Increased mitochondrial DNA in blood vessels and ragged-red fibers in mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes (MELAS)

Variations in mitochondrial DNA coding and D-loop region are associated with early embryonic development defects in infertile women

Mitochondrial DNA (mtDNA) plays a critical role in oocyte maturation, fertilization, and early embryonic development. Defects in mtDNA may determine the alteration of the mitochondrial function, affecting cellular oxidative phosphorylation and ATP supply, leading to impaired oocyte maturation, abnormal fertilization, and low embryonic developmental potential, ultimately leading to female infertility. This case-control study was established to investigate the correlation between mtDNA variations and early embryonic development defects. Peripheral blood was collected for next-generation sequencing from women who suffered the repeated failures of in vitro fertilization (IVF) and/or intracytoplasmic sperm injection (ICSI) cycles due to early embryonic development defects as well as in-house healthy controls, and the sequencing results were statistically analyzed for all subjects. This study found that infertile women with early embryonic development defects carried more mtDNA variants, especially in the D-loop region, ATP6 gene, and CYTB gene. By univariate logistic regression analysis, 16 mtDNA variants were associated with an increased risk of early embryonic development defects (OR > 1, p < 0.05). Furthermore, we identified 16 potentially pathogenic mtDNA variants only in infertile cases. The data proved that mtDNA variations were associated with early embryonic development defects in infertile Chinese women.

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.

Mitochondrial Dysfunction and Migraine

The molecular basis of migraine is still not completely understood. An impairment of mitochondrial oxidative metabolism might play a role in the pathophysiology of this disease, by influencing neuronal information processing. Biochemical assays of platelets and muscle biopsies performed in migraine sufferers have shown a decreased activity of the respiratory chain enzymes. Studies with phosphorus magnetic resonance spectroscopy (31P-MRS) have demonstrated an impairment of the brain oxidative energy metabolism both during and between migraine attacks. However, molecular genetic studies have not detected specific mitochondrial DNA (mtDNA) mutations in patients with migraine, although other studies suggest that particular genetic markers (i.e. neutral polymorphisms or secondary mtDNA mutations) might be present in some migraine sufferers. Further studies are still needed to clarify if migraine is associated with unidentified mutations on the mtDNA or on nuclear genes that code mitochondrial proteins. In this paper, we review morphological, biochemical, imaging and genetic studies which bear on the hypothesis that migraine may be related to mitochondrial dysfunction at least in some individuals.

When should MELAS (Mitochondrial myopathy, Encephalopathy, Lactic Acidosis, and Stroke-like episodes) be the diagnosis?

Mitochondrial myopathy, Encephalopathy, Lactic Acidosis, and Stroke-like episodes (MELAS) is a rare mitochondrial disorder. Diagnostic criteria for MELAS include typical manifestations of the disease: stroke-like episodes, encephalopathy, evidence of mitochondrial dysfunction (laboratorial or histological) and known mitochondrial DNA gene mutations. Clinical features of MELAS are not necessarily uniform in the early stages of the disease, and correlations between clinical manifestations and physiopathology have not been fully elucidated. It is estimated that point mutations in the tRNALeu(UUR) gene of the DNAmt, mainly A3243G, are responsible for more of 80% of MELAS cases. Morphological changes seen upon muscle biopsy in MELAS include a substantive proportion of ragged red fibers (RRF) and the presence of vessels with a strong reaction for succinate dehydrogenase. In this review, we discuss mainly diagnostic criterion, clinical and laboratory manifestations, brain images, histology and molecular findings as well as some differential diagnoses and current treatments.

Mitochondrial dysfunction and seizures: the neuronal energy crisis

Seizures are often the key manifestation of neurological diseases caused by pathogenic mutations in 169 of the genes that have so far been identified to affect mitochondrial function. Mitochondria are the main producers of ATP needed for normal electrical activities of neurons and synaptic transmission. Additionally, they have a central role in neurotransmitter synthesis, calcium homoeostasis, redox signalling, production and modulation of reactive oxygen species, and neuronal death. Hypotheses link mitochondrial failure to seizure generation through changes in calcium homoeostasis, oxidation of ion channels and neurotransmitter transporters by reactive oxygen species, a decrease in neuronal plasma membrane potential, and reduced network inhibition due to interneuronal dysfunction. Seizures, irrespective of their origin, represent an excessive acute energy demand in the brain. Accordingly, secondary mitochondrial dysfunction has been described in various epileptic disorders, including disorders that are mainly of non-mitochondrial origin. An understanding of the reciprocal relation between mitochondrial dysfunction and epilepsy is crucial to select appropriate anticonvulsant treatment and has the potential to open up new therapeutic approaches in the subset of epileptic disorders caused by mitochondrial dysfunction.

Cerebral metabolic abnormalities in A3243G mitochondrial DNA mutation carriers

OBJECTIVE

To establish cerebral metabolic features associated with the A3243G mitochondrial DNA mutation with proton magnetic resonance spectroscopic imaging ((1)H MRSI) and to assess their potential as prognostic biomarkers.

METHODS

In this prospective cohort study, we investigated 135 clinically heterogeneous A3243G mutation carriers and 30 healthy volunteers (HVs) with (1)H MRSI. Mutation carriers included 45 patients with mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS); 11 participants who would develop the MELAS syndrome during follow-up (converters); and 79 participants who would not develop the MELAS syndrome during follow-up (nonconverters). The groups were compared with respect to MRSI metabolic indices of 1) anaerobic energy metabolism (lactate), 2) neuronal integrity (N-acetyl-l-aspartate [NAA]), 3) mitochondrial function (NAA; lactate), 4) cell energetics (total creatine), and 5) membrane biosynthesis and turnover (total choline [tCho]).

RESULTS

Consistent with prior studies, the patients with MELAS had higher lactate (p < 0.001) and lower NAA levels (p = 0.01) than HVs. Unexpectedly, converters showed higher NAA (p = 0.042), tCho (p = 0.004), and total creatine (p = 0.002), in addition to higher lactate levels (p = 0.032), compared with HVs. Compared with nonconverters, converters had higher tCho (p = 0.015). Clinically, converters and nonconverters did not differ at baseline. Lactate and tCho levels were reliable biomarkers for predicting the risk of individual mutation carriers to develop the MELAS phenotype.

CONCLUSIONS

(1)H MRSI assessment of cerebral metabolism in A3243G mutation carriers shows promise in identifying disease biomarkers as well as individuals at risk of developing the MELAS phenotype.

Microangiopathy in the cerebellum of patients with mitochondrial DNA disease

Neuropathological findings in mitochondrial DNA disease vary and are often dependent on the type of mitochondrial DNA defect. Many reports document neuronal cell loss, demyelination, gliosis and necrotic lesions in post-mortem material. However, previous studies highlight vascular abnormalities in patients harbouring mitochondrial DNA defects, particularly in those with the m.3243A>G mutation in whom stroke-like events are part of the mitochondrial encephalopathy lactic acidosis and stroke-like episodes syndrome. We investigated microangiopathic changes in the cerebellum of 16 genetically and clinically well-defined patients. Respiratory chain deficiency, high levels of mutated mitochondrial DNA and increased mitochondrial mass were present within the smooth muscle cells and endothelial cells comprising the vessel wall in patients. These changes were not limited to those harbouring the m.3243A>G mutation frequently associated with mitochondrial encephalopathy, lactic acidosis and stroke-like episodes, but were documented in patients harbouring m.8344A>G and autosomal recessive polymerase (DNA directed), gamma (POLG) mutations. In 8 of the 16 patients, multiple ischaemic-like lesions occurred in the cerebellar cortex suggestive of vascular smooth muscle cell dysfunction. Indeed, changes in vascular smooth muscle and endothelium distribution and cell size are indicative of vascular cell loss. We found evidence of blood–brain barrier breakdown characterized by plasma protein extravasation following fibrinogen and IgG immunohistochemistry. Reduced immunofluorescence was also observed using markers for endothelial tight junctions providing further evidence in support of blood–brain barrier breakdown. Understanding the structural and functional changes occurring in central nervous system microvessels in patients harbouring mitochondrial DNA defects will provide an important insight into mechanisms of neurodegeneration in mitochondrial DNA disease. Since therapeutic strategies targeting the central nervous system are limited, modulating vascular function presents an exciting opportunity to lessen the burden of disease in these patients.

Mitochondrial diseases and the heart: an overview of molecular basis, diagnosis, treatment and clinical course

The mitochondrion is the main site of production of ATP that represents the source of energy for a large number of cellular processes. Mitochondrial diseases that result in a deficit in ATP production can affect almost every organ system with a large spectrum of clinical phenotypes. Cardiomyocytes are particularly vulnerable to limited ATP supply because of their large energy requirement. Abnormalities in the mitochondrial function are increasingly recognized in association with dilated and hypertrophic cardiomyopathy, cardiac conduction defects, endothelial dysfunction and coronary artery disease. Cardiologists should, therefore, be alerted to symptoms and signs suggestive of mitochondrial diseases and become familiar with the general issues related to multisystem disease management, genetic counseling and testing.

MELAS: clinical features, muscle biopsy and molecular genetics

OBJECTIVE: The aim of the study was to analyze a series of Brazilian patients suffering from MELAS. METHOD: Ten patients with MELAS were studied with correlation between clinical findings, laboratorial data, electrophysiology, histochemical and molecular features. RESULTS: Blood lactate was increased in eight patients. Brain image studies revealed a stroke-like pattern in all patients. Muscle biopsy showed ralled-red fibers (RRF) in 90% of patients on modified Gomori-trichrome and in 100% on succinate dehydrogenase stains. Cytochrome c oxidase stain analysis indicated deficient activity in one patient and subsarcolemmal accumulation in seven patients. Strongly succinate dehydrogenase-reactive blood vessels (SSV) occurred in six patients. The molecular analysis of tRNA Leu(UUR) gene by PCR/RLFP and direct sequencing showed the A3243G mutation on mtDNA in 4 patients. CONCLUSION: The muscle biopsy often confirmed the MELAS diagnosis by presence of RRF and SSV. Molecular analysis of tRNA Leu(UUR) gene should not be the only diagnostic criteria for MELAS.

Mitochondrial encephalopathy, lactic acidosis, and strokelike episodes: basic concepts, clinical phenotype, and therapeutic management of MELAS syndrome

Since the initial description almost 25 years ago, the syndrome of mitochondrial encephalopathy, lactic acidosis, and strokelike episodes (MELAS) has been a useful model to study the complex interplay of factors that define mitochondrial disease. This syndrome, most commonly caused by an A-to-G transition mutation at position 3243 of the mitochondrial genome, is typified by characteristic neurological manifestations including seizures, encephalopathy, and strokelike episodes, as well as other frequent secondary manifestations including short stature, cognitive impairment, migraines, depression, cardiomyopathy, cardiac conduction defects, and diabetes mellitus. In this review, we discuss the history, pathogenesis, clinical features, and diagnostic and management strategies of mitochondrial disease in general and of MELAS in particular. We explore features of mitochondrial genetics, including the concepts of heteroplasmy, mitotic segregation, and threshold effect, as a basis for understanding the variability and complicated inheritance patterns seen with this group of diseases. We also describe systemic manifestations of MELAS-associated mutations, including cardiac, renal, endocrine, gastrointestinal, and endothelial abnormalities and pathology, as well as the hypothetical role of derangements to COX enzymatic function in driving the unique pathology and clinical manifestations of MELAS. Although therapeutic options for MELAS and other mitochondrial diseases remain limited, and recent trials have been disappointing, we also consider current and potential therapeutic modalities.

Genotype-phenotype correlation of maternally inherited disorders due to mutations in mitochondrial DNA

Mitochondrial disorders are heterogeneous systemic ailments that are most often caused by maternal inheritance of a variety of mutations of the mitochondrial (mt) DNA. Paternal inheritance and somatic mutation are rare. The disorders are well recognized not only for the genotypic heterogeneity, but also the phenotypic variation among the affected members of a single family. The genotype-phenotype correlation of the diversity of the syndromic and non-syndromic features of mitochondrial disorders are discussed. Some aspects of the molecular mechanisms of this heterogeneity, and the histopathologic findings are highlighted.

Molecular neuropathology of MELAS: level of heteroplasmy in individual neurones and evidence of extensive vascular involvement

Mitochondrial DNA (mtDNA) disease is an important genetic cause of neurological disability. A variety of different clinical features are observed and one of the most common phenotypes is MELAS (Mitochondrial Myopathy, Encephalopathy, Lactic Acidosis and Stroke-like episodes). The majority of patients with MELAS have the 3243A>G mtDNA mutation. The neuropathology is dominated by multifocal infarct-like lesions in the posterior cortex, thought to underlie the stroke-like episodes seen in patients. To investigate the relationship between mtDNA mutation load, mitochondrial dysfunction and neuropathological features in MELAS, we studied individual neurones from several brain regions of two individuals with the 3243A>G mutation using dual cytochrome c oxidase (COX) and succinate dehydrogenase (SDH) histochemistry, and Polymerase Chain Reaction Restriction Fragment Lenght Polymorphism (PCR-RFLP) analysis. We found a low number of COX-deficient neurones in all brain regions. There appeared to be no correlation between the threshold level for the 3243A>G mutation to cause COX deficiency within single neurones and the degree of pathology in affected brain regions. The most severe COX deficiency associated with the highest proportion of mutated mtDNA was present in the walls of the leptomeningeal and cortical blood vessels in all brain regions. We conclude that vascular mitochondrial dysfunction is important in the pathogenesis of the stroke-like episodes in MELAS patients. As migraine is a commonly encountered feature in MELAS, we propose that coupling of the vascular mitochondrial dysfunction with cortical spreading depression (CSD) might underlie the selective distribution of ischaemic lesions in the posterior cortex in these patients.

Simultaneous detection and quantification of mitochondrial DNA deletion(s), depletion, and over-replication in patients with mitochondrial disease

Heterogeneous clinical expression of mitochondrial DNA (mtDNA) disorders depends on both qualitative and quantitative changes in mtDNA. We developed a sensitive and effective method that simultaneously detects mtDNA deletion(s) and quantifies total mtDNA content. The percentage of deletions and mtDNA content of 19 patients with single or multiple deletions were analyzed by real-time quantitative polymerase chain reaction (real-time qPCR) using TaqMan probes specific for mtDNA (tRNA leu(UUR), ND4, ATPase8, and D-loop regions) and nuclear DNA (AIB1, beta-2-microglobulin, and beta-actin). The proportion of deletion mutants determined by real-time qPCR was consistent with that determined by Southern analysis. Most patients with mtDNA deletions also demonstrated compensatory mtDNA over-replication. Multiple mtDNA deletions that were not detectable by Southern analysis due to low percentage of each deletion molecule were readily detected and quantified by real-time qPCR. Furthermore, 12 patients with clinical features and abnormal biochemical/histopathological results consistent with mitochondrial respiratory chain disorders without identified mtDNA mutations had either substantially depleted or significantly over-replicated mtDNA content, supporting the diagnosis of mitochondrial disease. Our results demonstrate that both qualitative and quantitative analyses are important in molecular diagnosis of mitochondrial diseases. The presence of deletion(s) and mtDNA depletion or compensatory over-replication can be determined simultaneously by real-time qPCR.

A juvenile case of MELAS with T3271C mitochondrial DNA mutation

We present here a patient with muscle fatigue and poor growth since the age of 6 y. The diagnosis of a mitochondrial disease was based on the presence of ragged red fibers in the muscle biopsy and on a combined defect of mitochondrial DNA-encoded respiratory enzymes. Epilepsia partialis continua with stroke-like episodes appeared 2 mo before death at the age of 18 and prompted a search for mitochondrial DNA mutations associated with mitochondrial encephalopathy, lactic acidosis, and stroke-like episodes. Minisequencing of the patient's DNA samples revealed a heteroplasmic T3271C mutation with a 78-94% mutation load in her fibroblasts or autopsy-derived tissue samples. This is the ninth reported non-Japanese patient with T3271C mutation. Our patient shows that despite very high proportion of mutant mtDNA, the T3271C mutation can give rise to mild symptoms in childhood and to a rapid terminal phase that simulates encephalitis.

Humanin detected in skeletal muscles of MELAS patients: a possible new therapeutic agent

Humanin (HN) was originally identified as an endogenous peptide that protects neuronal cells from apoptosis induced by various types of Alzheimer's disease-related insults. We have previously indicated that HN increases cellular ATP levels and speculated that this peptide may rescue energy-deficient cells in mitochondrial disorders. Here, we report, for the first time, increased HN expression in skeletal muscles from patients with mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes (MELAS). HN was strongly positive in all ragged-red fibers (RRFs) and some non-RRFs, and most of them were type 1 fibers generally requiring higher energy than type 2 fibers. HN in these fibers was localized in mitochondria. HN expression was also increased in small arteries that strongly reacted for succinate dehydrogenase. Our experiments on muscular TE671 cells indicated the possibility that synthesized HN increases cellular ATP levels by directly acting on mitochondria. From these in vivo and in vitro findings, we propose that HN expression might be induced in response to the energy crisis within affected fibers and vessels in MELAS muscles and further be a possible therapeutic candidate for MELAS.

Humanin improves impaired metabolic activity and prolongs survival of serum-deprived human lymphocytes

Humanin (HN) has been reported to be an endogenous peptide that exerts highly selective neuroprotection against cell death induced by various types of Alzheimer's disease-related insults. We previously proposed the much broader cytoprotective potential of HN from the result that HN suppressed serum-deprivation-induced death of rat pheochromocytoma cells. In this study, we showed that HN also suppressed death of human lymphocytes cultured under serum-deprived condition. Further, we revealed, by assaying metabolic activity and survival rate, that HN was a potent factor capable of increasing the metabolic activity of individual serum-deprived lymphocytes. To our knowledge, there is no report described about a rescue factor that increases the metabolic activity of individual serum-deprived cells and prolongs their survival. This novel feature of HN may enable us to apply this peptide for the management of diseases involving poor metabolic activity, such as mitochondria-related disorders and brain ischemia.

Simultaneous A8344G heteroplasmy and mitochondrial DNA copy number quantification in myoclonus epilepsy and ragged-red fibers (MERRF) syndrome by a multiplex molecular beacon based real-time fluorescence PCR

The association of a particular mitochondrial DNA (mtDNA) mutation with different clinical phenotypes is a well-known feature of mitochondrial diseases. A simple genotype-phenotype correlation has not been found between mutation load and disease expression. Tissue and intercellular mosaicism as well as mtDNA copy number are thought to be responsible for the different clinical phenotypes. As disease expression of mitochondrial tRNA mutations is mostly in postmitotic tissues, studies to elucidate disease mechanisms need to be performed on patient material. Heteroplasmy quantitation and copy number estimation using small patient biopsy samples has not been reported before, mainly due to technical restrictions. In order to resolve this problem, we have developed a robust assay that utilizes Molecular Beacons to accurately quantify heteroplasmy levels and determine mtDNA copy number in small samples carrying the A8344G tRNA(Lys) mutation. It provides the methodological basis to investigate the role of heteroplasmy and mtDNA copy number in determining the clinical phenotypes.

Neuroradiological features of six kindreds with MELAS tRNA(Leu) A2343G point mutation: implications for pathogenesis

OBJECTIVE

To determine the neuroradiological abnormalities associated with subjects carrying the mitochondrial myopathy, encephalopathy, lactic acidosis, and strokelike episodes (MELAS) tRNA(Leu)(UUR) A3243G point mutation

METHODS

Mitochondrial genetic analysis was performed on 24 subjects from six kindreds with the MELAS tRNA(Leu)(UUR) A3243G point mutation. Cerebral CT and MRI were performed on 24 patients and 15 patients respectively. Previous neuroradiological investigations including cerebral CT from four deceased members of the families were also reviewed. Histological examination of postmortem specimens of two patients within the kindreds was performed.

RESULTS

The commonest radiological finding was basal ganglia calcification. Other abnormalities included focal lesions and cerebellar and cerebral atrophy. Basal ganglia calcification was progressive, symmetric, and asymptomatic. Histologically, basal ganglia calcification in one patient was found to be in the pericapillary regions of the globus pallidus, with no neuronal involvement. Focal lesions most commonly involved the grey matter of the parietal and occipital lobes and cerebellum. Histopathological examination suggested that these were due to cellular rather than vascular dysfunction. Enlargement of the fourth ventricle was the first sign of cerebellar atrophy. Cerebral and cerebellar atrophy were only present with severe disease.

CONCLUSIONS

These radiological findings, when considered in the context of the clinical and pathological findings, seem to reflect two major disease processes: an intermittent abrupt loss of function associated with cell injury from which there is at least partial recovery and a slowly progressive degenerative process causing basal ganglia calcification, and cerebral and cerebellar atrophy. The clinical and radiological features resulting from these processes are distinctive and provide insight into the consequences of mitochondrial dysfunction on the brain.

Cerebral blood flow and oxygen metabolism before and after a stroke-like episode in patients with mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes (MELAS)

Cerebral blood flow and oxygen metabolism were examined in two patients with mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes (MELAS) using positron emission tomography (PET). Regional cerebral blood flow (rCBF), regional cerebral oxygen metabolic rate (rCMRO2) and regional oxygen extraction fraction (rOEF) were determined with the steady-state technique using oxygen-15-labeled tracers (15O2, C15O2 and C15O). Case 1, a 45-year-old woman, presented with abrupt onset of fluent aphasia. T2-weighted magnetic resonance imaging (MRI) showed a high signal intensity lesion in the left temporoparietal region. The first PET study on day 16 showed increased rCBF and decreased rCMRO2 in the temporal region. In the second PET study, on day 35, rCBF in the temporal region had decreased. Case 2 was a 19-year-old male; the second son of Case 1. He complained of transient blurring of vision, and then generalized tonic-clonic convulsion occurred. A PET study six days before this stroke-like episode demonstrated increased rCBF in both frontal lobes and putamen, where MRI showed lesions after the episode. Focal hyperemia of the lesion antedated and lasted for at least sixteen days after the stroke-like episode in these MELAS patients. These stroke-like episodes appear to be the result of metabolic dysfunction in neural tissue, although the role of an ischemic vascular event cannot be ruled out.

Single muscle fiber analysis of myoclonus epilepsy with ragged-red fibers

We examined two muscle biopsy specimens from a proband and her mother with myoclonus epilepsy with ragged-red fibers (MERRF), both obtained at an interval of about 10 years, using histochemistry, in situ hybridization, and single-fiber polymerase chain reaction. Total (wild-type and mutant) mitochondrial DNAs (mtDNAs) were greatly increased in ragged-red fibers (RRF) over non-RRF in all muscle specimens analyzed. The proportion of mutant mtDNA was also significantly higher in RRF than in non-RRF. By comparing the first and second muscle biopsied specimens in each patient, we found that while the proportion of RRF, cytochrome coxidase deficient fibers, and mutant DNA in muscle changed over a 10-year period, the proportion of wild-type and mutant mtDNAs in RRF and in non-RRF was similar between the two specimens. These results suggest that the ratio of wild-type to mutant mtDNAs in RRF and non-RRF in MERRF is at a steady state level in each muscle fiber, without replicative advantage of mutant mtDNA.

Clinical, physiological, and histological features in a kindred with the T3271C melas mutation

The majority of patients with MELAS syndrome (mitochondrial myopathy, encephalopathy, lactic acidosis, and strokelike episodes) have an A-->G mutation at nucleotide 3243 in mitochondrial transfer (t)RNA. To date there have only been 10 reported cases of MELAS syndrome in patients with a T-->C mutation at position 3271 of mitochondrial tRNA. Although many of the clinical features are similar between patients with these different mutations, it appears that the age at onset is later for the 3271 mutation. This report provides information from a North American kindred with the 3271 mutation (n = 6 proven; n = 2 probable; n = 3 possible) that adds clinical, physiological, histological, and molecular information to the pool of information on this rare disorder. Many of these features were similar to previous reports of both 3243 and 3271 patients. We conclude that the phenotypic expression of these different mutations are similar, but the age of onset for 3271 patients is later than for 3243 patients.

On the molecular etiology of decreased arachidonic (20:4n-6), docosapentaenoic (22:5n-6) and docosahexaenoic (22:6n-3) acids in Zellweger syndrome and other peroxisomal disorders

Alterations in the metabolism of arachidonic (20:4n-6), docosapentaenoic (22:5n-6), and docosahexaenoic (22:6n-3) acids and other polyunsaturated fatty acids in Zellweger syndrome and other peroxisomal disorders are reviewed. Previous proposals that peroxisomes are necessary for the synthesis of 22:6n-3 and 22:5n-6 are critically examined. The data suggest that 22:6n-3 is biosynthesized in mitochondria via a channelled carnitine-dependent pathway involving an n-3-specific delta-4 desaturase, while 20:4n-6, 20:5n-3 and 22:5n-6 are synthesized by both mitochondrial and microsomal systems; these pathways are postulated to be interregulated as compensatory-redundant systems. Present evidence suggests that 22:6n-3-containing phospholipids may be required for the biochemical events involved in successful neuronal migration and developmental morphogenesis, and as structural cofactors for the functional assembly and integration of a variety of membrane enzymes, receptors, and other proteins in peroxisomes and other subcellular organelles. A defect in the mitochondrial desaturation pathway is proposed to be a primary etiologic factor in the clinicopathology of Zellweger syndrome and other related disorders. Several implications of this proposal are examined relating to effects of pharmacological agents which appear to inhibit steps in this pathway, such as some hypolipidemics (fibrates), neuroleptics (phenothiazines and phenytoin) and prenatal alcohol exposure.

Mitochondrial DNA and RNA processing in MELAS

Mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes (MELAS), a maternally inherited disorder, is usually associated with a point mutation in mitochondrial DNA (mtDNA) at position 3,243 in the tRNA Leu(UUR) gene. To further study the pathogenesis of MELAS, we analyzed tissues from 8 MELAS-3,243 patients. Southern blot analysis showed an increase in the ratio of mtDNA to nuclear DNA in almost all tissues examined, implying that mitochondrial proliferation is ubiquitous and is not confined to ragged-red fibers in muscle. By northern blot analysis, we demonstrated increased steady-state levels of RNA 19, a polycistronic transcript corresponding to the 16S rRNA + tRNA Leu(UUR) + ND1 genes (which are contiguous in the mtDNA) in heart, kidney, and muscle. These results provide further evidence that altered mitochondrial nucleic acid metabolism may have pathogenic significance in MELAS.

Assessment of the distribution of mitochondrial ribosomal RNA in melas and in thrombotic cerebral infarcts by in situ hybridization

In situ hybridization to mitochondrial ribosomal RNA (rRNA) has been used to study the distribution of mitochondria in paraffin-embedded autopsy brain tissue from two patients with MELAS (mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes) and other organs from one of the patients. Comparison of in situ hybridization and electron microscopic findings in an antemortem biopsy specimen of pylorus from the latter patient showed a close correspondence between the distribution of hybridization signal on light microscopy and of mitochondria in ultrathin sections. Strong hybridization signal was present over smooth muscle fibres of the muscularis externa, which contained abnormal accumulations of mitochondria on electron microscopy. Hybridization to sections of skeletal muscle confirmed previous reports of 'ragged-red' fibres in this disorder and of mitochondrial accumulations in the walls of intramuscular blood vessels. To try to elucidate the role of vessel wall accumulation of mitochondria in the genesis of the stroke-like lesions, the distribution of mitochondrial rRNA was assessed in sections of brain from both of the cases of MFLAS and several cases of atherothrombotic cerebrovascular disease. Blood vessels in and adjacent to the cerebral lesions of MELAS showed strong hybridization signal with the mitochondrial probes, as was also seen in infarcts of various ages in the control brains. Only weak signal was present in the walls of blood vessels distant from the lesions, in both MELAS and control brains. These findings suggest that mitochondria accumulate in vascular endothelium and tunica media as a normal response to cerebral infarction or ischaemia. The accumulation of mitochondria in the cerebral lesions of MELAS may, at least in part, be a reaction to the destructive effects of the underlying metabolic dysfunction.

Marked increase in the number and variety of mitochondrial DNA rearrangements in aging human skeletal muscle

Several reports have shown that individual mitochondrial DNA (mtDNA) deletions accumulate with age. However, the overall extent of somatic mtDNA damage with age remains unclear. We have utilized full-length PCR to concurrently screen for multiple mtDNA rearrangements in total DNA extracted from skeletal muscle derived from physiologically normal individuals (n = 35). This revealed that both the number and variety of mtDNA rearrangements increases dramatically between young and old individuals (P < 0.0001). We further examined the mtDNA from both the younger and older subjects by Southern blot analysis and observed an age-related increase in mtDNA(s) comparable in size to mtDNA products unique to patients with known mtDNA deletions. These data imply that a wide spectrum of mtDNA rearrangements accumulate in old individuals, which correlates with the marked age related decrease in OXPHOS capacity observed in post-mitotic tissues.

Increase of mitochondria in vasa nervorum of cases with mitochondrial myopathy, Kearns-Sayre syndrome, progressive external ophthalmoplegia and MELAS

Previous studies on patients with mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes (MELAS) revealed accumulations of mitochondria in endothelial cells, smooth muscle cells of pial arterioles, and small intracerebral arteries up to 250 microns in diameter; in pericytes of capillaries, endothelial cells, and smooth muscle cells of small blood vessels in skeletal muscle; and according to preliminary results also in endothelial and smooth muscle cells of capillaries and arterioles in sural nerves. These mitochondria do not show the prominent paracrystalline inclusions which are seen in striated muscle fibres and led to the identification of this group of disorders. To corroborate our preliminary findings in peripheral nerves, additional cases have been evaluated morphometrically by electron microscopy including cases in which mitochondrial DNA (mtDNA) mutations have been identified. In fact, an increase of the mean number and an enlargement of the mean cross-sectional area of mitochondria was noted in endothelial and smooth muscle cells of endoneurial and epineurial arterioles, and in endothelial cells and in pericytes of capillaries in sural nerves of the 20 cases with mitochondrial disorders studied. This increase was statistically significant compared to the control group. However, due to heteroplasmia, which is a common feature in mitochondrial disorders, and because of the limited number of measurable blood vessels and cases, no significant differences could be detected between the various types of mitochondrial diseases which were characterized by different point mutations or deletions of mtDNA. Our findings suggest that the mitochondria play a significant role in the pathogenesis not only of myopathic and encephalopathic symptoms, but also in the pathogenesis of peripheral neuropathy which appears to be regularly associated with mitochondrial disorders.

A new mutation associated with MELAS is located in a mitochondrial DNA polypeptide-coding gene

We report a patient with mitochondrial encephalomyopathy, lactic acidosis and stroke-like episodes (MELAS) who harbored a novel missense mutation at mtDNA position 9957 in the gene specifying subunit III of cytochrome c oxidase (COX III). This T-->C transition converted Phe-251, a highly conserved amino acid in the C-terminus of the polypeptide, to Leu. The mutation, which was not present in 107 normal controls or in 57 patients with various mitochondrial diseases, was heteroplasmic in both muscle and blood of the proband and in blood from his asymptomatic mother. These results provide evidence that the MELAS clinical phenotype can be due not only to mutations in mtDNA-encoded tRNA genes, but in polypeptide-coding genes as well.

Mitochondrial DNA mutation and muscle pathology in mitochondrial myopathy, encephalopathy, lactic acidosis, and strokelike episodes

We sought a relationship between abnormalities of mitochondrial DNA (mtDNA) and muscle pathology in patients with mitochondrial myopathy, encephalopathy, lactic acidosis, and strokelike episodes (MELAS) at the single fiber level, using histochemistry, in situ hybridization, and single fiber PCR. Most type 1 ragged-red fibers (RRF) showed positive cytochrome c oxidase (COX) activity at the subsarcolemmal region, while type 2 RRF showed little COX activity. However, there was no difference in the amount of total (mutant and wild-type) mtDNAs and the proportion of mutant mtDNA between type 1 RRF and type 2 RRF. These observations suggest that mitochondrial proliferation and nuclear factors affect muscle pathology, including COX activity, in MELAS. Total mtDNAs were greatly increased in RRF. The proportion of mutant mtDNA was significantly higher in RRF than in non-RRF. The amount of both wild-type and mutant mtDNAs was increased in RRF in MELAS, which fact does not support the contention of a replicative advantage of mutant mtDNA. The proportion of mutant mtDNA was significantly higher in the strongly succinate dehydrogenase-reactive blood vessels (SSV) than in non-SSV. The similar morphological behavior in these vessels and fibers suggests that increased mutant mtDNA is responsible for mitochondrial proliferation and dysfunction in both tissues. It seems likely that systemic vascular abnormalities involving cerebral vessels lead to the evolution of strokelike episodes in MELAS.

Demonstration of mitochondrial ribosomal RNA in frozen and paraffin-embedded sections of skeletal muscle by in situ hybridization

We have used in situ hybridization with synthetic oligonucleotide probes to the 16S sub-unit of mitochondrial ribosomal RNA to detect mitochondrial accumulations in biopsies of skeletal muscle from 16 patients with mitochondrial myopathies. In all cases, the distribution of the hybridization signal matched the pattern of succinate dehydrogenase activity in adjacent cryostat sections. In situ hybridization also allowed visualization of mitochondrial accumulations in paraffin-embedded sections of skeletal muscle that had been fixed in either De Castro's solution or formalin. DNase pre-treatment did not significantly diminish the intensity of hybridization signal, suggesting that the hybridization is predominantly to RNA. The advantages of this technique are that it allows the investigation of mitochondrial disorders when only paraffin-embedded tissue is available and should also facilitate the demonstration of mitochondrial accumulations in tissues other than skeletal muscle.

Single muscle fiber analysis of mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes (MELAS)

We examined muscle sections from 3 patients with mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes (MELAS), using single-fiber polymerase chain reaction, histochemistry, and in situ hybridization. Most type 1 ragged-red fibers showed positive cytochrome c oxidase activity at the subsarcolemmal region, while type 2 ragged-red fibers had little cytochrome c oxidase activity. However, there was no difference in the amount of total (mutant and wild-type) mitochondrial DNAs (mtDNAs) and the proportion of mutant mtDNA between type 1 and type 2 ragged-red fibers. These observations suggest that mitochondrial proliferation and nuclear factors affect muscle pathology, including cytochrome c oxidase activity, in MELAS. Total mtDNAs were greatly increased in ragged-red fibers (about 5-17 times over those in non-ragged-red fibers). The proportion of mutant mtDNA was significantly higher in ragged-red fibers (88.1 +/- 5.5%) than in non-ragged-red fibers (63.2 +/- 21.6%). Thus, the amount of wild-type mtDNA as well as mutant mtDNA was increased in ragged-red fibers in MELAS, failing to support the contention of a replicative advantage of mutant mtDNA. The proportion of mutant mtDNA was significantly higher in the strongly succinate dehydrogenase-reactive blood vessels (83.2 +/- 4.2%) than in non-succinate dehydrogenase-reactive blood vessels (38.8 +/- 16.2%). It seems likely that systemic vascular abnormalities involving cerebral vessels lead to the evolution of stroke-like episodes in MELAS.

Mitochondrial DNA alterations and genetic diseases: a review

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