Familial amyotrophic lateral sclerosis and Cu/Zn superoxide dismutase mutation

Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disorder that involves mainly the motor neuron system. Five to 10 percent of the ALS cases are familial; most others are sporadic. Several mutations in the superoxide dismutase‐1 (SOD1) gene have recently been shown to be associated with about 20% of familial ALS patients. The reduced enzyme activity of many mutant SOD1 points to the possibility that a loss‐of‐function effect of the mutant enzyme is responsible for the pathogenesis of the disease. However, this conflicts with the autosomal dominant inheritance of SOD1 mutation‐associated ALS and the normal SOD1 activity in homozygous patients in a SOD1‐linked ALS family. Current biochemical investigations have provided evidence that mutant SOD1 may catalyze the peroxynitrite‐mediated nitration of protein tyrosine residues, release copper and zinc ions, facilitate apoptosis of neurons and have enhanced peroxidase activity. Immunocytochemical studies demonstrated the presence of intense SOD1 immunoreactivity in Lewy body‐like inclusions, which are characteristic features of a certain form of familial ALS with posterior column involvement, in the lower motor neurons of patients in ALS families with different SOD1 mutations. More recently, strains of transgenic mice expressing mutant SOD1 have been established. These mice clinicopathologically develop a motor neuron disease mimicking human ALS with the exception of pronounced intraneuronal vacuolar degeneration. The overexpression of wild‐type SOD1 in mice has failed to give rise to the disease. Only one transgene for mutant SOD1 is enough to cause motor neuron degeneration and the severity of clinical course correlates with the transgene copy number. These observations in SOD1‐linked familial ALS and its transgenic mouse model suggest a novel neurotoxic function of mutant SOD1.