Complete failure of insulin-transmitted signaling, but not obesity-induced insulin resistance, impairs respiratory chain function in muscle.

The role of mitochondrial dysfunction in the development of insulin resistance and type 2 diabetes remains controversial. In order to specifically define the relationship between insulin receptor (InsR) signaling, insulin resistance, hyperglycemia, hyperlipidemia and mitochondrial function, we analyzed mitochondrial performance of insulin-sensitive, slow-oxidative muscle in four different mouse models. In obese but normoglycemic ob/ob mice as well as in obese but diabetic mice under high-fat diet, mitochondrial performance remained unchanged even though intramyocellular diacylglycerols (DAGs), triacylglycerols (TAGs), and ceramides accumulated. In contrast, in muscle-specific InsR knockout (MIRKO) and streptozotocin (STZ)-treated hypoinsulinemic, hyperglycemic mice, levels of mitochondrial respiratory chain complexes and mitochondrial function were markedly reduced. In STZ, but not in MIRKO mice, this was caused by reduced transcription of mitochondrial genes mediated via decreased PGC-1α expression. We conclude that mitochondrial dysfunction is not causally involved in the pathogenesis of obesity-associated insulin resistance under normoglycemic conditions. However, obesity-associated type 2 diabetes and accumulation of DAGs or TAGs is not associated with impaired mitochondrial function. In contrast, chronic hypoinsulinemia and hyperglycemia as seen in STZ-treated mice as well as InsR deficiency in muscle of MIRKO mice lead to mitochondrial dysfunction. We postulate that decreased mitochondrial mass and/or performance in skeletal muscle of non-diabetic, obese or type 2 diabetic, obese patients observed in clinical studies must be explained by genetic predisposition, physical inactivity, or other still unknown factors.

Whole-body high-field MRI shows no skeletal muscle degeneration in young patients with recessive myotonia congenita.

BACKGROUND: Muscle magnetic resonance imaging (MRI) is the most sensitive method in the detection of dystrophic and non-dystrophic abnormalities within striated muscles. We hypothesized that in severe myotonia congenita type Becker muscle stiffness, prolonged transient weakness and muscle hypertrophy might finally result in morphologic skeletal muscle alterations reflected by MRI signal changes. AIM OF THE STUDY: To assess dystrophic and/or non-dystrophic alterations such as fatty or connective tissue replacement and muscle edema in patients with severe recessive myotonia congenita. METHODS: We studied three seriously affected patients with myotonia congenita type Becker using multisequence whole-body high-field MRI. All patients had molecular genetic testing of the muscle chloride channel gene (CLCN1). RESULTS: Molecular genetic analyses demonstrated recessive CLCN1 mutations in all patients. Two related patients were compound heterozygous for two novel CLCN1 mutations, Q160H and L657P. None of the patients showed skeletal muscle signal changes indicative of fatty muscle degeneration or edema. Two patients showed muscle bulk hypertrophy of thighs and calves in line with the clinical appearance. CONCLUSIONS: We conclude that (i) chloride channel dysfunction alone does not result in skeletal muscle morphologic changes even in advanced stages of myotonia congenita, and (ii) MRI skeletal muscle alterations in myotonic dystrophy must be clear consequences of the dystrophic disease process.

Rapid development of life-threatening complete atrioventricular block in Kearns-Sayre syndrome.

Kearns-Sayre syndrome is a rare mitochondrial disorder with defined diagnostic criteria. Knowledge of these diagnostic criteria and early diagnosis are important to ensure periodic electrocardiograms for identification of cardiac conduction disorders, which are the most important prognostic factor of the disease. We report on a 9-year-old girl with rapid development of a life-threatening complete atrioventricular block within 10 months and discuss the importance and time interval of regular electrocardiograms. Our patient survived by placing a temporary transvenous pacemaker lead followed by permanent pacemaker implantation a few days later.

Prenatal diagnosis of respiratory chain deficiency by direct mutation screening.

Respiratory chain deficiency (RCD) is responsible for a clinically heterogeneous group of early-onset untreatable disorders. Enzymological prenatal diagnosis (PD) can only be offered to a fraction of families. Moreover, due to the two-fold genetic origin of the respiratory chain (nuclear and mitochondrial DNA) and owing to the large number of nuclear genes involved in the respiratory chain assembly, maintenance and functioning, the identification of the disease causing gene in a given family remains challenging. Here, we report on PD of RCD by direct screening of NDUFV1, SDH-Fp, SCO1 and SURF1 mutations in five unrelated families with complex I, II and IV deficiency, respectively. The identification of the disease-causing gene in a given family with RCD is a major issue to provide both adequate genetic counselling and early, reliable PD.

Inversion of the circadian rhythm of melatonin in the Smith-Magenis syndrome.

OBJECTIVE: The objective was to determine the circadian rhythm of melatonin in the Smith-Magenis syndrome (SMS), which causes behavioral problems and sleep disturbance. STUDY DESIGN: Questionnaires, sleep consultations, and sleep diaries were obtained in 20 children with SMS (9 girls, 11 boys aged 4 to 17 years). Actigraphy, electroencephalography, and the circadian variations of plasma melatonin, cortisol, and growth hormone were recorded in 8 patients. Early sleep onset, early sleep offset, and sleep attack indicated sleep disturbance. RESULTS: All children with SMS had a phase shift of their circadian rhythm of melatonin. Time at onset of melatonin secretion was 6 AM +/- 2 (control group: 9 P.M. +/- 2). Peak time was 12 PM +/- 1 (control group: 3:30 AM +/- 1:30), and melatonin offset was at 8 PM +/- 1 (control group: 6 AM +/- 1). Behavioral problems correlated with the inverted circadian rhythm of melatonin. CONCLUSION: Considering that clock genes mediate the generation of circadian rhythms, we suggest that haploinsufficiency for a circadian system gene mapping to chromosome 17p11.2 may cause the inversion of the circadian rhythm of melatonin in SMS.

For debate: defective mitochondria, free radicals, cell death, aging-reality or myth-ochondria?

As both experimental evidence and theoretical considerations may suggest that free radicals and mitochondria might be associated as key factors in aging, these organelles have been implicated in various versions of the free radical theory of aging. However, except for a few cases, no evidence for a death process specifically activated in respiratory defective cells could be found in patients with a mitochondrial disorder, including those harboring high levels of mutant mtDNA associated with profound respiratory chain deficiencies. This and more recent evidence suggest that damages produced by free-radicals endogenously generated in the mitochondria result in a distinctive biochemical profile, only occur under exceptional conditions and that a dysfunction of the respiratory chain does not cause opening of the permeability transition pore and is not sufficient per se to trigger massive entrance of cells into death processes, neither apoptosis nor necrosis. Therefore, defective mitochondria and their particular genome, should not be considered as a major and primary source of free radicals either leading cells into a death cascade or resulting in an accelerated aging process.

A mutation in the human heme A:farnesyltransferase gene (COX10 ) causes cytochrome c oxidase deficiency.

Cytochrome c oxidase (COX) defects are found in a clinically and genetically heterogeneous group of mitochondrial disorders. To date, mutations in only two nuclear genes causing COX deficiency have been described. We report here a genetic linkage study of a consanguineous family with an isolated COX defect and subsequent identification of a mutation in a third nuclear gene causing a deficiency of the enzyme. A genome-wide search for homozygosity allowed us to map the disease gene to chromosome 17p13.1-q11.1 (Z (max)= 2.46; theta = 0.00 at the locus D17S799). This region encompasses two genes, SCO1 and COX10, encoding proteins involved in COX assembly. Mutation analysis followed by a complementation study in yeast permitted us to ascribe the COX deficiency to a homozygous missense mutation in the COX10 gene. This gene encodes heme A:farnesyltransferase, which catalyzes the first step in the conversion of protoheme to the heme A prosthetic groups of the enzyme. All three nuclear genes now linked to isolated COX deficiency are involved in the maturation and assembly of COX, emphasizing the major role of such genes in COX pathology.

Cerebral white matter disease in children may be caused by mitochondrial respiratory chain deficiency.

Several mitochondrial diseases are known to occasionally involve the cerebral white matter, namely Leigh syndrome, Kearns-Sayre syndrome, and MELAS syndrome, but in these cases the major finding is alteration in the basal ganglia and brainstem. Here we report on severe diffuse white matter involvement and respiratory chain enzyme deficiency or mitochondrial DNA rearrangement in 5 unrelated families. It is interesting that white matter lesions were the only abnormal neuroradiologic feature in 3 of the 5 families, and multiple small cyst-like white matter lesions were found in 2 of 5 probands. Respiratory chain deficiency should be considered in the diagnosis of severe white matter involvement in childhood.

Biochemical, genetic and immunoblot analyses of 17 patients with an isolated cytochrome c oxidase deficiency.

Mitochondrial respiratory chain defects involving cytochrome c oxidase (COX) are found in a clinically heterogeneous group of diseases, yet the molecular basis of these disorders have been determined in only a limited number of cases. Here, we report the clinical, biochemical and molecular findings in 17 patients who all had isolated COX deficiency and expressed the defect in cultured skin fibroblasts. Immunoblot analysis of mitochondrial fractions with nine subunit specific monoclonal antibodies revealed that in most patients, including in a patient with a novel mutation in the SURF1 gene, steady-state levels of all investigated COX subunits were decreased. Distinct subunit expression patterns were found, however, in different patients. The severity of the enzymatic defect matched the decrease in immunoreactive material in these patients, suggesting that the remnant enzyme activity reflects the amount of remaining holo-enzyme. Four patients presented with a clear defect of COX activity but had near normal levels of COX subunits. An increased affinity for cytochrome c was observed in one of these patients. Our findings indicate a genetic heterogeneity of COX deficiencies and are suggestive of a prominent involvement of nuclear genes acting on the assembly and maintenance of cytochrome c oxidase.

Effect of idebenone on cardiomyopathy in Friedreich's ataxia: a preliminary study.

BACKGROUND: Friedreich's ataxia is caused by a deficiency of frataxin, a protein involved in regulation of mitochondrial iron content. We have reported a combined deficiency of a Krebs-cycle enzyme, aconitase, and three mitochondrial respiratory-chain complexes in endomyocardial biopsy samples from patients with this disorder. All four enzymes share iron-sulphur cluster-containing proteins that are damaged by iron overload through generation of oxygen free radicals. We used an in-vitro system to elucidate the mechanism of iron-induced injury and to test the protective effects of various substances. On the basis of these results, we assessed the effect of idebenone (a free-radical scavenger) in three patients with Friedreich's ataxia. METHODS: Heart homogenates from patients with valvular stenosis were tested for respiratory-chain complex II activity, lipoperoxidation, and aconitase activity by spectrophotometric assays, in the presence of reduced iron (Fe2+), oxidised iron (Fe3+), desferrioxamine, ascorbic acid, and idebenone. The Friedreich's ataxia patients (aged 11 years, 19 years, and 21 years) underwent ultrasonographic heart measurements at baseline and after 4-9 months of idebenone (5 mg/kg daily). FINDINGS: Fe2+ (but not Fe3+) decreased complex II activity and increased lipoperoxidation in heart homogenate. Addition of ascorbate or desferrioxamine increased some of the iron-induced adverse effects. Idebenone protected against these effects. In the three patients, left-ventricular mass index decreased from baseline to 4-9 months of idebenone treatment (patient 1, 145 g to 114 g; patient 2, 215 g to 151 g; patient 3, 408 g to 279 g). INTERPRETATION: Our in-vitro data suggest that both iron chelators and antioxidant drugs that may reduce iron are potentially harmful in patients with Friedreich's ataxia. Conversely, our preliminary findings in patients suggest that idebenone protects heart muscle from iron-induced injury.

The neurogenic weakness, ataxia and retinitis pigmentosa (NARP) syndrome mtDNA mutation (T8993G) triggers muscle ATPase deficiency and hypocitrullinaemia.

UNLABELLED: Based on the study of three unrelated families, we report what we believe to be the first in vivo evidence of muscle ATPase deficiency in individuals carrying the neurogenic weakness, ataxia and retinitis pigmentosa (NARP) syndrome mtDNA mutation (T8993G). Since plasma citrulline was consistently low in 4/5 patients, we suggest that the NARP mutation caused complex V deficiency in the small intestine as well, thus reducing the availability of mitochondrial ATP required for citrulline synthesis. CONCLUSION: We suggest giving consideration to hypocitrullinaemia as a hallmark of the neurogenic weakness, ataxia and retinitis pigmentosa syndrome mutation and more generally of impaired oxidative phosphorylation in the small intestine in vivo.

A high rate (20%-30%) of parental consanguinity in cytochrome-oxidase deficiency.

By studying a large series of 157 patients, we found that complex I (33%), complex IV (28%), and complex I+IV (28%) deficiencies were the most common causes of respiratory chain (RC) defects in childhood. Truncal hypotonia (36%), antenatal (20%) and postnatal (31%) growth retardation, cardiomyopathy (24%), encephalopathy (20%), and liver failure (20%) were the main clinical features in our series. No correlation between the type of RC defect and the clinical presentation was noted, but complex I and complex I+IV deficiencies were significantly more frequent in cases of cardiomyopathy (P<.01) and hepatic failure (P<.05), respectively. The sex ratio (male/female) in our entire series was mostly balanced but was skewed toward males being affected with complex I deficiency (sex ratio R=1.68). Interestingly, a high rate of parental consanguinity was observed in complex IV (20%) and complex I+IV (28%) deficiencies. When parental consanguinity was related to geographic origin, an even higher rate of inbreeding was observed in North African families (76%, P<.01). This study gives strong support to the view that an autosomal recessive mode of inheritance is involved in most cases of mitochondrial disorders in childhood, a feature that is particularly relevant to genetic counseling for this devastating condition.

The consequences of a mild respiratory chain deficiency on substrate competitive oxidation in human mitochondria.

The competition between the respiratory substrates to gain access simultaneously to the respiratory chain depends on the dehydrogenase activity, the mitochondrial ubiquinone pool, and the oxidizing activity of the cytochrome segment. By studying the co-oxidation of NADH and succinate by control human liver homogenates, we found that a change in the balance between respiratory chain complex activities may affect significantly the ability of the mitochondria to oxidize one or the other substrate. Accordingly, in the particular case of a patient presenting with a partial complex I and IV deficiency, we observed a strongly reduced ability to oxidize NADH in the presence of succinate. It therefore appeared that even a slight imbalance between respiratory chain enzyme activities may result in a full blockade of a given substrate oxidation.