Exploring genetic variation in multiple sclerosis

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.
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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.