Mitochondrial myopathies. Clinical, morphological and biochemical aspects

Melatonin: an ancient molecule that makes oxygen metabolically tolerable

Melatonin is remarkably functionally diverse with actions as a free radical scavenger and antioxidant, circadian rhythm regulator, anti-inflammatory and immunoregulating molecule, and as an oncostatic agent. We hypothesize that the initial and primary function of melatonin in photosynthetic cyanobacteria, which appeared on Earth 3.5-3.2 billion years ago, was as an antioxidant. The evolution of melatonin as an antioxidant by this organism was necessary as photosynthesis is associated with the generation of toxic-free radicals. The other secondary functions of melatonin came about much later in evolution. We also surmise that mitochondria and chloroplasts may be primary sites of melatonin synthesis in all eukaryotic cells that possess these organelles. This prediction is made on the basis that mitochondria and chloroplasts of eukaryotes developed from purple nonsulfur bacteria (which also produce melatonin) and cyanobacteria when they were engulfed by early eukaryotes. Thus, we speculate that the melatonin-synthesizing actions of the engulfed bacteria were retained when these organelles became mitochondria and chloroplasts, respectively. That mitochondria are likely sites of melatonin formation is supported by the observation that this organelle contains high levels of melatonin that are not impacted by blood melatonin concentrations. Melatonin has a remarkable array of means by which it thwarts oxidative damage. It, as well as its metabolites, is differentially effective in scavenging a variety of reactive oxygen and reactive nitrogen species. Moreover, melatonin and its metabolites modulate a large number of antioxidative and pro-oxidative enzymes, leading to a reduction in oxidative damage. The actions of melatonin on radical metabolizing/producing enzymes may be mediated by the Keap1-Nrf2-ARE pathway. Beyond its direct free radical scavenging and indirect antioxidant effects, melatonin has a variety of physiological and metabolic advantages that may enhance its ability to limit oxidative stress.

Energy metabolism disorders in rare and common diseases. Toward bioenergetic modulation therapy and the training of a new generation of European scientists

Energy metabolism alterations are found in a large number of rare and common diseases of genetic or environmental origin. The number of patients that could benefit from bioenergetic modulation therapy (BIOMET) is therefore very important and includes individuals with pathologies as diverse as mitochondrial diseases, acute coronary syndrome, chronic kidney disease, asthma or even cancer. Although, the alteration of energy metabolism is disease specific and sometimes patient specific, the strategies for BIOMET could be common and target a series of bioenergetic regulatory mechanisms discussed in this article. An excellent training of scientists in the field of energy metabolism, related human diseases and drug discovery is also crucial to form a young generation of MDs, PHDs and Pharma or CRO-group leaders who will discover novel personalized bioenergetic medicines, through pharmacology, genetics, nutrition or adapted exercise training. The Mitochondrial European Educational Training (MEET) consortium was created to pursue this goal, and we dedicated here a special issue of Organelle in Focus (OiF) to highlight their objectives. A total of 10 OiFs articles constitute this Directed Issue on Mitochondrial Medicine. As part of this editorial article, we asked timely questions to the PR. Jan W. Smeitink, professor of Mitochondrial Medicine and CEO of Khondrion, a mitochondrial medicine company. He shared with us his objectives and strategies for the study of mitochondrial diseases and the identification of future treatments. This article is part of a Directed Issue entitled: Energy Metabolism Disorders and Therapies.

Multiple mitochondrial alterations in a case of myopathy

Mitochondrial alterations are the most common feature of human myopathies. A biopsy of quadriceps muscle from a 50-year-old woman exhibiting myopathic symptoms was examined by transmission electron microscopy. Biopsied fibers from quadriceps muscle displayed numerous subsarcolemmal mitochondria that contained crystalloids. Numbering 1-6 per organelle, these consisted of rows of punctuate densities measuring ∼0.34 nm; the parallel rows of these dots had a periodicity of ∼0.8 nm. The crystalloids were ensconced within cristae or in the outer compartment. Some mitochondria without crystalloids had circumferential cristae, leaving a membrane-free center that was filled with a farinaceous material. Other scattered fibrocyte defects included disruption of the contractile apparatus or its sporadic replacement by a finely punctuate material in some myofibers. Intramitochondrial crystalloids, although morphologically striking, do not impair organelle physiology to a significant degree, so the muscle weakness of the patient must originate elsewhere.

Adult rat cardiomyocytes in culture A model system to study the plasticity of the differentiated cardiac phenotype at the molecular and cellular levels

Adult rat cardiomyocytes (ARCs) in long-term culture, which show a distinct adaptive flexibility, are presented as a system to study cardiac cell hypertrophy in vitro. In the first 1-2 weeks after isolation, ARCs undergo a process of de- and redifferentiation during which the cell morphology is remodeled and the myofibrillar apparatus is restructured, accompanied by a cell enlargement. The growing cells spread and eventually establish new cell-cell contacts, which display newly formed intercalated discs; synchronous cell beating is resumed in the resulting tissuelike sheet. During myofibrillogenesis, the early fetal program of gene expression is reactivated for several genes, as is observed during hemodynamic overload hypertrophy. The cells resume hormonal activity and express atrial natriuretic factor (ANF); the expression pattern of ANF is also reminiscent of that seen in hypertrophy. In cells grown in a medium conditioned by 12-day ARCs, though, myofibrillogenesis is accelerated and accompanied by a downregulation of ANF. In a creatine-deficient medium, on the other hand, the ARCs display giant mitochondria with paracrystalline inclusions imitating a situation found, for example, in mitochondrial myopathies.

Blood metabolite data in response to maximal exercise in healthy subjects

Maximal exercise test with gas exchange measurement evaluates exercise capacities with maximal oxygen uptake (VO(2) max) measurement. Measurements of lactate (L), lactate/pyruvate ratio (L/P) and ammonium (A) during rest, exercise and recovery enhance interpretative power of maximal exercise by incorporating muscular metabolism exploration. Maximal exercise test with gas exchange measurement is standardized in cardiopulmonary evaluations but, no reference data of blood muscular metabolites are available to evaluate the muscular metabolism. We determined normal values of L, L/P and A during a standardized maximal exercise and recovery in 48 healthy sedentary volunteers and compared with results obtained in four patients with exercise intolerance and a mitochondrial disease. In healthy subjects, L, L/P and A rose during exercise. In 98% of them L, L/P or A decreased between the fifth and the fifteenth minutes of recovery. In mitochondrial patients, VO(2) max was normal or low, and L, L/P and A had the same evolution as normal subjects or showed no decrease during recovery. We gave normal L, L/P and A values, which establish references for a maximal exercise test with muscular metabolism exploration. This test is helpful for clinicians in functional evaluation, management and treatment of metabolic myopathy and would be a useful tool in diagnosis of metabolic myopathy.

Hyperactivation of oxidative mitochondrial metabolism in epithelial cancer cells in situ: visualizing the therapeutic effects of metformin in tumor tissue

We have recently proposed a new mechanism for explaining energy transfer in cancer metabolism. In this scenario, cancer cells behave as metabolic parasites, by extracting nutrients from normal host cells, such as fibroblasts, via the secretion of hydrogen peroxide as the initial trigger. Oxidative stress in the tumor microenvironment then leads to autophagy-driven catabolism, mitochondrial dys-function, and aerobic glycolysis. This, in turn, produces high-energy nutrients (such as L-lactate, ketones, and glutamine) that drive the anabolic growth of tumor cells, via oxidative mitochondrial metabolism. A logical prediction of this new "parasitic" cancer model is that tumor-associated fibroblasts should show evidence of mitochondrial dys-function (mitophagy and aerobic glycolysis). In contrast, epithelial cancer cells should increase their oxidative mitochondrial capacity. To further test this hypothesis, here we subjected frozen sections from human breast tumors to a staining procedure that only detects functional mitochondria. This method detects the in situ enzymatic activity of cytochrome C oxidase (COX), also known as Complex IV. Remarkably, cancer cells show an over-abundance of COX activity, while adjacent stromal cells remain essentially negative. Adjacent normal ductal epithelial cells also show little or no COX activity, relative to epithelial cancer cells. Thus, oxidative mitochondrial activity is selectively amplified in cancer cells. Although COX activity staining has never been applied to cancer tissues, it could now be used routinely to distinguish cancer cells from normal cells, and to establish negative margins during cancer surgery. Similar results were obtained with NADH activity staining, which measures Complex I activity, and succinate dehydrogenase (SDH) activity staining, which measures Complex II activity. COX and NADH activities were blocked by electron transport inhibitors, such as Metformin. This has mechanistic and clinical implications for using Metformin as an anti-cancer drug, both for cancer therapy and chemo-prevention. We also immuno-stained human breast cancers for a series of well-established protein biomarkers of metabolism. More specifically, we now show that cancer-associated fibroblasts over-express markers of autophagy (cathepsin B), mitophagy (BNIP3L), and aerobic glycolysis (MCT4). Conversely, epithelial cancer cells show the over-expression of a mitochondrial membrane marker (TOMM20), as well as key components of Complex IV (MT-CO1) and Complex II (SDH-B). We also validated our observations using a bioinformatics approach with data from > 2,000 breast cancer patients, which showed the transcriptional upregulation of mitochondrial oxidative phosphorylation (OXPHOS) in human breast tumors (p < 10(-20)), and a specific association with metastasis. Therefore, upregulation of OXPHOS in epithelial tumor cells is a common feature of human breast cancers. In summary, our data provide the first functional in vivo evidence that epithelial cancer cells perform enhanced mitochondrial oxidative phosphorylation, allowing them to produce high amounts of ATP. Thus, we believe that mitochondria are both the "powerhouse" and "Achilles' heel" of cancer cells.

Cardiac-specific induction of the transcriptional coactivator peroxisome proliferator-activated receptor gamma coactivator-1alpha promotes mitochondrial biogenesis and reversible cardiomyopathy in a developmental stage-dependent manner

Recent evidence has identified the peroxisome proliferator-activated receptor gamma coactivator-1alpha (PGC-1alpha) as a regulator of cardiac energy metabolism and mitochondrial biogenesis. We describe the development of a transgenic system that permits inducible, cardiac-specific overexpression of PGC-1alpha. Expression of the PGC-1alpha transgene in this system (tet-on PGC-1alpha) is cardiac-specific in the presence of doxycycline (dox) and is not leaky in the absence of dox. Overexpression of PGC-1alpha in tet-on PGC-1alpha mice during the neonatal stages leads to a dramatic increase in cardiac mitochondrial number and size coincident with upregulation of gene markers associated with mitochondrial biogenesis. In contrast, overexpression of PGC-1alpha in the hearts of adult mice leads to a modest increase in mitochondrial number, derangements of mitochondrial ultrastructure, and development of cardiomyopathy. The cardiomyopathy in adult tet-on PGC-1alpha mice is characterized by an increase in ventricular mass and chamber dilatation. Surprisingly, removal of dox and cessation of PGC-1alpha overexpression in adult mice results in complete reversal of cardiac dysfunction within 4 weeks. These results indicate that PGC-1alpha drives mitochondrial biogenesis in a developmental stage-dependent manner permissive during the neonatal period. This unique murine model should prove useful for the study of the molecular regulatory programs governing mitochondrial biogenesis and characterization of the relationship between mitochondrial dysfunction and cardiomyopathy and as a general model of inducible, reversible cardiomyopathy.

Mitochondrial DNA somatic mutations (point mutations and large deletions) and mitochondrial DNA variants in human thyroid pathology: a study with emphasis on Hürthle cell tumors

In an attempt to progress in the understanding of the relationship of mitochondrial DNA (mtDNA) alterations and thyroid tumorigenesis, we studied the mtDNA in 79 benign and malignant tumors (43 Hürthle and 36 non-Hürthle cell neoplasms) and respective normal parenchyma. The mtDNA common deletion (CD) was evaluated by semiquantitative polymerase chain reaction. Somatic point mutations and sequence variants of mtDNA were searched for in 66 tumors (59 patients) and adjacent parenchyma by direct sequencing of 70% of the mitochondrial genome (including all of the 13 OXPHOS system genes). We detected 57 somatic mutations, mostly transitions, in 34 tumors and 253 sequence variants in 59 patients. Follicular and papillary carcinomas carried a significantly higher prevalence of non-silent point mutations of complex I genes than adenomas. We also detected a significantly higher prevalence of complex I and complex IV sequence variants in the normal parenchyma adjacent to the malignant tumors. Every Hürthle cell tumor displayed a relatively high percentage (up to 16%) of mtDNA CD independently of the lesion's histotype. The percentage of deleted mtDNA molecules was significantly higher in tumors with D-loop mutations than in mtDNA stable tumors. Sequence variants of the ATPase 6 gene, one of the complex V genes thought to play a role in mtDNA maintenance and integrity in yeast, were significantly more prevalent in patients with Hürthle cell tumors than in patients with non-Hürthle cell neoplasms. We conclude that mtDNA variants and mtDNA somatic mutations of complex I and complex IV genes seem to be involved in thyroid tumorigenesis. Germline polymorphisms of the ATPase 6 gene are associated with the occurrence of mtDNA CD, the hallmark of Hürthle cell tumors.

The genetics and pathology of oxidative phosphorylation

The mitochondrial oxidative phosphorylation (OXPHOS) system is the final biochemical pathway in the production of ATP. The OXPHOS system consists of five multiprotein complexes, the individual subunits of which are encoded either by the mitochondrial or by the nuclear genome. Defects in the OXPHOS system result in devastating, mainly multisystem, diseases, and recent years have seen the description of the underlying genetic mutations in mitochondrial and nuclear genes. Advances in this arena have profited from progress in various genome projects, as well as improvements in our ability to create relevant animal models.

Altered mitochondrial sensitivity for ADP and maintenance of creatine-stimulated respiration in oxidative striated muscles from VDAC1-deficient mice

Voltage-dependent anion channels (VDACs) form the main pathway for metabolites across the mitochondrial outer membrane. The mouse vdac1 gene has been disrupted by gene targeting, and the resulting mutant mice have been examined for defects in muscle physiology. To test the hypothesis that VDAC1 constitutes a pathway for ADP translocation into mitochondria, the apparent mitochondrial sensitivity for ADP (Km(ADP)) and the calculated rate of respiration in the presence of the maximal ADP concentration (Vmax) have been assessed using skinned fibers prepared from two oxidative muscles (ventricle and soleus) and a glycolytic muscle (gastrocnemius) in control and vdac1(-/-) mice. We observed a significant increase in the apparent Km((ADP)) in heart and gastrocnemius, whereas the V(max) remained unchanged in both muscles. In contrast, a significant decrease in both the apparent Km((ADP)) and V(max) was observed in soleus. To test whether VDAC1 is required for creatine stimulation of mitochondrial respiration in oxidative muscles, the apparent Km((ADP)) and Vmax were determined in the presence of 25 mm creatine. The creatine effect on mitochondrial respiration was unchanged in both heart and soleus. These data, together with the significant increase in citrate synthase activity in heart, but not in soleus and gastrocnemius, suggest that distinct metabolic responses to altered mitochondrial outer membrane permeability occur in these different striated muscle types.

Abnormal mitochondrial morphology in sporadic Parkinson's and Alzheimer's disease cybrid cell lines

Diseases linked to defective mitochondrial function are characterized by morphologically abnormal, swollen mitochondria with distorted cristae. Several lines of evidence now suggest that sporadic forms of Parkinson's disease (PD) and Alzheimer's disease (AD) are linked to mitochondrial dysfunction arising from defects in mitochondrial DNA (mtDNA). Human neuroblastoma (SH-SY5Y) cells that are deficient in mtDNA (Rho(0)) were repopulated with mitochondria from AD or PD patients or age-matched controls. These cytoplasmic hybrid (cybrid) cell lines differ only in the source of their mtDNA. Differences between cybrid cell lines therefore arise from differences in mtDNA and provide a model for the study of how impaired mitochondrial function alters the mitochondria themselves and how these changes adversely affect the neuronal cells they occupy. Cybrid cell mitochondria were labeled with the mitochondrial membrane potential-sensitive dye, JC-1. Analysis of these JC-1 labeled mitochondria by confocal microscopy revealed that mitochondrial membrane potential was significantly reduced in both PD and AD cybrid cells when compared with controls. Ultrastructural examination showed that control cybrid cells contained small, morphologically normal, round or oval mitochondria with a dark matrix and regular distribution of cristae. PD cybrid cells contained a significant and increased percentage of mitochondria that were enlarged or swollen and had a pale matrix with few remaining cristae (0.26-0.65 microm(2)). AD cybrid cells also contained a significantly increased percentage of enlarged or swollen mitochondria (0.25-5.0 microm(2)) that had a pale matrix and few remaining cristae. Other pathological features such as crystal-like intramitochondrial inclusions and cytoplasmic inclusion bodies were also found in PD and AD cybrids. These observations suggest that transfer of PD or AD mtDNA into Rho(0) cells was sufficient to produce pathological changes in mitochondrial ultrastructure that are similar to those seen in other mitochondrial disorders. These data were reported in abstract form (Trimmer et al., 1998, Soc. Neurosci. Abstr. 24: 476).

PPAR gamma is required for placental, cardiac, and adipose tissue development

The nuclear hormone receptor PPAR gamma promotes adipogenesis and macrophage differentiation and is a primary pharmacological target in the treatment of type II diabetes. Here, we show that PPAR gamma gene knockout results in two independent lethal phases. Initially, PPAR gamma deficiency interferes with terminal differentiation of the trophoblast and placental vascularization, leading to severe myocardial thinning and death by E10.0. Supplementing PPAR gamma null embryos with wild-type placentas via aggregation with tetraploid embryos corrects the cardiac defect, implicating a previously unrecognized dependence of the developing heart on a functional placenta. A tetraploid-rescued mutant surviving to term exhibited another lethal combination of pathologies, including lipodystrophy and multiple hemorrhages. These findings both confirm and expand the current known spectrum of physiological functions regulated by PPAR gamma.

The impact of diabetes on CNS. Role of bioenergetic defects

To address the problem of the pathogenesis in diabetic neuropathy, rats were made diabetic by streptozotocin administration, and discrete brain regions, such as cortex, cerebellum, brainstem, thalamus, and hypothalamus, were sampled for assay of activities of electron transport chain complexes I-IV at 1 and 3 mo after induction of diabetes. Significant decrease was seen in activities of dinitrophenylhydrazine DNPH-coenzyme Q reductase (complex I), coenzyme Q cytochrome-c reductase (complex III), and cytochrome-c oxidase (complex IV) from discrete brain regions with more pronounced changes in complex I. The decline in the complex I, III, and IV activity was more severe in the 3-mo group. Succinate dehydrogenase (SDH) coenzyme Q reductase (complex II), which is an enzyme shared by tricarboxylic acid (TCA) cycle and electron transport chain, showed a significant increase under the same set of conditions. These results suggest that the bioenergetic impairment has an important role in the pathophysiology of diabetes.

The role of electron microscopy in the diagnosis of nonneoplastic muscle diseases

Accurate diagnosis of muscle disease is dependent on a careful clinical examination followed by the appropriate laboratory investigations, which in a contemporary diagnostic center should also include ultrastructural investigations. As is the case in other tissues, the interpretation of the ultrastructural abnormalities observed in muscle must take into consideration several factors, in particular the small sample size, possible artifacts, and the nonspecificity of changes. Despite the fact that the majority of ultrastructural changes seen in muscle are not specific, electron microscopic examination still provides important and unique clues regarding patterns of change that characterize certain disease entities. Since this detailed ultrastructural information cannot at present be obtained by any other means, it is anticipated that electron microscopy will still play a vital role in the diagnosis of the nonneoplastic muscle diseases, well into the twenty-first century.

Gas chromatographic-mass spectrometric metabolic profiling of patients with fatal infantile mitochondrial myopathy with de Toni-Fanconi-Debré syndrome

The metabolic profiles of three patients with fatal infantile mitochondrial myopathy with de Toni-Fanconi-Debré syndrome were studied by simultaneous analysis, after urease treatment of urinary organic acids, carbohydrates, polyols and amino acids using gas chromatography/mass spectrometry (GC/MS). All three patients persistently showed lactic aciduria, phosphaturia, glucosuria and generalized amino aciduria. This abnormal urinary metabolic profile was observed before the onset of any clinical symptoms, indicating that chemical diagnosis may be done presymptomatically. In one patient, the concentration of lactate increased in parallel with the severity of the clinical condition, whereas the urinary levels of 3-hydroxybutyrate, amino acids and glucose fluctuated and showed only a general tendency to increase with the clinical course. The above results suggest that simultaneous GC/MS analyses, without fractionation, of urinary metabolites facilitate not only the early chemical diagnosis either before or after the first onset, but also follow-up studies, providing an important index for the evaluation of the severity and clinical course in patients with this disorder.

Renal involvement in mitochondrial cytopathies

Mitochondrial cytopathies have long been regarded as neuromuscular diseases. However, an oxidative phosphorylation disorder may give rise to various symptoms in other organs or tissues which are dependent upon mitochondrial energy supply. A broad spectrum of clinical symptoms have been described in these patients, including renal symptoms. The most frequent is proximal tubular dysfunction with a more or less complete de Toni-Debré-Fanconi syndrome. A few patients have been reported with tubular acidosis, Bartter syndrome, chronic tubulointerstitial nephritis, or nephrotic syndrome. The diagnosis of a respiratory chain deficiency is difficult when only renal symptoms are present but should be easier when another seemingly unrelated symptom is observed. Metabolic screening for abnormal oxidoreduction status in plasma, including lactate/pyruvate and ketone body molar ratios, can help to identify patients for further investigations. These include the measurement of oxygen consumption by mitochondria, the assessment of mitochondrial respiratory enzyme activities by spectrophotometric studies, and, when possible, the molecular analysis of mitochondrial DNA. Any mode of inheritance can be observed: sporadic, autosomal dominant or recessive, or maternal inheritance. No satisfactory therapy is presently available for mitochondrial disorders.

Diagnosis of fatal infantile defects of the mitochondrial respiratory chain: age dependence and postmortem analysis of enzyme activities

We studied two diagnostic aspects of fatal infantile defects of the mitochondrial respiratory chain: the age dependence of muscle mitochondrial enzyme activities and the reliability of diagnosis from autopsy samples. In morphologically normal quadriceps muscle samples of 46 children between the ages of 3 days and 15 years, activities of complex I plus III (NADH:cytochrome c oxidoreductase) and complex II plus III (succinate:cytochrome c oxidoreductase) increased 2-fold during the first three years of life, while that of complex II (succinate dehydrogenase), complex IV (cytochrome c oxidase), and citrate synthase did not show significant correlation with age. We suggest that these changes are related to age and stress the importance of strictly age-matched controls when diagnosing a mitochondrial disease of early childhood. The value of autopsy samples in diagnostic studies was evaluated by comparing mitochondrial enzyme activities in quadriceps muscle from autopsies and from surgical biopsies. In quadriceps muscle mitochondria, all the enzyme activities studied remained stable for at least 3 h after death. Using age-matched controls and autopsy samples, we diagnosed a respiratory chain enzyme deficiency in two infants, and the defects were confirmed in cultured skin fibroblasts.

Cardiomyopathy in respiratory chain disorders

Disorders of mitochondrial oxidative phosphorylation may disturb cardiac energy metabolism and cause cardiomyopathy. Twenty one cases from the literature and one further patient with cardiomyopathy due to biochemically defined respiratory chain defects were reviewed for clinical course, morphology, and pathophysiological mechanisms of the cardiomyopathy. All cases showed concentric hypertrophy of the myocardium without an outflow tract obstruction. In most patients the cardiomyopathy was diagnosed early in infancy and showed rapid deterioration with death before the age of 2 years. Hypertrophy of the myocardium appears to result from swelling of the cardiomyocytes caused by accumulation of mitochondria and by morphologically abnormal megamitochondria.

Myopathy, lactic acidosis, and sideroblastic anemia: a new syndrome

We describe 2 sibs (brother and sister) with myopathy, sideroblastic anemia, lactic acidosis, mental retardation, microcephaly, high palate, high philtrum, distichiasis, and micrognathia. Very low levels of cytochromes a, b, and c were detected in the patients' muscle mitochondria. Deposition of iron within the mitochondria of bone marrow erythroblasts was observed on electron microscopy. Irregular and enlarged mitochondria with paracrystalline inclusions were also seen on electron microscopy of the patients' muscle specimen. Examination of DNA from the affected sibs showed no deletions in the mitochondrial DNA nor the mutations identified in the syndromes of mitochondrial myopathy, encephalopathy, lactic acidosis, and strokelike episodes (MELAS) or myoclonus, and epilepsy associated with rugged-red fibers (MERRF). Since the parents were first cousins and 2 of 6 sibs (male and female) were affected, we suggest that the syndrome expressed by our patients represents a previously unknown autosomal recessive disorder that includes mitochondrial myopathy, lactic acidosis, and sideroblastic anemia.

Non-Mendelian mitochondrial inheritance as a cause of progressive genetic sensorineural hearing loss

Awareness of non-Mendelian mitochondrial inheritance and of its role as an agent of genetic sensorineural hearing loss (SNHL) is recent. Mitochondria are passed on exclusively from the ovum to all the offspring of both sexes, a novel pattern of inheritance. Owing to the critical role of mitochondria in cellular energy metabolism, deletions or point mutations of the mitochondrial DNA often cause progressive SNHL and a variety of disorders in other organ systems (mitochondrial cytopathies). The clinical expression of mitochondrial diseases varies and depends on the proportion of mutated mitochondria in various body tissues, as well as the nature of the mutation or deletion. In order to determine how often SNHL occurs in mitochondrial diseases and what is its presenting symptom, and also whether SNHL is a marker for particular phenotypes, we carried out a review of published case reports of patients with an established diagnosis of mitochondrial disease. The review indicates that SNHL occurs at all ages and in virtually all variants of mitochondrial diseases. It is not clear whether SNHL is a marker for a more severe and more rapid course of disease; the lower prevalence of SNHL in descriptions of live patients than of those who had died may be an artifact of case selection reported in the literature. Mitochondrial disease needs to be considered in progressive hearing loss and better longitudinal audiometric study of established cases will be required to answer these questions.

Absence of cytochrome c oxidase activity in a boy with dysfunction of renal tubules, brain and muscle

We report on a boy who developed proximal renal tubular acidosis with loss of carnitine at the age of about 6 months. A few months later he began to suffer from progressive muscular weakness and neurological disturbances. Blood biochemistry showed elevated lactate and beta-hydroxybutyrate with increased lactate/pyruvate and beta-hydroxybutyrate/acetoacetate ratios. A high urinary excretion of lactate and citric acid cycle intermediates was found. These results indicated a defect of the mitochondrial respiratory chain. Analysis of biopsy material from skeletal muscle revealed low activities of all respiratory chain complexes. In muscle and fibroblasts cytochrome c-oxidase (complex IV) was absent. Despite high dose multi-vitamin therapy the boy died at the age of 30 months from central respiratory failure. At autopsy the neuropathological diagnosis of Leigh disease was made.

Mitochondrial creatine kinase activity in patients with disturbed energy generation in muscle mitochondria

Eleven patients with an established disturbance in muscle mitochondrial energy generation, in whom no defect in the pyruvate dehydrogenase complex or in the complexes of the respiratory chain could be detected, were investigated for a possible deficiency of mitochondrial creatine kinase (Mi-CK) (EC 2.7.3.2). Four patients with a defect in one of the complexes of the respiratory chain were also investigated for Mi-CK activity. In none of the investigated patients was Mi-CK deficiency found. Surprisingly, two of the four patients with a defect in one of the respiratory chain complexes showed enhanced activity of Mi-CK. It is concluded that Mi-CK deficiency is not frequently found as a primary defect in patients with disturbance in mitochondrial energy generation, but more patients should be examined to allow a definite conclusion.

Successful mitral valve replacement for MELAS

Successful mitral valve replacement for severe mitral regurgitation in a patient with mitochondrial encephalomyopathy is reported. Renal failure due to low cardiac output improved dramatically after mitral valve replacement and he was discharged 10 weeks after surgery. The surgical indication and timing for valvular dysfunction in patients with mitochondrial cardiomyopathy are discussed.

Problems with the biochemical diagnosis in mitochondrial (encephalo-)myopathies

Patients suffering from a mitochondrial (encephalo-)myopathy have a remarkable clinical heterogeneity. A reliable and extensive investigation must be performed in order to obtain a correct diagnosis, but many factors may influence the ultimate results of these investigations leading, under certain circumstances, to an incorrect diagnosis. Patients selection is of crucial importance. Metabolic examination of body fluids, particularly with respect to lactate accumulation, is used as a selection criterion for further examinations. Numerous aspects associated with this metabolic examination have been critically evaluated, including the phenomenon of other causes of lactic acidaemia apart from mitochondrial disorders. Correct performance of in vivo function tests may contribute to a reduction of the number of missed diagnoses. Selection of the controls for biochemical investigations must be accurately be performed to obtain reliable reference values. Knowledge of the age-dependency of the biochemical parameters is necessary for a correct interpretation. It goes without saying that the choice of the tissue for biochemical investigations is of utmost importance. Knowledge of the tissue-specific occurrence of some defects in the mitochondrial respiratory chain is necessary. The biochemical examinations can be performed both in biopsy and autopsy material but only under certain conditions. Diagnostic approach requires application of reliable biochemical methods which are described. One of the most intriguing aspects in the diagnosis of mitochondrial disorders is the significance of a defect in relation to the residual enzyme activity found in the patient. Moreover, attention is paid to relevant items such as the occurrence of multiple and secondary defects.(ABSTRACT TRUNCATED AT 250 WORDS)

Control of oxidative phosphorylation in rat muscle mitochondria: implications for mitochondrial myopathies

The control of oxidative phosphorylation has been studied in normal skeletal muscle isolated from the hind legs of rats. The control coefficients of different steps of oxidative phosphorylation on the flux of O2 consumption were determined by the inhibitor method and calculation was done according to the model of Gellerich et al. (FEBS Lett. 274 (1990) 167-170) using a non-linear regression fitting procedure. The respiration was recorded with pyruvate (+malate) and palmitoyl-L-carnitine (+malate) as respiratory substrates, which are the main substrates oxidized in the muscle. It appears that the control is broadly distributed among the different complexes of the respiratory chain, and of the ATP synthesis system. Our results also provide an explanation for the threshold effects often evidenced in the clinical manifestation of mitochondrial diseases.

Cerebral blood flow in mitochondrial myopathy, encephalopathy, lactic acidosis, and strokelike episodes

BACKGROUND AND PURPOSE

The precise mechanism of neurological symptoms with mitochondrial myopathy, encephalopathy, lactic acidosis, and strokelike episodes (MELAS) is still controversial. We investigated the correlation between strokelike episodes and cerebral blood flow in two patients with MELAS and discuss the pathogenesis of strokelike episodes with MELAS.

SUMMARY OF REPORT

Cerebral dynamic computed tomography and cerebral angiography were used to measure cerebral circulation in the first case, that of a 20-year-old woman with MELAS. The second subject was a 13-year-old female who was studied with xenon-enhanced computed tomography. The cerebral blood flow studies were performed 3-72 hours after the onset of strokelike episodes. Serial cerebral angiography, dynamic computed tomography, and xenon-enhanced computed tomography showed vasodilation localized in the affected cerebral cortexes during strokelike episodes, without any reduction in regional cerebral blood flow.

CONCLUSIONS

Our study suggests that the strokelike episodes associated with MELAS are different in origin from ischemic stroke.

A case of Kearns-Sayre syndrome with metaphyseal dysplasia

A case of mitochondrial encephalomyopathy (Kearns-Sayre syndrome) demonstrating bilateral symmetric metaphyseal dysplasia is reported. This represents a new association with the syndrome.

Translation rates of isolated liver mitochondria under conditions of hepatic mitochondrial proliferation

The hepatic mitochondrial content is increased in rats by treatment with the hypolipidaemic drug clofibrate and by administration of the cobalamin analogue hydroxycobalamin[c-lactam] (HCCL), an inhibitor of hepatic L-methylmalonyl-CoA mutase activity. As a first step in defining the mechanisms regulating liver mitochondrial contents in these models, the current studies were designed to test the hypothesis that hepatic mitochondrial proliferation is associated with enhanced translation rates of mitochondrial DNA gene products. Incorporation of [35S]methionine and [3H]leucine into protein was quantified in mitochondria isolated from control, clofibrate- and HCCL-treated rats. Use of multiple amino acid substrate concentrations permitted the maximal rate of translation (Vmax.) to be determined independent of endogenous amino acid concentrations. The Vmax. for methionine incorporation was not different in the models evaluated (0.062, 0.057 and 0.061 pmol/min per mg of mitochondrial protein in control, clofibrate- and HCCL-treated rats respectively). Similar results were obtained for leucine incorporation when absolute fractional radiolabel incorporation rates were analysed and when conventional Lineweaver-Burk analysis was employed. These results demonstrate no change in the intrinsic capacity of mitochondrial translation in these two models of hepatic mitochondrial proliferation.

Enzyme activities of the mitochondrial energy generating system in skeletal muscle tissue of preterm and fullterm neonates

Quadriceps muscle specimens from autopsy of 28 neonates (gestational age 25-42 weeks) were investigated to determine pyruvate and malate oxidation rates and several enzymes of the mitochondrial oxidative process. In general, the levels of all mitochondrial parameters measured, including carnitine levels, were lower in the neonates who died within the first week of life than those in the control group (age > 5 years). Pyruvate and malate oxidation rates (P < 0.05), activities of pyruvate dehydrogenase complex (P < 0.10) and succinate: cytochrome c oxidoreductase (P < 0.05) increased significantly with gestational age. Pyruvate oxidation rates (P < 0.05) as well as activities of citrate synthase (P < 0.05) and NADH:Q1 oxidoreductase (P < 0.05) were significantly lower in the group of very preterm infants at an age of 1-7 days compared with very preterm infants at an age between 3-8 weeks. We conclude from our study that special reference values are necessary for a correct biochemical diagnosis of mitochondrial encephalomyopathies in the neonatal period. Differences between preterm and fullterm children of the same age (1 week) indicate a maturational process in human muscle tissue during gestation. Comparison of two different age groups within the very preterm neonates point to a postnatal maturation of the mitochondrial energy metabolism, at least in preterm neonates.

Aplasia of the retinal vessels combined with optic nerve hypoplasia, neonatal epileptic seizures, and lactic acidosis due to mitochondrial complex I deficiency

A newborn male with mitochondrial complex I deficiency suffered from neonatal epileptic seizures, which later developed into infantile spasms. The infant was blind due to aplasia of the retinal vessels and hypoplasia of the optic nerve. There was congenital lactic acidosis, which persisted in later life. The boy was microcephalic and retarded. Muscular hypotonia later shifted to spasticity. Succinic acid was increased in urine. We assume that the aplasia of the retinal vessels is due to damage of the retinal ganglion cells caused by the mitochondrial disease in the first 3 to 4 months of pregnancy.

Mitochondrial diseases and myopathies: a series of muscle biopsy specimens with ultrastructural changes in the mitochondria

From 1986 to 1991, 472 muscle biopsy specimens from patients from different hospitals in Norway were examined. Of these, 364 were embedded for electron microscopy, and 194 were examined with electron microscopy. Ultrastructural alterations in the mitochondria were detected in 49 of these specimens. Characteristic electron microscopic findings included subsarcolemmal accumulation of abnormal mitochondria of various shapes and sizes, often containing electron-dense granules and sometimes lipid vacuoles in the mitochondria and diffusely electron-lucent matrix space. Paracrystalline inclusion bodies were seldom seen in specimens from young patients, but in some cases mitochondrial electron-dense granules at the cristae were found. These amorphous densities are consistent with lipoproteins, suggesting that they may represent an early stage of paracrystalline inclusions. Biochemical and genetic exploration of the patients with biopsy specimens suggesting mitochondrial disease indicated maternally genetic inheritance and an enzyme defect in the respiratory chain in 21 patients in two families. Three patients had MELAS syndrome, 7 Marinesco-Sjögren syndrome, and 2 Kearns-Sayre syndrome. Five family members had ptosis, cardiomyopathy, mild myopathy, and increased lactate in cerebrospinal fluid and serum. In addition to the diseases mentioned above, changes in the mitochondria were detected in other conditions such as Rett's syndrome (n = 1), ornithine transcarbamylase deficiency (n = 2), and hypothyroidism (n = 2) as well as in 3 patients with clinical and laboratory results indicative of inflammatory myopathy and 3 patients with clinical and laboratory findings consistent with peripheral neuropathy. It is concluded that, although ultrastructural changes in the mitochondria may represent unspecific findings, electron microscopic examination of muscle biopsy specimens is a useful screening method to select specimens for further biochemical analysis and to obtain an early and more precise diagnosis of the disease.

Mitochondrial disorders in pediatrics. Clinical, biochemical, and genetic implications

Disorders secondary to mitochondrial dysfunction are likely to be common in the population. The clinical aspects of these disorders are varied and may follow nonclassic inheritance patterns. These disorders serve to illustrate the importance of the clinical history and family history in directing diagnostic approaches.

A method for quantitative measurement of mitochondrial creatine kinase in human skeletal muscle

Defects in the mitochondrial energy generating system in patients with a mitochondrial myopathy are known to be localized in various enzyme complexes involved in energy production. Such a defect may exist at the level of mitochondrial creatine kinase (Mi-CK). On that account we have developed a method for measurement of the enzyme activity in human skeletal muscle biopsy material (greater than 10 mg). Interfering creatine kinase isoenzymes are removed by anion exchange and affinity chromatography. The activity of Mi-CK in reference skeletal muscle homogenates amounts to 240 +/- 88 mU/mg protein (30 +/- 8.0 mU/mg wet weight).

Combined deficiencies of the pyruvate dehydrogenase complex and enzymes of the respiratory chain in mitochondrial myopathies

In six patients with mitochondrial (encephalo-) myopathy investigations of skeletal muscle revealed a defect of pyruvate dehydrogenase complex (PDHC) in combination with one or more respiratory chain complex deficiencies. A combination of defects of this kind has not been reported previously. Five of the six patients presented within the 1st year of life and had a severe clinical course. Intrafamilial variability of the clinical course in dizygotic twins both suffering from a cytochrome c oxidase deficiency and one of them also from a PDHC deficiency suggests an additional effect of PDHC deficiency on the clinical symptoms. Immunoblot studies of PDHC in five of the patients revealed no abnormalities in their subunit pattern, rendering a defect of mitochondrial protein import or assembly unlikely. The finding of a combined PDHC and respiratory chain deficiency has implications for the diagnostic approach, for therapy and genetic counselling. The exact pathogenetic mechanism of this combination of defects remains to be elucidated.

Cytochrome c oxidase deficiency and long-chain acyl coenzyme A dehydrogenase deficiency with Leigh's subacute necrotizing encephalomyelopathy

A female infant was seen at the age of 2 months because of hypotonia, delayed motor development, and lactic acidosis, and she died at age 13 months due to respiratory failure. In a muscle specimen taken at 11 months and in a liver specimen obtained 1.5 hours postmortem, we found decreased activities of cytochrome c oxidase and long-chain acyl coenzyme A dehydrogenase. Neuropathological changes were typical for Leigh's subacute necrotizing encephalomyelopathy. To our knowledge, this is the first report of a combined defect of complex IV of the respiratory chain and of the long-chain specific acyl coenzyme A dehydrogenase of beta-oxidation in muscle and liver.

Clinical aspects of mitochondrial disorders

Mitochondrial disorders have long been regarded as neuromuscular diseases only. In fact, owing to the ubiquitous nature of the oxidative phosphorylation, a broad spectrum of clinical features should be expected in mitochondrial disorders. Here, we present eight puzzling observations which give support to the view that a disorder of oxidative phosphorylation can give rise to any symptom in any organ or tissue with any apparent mode of inheritance. Consequently, we suggest giving consideration to the diagnosis of a mitochondrial disorder when dealing with an unexplained association of symptoms, with an early onset and a rapidly progressive course involving seemingly unrelated organs. Determination of lactate/pyruvate and ketone body molar ratios in plasma can help to select patients at risk for this condition.

Mitochondrial encephalomyopathies in childhood. II. Clinical manifestations and syndromes

During a 4-year period 1984 to 1988, 20 children referred with manifestations of central nervous system or neuromuscular disease combined with hyperlactatemia were found to have a mitochondrial disease. Each diagnosis was based on the results of thorough biochemical and morphologic investigations. The patients were separated into one series with mainly encephalopathy (n = 14) and another with mainly myopathy (n = 6). The patients with encephalopathy had the following syndromes: Kearns-Sayre (n = 2), MERRF (myoclonus epilepsy and ragged red fibers; n = 2), MELAS (mitochondrial myopathy, encephalopathy, lactic acidosis, and strokelike episodes; n = 3), Alpers (n = 3), Leigh (n = 1), and other variants (n = 3). In patients with myopathy, three had hypertrophic nonobstructive cardiomyopathy. Ultrastructural abnormalities of mitochondria were the most common morphologic changes in the muscle biopsies. Complex I deficiency was most common in the patients with encephalopathy. All of the patients with myopathy had complex IV deficiency. Mutations of mitochondrial DNA were found in six patients with encephalopathy. We conclude that identification of defects at the DNA level and determination of the phenotypic expression with clinical, morphologic, and biochemical methods are fundamental for future rational classification of mitochondrial disorders.

Mitochondrial encephalomyopathies in childhood. I. Biochemical and morphologic investigations

During a 4-year period (1984 to 1988), 50 children referred with manifestations of central nervous system or neuromuscular disease combined with hyperlactatemia were subjected to investigations that aimed to identify and characterize children with mitochondrial disorders. Biochemical and morphologic investigations of quadriceps muscle biopsy tissue were done, including oximetric and spectrophotometric analysis of the respiratory chain function, enzyme histochemistry, electron microscopy, and analysis of mitochondrial DNA. A diagnosis of mitochondrial disease was based on the presence of at least two of five criteria: (1) abnormal results of oximetry, (2) abnormal results of spectrophotometry, (3) enzyme histochemical evidence of cytochrome x oxidase deficiency, (4) deletions or point mutations of mitochondrial DNA, and (5) abundant ultrastructurally abnormal mitochondria. With the combined biochemical and morphologic investigation, 20 of the children were found to have mitochondrial disorders. In an additional 10 children a mitochondrial disorder was neither excluded nor verified. Mitochondrial disorders are thus an important cause of central nervous system and neuromuscular disease in children with hyperlactatemia.

EEG findings in children and adolescents with mitochondrial encephalomyopathies: a study of 25 cases

EEG was studied in 25 children and adolescents with mitochondrial encephalomyopathies, defined on the basis of clinical, biochemical and morphological criteria. Twenty cases conformed to well-known mitochondrial syndromes: Alpers syndrome [6], Leigh syndrome [2], MERRF (myoclonus epilepsy and ragged red fibers) syndrome [3], MELAS (mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes) syndrome [5] and Kearns-Sayre syndrome [4]. Many patients were followed for several years with repeated EEG. In all, 112 EEG records were included in the study. A common feature of all the mitochondrial encephalomyopathic syndromes was slowing of the alpha rhythm. Epileptic discharges were seen in most syndromes. In spite of the small number of cases in each group, in Alpers, MERRF and MELAS syndromes we found sequential EEG patterns which seemed to be typical of the respective syndromes. In contrast, in Kearns-Sayre syndrome, a slow background rhythm was the only consistent finding. We conclude that EEG, especially repeated recordings, may be of help in the diagnostic evaluation of mitochondrial encephalomyopathies.