Search for Susceptibility Genes in Osteoarthritis

Osteoarthritis (OA) is a complex disease characterized by the destruction of the articular cartilage of the joints. The main symptoms are pain and disability, caused most probably by secondary synovitis. Secondary manifestations of OA are considered to be bone changes, osteophytes, subchondral sclerosis, and bone cysts. The suspected risk factors for OA include genetic predisposition, gender (greater in females), old age, obesity (also in non-weight-bearing joints), previous injury and various physical loading conditions. Prevalence of the disease varies between approximately 8 % and 60 %, dependent on the joint, but also largely on the age of study subjects, the population and OA definition. The disease affects the cartilage, bone, and synovium, but the etiology is still mainly unknown. Even the initiating events and tissue are unknown, but mechanical wear and tear is considered as the prime candidate for the root cause of the disease. The role of the genetic component in common skeletal diseases is well established but remains poorly understood. Heritability of hand and knee OA in women has been shown to range from 39 % to 65 %. Previous association studies have provided some evidence for genes affecting bone density, inflammation, composition and break down of the extracellular matrix. However, these results have been inconsistent. The objective of the present study was to identify predisposing genes for OA using single nucleotide polymorphism (SNP) markers. The study consisted of hand, knee and hip OA cases (n = 1466) and controls (n = 4475). A set of 25 candidate genes were studied in carefully selected case-control, family and twin settings. The objective of Study I was to pinpoint an OA-predisposing gene at the 2q11.2 region, which has previously been linked with hand OA using partially the same study sample (Leppävuori et al. 1999). In total 32 SNPs were genotyped in this region comprising 6 genes belonging to the interleukin 1 super-family. IL1R1 was associated with hand OA (p = 0.00091, with a significance threshold of p = 0.0021). The aim of Study II was to identify genetic variants in MMP8 and MMP9 genes that predispose to OA. Five different study cohorts including 1369 OA cases and 4445 controls were studied. Evidence for the role of MMP8 in knee OA was observed in one study sample (p = 0.0049, with a significance threshold of 0.0057) without statistically significant replication. The main association finding was found in the Finnish study sample for knee OA, while a similar tendency was observed in both the knee OA study sample of Spanish origin, and the hand OA sample of Finnish origin. The aim of Study III was to identify variants predisposing to hip OA from a preselected set of 25 biologically interesting candidate genes. Hips of the study subjects had been analyzed using magnetic resonance imaging (MRI). Variants in the COL9A2 and COL10A1 genes showed suggestive association (p = 0.0021 and p = 0.0015, with significance threshold of 0.00073). The change in the same codon of the COL9A2 has previously been shown to associate with a disc degeneration phenotype (Videman et al. 2009). In conclusion, the results of this thesis support the theory that inflammatory factors, cartilage breaking factors and originally poor quality of collagen associate with OA. Validation of all these results in larger study populations with more accurate genetic analysis are still needed for any further conclusions.