Mitochondrial respiratory chain deficiency correlates with the severity of neuropathology in sporadic Creutzfeldt-Jakob disease.

Mitochondrial dysfunction has been implicated in multiple neurodegenerative diseases but remains largely unexplored in Creutzfeldt-Jakob disease. Here, we characterize the mitochondrial respiratory chain at the individual neuron level in the MM1 and VV2 common molecular subtypes of sporadic Creutzfeldt-Jakob disease. Moreover, we investigate the associations between the mitochondrial respiratory chain and neuropathological markers of the disease.Brain tissue from individuals with sporadic Creutzfeldt-Jakob disease and age-matched controls were obtained from the brain collection of the Austrian Creutzfeldt-Jakob Surveillance. The mitochondrial respiratory chain was studied through a dichotomous approach of immunoreactivities in the temporal cortex and the hippocampal subregions of CA4 and CA3.We show that profound deficiency of all mitochondrial respiratory complexes (I-V) occurs in neurons of the severely affected temporal cortex of patients with Creutzfeldt-Jakob disease. This deficiency correlates strongly with the severity of neuropathological changes, including vacuolation of the neuropil, gliosis and disease associated prion protein load. Respiratory chain deficiency is less pronounced in hippocampal CA4 and CA3 regions compared to the temporal cortex. In both areas respiratory chain deficiency shows a predilection for the MM1 molecular subtype of Creutzfeldt-Jakob disease.Our findings indicate that aberrant mitochondrial respiration could be involved early in the pathogenesis of sporadic Creutzfeldt-Jakob disease and contributes to neuronal death, most likely via ATP depletion. Based on these results, we propose that the restricted MRI diffusion profile seen in the brain of patients with sporadic Creutzfeldt-Jakob disease might reflect cytotoxic changes due to neuronal respiratory chain failure and ATP loss.

Mitochondrial disruption in peroxisome deficient cells is hepatocyte selective but is not mediated by common hepatic peroxisomal metabolites.

The structural disruption of the mitochondrial inner membrane in hepatocytes lacking functional peroxisomes along with selective impairment of respiratory complexes and depletion of mitochondrial DNA was previously reported. In search for the molecular origin of these mitochondrial alterations, we here show that these are tissue selective as they do neither occur in peroxisome deficient brain nor in peroxisome deficient striated muscle. Given the hepatocyte selectivity, we investigated the potential involvement of metabolites that are primarily handled by hepatic peroxisomes. Levels of these metabolites were manipulated in L-Pex5 knockout mice and/or compared with levels in different mouse models with a peroxisomal ß-oxidation deficiency. We show that neither the deficiency of docosahexaenoic acid nor the accumulation of branched chain fatty acids, dicarboxylic acids or C27 bile acid intermediates are solely responsible for the mitochondrial anomalies. In conclusion, we demonstrate that peroxisomal inactivity differentially impacts mitochondria depending on the cell type but the cause of the mitochondrial destruction needs to be further explored.

The cytochrome bd-I respiratory oxidase augments survival of multidrug-resistant Escherichia coli during infection.

Nitric oxide (NO) is a toxic free radical produced by neutrophils and macrophages in response to infection. Uropathogenic Escherichia coli (UPEC) induces a variety of defence mechanisms in response to NO, including direct NO detoxification (Hmp, NorVW, NrfA), iron-sulphur cluster repair (YtfE), and the expression of the NO-tolerant cytochrome bd-I respiratory oxidase (CydAB). The current study quantifies the relative contribution of these systems to UPEC growth and survival during infection. Loss of the flavohemoglobin Hmp and cytochrome bd-I elicit the greatest sensitivity to NO-mediated growth inhibition, whereas all but the periplasmic nitrite reductase NrfA provide protection against neutrophil killing and promote survival within activated macrophages. Intriguingly, the cytochrome bd-I respiratory oxidase was the only system that augmented UPEC survival in a mouse model after 2 days, suggesting that maintaining aerobic respiration under conditions of nitrosative stress is a key factor for host colonisation. These findings suggest that while UPEC have acquired a host of specialized mechanisms to evade nitrosative stresses, the cytochrome bd-I respiratory oxidase is the main contributor to NO tolerance and host colonisation under microaerobic conditions. This respiratory complex is therefore of major importance for the accumulation of high bacterial loads during infection of the urinary tract.

A Transcript-Specific eIF3 Complex Mediates Global Translational Control of Energy Metabolism.

The multi-subunit eukaryotic translation initiation factor eIF3 is thought to assist in the recruitment of ribosomes to mRNA. The expression of eIF3 subunits is frequently disrupted in human cancers, but the specific roles of individual subunits in mRNA translation and cancer remain elusive. Using global transcriptomic, proteomic, and metabolomic profiling, we found a striking failure of Schizosaccharomyces pombe cells lacking eIF3e and eIF3d to synthesize components of the mitochondrial electron transport chain, leading to a defect in respiration, endogenous oxidative stress, and premature aging. Energy balance was maintained, however, by a switch to glycolysis with increased glucose uptake, upregulation of glycolytic enzymes, and strict dependence on a fermentable carbon source. This metabolic regulatory function appears to be conserved in human cells where eIF3e binds metabolic mRNAs and promotes their translation. Thus, via its eIF3d-eIF3e module, eIF3 orchestrates an mRNA-specific translational mechanism controlling energy metabolism that may be disrupted in cancer.

The Terminal Oxidase Cytochrome bd Promotes Sulfide-resistant Bacterial Respiration and Growth.

Hydrogen sulfide (H2S) impairs mitochondrial respiration by potently inhibiting the heme-copper cytochrome c oxidase. Since many prokaryotes, including Escherichia (E.) coli, generate H2S and encounter high H2S levels particularly in the human gut, herein we tested whether bacteria can sustain sulfide-resistant O2-dependent respiration. E. coli has three respiratory oxidases, the cyanide-sensitive heme-copper bo3 enzyme and two bd oxidases much less sensitive to cyanide. Working on the isolated enzymes, we found that, whereas the bo3 oxidase is inhibited by sulfide with half-maximal inhibitory concentration IC50 = 1.1 ± 0.1 µM, under identical experimental conditions both bd oxidases are insensitive to sulfide up to 58 µM. In E. coli respiratory mutants, both O2-consumption and aerobic growth proved to be severely impaired by sulfide when respiration was sustained by the bo3 oxidase alone, but unaffected by ≤200 µM sulfide when either bd enzyme acted as the only terminal oxidase. Accordingly, wild-type E. coli showed sulfide-insensitive respiration and growth under conditions favouring the expression of bd oxidases. In all tested conditions, cyanide mimicked the functional effect of sulfide on bacterial respiration. We conclude that bd oxidases promote sulfide-resistant O2-consumption and growth in E. coli and possibly other bacteria. The impact of this discovery is discussed.

Cerebellar ataxia and severe muscle CoQ10 deficiency in a patient with a novel mutation in ADCK3.

Inherited ataxias are a group of heterogeneous disorders in children or adults but their genetic definition remains still undetermined in almost half of the patients. However, CoQ10 deficiency is a rare cause of cerebellar ataxia and ADCK3 is the most frequent gene associated with this defect. We herein report a 48 year old man, who presented with dysarthria and walking difficulties. Brain magnetic resonance imaging showed a marked cerebellar atrophy. Serum lactate was elevated. Tissues obtained by muscle and skin biopsies were studied for biochemical and genetic characterization. Skeletal muscle biochemistry revealed decreased activities of complexes I+III and II+III and a severe reduction of CoQ10 , while skin fibroblasts showed normal CoQ10 levels. A mild loss of maximal respiration capacity was also found by high-resolution respirometry. Molecular studies identified a novel homozygous deletion (c.504del_CT) in ADCK3, causing a premature stop codon. Western blot analysis revealed marked reduction of ADCK3 protein levels. Treatment with CoQ10 was started and, after 1 year follow-up, patient neurological condition slightly improved. This report suggests the importance of investigating mitochondrial function and, in particular, muscle CoQ10 levels, in patients with adult-onset cerebellar ataxia. Moreover, clinical stabilization by CoQ10 supplementation emphasizes the importance of an early diagnosis.

Brain imaging in mitochondrial respiratory chain deficiency: combination of brain MRI features as a useful tool for genotype/phenotype correlations.

Mitochondrial diseases are characterised by a broad clinical and genetic heterogeneity that makes diagnosis difficult. Owing to the wide pattern of symptoms in mitochondrial disorders and the constantly growing number of disease genes, their genetic diagnosis is difficult and genotype/phenotype correlations remain elusive. Brain MRI appears as a useful tool for genotype/phenotype correlations. Here, we summarise the various combinations of MRI lesions observed in the most frequent mitochondrial respiratory chain deficiencies so as to direct molecular genetic test in patients at risk of such diseases. We believe that the combination of brain MRI features is of value to support respiratory chain deficiency and direct molecular genetic tests.

MTO1 mutations are associated with hypertrophic cardiomyopathy and lactic acidosis and cause respiratory chain deficiency in humans and yeast.

We report three families presenting with hypertrophic cardiomyopathy, lactic acidosis, and multiple defects of mitochondrial respiratory chain (MRC) activities. By direct sequencing of the candidate gene MTO1, encoding the mitochondrial-tRNA modifier 1, or whole exome sequencing analysis, we identified novel missense mutations. All MTO1 mutations were predicted to be deleterious on MTO1 function. Their pathogenic role was experimentally validated in a recombinant yeast model, by assessing oxidative growth, respiratory activity, mitochondrial protein synthesis, and complex IV activity. In one case, we also demonstrated that expression of wt MTO1 could rescue the respiratory defect in mutant fibroblasts. The severity of the yeast respiratory phenotypes partly correlated with the different clinical presentations observed in MTO1 mutant patients, although the clinical outcome was highly variable in patients with the same mutation and seemed also to depend on timely start of pharmacological treatment, centered on the control of lactic acidosis by dichloroacetate. Our results indicate that MTO1 mutations are commonly associated with a presentation of hypertrophic cardiomyopathy, lactic acidosis, and MRC deficiency, and that ad hoc recombinant yeast models represent a useful system to test the pathogenic potential of uncommon variants, and provide insight into their effects on the expression of a biochemical phenotype.

[Clinical, biochemical and genetic analysis of the mitochondrial disorders presenting with cardiac damage].

OBJECTIVE: Mitochondrial disease is a group of energy metabolic disorders, characterized by involvement of multisystem with high energy requirements. Encephalomyopathies are common clinical findings of the mitochondrial diseases. However, mitochondrial cardiac damage is not rare. In this study, the clinical, biological, and genetic analyses were performed in three patients with mitochondrial cardiac damage, in order to understand the characteristics of mitochondrial diseases. METHOD: Three girls presented with arrhythmia and cardiac enlargement from the age of 3, 4 and 8 years respectively. They were admitted into the Peking University First Hospital. Infection, autoimmune diseases, aminoacidopathies, organic acidurias, mitochondrial-fatty acid oxidation defects, and lysosomal storage disease were excluded by routine laboratory examinations and metabolic analysis for blood amino acids, acylcarnitines, urinary organic acids, and lysosome activity assay. Peripheral leukocytes mitochondrial respiratory chain enzyme I to V activities were measured by spectrophotometry. The entire sequence of the mitochondrial DNA was analyzed. RESULT: In two patients (case 1 and case 3), hypertrophic cardiomyopathy and grade I to grade II of cardiac function were found. One patient (case 2) was diagnosed with arrhythmia and grade I of cardiac function. Increased creatine phosphokinase and creatine kinase isoenzyme MB were observed. Mitochondrial respiratory chain complex deficiencies were indentified in the three patients. Patient 1 had combined deficiencies of complex III and V. The activity of complex I+III was 18.7 nmol/(min·mg mitochondrial protein) (control 84.4 ± 28.5). The activity of complex V was 20.4 nmol/(min·mg mitochondrial protein) (control 103.7 ± 29.2). In her mitochondrial gene, A14693G on tRNA(Glu) and T16519C on D-loop were found. Patient 2 had an isolated complex I deficiency. The activity was 22.0 nmol/(min·mg mitochondrial protein) (control 44.0 ± 5.4). A16183C, T16189C and G15043A mutations on D-loop were found. Patient 3 had a combined deficiency of complex IV and V. The activity of complex IV was 21.0 nmol/(min·mg mitochondrial protein) (control 54.1 ± 12.3). The activity of complex V was 23.2 nmol/(min·mg mitochondrial protein) (control 103.7 ± 29.2). C253T and C16187T mutations on D-loop were detected. Haplotype analysis showed that three patients belong to H2a2a. Improvement was observed after the treatment with L-carnitine, coenzyme Q10, vitamin C and E. At present, the patients are 7, 5 and 8 years old. Although excise intolerance still persists, they had a good general condition with normal school life. CONCLUSION: The mitochondrial diseases with cardiac damage show cardiomyopathy, arrhythmia and exercise intolerance. Many kinds of mitochondrial respiratory chain deficiency were observed. A14693G in mitochondrial tRNA(Glu) gene is probably one of the causes in China.

Starvation-induced autophagy is regulated by mitochondrial reactive oxygen species leading to AMPK activation.

Starvation is the most extensively studied condition that induces autophagy. Previous studies have demonstrated that starvation-induced autophagy is regulated by reactive oxygen species (ROS) such as superoxide (O(2)(·-)) but the source for ROS under starvation conditions and the downstream signaling pathways regulating autophagy are unclear. In this study, a cervical cancer HeLa cell line was generated that was deficient in mitochondrial electron transport chain (mETC) (HeLa ρ° cells). This resulted in endogenous levels of O(2)(·-) being significantly reduced and failed to be induced under starvation of glucose, L-glutamine, pyruvate, and serum (GP) or of amino acids and serum (AA) compared to wild type (wt) HeLa cells. In contrast, H(2)O(2) production failed to increase under GP starvation in both wild type and ρ° cells whereas it increased in wt cells but not in ρ° cells under AA starvation. GP or AA starvation induced autophagy was blocked in ρ° cells as determined by the amount of autophagosomes and autolysosomes. Autophagy is regulated by 5' adenosine monophosphate-activated protein kinase (AMPK) activation and AMPK is activated under starvation conditions. We demonstrate that ρ° cells and HeLa cells over expressing manganese-superoxide dismutase 2 (SOD2) cells fail to activate AMPK activation following starvation. This indicates that mitochondrial ROS might regulate AMPK activation. In addition, inhibiting AMPK activation either by siRNA or compound C resulted in reduced autophagy during starvation. Using a ROS scavenger NAC, AMPK activation is reduced under starvation condition and mTOR signaling is increased. Taken together, mitochondria-generated ROS induces autophagy mediated by the AMPK pathway under starvation conditions.

New mitochondrial tRNA HIS mutation in a family with lactic acidosis and stroke-like episodes (MELAS).

We report a new mutation in m.12146 A>G in the mt-tRNA(His) in a family with a remarkable clinical history having different degrees of lactic acidosis and stroke-like episodes. Biochemical measurements of a muscle biopsy established an isolated complex IV deficiency, while similar analysis of fibroblasts showed a combined complex I,III and IV deficiency. Transmitochondrial cybrid analysis proved that this tRNA(His) mutation causes the enzymatic deficiency. This family illustrates the complexity of the clinical, biochemical and genetic characteristics of a novel mtDNA encoded disorder, as well as the challenge to prove its pathogenicity.

An overview of a cohort of South African patients with mitochondrial disorders.

Mitochondrial disorders are frequently encountered inherited diseases characterized by unexplained multisystem involvement with a chronic, intermittent, or progressive nature. The objective of this paper is to describe the profile of patients with mitochondrial disorders in South Africa. Patients with possible mitochondrial disorders were accessed over 10 years. Analyses for respiratory chain and pyruvate dehydrogenase complex enzymes were performed on muscle. A diagnosis of a mitochondrial disorder was accepted only if an enzyme activity was deficient. Sixty-three patients were diagnosed with a mitochondrial disorder, including 40 African, 20 Caucasian, one mixed ancestry, and two Indian patients. The most important findings were the difference between African patients and other ethnicities: respiratory chain enzyme complexes CI+III or CII+III deficiencies were found in 52.5% of African patients, being of statistical significance (p value = 0.0061). They also presented predominantly with myopathy (p value = 0.0018); the male:female ratio was 1:1.2. Twenty-five (62.5%) African patients presented with varying degrees of a myopathy accompanied by a myopathic face, high palate, and scoliosis. Fourteen of these 25 also had ptosis and/or progressive external ophthalmoplegia. No patients of other ethnicities presented with this specific myopathic phenotype. Caucasian patients (16/20) presented predominantly with central nervous system involvement. Of the South African pediatric neurology patients, Africans are more likely to present with myopathy and CII+III deficiency, and Caucasian patients are more likely to present with encephalopathy or encephalomyopathy.

Approaches to finding the molecular basis of mitochondrial oxidative phosphorylation disorders.

Inherited disorders of mitochondrial oxidative phosphorylation are the most common group of inborn errors of metabolism and cause a wide range of clinical presentations. Mitochondrial DNA encodes 13 protein subunits required for oxidative phosphorylation plus 22 transfer RNAs and two ribosomal RNAs, and mutations in most of these genes cause human disease. Nuclear genes encode most of the protein subunits and all other proteins required for mitochondrial biogenesis and mitochondrial DNA replication and expression. Mutations in 64 nuclear genes and 34 mitochondrial genes are now known to cause mitochondrial disease and many novel mitochondrial disease genes await discovery. The genetic complexity of oxidative phosphorylation means that maternal, autosomal recessive, autosomal dominant and X-linked modes of inheritance can occur, along with de novo mutations. This complexity presents a challenge in planning efficient molecular genetic diagnosis of patients with suspected mitochondrial disease. In some situations, clinical phenotype can be strongly predictive of the underlying genotype. However, more often this is not the case and it is usually helpful, particularly with pediatric patients, to determine whether the activity of one or more of the individual oxidative phosphorylation enzymes is deficient before proceeding with mutation analysis. In this review we will summarize the genetic bases of mitochondrial disease and discuss some approaches to integrate information from clinical presentation, laboratory findings, family history, and imaging to guide molecular investigation.

Systematic evaluation of muscle coenzyme Q10 content in children with mitochondrial respiratory chain enzyme deficiencies.

Coenzyme Q10 content, pathology evaluation, and electron transport chain (ETC) enzyme analysis were determined in muscle biopsy specimens of 82 children with suspected mitochondrial myopathy. Data were stratified into three groups: "probable" ETC defects, "possible" ETC defects, and disease controls. Muscle total, oxidized, and reduced coenzyme Q10 concentrations were significantly decreased in the probable defect group. Stepwise logistic regression indicated that only total coenzyme Q10 was significantly associated with probable ETC defect. Receiver operator characteristic (ROC) analysis suggested that total muscle coenzyme Q10 was the best predictor of an ETC complex abnormality. Determination of muscle coenzyme Q10 deficiency in children with suspected mitochondrial disease may facilitate diagnosis and encourage earlier supplementation of this agent.

Fatal neonatal-onset mitochondrial respiratory chain disease with T cell immunodeficiency.

We present the clinical and laboratory features of a boy with a new syndrome of mitochondrial depletion syndrome and T cell immunodeficiency. The child suffered from severe recurrent infectious diseases, anemia, and thrombocytopenia. Clinically, he presented with severe psychomotor retardation, axial hypotonia, and a disturbed pain perception leading to debilitating biting of the thumb, lower lip, and tongue. Brain imaging showed hypoplasia of corpus callosum and an impaired myelinization of the temporo-occipital region with consecutive supratentorial hydrocephalus. Histologic examination of a skeletal muscle biopsy was normal. Biochemical investigation showed combined deficiency of respiratory chain complexes II+III and IV. MtDNA depletion was found by real-time PCR. No pathogenic mutations were identified in the TK2, SUCLA2, DGUOK, and ECGF1 genes. A heterozygous missense mutation was found in POLG1. The pathogenic relevance of this mutation is unclear. Interestingly, a lack of CD8(+) T lymphocytes as well as NK cells was also observed. The percentage of CD45RO-expressing cells was decreased in activated CD8(+) T lymphocytes. Activation of T lymphocytes via IL-2 was diminished. The occurrence of the immunologic deficiency in our patient with mtDNA depletion is a rare finding, implying that cells of the immune system might also be affected by mitochondrial disease.

Alterations of the mitochondrial proteome caused by the absence of mitochondrial DNA: A proteomic view.

The proper functioning of mitochondria requires that both the mitochondrial and the nuclear genome are functional. To investigate the importance of the mitochondrial genome, which encodes only 13 subunits of the respiratory complexes, the mitochondrial rRNAs and a few tRNAs, we performed a comparative study on the 143B cell line and on its Rho-0 counterpart, i.e., devoid of mitochondrial DNA. Quantitative differences were found, of course in the respiratory complexes subunits, but also in the mitochondrial translation apparatus, mainly mitochondrial ribosomal proteins, and in the ion and protein import system, i.e., including membrane proteins. Various mitochondrial metabolic processes were also altered, especially electron transfer proteins and some dehydrogenases, but quite often on a few proteins for each pathway. This study also showed variations in some hypothetical or poorly characterized proteins, suggesting a mitochondrial localization for these proteins. Examples include a stomatin-like protein and a protein sharing homologies with bacterial proteins implicated in tyrosine catabolism. Proteins involved in apoptosis control are also found modulated in Rho-0 mitochondria.

The importance of liver biopsy in the investigation of possible mitochondrial respiratory chain disease.

The diagnosis of mitochondrial respiratory chain deficiency is usually made by analysis of mitochondrial respiratory chain activity in muscle biopsy. We describe 4 patients in whom the diagnosis was based on mitochondrial respiratory chain deficiency in liver alone. In 3 patients, liver complex IV activity was deficient, and the 4th patient had liver complex I deficiency (relative to citrate synthase and complex II activity). The enzyme activities in skeletal muscle biopsies from these patients were normal or equivocal. The age at presentation and the neurological symptoms differed from one patient to another. All 3 patients with complex IV deficiency had non-specific white matter changes on brain MRI. None of the patients had clinical or biochemical evidence of liver disease. These findings illustrate the wide variety of presentations associated with liver mitochondrial respiratory chain deficiency. They also demonstrate the importance of mitochondrial respiratory chain enzyme analysis in liver, in addition to muscle, even in cases where the primary clinical deficit is neurological and there is no liver disease.

[Leigh Disease due to deficiency of mitochondrial respiratory chain complexes I, III and IV]. / Síndrome de Leigh con déficit de los complejos I, III y IV de la cadena respiratoria mitocondrial.

Leigh disease is a clinically heterogeneous and infrequent disorder in the pediatric age group. Inheritance is variable. It results from a genetic defect producing deficiencies in enzyme complexes and functional disturbance of the mitochondria. The prognosis is poor and effective treatment is lacking. We present the case of a 1-month-old boy with early manifestation and rapid progression of Leigh disease due to deficiency of mitochondrial respiratory chain complexes I, III and IV.

Síndrome de Leigh con déficit de los complejos I, III y IV de la cadena respiratoria mitocondrial / Leigh disease due to deficiency of mitochondrial respiratory chain complexes I, III and IV

La enfermedad de Leigh es un trastorno clínicamente heterogéneo y poco frecuente en la edad pediátrica, que presenta una forma de herencia variable. Se origina por una anomalía genética que condiciona déficit de complejos enzimáticos produciendo una alteración funcional mitocondrial. El pronóstico es malo y carece de tratamiento eficaz. Se presenta el caso de un lactante de un mes con aparición precoz y rápida evolución, en el que se halló un déficit de complejos I, III y IV de la cadena respiratoria mitocondrial

Leigh disease is a clinically heterogeneous and infrequent disorder in the pediatric age group. Inheritance is variable. It results from a genetic defect producing deficiencies in enzyme complexes and functional disturbance of the mitochondria. The prognosis is poor and effective treatment is lacking. We present the case of a 1-month-old boy with early manifestation and rapid progression of Leigh disease due to deficiency of mitochondrial respiratory chain complexes I, III and IV

Clinical spectrum, morbidity, and mortality in 113 pediatric patients with mitochondrial disease.

OBJECTIVES: The aim of this study was to elucidate the frequency of major clinical manifestations in children with mitochondrial disease and establish their clinical course, prognosis, and rates of survival depending on their clinical features. METHODS: We performed a retrospective review of the medical records of 400 patients who were referred for evaluation of mitochondrial disease. By use of the modified Walker criteria, only patients who were assigned a definite diagnosis were included in the study. RESULTS: A total of 113 pediatric patients with mitochondrial disease were identified. A total of 102 (90%) patients underwent a muscle biopsy as part of the diagnostic workup. A significant respiratory chain (RC) defect, according to the diagnostic criteria, was found in 71% of the patients who were evaluated. In this cohort, complex I deficiency (32%) and combined complex I, III, and IV deficiencies (26%) were the most common causes of RC defects, followed by complex IV (19%), complex III (16%), and complex II deficiencies (7%). Pathogenic mitochondrial DNA abnormalities were found in 11.5% of the patients. A substantial fraction (40%) of patients with mitochondrial disorders exhibited cardiac disease, diagnosed by Doppler echocardiography; however, the majority (60%) of patients had predominant neuromuscular manifestations. No correlation between the type of RC defect and the clinical presentation was observed. Overall, the mean age at presentation was 40 months. However, the mean age at presentation was 33 months in the cardiac group and 44 months in the noncardiac group. Twenty-six (58%) patients in the cardiac group exhibited hypertrophic cardiomyopathy, 29% had dilated cardiomyopathy, and the remainder (13%) had left ventricular noncompaction. Patients with cardiomyopathy had an 18% survival rate at 16 years of age. Patients with neuromuscular features but no cardiomyopathy had a 95% survival at the same age. CONCLUSIONS: This study gives strong support to the view that in patients with RC defects, cardiomyopathy is more common than previously thought and tends to follow a different and more severe clinical course. Although with a greater frequency than previously reported, mitochondrial DNA mutations were found in a minority of patients, emphasizing that most mitochondrial disorders of childhood follow a Mendelian pattern of inheritance.