• Bram P Prins, Ali Abbasi, Anson Wong, Ahmad Vaez, Ilja Nolte, Nora Franceschini, Philip E Stuart, Javier Guterriez Achury, Vanisha Mistry, Jonathan P Bradfield, Ana M Valdes, Jose Bras, Aleksey Shatunov, PAGE Consortium, International Stroke Genetics Consortium, Systemic Sclerosis consortium, Treat OA consortium, DIAGRAM Consortium, CARDIoGRAMplusC4D Consortium, ALS consortium, International Parkinson’s Disease Genomics Consortium, Autism Spectrum Disorder Working Group of the Psychiatric Genomics Consortium, CKDGen consortium, GERAD1 Consortium, International Consortium for Blood Pressure, Schizophrenia Working Group of the Psychiatric Genomics Consortium, Inflammation Working Group of the CHARGE Consortium, Chen Lu, Buhm Han, Soumya Raychaudhuri, Steve Bevan, Maureen D Mayes, Lam C Tsoi, Evangelos Evangelou, Rajan P Nair, Struan F A Grant, Constantin Polychronakos, Timothy R D Radstake, David A van Heel, Melanie L Dunstan, Nicholas W Wood, Ammar Al-Chalabi, Abbas Dehghan, Hakon Hakonarson, Hugh S Markus, James T Elder, Jo Knight, Dan E Arking, Timothy D Spector, Bobby P C Koeleman, Cornelia M van Duijn, Javier Martin, Andrew P Morris, Rinse K Weersma, Cisca Wijmenga, Patricia B Munroe, John R B Perry, Jennie G Pouget, Yalda Jamshidi, Harold Snieder, and Behrooz Z Alizadeh.
• Department of Epidemiology, University of Groningen, University Medical Center Groningen, Groningen, The Netherlands.
• PLoS Med. 2016 Jun 1; 13 (6): e1001976.

BackgroundC-reactive protein (CRP) is associated with immune, cardiometabolic, and psychiatric traits and diseases. Yet it is inconclusive whether these associations are causal.Methods And FindingsWe performed Mendelian randomization (MR) analyses using two genetic risk scores (GRSs) as instrumental variables (IVs). The first GRS consisted of four single nucleotide polymorphisms (SNPs) in the CRP gene (GRSCRP), and the second consisted of 18 SNPs that were significantly associated with CRP levels in the largest genome-wide association study (GWAS) to date (GRSGWAS). To optimize power, we used summary statistics from GWAS consortia and tested the association of these two GRSs with 32 complex somatic and psychiatric outcomes, with up to 123,865 participants per outcome from populations of European ancestry. We performed heterogeneity tests to disentangle the pleiotropic effect of IVs. A Bonferroni-corrected significance level of less than 0.0016 was considered statistically significant. An observed p-value equal to or less than 0.05 was considered nominally significant evidence for a potential causal association, yet to be confirmed. The strengths (F-statistics) of the IVs were 31.92-3,761.29 and 82.32-9,403.21 for GRSCRP and GRSGWAS, respectively. CRP GRSGWAS showed a statistically significant protective relationship of a 10% genetically elevated CRP level with the risk of schizophrenia (odds ratio [OR] 0.86 [95% CI 0.79-0.94]; p < 0.001). We validated this finding with individual-level genotype data from the schizophrenia GWAS (OR 0.96 [95% CI 0.94-0.98]; p < 1.72 × 10-6). Further, we found that a standardized CRP polygenic risk score (CRPPRS) at p-value thresholds of 1 × 10-4, 0.001, 0.01, 0.05, and 0.1 using individual-level data also showed a protective effect (OR < 1.00) against schizophrenia; the first CRPPRS (built of SNPs with p < 1 × 10-4) showed a statistically significant (p < 2.45 × 10-4) protective effect with an OR of 0.97 (95% CI 0.95-0.99). The CRP GRSGWAS showed that a 10% increase in genetically determined CRP level was significantly associated with coronary artery disease (OR 0.88 [95% CI 0.84-0.94]; p < 2.4 × 10-5) and was nominally associated with the risk of inflammatory bowel disease (OR 0.85 [95% CI 0.74-0.98]; p < 0.03), Crohn disease (OR 0.81 [95% CI 0.70-0.94]; p < 0.005), psoriatic arthritis (OR 1.36 [95% CI 1.00-1.84]; p < 0.049), knee osteoarthritis (OR 1.17 [95% CI 1.01-1.36]; p < 0.04), and bipolar disorder (OR 1.21 [95% CI 1.05-1.40]; p < 0.007) and with an increase of 0.72 (95% CI 0.11-1.34; p < 0.02) mm Hg in systolic blood pressure, 0.45 (95% CI 0.06-0.84; p < 0.02) mm Hg in diastolic blood pressure, 0.01 ml/min/1.73 m2 (95% CI 0.003-0.02; p < 0.005) in estimated glomerular filtration rate from serum creatinine, 0.01 g/dl (95% CI 0.0004-0.02; p < 0.04) in serum albumin level, and 0.03 g/dl (95% CI 0.008-0.05; p < 0.009) in serum protein level. However, after adjustment for heterogeneity, neither GRS showed a significant effect of CRP level (at p < 0.0016) on any of these outcomes, including coronary artery disease, nor on the other 20 complex outcomes studied. Our study has two potential limitations: the limited variance explained by our genetic instruments modeling CRP levels in blood and the unobserved bias introduced by the use of summary statistics in our MR analyses.ConclusionsGenetically elevated CRP levels showed a significant potentially protective causal relationship with risk of schizophrenia. We observed nominal evidence at an observed p < 0.05 using either GRSCRP or GRSGWAS-with persistence after correction for heterogeneity-for a causal relationship of elevated CRP levels with psoriatic osteoarthritis, rheumatoid arthritis, knee osteoarthritis, systolic blood pressure, diastolic blood pressure, serum albumin, and bipolar disorder. These associations remain yet to be confirmed. We cannot verify any causal effect of CRP level on any of the other common somatic and neuropsychiatric outcomes investigated in the present study. This implies that interventions that lower CRP level are unlikely to result in decreased risk for the majority of common complex outcomes.

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