.

Mitochondrial respiratory rates and activities of respiratory chain complexes correlate linearly with heteroplasmy of deleted mtDNA without threshold and independently of deletion size

To clarify the importance of deleted protein and tRNA genes on the impairment of mitochondrial function, we performed a quantitative analysis of biochemical, genetic and morphological findings in skeletal muscles of 16 patients with single deletions and 5 patients with multiple deletions of mtDNA. Clinically, all patients showed chronic progressive external ophthalmoplegia (CPEO). The size of deletions varied between 2.5 and 9 kb, and heteroplasmy between 31% and 94%. In patients with single deletions, the citrate synthase (CS) activity was nearly doubled. Decreased ratios of pyruvate- and succinate-dependent respiration were detected in fibers of all patients in comparison to controls. Inverse and linear correlations without thresholds were established between heteroplasmy and (i) CS referenced activities of the complexes of respiratory chain, (ii) CS referenced maximal respiratory rates, (iii) and cytochrome-c-oxidase (COX) negative fibers. In patients with single and multiple deletions, all respiratory chain complexes as well as the respiratory rates were decreased to a similar extent. All changes detected in patients with single deletions were independent of deletion size. In one patient, only genes of ND5, ND4L as well as tRNA(Leu(CUN)), tRNA(Ser(AGY)), and tRNA(His) were deleted. The pronounced decrease in COX activity in this patient points to the high pathological impact of these missing tRNA genes. The activity of nuclear encoded SDH was also significantly decreased in patients, but to a lesser extent. This is an indication of secondary disturbances of mitochondria at CPEO. In conclusion, we have shown that different deletions cause mitochondrial impairments of the same phenotype correlating with heteroplasmy. The missing threshold at the level of mitochondrial function seems to be characteristic for large-scale deletions were tRNA and protein genes are deleted.

Cardiomyopathy associated with neurologic disorders and mitochondrial phenotype

Cardiomyopathy and neuromuscular abnormalities may simultaneously coexist and present with defects in mitochondrial DNA and bioenergetic function. We sought to evaluate the relationship between clinical and mitochondrial phenotypes in 28 young patients with both cardiomyopathy and neurologic disorders including seizures, dystonia, ophthalmoplegia, Kearns-Sayre syndrome, Leigh disease, and Friedreich's ataxia. All tissues examined displayed marked defects in respiratory complex activities. Five patients had abundant large-scale mitochondrial DNA deletions and one patient displayed a pathogenic point mutation previously reported with mitochondrial cytopathy. In this cohort, patients with hypertrophic cardiomyopathy displayed a higher incidence of complex I defects, fewer DNA deletions and mitochondrial structural abnormalities and were less often associated with developmental delay phenotype compared with patients with dilated cardiomyopathy. Although structural abnormalities are present in a subset of patients, evaluation of respiratory enzyme activity appears to be most informative whether tissues examined were derived from heart or skeletal muscle. Defects in mitochondrial DNA and bioenergetics are frequently present in children with cardiomyopathy presenting with a variety of neurologic abnormalities and are amenable to biochemical and molecular analysis.

Expression pattern of mitochondrial respiratory chain enzymes in skeletal muscle of patients harboring the A3243G point mutation or large-scale deletions of mitochondrial DNA

To assess the detailed expression pattern of mitochondrial-encoded proteins in skeletal muscle of patients with mitochondrial diseases we performed determinations of cytochrome content and enzyme activities of respiratory chain complexes of 12 patients harboring large-scale deletions and of 10 patients harboring the A3243G mutation. For large-scale deletions we observed a mutation gene dose-dependent linear decline of cytochrome aa3 content, cytochrome c oxidase (COX) activity, and complex I activity. The content of cytochromes b and the complex III activity was either not affected or only weakly affected by the deletion mutation and did not correlate to the degree of heteroplasmy. In contrast, in skeletal muscle harboring the A3243G mutation all investigated enzymes containing mitochondrial-encoded subunits were equally affected by the mutation, but we observed milder enzyme deficiencies at a comparable mutation gene dose. The results of single fiber analysis of selected biopsies supported these findings but revealed differences in the distribution of COX deficiency. Whereas predominantly type I fibers were affected in A3243G and deletion CPEO biopsies, we observed in MELAS and KSS biopsies higher quantities of COX-deficient type 2 fibers. Our findings indicate different pathomechanisms of deletion and A3243G mutations.

Disorganization of the desmin cytoskeleton and mitochondrial dysfunction in plectin-related epidermolysis bullosa simplex with muscular dystrophy

Mutations of the human plectin gene (Plec1) cause autosomal recessive epidermolysis bullosa simplex with muscular dystrophy (EBS-MD). Here, we report on molecular mechanisms leading to severe dystrophic muscle alterations in EBS-MD. Analysis of a 25-yr-old EBS-MD patient carrying a novel homozygous 16-bp insertion mutation (13803ins16/13803ins16) close to the intermediate filament (IF) binding site of plectin showed severe disorganization of the myogenic IF cytoskeleton. Intermyofibrillar and subsarcolemmal accumulations of assembled but highly unordered desmin filaments may be attributed to impaired desmin binding capability of the mutant plectin. This IF pathology was also associated with severe mitochondrial dysfunction, suggesting that the muscle pathology of EBS-MD caused by IF disorganization leads not only to defects in mechanical force transduction but also to metabolic dysfunction. Beyond EBS-MD, our data may contribute to the understanding of other myopathies characterized by sarcoplasmic IF accumulations such as desminopathies or alpha-B-crystallinopathies.

The role of mitochondria in epileptogenesis

Mitochondrial dysfunction has gained considerable interest as a potential cause of epileptic seizures and therapy-resistant forms of severe epilepsy. Impairment of mitochondrial function has recently been observed in the seizure focus of human and experimental epilepsy. Additionally, a broad variety of mutation of mitochondrial DNA leading to the inhibition of mitochondrial respiratory chain or directly of mitochondrial adenosine triphosphate synthesis in epileptogenic areas of the human brain has been associated with epileptic phenotypes. Since mitochondrial oxidative phosphorylation provides the major source of adenosine triphosphate in neurons, and mitochondria participate in cellular Ca2+ homeostasis they can modulate neuronal excitability and synaptic transmission. Furthermore, mitochondria are intimately involved in pathways leading to the neuronal cell death characteristic for the areas of epileptogenesis.

Recognition of mitochondrial DNA deletion syndrome with non-neuromuscular multisystemic manifestation

PURPOSE

To correlate the molecular characteristics of the mtDNA deletions with clinical phenotypes.

METHODS

Southern analysis and polymerase chain reaction (PCR)/DNA sequencing were used to determine the size and location of deletions in 16 patients with mtDNA deletion syndrome. An additional 48 reported cases from the literature were also included in the statistical analysis.

RESULTS

The common 5-kb deletion is found in eight of nine patients with Kearns-Sayre syndrome (KSS), mitochondrial myopathies (MM), or progressive external ophthalmoplegia (PEO). The rare/novel deletions were found in six of seven patients with extra-neuromuscular multisystemic manifestations and infantile/early childhood onset.

CONCLUSIONS

Patients with mtDNA deletion syndrome who manifest non-neuromuscular multisystemic disorders at a very young age usually harbor mutant mtDNA with novel or rare deletions in every tissue analyzed. For this group of patients, it is possible to use the less invasive blood specimens instead of muscle biopsies for molecular diagnosis. Overwhelmingly, the common 5-kb deletion is mostly seen in the muscle specimens of patients with KSS and age of onset after the second decade of life.

Mitochondrial disorders

The most relevant contribution to the elucidation of the molecular basis of mitochondrial disorders has come from the discovery of an impressive and ever expanding number of mutations of mitochondrial DNA. However, known mutations of mtDNA only account for a fraction of all the mitochondrial disorders in both infants and adults. A number of recent clinical and molecular observations indicate that many syndromes are caused by abnormalities in nuclear genes related to oxidative phosphorylation. Nuclear genes encode hundreds of proteins involved in mitochondrial biogenesis and oxidative phosphorylation. Nevertheless, the identification of the nuclear genes responsible for oxidative phosphorylation-related disorders has proceeded at a much slower pace, compared with the discovery and characterization of mtDNA mutations. The reasons for such a gap are numerous, including the rarity of the syndromes, their genetic heterogeneity, and our ignorance of this nuclear gene repertoire in humans. This scenario is rapidly changing, thanks to the discovery of several oxidative phosphorylation-related human genes, and to the identification in some of them of mutations responsible for different clinical syndromes. In addition, animal models have recently been generated, which will offer the opportunity to understand better the pathogenesis of specific oxidative phosphorylation defects, and to test in a rational and controlled fashion therapeutic strategies for the treatment of these disorders.

Control of oxidative phosphorylation in skeletal muscle

The classical concept of ATP-demand control of energy metabolism in skeletal muscle has to be modified on the basis of studies showing the influence of additional controlling parameters (reducing equivalent supply, oxygen availability, proton leak, diffusion restrictions and the creatine kinase system) and on the basis of applications of metabolic control analysis showing very clearly multistep control. This concept of multistep control allows to quantify the individual influence of any parameter on mitochondrial oxidative phosphorylation and is extremely helpful to analyze the metabolic consequences of enzyme deficiencies in skeletal muscle occurring in mitochondrial myopathies.

Flux control of cytochrome c oxidase in human skeletal muscle

In the present work, by titrating cytochrome c oxidase (COX) with the specific inhibitor KCN, the flux control coefficient and the metabolic reserve capacity of COX have been determined in human saponin-permeabilized muscle fibers. In the presence of the substrates glutamate and malate, a 2.3 +/- 0.2-fold excess capacity of COX was observed in ADP-stimulated human skeletal muscle fibers. This value was found to be dependent on the mitochondrial substrate supply. In the combined presence of glutamate, malate, and succinate, which supported an approximately 1.4-fold higher rate of respiration, only a 1.4 +/- 0.2-fold excess capacity of COX was determined. In agreement with these findings, the flux control of COX increased, in the presence of the three substrates, from 0.27 +/- 0.03 to 0.36 +/- 0.08. These results indicate a tight in vivo control of respiration by COX in human skeletal muscle. This tight control may have significant implications for mitochondrial myopathies. In support of this conclusion, the analysis of skeletal muscle fibers from two patients with chronic progressive external ophthalmoplegia, which carried deletions in 11 and 49% of their mitochondrial DNA, revealed a substantially lowered reserve capacity and increased flux control coefficient of COX, indicating severe rate limitations of oxidative phosphorylation by this enzyme.