Therapeutic strategies for the treatment of amyotrophic lateral sclerosis (ALS) with mutations in angiogenin and superoxide dismutase 1

Abstract

Amyotrophic Lateral Sclerosis (ALS) is a fatal progressive neurodegenerative disorder. The cause of ALS is not completely understood. About 0.5-1% of ALS cases are associated with mutations in the angiogenin (ANG). These mutations are thought to cause disease through a loss of ANG function, but this hypothesis has not been evaluated statistically. In addition, the potential for ANG to promote disease has not been considered. With the goal of better defining the etiology of ANG-ALS, we assembled all clinical onset and disease duration data and determined if these were correlated with biochemical properties of ANG variants. Loss of ANG stability and ribonuclease activity were found to correlate with early ALS onset, confirming an aspect of the prevailing model of ANG-ALS. Conversely, loss of ANG stability and ribonuclease activity correlated with longer survival following diagnosis, which is inconsistent with the prevailing model. These results indicate that functional ANG appears to decrease the risk of developing ALS but exacerbate ALS once in progress. These findings are rationalized in terms of studies demonstrating that distinct mechanisms contribute to ALS onset and progression and propose that ANG replacement or stabilization would benefit pre-symptomatic ANG-ALS patients. However, this study challenges the prevailing hypothesis that augmenting ANG will benefit symptomatic ANG-ALS patients. Instead, our results suggest that the silencing of ANG activity may be beneficial for symptomatic ALS patients. ANG has an in vivo half-life of less than 2 hours, to use ANG as prophylactic for ANG-ALS multiple doses of ANG are required daily. To mitigate this problem, we evaluated cyclic thiosulfinates as a novel class of compounds for hydrogel synthesis to encapsulate ANG. We used cyclic thiosulfinates to cross-link PEG-thiol monomers and evaluated the safety of these hydrogels in vitro. Using alkylated bovine serum albumin as a surrogate to ANG (which has no free cysteines to cross-link) we demonstrated diffusion mediated sustained protein release. Finally, 2% of ALS cases were associated with mutations in superoxide dismutase 1 (SOD1). SOD1 is a homodimeric protein, in ALS-SOD1 variants the dimer destabilizes forming monomers which are prone to aggregation and are associated with toxicity. Previous studies demonstrated dimer stabilization as a viable therapeutic strategy but the cross-linkers used often form dead-end modifications with lone thiols making them harmful. Using alpha and beta lipoic acid (routinely used as dietary supplements) we demonstrated cyclic disulfides and cyclic thiosulfinates can efficiently cross-link SOD1 monomers while avoiding dead-end modifications of lone thiols.