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.