Prevalence of NUDT15 Genetic Variants and Incidence of Thiopurine-induced Leukopenia in Inflammatory Bowel Disease: A Systematic Review and Meta-analysis.

BACKGROUND AND AIMS: Nudix hydrolase 15 [NUDT15] genetic variants confer an increased risk of thiopurine-induced leukopenia [TIL]; however, their global prevalence in inflammatory bowel disease [IBD] patients is unknown. We aimed to evaluate the global prevalence of NUDT15 variants in IBD patients and incidence of TIL in these patients. METHODS: Six databases were searched from inception until July 2022. Studies reporting the frequency of any NUDT15 variant and/or frequency of leukopenia in adult IBD patients with these variants were included. A random effects model was performed to estimate the pooled prevalence of variants, incidence of early [≤8 weeks] and late [>8 weeks] leukopenia, and relative risk of developing leukopenia. RESULTS: Twenty studies comprising 5232 patients were included. The pooled prevalence of the *1/*3 c.415C > T C/T diplotype was 13% (95% confidence interval [CI]: 10-18%), *3/*3 c.415C > T T/T diplotype was 2% [95% CI: 1-2%], *1/*5 c.52G > A G/A diplotype was 2% [95% CI: 1-3%], and *1/*6 c.36_37insGGAGTC ins/- diplotype was 7% [95% CI: 4-12%]. The pooled prevalence of *1/*3 was high in Japanese [20%, 95% CI: 16-24%] and Chinese patients [18%, 95% CI: 12-27%]. The incidence of early leukopenia was 20% [95% CI: 16-26%] in *1/*3 patients, 99% [95% CI: 7-100%] in *3/*3 patients, and 49% [95% CI: 29-69%] in *1/*6 patients. The incidence of late leukopenia was 36% [95% CI: 26-49%] in *1/*3 patients. CONCLUSIONS: NUDT15 variants are common and strongly predict TIL in IBD patients. Pre-treatment NUDT15 genotyping should be considered particularly in Asian populations, to guide thiopurine dosing and prevent myelotoxicity.

Treatment adaptations and outcomes of patients experiencing inflammatory bowel disease flares during the early COVID-19 pandemic: the PREPARE-IBD multicentre cohort study.

BACKGROUND: The COVID-19 pandemic offered a unique opportunity to understand inflammatory bowel disease (IBD) management during unexpected disruption. This could help to guide practice overall. AIMS: To compare prescribing behaviour for IBD flares and outcomes during the early pandemic with pre-pandemic findings METHODS: We performed an observational cohort study comprising patients who contacted IBD teams for symptomatic flares between March and June 2020 in 60 National Health Service trusts in the United Kingdom. Data were compared with a pre-pandemic cohort after propensity-matching for age and physician global assessment of disease activity. RESULTS: We included 1864 patients in each of the pandemic and pre-pandemic cohorts. The principal findings were reduced systemic corticosteroid prescription during the pandemic in Crohn's disease (prednisolone: pandemic 26.5% vs. 37.1%; p < 0.001) and ulcerative colitis (UC) (prednisolone: pandemic 33.5% vs. 40.7%, p < 0.001), with increases in poorly bioavailable oral corticosteroids in Crohn's (pandemic 15.6% vs. 6.8%; p < 0.001) and UC (pandemic 11.8% vs. 5.2%; p < 0.001). Ustekinumab (Crohn's and UC) and vedolizumab (UC) treatment also significantly increased. Three-month steroid-free remission in each period was similar in Crohn's (pandemic 28.4% vs. 32.1%; p = 0.17) and UC (pandemic 36.4% vs. 40.2%; p = 0.095). Patients experiencing a flare and suspected COVID-19 were more likely to have moderately-to-severely active disease at 3 months than those with a flare alone. CONCLUSIONS: Despite treatment adaptations during the pandemic, steroid-free outcomes were comparable with pre-pandemic levels, although concurrent flare and suspected COVID-19 caused worse outcomes. These findings have implications for IBD management during future pandemics and for standard practice.

Ambulatory care management of 69 patients with acute severe ulcerative colitis in comparison to 695 inpatients: insights from a multicentre UK cohort study.

INTRODUCTION: Acute severe ulcerative colitis (ASUC) traditionally requires inpatient hospital management for intravenous therapies and/or colectomy. Ambulatory ASUC care has not yet been evaluated in large cohorts. AIMS: We used data from PROTECT, a UK multicentre observational COVID-19 inflammatory bowel disease study, to report the extent, safety and effectiveness of ASUC ambulatory pathways. METHODS: Adults (≥18 years old) meeting Truelove and Witts criteria between 1 January 2019-1 June 2019 and 1 March 2020-30 June 2020 were recruited to PROTECT. We used demographic, disease phenotype, treatment outcomes and 3-month follow-up data. Primary outcome was rate of colectomy during the index ASUC episode. Secondary outcomes included corticosteroid response, time to and rate of rescue or primary induction therapy, response to rescue or primary induction therapy, time to colectomy, mortality, duration of inpatient treatment and hospital readmission and colectomy within 3 months of index flare. We compared outcomes in three cohorts: (1) patients treated entirely in inpatient setting; ambulatory patients subdivided into; (2) patients managed as ambulatory from diagnosis and (3) patients hospitalised and subsequently discharged to ambulatory care for continued intravenous steroids. RESULTS: 37% (22/60) participating hospitals used ambulatory pathways. Of 764 eligible patients, 695 (91%) patients received entirely inpatient care, 15 (2%) patients were managed as ambulatory from diagnosis and 54 (7%) patients were discharged to ambulatory pathways. Aside from younger age in patients treated as ambulatory from diagnosis, no significant differences in disease or patient phenotype were observed. The rate of colectomy (15.0% (104/695) vs 13.3% (2/15) vs 13.0% (7/54), respectively, p=0.96) and secondary outcomes were similar among all three cohorts. Stool culture and flexible sigmoidoscopy were less frequently performed in ambulatory cohorts. Forty per cent of patients treated as ambulatory from diagnosis required subsequent hospital admission. CONCLUSIONS: In a post hoc analysis of one of the largest ASUC cohorts collected to date, we report an emerging UK ambulatory practice which challenges treatment paradigms. However, our analysis remains underpowered to detect key outcome measures and further studies exploring clinical and cost-effectiveness as well as patient and physician acceptability are needed. TRIAL REGISTRATION NUMBER: NCT04411784.

Inflammatory Bowel Disease Clinical Activity is Associated with COVID-19 Severity Especially in Younger Patients.

BACKGROUND AND AIMS: Age is a major prognostic factor for COVID-19 outcomes. The effect of inflammatory bowel disease [IBD] activity on COVID-19 is unclear. We examined the relationship between IBD activity and COVID-19 severity according to age. METHODS: We included IBD patients diagnosed with COVID-19, reported to SECURE-IBD between March 13, 2020 and August 3, 2021. Clinical IBD activity was measured by physician global assessment [PGA]. COVID-19-related outcomes were [1] intensive care unit [ICU] admission, ventilation or death, and [2] hospitalization. Using generalized estimating equations, we determined adjusted odds ratios [aOR, 95% confidence interval] for moderate and severe PGA vs clinical remission/mild PGA, controlling for demographics, medications and COVID-19 diagnosis period. We performed stratified analyses by age [≤50 vs >50 years]. RESULTS: Among 6078 patients, adverse COVID-19 outcomes were more common with active IBD: ICU/ventilation/death in 3.6% [175/4898] of remission/mild, 4.9% [45/920] of moderate and 8.8% [23/260] of severe [p < 0.001]; and hospitalization in 13% [649/4898] of remission/mild, 19% [178/920] of moderate and 38% [100/260] of severe [p < 0.001]. Stratified by decade, effect sizes were larger for younger patients. In patients ≤50 years, severe PGA was independently associated with ICU/ventilation/death (aOR 3.27 [1.15-9.30]) and hospitalization (aOR 4.62 [2.83-7.55]). In contrast, severe PGA was not independently associated with COVID-19 outcomes in those older than 50 years. CONCLUSIONS: Clinically active IBD may be a risk factor for severe COVID-19, particularly in younger patients. IBD disease control, including through medication compliance, and strategies to mitigate the risk of COVID-19 infection amongst patients with active IBD [e.g. distancing, immunization] are key to limit adverse COVID-19 outcomes.

Assessment, endoscopy, and treatment in patients with acute severe ulcerative colitis during the COVID-19 pandemic (PROTECT-ASUC): a multicentre, observational, case-control study.

BACKGROUND: There is a paucity of evidence to support safe and effective management of patients with acute severe ulcerative colitis during the COVID-19 pandemic. We sought to identify alterations to established conventional evidence-based management of acute severe ulcerative colitis during the early COVID-19 pandemic, the effect on outcomes, and any associations with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection and severe COVID-19 outcomes. METHODS: The PROTECT-ASUC study was a multicentre, observational, case-control study in 60 acute secondary care hospitals throughout the UK. We included adults (≥18 years) with either ulcerative colitis or inflammatory bowel disease unclassified, who presented with acute severe ulcerative colitis and fulfilled the Truelove and Witts criteria. Cases and controls were identified as either admitted or managed in emergency ambulatory care settings between March 1, 2020, and June 30, 2020 (COVID-19 pandemic period cohort), or between Jan 1, 2019, and June 30, 2019 (historical control cohort), respectively. The primary outcome was the proportion of patients with acute severe ulcerative colitis receiving rescue therapy (including primary induction) or colectomy. The study is registered with ClinicalTrials.gov, NCT04411784. FINDINGS: We included 782 patients (398 in the pandemic period cohort and 384 in the historical control cohort) who met the Truelove and Witts criteria for acute severe ulcerative colitis. The proportion of patients receiving rescue therapy (including primary induction) or surgery was higher during the pandemic period than in the historical period (217 [55%] of 393 patients vs 159 [42%] of 380 patients; p=0·00024) and the time to rescue therapy was shorter in the pandemic cohort than in the historical cohort (p=0·0026). This difference was driven by a greater use of rescue and primary induction therapies with biologicals, ciclosporin, or tofacitinib in the COVID-19 pandemic period cohort than in the historical control period cohort (177 [46%] of 387 patients in the COVID-19 cohort vs 134 [36%] of 373 patients in the historical cohort; p=0·0064). During the pandemic, more patients received ambulatory (outpatient) intravenous steroids (51 [13%] of 385 patients vs 19 [5%] of 360 patients; p=0·00023). Fewer patients received thiopurines (29 [7%] of 398 patients vs 46 [12%] of 384; p=0·029) and 5-aminosalicylic acids (67 [17%] of 398 patients vs 98 [26%] of 384; p=0·0037) during the pandemic than in the historical control period. Colectomy rates were similar between the pandemic and historical control groups (64 [16%] of 389 vs 50 [13%] of 375; p=0·26); however, laparoscopic surgery was less frequently performed during the pandemic period (34 [53%] of 64] vs 38 [76%] of 50; p=0·018). Five (2%) of 253 patients tested positive for SARS-CoV-2 during hospital treatment. Two (2%) of 103 patients re-tested for SARS-CoV-2 during the 3-month follow-up were positive 5 days and 12 days, respectively, after discharge from index admission. Both recovered without serious outcomes. INTERPRETATION: The COVID-19 pandemic altered practice patterns of gastroenterologists and colorectal surgeons in the management of acute severe ulcerative colitis but was associated with similar outcomes to a historical cohort. Despite continued use of high-dose corticosteroids and biologicals, the incidence of COVID-19 within 3 months was low and not associated with adverse COVID-19 outcomes. FUNDING: None.

Quality improvement project identifies factors associated with delay in IBD diagnosis.

BACKGROUND: Delay in the diagnosis of inflammatory bowel disease (IBD) is common and contemporary UK studies are lacking. AIM: To determine factors associated with, and the consequences of, a prolonged time to diagnosis in IBD. METHODS: This quality improvement study included 304 adults with a new IBD diagnosis made between January 2014 and December 2017 across 49 general practices (GP) and gastroenterology secondary care services. Outcome measures were demographic, clinical and laboratory factors associated with a delayed time, defined as greater than upper quartile, to: (a) patient presentation (b) GP referral (c) secondary care diagnosis, and factors associated with a complicated disease course (hospitalisation and/or surgery and/or biologic treatment) in the year after diagnosis. RESULTS: The median [IQR] diagnosis sub-intervals were: (a) patient = 2.1 months [0.9-5.1]; (b) GP = 0.3 months [0.0-0.9]; (c) secondary care = 1.1 months [0.5-2.1]. 50% of patients were diagnosed within 4 months and 92% were diagnosed within 2 years of symptom onset. Diagnostic delay was more common in Crohn's disease (7.6 months [3.1-15.0]) than ulcerative colitis (3.3 months [1.9-7.3]) (P < 0.001). Patients who presented as an emergency (P < 0.001) but not those with a delayed overall time to diagnosis (P = 0.35) were more likely to have a complicated disease course. CONCLUSION: Time to patient presentation is the largest component of time to IBD diagnosis. Emergency presentation is common and, unlike a delayed time to diagnosis, is associated with a complicated disease course.

Clinical Features and Genetic Risk of Demyelination Following Anti-TNF Treatment.

BACKGROUND: Anti-TNF exposure has been linked to demyelination events. We sought to describe the clinical features of demyelination events following anti-TNF treatment and to test whether affected patients were genetically predisposed to multiple sclerosis [MS]. METHODS: We conducted a case-control study to describe the clinical features of demyelination events following anti-TNF exposure. We compared genetic risk scores [GRS], calculated using carriage of 43 susceptibility loci for MS, in 48 cases with 1219 patients exposed to anti-TNF who did not develop demyelination. RESULTS: Overall, 39 [74%] cases were female. The median age [range] of patients at time of demyelination was 41.5 years [20.7-63.2]. The median duration of anti-TNF treatment was 21.3 months [0.5-99.4] and 19 [36%] patients were receiving concomitant immunomodulators. Most patients had central demyelination affecting the brain, spinal cord, or both. Complete recovery was reported in 12 [23%] patients after a median time of 6.8 months [0.1-28.7]. After 33.0 months of follow-up, partial recovery was observed in 29 [55%] patients, relapsing and remitting episodes in nine [17%], progressive symptoms in three [6%]: two [4%] patients were diagnosed with MS. There was no significant difference between MS GRS scores in cases (mean -3.5 × 10-4, standard deviation [SD] 0.0039) and controls [mean -1.1 × 10-3, SD 0.0042] [p = 0.23]. CONCLUSIONS: Patients who experienced demyelination events following anti-TNF exposure were more likely female, less frequently treated with an immunomodulator, and had a similar genetic risk to anti-TNF exposed controls who did not experience demyelination events. Large prospective studies with pre-treatment neuroimaging are required to identify genetic susceptibility loci.

Primary care faecal calprotectin testing in children with suspected inflammatory bowel disease: a diagnostic accuracy study.

OBJECTIVE: To determine the diagnostic accuracy of calprotectin to diagnose inflammatory bowel disease (IBD) in children in whom general practitioners (GPs) suspected IBD. DESIGN: Prospective observational cohort study of a new calprotectin-based primary care referral pathway. SETTING: 48 GP practices and gastroenterology secondary care services at the Royal Devon and Exeter NHS Foundation Trust in the South-West of England, UK. PATIENTS: 195 children aged between 4 and 18 years referred on the pathway between January 2014 and August 2017 for investigation of gastrointestinal symptoms were included. INTERVENTIONS: Primary-care-driven faecal calprotectin testing. Primary and secondary care records over 12 months from the point of calprotectin testing were used as the reference standard. MAIN OUTCOME MEASURES: Diagnostic accuracy of calprotectin testing to detect IBD. RESULTS: 7% (13/195) tested patients were diagnosed with IBD. Using our prespecified cut-off of 100 µg/g, calprotectin had a diagnostic accuracy of 91% (95% CI 86% to 95%) with a sensitivity for distinguishing IBD from non-IBD of 100% (95% CI 75% to 100%), a specificity of 91% (95% CI 85% to 94%), a positive predictive value of 43% (95% CI 25% to 63%) and a negative predictive value of 100% (95% CI 98% to 100%). Calprotectin testing had no effect on the time to diagnosis, but a negative test contributed to saved referrals and was associated with fewer diagnostic tests in secondary care. CONCLUSIONS: Calprotectin testing of children with suspected IBD in primary care accurately distinguishes IBD from a functional gut disorder, reduces secondary care referrals and associated diagnostic healthcare utilisation.

Massively parallel variant characterization identifies NUDT15 alleles associated with thiopurine toxicity.

As a prototype of genomics-guided precision medicine, individualized thiopurine dosing based on pharmacogenetics is a highly effective way to mitigate hematopoietic toxicity of this class of drugs. Recently, NUDT15 deficiency was identified as a genetic cause of thiopurine toxicity, and NUDT15-informed preemptive dose reduction was quickly adopted in clinical settings. To exhaustively identify pharmacogenetic variants in this gene, we developed massively parallel NUDT15 function assays to determine the variants' effect on protein abundance and thiopurine cytotoxicity. Of the 3,097 possible missense variants, we characterized the abundance of 2,922 variants and found 54 hotspot residues at which variants resulted in complete loss of protein stability. Analyzing 2,935 variants in the thiopurine cytotoxicity-based assay, we identified 17 additional residues where variants altered NUDT15 activity without affecting protein stability. We identified structural elements key to NUDT15 stability and/or catalytical activity with single amino acid resolution. Functional effects for NUDT15 variants accurately predicted toxicity risk alleles in patients treated with thiopurines with far superior sensitivity and specificity compared to bioinformatic prediction algorithms. In conclusion, our massively parallel variant function assays identified 1,152 deleterious NUDT15 variants, providing a comprehensive reference of variant function and vastly improving the ability to implement pharmacogenetics-guided thiopurine treatment individualization.

HLA-DQA1*05 Carriage Associated With Development of Anti-Drug Antibodies to Infliximab and Adalimumab in Patients With Crohn's Disease.

BACKGROUND & AIMS: Anti-tumor necrosis factor (anti-TNF) therapies are the most widely used biologic drugs for treating immune-mediated diseases, but repeated administration can induce the formation of anti-drug antibodies. The ability to identify patients at increased risk for development of anti-drug antibodies would facilitate selection of therapy and use of preventative strategies. METHODS: We performed a genome-wide association study to identify variants associated with time to development of anti-drug antibodies in a discovery cohort of 1240 biologic-naïve patients with Crohn's disease starting infliximab or adalimumab therapy. Immunogenicity was defined as an anti-drug antibody titer ≥10 AU/mL using a drug-tolerant enzyme-linked immunosorbent assay. Significant association signals were confirmed in a replication cohort of 178 patients with inflammatory bowel disease. RESULTS: The HLA-DQA1*05 allele, carried by approximately 40% of Europeans, significantly increased the rate of immunogenicity (hazard ratio [HR], 1.90; 95% confidence interval [CI], 1.60-2.25; P = 5.88 × 10-13). The highest rates of immunogenicity, 92% at 1 year, were observed in patients treated with infliximab monotherapy who carried HLA-DQA1*05; conversely the lowest rates of immunogenicity, 10% at 1 year, were observed in patients treated with adalimumab combination therapy who did not carry HLA-DQA1*05. We confirmed this finding in the replication cohort (HR, 2.00; 95% CI, 1.35-2.98; P = 6.60 × 10-4). This association was consistent for patients treated with adalimumab (HR, 1.89; 95% CI, 1.32-2.70) or infliximab (HR, 1.92; 95% CI, 1.57-2.33), and for patients treated with anti-TNF therapy alone (HR, 1.75; 95% CI, 1.37-2.22) or in combination with an immunomodulator (HR, 2.01; 95% CI, 1.57-2.58). CONCLUSIONS: In an observational study, we found a genome-wide significant association between HLA-DQA1*05 and the development of antibodies against anti-TNF agents. A randomized controlled biomarker trial is required to determine whether pretreatment testing for HLA-DQA1*05 improves patient outcomes by helping physicians select anti-TNF and combination therapies. ClinicalTrials.gov ID: NCT03088449.

Predictors of anti-TNF treatment failure in anti-TNF-naive patients with active luminal Crohn's disease: a prospective, multicentre, cohort study.

BACKGROUND: Anti-TNF drugs are effective treatments for the management of Crohn's disease but treatment failure is common. We aimed to identify clinical and pharmacokinetic factors that predict primary non-response at week 14 after starting treatment, non-remission at week 54, and adverse events leading to drug withdrawal. METHODS: The personalised anti-TNF therapy in Crohn's disease study (PANTS) is a prospective observational UK-wide study. We enrolled anti-TNF-naive patients (aged ≥6 years) with active luminal Crohn's disease at the time of first exposure to infliximab or adalimumab between March 7, 2013, and July 15, 2016. Patients were evaluated for 12 months or until drug withdrawal. Demographic data, smoking status, age at diagnosis, disease duration, location, and behaviour, previous medical and drug history, and previous Crohn's disease-related surgeries were recorded at baseline. At every visit, disease activity score, weight, therapy, and adverse events were recorded; drug and total anti-drug antibody concentrations were also measured. Treatment failure endpoints were primary non-response at week 14, non-remission at week 54, and adverse events leading to drug withdrawal. We used regression analyses to identify which factors were associated with treatment failure. FINDINGS: We enrolled 955 patients treated with infliximab (753 with originator; 202 with biosimilar) and 655 treated with adalimumab. Primary non-response occurred in 295 (23·8%, 95% CI 21·4-26·2) of 1241 patients who were assessable at week 14. Non-remission at week 54 occurred in 764 (63·1%, 60·3-65·8) of 1211 patients who were assessable, and adverse events curtailed treatment in 126 (7·8%, 6·6-9·2) of 1610 patients. In multivariable analysis, the only factor independently associated with primary non-response was low drug concentration at week 14 (infliximab: odds ratio 0·35 [95% CI 0·20-0·62], p=0·00038; adalimumab: 0·13 [0·06-0·28], p<0·0001); the optimal week 14 drug concentrations associated with remission at both week 14 and week 54 were 7 mg/L for infliximab and 12 mg/L for adalimumab. Continuing standard dosing regimens after primary non-response was rarely helpful; only 14 (12·4% [95% CI 6·9-19·9]) of 113 patients entered remission by week 54. Similarly, week 14 drug concentration was also independently associated with non-remission at week 54 (0·29 [0·16-0·52] for infliximab; 0·03 [0·01-0·12] for adalimumab; p<0·0001 for both). The proportion of patients who developed anti-drug antibodies (immunogenicity) was 62·8% (95% CI 59·0-66·3) for infliximab and 28·5% (24·0-32·7) for adalimumab. For both drugs, suboptimal week 14 drug concentrations predicted immunogenicity, and the development of anti-drug antibodies predicted subsequent low drug concentrations. Combination immunomodulator (thiopurine or methotrexate) therapy mitigated the risk of developing anti-drug antibodies (hazard ratio 0·39 [95% CI 0·32-0·46] for infliximab; 0·44 [0·31-0·64] for adalimumab; p<0·0001 for both). For infliximab, multivariable analysis of immunododulator use, and week 14 drug and anti-drug antibody concentrations showed an independent effect of immunomodulator use on week 54 non-remission (odds ratio 0·56 [95% CI 0·38-0·83], p=0·004). INTERPRETATION: Anti-TNF treatment failure is common and is predicted by low drug concentrations, mediated in part by immunogenicity. Clinical trials are required to investigate whether personalised induction regimens and treatment-to-target dose intensification improve outcomes. FUNDING: Guts UK, Crohn's and Colitis UK, Cure Crohn's Colitis, AbbVie, Merck Sharp and Dohme, Napp Pharmaceuticals, Pfizer, and Celltrion.

Association of Genetic Variants in NUDT15 With Thiopurine-Induced Myelosuppression in Patients With Inflammatory Bowel Disease.

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 × 109/L or less or a decline in absolute neutrophil cell count to 1.0 × 109/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 × 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 × 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 × 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.