Genetic variations influencing susceptibility to multiple sclerosis

Abstract

Multiple sclerosis (MS) is a chronic neurological disease affecting approximately 12 000 individuals in Sweden. Epidemiological studies have shown that most likely several genetic variations contribute and interact with each other as well as with environmental factors in the development of MS. The disease is characterized by focal infiltration of inflammatory cells in the brain and spinal cord, demyelination of axons as well as neurodegeneration.

In this thesis we examined the risk of MS in relation to genetic variations in selected candidate genes and gene expression induced by neurodegeneration in an experimental animal model disease.

Study I did not show any association in MS with the two candidate genes NOTCH4 and tumour necrosis factor α (TNF), both located in the human leukocyte antigen (HLA) complex on chromosome 6p21.3. We found a difference in carriage count for one single nucleotide polymorphism (SNP) studied in the NOTCH4 gene but this was shown to be secondary to HLA-DRB1*15, the well known risk factor for MS in the HLA area.

Study II was a two-stage analysis, in which we determined genotypes, in up to 672 MS patients and 672 controls, for 123 SNPs in 66 genes. Genes were chosen based on their chromosomal positions, suggestively linked to MS or other autoimmune disorders, or on the basis of biological function hypothesized to be important in MS. In stage one, 22 genes contained at least one SNP for which the carriage rate for one allele differed significantly between patients and controls. After additional genotyping in the second stage, two genes conferred susceptibility to MS: LAG3 on chromosome 12p13, and IL7R on 5p13. LAG3 inhibits activated T cells, while IL7R is necessary for the maturation of T and B cells.

In Study III we examined nerve injury-induced neurodegeneration in the ventral root avulsion (VRA) model using Affymetrix oligonucleotide arrays (interrogating approximately 7000 full-length rat sequences and 1000 EST clusters) in two inbred rat strains differing in degree of nerve cell loss after injury. Gene expression was determined in the spinal cord of naïve animals and at two time points after injury. Differential regulation was found in 278 genes, whereof 245 were regulated by the injury and 68 differed significantly between strains. Principal component analysis revealed a common injury response pattern significantly modified by genetic background. A central finding was that inflammatory genes comprised the largest group of genes induced by injury and that these transcripts prevailed in the strain most susceptible to nerve cell loss. In addition, levels of the strain regulated genes C1qb and Timp1 correlated with degree of nerve cell loss in a group of genetically heterogeneous animals. These results suggest a link between the inflammatory response elicited by nerve injury and subsequent neurodegeneration.

In Study IV we investigated, the CD74 (also known as invariant chain) gene on chromosome 5q33 and Metallothionein (MT) genes on chromosome 16q, for a possible importance in determining susceptibility to MS based on their upregulated transcription as well as difference in regulation influenced by genetic background in study III. Seven SNPs in the CD74 gene and 9 SNPs in the MT cluster were analyzed in 890 MS patients and 775 controls. Three SNPs belonging to the same linkage disequilibrium block in the MT region were found to be associated with the risk of MS as well as one haplotype. MTs are heavy metal binding proteins that are transcriptionally regulated both by heavy metals and glucocorticoids. They are found throughout the mammalian body, including the CNS and are rapidly induced following many types of CNS insults where being strongly neuroprotective. One SNP in CD74 showed borderline signs of association not supported by other SNPs in the same LD block.