SOD1 Enzymatic Activity in CSF from ALS patients with and without SOD1 mutations

Abstract

Superoxide dismutase-1 (SOD1) mutations are a common cause of amyotrophic lateral sclerosis (ALS). Intrathecal gene therapy using the antisense-oligo-nucleotide drug tofersen to reduce SOD1 expression shows significant effects on disease progression and has recently been approved in the United States and the European Union. However, the discovery of children homozygous for four different inactivating SOD1 mutations developing the Infantile SOD1 Deficiency Syndrome (ISODDES) with injury to both upper and lower motor systems suggests that low SOD1 activities may be deleterious in humans. Monitoring SOD1 activity in cerebrospinal fluid (CSF) from tofersen-treated patients is advisable but difficult due to low levels and the presence of the isoenzyme SOD3. We here present a method to efficiently remove SOD3 from CSF using highly specific immobilized antibodies and subsequent measurement of the SOD activity as a sensitive assay for the SOD1 activity in CSF. We applied the method on CSF samples from ALS patients and controls and used paired erythrocyte samples for comparison. In ALS patients with wildtype SOD1, the SOD1 activity in CSF was the same as in controls, but patients with mutant SOD1 show lower activity in CSF, even patients with mutants previously reported to have full activity in erythrocytes. Activity variation is large among patients carrying the same SOD1 mutation and larger than seen in erythrocytes and in post-mortem central nervous system tissue. SOD1 in CSF shows a high specific activity, indicating that it is mainly native. Lastly, we identified a discrepancy between the SOD1 activity and protein level measured with ELISA in both CSF and erythrocytes. Since antibodies for SOD1 ELISA-quantification are raised against the native wildtype enzyme, the content of mutant SOD1s may be underestimated. Direct analysis of SOD1 enzymatic activity in CSF is therefore a more reliable way to monitor the effect of SOD1-lowering compounds.