Comprehensive analysis to explain reduced or increased SOD1 enzymatic activity in ALS patients and their relatives

Abstract

Objective: To characterise stabilities in erythrocytes of mutant SOD1 proteins, compare SOD1 enzymatic activities between patients with different genetic causes of ALS and search for underlying causes of deviant SOD1 activities in individuals lacking SOD1 mutations.

Methods: Blood samples from 4072 individuals, ALS patients with or without a SOD1 mutation, family members and controls were studied. Erythrocyte SOD1 enzymatic activities normalised to haemoglobin content were determined, and effects of haemoglobin disorders on dismutation assessed. Coding SOD1 sequences were analysed by Sanger sequencing, exon copy number variations by fragment length analysis and by TaqMan Assay.

Results: Of the 44 SOD1 mutations found, 75% caused severe destabilisation of the mutant protein but in 25% it was physically stable. Mutations producing structural changes caused halved erythrocyte SOD1 activities. There were no differences in SOD1 activities between patients without a SOD1 mutation and control individuals or carriers of TBK1 mutations and C9orf72^HRE. In the low and high SOD1 activity groups no deviations were found in exon copy numbers and intron gross structures. Thalassemias and iron deficiency were associated with increased SOD1 activity/haemoglobin ratios.

Conclusion: Adjunct erythrocyte SOD1 activity analysis reliably signals destabilising SOD1 mutations including intronic mutations that are missed by exon sequencing.

Keywords:

• Amyotrophic lateral sclerosis
• superoxide dismutase
• mutation
• enzymatic activity

Acknowledgements

We thank the many people who kindly provided samples for this study, including the neurologists that collected them. Philippe Couratier provided material with the intron 4-pseudoexon mutation. The study was supported by the Swedish Research Council, the Knut and Alice Wallenberg Foundation, the Bertil Hållsten Foundation, the Torsten and Ragnar Söderberg Foundation, the Swedish Brain Fund, the Stratneuro Initiative, Västerbotten County Council, and the Kempe Foundations. We thank Jonathan Gilthorpe, Matthew Marklund, Mikael Oliveberg, Jens Danielsson and Anna Wuolikainen for valuable discussions and Eva Jonsson, Helena Alstermark and Ann-Charloth Nilsson for skilful technical assistance.

Declaration of interest

The authors declare no conflicts of interest.

Supplementary material available online