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Objectives To find causal genes for rheumatoid arthritis (RA) and its seropositive (RF and/or ACPA positive) and seronegative subsets. Methods We performed a genome-wide association study (GWAS) of 31 313 RA cases (68% seropositive) and similar to 1 million controls from Northwestern Europe. We searched for causal genes outside the HLA-locus through effect on coding, mRNA expression in several tissues and/or levels of plasma proteins (SomaScan) and did network analysis (Qiagen). Results We found 25 sequence variants for RA overall, 33 for seropositive and 2 for seronegative RA, altogether 37 sequence variants at 34 non-HLA loci, of which 15 are novel. Genomic, transcriptomic and proteomic analysis of these yielded 25 causal genes in seropositive RA and additional two overall. Most encode proteins in the network of interferon-alpha/beta and IL-12/23 that signal through the JAK/STAT-pathway. Highlighting those with largest effect on seropositive RA, a rare missense variant in STAT4 (rs140675301-A) that is independent of reported non-coding STAT4-variants, increases the risk of seropositive RA 2.27-fold (p=2.1x10(-9)), more than the rs2476601-A missense variant in PTPN22 (OR=1.59, p=1.3x10(-160)). STAT4 rs140675301-A replaces hydrophilic glutamic acid with hydrophobic valine (Glu128Val) in a conserved, surface-exposed loop. A stop-mutation (rs76428106-C) in FLT3 increases seropositive RA risk (OR=1.35, p=6.6x10(-11)). Independent missense variants in TYK2 (rs34536443-C, rs12720356-C, rs35018800-A, latter two novel) associate with decreased risk of seropositive RA (ORs=0.63-0.87, p=10(-9)-10(-27)) and decreased plasma levels of interferon-alpha/beta receptor 1 that signals through TYK2/JAK1/STAT4. Conclusion Sequence variants pointing to causal genes in the JAK/STAT pathway have largest effect on seropositive RA, while associations with seronegative RA remain scarce., Funding Agencies|NORDFORSK [90825]; Swedish Research Council [2018-02803]; Swedish innovation Agency (Vinnova); Innovationsfonden; The Research Council of Norway; Region Stockholm-Karolinska Institutet; Region Vasterbotten (ALF); Danish Rheumatism Association [R194-A6956, A1923, A3037, A3570]; Swedish Brain Foundation; Nils and Bibbi Jensens Foundation; Knut and Alice Wallenberg Foundation; Margaretha af Ugglas Foundation; South-Eastern Heath Region of Norway; Health Research Fund of Central Denmark Region; Region of Southern Denmark; A.P. Moller Foundation for the Advancement of Medical Science; Colitis-Crohn Foreningen; Novo Nordisk Foundation [NNF15OC0016932]; Aase og Ejnar Danielsens Fond; Beckett-Fonden; Augustinus Fonden; Knud and Edith Eriksens Mindefond; Laege Sofus Carl Emil Friis and Hustru Olga Doris Friis Legat; Psoriasis Forskningsfonden; University of Aarhus; Region of Southern Denmarks PhD Fund [12/7725]; Department of Rheumatology, Frederiksberg Hospital; Research Council of Norway [229624, 223273]; South East and Western Norway Health Authorities; ERC AdG project SELECTionPREDISPOSED; Stiftelsen Kristian Gerhard Jebsen; Trond Mohn Foundation; Novo Nordisk Foundation; University of Bergen

BACKGROUND: The prevalence of iron depletion is high among premenopausal women who donate blood frequently. Studies in nondonor populations indicate that iron deficiency anemia is associated with an increased risk of low birth weight. This prompts concerns that iron deficiency induced by frequent blood donation might impair subsequent fetal development. STUDY DESIGN AND METHODS: The aim of this study was to assess whether prepregnancy donation intensity affects the birth weight of singletons born at term (gestational week 38 or later) to nulliparous female donors in Denmark. We identified 293,897 first live singleton births to Danish women between 1997 and 2012 with complete information on gestational age, birth weight, child sex, parental age, maternal smoking status during pregnancy, and parental education length and annual income. Linear regression analysis was applied, with birth weight as outcome, number of donations within the 3 years before pregnancy as the explanatory variable, and confounding variables as described. RESULTS: Birth weight among children of low-intensity donors (n = 22,120) was 12.6 g (95% confidence interval, 6.7–18.6) higher than nondonors (n = 268,253) after controlling for the above-mentioned factors. The higher birth weight among low-intensity donors can be explained by the healthy donor effect. In fully adjusted analyses, birth weight among children of high-intensity donors (n = 3,524) was 20.2 g (95% confidence interval, 5.1–35.3 g) lower compared with low-intensity donors. This reduced birth weight among high-intensity donors compared to low-intensity donors may reflect blood donation–induced iron deficiency. CONCLUSIONS: Our results show that high prepregnancy donation intensity is inversely associated with birth weight of singletons born at term to nulliparous women.

BACKGROUND: Genomewide association studies have reported alleles in the ABO locus to be associated with ferritin levels. These studies warrant the investigation of a possible association between the ABO blood group and ferritin levels. We aimed to explore if ABO blood group is associated with iron stores expressed as ferritin levels. STUDY DESIGN AND METHODS: Ferritin levels were measured at least once for 30,595 Danish Blood Donor Study participants. Linear regression analyses were performed with the ABO blood group as explanatory variable and adjusted for age, number of donations 3 years before the ferritin measurement, and time since latest donation. In addition, a subanalysis was performed on 15,280 individuals in which further adjustments for body mass index, smoking status, and C-reactive protein levels were possible. Furthermore, logistic regression analyses were performed to determine if ABO blood group was associated with a ferritin level of less than 15 ng/mL. RESULTS: Non-O blood group donors had lower ferritin levels than blood group O donors, regardless of sex. Accordingly, risk of ferritin level of less than 15 ng/mL was increased for individuals with non-O blood group compared with O blood group. In subanalyses similar associations were observed, albeit in women the association between blood group and risk of a ferritin level below 15 ng/mL was no longer significant. ABO blood group was not associated with red blood cell indices such as mean cell volume and mean cell hemoglobin content. CONCLUSION: Donors with non-O blood group have lower ferritin levels than donors with other blood groups.