Mitochondrial myopathies: clinical and biochemical features of 30 patients with major deletions of muscle mitochondrial DNA

Mechanisms and pathologies of human mitochondrial DNA replication and deletion formation

Human mitochondria possess a multi-copy circular genome, mitochondrial DNA (mtDNA), that is essential for cellular energy metabolism. The number of copies of mtDNA per cell, and their integrity, are maintained by nuclear-encoded mtDNA replication and repair machineries. Aberrant mtDNA replication and mtDNA breakage are believed to cause deletions within mtDNA. The genomic location and breakpoint sequences of these deletions show similar patterns across various inherited and acquired diseases, and are also observed during normal ageing, suggesting a common mechanism of deletion formation. However, an ongoing debate over the mechanism by which mtDNA replicates has made it difficult to develop clear and testable models for how mtDNA rearrangements arise and propagate at a molecular and cellular level. These deletions may impair energy metabolism if present in a high proportion of the mtDNA copies within the cell, and can be seen in primary mitochondrial diseases, either in sporadic cases or caused by autosomal variants in nuclear-encoded mtDNA maintenance genes. These mitochondrial diseases have diverse genetic causes and multiple modes of inheritance, and show notoriously broad clinical heterogeneity with complex tissue specificities, which further makes establishing genotype-phenotype relationships challenging. In this review, we aim to cover our current understanding of how the human mitochondrial genome is replicated, the mechanisms by which mtDNA replication and repair can lead to mtDNA instability in the form of large-scale rearrangements, how rearranged mtDNAs subsequently accumulate within cells, and the pathological consequences when this occurs.

Mitochondrial DNA competition: starving out the mutant genome

High levels of pathogenic mitochondrial DNA (mtDNA) variants lead to severe genetic diseases, and the accumulation of such mutants may also contribute to common disorders. Thus, selecting against these mutants is a major goal in mitochondrial medicine. Although mutant mtDNA can drift randomly, mounting evidence indicates that active forces play a role in the selection for and against mtDNA variants. The underlying mechanisms are beginning to be clarified, and recent studies suggest that metabolic cues, including fuel availability, contribute to shaping mtDNA heteroplasmy. In the context of pathological mtDNAs, remodeling of nutrient metabolism supports mitochondria with deleterious mtDNAs and enables them to outcompete functional variants owing to a replicative advantage. The elevated nutrient requirement represents a mutant Achilles' heel because small molecules that restrict nutrient consumption or interfere with nutrient sensing can purge cells of deleterious mtDNAs and restore mitochondrial respiration. These advances herald the dawn of a new era of small-molecule therapies to counteract pathological mtDNAs.

Progressive external ophthalmoplegia

Progressive external ophthalmoplegia (PEO), characterized by ptosis and impaired eye movements, is a clinical syndrome with an expanding number of etiologically distinct subtypes. Advances in molecular genetics have revealed numerous pathogenic causes of PEO, originally heralded in 1988 by the detection of single large-scale deletions of mitochondrial DNA (mtDNA) in skeletal muscle of people with PEO and Kearns-Sayre syndrome. Since then, multiple point variants of mtDNA and nuclear genes have been identified to cause mitochondrial PEO and PEO-plus syndromes, including mitochondrial neurogastrointestinal encephalomyopathy (MNGIE) and sensory ataxic neuropathy dysarthria ophthalmoplegia (SANDO). Intriguingly, many of those nuclear DNA pathogenic variants impair maintenance of the mitochondrial genome causing downstream mtDNA multiple deletions and depletion. In addition, numerous genetic causes of nonmitochondrial PEO have been identified.

[Chronic progressive external ophthalmoplegia that could not be diagnosed by biceps muscle biopsy, but was genetically diagnosed by extraocular muscle biopsy]

A 48-year-old Japanese male experienced slowly progressive diplopia. He had no family history and was negative for the edrophonium chloride test. Blood analysis showed elevated lactic acid and pyruvic acid levels, suggesting mitochondrial disease. A muscle biopsy from the biceps brachii was performed, but no pathological or genetical mitochondrial abnormalities were detected. Subsequently, he underwent muscle plication for diplopia in which the right inferior rectus muscle was biopsied. Genetic examination of genomic DNA extracted from the extraocular muscle tissue revealed multiple mitochondrial gene deletions, with a heteroplasmy rate of approximately 35%, resulting in the diagnosis of chronic progressive external ophthalmoplegia. In mitochondrial diseases, the tissue distribution of mitochondria with disease-associated variants in mtDNA should be noted, and it is important to select the affected muscle when performing a biopsy for an accurate diagnosis.

Highly asymmetrical distribution of muscle wasting correlates to the heteroplasmy in a patient carrying a large-scale mitochondrial DNA deletion: a novel pathophysiological mechanism for explaining asymmetry in mitochondrial myopathies

Mitochondrial diseases are a heterogeneous group of pathologies, caused by missense mutations, sporadic large-scale deletions of mitochondrial DNA (mtDNA) or mutations of nuclear maintenance genes. We report the case of a patient in whom extended muscle pathology, biochemical and genetic mtDNA analyses have proven to be essential to elucidate a unique asymmetrical myopathic presentation. From the age of 34 years on, the patient has presented with oculomotor disorders, right facial peripheral palsy and predominantly left upper limb muscle weakness and atrophy. By contrast, he displayed no motor weakness on the right hemi-body, and no sensory symptoms, cerebellar syndrome, hypoacusis, or parkinsonism. Cardiac function was normal. CK levels were elevated (671 UI/L). Electroneuromyography (ENMG) and muscle MRI showed diffuse myogenic alterations, more pronounced on the left side muscles. Biopsy of the left deltoid muscle showed multiple mitochondrial defects, whereas in the right deltoid, mitochondrial defects were much less marked. Extended mitochondrial biochemical and molecular workup revealed a unique mtDNA deletion, with a 63.4% heteroplasmy load in the left deltoid, versus 8.1% in the right one. This case demonstrates that, in mitochondrial myopathies, heteroplasmy levels may drastically vary for the same type of muscle, rising the hypothesis of a new pathophysiological mechanism explaining asymmetry in hereditary myopathies.

Recent Advances in Mitochondrial Diseases: from Molecular Insights to Therapeutic Perspectives

Mitochondria are double-membraned cytoplasmic organelles that are responsible for the production of energy in eukaryotic cells. The process is completed through oxidative phosphorylation (OXPHOS) by the respiratory chain (RC) in mitochondria. Thousands of mitochondria may be present in each cell, depending on the function of that cell. Primary mitochondria disorder (PMD) is a clinically heterogeneous disease associated with germline mutations in mitochondrial DNA (mtDNA) and/or nuclear DNA (nDNA) genes, and impairs mitochondrial structure and function. Mitochondrial dysfunction can be detected in early childhood and may be severe, progressive and often multi-systemic, involving a wide range of organs. Understanding epigenetic factors and pathways mutations can help pave the way for developing an effective cure. However, the lack of information about the disease (including age of onset, symptoms, clinical phenotype, morbidity and mortality), the limits of current preclinical models and the wide range of phenotypic presentations hamper the development of effective medicines. Although new therapeutic approaches have been introduced with encouraging preclinical and clinical outcomes, there is no definitive cure for PMD. This review highlights recent advances, particularly in children, in terms of etiology, pathophysiology, clinical diagnosis, molecular pathways and epigenetic alterations. Current therapeutic approaches, future advances and proposed new therapeutic plans will also be discussed.

Kearns–Sayre syndrome with a novel large-scale deletion: a case report

Background

Kearns–Sayre syndrome (KSS) is a rare, multisystem mitochondrial encephalomyopathy. We report a case of KSS with a novel 7.6-kb deletion as assessed through a long-range polymerase chain reaction (PCR) study in the blood. In addition, optical coherence tomography angiography (OCTA) confirmed deep retinal capillary atrophy for the first time.

Case presentation

A 13-year-old patient presented with progressive vision loss and difficulty with eye opening and was diagnosed with progressive external ophthalmoplegia and retinitis pigmentosa (RP). The patient also experienced heart block, vestibular dysfunction, growth retardation and multiple demyelinating lesions. A long-range PCR study in the blood revealed a large-scale Chrm: 6341–13,993 deletion, which was first reported and broadened the genetic spectrum of this disease. The patient underwent complete ophthalmic examination, medical history review and gene detection, resulting in a confirmation of the diagnosis of KSS. The patient was given a pair of applicable glasses to wear and was followed up every 3 months. An implantable pacemaker was also installed based on the advice of the physician.

Conclusions

We reported a novel large-scale deletion in the mitochondrial DNA of KSS, and OCTA was used for the first time to confirm deep retinal capillary atrophy. Furthermore, because ophthalmic symptoms are often the primary manifestation of KSS, the relationship between ophthalmology and mitochondrial diseases should be emphasised.

Mitochondrial DNA Copy Number and Developmental Origins of Health and Disease (DOHaD)

Mitochondrial dysfunction is known to contribute to mitochondrial diseases, as well as to a variety of aging-based pathologies. Mitochondria have their own genomes (mitochondrial DNA (mtDNA)) and the abnormalities, such as point mutations, deletions, and copy number variations, are involved in mitochondrial dysfunction. In recent years, several epidemiological studies and animal experiments have supported the Developmental Origin of Health and Disease (DOHaD) theory, which states that the environment during fetal life influences the predisposition to disease and the risk of morbidity in adulthood. Mitochondria play a central role in energy production, as well as in various cellular functions, such as apoptosis, lipid metabolism, and calcium metabolism. In terms of the DOHaD theory, mtDNA copy number may be a mediator of health and disease. This paper summarizes the results of recent epidemiological studies on the relationship between environmental factors and mtDNA copy number during pregnancy from the perspective of DOHaD theory. The results of these studies suggest a hypothesis that mtDNA copy number may reflect environmental influences during fetal life and possibly serve as a surrogate marker of health risks in adulthood.

Cellular mechanisms of mtDNA heteroplasmy dynamics

Heteroplasmy refers to the coexistence of more than one variant of the mitochondrial genome (mtDNA). Mutated or partially deleted mtDNAs can induce chronic metabolic impairment and cause mitochondrial diseases when their heteroplasmy levels exceed a critical threshold. These mutant mtDNAs can be maternally inherited or can arise de novo. Compelling evidence has emerged showing that mutant mtDNA levels can vary and change in a nonrandom fashion across generations and amongst tissues of an individual. However, our lack of understanding of the basic cellular and molecular mechanisms of mtDNA heteroplasmy dynamics has made it difficult to predict who will inherit or develop mtDNA-associated diseases. More recently, with the advances in technology and the establishment of tractable model systems, insights into the mechanisms underlying the selection forces that modulate heteroplasmy dynamics are beginning to emerge. In this review, we summarize evidence from different organisms, showing that mutant mtDNA can experience both positive and negative selection. We also review the recently identified mechanisms that modulate heteroplasmy dynamics. Taken together, this is an opportune time to survey the literature and to identify key cellular pathways that can be targeted to develop therapies for diseases caused by heteroplasmic mtDNA mutations.

Mitochondrial genome stability in human: understanding the role of DNA repair pathways

Mitochondria are semiautonomous organelles in eukaryotic cells and possess their own genome that replicates independently. Mitochondria play a major role in oxidative phosphorylation due to which its genome is frequently exposed to oxidative stress. Factors including ionizing radiation, radiomimetic drugs and replication fork stalling can also result in different types of mutations in mitochondrial DNA (mtDNA) leading to genome fragility. Mitochondria from myopathies, dystonia, cancer patient samples show frequent mtDNA mutations such as point mutations, insertions and large-scale deletions that could account for mitochondria-associated disease pathogenesis. The mechanism by which such mutations arise following exposure to various DNA-damaging agents is not well understood. One of the well-studied repair pathways in mitochondria is base excision repair. Other repair pathways such as mismatch repair, homologous recombination and microhomology-mediated end joining have also been reported. Interestingly, nucleotide excision repair and classical nonhomologous DNA end joining are not detected in mitochondria. In this review, we summarize the potential causes of mitochondrial genome fragility, their implications as well as various DNA repair pathways that operate in mitochondria.

Thermodynamic analysis of DNA hybridization signatures near mitochondrial DNA deletion breakpoints

Broad evidence in the literature supports double-strand breaks (DSBs) as initiators of mitochondrial DNA (mtDNA) deletion mutations. While DNA misalignment during DSB repair is commonly proposed as the mechanism by which DSBs cause deletion mutations, details such as the specific DNA repair errors are still lacking. Here, we used DNA hybridization thermodynamics to infer the sequence lengths of mtDNA misalignments that are associated with mtDNA deletions. We gathered and analyzed 9,921 previously reported mtDNA deletion breakpoints in human, rhesus monkey, mouse, rat, and Caenorhabditis elegans. Our analysis shows that a large fraction of mtDNA breakpoint positions can be explained by the thermodynamics of short ≤ 5-nt misalignments. The significance of short DNA misalignments supports an important role for erroneous non-homologous and micro-homology-dependent DSB repair in mtDNA deletion formation. The consistency of the results of our analysis across species further suggests a shared mode of mtDNA deletion mutagenesis.

Using urine to diagnose large-scale mtDNA deletions in adult patients

OBJECTIVE

The aim of this study was to evaluate if urinary sediment cells offered a robust alternative to muscle biopsy for the diagnosis of single mtDNA deletions.

METHODS

Eleven adult patients with progressive external ophthalmoplegia and a known single mtDNA deletion were investigated. Urinary sediment cells were used to isolate DNA, which was then subjected to long-range polymerase chain reaction. Where available, the patient`s muscle DNA was studied in parallel. Breakpoint and thus deletion size were identified using both Sanger sequencing and next generation sequencing. The level of heteroplasmy was determined using quantitative polymerase chain reaction.

RESULTS

We identified the deletion in urine in 9 of 11 cases giving a sensitivity of 80%. Breakpoints and deletion size were readily detectable in DNA extracted from urine. Mean heteroplasmy level in urine was 38% ± 26 (range 8 - 84%), and 57% ± 28 (range 12 - 94%) in muscle. While the heteroplasmy level in urinary sediment cells differed from that in muscle, we did find a statistically significant correlation between these two levels (R = 0.714, P = 0.031(Pearson correlation)).

INTERPRETATION

Our findings suggest that urine can be used to screen patients suspected clinically of having a single mtDNA deletion. Based on our data, the use of urine could considerably reduce the need for muscle biopsy in this patient group.

A comprehensive overview of mitochondrial DNA 4977-bp deletion in cancer studies

Mitochondria are cellular machines essential for energy production. The biogenesis of mitochondria is a highly complex and it depends on the coordination of the nuclear and mitochondrial genome. Mitochondrial DNA (mtDNA) mutations and deletions are suspected to be associated with carcinogenesis. The most described mtDNA deletion in various human cancers is called the 4977-bp common deletion (mDNA⁴⁹⁷⁷) and it has been explored since two decades. In spite of that, its implication in carcinogenesis still unknown and its predictive and prognostic impact remains controversial. This review article provides an overview of some of the cellular and molecular mechanisms underlying mDNA⁴⁹⁷⁷ formation and a detailed summary about mDNA⁴⁹⁷⁷ reported in various types of cancers. The current knowledges of mDNA⁴⁹⁷⁷ as a prognostic and predictive marker are also discussed.

QIL1-dependent assembly of MICOS complex-lethal mutation in C19ORF70 resulting in liver disease and severe neurological retardation

Seven subunits of the mitochondrial contact site and cristae junction (CJ) organizing system (MICOS) in humans have been recently described in function and structure. QIL1 (also named MIC13) is a small complex that is crucial for the maintenance and assembling of MICOS. A novel mutation of an essential splice site in the C19orf70 gene encoding QIL1 induces severe mitochondrial encephalopathy, hepatopathy and lactate acidosis consistent with psychomotor retardation. In addition, bilateral kidney stones were observed. Disassembly of MICOS complex subunits displays lack of MIC10-MIC26-MIC27-QIL1 subcomplex, resulting in aberrant cristae structure and a loss of cristae junctions and contact sites. In liver and muscle tissue, the activity of the respiratory chain complexes (OXPHOS) was severely impaired. Defects in MICOS complex do not only affect mitochondrial architecture, but also mitochondrial fusion, metabolic signalling, lipid trafficking and cellular electric homeostasis.

Disclosing the functional changes of two genetic alterations in a patient with Chronic Progressive External Ophthalmoplegia: Report of the novel mtDNA m.7486G>A variant

Chronic Progressive External Ophthalmoplegia (CPEO) is characterized by ptosis and ophthalmoplegia and is usually caused by mitochondrial DNA (mtDNA) deletions or mt-tRNA mutations. The aim of the present work was to clarify the genetic defect in a patient presenting with CPEO and elucidate the underlying pathogenic mechanism. This 62-year-old female first developed ptosis of the right eye at the age of 12 and subsequently the left eye at 45 years, and was found to have external ophthalmoplegia at the age of 55 years. Histopathological abnormalities were detected in the patient's muscle, including ragged-red fibres, a mosaic pattern of COX-deficient muscle fibres and combined deficiency of respiratory chain complexes I and IV. Genetic investigation revealed the "common deletion" in the patient's muscle and fibroblasts. Moreover, a novel, heteroplasmic mt-tRNASer(UCN) variant (m.7486G>A) in the anticodon loop was detected in muscle homogenate (50%), fibroblasts (11%) and blood (4%). Single-fibre analysis showed segregation with COX-deficient fibres for both genetic alterations. Assembly defects of mtDNA-encoded complexes were demonstrated in fibroblasts. Functional analyses showed significant bioenergetic dysfunction, reduction in respiration rate and ATP production and mitochondrial depolarization. Multilamellar bodies were detected by electron microscopy, suggesting disturbance in autophagy. In conclusion, we report a CPEO patient with two possible genetic origins, both segregating with biochemical and histochemical defect. The "common mtDNA deletion" is the most likely cause, yet the potential pathogenic effect of a novel mt-tRNASer(UCN) variant cannot be fully excluded.

Altered skeletal muscle (mitochondrial) properties in patients with mitochondrial DNA single deletion myopathy

Background

Mitochondrial myopathy severely affects skeletal muscle structure and function resulting in defective oxidative phosphorylation. However, the major pathomechanisms and therewith effective treatment approaches remain elusive. Therefore, the aim of the present study was to investigate disease-related impairments in skeletal muscle properties in patients with mitochondrial myopathy. Accordingly, skeletal muscle biopsies were obtained from six patients with moleculargenetically diagnosed mitochondrial myopathy (one male and five females, 53 ± 9 years) and eight age- and gender-matched healthy controls (two males and six females, 58 ± 14 years) to determine mitochondrial respiratory capacity of complex I-V, mitochondrial volume density and fiber type distribution.

Results

Mitochondrial volume density (4.0 ± 0.5 vs. 5.1 ± 0.8 %) as well as respiratory capacity of complex I-V were lower (P < 0.05) in mitochondrial myopathy and associated with a higher (P < 0.001) proportion of type II fibers (65.2 ± 3.6 vs. 44.3 ± 5.9 %). Additionally, mitochondrial volume density and maximal oxidative phosphorylation capacity correlated positively (P < 0.05) to peak oxygen uptake.

Conclusion

Mitochondrial myopathy leads to impaired mitochondrial quantity and quality and a shift towards a more glycolytic skeletal muscle phenotype.

Role of p53, Mitochondrial DNA Deletions, and Paternal Age in Autism: A Case-Control Study

BACKGROUND

The tumor suppressor p53 responds to a variety of environmental stressors by regulating cell cycle arrest, apoptosis, senescence, DNA repair, bioenergetics and mitochondrial DNA (mtDNA) copy number maintenance. Developmental abnormalities have been reported in p53-deficient mice, and altered p53 and p53-associated pathways in autism (AU). Furthermore, via the Pten-p53 crosstalk, Pten haploinsufficient-mice have autisticlike behavior accompanied by brain mitochondrial dysfunction with accumulation of mtDNA deletions.

METHODS

mtDNA copy number and deletions, and p53 gene copy ratios were evaluated in peripheral blood monocytic cells from children aged 2-5 years with AU (n = 66), race-, gender-, and age-matched typically neurodeveloping children (n = 46), and both parents from each diagnostic group, recruited by the Childhood Autism Risk from Genes and Environment study at the University of California, Davis.

RESULTS

mtDNA deletions and higher p53 gene copy ratios were more common in children with AU and their fathers. The incidence of mtDNA deletions in fathers of children with AU was increased 1.9-fold over fathers of typically neurodeveloping children, suggesting a role for deficient DNA repair capacity not driven by paternal age. Deletions in mtDNA and altered p53 gene copy ratios seem to result from genetics (children with severity scores ≥8) and/or act in concert with environmental factors (children with 6-7 severity scores).

CONCLUSIONS

Given pro- and antioxidant activities of p53, and associations of genomic instability with disorders other than AU, our study suggests a link between DNA repair capacity, genomic instability in the 17p13.1 region influenced by environmental triggers, and AU diagnosis.

Insulin-like growth factor 1 (IGF-1) therapy: Mitochondrial dysfunction and diseases

This review resumes the association between mitochondrial function and diseases, especially neurodegenerative diseases. Additionally, it summarizes the major role of IGF-1 as a mitochondrial protector, as studied in several experimental models (cirrhosis, aging …). The contribution of mitochondrial dysfunction to impairments in insulin metabolic signaling is also suggested by gene array analysis showing that reductions in gene expression, that regulates mitochondrial ATP production, are associated with insulin resistance and type 2 diabetes mellitus. Moreover, reductions in oxidative capacity of mitochondrial electron transport chain are manifested in obese, insulin-resistant and diabetic patients. Genetic and environmental factors, oxidative stress, and alterations in mitochondrial biogenesis can adversely affect mitochondrial function, leading to insulin resistance and several pathological conditions, such as type 2 diabetes. Finally, it remains essential to know the exact mechanisms involved in mitochondrial generation and metabolism, mitophagy, apoptosis, and oxidative stress to establish new targets in order to develop potentially effective therapies. One of the newest targets to recover mitochondrial dysfunction could be the administration of IGF-1 at low doses. In the last years, it has been observed that IGF-1 therapy has several beneficial effects: restores physiological IGF-1 levels; improves insulin resistance and lipid metabolism; exerts mitochondrial protection; and has hepatoprotective, neuroprotective, antioxidant and antifibrogenic effects. In consequence, treatment of mitochondrial dysfunctions with low doses of IGF-1 could be a powerful and useful effective therapy to restore normal mitochondrial functions.

Paediatric single mitochondrial DNA deletion disorders: an overlapping spectrum of disease

BACKGROUND

Single large-scale mitochondrial DNA (mtDNA) deletions (SLSMDs) are amongst the most frequently diagnosed mtDNA disorders in childhood, yet their natural history remains poorly understood. We report the natural history of a large multicentre cohort of such children.

METHODS

We reviewed case notes from three different UK centres to determine the clinical course of 34 patients (16 female, 18 male) with childhood-onset mitochondrial disease caused by SLSMDs. Kaplan-Meier analysis was used to compare survival of patients presenting with haematological features (Pearson syndrome) and those with nonhaematological presentations.

RESULTS

The most frequent initial presentation was with isolated ptosis (16/34, 47%). Eleven (32%) patients presented with transfusion-dependent anaemia soon after birth and were diagnosed with Pearson syndrome, whilst ten were classified as having Kearns-Sayre syndrome, three as having progressive external ophthalmoplegia (PEO) and seven as having PEO-plus. Three patients did not conform to any specific mitochondrial syndrome. The most frequently affected organ during the disease course was the kidney, with documented tubular or glomerular dysfunction in 17 of 20 (85%) cases who had detailed investigations. SLSMDs were present in blood and/or urine cells in all cases tested, indicating that muscle biopsy is not necessary for diagnosis in the paediatric age range. Kaplan-Meier survival analysis revealed significantly worse mortality in patients with Pearson syndrome compared with the rest of the cohort.

CONCLUSIONS

Mitochondrial disease caused by SLSMDs is clinically heterogeneous, and not all cases conform to a classical mitochondrial syndrome. Multisystem disease is the norm, with anaemia, renal impairment and endocrine disturbance being the most frequent extraneurological features. SLSMDs should be considered in the differential diagnosis of all children presenting with ptosis.

Peripheral neuropathy predicts nuclear gene defect in patients with mitochondrial ophthalmoplegia

Mitochondrial ophthalmoplegia is a genetically heterogeneous disorder. Horga et al. investigate whether peripheral neuropathy can predict the underlying genetic defect in patients with progressive external ophthalmoplegia. Results indicate that neuropathy is highly predictive of a nuclear DNA defect and that it is rarely associated with single mitochondrial DNA deletions.

Progressive external ophthalmoplegia is a common clinical feature in mitochondrial disease caused by nuclear DNA defects and single, large-scale mitochondrial DNA deletions and is less frequently associated with point mutations of mitochondrial DNA. Peripheral neuropathy is also a frequent manifestation of mitochondrial disease, although its prevalence and characteristics varies considerably among the different syndromes and genetic aetiologies. Based on clinical observations, we systematically investigated whether the presence of peripheral neuropathy could predict the underlying genetic defect in patients with progressive external ophthalmoplegia. We analysed detailed demographic, clinical and neurophysiological data from 116 patients with genetically-defined mitochondrial disease and progressive external ophthalmoplegia. Seventy-eight patients (67%) had a single mitochondrial DNA deletion, 12 (10%) had a point mutation of mitochondrial DNA and 26 (22%) had mutations in either POLG, C10orf2 or RRM2B, or had multiple mitochondrial DNA deletions in muscle without an identified nuclear gene defect. Seventy-seven patients had neurophysiological studies; of these, 16 patients (21%) had a large-fibre peripheral neuropathy. The prevalence of peripheral neuropathy was significantly lower in patients with a single mitochondrial DNA deletion (2%) as compared to those with a point mutation of mitochondrial DNA or with a nuclear DNA defect (44% and 52%, respectively; P < 0.001). Univariate analyses revealed significant differences in the distribution of other clinical features between genotypes, including age at disease onset, gender, family history, progressive external ophthalmoplegia at clinical presentation, hearing loss, pigmentary retinopathy and extrapyramidal features. However, binomial logistic regression analysis identified peripheral neuropathy as the only independent predictor associated with a nuclear DNA defect (P = 0.002; odds ratio 8.43, 95% confidence interval 2.24–31.76). Multinomial logistic regression analysis identified peripheral neuropathy, family history and hearing loss as significant predictors of the genotype, and the same three variables showed the highest performance in genotype classification in a decision tree analysis. Of these variables, peripheral neuropathy had the highest specificity (91%), negative predictive value (83%) and positive likelihood ratio (5.87) for the diagnosis of a nuclear DNA defect. These results indicate that peripheral neuropathy is a rare finding in patients with single mitochondrial DNA deletions but that it is highly predictive of an underlying nuclear DNA defect. This observation may facilitate the development of diagnostic algorithms. We suggest that nuclear gene testing may enable a more rapid diagnosis and avoid muscle biopsy in patients with progressive external ophthalmoplegia and peripheral neuropathy.

Disease progression in patients with single, large-scale mitochondrial DNA deletions

Single, large-scale deletions of mitochondrial DNA are a common cause of mitochondrial disease and cause a broad phenotypic spectrum ranging from mild myopathy to devastating multi-system syndromes such as Kearns-Sayre syndrome. Studies to date have been inconsistent on the value of putative predictors of clinical phenotype and disease progression such as mutation load and the size or location of the deletion. Using a cohort of 87 patients with single, large-scale mitochondrial DNA deletions we demonstrate that a variety of outcome measures such as COX-deficient fibre density, age-at-onset of symptoms and progression of disease burden, as measured by the Newcastle Mitochondrial Disease Adult Scale, are significantly (P < 0.05) correlated with the size of the deletion, the deletion heteroplasmy level in skeletal muscle, and the location of the deletion within the genome. We validate these findings with re-analysis of 256 cases from published data and clarify the previously conflicting information of the value of these predictors, identifying that multiple regression analysis is necessary to understand the effect of these interrelated predictors. Furthermore, we have used mixed modelling techniques to model the progression of disease according to these predictors, allowing a better understanding of the progression over time of this strikingly variable disease. In this way we have developed a new paradigm in clinical mitochondrial disease assessment and management that sidesteps the perennial difficulty of ascribing a discrete clinical phenotype to a broad multi-dimensional and progressive spectrum of disease, establishing a framework to allow better understanding of disease progression.

Prevalence of rare mitochondrial DNA mutations in mitochondrial disorders

Background

Mitochondrial DNA (mtDNA) diseases are rare disorders whose prevalence is estimated around 1 in 5000. Patients are usually tested only for deletions and for common mutations of mtDNA which account for 5–40% of cases, depending on the study. However, the prevalence of rare mtDNA mutations is not known.

Methods

We analysed the whole mtDNA in a cohort of 743 patients suspected of manifesting a mitochondrial disease, after excluding deletions and common mutations. Both heteroplasmic and homoplasmic variants were identified using two complementary strategies (Surveyor and MitoChip). Multiple correspondence analyses followed by hierarchical ascendant cluster process were used to explore relationships between clinical spectrum, age at onset and localisation of mutations.

Results

7.4% of deleterious mutations and 22.4% of novel putative mutations were identified. Pathogenic heteroplasmic mutations were more frequent than homoplasmic mutations (4.6% vs 2.8%). Patients carrying deleterious mutations showed symptoms before 16 years of age in 67% of cases. Early onset disease (<1 year) was significantly associated with mutations in protein coding genes (mainly in complex I) while late onset disorders (>16 years) were associated with mutations in tRNA genes. MTND5 and MTND6 genes were identified as ‘hotspots’ of mutations, with Leigh syndrome accounting for the large majority of associated phenotypes.

Conclusions

Rare mitochondrial DNA mutations probably account for more than 7.4% of patients with respiratory chain deficiency. This study shows that a comprehensive analysis of mtDNA is essential, and should include young children, for an accurate diagnosis that is now accessible with the development of next generation sequencing technology.

Mitochondrial DNA deletions in muscle satellite cells: implications for therapies

Progressive myopathy is a major clinical feature of patients with mitochondrial DNA (mtDNA) disease. There is limited treatment available for these patients although exercise and other approaches to activate muscle stem cells (satellite cells) have been proposed. The majority of mtDNA defects are heteroplasmic (a mixture of mutated and wild-type mtDNA present within the muscle) with high levels of mutated mtDNA and low levels of wild-type mtDNA associated with more severe disease. The culture of satellite cell-derived myoblasts often reveals no evidence of the original mtDNA mutation although it is not known if this is lost by selection or simply not present in these cells. We have explored if the mtDNA mutation is present in the satellite cells in one of the commonest genotypes associated with mitochondrial myopathies (patients with single, large-scale mtDNA deletions). Analysis of satellite cells from eight patients showed that the level of mtDNA mutation in the satellite cells is the same as in the mature muscle but is most often subsequently lost during culture. We show that there are two periods of selection against the mutated form, one early on possibly during satellite cell activation and the other during the rapid replication phase of myoblast culture. Our data suggest that the mutations are also lost during rapid replication in vivo, implying that strategies to activate satellite cells remain a viable treatment for mitochondrial myopathies in specific patient groups.

Mitochondrial common deletion, a potential biomarker for cancer occurrence, is selected against in cancer background: a meta-analysis of 38 studies

Mitochondrial dysfunction has been long proposed to play a major role in tumorigenesis. Mitochondrial DNA (mtDNA) mutations, especially the mtDNA 4,977 bp deletion has been found in patients of various types of cancer. In order to comprehend the mtDNA 4,977 bp deletion status in various cancer types, we performed a meta-analysis composed of 33 publications, in which a total of 1613 cancer cases, 1516 adjacent normals and 638 healthy controls were included. When all studies were pooled, we found that cancerous tissue carried a lower mtDNA 4,977 bp deletion frequency than adjacent non-cancerous tissue (OR = 0.43, 95% CI = 0.20-0.92, P = 0.03 for heterogeneity test, I(2) = 91.5%) among various types of cancer. In the stratified analysis by cancer type the deletion frequency was even lower in tumor tissue than in adjacent normal tissue of breast cancer (OR = 0.19, 95% CI = 0.06-0.61, P = 0.005 for heterogeneity test, I(2)= 82.7%). Interestingly, this observation became more significant in the stratified studies with larger sample sizes (OR = 0.70, 95% CI = 0.58-0.86, P = 0.0005 for heterogeneity test, I(2) = 95.1%). Furthermore, when compared with the normal tissue from the matched healthy controls, increased deletion frequencies were observed in both adjacent non-cancerous tissue (OR = 3.02, 95% CI = 2.13-4.28, P<0.00001 for heterogeneity test, I(2)= 53.7%), and cancerous tissue (OR = 1.36, 95% CI = 1.04-1.77, P = 0.02 for heterogeneity test, I(2)= 83.5%). This meta-analysis suggests that the mtDNA 4,977 bp deletion is often found in cancerous tissue and thus has the potential to be a biomarker for cancer occurrence in the tissue, but at the same time being selected against in various types of carcinoma tissues. Larger and better-designed studies are still warranted to confirm these findings.

Evidence of reactive oxygen species-mediated damage to mitochondrial DNA in children with typical autism

Background

The mitochondrial genome (mtDNA) is particularly susceptible to damage mediated by reactive oxygen species (ROS). Although elevated ROS production and elevated biomarkers of oxidative stress have been found in tissues from children with autism spectrum disorders, evidence for damage to mtDNA is lacking.

Findings

mtDNA deletions were evaluated in peripheral blood monocytic cells (PBMC) isolated from 2–5 year old children with full autism (AU; n = 67), and typically developing children (TD; n = 46) and their parents enrolled in the CHildhood Autism Risk from Genes and Environment study (CHARGE) at University of California Davis. Sequence variants were evaluated in mtDNA segments from AU and TD children (n = 10; each) and their mothers representing 31.2% coverage of the entire human mitochondrial genome. Increased mtDNA damage in AU children was evidenced by (i) higher frequency of mtDNA deletions (2-fold), (ii) higher number of GC→AT transitions (2.4-fold), being GC preferred sites for oxidative damage, and (iii) higher frequency of G,C,T→A transitions (1.6-fold) suggesting a higher incidence of polymerase gamma incorporating mainly A at bypassed apurinic/apyrimidinic sites, probably originated from oxidative stress. The last two outcomes were identical to their mothers suggesting the inheritance of a template consistent with increased oxidative damage, whereas the frequency of mtDNA deletions in AU children was similar to that of their fathers.

Conclusions

These results suggest that a combination of genetic and epigenetic factors, taking place during perinatal periods, results in a mtDNA template in children with autism similar to that expected for older individuals.

Progressive external ophthalmoplegia in southwestern Finland: a clinical and genetic study

BACKGROUND

Progressive external ophthalmoplegia (PEO) is a common phenotype of mitochondrial disease. Molecular etiologies include sporadic, large-scale deletions in mitochondrial DNA (mtDNA), multiple mtDNA deletions secondary to autosomal dominant or recessive mutations and mtDNA point mutations.

METHODS

We studied the prevalence and clinical and genetic characteristics of PEO in a defined population in southwestern Finland. A total of 620 patients were first identified from the patient registry at the Turku University Hospital over an 18-year period. The medical records of these patients were scrutinized, and those with clinical features compatible with PEO were ascertained.

RESULTS

We identified 10 patients with possible PEO. The patients were examined clinically, and DNA was analyzed for mtDNA deletions and for the m.3243A>G and m.8344A>G mtDNA point mutations. The ANT1, PEO1, POLG1 and POLG2 genes were sequenced. We confirmed the clinical diagnosis of PEO in 6 patients. Large-scale mtDNA deletions were detected in 3 out of 6 PEO patients and mutations in the POLG1 gene in 1 out of 6. We did not find any mutations in the ANT1, PEO1 or POLG2 genes.

CONCLUSIONS

Our results suggest that molecular investigation of patients with PEO, either sporadic or familial, should start with an analysis for mtDNA deletions, followed by an analysis of the POLG1 gene.

Multisystem disorder in late-onset chronic progressive external ophthalmoplegia

INTRODUCTION

Chronic progressive external ophthalmoplegia (CPEO) is a mitochondrial syndrome on a disease spectrum with Kearns-Sayre syndrome (KSS). Clinical presentation is variable and our experience suggested that phenotypic differences exist in CPEO with onset after age 20.

METHODS

This descriptive study is a retrospective chart review of 40 patients with late-onset CPEO. Clinical features, laboratory and neurophysiology results were reviewed.

RESULTS

Multisystem dysfunction was very common in this series. Gastrointestinal dysfunction was more common than expected (60%) as was migraine headache (40%). Clinical characteristics on the KSS disease spectrum were uncommon in this series with only 2.5% having pigmentary retinopathy, 5% with cardiac conduction abnormality, and 22.5% having endocrinopathy (most often thyroid dysfunction rather than diabetes). Neurophysiology abnormalities included length-dependent axonal polyneuropathy in 44% (sometimes subclinical) and myopathic EMG changes in 26%. Exposure to sources of acquired mitochondrial toxicity including cigarette use and hepatitis C infection were more common than expected in this series.

DISCUSSION

Phenotype was different in this late-onset series compared with previous reports in CPEO patients. In this series of late-onset patients, multi-organ dysfunction was more common than previously reported in CPEO, and some classical mitochondrial manifestations, such as pigmentary retinopathy were rare. We suggest that acquired mitochondrial toxicity may have a role in the pathogenesis of adult-onset CPEO.

Modeling mitochondrial encephalomyopathy in Drosophila

Mitochondrial encephalomyopathies are disturbingly complex and devastating diseases, reflecting the underlying importance of the affected organelle. Therapeutic approaches for these diseases remain limited due to a poor understanding of disease pathogenesis resulting largely from a lack of tractable model systems in which to study these diseases. This is especially so for disease conditions resulting from mutations directly affecting the mitochondrial genome. Recent studies using Drosophila to develop genetic models with endogenous mitochondrial mutations suggest the fruit fly will contribute significantly to our understanding of mitochondrial disease pathogenesis and the development of novel therapeutic avenues.

A novel mutation in the tRNAIle gene (MTTI) affecting the variable loop in a patient with chronic progressive external ophthalmoplegia (CPEO)

We describe a 62-year-old woman with chronic progressive external ophthalmoplegia (CPEO), multiple lipomas, diabetes mellitus, and a novel mitochondrial DNA (mtDNA) mutation at nucleotide 4302 (4302A>G) of the tRNA(Ile) gene (MTTI). This is the first mutation at position 44 in the variable loop (V loop) of any mitochondrial tRNA. The muscle biopsy revealed 10% ragged-red/ragged-blue fibers and 25% cytochrome c oxidase (COX)-deficient fibers. No deletions or duplications were detected by Southern blot analysis. The 4302A>G transition was present only in the patient's muscle and single-fiber analysis revealed significantly higher levels of the mutation in COX-deficient than in normal fibers. Like tRNA(Leu(UUR)), tRNA(Ile) appears to be a "hot spot" for mtDNA mutations causing CPEO.

Role of mitochondrial DNA 4977-bp deletions in esophageal cancer susceptibility and prognosis in a northern Indian population

The mitochondrial DNA 4977-bp deletion (Delta upmtDNA4977) has been explored in various cancers, but its predictive or prognostic role in esophageal cancer is poorly understood. The objective of the present study was to investigate a possible role of Delta upmtDNA4977 in susceptibility and prognosis of esophageal cancer in a northern Indian population. The study was performed in 39 histopathologically confirmed cases with esophageal cancer. Tumor, normal tissues, and intravenous blood samples were taken for detection of Delta upmtDNA4977 through a duplex polymerase chain reaction technique. Delta upmtDNA4977 was detected in two tumors and one adjacent normal tissue sample, but in none of the blood samples. All three patients with Delta upmtDNA4977 were male, with squamous cell carcinoma in the middle third of the esophagus. Survival analysis suggested a role of Delta upmtDNA4977 in prognosis of esophageal cancer patients. Despite the low frequency of Delta upmtDNA4977 in esophageal cancer patients of northern India, this feature may have a role in esophageal cancer progression and prediction of survival outcome.

Update on Chronic Progressive External Ophthalmoplegia

Mitochondrial encephalomyopathies are clinically and genetically heterogeneous disorders. External ophthalmoplegia is the most frequent symptom. Other frequently involved tissues and organs include the retina, heart, limb muscles, peripheral and central nervous system, inner ear and endocrine system. The diagnosis is based on the finding of elevated serum lactate, the characteristic histopathological changes in the muscle biopsy, and decreased activities of mitochondrial respiratory chain enzymes. In many cases, the underlying molecular defect in the mtDNA can be identified. The efficacy of pharmacological therapies (e.g., coenzyme Q) has not been established so far. Symptomatic ophthalmological treatment includes ptosis and strabismus surgery. Early cardiac pacemaker implantation may be life-saving.

Complex I and ATP content deficiency in lymphocytes from Friedreich's ataxia

BACKGROUND

Friedreich's ataxia (FRDA) is an inherited recessive disorder characterized by progressive neurological disability and heart abnormalities. A deficiency in the protein frataxin causes this disease. Frataxin deficiency leads to progressive iron accumulation in mitochondria, excessive free radical production and dysfunction of respiratory chain complexes. The expansion (GAA) repeat in the first intron causes decreased frataxin expression by interfering with transcription.

METHODS

Activity of mitochondrial respiratory chain complex I (measured as NADH ferricyanide reductase) and intracellular ATP measurement was performed on lymphocyte of FRDA patients (n=12) and control subjects (n=25).

RESULTS

Our findings showed that complex I activity and intracellular ATP were significantly reduced (P=0.001) in patients compared with controls and we found strong correlation between complex I activity and intracellular ATP content in FRDA patients (r=0.93; P<0.002). 8.6 and 9.0 kb deletion in mtDNA was detected in 9 patients out of 12 (75%) by multiplex polymerase chain reaction (PCR) and Southern blot analysis.

CONCLUSIONS

This study suggested that a biochemical defect in complex I activity and ATP production, which may be due to iron accumulation in mitochondria, can be involved in age of onset of FRDA.

Mice expressing an error-prone DNA polymerase in mitochondria display elevated replication pausing and chromosomal breakage at fragile sites of mitochondrial DNA

Expression of a proof-reading deficient form of mitochondrial DNA (mtDNA) polymerase γ, POLG, causes early death accompanied by features of premature ageing in mouse. However, the mechanism of cellular senescence remains unresolved. In addition to high levels of point mutations of mtDNA, the POLG mutator mouse harbours linear mtDNAs. Using one- and two-dimensional agarose gel electrophoresis, we show that the linear mtDNAs derive from replication intermediates and are indicative of replication pausing and chromosomal breakage at the accompanying fragile sites. Replication fork arrest is not random but occurs at specific sites close to two cis-elements known as O_H and O_L. Pausing at these sites may be enhanced in the case of exonuclease-deficient POLG owing to delayed resumption of DNA replication, or replisome instability. In either case, the mtDNA replication cycle is perturbed and this might explain the progeroid features of the POLG mutator mouse.

Maternally inherited diabetes mellitus, deafness, chronic progressive external ophthalmoplegia and myopathy as the result of A3243G mutation of mtDNA

A mitokondriális encephalomyopathiát, laktátacidózist, stroke-szerű epizódokat (MELAS-szindrómát) a leggyakrabban a mitokondriális genom A3243G-mutációja okozza. A mitokondriális DNS (mtDNS) A3243G-szubsztitúciója számos egyéb klinikai tünet, tünetcsoport hátterében is előfordulhat. Jelen közleményünkben egy 33 éves nő esetét ismertetjük, akinél a serdülőkori ophthalmoplegia externa, 19 éves korban, szülést követően jelentkező generalizált izomgyengeség, terhelési intolerancia, progresszív hypacusis és diabetes mellitus hátterében a szövettani vizsgálat mitokondriális betegséget igazolt. A genetikai analízis az mtDNS tRNS Leu(UUR) génjében heteroplazmikus formában A3243G-cserét talált. Esetünkben a klasszikus MELAS-fenotípus nem jelentkezett, az irodalmi adatok alapján azonban az A3243G-mutációhoz izoláltan társuló maternális öröklődésű diabetes mellitus, progresszív nagyothallás, krónikus progresszív ophthalmoplegia externa (CPEO), terhelési intolerancia és myopathia együttesen fordult elő. A családon belüli fenotípus egységes, a proband szintén A3243G-mutációt hordozó leányának is ptosisa, terhelési intoleranciája és myopathiája van. Az esetleírás kapcsán rövid áttekintést nyújtunk az A3243G-mutációkhoz társuló változatos klinikai fenotípusokról, valamint a CPEO-szindróma hátterében álló mtDNS-alterációkról.

Presentation and diagnosis of mitochondrial disorders in children

The first disorder of mitochondrial function was described by Luft in 1959. Over the ensuing decades, multiple cases of mitochondrial dysfunction were reported, and the term "mitochondrial disorder" arose to describe any defect in the mitochondrial electron transport chain. The sequence of the mitochondrial genome was elucidated in 1981 by Anderson et al., and during the next 20 years, >200 pathogenic point mutations, deletions, insertions, and rearrangements were described. Most of the original cases were adults, and the diagnosis of a mitochondrial disorder in an adult patient became relatively straightforward. Adults present with well-defined "mitochondrial syndromes" and generally carry mitochondrial DNA mutations that are easily identified. Children with mitochondrial disorders are much harder to define. Children are more likely to have a nuclear DNA mutation, whereas the "classic" syndromic findings tend to be absent. This review describes both the varying presentations of mitochondrial disorders and the common laboratory, imaging, and pathologic findings related to children.

The role of mitochondrial DNA large deletion for the development of presbycusis in Fischer 344 rats

Age-related hearing loss, or presbycusis, has been associated with large-scale mitochondrial DNA (mtDNA) deletion in previous studies. However, the role of this mtDNA damage in presbycusis is still not clear because the deletion in inner ears has not been measured quantitatively and analyzed in parallel with the time course of presbycusis. In the present study, the deletion was quantified using quantitative real-time PCR (qRT-PCR) in male Fischer 344 rats of different ages. It was found that the deletion increased quickly during young adulthood and reached over 60% at 6 months of age. However, a significant hearing loss was not seen until after 12 months of age. The results suggest that the existence of the deletion per se does not necessarily imply cochlear damage, but rather a critical level of the accumulated deletion seems to precede the hearing loss. The long delay may indicate the involvement of mechanisms other than mtDNA deletion in the development of presbycusis.

DNA delivery to the mitochondria sites using mitochondrial leader peptide conjugated polyethylenimine

Some genetic diseases are associated with the defects of the mitochondrial genome. Direct DNA delivery to the mitochondrial matrix has been suggested as an approach for mitochondrial gene therapy for these diseases. We hypothesized that a mitochondrial leader peptide (LP) conjugated polyethylenimine (PEI) could deliver DNA to the mitochondrial sites. PEI-LP was synthesized by the conjugation of LP to PEI using disulfide bond. The complex formation of PEI-LP with DNA was confirmed by a gel retardation assay. In this study, DNA was completely retarded at a 0.4/1 PEI-LP/DNA weight ratio. In vitro delivery tests into isolated mitochondria or living cells were performed with rhodamin-labeled DNA and PEI-LP. In vitro cell-free delivery assay with isolated mitochondria showed that PEI-LP/DNA complexes were localized at mitochondria sites. Furthermore, the PEL-LP/DNA complexes were localized at the mitochondrial sites in living cells. However, a control carrier, PEI, did not show this effect. In addition, MTT assay showed that PEI-LP showed lower cytotoxicity than PEI. These results suggest that PEI-LP can deliver DNA to the mitochondrial sites and may be useful for the development of mitochondrial gene therapy.

Chronic progressive ophthalmoplegia with large-scale mtDNA rearrangement: can we predict progression?

The prognosis of chronic progressive ophthalmoplegia with large-scale mitochondrial DNA (mtDNA) may strikingly vary from mild slowly progressive myopathy to severe multi-organ involvement. Evaluation of the disease course at the beginning of the disease is reputed impossible. To address the existence of predictive prognostic clues of these diseases, we classified 69 patients with chronic progressive ophthalmoplegia and large size mtDNA deletion into two groups according to the presence of manifestations from brain, inner ear or retina. These manifestations were present in 29 patients (CPEO/+N group) and absent in 40 patients (CPEO/-N group). We retrospectively established the clinical history of the patients and characterized their genetic alteration (amount of residual normal mtDNA molecules, site, size and percentage of the mtDNA deletion in 116 DNA samples from muscle, blood, urinary and buccal cells). In both clinical groups, the disease was progressive and heart conduction defects were frequent. We show that the CPEO/+N phenotype segregated with severe prognosis in term of rate of progression, multi-organs involvement and rate of survival. Age at onset appeared a predictive factor. The risk to develop a CPEO/+N phenotype was high when onset was before 9 years of age and low when onset was after 20 years of age. The presence and proportion of the mtDNA deletion in blood was also significantly associated with the CPEO/+N phenotype. This study is the first to establish the natural history of chronic ophthalmoplegia with mtDNA deletion in a large series of patients and to look for parameters potentially predictive of the patients' clinical course.

Chronic Progressive External Ophthalmoplegia

INTRODUCTION

Chronic progressive external ophthalmoplegia is a common mitochondrial disease that shares clinical, enzymatic, and genetic features with other mitochondrial disorders. Effective treatment does not exist, and corrective surgery of the ptosis as a palliative measure is a treatment option.

PATIENTS AND METHODS

This was a retrospective study of 10 years' duration gathering patients with the diagnosis. Information related to clinical features, ancillary tests, and genetic data was obtained from our patients.

RESULTS

Six patients were identified with this disease, 5 of them women, aged 44 to 72 years. All patients presented with ptosis, and in 50% of the patients it was asymmetric. Half of the patients noted mild dysphagia for liquids. The CPK and acetylcholine receptor antibody levels were normal. Jitter was increased in half of the patients and ragged-red fibers were present in 5 of them. The most common enzyme alteration was the combined deficit of complexes I and IV. Familial forms were not found among our patients. The most common genetic anomaly was a single deletion in the mitochondrial DNA.

CONCLUSION

Knowledge of this disorder enables us to avoid the use of drugs with significant side effects in cases of ptosis and ophthalmoplegia that do not respond to anticholinesterases.

Orbital magnetic resonance imaging of extraocular muscles in chronic progressive external ophthalmoplegia: specific diagnostic findings

INTRODUCTION

Chronic progressive external ophthalmoplegia (CPEO) is characterized by slowly progressive bilateral ophthalmoplegia and blepharoptosis. Molecular diagnosis is problematic because sporadic mitochondrial DNA deletions can be causative. We sought findings using magnetic resonance imaging (MRI) that might support the diagnosis of CPEO.

METHODS

Two men (ages 31 and 47 years) and 3 women (ages 40-49 years) with CPEO and symptom durations of 8 months to 28 years underwent high-resolution (2-mm slice thickness, 312 micron pixels), surface coil, T1-weighted orbital MRI in coronal planes. Images were analyzed quantitatively to determine extraocular muscle (EOM) sizes and were compared with 10 age- and gender-matched normal volunteers, one subject with myasthenia gravis, and with 30 subjects having EOM paralysis caused by oculomotor, trochlear,0 and abducens neuropathies.

RESULTS

EOM function was clinically diminished in CPEO, most markedly for the superior rectus (SR) and levator muscles. All EOMs in CPEO exhibited unusual qualitative T1 MRI signal abnormalities. Unlike the profound EOM atrophy typical of neurogenic paralysis, anterior volumes of medial rectus, lateral rectus, and inferior rectus muscles in CPEO were not smaller than normal (p>0.003). Anterior volumes of the SR muscle-levator complex and superior oblique were significantly reduced (p<0.003). Denervated EOMs exhibited statistically significant volume reduction when compared with normal and CPEO groups. Volume of the SR muscle-levator complex was the same in subjects with CPEO and oculomotor palsies.

CONCLUSIONS

CPEO is associated with minimal EOM volume reduction despite clinically severe weakness. This combination of findings may be specific for CPEO and could resolve the diagnostic dilemma in difficult cases.

Identification and sizing of GAA trinucleotide repeat expansion, investigation for D-loop variations and mitochondrial deletions in Iranian patients with Friedreich's ataxia

Friedreich's Ataxia (FA) is the commonest genetic cause of ataxia and is associated with the expansion of a GAA repeat in intron 1 of the frataxin gene. Iron accumulation in the mitochondria of patients with FA would result in hypersensitivity to oxidative stress. Mitochondrial DNA (mtDNA) could be considered a candidate modifier factor for FA disease, since mitochondrial oxidative stress is thought to be involved in the pathogenesis of this disease. We studied 25 Iranian patients (16 females and 9 males) from 12 unrelated families. DNA from each patient was extracted and frequency and length of (GAA)(n) repeat was analyzed using a long-range polymerase chain reaction (PCR) test. Also we investigated impact of GAA size on neurological findings, age of onset and disease development. In order to identify polymorphic sites and genetic background, the sequence of two hypervariable regions (HVR-I and HVR-II) of mtDNA was obtained from FA patients harbouring GAA trinucletide expansions. Alignment was made with the revised cambridge reference sequence (rCRS) and any differences recorded as single base substitution (SBS), insertions and deletions. Homozygous GAA expansion was found in 21 (84%) of all cases. In four cases (16%), no expansion was observed, ruling out the diagnosis of Friedreich's ataxia. In cases with GAA expansions, ataxia, scoliosis and pes cavus, cardiac abnormalities and some neurological findings occurred more frequently than in our patients without GAA expansion. Molecular analysis was imperative for diagnosis of Friedreich's ataxia, not only for typical cases, but also for atypical ones. Diagnosis bases only on clinical findings is limited, however, it aids in better screening for suspected cases that should be tested. Our results showed that the rate of D-loop variations was higher in FA patients than control (P<0.05). mtDNA deletions were present in 76% of our patients representing mtDNA damage, which may be due to iron accumulation in mitochondria.

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.