Cindy Boer

122 | Chapter 3.1 ▲ Figure 1: GWAS results for association with the KL sum score in the discovery phase. Manhattan plot for association with the KL sum score, adjusted for age and sex, in the discovery cohorts of RSI, RSII and RSIII. The −log10 p-values, for each of the ~11million SNPs analysed (remaining after EASYQC quali- ty control) as part of the genome-wide association with the KL sum score, plotted against their position per chromosome. The red dotted horizontal line corresponds to the genome-wide significant threshold (p-value=5×10−08). The dotted grey line corresponds to the selection for replication threshold (p-val- ue=5×10−06). SNP location represented by [], if the SNP is localised intergenic the dashes denotes the distance, -≤10kb, --≤100kb, ---≤1000kb, ----≤1Mkb, -----≥1Mkb. GWAS, genome-wide association stud- ies; KL, Kellgren and Lawrence score Next, we investigated the association of rs4764133 with bilateral severe hand OA and bilateral finger OA using the discovery set (RSI, RSII and RSIII). We found a strong association with finger OA (p-value=3.09x10 –08 , OR=1.25) and nominal significant asso- ciation with severe hand OA (p-value=2.80x10 –02 , OR=1.36), which has a low frequency in the population ( Supplementary Table 4. ). To see if rs4764133 also confers risk for other forms of OA, that is, OA of the hip and knee, we used the GWAS summary data of the treat OA consortium [27] and the recently published minimal joint space width of the hip (mJSW) meta-analysis.[18] No association was found between rs4764133 and hip or knee OA ( Supplementary Table 4. ). However, we did find a nominal significant association between rs1049897 and cartilage thickness in the hip joint(mJSW) (r 2 =0.98 with rs4764133) (p value=1.28x10 –02 , Beta=−0.398). Gene expression analyses In order to identify potential causal genes located in the LD block surrounding rs4764133, we assessed gene expression of MGP , ERP27 , ART4 , SMOC3 (C12orf69) and C12orf60 in articular cartilage, the primary OA affected tissue. RNA sequencing was ob- tained on articular cartilage from patients with primary OA who had total joint replace- ment surgeries of either the knee (n=25) or hip (n=22) joint. Expression levels of ERP27 , C12orf60 , ART4 and SMOC3 were substantially lower than MGP expression levels