Rapid exome sequencing as a first-tier test in neonates with suspected genetic disorder: results of a prospective multicenter clinical utility study in the Netherlands.

The introduction of rapid exome sequencing (rES) for critically ill neonates admitted to the neonatal intensive care unit has made it possible to impact clinical decision-making. Unbiased prospective studies to quantify the impact of rES over routine genetic testing are, however, scarce. We performed a clinical utility study to compare rES to conventional genetic diagnostic workup for critically ill neonates with suspected genetic disorders. In a multicenter prospective parallel cohort study involving five Dutch NICUs, we performed rES in parallel to routine genetic testing for 60 neonates with a suspected genetic disorder and monitored diagnostic yield and the time to diagnosis. To assess the economic impact of rES, healthcare resource use was collected for all neonates. rES detected more conclusive genetic diagnoses than routine genetic testing (20% vs. 10%, respectively), in a significantly shorter time to diagnosis (15 days (95% CI 10-20) vs. 59 days (95% CI 23-98, p < 0.001)). Moreover, rES reduced genetic diagnostic costs by 1.5% (€85 per neonate). CONCLUSION:  Our findings demonstrate the clinical utility of rES for critically ill neonates based on increased diagnostic yield, shorter time to diagnosis, and net healthcare savings. Our observations warrant the widespread implementation of rES as first-tier genetic test in critically ill neonates with disorders of suspected genetic origin. WHAT IS KNOWN: • Rapid exome sequencing (rES) enables diagnosing rare genetic disorders in a fast and reliable manner, but retrospective studies with neonates admitted to the neonatal intensive care unit (NICU) indicated that genetic disorders are likely underdiagnosed as rES is not routinely used. • Scenario modeling for implementation of rES for neonates with presumed genetic disorders indicated an expected increase in costs associated with genetic testing. WHAT IS NEW: • This unique prospective national clinical utility study of rES in a NICU setting shows that rES obtained more and faster diagnoses than conventional genetic tests. • Implementation of rES as replacement for all other genetic tests does not increase healthcare costs but in fact leads to a reduction in healthcare costs.

Recommendations for whole genome sequencing in diagnostics for rare diseases.

In 2016, guidelines for diagnostic Next Generation Sequencing (NGS) have been published by EuroGentest in order to assist laboratories in the implementation and accreditation of NGS in a diagnostic setting. These guidelines mainly focused on Whole Exome Sequencing (WES) and targeted (gene panels) sequencing detecting small germline variants (Single Nucleotide Variants (SNVs) and insertions/deletions (indels)). Since then, Whole Genome Sequencing (WGS) has been increasingly introduced in the diagnosis of rare diseases as WGS allows the simultaneous detection of SNVs, Structural Variants (SVs) and other types of variants such as repeat expansions. The use of WGS in diagnostics warrants the re-evaluation and update of previously published guidelines. This work was jointly initiated by EuroGentest and the Horizon2020 project Solve-RD. Statements from the 2016 guidelines have been reviewed in the context of WGS and updated where necessary. The aim of these recommendations is primarily to list the points to consider for clinical (laboratory) geneticists, bioinformaticians, and (non-)geneticists, to provide technical advice, aid clinical decision-making and the reporting of the results.

Stepwise ABC system for classification of any type of genetic variant.

The American College of Medical Genetics and Genomics and the Association for Molecular Pathology (ACMG-AMP) system for variant classification is score based with five classes: benign, likely benign, variant of unknown significance (VUS), likely pathogenic, and pathogenic. Here, we present a variant classification model that can be an add-on or alternative to ACMG classification: A stepwise system that can classify any type of genetic variant (e.g., hypomorphic alleles, imprinted alleles, copy number variants, runs of homozygosity, enhancer variants, and variants related to traits). We call it the ABC system because classification is first functional (A), then clinical (B), and optionally a standard comment that fits the clinical question is selected (C). Both steps A and B have 1-5 grading when knowledge is sufficient, if not, class "zero" is assigned. Functional grading (A) only concerns biological consequences with the stages normal function (1), likely normal function (2), hypothetical functional effect (3), likely functional effect (4), and proven functional effect (5). Clinical grading (B) is genotype-phenotype focused with the stages "right type of gene" (1), risk factor (2), and pathogenic (3-5, depending on penetrance). Both grades are listed for each variant and combined to generate a joint class ranging from A to F. Importantly, the A-F classes are linked to standard comments, reflecting laboratory or national policy. In step A, the VUS class is split into class 0 (true unknown) and class 3 (hypothetical functional effect based on molecular predictions or de novo occurrence), providing a rationale for variant-of-interest reporting when the clinical picture could fit the finding. The system gives clinicians a better guide to variant significance.

Natural Exon Skipping Sets the Stage for Exon Skipping as Therapy for Dystrophic Epidermolysis Bullosa.

Dystrophic epidermolysis bullosa (DEB) is a devastating blistering disease affecting skin and mucous membranes. It is caused by pathogenic variants in the COL7A1 gene encoding type VII collagen, and can be inherited dominantly or recessively. Recently, promising proof-of-principle has been shown for antisense oligonucleotide (AON)-mediated exon skipping as a therapeutic approach for DEB. However, the precise phenotypic effect to be anticipated from exon skipping, and which patient groups could benefit, is not yet clear. To answer these questions, we studied new clinical and molecular data on seven patients from the Dutch EB registry and reviewed the literature on COL7A1 exon skipping variants. We found that phenotypes associated with dominant exon skipping cannot be distinguished from phenotypes caused by other dominant DEB variants. Recessive exon skipping phenotypes are generally relatively mild in the spectrum of recessive DEB. Therefore, for dominant DEB, AON-mediated exon skipping is unlikely to ameliorate the phenotype. In contrast, the overall severity of phenotypes associated with recessive natural exon skipping pivots toward the milder end of the spectrum. Consequently, we anticipate AON-mediated exon skipping for recessive DEB caused by bi-allelic null variants should lead to a clinically relevant improvement of this devastating phenotype.

Regarding the rights and duties of Clinical Laboratory Geneticists in genetic healthcare systems; results of a survey in over 50 countries.

Specialists of human genetic diagnostics can be divided into four groups: Medical Geneticists (MDG), Genetic Nurses and/or Counsellors (GN/GC), Clinical Laboratory Geneticists (CLG) and Laboratory Genetics Technicians (LGT). While the first two groups are in direct patient contact, the work of the latter two, of equal importance for patient care, are often hidden as they work behind the scenes. Herein the first study on the rights and duties of CLGs is presented. We present the results of a survey performed in 35 European and 18 non-European countries with 100 participating specialists. A national CLG title is available in 60% of European countries, and in 77% of the surveyed European countries a CLG can be the main responsible head of the laboratory performing human genetic tests. However, in only 20% of European countries is a lab-report valid with only a CLGs' signature - even though the report is almost always formulated by the CLG, and an interpretation of the obtained results in a clinical context by the CLG is expected in nearly 90% of European countries. Interestingly, CLGs see patients in 30% of European countries, and are also regularly involved in student education. Overall, the CLG profession includes numerous duties, which are quite similar in all regions of the world. Strikingly, the CLG's rights and responsibilities of leading a lab, or signing a report are regulated differently according to country specific regulations. Overall, the CLG is a well-recognized profession worldwide and often working within a multidisciplinary team of human genetic diagnostics professionals.

Genotype-Guided Thiopurine Dosing Does not Lead to Additional Costs in Patients With Inflammatory Bowel Disease.

BACKGROUND AND AIMS: Decreased thiopurine S-methyltransferase [TPMT] enzyme activity increases the risk of haematological adverse drug reactions [ADRs] in patients treated with thiopurines. Clinical studies have shown that in patients with inflammatory bowel disease [IBD], pharmacogenetic TPMT-guided thiopurine treatment reduces this risk of ADRs. The aim of this study was to investigate whether this intervention impacts on healthcare costs and/or quality of life. METHODS: An a priori defined cost-effectiveness analysis was conducted in the Thiopurine response Optimization by Pharmacogenetic testing in Inflammatory bowel disease Clinics [TOPIC] trial, a randomized controlled trial performed in 30 Dutch hospitals. Patients diagnosed with IBD [age ≥18 years] were randomly assigned to the intervention [i.e. pre-treatment genotyping] or control group. Total costs in terms of volumes of care, and effects in quality-adjusted life years [QALYs], based on EuroQol-5D3L utility scores, were measured for 20 weeks. Mean incremental cost savings and QALYs with confidence intervals were calculated using non-parametric bootstrapping with 1000 replications. RESULTS: The intervention group consisted of 381 patients and the control group 347 patients. The mean incremental cost savings were €52 per patient [95% percentiles -682, 569]. Mean incremental QALYs were 0.001 [95% percentiles -0.009, 0.010]. Sensitivity analysis showed that the results were robust for potential change in costs of screening, costs of biologicals and costs associated with productivity loss. CONCLUSIONS: Genotype-guided thiopurine treatment in IBD patients reduced the risk of ADRs among patients carrying a TPMT variant, without increasing overall healthcare costs and resulting in comparable quality of life, as compared to standard treatment.

Patients' beliefs about medicine are associated with early thiopurine discontinuation in patients with inflammatory bowel diseases.

BACKGROUND: Patients' beliefs about medicine may either reflect the necessity for treatment or concerns regarding the treatment. We explored the extent to which these beliefs have an effect on thiopurine metabolite levels and premature discontinuation in patients with inflammatory bowel disease (IBD). PATIENTS AND METHODS: Patients enrolled in the 'Thiopurine response Optimization by Pharmacogenetic testing in Inflammatory Bowel Disease Clinics' (TOPIC) trial were asked to complete the Beliefs about Medicine Questionnaire (BMQ) 4 weeks after thiopurine initiation. The BMQ measures perceptions about treatment necessity and concerns. On the basis of the necessity and concern scores, patients can be categorized as accepting, ambivalent, indifferent, or skeptical. The thiopurine discontinuation rates for these belief subgroups were compared by Kaplan-Meier curves. Furthermore, clinical response and metabolite levels were compared between the belief subgroups. RESULTS: A total of 767 patients with IBD started thiopurine treatment, of whom 576 (75%) completed the BMQ. Patients could be classified as accepting (34%), indifferent (17%), ambivalent (34%), or skeptical (15%). Compared with patients in the accepting group (discontinuation rate 22%), patients with an indifferent (35%; P=0.02), ambivalent (37%; P<0.01), or skeptical belief (54%; P<0.01) had higher thiopurine discontinuation rates. No differences were observed in the steady-state thiopurine metabolite levels between the different belief subgroups. CONCLUSION: Patients with a low perceived treatment necessity or high concerns toward IBD treatment were more likely to discontinue thiopurine treatment prematurely. Extra attention toward these patients might prevent premature discontinuation.

Detection of clinically relevant copy-number variants by exome sequencing in a large cohort of genetic disorders.

PURPOSE: Copy-number variation is a common source of genomic variation and an important genetic cause of disease. Microarray-based analysis of copy-number variants (CNVs) has become a first-tier diagnostic test for patients with neurodevelopmental disorders, with a diagnostic yield of 10-20%. However, for most other genetic disorders, the role of CNVs is less clear and most diagnostic genetic studies are generally limited to the study of single-nucleotide variants (SNVs) and other small variants. With the introduction of exome and genome sequencing, it is now possible to detect both SNVs and CNVs using an exome- or genome-wide approach with a single test. METHODS: We performed exome-based read-depth CNV screening on data from 2,603 patients affected by a range of genetic disorders for which exome sequencing was performed in a diagnostic setting. RESULTS: In total, 123 clinically relevant CNVs ranging in size from 727 bp to 15.3 Mb were detected, which resulted in 51 conclusive diagnoses and an overall increase in diagnostic yield of ~2% (ranging from 0 to -5.8% per disorder). CONCLUSIONS: This study shows that CNVs play an important role in a broad range of genetic disorders and that detection via exome-based CNV profiling results in an increase in the diagnostic yield without additional testing, bringing us closer to single-test genomics.Genet Med advance online publication 27 October 2016.

More Dose-dependent Side Effects with Mercaptopurine over Azathioprine in IBD Treatment Due to Relatively Higher Dosing.

BACKGROUND: There are substantial global differences in the preference for mercaptopurine (MP) or its prodrug azathioprine (AZA) as first-choice thiopurine to treat inflammatory bowel diseases. Studies comparing both agents are scarce. Our aim was to compare AZA and MP in thiopurine-naive patients with inflammatory bowel disease for the frequency of side effects and efficacy. METHODS: Post hoc analysis of the "Thiopurine response Optimization by Pharmacogenetic testing in Inflammatory bowel disease Clinics" (TOPIC) trial, in which thiopurine-naive patients with inflammatory bowel disease with an indication for a thiopurine were randomized for a genotype-based dose versus standard of care. For this study, Cox proportional hazard ratios (HRs) were calculated to compare AZA and MP for discontinuation rates within 5 months, incidence of hepatotoxicity, leukopenia, and gastrointestinal side effects. Treatment efficacy was compared by logistic regression. RESULTS: Patient characteristics were similar for patients treated with AZA (n = 494, 64.4%) and MP (n = 273, 35.6%), yet patients with MP were relatively higher dosed compared with those on AZA. Discontinuation rates within 5 months were not different, 39.3% (AZA) and 38.1% (MP), HR 0.92 (95% confidence interval, 0.72-1.17; P = 0.50); however, patients on MP were more often subjected to dose reductions (30% versus 14%, P < 0.01). Higher rates of hepatotoxicity, HR 1.93 (95% confidence interval, 1.35-2.76; P < 0.01) and leukopenia, HR 2.55 (95% confidence interval, 1.51-4.30; P < 0.01) were observed with MP, which annulled in a secondary analysis with adjustment for the higher dose and metabolite levels. CONCLUSIONS: Patients treated with MP were relatively higher dosed, which resulted in more dose-dependent side effects and a higher rate of dose reductions.

Critical points for an accurate human genome analysis.

Next-generation sequencing is radically changing how DNA diagnostic laboratories operate. What started as a single-gene profession is now developing into gene panel sequencing and whole-exome and whole-genome sequencing (WES/WGS) analyses. With further advances in sequencing technology and concomitant price reductions, WGS will soon become the standard and be routinely offered. Here, we focus on the critical steps involved in performing WGS, with a particular emphasis on points where WGS differs from WES, the important variables that should be taken into account, and the quality control measures that can be taken to monitor the process. The points discussed here, combined with recent publications on guidelines for reporting variants, will facilitate the routine implementation of WGS into a diagnostic setting.

European registration process for Clinical Laboratory Geneticists in genetic healthcare.

Tremendous progress in genetics and genomics led to a wide range of healthcare providers, genetic tests, and more patients who can benefit from these developments. To guarantee and improve the quality of genetic testing, a unified European-based registration for individuals qualified in biomedicine was realized. Therefore a Europe-wide recognition of the profession 'European registered Clinical Laboratory Geneticist (ErCLG)' based on a syllabus of core competences was established which allows for harmonization in professional education. The 'European Board of Medical Genetics division - Clinical Laboratory Geneticist' provides now since 3 years the possibility to register as an ErCLG. Applicants may be from all European countries and since this year also from outside of Europe. Five subtitles reflect the exact specialty of each ErCLG, who can reregister every 5 years. A previously not possible statistics based on ~300 individuals from 19 countries as holders of an ErCLG title provides interesting insights into the professionals working in human genetics. It could be substantiated that there are around twice as many females than males and that a PhD title was achieved by 80% of registered ErCLGs. Also most ErCLGs are still trained as generalists (66%), followed by such ErCLGs with focus on molecular genetics (23%); the remaining are concentrated either on clinical (6%), tumor (4%) or biochemical genetics (1%). In conclusion, besides MDs and genetic counselors/nurses an EU-wide recognition system for Clinical Laboratory Geneticist has been established, which strengthens the status of specialists working in human genetic diagnostics in Europe and worldwide.

Diagnostic exome sequencing in 266 Dutch patients with visual impairment.

Inherited eye disorders have a large clinical and genetic heterogeneity, which makes genetic diagnosis cumbersome. An exome-sequencing approach was developed in which data analysis was divided into two steps: the vision gene panel and exome analysis. In the vision gene panel analysis, variants in genes known to cause inherited eye disorders were assessed for pathogenicity. If no causative variants were detected and when the patient consented, the entire exome data was analyzed. A total of 266 Dutch patients with different types of inherited eye disorders, including inherited retinal dystrophies, cataract, developmental eye disorders and optic atrophy, were investigated. In the vision gene panel analysis (likely), causative variants were detected in 49% and in the exome analysis in an additional 2% of the patients. The highest detection rate of (likely) causative variants was in patients with inherited retinal dystrophies, for instance a yield of 63% in patients with retinitis pigmentosa. In patients with developmental eye defects, cataract and optic atrophy, the detection rate was 50, 33 and 17%, respectively. An exome-sequencing approach enables a genetic diagnosis in patients with different types of inherited eye disorders using one test. The exome approach has the same detection rate as targeted panel sequencing tests, but offers a number of advantages. For instance, the vision gene panel can be frequently and easily updated with additional (novel) eye disorder genes. Determination of the genetic diagnosis improved the clinical diagnosis, regarding the assessment of the inheritance pattern as well as future disease perspective.

The diagnostic yield of whole-exome sequencing targeting a gene panel for hearing impairment in The Netherlands.

Hearing impairment (HI) is genetically heterogeneous which hampers genetic counseling and molecular diagnosis. Testing of several single HI-related genes is laborious and expensive. In this study, we evaluate the diagnostic utility of whole-exome sequencing (WES) targeting a panel of HI-related genes. Two hundred index patients, mostly of Dutch origin, with presumed hereditary HI underwent WES followed by targeted analysis of an HI gene panel of 120 genes. We found causative variants underlying the HI in 67 of 200 patients (33.5%). Eight of these patients have a large homozygous deletion involving STRC, OTOA or USH2A, which could only be identified by copy number variation detection. Variants of uncertain significance were found in 10 patients (5.0%). In the remaining 123 cases, no potentially causative variants were detected (61.5%). In our patient cohort, causative variants in GJB2, USH2A, MYO15A and STRC, and in MYO6 were the leading causes for autosomal recessive and dominant HI, respectively. Segregation analysis and functional analyses of variants of uncertain significance will probably further increase the diagnostic yield of WES.

Early Assessment of Thiopurine Metabolites Identifies Patients at Risk of Thiopurine-induced Leukopenia in Inflammatory Bowel Disease.

BACKGROUND AND AIMS: Only a quarter of thiopurine-induced myelotoxicity in inflammatory bowel disease [IBD] patients is related to thiopurine S-methyltransferase deficiency. We determined the predictive value of 6-thioguanine nucleotide [6-TGN] and 6-methylmercaptopurine ribonucleotide [6-MMPR] concentrations 1 week after initiation [T1] for development of leukopenia during the first 8 weeks of thiopurine treatment. METHODS: The study was performed in IBD patients starting thiopurine therapy as part of the Dutch randomized controlled TOPIC trial [ClinicalTrials.gov NCT00521950]. Blood samples for metabolite measurement were collected at T1. Leukopenia was defined by leukocyte counts of <3.0 × 109/L. For comparison, patients without leukopenia who completed the 8 weeks on the stable dose were selected from the first 272 patients of the TOPIC trial. RESULTS: Thirty-two patients with, and 162 patients without leukopenia were analysed. T1 threshold 6-TGN concentrations of 213 pmol/8 × 108 erythrocytes and 3525 pmol/8 × 108 erythrocytes for 6-MMPR were defined: patients exceeding these values were at increased leukopenia risk (odds ratio [OR] 6.2 [95% CI: 2.8-13.8] and 5.9 [95% CI: 2.7-13.3], respectively). Leukopenia rates were higher in patients treated with mercaptopurine, compared with azathioprine (OR 7.3 [95% CI: 3.1-17.0]), and concurrent anti-TNF therapy (OR 5.1 [95% CI: 1.6-16.4]). Logistic regression analysis of thiopurine type, threshold concentrations, and concurrent anti-tumour necrosis factor [TNF] therapy revealed that elevations of both T1 6-TGN and 6-MMPR resulted in the highest risk for leukopenia, followed by exceeding only the T1 6-MMPR or 6-TGN threshold concentration (area under the curve 0.84 [95% CI: 0.76-0.92]). CONCLUSIONS: In ~80% of patients, leukopenia could be explained by T1 6-TGN and/or 6-MMPR elevations. Validation of the predictive model is needed before implementing in clinical practice.

Clinical exome sequencing for cerebellar ataxia and spastic paraplegia uncovers novel gene-disease associations and unanticipated rare disorders.

Cerebellar ataxia (CA) and hereditary spastic paraplegia (HSP) are two of the most prevalent motor disorders with extensive locus and allelic heterogeneity. We implemented clinical exome sequencing, followed by filtering data for a 'movement disorders' gene panel, as a generic test to increase variant detection in 76 patients with these disorders. Segregation analysis or phenotypic re-evaluation was utilized to substantiate findings. Disease-causing variants were identified in 9 of 28 CA patients, and 8 of 48 HSP patients. In addition, possibly disease-causing variants were identified in 1 and 8 of the remaining CA and HSP patients, respectively. In 10 patients with CA, the total disease-causing or possibly disease-causing variants were detected in 8 different genes, whereas 16 HSP patients had such variants in 12 different genes. In the majority of cases, the identified variants were compatible with the patient phenotype. Interestingly, in some patients variants were identified in genes hitherto related to other movement disorders, such as TH variants in two siblings with HSP. In addition, rare disorders were uncovered, for example, a second case of HSP caused by a VCP variant. For some patients, exome sequencing results had implications for treatment, exemplified by the favorable L-DOPA treatment in a patient with HSP due to ATP13A2 variants (Parkinson type 9). Thus, clinical exome sequencing in this cohort of CA and HSP patients suggests broadening of disease spectra, revealed novel gene-disease associations, and uncovered unanticipated rare disorders. In addition, clinical exome sequencing results have shown their value in guiding practical patient management.

Guidelines for diagnostic next-generation sequencing.

We present, on behalf of EuroGentest and the European Society of Human Genetics, guidelines for the evaluation and validation of next-generation sequencing (NGS) applications for the diagnosis of genetic disorders. The work was performed by a group of laboratory geneticists and bioinformaticians, and discussed with clinical geneticists, industry and patients' representatives, and other stakeholders in the field of human genetics. The statements that were written during the elaboration of the guidelines are presented here. The background document and full guidelines are available as supplementary material. They include many examples to assist the laboratories in the implementation of NGS and accreditation of this service. The work and ideas presented by others in guidelines that have emerged elsewhere in the course of the past few years were also considered and are acknowledged in the full text. Interestingly, a few new insights that have not been cited before have emerged during the preparation of the guidelines. The most important new feature is the presentation of a 'rating system' for NGS-based diagnostic tests. The guidelines and statements have been applauded by the genetic diagnostic community, and thus seem to be valuable for the harmonization and quality assurance of NGS diagnostics in Europe.

Identification of Patients With Variants in TPMT and Dose Reduction Reduces Hematologic Events During Thiopurine Treatment of Inflammatory Bowel Disease.

BACKGROUND & AIMS: More than 20% of patients with inflammatory bowel disease (IBD) discontinue thiopurine therapy because of severe adverse drug reactions (ADRs); leukopenia is one of the most serious ADRs. Variants in the gene encoding thiopurine S-methyltransferase (TPMT) alter its enzymatic activity, resulting in higher levels of thiopurine metabolites, which can cause leukopenia. We performed a prospective study to determine whether genotype analysis of TPMT before thiopurine treatment, and dose selection based on the results, affects the outcomes of patients with IBD. METHODS: In a study performed at 30 Dutch hospitals, patients were assigned randomly to groups that received standard treatment (control) or pretreatment screening (intervention) for 3 common variants of TPMT (TPMT*2, TPMT*3A, and TPMT*3C). Patients in the intervention group found to be heterozygous carriers of a variant received 50% of the standard dose of thiopurine (azathioprine or 6-mercaptopurine), and patients homozygous for a variant received 0%-10% of the standard dose. We compared, in an intention-to-treat analysis, outcomes of the intervention (n = 405) and control groups (n = 378) after 20 weeks of treatment. Primary outcomes were the occurrence of hematologic ADRs (leukocyte count < 3.0*10(9)/L or reduced platelet count < 100*10(9)/L) and disease activity (based on the Harvey-Bradshaw Index for Crohn's disease [n = 356] or the partial Mayo score for ulcerative colitis [n = 253]). RESULTS: Similar proportions of patients in the intervention and control groups developed a hematologic ADR (7.4% vs 7.9%; relative risk, 0.93; 95% confidence interval, 0.57-1.52) in the 20 weeks of follow-up evaluation; the groups also had similar mean levels of disease activity (P = .18 for Crohn's disease and P = .14 for ulcerative colitis). However, a significantly smaller proportion of carriers of the TPMT variants in the intervention group (2.6%) developed hematologic ADRs compared with patients in the control group (22.9%) (relative risk, 0.11; 95% confidence interval, 0.01-0.85). CONCLUSIONS: Screening for variants in TPMT did not reduce the proportions of patients with hematologic ADRs during thiopurine treatment for IBD. However, there was a 10-fold reduction in hematologic ADRs among variant carriers who were identified and received a dose reduction, compared with variant carriers who did not, without differences in treatment efficacy. ClinicalTrials.gov number: NCT00521950.