Amyotrophic lateral sclerosis (ALS) is an adult-onset fatal neurodegenerative disease characterized by progressive loss of motor neurons in the motor cortex, the brainstem and the ventral horns of the spinal cord. This is clinically manifested as progressive paresis and wasting of the skeletal muscles. Inevitably, the patient expires when the respiratory muscles becomes paralyzed. In Sweden about 180-200 cases of ALS are diagnosed yearly, and the mean survival time from onset is 2.5-3 years. The disease has world-wide distribution but the aetiology is unknown.
In 1993 the finding of missense mutations in the gene enconding the free radical scavenging enzyme CuZn-superoxide dismutase (CuZn-SOD) was reported in a subgroup of North American patients with familial ALS (5- 10% of ALS cases are presumed to be heriditary) suggesting that defective metabolism of superoxide free radicals may be the cause of some cases of ALS.
Objectives of the study:
(I) To search for mutations in the CuZn-SOD gene in ALS patients in Scandinavia.
(II) To investigate whether mutations in the CuZn-SOD gene is associated with cases of sporadic ALS.
(III) To compare the phenotype of ALS patients with different CuZn-SOD mutations.
(IV) To characterize the metabolism of 027 in ALS patients with different CuZn-SOD mutations.
Methodology: Blood samples were collected from patients with adult-onset motor neuron disease living in Denmark, Finland, Norway and Sweden and screened for CuZn-SOD mutations using PCR-SSCP. In cases with abnormal SSCP- pattern, nucleotide sequencing was performed to identify the mutation. Also, the activity of CuZn-SOD and other free radical scavenging enzymes was measured in the blood. When possible, the patients with a CuZn-SOD mutation were examined and the disease history collected.
Results:
(I) 9.6% of 427 Scandinavian ALS patients were found to have a CuZn-SOD mutation. Five different mutations were found with different genetic and phenotypic characteristics including the first-ever mutation in homozygous form, Asp90Ala. (II) CuZn-SOD mutations were found in 14 patients with apparently sporadic ALS, and in 12 of 51 families with recognized familial ALS.
(III) The phenotype of patients with the five different CuZn-SOD mutations varied spanning from rapidly progressive disease with only lower motor neuron signs to very slowly evolving disease with affection of both the upper- and lower motor neuron systems. However, the 46 patients homozygous for the D90A CuZn-SOD mutation all showed a remarkable consistent phenotype.
(IV) The blood activities of CuZn-SOD, EC-SOD and glutathion peroxidase were essentially normal in 44 individuals homozygous for the D90A CuZn-SOD mutation and did not differ from 114 healthy relatives heterozygous for the same mutation. For the other mutations, no association was found between disease phenotype and loss of CuZn-SOD activity as measured in erythrocyte-hemolysate.
Conclusions:
(I) CuZn-SOD mutations, in particular the D90A CuZn-SOD mutation in homozygous form, are relatively frequent among ALS patients in Scandinavia.
(II) Apparently sporadic ALS cases may be associated with a CuZn-SOD mutation.
(III) Patients homozygous for the D90A CuZn-SOD mutation constitutes a well-defined distinct entity of ALS in Scandinavia. (IV) No evidence has been found indicating that the CuZn-SOD mutations confer pathological oxidative stress, rather the mutated CuZn-SOD enzyme causes ALS by a cytotoxic gain-of-function process.