Protein Disulphide Isomerase is protective against DNA damage in Amyotrophic Lateral Sclerosis

Maintaining genomic integrity is essential for all organisms and for the transfer of genetic information from one generation to another. However, DNA undergoes continuous attack from many sources, which can lead to DNA damage and therefore genomic instability. The DNA damage response refers to signalling pathways that aim to detect and repair this damage. DNA damage increases significantly during ageing, which is the biggest risk factor for neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS). ALS is a fatal, rapidly progressing condition in which both the upper and lower motor neurons degenerate. There are few effective treatments for ALS, and whilst DNA damage has been increasingly described in pathophysiology, there are currently no effective ways to prevent damage or enhance DNA repair. Our group has previously shown that a novel protein chaperone, protein disulphide isomerase (PDI), is protective against multiple pathologies associated with ALS. PDI is unique because unlike other chaperones it also possesses oxidoreductase activity, which modulates redox regulation. However, whilst it has been established that PDI is protective against events associated with proteostasis in ALS, it has not been shown that PDI is protective against DNA damage.  The studies from this thesis revealed that PDI is protective against DNA damage following etoposide or H2O2 treatment using an in vitro neuroblastoma cell line in which endogenous PDI was knocked down with siRNA. It was also shown that PDI is protective against apoptosis induced by DNA damage. Furthermore, PDI was found to translocate to the nucleus following DNA damage induced by etoposide, implying it has a direct rather than indirect role. Two ALS risk-causing mutants that display impaired redox activity, PDI-D292N and PDI-R300H, along with a PDI-Quad mutant lacking the redox active cystine sites, were also investigated. Whilst PDI-R300H displayed some residual protective activity against DNA damage, PDI-D292N and the Quad mutant were not protective. Hence these results indicate that the redox activity of PDI is protective against DNA damage, but this is perturbed in ALS. By defining these mechanisms further in future studies, these results can lead to design future therapeutics for ALS based on PDI that can prevent DNA damage.