Single Large-scale mtDNA Deletions Research Articles

The clinical, myopathological, and genetic analysis of 155 Chinese mitochondrial ophthalmoplegia patients with mitochondrial DNA single large deletions.

Progressive external ophthalmoplegia (PEO) is a common subtype of mitochondrial encephalomyopathy. The study aimed to investigate the relationship between mitochondrial DNA (mtDNA) abnormalities, muscle pathology, and clinical manifestations in Chinese patients with single large-scale mtDNA deletion presenting with PEO. This is a retrospective single-center study. Patients with PEO who had a single large deletion in mitochondrial DNA were included in this study. The associations were analyzed between mtDNA deletion patterns, myopathological changes, and clinical characteristics. In total, 155 patients with mitochondrial PEO carrying single large-scale mtDNA mutations were enrolled, including 137 chronic progressive external ophthalmoplegia (CPEO) and 18 Kearns-Sayre syndrome (KSS) patients. The onset ages were 9.61 ± 4.12 in KSS and 20.15 ± 9.06 in CPEO. The mtDNA deletions ranged from 2225 bp to 9131 bp, with m.8470_13446del being the most common. The KSS group showed longer deletions than the CPEO group (p = 0.004). Additionally, a higher number of deleted genes encoding respiratory chain complex subunits (p = 0.001) and tRNA genes (p = 0.009) were also observed in the KSS group. A weak negative correlation between the mtDNA deletion size and ages of onset (p < 0.001, r = -0.369) was observed. The proportion of ragged red fibers, ragged blue fibers, and cytochrome c negative fibers did not correlate significantly with onset ages (p > 0.05). However, a higher percentage of abnormal muscle fibers corresponds to an increased prevalence of exercise intolerance, limb muscle weakness, dysphagia, and cerebellar ataxia. We reported a large Chinese cohort consisting of mitochondrial PEO patients with single large-scale mtDNA deletions. Our results demonstrated that the length and locations of mtDNA deletions may influence onset ages and clinical phenotypes. The severity of muscle pathology could not only indicate diagnosis but also may be associated with clinical manifestations beyond the extraocular muscles.

Single-cell multi-omics of mitochondrial DNA disorders reveals dynamics of purifying selection across human immune cells.

Pathogenic mutations in mitochondrial DNA (mtDNA) compromise cellular metabolism, contributing to cellular heterogeneity and disease. Diverse mutations are associated with diverse clinical phenotypes, suggesting distinct organ- and cell-type-specific metabolic vulnerabilities. Here we establish a multi-omics approach to quantify deletions in mtDNA alongside cell state features in single cells derived from six patients across the phenotypic spectrum of single large-scale mtDNA deletions (SLSMDs). By profiling 206,663 cells, we reveal the dynamics of pathogenic mtDNA deletion heteroplasmy consistent with purifying selection and distinct metabolic vulnerabilities across T-cell states in vivo and validate these observations in vitro. By extending analyses to hematopoietic and erythroid progenitors, we reveal mtDNA dynamics and cell-type-specific gene regulatory adaptations, demonstrating the context-dependence of perturbing mitochondrial genomic integrity. Collectively, we report pathogenic mtDNA heteroplasmy dynamics of individual blood and immune cells across lineages, demonstrating the power of single-cell multi-omics for revealing fundamental properties of mitochondrial genetics.

Phenotyping mitochondrial DNA-related diseases in childhood: A cohort study of 150 patients.

Mitochondrial diseases (MDs) are heterogeneous disorders caused by mutations in nuclear DNA (nDNA) or mitochondrial DNA (mtDNA) associated with specific syndromes. However, especially in childhood, patients often display heterogeneity. Several reports on the biochemical and molecular profiles in children have been published, but studies tend not to differentiate between mtDNA- and nDNA-associated diseases, and focus is often on a specific phenotype. Thus, large cohort studies specifically focusing on mtDNA defects in the pediatric population are lacking. We reviewed the clinical, metabolic, biochemical, and neuroimaging data of 150 patients with MDs due to mtDNA alterations collected at our neurological institute over the past 20 years. mtDNA impairment is less frequent than nDNA impairment in pediatric MDs. Ocular involvement is extremely frequent in our cohort, as is classical Leber hereditary optic neuropathy, especially with onset before 12 years of age. Extraneurological manifestations and isolated myopathy appear to be rare, unlike adult phenotypes. Deep gray matter involvement, early disease onset, and specific phenotypes, such as Pearson syndrome and Leigh syndrome, represent unfavorable prognostic factors. Phenotypes related to single large scale mtDNA deletions appear to be very frequent in the pediatric population. Furthermore, we report for the first time an mtDNA pathogenic variant associated with cavitating leukodystrophy. We report on a large cohort of pediatric patients with mtDNA defects, adding new data on the phenotypical characterization of mtDNA defects and suggestions for diagnostic workup and therapeutic approach.

Modeling mitochondrial DNA diseases: from base editing to pluripotent stem-cell-derived organoids.

Mitochondrial DNA (mtDNA) diseases are multi-systemic disorders caused by mutations affecting a fraction or the entirety of mtDNA copies. Currently, there are no approved therapies for the majority of mtDNA diseases. Challenges associated with engineering mtDNA have in fact hindered the study of mtDNA defects. Despite these difficulties, it has been possible to develop valuable cellular and animal models of mtDNA diseases. Here, we describe recent advances in base editing of mtDNA and the generation of three-dimensional organoids from patient-derived human-induced pluripotent stem cells (iPSCs). Together with already available modeling tools, the combination of these novel technologies could allow determining the impact of specific mtDNA mutations in distinct human cell types and might help uncover how mtDNA mutation load segregates during tissue organization. iPSC-derived organoids could also represent a platform for the identification of treatment strategies and for probing the in vitro effectiveness of mtDNA gene therapies. These studies have the potential to increase our mechanistic understanding of mtDNA diseases and may open the way to highly needed and personalized therapeutic interventions.

Pathogenic mtDNA variants, in particular single large-scale mtDNA deletions, are strongly associated with post-lingual onset sensorineural hearing loss in primary mitochondrial disease.

In this retrospective cohort study of 193 consecutive subjects with primary mitochondrial disease (PMD) seen at the Children's Hospital of Philadelphia Mitochondrial Medicine Frontier Program, we assessed prevalence, severity, and time of onset of sensorineural hearing loss (SNHL) for PMD cases with different genetic etiologies. Subjects were grouped by genetic diagnosis: mitochondrial DNA (mtDNA) pathogenic variants, single large-scale mtDNA deletions (SLSMD), or nuclear DNA (nDNA) pathogenic variants. SNHL was audiometrically confirmed in 27% of PMD subjects (20% in mtDNA pathogenic variants, 58% in SLSMD and 25% in nDNA pathogenic variants). SLSMD had the highest odds ratio for SNHL. SNHL onset was post-lingual in 79% of PMD cases, interestingly including all cases with mtDNA pathogenic variants and SLSMD, which was significantly different from PMD cases caused by nDNA pathogenic variants. SNHL onset during school age was predominant in this patient population. Regular audiologic assessment is important for PMD patients, and PMD of mtDNA etiology should be considered as a differential diagnosis in pediatric patients and young adults with post-lingual SNHL onset, particularly in the setting of multi-system clinical involvement. Pathogenic mtDNA variants and SLSMD are less likely etiologies in subjects with congenital, pre-lingual onset SNHL.

Advanced approach for comprehensive mtDNA genome testing in mitochondrial disease

Mitochondrial disease diagnosis requires interrogation of both nuclear and mitochondrial (mtDNA) genomes for single-nucleotide variants (SNVs) and copy number alterations, both in the proband and often maternal relatives, together with careful phenotype correlation. We developed a comprehensive mtDNA sequencing test (‘MitoGenome’) using long-range PCR (LR-PCR) to amplify the full length of the mtDNA genome followed by next generation sequencing (NGS) to accurately detect SNVs and large-scale mtDNA deletions (LSMD), combined with droplet digital PCR (ddPCR) for LSMD heteroplasmy quantification. Overall, MitoGenome tests were performed on 428 samples from 394 patients with suspected or confirmed mitochondrial disease. The positive yield was 11% (43/394), including 34 patients with pathogenic or likely pathogenic SNVs (the most common being m.3243A > G in 8/34 (24%) patients), 8 patients with single LSMD, and 3 patients with multiple LSMD exceeding 10% heteroplasmy levels. Two patients with both LSMD and pathogenic SNV were detected. Overall, this LR-PCR/NGS assay provides a highly accurate and comprehensive diagnostic method for simultaneous mtDNA SNV detection at heteroplasmy levels as low as 1% and LSMD detection at heteroplasmy levels below 10%. Inclusion of maternal samples for variant classification and ddPCR to quantify LSMD heteroplasmy levels further enables accurate pathogenicity assessment and clinical correlation interpretation of mtDNA genome sequence variants and copy number alterations.

Large-single scale mitochondrial DNA deletions in different tissues in Kearns-Sayre syndrome

Objective To investigate the clinical significance of different samples (the peripheral blood, urine and skeletal muscle) that could be detected the large-scale single deletions directly by using next-generation sequencing in the diagnosis of Kearns-Sayre syndrome (KSS) by concluding the clinical and genetic features of KSS, in order to explore a non-invasive method for diagnosis. Methods The clinical data, skeletal muscle′s pathology and enzymology and genetic results of individuals with KSS, who were hospitalized from October 2016 to October 2017 in Department of Neurology, Beijing Children′s Hospital, Capital Medical University, were collected.The gene tests were performed by using next generation sequencing technology and long-PCR technology of mitochondrial DNA(mtDNA) and the whole exon in the peripheral blood, urine and skeletal muscle. Results Four patients were all consistent with the diagnosis criteria of KSS, among whom the age of onset was 8.2 years old on average, and the initial symptoms were statue, ptosis, headache and vomiting, and visual impairment.The common symptoms of the 4 cases were ophthalmoplegia, exercise intolerance, development delay, loss of appetite, hypotonia, muscle weakness, with cerebrospinal fluid protein concentration over 1 000 mg/L, the cerebral magnetic resonance imaging showed that abnormal signals in the brainstem, in addition, white matter, thalamus, basal ganglia, cerebrum and cerebellum atrophy could be found.Moreover, 3 cases had cardiac conduction block.Two cases had maternal family history.Molecular analysis of the 4 cases revealed the large-scale single deletions of mtDNA from the peripheral blood, the urine, the skeletal muscle through the next-generation sequencing, which were m. 6460-15590(9 131 bp del), m.8482-13446(4 964 bp del), m.6831-14981(8 151 bp del), m.7983-15495(7 513 bp del), respectively.Among 3 cases who did pedigree analysis, only the mother of case 4 was detected with the same variation of the proband. Conclusions KSS is a rare mitochondrial disease, which could be detected with the single large scale mtDNA deletions in the peripheral blood, urine and skeletal muscle.With the development of the methodology, the diagnosis of KSS maybe no longer than depends on the muscle biopsy with the next-generation sequencing.And the possibility to get the positive results in the peripheral blood and urine by the non-invasive method could improve the molecular diagnosis of KSS. Key words: Kearns-Sayre syndrome; Large-single scale mtDNA deletions; Next generation sequencing; Peripheral blood; Urine

MELAS and Kearns-Sayre overlap syndrome due to the mtDNA m. A3243G mutation and large-scale mtDNA deletions.

This paper reported an unusual manifestation of a 19-year-old Chinese male patient presented with a complex phenotype of mitochondrial encephalomyopathy, lactic acidosis and stroke-like episodes (MELAS) syndrome and Kearns–Sayre syndrome (KSS). He was admitted to our hospital with the chief complaint of “acute fever, headache and slow reaction for 21 days”. He was initially misdiagnosed as “viral encephalitis”. This Chinese man with significant past medical history of intolerating fatigue presented paroxysmal neurobehavioral attacks that started about 10 years ago. During this span, 3 or 4 attack clusters were described during which several attacks occurred over a few days. The further examination found that the hallmark signs of this patient included progressive myoclonus epilepsy, cerebellar ataxia, hearing loss, myopathic weakness, ophthalmoparesis, pigmentary retinopathy and bifascicular heart block (Wolff–Parkinson–White syndrome). By young age the disease progression is characterized by the addition of migraine, vomiting, and stroke-like episodes, symptoms of MELAS expression, which indicated completion of the MELAS/KSS overlap syndrome. The m. A3243G mitochondrial DNA mutation and single large-scale mtDNA deletions were found in this patient. This mutation has been reported with MELAS, KSS, myopathy, deafness and mental disorder with cognitive impairment. This is the first description with a MELAS/KSS syndrome in Chinese.

0454: Long-term cardiac prognosis and risk stratification in 260 adults presenting with mitochondrial diseases

To assess the long-term cardiac prognosis of adults with mitochondrial diseases. Between January 2000 and May 2014, we retrospectively included in this study 260 consecutive patients (60% women) ≥18 years of age, (interquartile range [IQR]; 31 to 54), with genetically proven mitochondrial diseases, including 109 with mitochondrial DNA (mtDNA) single large-scale deletions, 64 with the m.3243A>G mutation in MT-TL1, 51 with other mtDNA point mutations, and 36 patients with nuclear genes mutations. Cardiac involvement was present at baseline in 81 patients (30%). Single and multiple variable analyses were performed in search of predictors of major adverse cardiac event (MACE), and hazard ratios (HR) and 95% confidence intervals (CI) were calculated. Over a median follow-up of 7 years [3.6 to 11.7], 27 patients (10%) suffered a MACE, defined as sudden death, death due to heart failure (HF), resuscitated cardiac arrest, 3 rd degree atrioventricular block, sinus node dysfunction, cardiac transplantation, or hospitalization for management of HF. Patients with single large-scale mtDNA deletions or m.3243A>G mutations had the highest incidence of MACE. By multiple variable analysis, intraventricular conduction block (HR=16.9; 95% CI: 7.2 to 39.4), diabetes (HR=7.0; 95% CI: 2.9 to 16.7), premature ventricular complexes (HR=3.6; 95% CI: 1.4 to 9.2) and left ventricular (LV) hypertrophy (HR=2.5; 95% CI: 1.1 to 5.8) were independent predictors of MACE. In patients with 0, 1, and ≥2 risk factors, the incidence of MACE was 1.7, 15 and 42% respectively. Patients with mitochondrial diseases are at high risk of MACE, independently predicted by intraventricular conduction block, diabetes, ventricular prematurity and LV hypertrophy.

Abstract 15785: Long-term Cardiac Prognosis and Risk Stratification in 260 Adults Presenting With Mitochondrial Diseases

Introduction: The long-term cardiac prognosis of adults with mitochondrial diseases is unknown. Methods and Results: Between January 2000 and May 2014, we retrospectively included in this study 260 consecutive patients (60% women) ≥18 years of age, (interquartile range [IQR]; 31 to 54), with genetically proven mitochondrial diseases, including 109 with mitochondrial DNA (mtDNA) single large-scale deletions, 64 with the m.3243A&gt;G mutation in MT-TL1 , 51 with other mtDNA point mutations, and 36 patients with nuclear genes mutations. Cardiac involvement was present at baseline in 81 patients (30%). Single and multiple variable analyses were performed in search of predictors of major adverse cardiac event (MACE), and hazard ratios (HR) and 95% confidence intervals (CI) were calculated. Over a median follow-up of 7 years [3.6 to 11.7], 27 patients (10%) suffered a MACE, defined as sudden death, death due to heart failure (HF), resuscitated cardiac arrest, 3 rd degree atrioventricular block, sinus node dysfunction, cardiac transplantation, or hospitalization for management of HF. Patients with single large-scale mtDNA deletions or m.3243A&gt;G mutations had the highest incidence of MACE. By multiple variable analysis, intraventricular conduction block (HR = 16.9; 95% CI: 7.2 to 39.4), diabetes (HR = 7.0; 95% CI: 2.9 to 16.7), premature ventricular complexes (HR = 3.6; 95% CI: 1.4 to 9.2) and left ventricular (LV) hypertrophy (HR = 2.5; 95% CI: 1.1 to 5.8) were independent predictors of MACE. In patients with 0, 1, and ≥2 risk factors, the incidence of MACE was 1.7, 15 and 42% respectively. Conclusions: Patients with mitochondrial diseases are at high risk of MACE, independently predicted by intraventricular conduction block, diabetes, ventricular prematurity and LV hypertrophy.

Long-term cardiac prognosis and risk stratification in 260 adults presenting with mitochondrial diseases.

The aim of this study is to assess the long-term cardiac prognosis of adults with mitochondrial diseases. Between January 2000 and May 2014, we retrospectively included in this study 260 consecutive patients (60% women) ≥18 years (interquartile range 31-54), with genetically proven mitochondrial diseases, including 109 with mitochondrial DNA (mtDNA) single large-scale deletions, 64 with the m.3243A>G mutation in MT-TL1, 51 with other mtDNA point mutations, and 36 patients with nuclear gene mutations. Cardiac involvement was present at baseline in 81 patients (30%). Single and multiple variable analyses were performed in search of predictors of major adverse cardiac events (MACEs), and hazard ratios (HRs) and 95% confidence intervals (CI) were calculated. Over a median follow-up of 7 years (3.6-11.7), 27 patients (10%) suffered a MACE, defined as sudden death, death due to heart failure (HF), resuscitated cardiac arrest, third-degree atrioventricular block, sinus node dysfunction, cardiac transplantation, or hospitalization for management of HF. Patients with single large-scale mtDNA deletions or m.3243A>G mutations had the highest incidence of MACE. By multiple variable analysis, intraventricular conduction block (HR = 16.9; 95% CI: 7.2-39.4), diabetes (HR = 7.0; 95% CI: 2.9-16.7), premature ventricular complexes (HR = 3.6; 95% CI: 1.4-9.2), and left ventricular (LV) hypertrophy (HR = 2.5; 95% CI: 1.1-5.8) were independent predictors of MACEs. In patients with zero, one, and two or more risk factors, the incidences of MACE were 1.7, 15 and 42%, respectively. Patients with mitochondrial diseases are at high risk of MACE, independently predicted by intraventricular conduction block, diabetes, ventricular prematurity, and LV hypertrophy.

0370 : Long-term cardiac prognosis and risk stratification in 260 adults presenting with mitochondrial diseases

The long-term cardiac prognosis of adults with mitochondrial diseases is unknown. Between January 2000 and May 2014, we retrospectively included in this study 260 consecutive patients (60% women) ε18 years of age, [interquartile range (IQR) = 31 to 54], with genetically proven mitochondrial diseases, including 109 with mtDNA single large-scale deletions, 64 with the m.3243A>G mutation in MT-TL1, 51 with other mtDNA point mutations, and 36 patients with nuclear genes mutations. Cardiac involvement was present at baseline in 81 patients (30%). Single and multiple variable analyses were performed in search of predictors of major adverse cardiac event (MACE), and hazard ratios (HR) and 95% confidence intervals (CI) were calculated. Over a median follow-up of 7 years (IQR = 3.6 to 11.7), 27 patients (10%) experienced a MACE, defined as sudden death, death due to heart failure (HF), resuscitated cardiac arrest, 3rd degree atrioventricular block, sinus node dysfunction, cardiac transplantation, or hospitalization for management of HF. Patients with single large-scale mtDNA deletions or m.3243A>G mutations had the highest incidence of MACE. By multiple variable analysis, intraventricular conduction block (HR = 16.9; 95% CI: 7.2 to 39.4), diabetes (HR = 7.0; 95% CI: 2.9 to 16.7), premature ventricular complexes (HR = 3.6; 95% CI: 1.4 to 9.2) and left ventricular hypertrophy (HR=2.5; 95% CI: 1.1 to 5.8) were independent predictors of MACE. In patients with 0, 1, and ε2 risk factors, the incidence of MACE was 1.7, 15 and 42% respectively. Patients with mitochondrial diseases are at high risk of MACE, independently predicted by intraventricular conduction block, diabetes, ventricular prematurity and left ventricular hypertrophy.

The investigation and diagnosis of pathogenic mitochondrial DNA mutations in human urothelial cells

Patients with mitochondrial DNA disease are amongst the most challenging to diagnose and manage given the striking phenotypic and genetic heterogeneity, which characterise these conditions. Recently, we and others have demonstrated the m.3243A>G mutation, one of the most common mitochondrial DNA pathogenic mutations, is present at clinically relevant levels in urinary epithelium, thus providing a practical, non-invasive test for diagnosis and mutation screening. In this study we further evaluate the use of these cells in detecting the m.3243A>G mutation, other mtDNA tRNA gene point mutations including the m.8344A>G mutation and single large-scale mtDNA deletions. We observe a robust relationship between m.3243A>G levels in urothelial cells and clinically affected tissues that does not change with time. Conversely, single large-scale mtDNA deletions can be detected in urothelial cells, with higher levels present in younger patients with more severe disease, but generally mtDNA deletion levels are not representative of those seen in a clinically affected tissue. Our results have implications for the diagnosis, management and counselling of families with mtDNA disease.

Response To Prolonged Endurance Training and Detraining In Mitochondrial Myopathy

Patients with mitochondrial myopathies (MM) due to mtDNA mutations in skeletal muscle often suffer disabling exercise intolerance. Our previous studies suggest that short duration (14 weeks) endurance training improves exercise capacity and increases the activity of rate-limiting enzymes. The effect of a longer duration exercise training has not been studied. PURPOSE: To assess the effect of 6 months of regular endurance training on exercise capacity and to determine if training adaptations are maintained after a prolonged period of self-selected activity in a patient with severe muscle oxidative impairment due to a mitochondrial myopathy. METHODS: A 26-year male with a heteroplasmic single large-scale deletion of mtDNA underwent 26 weeks of cycle exercise training followed by a 52 week period of self-selected activity level. Peak work, oxygen uptake (VO2), and submaximal heart rate and blood lactate were determined at baseline, after training and following 52 weeks of self-determined activity. RESULTS: Compared to baseline, training increased peak work from 110 to 130 watts and VO2 from 1.527 to 1.837 L/min. During submaximal (50 watt) exercise, peak heart rate decreased from 153 to 120 bpm and venous lactate from 6.41 to 3.25 mM. After 52 weeks of self-selected activity following 6 months of endurance training, peak work and VO2 fell to 110 watts and 1.684 L/min respectively. During submaximal exercise, after the self-selected activity period compared to post training, heart rate (133 vs. 120 bpm) and lactate levels (4.36 vs. 3.25 mM) were higher. CONCLUSION: Endurance training in MM improved work and oxidative capacity supporting the efficacy of this therapy in patients with mitochondrial myopathy due to single large-scale mtDNA deletions. Despite the substantial benefits of exercise training, the patient did not choose to maintain regular exercise training and after the 52 week period of self-selected activity, exercise capacity declined to near pre-training levels indicating that deconditioning contributes to exercise limitations in these patients and that the applicability of exercise training as therapy is limited by patient motivation.

Endurance training and detraining in mitochondrial myopathies due to single large-scale mtDNA deletions

At present there are limited therapeutic interventions for patients with mitochondrial myopathies. Exercise training has been suggested as an approach to improve physical capacity and quality of life but it is uncertain whether it offers a safe and effective treatment for patients with heteroplasmic mitochondrial DNA (mtDNA) mutations. The objectives of this study were to assess the effects of exercise training and detraining in eight patients with single, large-scale mtDNA deletions to determine: (i) the efficacy and safety of endurance training (14 weeks) in this patient population; (ii) to determine the effect of more prolonged (total of 28 weeks) exercise training upon muscle and cardiovascular function and (iii) to evaluate the effect of discontinued training (14 weeks) upon muscle and cardiovascular function. Our results show that: (i) 14 weeks of exercise training significantly improved tolerance of submaximal exercise and peak capacity for work, oxygen utilization and skeletal muscle oxygen extraction with no change in the level of deleted mtDNA; (ii) continued training for an additional 14 weeks maintained these beneficial adaptations; (iii) the cessation of training (detraining) resulted in loss of physiological adaptation to baseline capacity with no overall change in mutation load. Patients' self assessment of quality of life as measured by the SF-36 questionnaire improved with training and declined with detraining. Whilst our findings of beneficial effects of training on physiological outcome and quality of life without increases in the percentage of deleted mtDNA are encouraging, we did not observe changes in mtDNA copy number. Therefore there remains a need for longer term studies to confirm that endurance exercise is a safe and effective treatment for patients with mitochondrial myopathies. The effects of detraining clearly implicate physical inactivity as an important mechanism in reducing exercise capacity and quality of life in patients with mitochondrial myopathy.

Prevalence of large-scale mitochondrial DNA deletions in an adult Finnish population

Large-scale mitochondrial DNA (mtDNA) deletions are associated with clinical conditions such as Kearns-Sayre syndrome and chronic progressive external ophthalmoplegia in adults and Pearson syndrome in children. Reported case series have suggested that deletions are not uncommon in the population, but their prevalence has not been documented. The authors ascertained patients with clinical features associated with mtDNA deletions in a defined adult population in northern Finland. Buccal epithelial samples were requested from each patient fulfilling the selection criteria, and full-length mtDNA was amplified using the long PCR method. Deletion breakpoints were identified using sequencing. Patients with deletions were examined clinically. The authors identified four patients with single large-scale mtDNA deletions. The prevalence of deletions was calculated to be 1.6/100,000 in the adult population in the province of Northern Ostrobothnia (0.0 to 3.2; 95% CI). Analysis of incident cases from a neighboring province revealed two patients with deletions and yielded a similar population frequency. The frequency of large-scale mitochondrial DNA deletions is similar among populations, suggesting that there is a constant rate of new deletions.

Mitochondrial DNA Analysis in Ocular Myopathy

We analyzed 29 patients with progressive external ophthalmoparesis (PEO) either alone or as part of a multisystem disorder. Ragged-red fibers were very abundant (10–20%) in 15 patients, and many of them were also cytochrome c oxidase-negative. Biochemical analysis of the respiratory chain showed partial defects of single or multiple complexes in 18 patients (64%). Eleven PEO patients (38%) harbored single large-scale mtDNA deletions in muscle, which averaged 5.4 kb in size and 47% in relative abundance. One PEO patient harbored the A3243G mutation (MELAS mutation) in muscle (63%). Our findings, the first reported in Portuguese patients, confirm that single large-scale mtDNA deletions are a significant cause of PEO. Although ophthalmoparesis was the main clinical feature in the majority of patients, the clinical spectrum is broad, ranging from severe encephalopathy of childhood to a milder, though disabling, muscle weakness in adults.

Atypical clinical presentations associated with the MELAS mutation at position 3243 of human mitochondrial DNA

Mitochondrial encephalopathy, lactic acidosis and stroke-like episodes (MELAS) is commonly associated with an A → G transition at position 3243 of the mitochondrial DNA. To determine the diversity of clinical syndromes associated with this mutation, 91 patients with mitochondrial encephalomyopathies that did not conform to the MELAS phenotype were screened. Twenty one patients with the 3243 mutation, most of whom had progressive external ophthalmoplegia (PEO) were found. Clinical features did not distinguish PEO patients with the 3243 mutation from those with large-scale deletions of mtDNA. However, most cases with single large-scale mtDNA deletions were sporadic, whereas most patients with the 3243 mutation had affected maternal relatives. Histochemical studies of muscle showed that cytochrome c oxidase (COX) deficiency was more severe in patients with PEO than in patients with typical MELAS, even though PEO patients had a lower percentage of mutant genomes in muscle. These data imply that the 3243 mutation is a major cause of familial PEO, and suggests that the threshold number of mtDNAs harboring the 3243 mutation necessary to affect a particular tissue vary in different patients. The proportion of mutant genomes in combination with other, still undefined, tissue-specific modulating factors seem to determine the overall clinical syndrome.