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Exploring genetic variation in multiple sclerosis
thesisposted on 2017-02-03, 03:58 authored by Jensen, Cathy Joanna
Recent genome wide association studies (GWAS) into factors influencing multiple sclerosis (MS) have uncovered loci that affect susceptibility to multiple sclerosis. These discoveries add to existing evidence that genetic factors are important in MS. The work in this thesis continues investigation of the mechanisms in which some of these genetic factors may influence susceptibility to MS, and the relationship between MS susceptibility and severity. The myelin oligodendrocyte glycoprotein (MOG) is an integral membrane protein expressed exclusively in oligodendrocytes. Peptides from the MOG protein sequence can induce an acute, central nervous system (CNS) restricted autoimmune condition in susceptible animals called experimental autoimmune encephalomyelitis (EAE) which resembles some aspects of MS. Candidate gene studies have been equivocal in suggesting a role for MOG in MS, however a recent, large study supports an association between a polymorphism in MOG (rs3130253) and susceptibility to MS. These studies demonstrate that the MS-associated polymorphism is associated with changes in the processing of the MOG pre-mRNA transcript that favour an increase in exon 2 to exon 3 splicing. A plausible outcome from increased exon 2/3 splicing of the MOG pre-mRNA is increased expression of MOG protein isoforms containing exons 1–3, which includes the extracellular and transmembrane domains, on the surface of oligodendrocytes. As the extracellular domain in an important epitope in MS and EAE, an alteration in the expression of MOG protein may affect MS susceptibility. C-type lectin family 16, member A (CLEC16A) is a gene that has been associated with susceptibility to MS and to other autoimmune diseases. Beyond these associations, very little is known about this gene. The data in this thesis provides a basic characterisation of the molecular biology of CLEC16A. This thesis demonstrates that there is brain-tissue specific splicing of CLEC16A pre-mRNA that is conserved between the mouse and the human; however no differences in splicing were correlated with the disease-associated polymorphisms in brain tissue. A yeast-2-hybrid screen against a mouse cortical library found that the FPL domain of CLEC16A interacted with HGF-regulated tyrosine kinase substrate (Hrs) and Alix, members of the endosomal sorting complex required for transport (ESCRT) pathway, involved in sorting and trafficking proteins to the late endosome and lysosome. The studies regarding CLEC16A presented in this thesis contribute to the knowledge about the biology of CLEC16A, and suggest a function for the protein. xiii There is potential that recently discovered genetic susceptibility factors could modulate MS disease severity, as demonstrated previously for the MS risk allele HLA-DR15. This hypothesis was investigated in a cohort of 1006 well characterised MS patients from South- Eastern Australia. SNPs from seven loci (CLEC16A, IL2RA, IL7R, EVI5/RPL5, CD58, CD40 and chromosome 12q13-14) were assessed for association with five measures of disease severity incorporating disability, age of onset, cognition and brain atrophy. Trends towards association were observed between the EVI5/RPL5 risk SNP and time between first demyelinating event and relapse, and between the CD40 risk SNP and symbol digit test score. No associations were significant after correction for multiple testing. This data does not provide evidence for the hypothesis that these MS disease risk-associated SNPs influence disease severity. Collectively, the data presented in this thesis explores some aspects of genetic variation in MS.