DNA Methylation in Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease primarily characterised by motor neurone (MN) degeneration and death. ALS can be sporadic (sALS) or familial, with a number of associated gene mutations, including C9orf72 (C9ALS). Previous studies have found impaired gene expression, implicating pathways involved with RNA metabolism and inflammation/immune response. DNA methylation is an epigenetic mechanism whereby a methyl group is attached to a cytosine (5mC), usually resulting in gene expression repression. 5mC can further be oxidised to 5-hydroxymethylcytosine (5hmC). DNA methylation has been studied in other neurodegenerative diseases, but little work has been conducted in ALS. The aim of this thesis is to elucidate DNA methylation’s role (if any) in the decline of MNs without the interactions from other cell types, which may mask MN-specific DNA methylation changes. Immunohistochemical analysis found higher levels of 5mC and 5hmC in ALS in the residual lower motor neurones (LMNs) of the spinal cord, with C9ALS displaying the highest global methylation. Interestingly, in LMNs with TDP43 pathology, a loss of 5mC and 5hmC from the nucleus was observed. LMNs were then extracted from a subset of the same cases using laser capture microdissection (LCM). Following this, DNA was extracted from the LMNs and underwent analysis using the MethylationEPIC array. Results indicated a global hypermethylation in C9ALS, with both hypermethylation and hypomethylation detected at the single gene level in ALS. GO and pathway analysis implicated RNA metabolism changes. The MethylationEPIC dataset was then compared to pre-existing mRNA expression data, with overlapping hits undergoing pathway analysis. Changes were found in cell signalling, inflammation and immune response and cell death/apoptosis. In conclusion, DNA methylation is a contributory factor in ALS. Data presented in this thesis suggests that hypermethylation is a prominent factor. Further studies are warranted to further understand the role of DNA methylation in ALS.