Shamima Rahman

Deficiency of the ADP-Forming Succinyl-CoA Synthase Activity Is Associated with Encephalomyopathy and Mitochondrial DNA Depletion

The mitochondrial DNA (mtDNA) depletion syndrome is a quantitative defect of mtDNA resulting from dysfunction of one of several nuclear-encoded factors responsible for maintenance of mitochondrial deoxyribonucleoside triphosphate (dNTP) pools or replication of mtDNA. Markedly decreased succinyl-CoA synthetase activity due to a deleterious mutation in SUCLA2, the gene encoding the beta subunit of the ADP-forming succinyl-CoA synthetase ligase, was found in muscle mitochondria of patients with encephalomyopathy and mtDNA depletion. Succinyl-CoA synthetase is invariably in a complex with mitochondrial nucleotide diphosphate kinase; hence, we propose that a defect in the last step of mitochondrial dNTP salvage is a novel cause of the mtDNA depletion syndrome.

Decrease of 3243 A→G mtDNA Mutation from Blood in MELAS Syndrome: A Longitudinal Study

It is widely held that changes in the distribution of mutant mtDNAs underlie the progressive nature of mtDNA diseases, but there are few data documenting such changes. We compared the levels of 3243 A-->G mutant mtDNA in blood at birth from Guthrie cards and at the time of diagnosis in a blood DNA sample from patients with mitochondrial encephalopathy, lactic acidosis, and strokelike episodes (MELAS) syndrome. Paired blood DNA samples separated by 9-19 years were obtained from six patients with MELAS. Quantification of mutant load, by means of a solid-phase minisequencing technique, demonstrated a decline (range 12%-29%) in the proportion of mutant mtDNA in all cases (P=.0015, paired t-test). These results suggest that mutant mtDNA is slowly selected from rapidly dividing blood cells in MELAS.

A Missense Mutation of Cytochrome Oxidase Subunit II Causes Defective Assembly and Myopathy

We report the first missense mutation in the mtDNA gene for subunit II of cytochrome c oxidase (COX). The mutation was identified in a 14-year-old boy with a proximal myopathy and lactic acidosis. Muscle histochemistry and mitochondrial respiratory-chain enzymology demonstrated a marked reduction in COX activity. Immunohistochemistry and immunoblot analyses with COX subunit-specific monoclonal antibodies showed a pattern suggestive of a primary mtDNA defect, most likely involving CO II, for COX subunit II (COX II). mtDNA-sequence analysis demonstrated a novel heteroplasmic T-->A transversion at nucleotide position 7,671 in CO II. This mutation changes a methionine to a lysine residue in the middle of the first N-terminal membrane-spanning region of COX II. The immunoblot studies demonstrated a severe reduction in cross-reactivity, not only for COX II but also for the mtDNA-encoded subunit COX III and for nuclear-encoded subunits Vb, VIa, VIb, and VIc. Steady-state levels of the mtDNA-encoded subunit COX I showed a mild reduction, but spectrophotometric analysis revealed a dramatic decrease in COX I-associated heme a3 levels. These observations suggest that, in the COX protein, a structural association of COX II with COX I is necessary to stabilize the binding of heme a3 to COX I.

Prevalence of Mitochondrial 1555A→G Mutation in European Children

To the Editor: Aminoglycoside antibiotics are used worldwide to treat gram-negative sepsis. Since these drugs are ototoxic and nephrotoxic, drug levels are closely monitored. However, their effect ...

Reactive oxygen species, oxidative stress, and cell death correlate with level of CoQ10 deficiency

Coenzyme Q10 (CoQ10) is essential for electron transport in the mitochondrial respiratory chain and antioxidant defense. The relative importance of respiratory chain defects, ROS production, and apoptosis in the pathogenesis of CoQ10 deficiency is unknown. We determined previously that severe CoQ10 deficiency in cultured skin fibroblasts harboring COQ2 and PDSS2 mutations produces divergent alterations of bioenergetics and oxidative stress. Here, to better understand the pathogenesis of CoQ10 deficiency, we have characterized the effects of varying severities of CoQ10 deficiency on ROS production and mitochondrial bioenergetics in cells harboring genetic defects of CoQ10 biosynthesis. Levels of CoQ10 seem to correlate with ROS production;10‐15% and >60% residual CoQ10 are not associated with significant ROS production, whereas 30‐50% residual CoQ10 is accompanied by increased ROS production and cell death. Our results confirm that varying degrees of CoQ10 deficiency cause variable defects of ATP synthesis and oxidative stress. These findings may lead to more rational therapeutic strategies for CoQ10 deficiency.— Quinzii, C. M., Lopez, L. C., Gilkerson, R. W., Dorado, B., Coku, J., Naini, A. B., Lagier‐Tourenne, C., Schuelke, M., Salviati, L., Carrozzo, R., Santorelli, F., Rahman, S., Tazir, M., Koenig, M., DiMauro, S., Hirano, M. Reactive oxygen species, oxidative stress, and cell death correlate with level of CoQ10 deficiency. FASEB J. 24, 3733–3743 (2010). www.fasebj.org

Treatable childhood neuronopathy caused by mutations in riboflavin transporter RFVT2

Childhood onset motor neuron diseases or neuronopathies are a clinically heterogeneous group of disorders. A particularly severe subgroup first described in 1894, and subsequently called Brown-Vialetto-Van Laere syndrome, is characterized by progressive pontobulbar palsy, sensorineural hearing loss and respiratory insufficiency. There has been no treatment for this progressive neurodegenerative disorder, which leads to respiratory failure and usually death during childhood. We recently reported the identification of SLC52A2, encoding riboflavin transporter RFVT2, as a new causative gene for Brown-Vialetto-Van Laere syndrome. We used both exome and Sanger sequencing to identify SLC52A2 mutations in patients presenting with cranial neuropathies and sensorimotor neuropathy with or without respiratory insufficiency. We undertook clinical, neurophysiological and biochemical characterization of patients with mutations in SLC52A2, functionally analysed the most prevalent mutations and initiated a regimen of high-dose oral riboflavin. We identified 18 patients from 13 families with compound heterozygous or homozygous mutations in SLC52A2. Affected individuals share a core phenotype of rapidly progressive axonal sensorimotor neuropathy (manifesting with sensory ataxia, severe weakness of the upper limbs and axial muscles with distinctly preserved strength of the lower limbs), hearing loss, optic atrophy and respiratory insufficiency. We demonstrate that SLC52A2 mutations cause reduced riboflavin uptake and reduced riboflavin transporter protein expression, and we report the response to high-dose oral riboflavin therapy in patients with SLC52A2 mutations, including significant and sustained clinical and biochemical improvements in two patients and preliminary clinical response data in 13 patients with associated biochemical improvements in 10 patients. The clinical and biochemical responses of this SLC52A2-specific cohort suggest that riboflavin supplementation can ameliorate the progression of this neurodegenerative condition, particularly when initiated soon after the onset of symptoms.

Treatment of CoQ10 Deficient Fibroblasts with Ubiquinone, CoQ Analogs, and Vitamin C: Time- and Compound-Dependent Effects

Background Coenzyme Q10 (CoQ10) and its analogs are used therapeutically by virtue of their functions as electron carriers, antioxidant compounds, or both. However, published studies suggest that different ubiquinone analogs may produce divergent effects on oxidative phosphorylation and oxidative stress. Methodology/Principal Findings To test these concepts, we have evaluated the effects of CoQ10, coenzyme Q2 (CoQ2), idebenone, and vitamin C on bioenergetics and oxidative stress in human skin fibroblasts with primary CoQ10 deficiency. A final concentration of 5 µM of each compound was chosen to approximate the plasma concentration of CoQ10 of patients treated with oral ubiquinone. CoQ10 supplementation for one week but not for 24 hours doubled ATP levels and ATP/ADP ratio in CoQ10 deficient fibroblasts therein normalizing the bioenergetics status of the cells. Other compounds did not affect cellular bioenergetics. In COQ2 mutant fibroblasts, increased superoxide anion production and oxidative stress-induced cell death were normalized by all supplements. Conclusions/Significance These results indicate that: 1) pharmacokinetics of CoQ10 in reaching the mitochondrial respiratory chain is delayed; 2) short-tail ubiquinone analogs cannot replace CoQ10 in the mitochondrial respiratory chain under conditions of CoQ10 deficiency; and 3) oxidative stress and cell death can be counteracted by administration of lipophilic or hydrophilic antioxidants. The results of our in vitro experiments suggest that primary CoQ10 deficiencies should be treated with CoQ10 supplementation but not with short-tail ubiquinone analogs, such as idebenone or CoQ2. Complementary administration of antioxidants with high bioavailability should be considered if oxidative stress is present.

Prevalence and natural history of heart disease in adults with primary mitochondrial respiratory chain disease

The prevalence and natural history of cardiovascular disease in adult patients with respiratory chain disease (RCD) is poorly characterized. We sought to determine the frequency and natural history of cardiac disease in patients with primary RCD.

Diagnosis of mitochondrial DNA depletion syndromes

Mitochondrial DNA (mtDNA) depletion syndromes (MDDS) are a group of clinically heterogeneous autosomal recessive disorders characterised by a severe quantitative reduction of total mtDNA, the genetic material present exclusively within mitochondria. mtDNA is a 16.5 kb circular genome, encoding 13 subunits of the respiratory chain and 24 RNA molecules necessary for the intramitochondrial translation of these 13 proteins. Unlike nuclear DNA, where every cell contains two copies of each gene (one copy from each parent), mtDNA is a multicopy genome, each cell containing thousands of copies. mtDNA depletion has been defined as a residual mtDNA copy number of <30% compared with age-matched controls,1 2 but mtDNA levels are often <10%, and sometimes as little as 1–2%, of controls, particularly in the hepatocerebral form of the disease. Recognised clinical presentations of MDDS include early-onset hepatocerebral disease overlapping with Alpers syndrome, isolated myopathy, encephalomyopathy and the MNGIE (mitochondrial neurogastrointestinal encephalomyopathy) syndrome (table 1). View this table: Table 1 The mitochondrial DNA (mtDNA) depletion syndromes: genes, phenotypes and pathogenic mechanisms MDDS may be caused by recessive defects in proteins involved in mtDNA replication (making copies of mtDNA) or in proteins that synthesise deoxyribonucleoside triphosphates (dNTPs) for incorporation into mtDNA (table 1). Both types of defect are likely to cause stalling of the replication complex,3 a mechanism that is also important in the generation of multiple mtDNA deletions, another defect of mtDNA maintenance. Alpers syndrome and related hepatocerebral disorders constitute the most common subgroup of MDDS, caused by mutations in the POLG gene encoding the catalytic subunit of the mitochondrial DNA polymerase γ,4 5 the enzyme responsible for mtDNA replication. Alpers syndrome is characterised by intractable epilepsy and liver dysfunction, but either problem can present in isolation. In patients with POLG mutations and Alpers syndrome, the degree of tissue mtDNA depletion has some correlation with the clinical severity.6 As well as profound tissue depletion of …

Ototoxicity caused by aminoglycosides

Is severe and permanent in genetically susceptible people