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Funding Information: reported serving as a consultant for AbbVie UK; receiving honoraria from Falk and AbbVie UK; receiving grants from Crohn’s & Colitis UK and Tillott’s Pharmaceuticals; having a fellowship from the UK National Institute for Health Research; and receiving travel reimbursement from Merck Sharp & Dohme and Norgine. Dr Heap reported receiving travel reimbursement from AbbVie; and being a current employee of AbbVie and owning stock in the company. Dr Andersen reported receiving personal fees from Merck Sharp & Dohme and Janssen. Dr Ananthakrishnan reported receiving a grant from Pfizer; and receiving personal fees from Takeda. Dr Beaugerie reported receiving advisory board fees from Allergan, Janssen, and Pfizer; receiving a grant from Hospira; and receiving grants and honoraria from AbbVie, Merck Sharp & Dohme, Ferring, Takeda, and Tillott’s Pharmaceuticals. Dr Cummings reported receiving personal fees from AbbVie, Takeda, Biogen, Janssen, Merck Sharp & Dohme, Amgen, Hakim Pharmaceuticals, and Pfizer/Hospira; and receiving grants from Takeda, Biogen, AstraZeneca, and Pfizer/Hospira. Dr Halfvarson reported receiving personal fees from AbbVie, Hospira, Janssen, Medivir, Merck Sharp & Dohme, Pfizer, RenapharmaVifor, Takeda, Tillott’s Pharmaceuticals, Celgene, Sandoz, and Shire; and receiving grants from Janssen, Merck Sharp & Dohme, and Takeda. Dr Hart reported receiving advisory board fees from AbbVie, Atlantic, Bristol-Myers Squibb, Celltrion, Janssen, Merck Sharp & Dohme, Pfizer, Shire, and Takeda; receiving honoraria from Falk and Ferring; and receiving a grant from Takeda. Dr Irving reported receiving personal fees from Janssen, AbbVie, Takeda, Ferring, Pfizer, Lilly, Merck Sharp & Dohme, Samsung, and Sandoz; and receiving grants from Takeda and Merck Sharp & Dohme. Dr Lindsay reported receiving advisory board fees from Atlantic Health, AbbVie UK/global, Merck Sharp & Dohme, Shire UK, Vifor Pharma, Ferring International, Celltrion, Takeda, Napp, Pfizer, and Janssen; serving as a consultant for AbbVie UK/global, Takeda, and Pfizer; receiving grants from Shire UK, AbbVie UK/global, Warner Chilcott, Funding Information: Takeda, Hospira, Ferring International, and Merck Sharp & Dohme; receiving honoraria from Takeda, Cornerstones US, Tillott’s Pharmaceuticals, Napp, Shire International, Janssen, AbbVie, and Pfizer; and receiving travel reimbursement from AbbVie UK, Merck Sharp & Dohme, Warner Chilcott, Takeda, and Shire International. Dr McGovern reported receiving grants from the National Institutes of Health, Helmsley Charitable Trust, and Janssen; and serving as a consultant for Pfizer, Q Biologics, Cidara, Gilead, and Janssen. Dr Seksik reported receiving advisory board fees from Astellas; receiving honoraria from Takeda, AbbVie, and Ferring; and receiving grants from Merck Sharp & Dohme and Biocodex. Dr Sokol reported receiving grants from Biocodex, Danone, and BiomX; serving as a consultant for Enterome, Takeda, AbbVie, Roche, Amgen, Danone, BiomX, Ferring, Bristol-Myers Squibb, Astellas, Merck Sharp & Dohme, Novartis, Tillott’s Pharmaceuticals, and Biose; and being the co-founder of Nextbiotix. Dr Annese reported receiving advisory board fees from Takeda, AbbVie, and Medtronic; and receiving honoraria from Janssen, Takeda, AbbVie, and Medtronic. Dr Weersma reported receiving grants from Takeda, Ferring, and Tramedico; and receiving personal fees from AbbVie. Dr Goodhand reported receiving honoraria from Falk, AbbVie, and Shield Therapeutics. Dr Kennedy reported serving as a consultant for Falk; receiving honoraria from Falk, Allergan, Pharmacosmos, and Takeda; and being a deputy editor of Alimentary Pharmacology & Therapeutics. Dr Ahmad reported receiving unrestricted grants, advisory board fees, speaker honoraria, and support to attend international meetings from AbbVie, Merck Sharp & Dohme, Janssen, Takeda, Ferring, Tillott’s Pharmaceuticals, Ferring, Pfizer, Napp, Celltrion, and Hospira. No other disclosures were reported. Funding Information: Immunobiology Research Institute, Cedars-Sinai Medical Center, Los Angeles, California), Alistair McNair, PhD (Queen Elizabeth Hospital, London, UK), Anita Modi, MD (Luton and Dunstable University Hospital, Luton, UK), Kevin Monahan, PhD (West Middlesex University Hospital, Middlesex, UK), Alex Moran, MD (Northern Devon Healthcare Trust, Barnstaple, UK), Mary-Anne Morris, MD (Norfolk and Norwich University Hospitals NHS Foundation Trust, Norwich, UK), Marianne Mortimore, MBBS (Mater Research Institute, University of Queensland, South Brisbane, Australia), Craig Mowat, MD (Ninewells Hospital, NHS Tayside, Dundee, UK), Rafeeq Muhammed, MD (Birmingham Children's Hospital, Birmingham, UK), Charles D. R. Murray, PhD (Royal Free Hospital, Royal Free London NHS Foundation Trust, London, UK), Hanlie Olivier (IBD Pharmacogenetics Group, University of Exeter, Exeter, UK), Timothy R. Orchard, DM (Imperial College Healthcare NHS Trust, London, UK), Simon Panter, MD (South Tyneside District Hospital, South Tyneside, UK), Vinod Patel, MBBS (Tameside and Glossop Integrated Care NHS Foundation Trust, Ashton-under-Lyne, UK), Rosemary Phillips, MD (Princess Alexandra Hospital, Essex, UK), Neeraj Prasad, MSc (Wrightington Hospital, Wrightington, UK), Cathryn Preston, MBChB (Bradford Royal Infirmary, Bradford, UK), Graham Radford-Smith, PhD (Royal Brisbane and Women’s Hospital, Brisbane, Australia), Praveen Rajasekhar, MD (Northumbria NHS Trust, Tyne and Wear, UK), Dipak Roy, PhD (Tameside and Glossop Integrated Care NHS Foundation Trust, Ashton-under-Lyne, UK), Rebecca Saich, PhD (Basingstoke and North Hampshire Hospital, Basingstoke, UK), Jack Satsangi, PhD (Western General Hospital, NHS Lothian, Edinburgh, UK), Stefan Schreiber, PhD (Kiel University, Kiel, Germany), Sandip Sen, MD (Royal Stoke University Hospital, Stoke-on-Trent, UK), Neil Shah, MD (Great Ormond Street Hospital for Children NHS Foundation Trust, London, UK), Richard Shenderay, MBBS (Airedale NHS Foundation Trust, Keighley, UK), Acuth Shenoy, MD (Colchester Hospital University NHS Foundation Trust, Colchester, UK), James Shutt, DM (Dorset County Hospital NHS Foundation Trust, Dorchester, UK), Mark Silverberg, PhD (Mount Sinai Hospital, Toronto, Ontario, Canada), Alison Simmons, PhD (Oxford University Hospitals, Oxford, UK), Jonathan Simmons, DM (Royal Berkshire Hospital, Royal Berkshire NHS Foundation Trust, Reading, UK), Salil Singh, PhD (Bolton NHS Foundation Trust, Bolton, UK), Malcolm Smith, MBChB (Aberdeen Royal Infirmary, Aberdeen, UK), Mark Smith, MD (Shrewsbury and Telford Hospital NHS Trust, Shrewsbury, UK), Melissa Smith, MB (Royal Sussex County Hospital, Brighton, UK), Jonathon A. Snook, DPhil (Poole Hospital NHS Foundation Trust, Poole, UK), Sunil Sonwalker, MD (Calderdale Royal Hospital, Halifax, UK), Christine R. Stevens, PhD (Broad Institute, Harvard University, Cambridge, Massachusetts), Giacomo Sturniolo, PhD (Univerita di Padova, Padova, Italy), Sreedhar Subramanian, MD (Royal Liverpool and Broadgreen University Hospitals NHS Trust, Liverpool, UK), Amanda Thomas, MBBS (Department of Gastroenterology, Royal Devon and Exeter Hospital NHS Foundation Trust, Exeter, UK), Mark Tighe, BM (Poole Hospital NHS Foundation Trust, Poole, UK), Franco Torrente, MD (Department of Gastroenterology, Addenbrooke’s Hospital, Cambridge University Hospitals NHS Foundation Trust, Cambridge, UK), Mark Tremelling, MD (Norfolk and Norwich University Hospitals NHS Foundation Trust, Norwich, UK), Epameinondas Tsianos, PhD (University Hospital of Ioannina, Ioannina, Greece), Deven Vani, MD (Mid Yorkshire Hospitals NHS Trust, Wakefield, UK), Alissa Walsh, MBBS (St Vincent’s Hospital, Sydney, Australia), Gillian Watermeyer, MBChB (Groote Schuur Hospital, Cape Town, South Africa), David Watts, MBChB (Forth Valley Royal Hospital, Larbert, UK), Gill Watts, MD (Wythenshawe Hospital, South Manchester, UK), Sean Weaver, PhD (Royal Bournemouth General Hospital, Bournemouth, UK), Emma Wesley, MBBS (Musgrove Park Hospital, Taunton and Somerset NHS Hospitals, Taunton, UK), Anne Willmott, MBChB (Leicester Royal Infirmary-Paediatric, Leicester, UK), Karen Yearsley, BM (Nevill Hall Hospital, Abergavenny, UK), Veena Zambar, MBBS (Leeds General Infirmary, Leeds, UK), and Sebastian Zeissig, MD (University Medical Center Schleswig-Hostein, Kiel, Germany). These individuals identified and recruited patient s to the study and provided comments on a draft of the manuscript. Funding Information: Adverse Events Consortium funded the sample collection and genotyping at the Broad Institute. The UK National Institute for Health Research provided research nurse support to facilitate recruitment at all UK research sites. Crohn’s & Colitis UK and forCrohns provided funding support and publicized this study to their members. The Exeter National Institute for Health Research Clinical Research Facility provided DNA storage and management. Institutional strategic support award WT097835MF from Wellcome Trust supported the management of the study. Samples from Cedars-Sinai were collected and processed through the MIRIAD biobank that was funded by grant P01DK046763 from the National Institutes of Health. Publisher Copyright: © 2019 American Medical Association. All rights reserved. IMPORTANCE Use of thiopurines may be limited by myelosuppression. TPMT pharmacogenetic testing identifies only 25% of at-risk patients of European ancestry. Among patients of East Asian ancestry, NUDT15 variants are associated with thiopurine-induced myelosuppression (TIM). OBJECTIVE To identify genetic variants associated with TIM among patients of European ancestry with inflammatory bowel disease (IBD). DESIGN, SETTING, AND PARTICIPANTS Case-control study of 491 patients affected by TIM and 679 thiopurine-tolerant unaffected patients who were recruited from 89 international sites between March 2012 and November 2015. Genome-wide association studies (GWAS) and exome-wide association studies (EWAS) were conducted in patients of European ancestry. The replication cohort comprised 73 patients affected by TIM and 840 thiopurine-tolerant unaffected patients. EXPOSURES Genetic variants associated with TIM. MAIN OUTCOMES AND MEASURES Thiopurine-induced myelosuppression, defined as a decline in absolute white blood cell count to 2.5 x 10(9)/L or less or a decline in absolute neutrophil cell count to 1.0 x 10(9)/L or less leading to a dose reduction or drug withdrawal. RESULTS Among 1077 patients (398 affected and 679 unaffected; median age at IBD diagnosis, 31.0 years [interquartile range, 21.2 to 44.1 years]; 540 [50%] women; 602 [56%] diagnosed as having Crohn disease), 919 (311 affected and 608 unaffected) were included in the GWAS analysis and 961 (328 affected and 633 unaffected) in the EWAS analysis. The GWAS analysis confirmed association of TPMT (chromosome 6, rs11969064) with TIM (30.5% [95/311] affected vs 16.4% [100/608] unaffected patients; odds ratio [OR], 2.3 [95% CI, 1.7 to 3.1], P = 5.2 x 10(-9)). The EWAS analysis demonstrated an association with an in-frame deletion in NUDT15 (chromosome 13, rs746071566) and TIM (5.8% [19/328] affected vs 0.2% [1/633] unaffected patients; OR, 38.2 [95% CI, 5.1 to 286.1], P = 1.3 x 10(-8)), which was replicated in a different cohort (2.7% [2/73] affected vs 0.2% [2/840] unaffected patients; OR, 11.8 [95% CI, 1.6 to 85.0], P = .03). Carriage of any of 3 coding NUDT15 variants was associated with an increased risk (OR, 27.3 [95% CI, 9.3 to 116.7], P = 1.1 x 10(-7)) of TIM, independent of TPMT genotype and thiopurine dose. CONCLUSIONS AND RELEVANCE Among patients of European ancestry with IBD, variants in NUDT15 were associated with increased risk of TIM. These findings suggest that NUDT15 genotyping may be considered prior to initiation of thiopurine therapy; however, further study including additional validation in independent cohorts is required.

Pancreatitis occurs in approximately 4% of patients treated with the thiopurines azathioprine or mercaptopurine. Its development is unpredictable and almost always leads to drug withdrawal. We identified patients with inflammatory bowel disease (IBD) who had developed pancreatitis within 3 months of starting these drugs from 168 sites around the world. After detailed case adjudication, we performed a genome-wide association study on 172 cases and 2,035 controls with IBD. We identified strong evidence of association within the class II HLA region, with the most significant association identified at rs2647087 (odds ratio 2.59, 95% confidence interval 2.07-3.26, P = 2 x 10(-16)). We replicated these findings in an independent set of 78 cases and 472 controls with IBD matched for drug exposure. Fine mapping of the H LA region identified association with the HLA-DQA1*02:01-HLA-DRB1*07:01 haplotype. Patients heterozygous at rs2647087 have a 9% risk of developing pancreatitis after administration of a thiopurine, whereas homozygotes have a 17% risk.