Multiple sclerosis linkage analysis and DNA variation in a complex trait

Abstract

Multiple sclerosis is a life-long disease affecting approximately 11.000 individuals in Sweden today. It is the second most common cause of neurological disability amongst young adults, only outnumbered by traffic accidents. Genetic epidemiological studies indicate that genetic factors contribute to the susceptibility of MS. So far the only convincing findings of genetic contribution, however, is the HLA association. The small effects that each genetic factor is likely to contribute is thought to be rather small and thus difficult to map.

In this thesis we have investigated chromosomal areas and candidate genes that influence susceptibility and severity to MS in different types of materials and with different approaches.

In study I the aim was to investigate the three NOS genes NOS 1, NOS 2A and NOS 3 with regard to susceptibility and severity to MS in a case control dataset consisting of 92 severe cases and 92 mild cases and 148 controls. The aim was also to investigate the chromosomal regions of NOS 2A and NOS 3 in regard to linkage to a disease susceptibility locus for MS in a sib-pair dataset. NO is an important mediator in the immune system, it is synthesized by the three NOS genes and there are several studies indicating a role for NO in MS. We did not find any evidence for association either to susceptibility or severity of the NOS genes to MS. We did not detect any linkage to this region either.

In study II we performed a genome-wide linkage analysis on a consanguineous kinship displaying a possible monogenic inheritance pattern. The result from the screen gave one peak on the long arm of chromosome 9 between markers D9S175 and D9S261. The highest LOD score revealed a suggestive LOD score of 2.29 (P value 0.0009) close to marker D9S1790. This area was fine mapped with 40 additional microsatellite markers. Reconstruction of haplotypes showed that four out of five affected family members were identically homozygous for a haplotype spanning approximately 43 cM. The fifth affected family member and all unaffected subjects were heterozygous for this haplotype.

In study III we performed a genome-wide TDT analysis on an MS dataset collected in a geographically isolated area consisting of 54 patients and 114 healthy family members. Eight of these families originated from Lysvik, a small village with less than 1800 inhabitants. Investigation of Swedish church records showed that six of these families originated from a common ancestor born in Savolaks in Finland in the 16th centrury. Five regions were found to be in transmission disequilibrium: 2q21-33, 6p25-23, 6q25-27, 14q24-31 and 17q22. The most interesting region was 14q24-31, where several di-marker haplotypes were in transmission disequilibrium and one marker also positive in the single marker TDT.

In study IV we investigated the gene encoding C5a located in the area of interest from study II. The anaphylatoxin C5a is a potent inflammatory mediator of the complement system, a key component of the innate immune system. Two different materials were analysed: first two members from the consanguineous kinship were selected for sequencing of C5a, one affected and one healthy family member; and then 5 SNP were investigated in a case control dataset consisting of 558 cases and 558 controls. We did not find any deviation in the sequence between the affected family member, the unaffected family member and the reference sequence, nor did we find evidence that DNA variation in C5a has a genetic effect on MS in the case-control material.