Identification of genes that regulate arthritis and IgE production in rat and human

Abstract

Multiple genome-wide scans have identified quantitative trait loci (QTLs) that influence inflammatory diseases on chromosome 4q42 in DA rats. Several of the QTLs overlap with the QTL designated Oia2 (Oil-induced arthritis-2), which was originally identified in a genomewide linkage analysis in oil-induced arthritis (OIA), a model that shows similarities to rheumatoid arthritis (RA) in human.

When initiating this thesis, the major candidate disease gene(s) within Oia2 was the natural killer cell gene complex (NKC), because the DA NKC encodes abberant NK cell-mediated natural toxicity that could be suspected to influence arthritis and other immunopathologies.

The main aim with this thesis was to identify genes within Oia2 that regulate immunopathologies, as this could provide new knowledge of disease pathways leading to RA and possibly other inflammatory conditions in human.

Congenic and subcongenic strains were established by transfer of chromosomal intervals covering the Oia2-region, between DA rats and arthritis-resistant MHC-identical PVG.1AV1 and LEW.1AV1 rats. A novel congenic strain DA.C4R3(PVG), harboring PVG alleles in a 16.8 Mb Oia2 interval on DA background, deviated from DA in both arthritis susceptibility and total serum lgE levels. It was resistant to OIA and showed protection against arthritis induced by both collagen type II (CIA) and squalene (SIA). Serum levels of lgE in naïve DA.C4R3(PVG) were increased compared to DA. Genetic fine-mapping in 16 congenic and subcongenic strains in oil-induced arthritis demonstrated that the NKC is situated outside the arthritis-regulating region, thus excluding NKC as being Oia2.

Rats made congenic for Oia2 in a 1.44 Mb region, and designated DA.C4R1 1(PVG), were resistant to OIA and had lower levels of lgE compared to DA. This suggests that the larger 16.8 Mb interval in DA.C4R3(PVG) contains more than one gene that regulates serum lgE levels, which is a phenotype that may correlate with asthma. Subsequently, an Oia2 congenic strain was produced, DA.C4R17(PVG), which was arthritis resistant, and which defines a C-type lectin-like receptor gene complex, designated Antigen Presenting Lectin-like receptor gene Complex (APLEC). Sequencing of the APLEC cDNAs from DA and PVG.1AV1 revealed a nonsense mutation in the gene for Dendritic Cell Activating Receptor 1 (Dcar1) in the DA rat, and RTPCR displayed significantly reduced mRNA levels of Dcar1 in lymph nodes of DA.

In humans, the homologous APLEC genes located on chromosome 12p13 were tested in case-control materials. Frequency comparisons of single nucleotide polymorphisms (SNPs) of the Dendritic Cell Immunoreceptor (DCIR), which is closely related to rat Dcar1, revealed association with RA as well as with total IgE-levels in atopic asthma. In addition, the DCIR was also associated with forced expiratory vital capacity (FVC), which demonstrates influence on asthma severity.

In conclusion, genes encoded from APLEC influence both arthritis and total IgE-levels in both humans and rats.