Genetic biochemical and pathophysiological characterization of a familial mitochondrial encephalomyopathy (MERRF)

Cardiovascular Outcomes in Patients With Mitochondrial Disease in the United States: A Propensity Score Analysis

Mitochondrial disease comprises a wide range of genetic disorders caused by mitochondrial dysfunction. Its rarity, however, has limited the ability to assess its effects on clinical outcomes. To evaluate this relationship, we collected data from the 2016 National Inpatient Sample, which includes data from >7 million hospital stays. We identified 705 patients (mean age, 22 ± 20.7 yr; 54.2% female; 67.4% white) whose records included the ICD-10-CM code E88.4. We also identified a propensity-matched cohort of 705 patients without mitochondrial disease to examine the effect of mitochondrial disease on major adverse cardiovascular events, including all-cause in-hospital death, cardiac arrest, and acute congestive heart failure. Patients with mitochondrial disease were at significantly greater risk of major adverse cardiovascular events (odds ratio [OR]=2.42; 95% CI, 1.29-4.57; P=0.005), systolic heart failure (OR=2.37; 95% CI, 1.08-5.22; P=0.027), and all-cause in-hospital death (OR=14.22; 95% CI, 1.87-108.45; P<0.001). These findings suggest that mitochondrial disease significantly increases the risk of inpatient major adverse cardiovascular events.

MERRF Classification: Implications for Diagnosis and Clinical Trials

BACKGROUND

Given the etiologic heterogeneity of disease classification using clinical phenomenology, we employed contemporary criteria to classify variants associated with myoclonic epilepsy with ragged-red fibers (MERRF) syndrome and to assess the strength of evidence of gene-disease associations. Standardized approaches are used to clarify the definition of MERRF, which is essential for patient diagnosis, patient classification, and clinical trial design.

METHODS

Systematic literature and database search with application of standardized assessment of gene-disease relationships using modified Smith criteria and of variants reported to be associated with MERRF using modified Yarham criteria.

RESULTS

Review of available evidence supports a gene-disease association for two MT-tRNAs and for POLG. Using modified Smith criteria, definitive evidence of a MERRF gene-disease association is identified for MT-TK. Strong gene-disease evidence is present for MT-TL1 and POLG. Functional assays that directly associate variants with oxidative phosphorylation impairment were critical to mtDNA variant classification. In silico analysis was of limited utility to the assessment of individual MT-tRNA variants. With the use of contemporary classification criteria, several mtDNA variants previously reported as pathogenic or possibly pathogenic are reclassified as neutral variants.

CONCLUSIONS

MERRF is primarily an MT-TK disease, with pathogenic variants in this gene accounting for ~90% of MERRF patients. Although MERRF is phenotypically and genotypically heterogeneous, myoclonic epilepsy is the clinical feature that distinguishes MERRF from other categories of mitochondrial disorders. Given its low frequency in mitochondrial disorders, myoclonic epilepsy is not explained simply by an impairment of cellular energetics. Although MERRF phenocopies can occur in other genes, additional data are needed to establish a MERRF disease-gene association. This approach to MERRF emphasizes standardized classification rather than clinical phenomenology, thus improving patient diagnosis and clinical trial design.

[Cardiac manifestations of mitochondrial diseases]

Mitochondrial diseases are multi-system disorders in relation with mitochondrial DNA and/or nuclear DNA abnormalities. Clinical pictures are heterogeneous, involving endocrine, cardiac, neurologic or sensory systems. Cardiac involvements are morphological and electrical disturbances. Prognosis is worsened in case of cardiac impairment. Treatments are related to the type of cardiac dysfunction including medication or pacemaker implantation.

Cardiac involvement in mitochondrial DNA disease: clinical spectrum, diagnosis, and management

Mitochondrial disease refers to a heterogenous group of genetic disorders that result from dysfunction of the final common pathway of energy metabolism. Mitochondrial DNA mutations affect key components of the respiratory chain and account for the majority of mitochondrial disease in adults. Owing to critical dependence of the heart on oxidative metabolism, cardiac involvement in mitochondrial disease is common and may occur as the principal clinical manifestation or part of multisystem disease. Recent advances in our understanding of the clinical spectrum and genetic aetiology of cardiac involvement in mitochondrial DNA disease have important implications for cardiologists in terms of the investigation and multi-disciplinary management of patients.

Hearing loss in children with mitochondrial disorders

OBJECTIVE

At least 1-5 children per 1000 suffer from congenital hearing loss, and 50% of these cases can be attributed to genetic causes. It has been estimated that 1% of pre-lingual hearing loss is due to mutations in mitochondrial DNA. Previous literature reports audiometric data for few patients, usually less than 20 per study. The goal of this study was to characterize the hearing loss associated with mitochondrial mutations and determine whether previously characterized patterns of hearing loss in these patients (progressive, sensorineural, high frequency losses) are found in our population as well.

METHODS

An IRB-approved retrospective chart review of the electronic medical records in the Nemours/Alfred I. dupont Hospital for Children system from January 2004 to October 2009 (a five-year period) was undertaken using ICD-9 codes 277.87 (mitochondrial disorder) and 359.89BA (mitochondrial myopathy). These 149 records were then evaluated for audiologic data, resulting in 26 charts with both a mitochondrial disorder and hearing evaluation.

RESULTS

Of 26 patients with known mitochondrial disorders and audiometric documentation, 15 (58%) had hearing loss, and 11 patients had normal hearing (42%). Ten patients had sensorineural hearing loss (38%), two patients had conductive hearing loss (7.7%), one patient had a mixed hearing loss (3.8%), and two patients had an as yet undefined hearing loss (ABR had not yet been performed at the time of this study) (7.7%).

CONCLUSION

In comparison with previous studies, generally including less than 20 patients, this is one of the largest collections of audiometric data on children with mitochondrial disorders. Unlike prior studies describing a progressive, sensorineural loss across all frequencies or mainly affecting high frequencies, the hearing loss in our patients was more variable including low frequency losses, mid-frequency losses, and conductive losses and was often not progressive or even improved. Our overall 38% rate of sensorineural hearing loss correlates well with previous case series; this study clearly justifies the use of routine audiometric screening in children with mitochondrial disorders, including use of ABR and OAEs as ASND can be seen in this population, as well as repeat testing over time to evaluate for progression.

Mitochondrial DNA mutations and human disease

Mitochondrial disorders are a group of clinically heterogeneous diseases, commonly defined by a lack of cellular energy due to oxidative phosphorylation (OXPHOS) defects. Since the identification of the first human pathological mitochondrial DNA (mtDNA) mutations in 1988, significant efforts have been spent in cataloguing the vast array of causative genetic defects of these disorders. Currently, more than 250 pathogenic mtDNA mutations have been identified. An ever-increasing number of nuclear DNA mutations are also being reported as the majority of proteins involved in mitochondrial metabolism and maintenance are nuclear-encoded. Understanding the phenotypic diversity and elucidating the molecular mechanisms at the basis of these diseases has however proved challenging. Progress has been hampered by the peculiar features of mitochondrial genetics, an inability to manipulate the mitochondrial genome, and difficulties in obtaining suitable models of disease. In this review, we will first outline the unique features of mitochondrial genetics before detailing the diseases and their genetic causes, focusing specifically on primary mtDNA genetic defects. The functional consequences of mtDNA mutations that have been characterised to date will also be discussed, along with current and potential future diagnostic and therapeutic advances.

Leigh disease with mitochondrial DNA A8344G mutation: case report and brief review

Leigh disease, subacute necrotizing encephalomyelopathy, is a neurodegenerative disorder often seen in infancy or childhood but rarely reported in adults. Genetic heterogeneity is well recognized, and the associated etiologies include both mitochondrial and nuclear DNA defects. We describe an infant presenting with developmental delay and then progressive multisystem disorder and neuroradiologic features of Leigh disease. He and his maternal relatives all have the A8344G mitochondrial DNA mutation. However, only minor clinical features are seen in his maternal relatives, with migraine being the most common problem. Additionally the A8344G mitochondrial DNA mutation is associated with spinocerebellar degeneration, other nonspecific mitochondrial encephalomyopathies, atypical Charcot-Marie-Tooth disease, and progressive external ophthalmoplegia. The A8344G mitochondrial DNA mutation may present with Leigh disease or other different atypical clinical features without myoclonic epilepsy and ragged red fibers.

Cardiovascular Complications of Neuromuscular Disorders

In the past decade, advances in molecular genetics have shown that many familial neuromuscular and cardiovascular diseases share a common pathophysiology. They are caused by inherited mutations in the cellular cytoskeleton of cardiac and skeletal muscle cells. The clinical manifestation of cardiac disease in neuromuscular disorders is common and their management should include both periodic cardiac assessment and appropriate symptomatic and definitive therapy. Dilated cardiomyopathy is a common complication of neuromuscular diseases. Cardiac function may decline progressively as part of the natural history of the disease, but current medical therapy, including angiotensin-converting enzyme inhibitors, beta-blockers, and diuretics, can be used to alleviate symptoms of left ventricular dysfunction. Conduction disturbances may be an important cause of mortality, especially in patients with Emery Dreifuss muscular dystrophy, Kearns-Sayre syndrome, and myotonic dystrophy, and thus pacemaker implantation can be life-saving. Rhythm disturbances, such as atrial fibrillation and ventricular tachyarrhythmias, have been reported in patients with neuromuscular diseases. Treatment is based on preventing sudden death and embolic phenomena and cardioverting or controlling atrial fibrillation. In these patients, problems may arise with anticoagulation and antiarrhythmic therapy due to the inherent locomotor instability associated with the disease, and the presence of concomitant atrioventricular disease. Although uncommon, hypertrophic cardiomyopathy may be a feature of some neuromuscular disorders. Patients should undergo regular risk stratification for sudden cardiac death and symptoms such as heart failure can be treated with medical therapy.

Comparative proteomics as a new tool for exploring human mitochondrial tRNA disorders

More than 70 different point mutations in human mitochondrial tRNA genes are correlated with severe disorders, including fatal cardiopathies, encephalopathies, myopathies, and others. So far, investigation of the molecular impact(s) of mutations has focused on the affected tRNA itself by seeking structural and/or functional perturbations capable of interfering with synthesis of the 13 mitochondrion-encoded subunits of respiratory chain complexes. Here, a proteomic approach was used to investigate whether such mutations would affect the pattern of mitochondrial proteins at a broader level. Analysis of several hundred mitochondrial proteins from sibling cybrid cell lines by two-dimensional electrophoresis, an approach that takes into account all regulatory steps of mitochondrial and nuclear gene expression, indeed reveals a number of up- and downregulated proteins when healthy and single-point-mutation-carrying mitochondria representative of either MELAS or MERRF syndrome were compared. Assignment by mass spectrometry of the two proteins which exhibit obvious large quantitative decreases in the levels of both pathologic mitochondria identified nuclear-encoded subunits of cytochrome c oxidase, a respiratory chain complex. This clearly shows a linkage between the effects of mutations in mitochondrial tRNA genes and the steady-state level of nuclear-encoded proteins in mitochondria. It opens new routes toward a large-scale exploration of potential proteic partners involved in the genotype-phenotype correlation of mitochondrial disorders.

Thyroid hormone regulates oxidative phosphorylation in the cerebral cortex and striatum of neonatal rats

We have previously shown that thyroid hormone (T(3)) regulates mitochondrial gene expression, morphology and transmembrane potential in the developing brain. Here, we have analysed the effect of thyroid hormone on mitochondrial function in different brain regions. For this purpose we have determined, in control, hypothyroid and T(3)-treated hypothyroid neonatal rats, the rate of oxidative phosphorylation in isolated mitochondria and the activity of the respiratory complexes in tissue homogenates. Our results showed a decrease in oxidative phosphorylation rate (only in the presence of NADH-generating substrates) and mitochondrial complexes I and III activity in the cerebral cortex and striatum of hypothyroid neonates, but not in the other areas analysed (hippocampus, cerebellum, thalamus, mid brain and brain stem). In parallel with mitochondrial activity, the levels of mitochondrially encoded transcripts were decreased only in the cerebral cortex and striatum of hypothyroid rats. The administration of T(3) corrected all these parameters. In summary, this study showed a down-regulation of mitochondrial gene expression accompanied by a decrease in mitochondrial activity in the cerebral cortex and striatum of developing hypothyroid neonatal rats.

Clinical differences in patients with mitochondriocytopathies due to nuclear versus mitochondrial DNA mutations

Defects in oxidative phosphorylation (OXPHOS) are genetically unique because the different components involved in this process, respiratory chain enzyme complexes (I, III, and IV) and complex V, are encoded by nuclear and mitochondrial genome. The objective of the study was to assess whether there are clinical differences in patients suffering from OXPHOS defects caused by nuclear or mitochondrial DNA (mtDNA) mutations. We studied 16 families with > or = two siblings with a genetically established OXPHOS deficiency, four due to a nuclear gene mutation and 12 due to a mtDNA mutation. Siblings with a nuclear gene mutation showed very similar clinical pictures that became manifest in the first years (ranging from first months to early childhood). There was a severe progressive course. Seven of the eight children died in their first decade. Conversely, siblings with a mtDNA mutation had clinical pictures that varied from almost alike to very distinct. They became symptomatic at an older age (ranging from childhood to adulthood), with the exception of defects associated with Leigh or Leigh-like phenotype. The clinical course was more gradual and relatively less severe; four of the 26 patients died, one in his second year, another in her second decade and two in their sixth decade. There are differences in age at onset, severity of clinical course, outcome, and intrafamilial variability in patients affected of an OXPHOS defect due to nuclear or mtDNA mutations. Patients with nuclear mutations become symptomatic at a young age, and have a severe clinical course. Patients with mtDNA mutations show a wider clinical spectrum of age at onset and severity. These differences may be of importance regarding the choice of which genome to study in affected patients as well as with respect to genetic counseling.

Mitochondrial defects in cardiomyopathy and neuromuscular disease

Over the past 11 years, a considerable body of evidence has accumulated implicating defects in the mitochondrial energy-generating pathway, oxidative phosphorylation, in a wide variety of degenerative diseases including myopathy and cardiomyopathy. Most classes of pathogenic mitochondrial DNA mutations affect the heart, in association with a variety of other clinical manifestations that can include skeletal muscle, the central nervous system (including eye), the endocrine system, and the renal system. To better understand the pathophysiologic basis of mitochondrial diseases and their role in myopathy and cardiomyopathy, several mouse models of mitochondrial disease have been prepared. Mitochondrial DNA mutations from cultured cells have been introduced into mice; nuclear DNA genes involved in mitochondrial energy production and reactive oxygen species detoxification have been genetically inactivated, which resulted in mice with hypertrophic and dilated cardiomyopathy, respectively. Physiologic characterization of these mice has confirmed the importance of decreased mitochondrial energy production, increased mitochondrial reactive oxygen species production, and the mitochondrial initiation of apoptosis in mitochondrial disease. With these insights, new therapeutic approaches for neuromuscular and cardiac disease have been suggested.

Utility of multimodal evoked potential study and electroencephalography in mitochondrial encephalomyopathy

We performed electroencephalography (EEG) and multimodal evoked potential (EP) studies in 16 patients with various forms of mitochondrial encephalomyopathy (ME). The electrophysiological investigations revealed signs of involvement of the peripheral and central nervous system (CNS) in 14 patients, with a high incidence of visual-EP (VEP) alterations, indicative of visual pathway vulnerability in mitochondrial diseases. No specific pattern of abnormalities emerged and, in particular, clinical and laboratory findings did not correlate with each other. EP (particularly VEP and electroretinogram) investigations should be part of the diagnostic work-up of patients with mitochondrial disorders in order to better characterize the clinical picture, disclose involvement of specific sensory systems of the CNS, and assess patients with atypical clinical presentations.

Pathophysiology of the MELAS 3243 transition mutation

Single base substitutions of the mitochondrial genome are associated with a variety of metabolic disorders. The myopathy, encephalopathy, lactic acidosis, stroke-like episodes syndrome, most frequently associated with an A to G transition mutation at position 3243 of the mitochondrial tRNALeu(UUR) gene, is characterized by biochemical and structural alterations of mitochondria. To investigate the pathophysiology of the mutation, we established distinct Epstein-Barr virus-transformed B-cell lines for analyses that harbored 30-70% of the mutated genome. Interestingly, neither an alteration of the processing of primary transcripts nor a general impairment of individual mitochondrial protein subunit synthesis rates could be observed. Nevertheless a marked decrease of cytochrome-c oxidase activity and reduced content of mitochondrial encoded subunits in the assembled respiratory complex IV was recorded on the cell line harboring 70% mutated mtDNA. Quantitative analysis of incorporation rates of the amino acid leucine into newly synthesized mitochondrial proteins, representing the functionality of the tRNALeu(UUR) in protein biosynthesis, revealed a specific decrease of this amino acid in distinct mitochondrial translation products. This observation was supported by a variation in the proteolytic fingerprint pattern. Our results suggest that the malfunctioning mitochondrial tRNALeu(UUR) leads to an alteration of amino acid incorporation into the mitochondrially synthesized subunits of the oxidative phosphorylation system, thus altering it's structure and function.

Gene dosage effect in one family with myoclonic epilepsy and ragged-red fibers (MERRF)

OBJECTIVES

We analyzed the percentage of mitochondrial DNA (mtDNA) heteroplasmy in blood samples of 13 individuals belonging to a three family generation of myoclonic epilepsy with ragged-red fibers (MERRF) and compared the 5 affected patients and the 8 unaffected relatives.

MATERIAL AND METHODS

DNA was extracted from blood and muscle of the proband and from blood of 12 maternal relatives. A PCR restriction analysis method was used to detect the mutation.

RESULTS

The proband had the complete MERRF phenotype. The phenotype in three other individuals in the maternal lineage was consistent with the MERRF syndrome. The remaining were asymptomatic. The np 8344 mutation was observed in muscle and blood of the proband, and in blood from every one of 12 maternal relatives, ranging from 44% to 83% of mutated genomes. Symptomatic individuals had higher levels (P < 0.001) of mutated mtDNA than asymptomatic maternal relatives. However, high proportions of mutant genomes (up to 63%) were found in asymptomatic relatives.

CONCLUSIONS

Although there seems to be a gene dosage effect in MERRF, we found no absolute relationship between the relative proportion of mutant genomes in blood and clinical severity. Factors other than gene dosage in blood may account for the differences in clinical phenotype.

Mitochondrial dysfunction with myoclonus epilepsy and ragged-red fibers point mutation in nerve, muscle, and adipose tissue of a patient with multiple symmetric lipomatosis

We report a 64-year-old man presenting with multiple symmetric lipomatosis (MSL) and mitochondrial encephalomyoneuropathy. The diagnosis of a mitochondrial cytopathy was based on the typical clinical symptoms and signs, including chronic progressive external ophthalmoplegia, hearing impairment, cerebellar ataxia, proximal myopathy, and polyneuropathy, and on molecular genetic and histological examinations. As a unique finding, the A-->G(8344) myoclonus epilepsy and ragged-red fibers point mutation was found in peripheral nerve, muscle, and adipose tissue. Muscle biopsy revealed multiple ragged-red fibers and other morphological signs of a mitochondrial myopathy. Sural nerve biopsy demonstrated a mixed axonal and demyelinating neuropathy with extensive loss of myelinated fibers and conspicuous onion bulb formations, as well as structural mitochondrial abnormalities on electron microscopy. These findings clearly demonstrate mitochondrial dysfunction in muscle, adipose tissue, and for the first time also in nervous tissue of an MSL patient, and strongly support the concept of mitochondrial cytopathy as one of the possible causes of multiple symmetric lipomatosis.

Biochemical analysis of fibroblasts from patients with cytochrome c oxidase-associated Leigh syndrome

Cultured skin fibroblasts from four patients with Leigh syndrome and cytochrome c oxidase deficiency were studied. Mitochondrial DNA (mtDNA) analysis excluded large-scale deletions and known point mutations associated with Leigh syndrome. The COX activities were reduced to 18-44% of healthy probands, when measured in the presence of laurylmaltoside. COX activity from patients was shown to be more temperature sensitive than COX activity from control cells. In order to determine the subunit composition of COX immunoblotting studies were performed using mono- and polyclonal antibodies to distinct subunits. A monoclonal antibody to subunit IV crossreacted with two unknown proteins of higher apparent molecular weight in mitochondria from three patients, but not in mitochondria from control and the fourth patient. Quantification of immunoreactivity revealed a decrease of subunits II/III and IV parallel to the determined enzyme activity. In contrast, a variable amount of subunit VIIa (and/or VIIb) was found in mitochondria from different patients. The results indicate a defective COX holoenzyme complex in patients with Leigh syndrome and suggest different molecular origins of the defect.

Investigation on the mitochondrial transfer RNA(Leu)(UUR) in blood cells from patients with cluster headache

Various mutations in the mitochondrial tRNA(Leu)(UUR) gene give rise to a variety of neurological disorders. Among these, mitochondrial encephalomyopathy, lactic acidosis and stroke-like episodes (MELAS syndrome) are frequently associated with a tRNA(Leu)(UUR) mutation at nucleotide position 3243 of the mitochondrial DNA. A supplementary clinical feature seen in these patients is headache in early life. Recently, a tRNA(Leu)(UUR) mutation at nucleotide position 3243 has been found in a patient presenting with cluster headache. This led us to examine the mitochondrial genomes of 22 patients presenting with cluster headache. None of the patients harboured the reported tRNA(Leu)(UUR) mutation or any other length variations of the mtDNA. Cluster headache is most likely not causally associated with the A3243G mutation of the mitochondrial DNA.

The 8,344 mutation in mitochondrial DNA: a comparison between the proportion of mutant DNA and clinico-pathologic findings

Ten patients, two men and eight women with mitochondrial encephalomyopathy, had an A-G mutation at nucleotide pair 8,344 in the mitochondrial DNA, the most common genetic defect in myoclonus epilepsy with ragged-red fibers (MERRF). Eight patients had the clinical and pathologic characteristics of MERRF including myoclonus, seizures, cerebellar ataxia and myopathy with ragged-red fibers. Two patients had atypical symptoms such as early onset of fatal cardiac failure and late onset of rapid mental deterioration, respectively. The striking feature in our patients with the 8,344 mutation cardiac involvement and two developed progressive heart failure. In the typical MERRF patients, the proportion of mutant mitochondrial DNA in their skeletal muscles, quantified by a single strand conformation polymorphism analysis, was above 85%. However, there was no significant correlation between clinical severity, histopathological findings and the proportion of mutant mtDNA in muscle biopsy samples, suggesting that non-ragged-red fibers play an important role in the phenotype expression of the mutants.

Human aging is associated with stochastic somatic mutations of mitochondrial DNA

Deletions and point mutations of mitochondrial DNA (mtDNA), which are characteristic of various human mitochondrial diseases, have been identified mainly in postmitotic tissues like brain, heart and skeletal muscle of healthy humans of advanced age but not in young people. An exponential increase with age was described for deletions of mtDNA. This paper reviews the molecular basis and experimental results on mutations of mtDNA in patients with mitochondrial diseases and in aged individuals. In addition new data on the exponential increase of point mutations of mtDNA, characteristic for MERRF and MELAS disease, in extraocular muscle from elderly humans are shown. Finally the 'mitochondrial hypothesis on aging' based on stochastic somatic mutations of mtDNA is presented.

Tissue distribution and disease manifestations of the tRNA(Lys) A-->G(8344) mitochondrial DNA mutation in a case of myoclonus epilepsy and ragged red fibres

This man with myoclonus epilepsy and ragged red fibres (MERRF) syndrome due to the tRNA(Lys) A-->G(8344) mutation of mitochondrial DNA (mtDNA) died of bronchopneumonia at 18 years of age. He had progressive clinical symptoms from 6 months of age manifesting as ataxia, myoclonic seizures, and muscle weakness. A post-mortem examination revealed 91-99% mutated mtDNA in all 32 examined tissue samples, including various organs and different brain regions. The brain appeared without macroscopic changes, but microscopic examination showed degeneration with loss of nerve cells and gliosis affecting the globus pallidus, substantia nigra, red nucleus, dentate nucleus, inferior olivary nucleus, cerebellar cortex, and the spinal cord. Skeletal muscle showed cytochrome c oxidase deficient muscle fibres with proliferation of mitochondria. In addition to pathological changes of muscle and brain there were few morphological changes that could be attributed to his mitochondrial disease. These data support the concept that in patients with the tRNA(Lys) A-->G(8344) mutation who are manifesting disease there are high levels of mutated mtDNA in all tissues, but only some tissues and brain regions are vulnerable.

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

BACKGROUND

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

METHODS AND RESULTS

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

CONCLUSIONS

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

Transfection of mitochondria: strategy towards a gene therapy of mitochondrial DNA diseases

Successes in classical gene therapies have been achieved by placing a corrected copy of a defective nuclear gene in cells. A similar gene replacement approach for a mutant mitochondrial genome is invariably linked to the use of a yet unavailable mitochondrial transfection vector. Here we show that DNA coupled covalently to a short mitochondrial leader peptide (chimera) can enter mitochondria via the protein import pathway, opening a new way for gene-, antisense-RNA- or antisense-DNA-delivery in molecular therapies. The import behavior of the purified chimera, composed of the amino-terminal leader peptide of the rat ornithine transcarbamylase (OTC) and a double stranded DNA molecule (17 bp or 322 bp), was tested by incubating with coupled and 'energized' rat liver mitochondria in the presence of reticulocyte lysate. The chimera was translocated with a high efficiency into the matrix of mitochondria utilizing the protein import pathway, independent from the size of its passenger DNA.

Immunohistochemical analysis of muscle cytochrome c oxidase deficiency in children

Despite the demonstration of a clear biochemical defect, the genetic alterations causing childhood forms of cytochrome c oxidase (COX) deficiency remain unknown. The double genetic origin (nuclear and mitochondrial DNA), and the complexity of COX enzyme structure and regulation, indicate the need for genetic investigations of the molecular structure of individual COX subunits. In the present study a new monoclonal antibody, which reacts exclusively with heart-type human COX subunit VIIa (VIIa-H), and other monoclonal antibodies against human COX subunits, were used in the immunohistochemical analysis of skeletal muscle from children with different forms of mitochondrial myopathy with COX deficiency. By immunohistochemical investigation a normal reaction was seen with antibodies to COX subunits IV, Va+Vb, and VIa+VIc in all four cases, and in two cases with antibodies to COX VIIa-H and VIIa+VIIb. In muscle from a fatal infantile case with cardiac and skeletal muscle involvement, no immunohistochemical reaction was seen with the monoclonal antibody against the tissue-specific subunit VIIa-H. In muscle from an 11-year-old boy with exclusive muscular symptoms and signs, immunohistological reactions were absent with COX subunit VIIa-H and COX subunits VIIa+VIIb, and slightly decreased with COX subunit II, thus demonstrating a different molecular mechanism in each case. It is concluded that the molecular basis of COX deficiency in childhood may vary greatly between patients.

Pathogenetic aspects of the A8344G mutation of mitochondrial DNA associated with MERRF syndrome and multiple symmetric lipomas

Myoclonus epilepsy and ragged-red fibers syndrome (MERRF) is caused by a heteroplasmic mutation at nucleotide 8344 (A8344G) of the tRNA(Lys) gene of mitochondrial DNA (mtDNA). This mutation impairs mitochondrial protein synthesis and causes a respiratory chain dysfunction. The risk for transmission of the A8344G mutation from mother to child is dependent on the levels of mutated mtDNA in the mother and above a threshold level of 35-40% the mutation is transmitted to all children. The progression of symptoms in MERRF can be explained by a gene dosage effect with accumulation over time of mutated mtDNA. High levels of mutated mtDNA, ultrastructurally abnormal mitochondria, and a clonal deletion on chromosome 6 are found in lipomas associated with MERRF. These findings indicate that there is a respiratory chain dysfunction in the lipomas and that lipomas may be a manifestation of the A8344G mutation.

Cortical reflex myoclonus in patients with the mitochondrial DNA transfer RNA(Lys)(8344) (MERRF) mutation

Five patients from three families with the syndrome of myoclonic epilepsy and ragged red fibres (MERRF), associated with the mitochondrial DNA point mutation at position 8344, were studied neurophysiologically to determine the characteristics of their myoclonus. The findings were those of cortical reflex myoclonus, with enlarged cortical somatosensory evoked potentials and late reflex responses to peripheral nerve stimulation. Electroencephalography showed paroxysmal spike and polyspike and wave discharges, with photic sensitivity. This pattern of electrophysiological abnormalities was uniform, despite considerable variation in severity of myoclonus. Although a consistent finding, cortical reflex myoclonus is not specific to MERRF amongst myoclonic syndromes.

Mitochondrial DNA mutation underlying Leigh's syndrome: clinical, pathological, biochemical, and genetic studies of a patient presenting with progressive myoclonic epilepsy

An 18-year-old male patient presented with clinical and radiological evidence of Leigh's syndrome (LS), having developed progressive myoclonic epilepsy and ataxia 11 years previously. Muscle biopsy showed cytochrome oxidase deficiency but no ragged red fibres. Autopsy confirmed the diagnosis of LS; there was additional degenerative change in the cerebellum and dentate and olivary nuclei, and an axonal peripheral neuropathy. Biochemical studies showed reduced activity of complexes I and IV of the respiratory chain in mitochondria from heart, liver and kidney. The mutation of mitochondrial DNA (mtDNA) at position 8344, commonly associated with the syndrome of myoclonic epilepsy and ragged red fibres, was detected in the patient's blood and was present in muscle, brain, liver, heart, and kidney in uniformly high amounts. It is clear that LS is genetically heterogeneous and represents one of the most severe phenotypes of a number of different mtDNA defects.

A new mtDNA mutation in the tRNA(Leu(UUR)) gene associated with maternally inherited cardiomyopathy

We report a new mutation, a C to T transition at nt 3303 of mtDNA, in seven members of a family with cardiomyopathy and myopathy: the proband and two siblings had fatal infantile cardiomyopathy, whereas in three maternal relatives the disease manifested later in life as sudden cardiac death or as mitochondrial myopathy with cardiomyopathy. The mutation was homoplasmic in all tissues (including blood) from the proband and her brother, but heteroplasmic in blood from five oligosymptomatic or asymptomatic maternal relatives. This mutation disrupts a conserved base pair in the aminoacyl stem of the tRNA(Leu(UUR)). None of 70 controls carried the mutation. Our data indicate that this mutation is the genetic cause of the disorder in this family, and confirm that the tRNA(Leu(UUR)) is a "hot spot" for mutations in mtDNA.

Myoclonic epilepsy with ragged-red fibers (MERRF) syndrome: report of a Chinese family with mitochondrial DNA point mutation in tRNA(Lys) gene

We report myoclonic epilepsy with ragged-red fibers (MERRF) syndrome in a Chinese family with confirmed mitochondrial DNA point mutation. Six members of the family including the grandmother, two siblings, and three grandchildren were affected. Among them, action myoclonus was seen in five; short stature, muscle weakness, and mental retardation in four; lactic acidosis, hearing impairment, and ataxia in two; and seizures in one. Muscle biopsy from two affected siblings revealed ragged-red fibers and abundant subsarcolemmal mitochondria with paracrystalline inclusions. Pedigree analysis suggests a maternal transmission. Analysis of mitochondrial DNA showed a point mutation from A to G at the 8344th nucleotide position located in the tRNA(Lys) gene. To our knowledge, this is the first report of MERRF syndrome with such genetic defect from a Chinese family. The present and previous reports support the notion that mitochondrial DNA point mutation at the 8344th nucleotide position is the most common cause of MERRF syndrome.

Human aging is associated with various point mutations in tRNA genes of mitochondrial DNA

In extraocular muscle tissue of elderly humans small amounts of point mutations in tRNA genes of mitochondrial DNA (mtDNA) were identified by point mutation-specific PCR. These mutations were not found in navel-string samples from newborns. While the mutations in tRNA(Leu(UUR)) (np 3243) and tRNA(Gly) (np 10006), previously identified in patients with MELAS and CIPO, respectively, were found in most elderly people, the mutations in tRNA(Ser(GCU)O (np 12246) and tRNA(Asn) (np 5692), identified in patients with CIPO and CPEO, respectively, were found only in two of 15 tissue samples from different individuals. The data suggest that some nucleotides of mtDNA represent "hot spots" for somatic mutations, which contribute to human aging.

Myoclonus epilepsy and ragged-red fibers: blood mitochondrial DNA heteroplasmy in affected and asymptomatic members of a family

By a rapid PCR-based method to assess the 8344 mtDNA mutation associated with MERRF disease, we have studied DNA from blood samples of 10 individuals belonging to a family spanning four generations in which one patient showed the complete MERRF phenotype, three other members were less severely affected, while the remaining were unaffected. The percentage of mutant mtDNA was quantified by laser-densitometric scanning of the negative photographic sheets of the agarose gels. The results showed that the MERRF patient had 53% of mutated mtDNA while the two less affected patients had 62% and 14% of mutated mtDNA, respectively. However, a high percentage of mutated genomes (up to 64%) was also found in some unaffected relatives. These results show that although on one hand the mutation is probably the primary cause of the disease, on the other hand the relative amount of mutated mtDNA in blood samples is not indicative of its clinical expression.

Merrf family with 8344 mutation in tRNA (lys). Evidence of a mitochondrial vasculopathy in muscle biopsies

This article reports a new MERRF family. The mother, regarded as suffering from Ramsay-Hunt Syndrome, and her three daughters, had the same clinical pattern: myoclonic epilepsy and ataxia. Two daughters were studied on morphological, biochemical and molecular genetic levels. Muscle biopsies showed ragged-red fibres and mitochondrial vasculopathy. Arterioles were strongly SDH-reactive and COX-negative. By electron microscopy, abnormal mitochondria were observed in skeletal muscle fibres, in smooth muscle fibres of intramuscular vessels and in sweat gland epithelium. The study of the respiratory chain showed complex IV and I + IV deficiency, respectively. Mitochondrial tRNA (lys) mutation at position 8344 was pointed out as previously reported in the MERRF syndrome.

A mitochondrial tRNA anticodon swap associated with a muscle disease

We have identified an unusual mitochondrial (mt) tRNA mutation in a seven year-old girl with a pure myopathy. This G to A transition at mtDNA position 15990 changed the anticodon normally found in proline tRNAs (UGG) to the one found in serine tRNAs (UGA), and is the first pathogenic anticodon alteration described in a higher eukaryote. The mutant mtDNA was heteroplasmic (85% mutant) in muscle but was undetectable in white blood cells from the patient and her mother. Analysis of single muscle fibres indicated that mutant mtDNAs severely impaired mitochondrial protein synthesis and respiratory chain activity, but only when present at greater than 90%. The recessive behaviour of this mtDNA alteration may explain the patient's relatively mild clinical phenotype.

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

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

Infantile familial cardiomyopathy due to mitochondrial complex I and IV associated deficiency

Two brothers, aged 27 and 20 months, born from consanguineous healthy parents, presented with cardiomyopathy, lactic acidosis and carnitine abnormalities in serum and muscle, without clinical evidence of muscle involvement. The histochemical reaction for cytochrome c oxidase (COX) activity was negative in all muscle fibres, although the holoenzyme and subunits were present at a normal level, as shown by immunocytochemistry. The COX activity was, respectively, 5 and 25% of control values, in muscle biopsies. Partial deficiency of complex IV was confirmed in fresh isolated muscle mitochondria from patient 2 and was associated with a defect of complex I. Patient 1 died at age 3 yr 6 months. Partial improvement of cardiomyopathy in patient 2 was obtained under carnitine therapy, but seizures occurred and CT scan and magnetic resonance imaging (MRI) revealed thalamic hypodensity. Thus, the disorder appears to be progressive despite the clinical stabilization of the cardiomyopathy. This further demonstrates the complexity and clinical heterogeneity of combined respiratory chain complex deficiencies.

Use of single strand conformation polymorphism analysis to detect point mutations in human mitochondrial DNA

Myoclonus epilepsy with ragged-red fibers (MERRF) has been shown to be associated with a specific point mutation at the nucleotide 8344 in the tRNA(Lys) gene of mitochondrial DNA (mtDNA). We screened 6 patients with clinically diagnosed MERRF and 1 patient with ocular myopathy for point mutations in the tRNA(Lys) gene, using single strand conformation polymorphism (SSCP) analysis, which can detect even a 1-basepair difference between 2 DNA sequences. Using SSCP and consequent DNA sequencing, we identified the known MERRF mutation in 4 out of 6 MERRF patients, as well as in 1 patient with a new clinical phenotype associated with this mutation: progressive external ophthalmoplegia, muscle weakness and a lipoma, but no myoclonus or epilepsy. Two of the patients with clinical MERRF had neither the MERRF-mutation nor any other mutations in the tRNA(Lys) gene. Using SSCP analysis, we also detected a new polymorphism in 1 patient. Thus, SSCP analysis can be applied to search effectively and rapidly for point mutations or polymorphisms in mitochondrial DNA.

Analysis of the tissue distribution and inheritance of heteroplasmic mitochondrial DNA point mutation by denaturing gradient gel electrophoresis in MERRF syndrome

MERRF (Myoclonic Epilepsy and Ragged-Red Fibres) syndrome is one of the maternally inherited diseases for which a mitochondrial DNA (mtDNA) point mutation has recently been identified. The mutation is always heteroplasmic, that is normal and mutant mtDNA coexist within the same individual. We studied mtDNA heteroplasmy in two families with MERRF syndrome, using a denaturing gradient gel electrophoresis technique that avoids the errors in the evaluation of wild/mutant mtDNA ratios caused by restriction enzyme cutting in the situation of amplification of a heteroplasmic DNA. In two patients, the proportion of muscle mutant mtDNA was in agreement with the severity of muscle mitochondrial proliferation, energy defect and fibre type I predominance. In nine patients from three generations of one family, mutant mtDNA proportion in leukocytes was in relative agreement with the clinical severity of the disease. Transmission of mutant mtDNA through these three generations did not show any tendency toward homoplasmy. Homogeneity of the mutant mtDNA proportion among different tissues from one patient was demonstrated in brain, liver, muscle and heart but a possibility of divergence of the mutant mtDNA proportion during mitosis was documented in cultured skin fibroblasts.