Mitochondrial abnormalities in remote tissues of patients with amyotrophic lateral sclerosis
Amyotrophic Lateral Sclerosis (ALS) is a neurodegenerative condition which is almost universally fatal. Some sufferers have an identifiable genetic mutation (<2%), but the majority of cases are sporadic (SALS). There is a body of evidence suggesting involvement of oxidative stress and mitochondrial abnormalities in the pathogenesis of ALS. Mitochondrial function was assessed in remote tissues (muscle, cultured myoblasts, cultured fibroblasts) from patients with ALS and controls. Muscle tissue was examined for histological features consistent with a mitochondrial disorder. Mitochondrial respiratory chain function was measured in mitochondrial homogenates. Mitochondrial protein expression was determined using immunofluorescent antibodies to a mitochondrial DNA (mtDNA)-encoded respiratory chain enzyme subunit with image analysis. MtDNA quantity and quality was assessed using Southern blot and long-range PCR. Analysis of inheritance patterns was performed using a database of familial ALS cases. No mitochondrial abnormalities were identified using these techniques. There was no evidence of anticipation or a sex effect on inheritance. In order to look for a difference in susceptibility to oxidative stress, cell cultures were examined for markers of oxidative damage and apoptosis, following growth with a free radical generating agent (paraquat). These studies demonstrated an increased susceptibility to oxidative damage in patient myoblasts and fibroblasts. This effect was not seen in cybrid studies combining mtDNA from patient platelets with experimental cells devoid of mtDNA (p cells), where no difference in oxidative damage was seen, confirming the absence of a systemic mtDNA abnormality. These results suggest that patients with SALS have an increased systemic sensitivity to oxidative stress which becomes pathological in tissues, such as motor neurons, which are considered to be vulnerable to such stress due to their metabolic activity and structure. This altered sensitivity is due to a factor other than a mitochondrial DNA abnormality, such as a nuclear DNA mutation or an unidentified environmental factor.