Correlation between clinical and molecular features in two MELAS families

Diagnosis of adult-onset MELAS syndrome in a 63-year-old patient with suspected recurrent strokes - a case report

BACKGROUND

Mitochondrial encephalomyopathy, lactic acidosis and stroke-like episodes (MELAS) is a mitochondrial cytopathy caused by mutations in mitochondrial DNA. Clinical manifestation is typically before the age of 40.

CASE PRESENTATION

We present the case of a 63-year-old female in whom the symptoms of MELAS were initially misdiagnosed as episodes of recurrent ischemic strokes. Brain imaging including MRI, clinical and laboratory findings that lent cues to the diagnosis of MELAS are discussed. In addition, MRI findings in MELAS in comparison to imaging mimics of MELAS are presented.

CONCLUSIONS

This case underscores the importance of considering MELAS as a potential cause of recurrent stroke-like events if imaging findings are untypical for cerebral infarction, even among middle-aged patients with vascular risk factors.

Mitochondrial DNA m.3243A > G heteroplasmy affects multiple aging phenotypes and risk of mortality

Mitochondria contain many copies of a circular DNA molecule (mtDNA), which has been observed as a mixture of normal and mutated states known as heteroplasmy. Elevated heteroplasmy at a single mtDNA site, m.3243A > G, leads to neurologic, sensory, movement, metabolic, and cardiopulmonary impairments. We measured leukocyte mtDNA m.3243A > G heteroplasmy in 789 elderly men and women from the bi-racial, population-based Health, Aging, and Body Composition Study to identify associations with age-related functioning and mortality. Mutation burden for the m.3243A > G ranged from 0-19% and elevated heteroplasmy was associated with reduced strength, cognitive, metabolic, and cardiovascular functioning. Risk of all-cause, dementia and stroke mortality was significantly elevated for participants in the highest tertiles of m.3243A > G heteroplasmy. These results indicate that the accumulation of a rare genetic disease mutation, m.3243A > G, manifests as several aging outcomes and that some diseases of aging may be attributed to the accumulation of mtDNA damage.

Mitochondrial DNA sequence characteristics modulate the size of the genetic bottleneck

With a combined carrier frequency of 1:200, heteroplasmic mitochondrial DNA (mtDNA) mutations cause human disease in ∼1:5000 of the population. Rapid shifts in the level of heteroplasmy seen within a single generation contribute to the wide range in the severity of clinical phenotypes seen in families transmitting mtDNA disease, consistent with a genetic bottleneck during transmission. Although preliminary evidence from human pedigrees points towards a random drift process underlying the shifting heteroplasmy, some reports describe differences in segregation pattern between different mtDNA mutations. However, based on limited observations and with no direct comparisons, it is not clear whether these observations simply reflect pedigree ascertainment and publication bias. To address this issue, we studied 577 mother–child pairs transmitting the m.11778G>A, m.3460G>A, m.8344A>G, m.8993T>G/C and m.3243A>G mtDNA mutations. Our analysis controlled for inter-assay differences, inter-laboratory variation and ascertainment bias. We found no evidence of selection during transmission but show that different mtDNA mutations segregate at different rates in human pedigrees. m.8993T>G/C segregated significantly faster than m.11778G>A, m.8344A>G and m.3243A>G, consistent with a tighter mtDNA genetic bottleneck in m.8993T>G/C pedigrees. Our observations support the existence of different genetic bottlenecks primarily determined by the underlying mtDNA mutation, explaining the different inheritance patterns observed in human pedigrees transmitting pathogenic mtDNA mutations.

A wide range of 3243A>G/tRNALeu(UUR) (MELAS) mutation loads may segregate in offspring through the female germline bottleneck

Segregation of mutant mtDNA in human tissues and through the germline is debated, with no consensus about the nature and size of the bottleneck hypothesized to explain rapid generational shifts in mutant loads. We investigated two maternal lineages with an apparently different inheritance pattern of the same pathogenic mtDNA 3243A>G/tRNA^Leu(UUR) (MELAS) mutation. We collected blood cells, muscle biopsies, urinary epithelium and hair follicles from 20 individuals, as well as oocytes and an ovarian biopsy from one female mutation carrier, all belonging to the two maternal lineages to assess mutant mtDNA load, and calculated the theoretical germline bottleneck size (number of segregating units). We also evaluated “mother-to-offspring” segregations from the literature, for which heteroplasmy assessment was available in at least three siblings besides the proband. Our results showed that mutation load was prevalent in skeletal muscle and urinary epithelium, whereas in blood cells there was an inverse correlation with age, as previously reported. The histoenzymatic staining of the ovarian biopsy failed to show any cytochrome-c-oxidase defective oocyte. Analysis of four oocytes and one offspring from the same unaffected mother of the first family showed intermediate heteroplasmic mutant loads (10% to 75%), whereas very skewed loads of mutant mtDNA (0% or 81%) were detected in five offspring of another unaffected mother from the second family. Bottleneck size was 89 segregating units for the first mother and 84 for the second. This was remarkably close to 88, the number of “segregating units” in the “mother-to-offspring” segregations retrieved from literature. In conclusion, a wide range of mutant loads may be found in offspring tissues and oocytes, resulting from a similar theoretical bottleneck size.

Preventing the transmission of pathogenic mitochondrial DNA mutations: Can we achieve long-term benefits from germ-line gene transfer?

Mitochondrial medicine is one of the few areas of genetic disease where germ-line transfer is being actively pursued as a treatment option. All of the germ-line transfer methods currently under development involve some carry-over of the maternal mitochondrial DNA (mtDNA) heteroplasmy, potentially delivering the pathogenic mutation to the offspring. Rapid changes in mtDNA heteroplasmy have been observed within a single generation, and so any ‘leakage’ of mutant mtDNA could lead to mtDNA disease in future generations, compromising the reproductive health of the first generation, and leading to repeated interventions in subsequent generations. To determine whether this is a real concern, we developed a model of mtDNA heteroplasmy inheritance by studying 87 mother–child pairs, and predicted the likely outcome of different levels of ‘mutant mtDNA leakage’ on subsequent maternal generations. This showed that, for a clinical threshold of 60%, reducing the proportion of mutant mtDNA to <5% dramatically reduces the chance of disease recurrence in subsequent generations, but transmitting >5% mutant mtDNA was associated with a significant chance of disease recurrence. Mutations with a lower clinical threshold were associated with a higher risk of recurrence. Our findings provide reassurance that, at least from an mtDNA perspective, methods currently under development have the potential to effectively eradicate pathogenic mtDNA mutations from subsequent generations.

Hypothyroidism decreases the biogenesis in free mitochondria and neuronal oxygen consumption in the cerebral cortex of developing rats

Thyroid hormone plays a critical role in mitochondrial biogenesis in two areas of the developing brain, the cerebral cortex and the striatum. Here we analyzed, in the cerebral cortex of neonatal rats, the effect of hypothyroidism on the biogenesis in free and synaptosomal mitochondria by analyzing, in isolated mitochondria, the activity of respiratory complex I, oxidative phosphorylation, oxygen consumption, and the expression of mitochondrial genome. In addition, we studied the effect of thyroid hormone in oxygen consumption in vivo by determining metabolic flow through (13)C nuclear magnetic resonance spectroscopy. Our results clearly show that in vivo, hypothyroidism markedly reduces oxygen consumption in the neural population of the cerebral cortex. This effect correlates with decreased free mitochondria biogenesis. In contrast, no effect was observed in the biogenesis in synaptosomal mitochondria. The parameters analyzed were markedly improved after T(3) administration. These results suggest that a reduced biogenesis and the subsequent reduction of respiratory capacity in free mitochondria could be the underlying cause of decreased oxygen consumption in the neurons of the cerebral cortex of hypothyroid neonates.

Longitudinal changes of mtDNA A3243G mutation load and level of functioning in MELAS

Mitochondrial encephalopathy, lactic acidosis, and stroke-like episodes (MELAS), one of the most common mitochondrial multisystemic diseases, is most commonly associated with an A-to-G transition at nucleotide position 3243 (A3243G) in mitochondrial DNA. We studied 34 individuals harboring the A3243G mutation for up to 7 years; 17 had the full MELAS phenotype and 17 who were classified as "carrier relatives" because they were either asymptomatic or had some symptoms suggestive of mitochondrial disease but no seizures or strokes. Using the sensitive real-time polymerase chain reaction to quantify the A3243G mutation, we confirmed that the percent mutation decreases progressively in DNA isolated from blood: the average percent decrease was 0.5% per year for fully symptomatic patients and 0.2% per year for oligosymptomatic carrier relatives. We also correlated mutant loads with functional status estimated by the Karnofksky score: even though the mutation load decreases, the level of functioning worsens in fully symptomatic patients, whereas the level of functioning of carrier relatives remains largely unchanged. This study suggests that A3243G mutant load in DNA isolated from blood is neither useful for prognosis nor for functional assessment.

Batteries not included: diagnosis and management of mitochondrial disease

In 1998, Wallace et al. (Science 1988; 242: 1427-30) published evidence that the mutation m.11778G>A was responsible for causing Leber's hereditary optic neuropathy. This was the first account of a mitochondrial DNA mutation being irrefutably linked with a human disease and was swiftly followed by a report from Holt et al. (Nature 1988; 331: 717-9) identifying deletions in mitochondrial DNA as a cause for myopathy. During the subsequent 20 years there has been an exponential growth in 'mitochondrial medicine', with clinical, biochemical and genetic characterizations of a wide range of mitochondrial diseases and evidence implicating mitochondria in a host of many other clinical conditions including ageing, neurodegenerative illness and cancer. In this review we shall focus on the diagnosis and management of mitochondrial diseases that lead directly or indirectly to disruption of the process of oxidative phosphorylation.

Selection against pathogenic mtDNA mutations in a stem cell population leads to the loss of the 3243A-->G mutation in blood

The mutation 3243A-->G is the most common heteroplasmic pathogenic mitochondrial DNA (mtDNA) mutation in humans, but it is not understood why the proportion of this mutation decreases in blood during life. Changing levels of mtDNA heteroplasmy are fundamentally related to the pathophysiology of the mitochondrial disease and correlate with clinical progression. To understand this process, we simulated the segregation of mtDNA in hematopoietic stem cells and leukocyte precursors. Our observations show that the percentage of mutant mtDNA in blood decreases exponentially over time. This is consistent with the existence of a selective process acting at the stem cell level and explains why the level of mutant mtDNA in blood is almost invariably lower than in nondividing (postmitotic) tissues such as skeletal muscle. By using this approach, we derived a formula from human data to correct for the change in heteroplasmy over time. A comparison of age-corrected blood heteroplasmy levels with skeletal muscle, an embryologically distinct postmitotic tissue, provides independent confirmation of the model. These findings indicate that selection against pathogenic mtDNA mutations occurs in a stem cell population.

Mitochondrial Disease—Its Impact, Etiology, and Pathology

Mitochondria are ubiquitous organelles that are intimately involved in many cellular processes, but whose principal task is to provide the energy necessary for normal cell functioning and maintenance. Disruption of this energy supply can have devastating consequences for the cell, organ, and individual. Over the last two decades, mutations in both mitochondrial DNA (mtDNA) and nuclear DNA have been identified as causative in a number of well-characterized clinical syndromes, although for mtDNA mutations in particular, this relationship between genotype and phenotype is often not straightforward. Despite this, a number of epidemiological studies have been undertaken to assess the prevalence of mtDNA mutations and these have highlighted the impact that mtDNA disease has on both the community and individual families. Although there has been considerable improvement in the diagnosis of mitochondrial disorders, disappointingly this has not been matched by developments toward effective treatment. Nevertheless, our understanding of mitochondrial biology is gathering pace and progress in this area will be crucial to devising future treatment strategies. In addition to mitochondrial disease, evidence for a central role of mitochondria in other processes, such as aging and neurodegeneration, is slowly accumulating, although their role in cancer remains controversial. In this chapter, we discuss these issues and offer our own views based on our cumulative experience of investigating and managing these diseases over the last 20 years.

A mitochondrial DNA mutation in a patient with an extensive family history of Duchenne muscular dystrophy

One challenge in the molecular diagnosis of mitochondrial DNA (mtDNA) disorders is detection of a low percentage of mutant heteroplasmy. We report a patient who had a delayed molecular diagnosis of mitochondrial encephalopathy, lactic acidosis, and stroke-like episodes (MELAS) syndrome due to the complication of an extensive family history of another neuromuscular disease, Duchenne muscular dystrophy, and the failure to detect a low proportion of mutant A3243G mtDNA with a polymerase chain reaction (PCR)/restriction fragment length polymorphism (RFLP)/ethidium bromide detection method. Using an improved, more sensitive allele-specific oligonucleotide (ASO) radioactive dot-blot hybridization method, a low degree of A3243G heteroplasmy was detected in several tissues from this patient. This case underscores the importance of a sensitive mutation detection method and the need for a search for mtDNA mutations if the patient's clinical symptoms suggest a mitochondrial disorder despite the family background of another neuromuscular disease.

Oxidative capacity correlates with muscle mutation load in mitochondrial myopathy

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

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.

Genotype-phenotype correlations and differential diagnosis in autosomal dominant macular disease

In the past few years, great progress has been made in the understanding of macular diseases. A number of disease-causing genes have been cloned, and numerous loci for other conditions have been mapped. The purpose of this article is to provide an overview of the current understanding of the genotype-phenotype correlations in autosomal dominant macular diseases with an emphasis on differential diagnostic issues. Whenever possible, the molecular correlates have been reviewed and the implications for age-related macular degeneration have been discussed. The many similarities of these diseases to age-related macular degeneration of the atrophic or exudative type, which can be misleading in elderly subjects, have also been addressed. While some conditions yield disease truly confined to the macula, others show widespread retinal involvement on functional testing. Clear-cut genotype-phenotype correlations are possible only for some forms of macular diseases. To further complicate the diagnostic process, there is a considerable degree of clinical overlap between many of them, making the differential diagnostic process potentially challenging. Functional testing, careful assessment of family history and extensive family work-up are essential in differentiating at the clinical level most, but not all, of these disease entities. Awareness of all of these conditions is required to direct correctly diagnostic investigations, to formulate an accurate prognosis, and for proper genetic counseling.

De novo mutation in the mitochondrial tRNALeu(UUR) gene (A3243G) with rapid segregation resulting in MELAS in the offspring

A 14-year-old Chinese boy with a normal perinatal and early developmental history presented at 5 years of age with migraine, intractable epilepsy, ataxia, supraventricular tachycardia, paralytic ileus and progressive mental deterioration. Computerized tomography revealed multiple cerebral infarcts in the parieto-occipital region without basal ganglial calcification. Magnetic resonance imaging showed increased signal intensity in T2 weighted images in the same regions. A cerebral digital subtraction angiogram was normal. Venous lactate, pyruvate, lactate to pyruvate ratio and cerebrospinal fluid lactate were elevated. Muscle biopsy did not reveal any ragged red fibres; dinucleotide-tetrazolium reductase activity was normal. Mitochondrial DNA analysis detected an adenine to guanine mutation at nucleotide position 3243 of tRNALeu(UUR). All four tissues analysed demonstrated heteroplasmy: leucocyte 56%, hair follicle 70%; buccal cell 64%; muscle 54%. The mother and brother of the proband, both asymptomatic, were also found to have a heteroplasmic A3243G mutation in the leucocytes, hair follicle and buccal cells. Other members of the maternal lineage, including the maternal grandmother, did not have the mutation. This report describes a patient with mitochondrial encephalopathy, lactic acidosis, stroke-like episodes, who presented with multisystem involvement. The absence of ragged red fibres in muscle biopsy did not preclude the diagnosis. Mutational analysis of mitochondrial DNA conveniently confirmed the diagnosis of the disorder. A de novo mutation is demonstrated in this family.

Follow-up in carriers of the 'MELAS' mutation without strokes

Eight carriers of the A3243G mutation of mitochondrial DNA without stroke-like episodes were monitored for up to 7 years in clinical and metabolic studies, by magnetic resonance imaging (MRI) and positron emission tomography (PET). None developed mitochondrial encephalopathy (MELAS), but 2 developed diabetes mellitus, 1 terminal kidney failure and 2 cardiomyopathy. One patient improved markedly under ubiquinone. Electroencephalography showed progressive slowing in 2 cases, but electrophysiological tests and MRI were otherwise noncontributary. PET showed widespread cortical and basal ganglion metabolic deficits in 6 cases. We conclude that internal medical complications are more common than MELAS in adult carriers of the mutation. PET findings, firstly reported in such patients, suggest that chronic subclinical encephalopathy is very frequent, and PET may play a role in monitoring in the future.

Random mitotic segregation of mitochondrial DNA in MELAS syndrome

We describe the heterogeneity of clinical features and molecular genetic characteristics of the probands and other members in two families with mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes (MELAS) syndrome. A point mutation at the 3243rd nucleotide position of mtDNA was found only in some of the maternal lineage members of the two families. Furthermore, the proportions of mutant mtDNA were varied and found only in some tissues of the individuals. Intriguingly, in some subjects, the mutant mtDNA was found in blood cells or hair follicles but was absent in muscles. The data do not support the notion of a selective advantage of wild-type mtDNA to rapidly replicating cells. We suggest that a rapid replicative segregation may occur in early embryogenesis.

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.

Did de novo MELAS common mitochondrial DNA point mutation (mtDNA 3243, A-->G transition) occur in the mother of a proband of a Japanese MELAS pedigree?

MELAS is a major maternally inherited mitochondrial (mt) encephalomyopathy of which 80% of cases are associated with mtDNA point mutation (mtDNA 3243, A-->G transition) which exists under heteroplasmic conditions with wild-type mtDNA. The origin of this mutation remains obscure in the reported pedigrees. I analyzed this mutation in a Japanese MELAS pedigree by PCR. The proband had typical MELAS features. The proband's mother was oligosymptomatic (fatigability, nerve deafness and diabetes mellitus). The proband's maternal grandmother was diagnosed as having senile dementia of the Alzheimer type clinically. The brother of the proband's mother was healthy. The ratios of this mutation in muscle and leukocytes of the proband and his mother were 89%, 36%, 79% and 10%, respectively. There were no mutations in muscle and leukocytes of the proband's maternal grandmother and his mother's brother. These results showed the possibility that this mutation occurred in the proband's mother.

Clinical spectrum of the MELAS mutation in a large pedigree

INTRODUCTION

MELAS is most often due to an mentally transmitted A-G transition mutation of mitochondrial DNA (mtDNA) at position 3243. In this study we report on the clinical spectrum associated with the mutation in the largest family reported so far.

PATIENTS AND METHODS

In a family with three MELAS cases we identified 47 persons at risk for the mutation; sufficient data was available on 29. Mitochondrial disease was diagnosed in two of 9 deceased numbers (posthumous molecular analysis in one); 27 surviving family members underwent examination and 25 a molecular analysis of mt DNA from lymphoblasts. Then had a muscle biopsy and two were later autopsied.

RESULTS

All 26 cases investigated by molecular analysis showed the mutation at position 3243. The 18 symptomatic patients without stroke-like episodes had sensorineural hearing loss in 15 cases, diabetes in 6, nephropathy in 7, mild myopathy in 4, cardiomyopathy in 2, cerebellar disease in 4 and mental retardation in 2 cases. Eight carriers were asymptomatic. Autopsy showed > 80% mutant mt DNA in all tissues except blood (20%) examined in a MELAS patients, but < 20 mutant mt DNA in all tissues except lever (40%) and kidney (70%) in a patient with hepatopathy, renal failure and diabetes. Histologic and biochemical studies of muscle biopsy were often non-informative.

CONCLUSIONS

The mutation of mt DNA at position 3243 causes a multisystem disorder with a variable phenotype due to heteroplasmy. Most carriers are oligosymptomatic with hearing loss and a variety of neurological and internal medical symptoms. Diabetes, cardiomyopathy and renal disease, which is newly reported here for this mutation, are frequent. The blood test is a reliable screening tool in affected families, but is of prognostic value only combined with examination of other tissues.

Complementation and segregation behavior of disease-causing mitochondrial DNA mutations in cellular model systems

The recent development of cellular models of mitochondrial DNA-linked diseases by transfer of patient-derived mitochondria into human mtDNA-less (rho o) cells has provided a valuable tool for investigating the complementation and segregation of mtDNA mutations. In transformants carrying in heteroplasmic form the mitochondrial tRNA(Lys) gene 8344 mutation or tRNA(Leu(UUR)) gene 3243 mutation associated, respectively, with the MERRF or the MELAS encephalomyopathy, full protection of the cells against the protein synthesis and respiration defects caused by the mutations was observed when the wild-type mtDNA exceeded 10% of the total complement. In the MERRF transformants, the protective effect of wild-type mtDNA was shown to involve interactions of the mutant and wild-type gene products, probably coexisting within the same organelle from the time of the mutation event. In striking contrast, in experiments in which two mtDNAs carrying either the MERRF or the MELAS mutation were sequentially introduced within distinct organelles into the same rho o cells, no evidence of cooperation between their products was observed. These results pointed to the phenotypic independence of the two genomes. A similar conclusion was reached in experiments in which a chloramphenicol (CAP) resistance-conferring mtDNA mutation was introduced into CAP-sensitive cells. In the area of segregation of mtDNA mutations, in unstable heteroplasmic MELAS transformants, observations were made which pointed to a replicative advantage of mutant molecules, leading to a rapid shift of the genome towards the mutant type. These results are consistent with a model in which the mitochondrion, rather than the mtDNA molecule, is the segregating unit.

Germ-line therapy to cure mitochondrial disease: protocol and ethics of in vitro ovum nuclear transplantation

The combination of genuine ethical concerns and fear of learning to use germ-line therapy for human disease must now be confronted. Until now, no established techniques were available to perform this treatment on a human. Through an integration of several fields of science and medicine, we have developed a nine step protocol at the germ-line level for the curative treatment of a genetic disease. Our purpose in this paper is to provide the first method to apply germ-line therapy to treat those not yet born, who are destined to have a life threatening, or a severely debilitating genetic disease. We hope this proposal will initiate the process of a thorough analysis from both the scientific and ethical communities. As such, this proposal can be useful for official groups studying the advantages and disadvantages of germ-line therapy.

MELAS syndrome: correlation between clinical features and molecular genetic analysis

The clinical manifestations and mitochondrial DNA (mtDNA) mutations in a Taiwanese family with a female proband exhibiting mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes syndrome are reported. Clinically, the proband had a stroke-like episode with right hemiparesis, hemianopsia and mental dysfunction as well as short stature, hearing impairments, and elevated lactate levels. Brain magnetic resonance images showed multiple increased signal intensities over the left frontal, parietal and temporal areas. There were no ragged-red fibers, but paracrystalline inclusion bodies were shown in the muscle biopsies under electron microscopic examination. A deficiency of NADH-CoQ reductase was also found in biochemical studies of the muscles. The family survey revealed no abnormal findings except for headache and episodic vomiting in her mother. The molecular analysis of mtDNA disclosed a mutation from A to G at the nucleotide pair 3243 of the mitochondrial transfer RNA(Leu) gene in the blood, hair follicles and/or muscle of the maternal relatives. A characteristic finding of the MELAS family is variation of percentage of mutated mtDNA in various tissues and individuals. However, a higher proportion of mutated mtDNA was noted in the proband than that in the asymptomatic or oligosymptomatic family members. From the data, the variable clinical phenotypes in this MELAS family can be explained at least partly, by the different proportions of mutant mtDNA in the target tissues of the proband and maternal relatives.

Extreme variability of clinical symptoms among sibs in a MELAS family correlated with heteroplasmy for the mitochondrial A3243G mutation

In a family with mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes with extremely varying clinical expression, we have identified the A3243G heteroplasmic point mutation in mitochondrial DNA. The degree of severity of the clinical symptoms in the various family members was reflected in the relative quantity of mutated mitochondrial DNA in different tissues. The biochemical activity of complex I of the respiratory chain in muscle was decreased in some members of this family.

MELAS syndrome with mitochondrial tRNA(Leu(UUR)) gene mutation in a Chinese family

The clinical features of a patient in a Chinese family with mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes (MELAS syndrome) are reported. The study revealed that hearing and visual impairments and miscarriages may be early clinical presentations in MELAS. A heteroplasmic A to G transition in the tRNA(Leu(UUR)) gene was noted at the nucleotide pair 3243 in the mitochondrial DNA of muscle, blood, and hair follicles of the proband and his maternal relatives. Quantitative analysis of the mutated mitochondrial DNA revealed variable proportions in different tissues and subjects of maternal lineage in the family. Muscle tissue contained a higher proportion of the mutant mitochondria than other tissues examined. The function of the reproductive system of the proband seems to be impaired. In one clinically healthy sibling, the 3243rd point mutation was found in sperm mitochondrial DNA, although sperm motility was not affected. It seems that biochemical defects in mitochondrial respiration and oxidative phosphorylation are tissue specific expressions of the 3243rd point mutation in the mitochondrial DNA of the affected target tissues.

Mitochondrial myopathy, encephalopathy, lactic acidosis, and strokelike episodes (MELAS): current concepts

Mitochondrial encephalomyopathy, lactic acidosis, and strokelike episodes (MELAS) syndrome is one of many mitochondrially inherited multisystem diseases. The features of 110 reported mitochondrial encephalomyopathy, lactic acidosis, and strokelike episodes patients are reviewed to define the clinical spectrum of this disease. The clinical disorder, in addition to emerging concepts of genetic etiology, is promoting our understanding of mitochondrial functions. New knowledge may lead to more rational therapies. Finally, the recent revolution in the study of mitochondrial diseases may further our understanding of other degenerative disorders and even aging.

Mitochondrial cytopathies

Defects of the mitochondrial respiratory chain and mutations of mitochondrial DNA have now been associated with a wide range of human diseases. The precise pathogenetic mechanisms by which these biochemical abnormalities induce tissue dysfunction are not understood. The identification of a mutation in the proline anticodon and in the 12S RNA genes of mitochondrial DNA are interesting new additions to the catalogue of pathogenetic mutations of this genome. The recent demonstration of nuclear complementation of mitochondrial DNA depletion provides the opportunity to identify nuclear genes involved in mitochondrial DNA replication. The possible role for mitochondrial deficiencies in certain neurodegenerative diseases and in the ageing process have given additional momentum to research in this area. Treatment for the mitochondrial 'cytopathies' remains disappointing and improvement in this area awaits a better understanding of their aetiology.

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

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