Genetic regulation of nerve injury-induced neurodegeneration and inflammation

Abstract

Neurodegeneration and inflammation in the central nervous system (CNS) are hallmarks of several neurological disorders, including multiple sclerosis (MS), Alzheimer’s disease and Parkinson’s disease. The susceptibility of an individual to these conditions is complex, i.e. influenced by both genetic and environmental factors. To study the genetic component of complex traits, experimental models are valuable tools to control for the impact of environment and to perform genetic mapping in large sample size intercrosses between inbred strains. The studies included in this thesis are based on the finding that inbred rat strains respond differently to nerve injury both with regard to degree of neurodegeneration and inflammatory responses, and aim at describing the phenotypic differences between strains in response to nerve injury in order to identify genetic regions regulating these parameters. The ultimate goal is to identify candidate genes of relevance to human disease.

We first performed a genome-wide linkage study of the responses to nerve injury by ventral root avulsion (VRA) in an F2 intercross between DA and PVG rat strains. This identified four loci regulating the degree of neurodegeneration (Vra1, 2), T cell infiltration (Vra2, 3) and major histocompatibility complex (MHC) class II expression (Vra4). From these results, we can conclude that the complex responses to nerve injury can be genetically dissected and are regulated by independent (Vra1, 3, 4), as well as linked or identical loci (Vra2). The Vra4 locus displayed a very strong linkage to MHC class II expression by microglia after injury (logarithm of odds, LOD 27.4).

Next, to position a candidate gene, Vra4 was fine-mapped by use of an advanced intercross line between DA and PVGav1 strains. By additional use of haplotype maps, sequencing and expression analysis of genes in the region, the MHC class II transactivator, Mhc2ta was identified as the candidate gene. A polymorphism in the corresponding human gene, MHC2TA was found to mediate differential expression of MHC class II transcripts and was genetically associated to the susceptibility to the three inflammatory disorders MS, rheumatoid arthritis and myocardial infarction.

In the third study, congenic rats where the Vra4 region harboring Mhc2ta had been transferred from PVGav1 to DA and vice versa, were studied with regard to MHC class II expression in the CNS and susceptibility to the MS model experimental autoimmune encephalomyelitis (EAE) induced by myelin oligodendrocyte glycoprotein. The expression of MHC class II was determined by the Vra4 allele and was thus reversed in the congenics compared to their respective background strain. In addition, Vra4 alleles from PVGav1 transferred to the susceptible DA background genome conferred significant protection from clinical manifestations of EAE. The phenotypic differences between the DA and PVG were further studied by analyzing the global gene transcription levels with microarrays. This identified a common response to VRA at a transcriptional level as well as strain specific patterns with inflammatory genes prevailing in the DA rats. In addition, two genes differing in expression between the strains, C1qb and Timp1 correlated to the degree of neurodegeneration in genetically heterogeneous animals.

In conclusion; neurodegeneration and inflammation in the CNS can be genetically dissected in rat strains displaying phenotypic differences in the response to nerve injury, and identified candidate genes can be of relevance to human disease.