Clinical and molecular heterogeneity of cytochrome c oxidase deficiency in the newborn

Morphological Methods in the Diagnosis of Mitochondrial Encephalomyopathies: The Role of Electron Microscopy

Mitochondrial encephalomyopathies (MEs) encompass a heterogeneous group of disorders that frequently present a diagnostic challenge to clinicians. Historically, MEs were diagnosed by finding ragged red fibers in the muscle biopsy and confirmatory evidence was provided by the presence of numerical and/or ultrastructural abnormalities in mitochondria. In most centers diagnosis involves clinical evaluation and the morphological, histochemical, and biochemical investigation of a skeletal muscle biopsy. However, with the availability of mitochondrial DNA analysis, the necessity and role of morphological methods and, in particular, electron microscopy has been questioned. The aim of this study was to delineate the role of electron microscopy in the diagnosis of MEs.

Mitochondrial encephalomyopathies: diagnostic approach

Mitochondrial diseases have extremely heterogeneous clinical presentations due to the ubiquitous nature of mitochondria and the dual genetic control of the respiratory chain. Thus, mitochondrial disorders can be multisystemic (mitochondrial encephalomyopathies) or confined to a single tissue, and they can be sporadic or transmitted by mendelian or maternal inheritance. Mendelian disorders are usually inherited as autosomal recessive traits, tend to present earlier in life, and usually "breed true" in each family. By contrast, mitochondrial DNA-related diseases usually start later and vary in their presentation within members of the same family. Precise diagnosis is often a challenge; we go through the traditional steps of the diagnostic process, trying to highlight clues to mitochondrial dysfunction in the family history, physical and neurological examinations, routine and special laboratory tests, and histo-chemical and biochemical results of the muscle biopsy. The ultimate goal is to reach, whenever possible, a definitive molecular diagnosis, which permits rational genetic counseling and a prenatal diagnosis.

The effect of short-term dimethylglycine treatment on oxygen consumption in cytochrome oxidase deficiency: a double-blind randomized crossover clinical trial

OBJECTIVE

To study the effectiveness of dimethylglycine (DMG) on oxygen consumption (VO(2)) in children with Saguenay-Lac-Saint-Jean cytochrome-c oxidase (SLSJ-COX) deficiency (OMIM 220111).

STUDY DESIGN

In a crossover randomized double-blind clinical trial, 5 children with SLSJ-COX deficiency, who were stable and old enough to comply with VO(2) measurement, were treated with placebo or DMG for 3 days, and with the alternate treatment after a 2-week washout period. VO(2) was measured by indirect calorimetry before and after treatment. Dietary caloric intake was calculated for 3 days before each measurement. Mean caloric intakes per day were 1562 and 1342 kcal x m(-2) before and during placebo, 1,336 and 1,380 before and during DMG, respectively.

RESULTS

DMG was well tolerated and, in all cases, resulted in markedly increased blood DMG levels (617 + 203 mmol x L(-1)), versus 0 to 2 mmol x L(-1) without treatment. Mean VO(2) was lower after administration of either DMG (-1 +/- 3 mL x min(-1) x m(-2)) or placebo (-6 +/- 4), but neither difference was statistically significant. There was no detectable effect of DMG treatment on blood levels of lactate, pyruvate, bicarbonate, or pH. VO(2) values of patients (range, 101-135 mL x min(-1) x m(-2)) were lower than published norms (150-160 mL x min(-1) x m(-2)).

CONCLUSION

This study suggests that treatment with DMG does not substantially change VO(2) in children with SLSJ-COX deficiency.

Electron transport chain defects in heart failure

In recent years, the possibility that disorders of cardiac metabolism play a role in the mechanisms that lead to ventricular dilatation and dysfunction in heart failure has attracted much attention. Electron transport chain is constituted by a series of multimeric protein complexes, located in the inner mitochondrial membranes, whose genes are distributed over both nuclear and mitochondrial DNA. Its normal function is essential to provide the energy for cardiac function. Many studies have described abnormalities in mitochondrial DNA genes encoding for electron transport chain (ETC) in dilated cardiomyopathies. In some cases, heart failure is one more or less relevant symptom among other multisystem manifestations characteristic of mitochondrial encephalomyopathies, being heart failure imputable to a primary mitochondrial disease. In the case of idiopathic dilated cardiomyopathies (IDC), many mitochondrial abnormalities have also been described using hystological, biochemical or molecular studies. The importance of such findings is under debate. The great variability in the mitochondrial abnormalities described has prompted the proposal that mitochondrial dysfunction could be a secondary phenomenon in IDC, and not a primary one. Among other possible explanations for such findings, the presence of an increased oxidative damage due to a free radical excess has been postulated. In this setting, the dysfunction of ETC could be a consequence, but also a cause of the presence of an increased free radical damage. Independently of its origin, ETC dysfunction may contribute to the persistence and worsening of heart failure. If this hypothesis, still to be proven, was certain, the modulation of cardiac metabolism could be an interesting approach to treat IDC. The precise mechanisms that lead to ventricular dilatation and dysfunction in heart failure are still nowadays poorly understood. Circumstances such as cytotoxic insults, viral infections, immune abnormalities, contractile protein defects, ischemic factors and familial conditions have been thoroughly investigated [1]. It is possible that several mechanisms combine to produce the clinical syndrome of heart failure. In recent years the possibility that disorders of energy metabolism, either isolated or in combination with the other aforementioned factors, may play a role in the development of heart failure in susceptible patients has attracted much attention. The present paper reviews the current knowledge on mitochondrial function in the failing myocardium. We restrain our discussion to heart failure where an impaired inotropic state leads to a weakened systolic contraction (i.e. the so-called systolic heart failure). Idiopathic dilated cardiomyopathy (IDC) is the prototype of the conditions under discussion. Other circumstances where a defect in myocardial contraction is due to a chronic excessive work load (i.e., hypertension, valvular or congenital heart diseases), and states in which the principal abnormality involves impaired relaxation of the ventricle (i.e. diastolic heart failure), as well as mitochondrial defects outside the electron transport chain (i.e., defects in Krebs cycle or beta-oxidation of fatty acids) are only approached circumstantially.

Characterization of the murine gene for subunit VIIaL of cytochrome c oxidase

Mammalian cytochrome c oxidase consists of thirteen subunits, ten encoded by the nuclear genome and three by the mitochondrial DNA. In several species, two isoforms have been isolated for nuclear-encoded subunits VIa, VIIa and VIII: an ubiquitous L (liver) form and a heart- and skeletal-muscle specific H form. The gene for murine cytochrome c oxidase subunit VIIa-L (Cox7aL) and its promoter region were isolated, sequenced and analysed. The coding region is split in four exons spanning 4.1 kbp and the promoter carries potential binding sites for Sp1, NRF1 and NRF2 transcription factors. Transcriptional activity of the promoter in reporter assays suggested an ubiquitous expression in mouse tissues.

A novel syndrome affecting multiple mitochondrial functions, located by microcell-mediated transfer to chromosome 2p14-2p13

We have studied cultured skin fibroblasts from three siblings and one unrelated individual, all of whom had fatal mitochondrial disease manifesting soon after birth. After incubation with 1 mM glucose, these four cell strains exhibited lactate/pyruvate ratios that were six times greater than those of controls. On further analysis, enzymatic activities of the pyruvate dehydrogenase complex, the 2-oxoglutarate dehydrogenase complex, NADH cytochrome c reductase, succinate dehydrogenase, and succinate cytochrome c reductase were severely deficient. In two of the siblings the enzymatic activity of cytochrome oxidase was mildly decreased (by approximately 50%). Metabolite analysis performed on urine samples taken from these patients revealed high levels of glycine, leucine, valine, and isoleucine, indicating abnormalities of both the glycine-cleavage system and branched-chain alpha-ketoacid dehydrogenase. In contrast, the activities of fibroblast pyruvate carboxylase, mitochondrial aconitase, and citrate synthase were normal. Immunoblot analysis of selected complex III subunits (core 1, cyt c(1), and iron-sulfur protein) and of the pyruvate dehydrogenase complex subunits revealed no visible changes in the levels of all examined proteins, decreasing the possibility that an import and/or assembly factor is involved. To elucidate the underlying molecular defect, analysis of microcell-mediated chromosome-fusion was performed between the present study's fibroblasts (recipients) and a panel of A9 mouse:human hybrids (donors) developed by Cuthbert et al. (1995). Complementation was observed between the recipient cells from both families and the mouse:human hybrid clone carrying human chromosome 2. These results indicate that the underlying defect in our patients is under the control of a nuclear gene, the locus of which is on chromosome 2. A 5-cM interval has been identified as potentially containing the critical region for the unknown gene. This interval maps to region 2p14-2p13.