Paul E. Hart

Mitochondrial dysfunction and free radical damage in the Huntington R6/2 transgenic mouse.

Huntingtons disease is a progressive neurodegenerative disease caused by an abnormally expanded (>36) CAG repeat within the ITI5 gene encoding a widely expressed 349‐kd protein, huntingtin. The medium spiny neurons of the caudate preferentially degenerate in Huntingtons disease, with the presence of neuronal intranuclear inclusions. Excitotoxicity is thought to be important in the pathogenesis of Huntingtons disease; the recently described mitochondrial respiratory chain and aconitase defects in Huntingtons disease brain are consistent with this hypothesis. A transgenic mouse model (R6/2) of Huntingtons disease develops a movement disorder, muscle wasting, and premature death at about 14 to 16 weeks. Selective neuronal death in these mice is not seen until 14 weeks. Biochemical analysis of R6/2 mouse brain at 12 weeks demonstrated a significant reduction in aconitase and mitochondrial complex IV activities in the striatum and a decrease in complex IV activity in the cerebral cortex. Increased immunostaining for inducible nitric oxide synthase and nitrotyrosine was seen in the transgenic mouse model but not control mouse brains. These results extend the parallels between Huntingtons disease and the transgenic mouse model to biochemical events and suggest complex IV deficiency and elevated nitric oxide and superoxide radical generation precede neuronal death in the R6/2 mouse and contribute to pathogenesis. Ann Neurol 2000; 47:80–86

Antioxidant treatment improves in vivo cardiac and skeletal muscle bioenergetics in patients with friedreich's ataxia

Friedreichs ataxia (FA) is the most common form of autosomal recessive spinocerebellar ataxia and is often associated with a cardiomyopathy. The disease is caused by an expanded intronic GAA repeat, which results in deficiency of a mitochondrial protein called frataxin. In the yeast YFH1 knockout model of the disease there is evidence that frataxin deficiency leads to a severe defect of mitochondrial respiration, intramitochondrial iron accumulation, and associated production of oxygen free radicals. Recently, the analysis of FA cardiac and skeletal muscle samples and in vivo phosphorus magnetic resonance spectroscopy (31P‐MRS) has confirmed the deficits of respiratory chain complexes in these tissues. The role of oxidative stress in FA is further supported by the accumulation of iron and decreased aconitase activities in cardiac muscle. We used 31P‐MRS to evaluate the effect of 6 months of antioxidant treatment (Coenzyme Q10 400 mg/day, vitamin E 2,100 IU/day) on cardiac and calf muscle energy metabolism in 10 FA patients. After only 3 months of treatment, the cardiac phosphocreatine to ATP ratio showed a mean relative increase to 178% (p = 0.03) and the maximum rate of skeletal muscle mitochondrial ATP production increased to 139% (p = 0.01) of their respective baseline values in the FA patients. These improvements, greater in prehypertrophic hearts and in the muscle of patients with longer GAA repeats, were sustained after 6 months of therapy. The neurological and echocardiographic evaluations did not show any consistent benefits of the therapy after 6 months. This study demonstrates partial reversal of a surrogate biochemical marker in FA with antioxidant therapy and supports the evaluation of such therapy as a disease‐modifying strategy in this neurodegenerative disorder.

Coenzyme Q10 and vitamin E deficiency in Friedreich’s ataxia: predictor of efficacy of vitamin E and coenzyme Q10 therapy

Background and purpose:  A pilot study of high dose coenzyme Q10 (CoQ10)/vitamin E therapy in Friedreich’s ataxia (FRDA) patients resulted in significant clinical improvements in most patients. This study investigated the potential for this treatment to modify clinical progression in FRDA in a randomized double blind trial.

Role of oxidative damage in Friedreich's ataxia.

Plasma malondialdehyde (MDA) levels were raised in Friedreichs ataxia (FRDA) patients. These levels correlated with increasing age and disease duration, suggesting lipid peroxidation increased with disease progression. Using fibroblasts from FRDA patients we observed that GSH levels and aconitase activities were normal, suggesting their antioxidant status was unchanged. When exposed to various agents to increase free radical generation we observed that intracellular superoxide generation induced by paraquat caused enhanced oxidative damage. This correlated with the size of the GAA1 expansion, suggesting decreased frataxin levels may render the cells more vulnerable to mild oxidative stress. More severe oxidative stress induced by hydrogen peroxide caused increased cell death in FRDA fibroblasts but was not significantly different from control cells. We propose that abnormal respiratory chain function and iron accumulation may lead to a progressive increase in oxidative damage, but increased sensitivity to free radicals may not require detectable respiratory chain dysfunction.

International cooperative ataxia rating scale (ICARS): Appropriate for studies of Friedreich's ataxia?

Clinicians require scientifically rigorous, clinically meaningful rating scales to evaluate the health impact of disease and treatment that cannot be measured using conventional laboratory instruments. This study evaluated the psychometric properties of the International Cooperative Ataxia Rating Scale (ICARS), a commonly used clinician‐rated measure, in Friedreichs ataxia (FRDA). People with confirmed FRDA were assessed by using the ICARS. Two assumptions of its measurement model were tested: the legitimacy of reporting ICARS scores in FRDA, and the acceptability, reliability, and validity of total and subscale scores. Seventy‐seven people with FRDA were assessed. The ICARS total score effectively satisfied all psychometric criteria tested. The posture and gait disturbances subscale also performed well. The other three subscales did not pass standard criteria for tests of scaling assumptions, reliability, and validity. This small study recommends only the use of the ICARS total score as a measure of FRDA. However, the extent to which this score quantifies the true extent of FRDA remains uncertain as our validity testing was limited, partly by the lack of appropriate validating measures. Further validity testing, and examination of responsiveness, is required before the ICARS can be recommended as an outcome measure for treatment trials of FDRA.

MELAS mitochondrial DNA mutation A3243G reduces glutamate transport in cybrids cell lines

MELAS (mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes) is commonly associated with the A3243G mitochondrial DNA (mtDNA) mutation encoding the transfer RNA of leucine (UUR) (tRNA (Leu(UUR))). The pathogenetic mechanisms of this mutation are not completely understood. Neuronal functions are particularly vulnerable to alterations in oxidative phosphorylation, which may affect the function of the neurotransmitter glutamate, leading to excitotoxicity. In order to investigate the possible effects of A3243G upon glutamate homeostasis, we assessed glutamate uptake in osteosarcoma-derived cytoplasmic hybrids (cybrids) expressing high levels of this mutation. High-affinity Na(+)-dependent glutamate uptake was assessed as radioactive [(3)H]-glutamate influx mediated by specific excitatory amino acid transporters (EAATs). The maximal rate (V(max)) of Na(+)-dependent glutamate uptake was significantly reduced in all the mutant clones. Although the defect did not relate to either the mutant load or magnitude of oxidative phosphorylation defect, we found an inverse relationship between A3243G mutation load and mitochondrial ATP synthesis, without any evidence of increased cellular or mitochondrial free radical production in these A3243G clones. These data suggest that a defect of glutamate transport in MELAS neurons may be due to decreased energy production and might be involved in mediating the pathogenic effects of the A3243G mtDNA mutation.

Chapter 2 Clinical Features of the Mitochondrial Encephalomyopathies

Publisher Summary This chapter describes the clinical features of the mitochondrial encephalomyopathies. Mitochondrial dysfunction is implicated in a rapidly increasing number and variety of disorders. These have been categorized into class I and class II oxidative phosphorylation defects. The former represents the archetypal mitochondrial encephalomyopathies on which this chapter focuses. These result from mutations of mitochondrial DNA (mtDNA) or mutations of nuclear genes that encode subunits of the respiratory chain and oxidative phosphorylation system (OXPHOS). The mitochondrial encephalomyopathies are a diverse group of disorders. Neurologic features may reflect dysfunction of any part of the neuraxis. Their clinical features encompass a broad range of common neurologic symptoms, including dementia, psychiatric disease, developmental delay, stroke, epilepsy, neuropathy, and muscle disease. They are multisystem disorders and may manifest with cardiac, endocrine, gastrointestinal, hepatic, renal, or hematologic involvement. Another key feature of the mitochondrial encephalomyopathies is their marked phenotypic and genotypic diversity. A specific mtDNA defect may cause markedly varied phenotypes in different individuals or even within a single family.

Mitochondria: Aspects for neuroprotection

The understanding of mitochondrial biology and, subsquently, the role of mitochondrial pathology in human disease has increased exponentially over the past 30 years. As insight has increased, so attention has begun to shift to the possibilities for treating mitochondrially based disorders. There are a number of archetypal mitochondrial diseases, each associated with specific mitochondrial DNA mutations, deletions, or depletions. In addition there are a number of disorders, mainly neurodegenerative in nature, in which mitochondrial dysfunction appears to play a pivotal role. Mitochondrial structure and function are discussed. Treatment of the archetypal mitochondrial disorders and other neurogenerative conditions is reviewed, with specific emphasis on the prospects for neuroprotection. Drug Dev. Res. 46:57–66, 1998.