Efficacy of Tocilizumab Monotherapy Versus Tocilizumab and Methotrexate Combination Therapy in the Prevention of Radiographic Progression in Rheumatoid Arthritis: An Analysis Using Individual Patient Data From Multiple Clinical Trials.

OBJECTIVE: To compare the effects of preventing radiographic progression (in its 3 components) of tocilizumab (TCZ) monotherapy with those of TCZ and methotrexate (MTX) in combination therapy (TCZ + MTX), and to evaluate possible effect modifiers in this model. METHODS: Randomized trials that compared TCZ monotherapy to TCZ + MTX combination therapy for differences in radiographic progression were analyzed on an individual patient data level using mixed-effects models, and data were collected from 820 subjects with either early rheumatoid arthritis (RA) or established RA. Outcomes were classified as the absence of radiographic progression after 2 years (i.e., preventing radiographic progression) as measured by total Sharp/van der Heijde score (SHS), erosion score, and joint space narrowing (JSN) score. Effect modification by baseline joint damage, disease duration, and Disease Activity Score in 28 joints (DAS28) was studied. RESULTS: Overall, TCZ + MTX combination therapy was more effective in preventing radiographic progression compared to TCZ monotherapy, which was measured by total SHS score. However, in patients with early RA who had more joint damage compared to those with less joint damage at baseline (relative risk [RR] 1.02 versus RR 0.91, respectively) or in patients with a lower DAS28 score compared to those with a higher DAS28 score (RR 1.04 versus RR 0.92, respectively) at baseline, this advantage disappeared. In patients with established RA, the advantage of TCZ + MTX versus TCZ alone in the prevention of radiographic progression disappeared with a longer disease duration at baseline (RR 1.04 versus 0.83). Results of erosion scores as an outcome were in line with these findings, though findings for JSN scores were less clear. CONCLUSION: Combination therapy with TCZ + MTX is more effective in preventing radiographic progression compared to TCZ monotherapy, but the effectiveness of TCZ monotherapy may approximate the effectiveness of TCZ + MTX in patients with early RA who have more joint damage and/or a lower DAS28 at baseline and in patients with established RA who have longer disease duration.

Effect on Costs and Quality-adjusted Life-years of Treat-to-target Treatment Strategies Initiating Methotrexate, or Tocilizumab, or Their Combination in Early Rheumatoid Arthritis.

OBJECTIVE: Our study aimed to evaluate the cost effectiveness of initiating tocilizumab (TCZ) ± methotrexate (MTX) versus initiating MTX as treat-to-target treatment strategies over 5 years in early disease-modifying antirheumatic drug (DMARD)-naïve rheumatoid arthritis (RA). METHODS: Data on resource use were collected with questionnaires at baseline, 3, 6, 12, and 24 months, and yearly thereafter, and were converted to costs using Dutch reference prices. Quality-adjusted life-years (QALY) were calculated using the EQ5D5L, with utility based on Dutch tariff or estimated by the Health Assessment Questionnaire. To account for missing cost data and QALY data and for sample uncertainty, first bootstraps (10,000 samples) were obtained. Second, single imputation using chained equations nested within these bootstrap samples was performed. An economic evaluation was performed for TCZ + MTX and TCZ, compared to MTX, as initial treatment in a treat-to-target strategy from a healthcare and societal perspective over 5 years. Several sensitivity analyses were performed. RESULTS: Mean differences in QALY were small and not significant (TCZ + MTX vs MTX: 0.06, 95% CI -0.02 to 0.13; TCZ vs. MTX: -0.03, 95% CI -0.05 to 0.11). Limited savings in indirect nonhealthcare costs and productivity loss costs (for TCZ only) were observed, but these did not compensate for the higher medication costs. Sensitivity analyses did not materially change these findings, although lower-priced TCZ, or reserving TCZ as initial therapy for prognostically unfavorable RA patients, improved cost effectiveness considerably but did not individually lead to a strategy being cost effective. CONCLUSION: Based on our analyses, early initiation of TCZ + MTX is not cost effective compared to MTX initiation in a step-up treat-to-target treatment strategy over 5 years in early RA patients.

Comprehensive exploratory autoantibody profiling in patients with early rheumatoid arthritis treated with methotrexate or tocilizumab.

BACKGROUND: We sought to identify immunoglobin G autoantibodies predictive of early treatment response to methotrexate, the recommended first-line therapy for patients with newly diagnosed rheumatoid arthritis, and to the interleukin-6 receptor inhibitor biologic tocilizumab, initiated as the first disease-modifying anti-rheumatic drug. MATERIALS AND METHODS: In baseline sera of a subset of patients with newly diagnosed rheumatoid arthritis in the U-Act-Early study, selected based on specific responder/non-responder criteria using the Disease Activity Score assessing 28 joints (DAS28) within the first 20 weeks, we measured immunoglobin G antibody reactivity against 463 protein antigens and performed supervised cluster analysis to identify predictive autoantibodies for treatment response. The analysis subset comprised 56 patients in the methotrexate arm (22 responders, 34 non-responders) and 50 patients in the tocilizumab arm (34 responders, 16 non-responders). For comparison, these analyses were also performed in 50 age- and gender-matched healthy controls. RESULTS: Increased reactivity in responders versus non-responders was found in the methotrexate arm against two antigens-DOT1-like histone lysine methyltransferase (p = 0.009) and tropomyosin (p = 0.003)-and in the tocilizumab arm against one antigen-neuro-oncological ventral antigen 2 (p = 0.039). Decreased reactivity was detected against two antigens in the methotrexate arm-G1 to S phase transition 2 (p = 0.023) and the zinc finger protein ZPR1 (p = 0.021). Reactivity against the identified antigens was not statistically significant in either treatment arm for patients with rheumatoid factor-positive versus-negative or anti-cyclic citrullinated test-positive versus test-negative rheumatoid arthritis (p ≥ 0.06). CONCLUSIONS: Comprehensive profiling of baseline sera revealed several novel immunoglobin G autoantibodies associated with early treatment response to methotrexate and to tocilizumab in disease-modifying anti-rheumatic drug-naive patients with rheumatoid arthritis. These findings could eventually yield clinically relevant predictive markers, if corroborated in different patient cohorts, and may facilitate future benefit in personalised healthcare.

Effectiveness and safety over 3 years after the 2-year U-Act-Early trial of the strategies initiating tocilizumab and/or methotrexate.

OBJECTIVES: U-Act-Early was a 2-year, randomized placebo controlled, double-blind trial, in which DMARD-naïve early RA patients were treated to the target of sustained remission (SR). Two strategies initiating tocilizumab (TCZ), with and without methotrexate (MTX), were more effective than a strategy initiating MTX. The aim of the current study was to determine longer-term effectiveness in daily clinical practice. METHODS: At the end of U-Act-Early, patients were included in a 3-year post-trial follow-up (PTFU), in which treatment was according to standard care and data were collected every 3 months during the first year and every 6 months thereafter. Primary end point was disease activity score assessing 28 joints (DAS28) over time. Mixed effects models were used to compare effectiveness between initial strategy groups, correcting for relevant confounders. Between the groups as randomized, proportions of patients were tested for DMARD use, SR and radiographic progression of joint damage. RESULTS: Of patients starting U-Act-Early, 226/317 (71%) participated in the PTFU. Over the total 5 years, mean DAS28 was similar between groups (P > 0.20). During U-Act-Early, biologic DMARD use decreased in both TCZ initiation groups and increased in the MTX initiation group, but during follow-up this trend did not continue. SR was achieved at least once in 99% of patients. Of the 226 patients, only 30% had any radiographic progression over 5 years, without significant differences between the groups. CONCLUSION: Although in the short-term the strategies initiating TCZ yielded the most clinical benefit, in the longer-term differences in important clinical outcomes between the strategies disappeared, probably due to continuation of the treat-to-target principle.

Baseline metabolic profiles of early rheumatoid arthritis patients achieving sustained drug-free remission after initiating treat-to-target tocilizumab, methotrexate, or the combination: insights from systems biology.

BACKGROUND: We previously identified, in newly diagnosed rheumatoid arthritis (RA) patients, networks of co-expressed genes and proteomic biomarkers associated with achieving sustained drug-free remission (sDFR) after treatment with tocilizumab- or methotrexate-based strategies. The aim of this study was to identify, within the same patients, metabolic pathways important for achieving sDFR and to subsequently study the complex interactions between different components of the biological system and how these interactions might affect the therapeutic response in early RA. METHODS: Serum samples were analyzed of 60 patients who participated in the U-Act-Early trial (ClinicalTrials.gov number NCT01034137) and initiated treatment with methotrexate, tocilizumab, or the combination and who were thereafter able to achieve sDFR (n = 37); as controls, patients were selected who never achieved a drug-free status (n = 23). Metabolomic measurements were performed using mass spectrometry on oxidative stress, amine, and oxylipin platforms covering various compounds. Partial least square discriminant analyses (PLSDA) were performed to identify, per strategy arm, relevant metabolites of which the biological pathways were studied. In addition, integrative analyses were performed correlating the previously identified transcripts and proteins with the relevant metabolites. RESULTS: In the tocilizumab plus methotrexate, tocilizumab, and methotrexate strategy, respectively, 19, 13, and 12 relevant metabolites were found, which were subsequently used for pathway analyses. The most significant pathway in the tocilizumab plus methotrexate strategy was "histidine metabolism" (p < 0.001); in the tocilizumab strategy it was "arachidonic acid metabolism" (p = 0.018); and in the methotrexate strategy it was "arginine and proline metabolism" (p = 0.022). These pathways have treatment-specific drug interactions with metabolites affecting either the signaling of interleukin-6, which is inhibited by tocilizumab, or affecting protein synthesis from amino acids, which is inhibited by methotrexate. CONCLUSION: In early RA patients treated-to-target with a tocilizumab- or methotrexate-based strategy, several metabolites were found to be associated with achieving sDFR. In line with our previous observations, by analyzing relevant transcripts and proteins within the same patients, the metabolic profiles were found to be different between the strategy arms. Our metabolic analysis further supports the hypothesis that achieving sDFR is not only dependent on predisposing biomarkers, but also on the specific treatment that has been initiated. TRIAL REGISTRATION: ClinicalTrials.gov, NCT01034137 . Registered on January 2010.

Inadequate response to treat-to-target methotrexate therapy in patients with new-onset rheumatoid arthritis: development and validation of clinical predictors.

OBJECTIVE: To identify and validate clinical baseline predictors associated with inadequate response (IR) to methotrexate (MTX) therapy in newly diagnosed patients with rheumatoid arthritis (RA). METHODS: In U-Act-Early, 108 disease-modifying antirheumatic drug (DMARD)-naive patients with RA were randomised to initiate MTX therapy and treated to target until sustained remission (disease activity score assessing 28 joints (DAS28) <2.6 with four or less swollen joints for ≥24 weeks) was achieved. If no remission, hydroxychloroquine was added to the treatment regimen (ie, 'MTX+') and replaced by tocilizumab if the target still was not reached thereafter. Regression analyses were performed to identify clinical predictors for IR, defined as needing addition of a biological DMARD, to 'MTX+'. Data from the treatment in the Rotterdam Early Arthritis Cohort were used for external validation of the prediction model. RESULTS: Within 1 year, 56/108 (52%) patients in U-Act-Early showed IR to 'MTX+'. DAS28 (adjusted OR (ORadj) 2.1, 95% CI 1.4 to 3.2), current smoking (ORadj 3.02, 95% CI 1.1 to 8.0) and alcohol consumption (ORadj 0.4, 95% CI 0.1 to 0.9) were identified as baseline predictors. The area under the receiver operator characteristic curve (AUROC) of the prediction model was 0.75 (95% CI 0.66 to 0.84); the positive (PPV) and negative predictive value (NPV) were 65% and 80%, respectively. When applying the model to the validation cohort, the AUROC slightly decreased to 0.67 (95% CI 0.55 to 0.79) and the PPV and NPV to 54% and 80%, respectively. CONCLUSION: Higher DAS28, current smoking and no alcohol consumption are predictive factors for IR to step-up 'MTX+' in DMARD-naive patients with new-onset RA. TRIAL REGISTRATION: NCT01034137; Post-results, ISRCTN26791028; Post-results.

Explorative analyses of protein biomarkers in patients with early rheumatoid arthritis achieving sustained drug-free remission after treatment with tocilizumab- or methotrexate-based strategies: from transcriptomics to proteomics.

OBJECTIVES: Previously, we identified networks of co-expressed genes related to achieving sustained drug-free remission (sDFR). The aim of the present exploratory analysis was to identify inflammatory proteins associated with achieving sDFR and their enriched biological pathways, and compare these pathways with those found in the previous transcriptomic analyses. METHODS: Serum samples were used from 60 patients who participated in the U-Act-Early trial and were treated-to-target with tocilizumab plus methotrexate, or tocilizumab or methotrexate; 37 achieved sDFR (≥3 months drug-free) and 23 did not (controls). Luminex® multi-analyte profiling (xMAP)® was used to measure 85 proteins. Partial least square discriminant analyses (PLSDA) identified proteins associated with achieving sDFR within each strategy arm, which were thereafter used for pathway analyses. RESULTS: PLSDA identified 9, 14 and 13 relevant proteins in the tocilizumab plus methotrexate, tocilizumab and methotrexate arm, respectively and pathway analyses thereafter identified respectively 49, 88 and 117 significantly enriched gene ontology (GO) terms. When comparing these terms with those previously found in the transcriptomic analyses, corresponding pathways were related in the tocilizumab arm to activity of leukocytes; in the methotrexate arm to response of stimuli and regulation of the Janus kinase signal transducer and activator of transcription (JAK-STAT) pathway. In the tocilizumab plus methotrexate arm, no corresponding enriched pathways were found. CONCLUSIONS: Multiple proteins were associated with achieving sDFR and several biological pathways corresponded, mainly in the methotrexate arm, with our previous transcriptomic findings potentially providing further insights into gene expression and protein translation in newly diagnosed RA patients.

Radiographic joint damage in early rheumatoid arthritis patients: comparing tocilizumab- and methotrexate-based treat-to-target strategies.

OBJECTIVE: To evaluate the progression of erosions and joint space narrowing (JSN) in feet and hands in the U-Act-Early trial. METHODS: In this trial, 317 newly diagnosed DMARD-naïve RA patients initiated randomly tocilizumab, or step-up MTX or a combination of the two. Radiographs were scored at baseline and after 52 and 104 weeks using the Sharp-van der Heijde erosion and JSN score. Between the strategy arms, changes from baseline and the proportions of patients without radiographic progression (change from baseline ≤0) were compared. RESULTS: Mean changes from baseline in erosion and JSN scores for the whole study population were after 52 weeks 0.59 and 0.18 and after 104 weeks 0.70 and 0.50, respectively. For JSN, at both time points no differences in progression were found between strategies (P ⩾ 0.09). For erosions, the progression was significantly lower at week 104 in both tocilizumab arms when compared with the MTX arm ((p≤0.023). Less progression of erosions in the feet was found after 104 weeks in both tocilizumab arms (P ⩽ 0.046); this was not significant for the hands (P ⩾ 0.11). The proportion of patients without progression in erosions was higher in the tocilizumab arms at week 52 (tocilizumab plus MTX: 87%, P = 0.038; tocilizumab: 81%, P = 0.29) and 104 (tocilizumab plus MTX: 85%, P = 0.001; tocilizumab: 77%, P = 0.028), compared with the MTX arm (74 and 60%, respectively). CONCLUSION: In DMARD-naïve early RA patients, initiating a tocilizumab-based treat-to-target strategy inhibits the progression of erosions, especially in the feet, more compared with initiation of a step-up MTX strategy. TRIAL REGISTRATION: ClinicalTrials.gov, https://clinicaltrials.gov, NCT01034137.

Patient-reported outcomes in newly diagnosed early rheumatoid arthritis patients treated to target with a tocilizumab- or methotrexate-based strategy.

Objective: To evaluate the effect of initiation of tocilizumab, with or without MTX, compared with MTX alone on patient-reported outcomes (PROs), in DMARD-naïve patients with early RA. Methods: In U-Act-Early, patients initiated treat-to-target step-up MTX, tocilizumab or tocilizumab plus MTX therapy. PROs assessed included the Functional Assessment of Chronic Illness Therapy-Fatigue, 36-item Short Form (SF-36), five dimensional EuroQol (EQ-5D) and the Revised Illness Perception Questionnaire. Differences between strategy groups over time and proportions of patients exceeding minimum clinically important differences (MCID) were evaluated. Results: During the 2-year study period, significant improvements were found in the tocilizumab strategies in the SF-36 physical component score (tocilizumab, P = 0.012; tocilizumab plus MTX, P = 0.044) and EQ-5D score (tocilizumab plus MTX, P = 0.020) when compared with the MTX strategy. No significant differences were noted in other PROs (P ⩾ 0.052, except for the domain 'identity' in the Illness Perception Questionnaire; tocilizumab vs MTX, P = 0.048). The proportions of patients achieving MCID in SF-36 physical component score were significantly higher at 12 and 52 weeks (P ⩽ 0.049) in the tocilizumab arms when compared with the MTX arm. At week 24, the proportion achieving MCID in EQ-5D was significantly higher in the tocilizumab plus MTX arm vs the MTX arm (P = 0.045). Conclusion: Initiation of treat-to-target tocilizumab therapy resulted in significantly improved PROs, especially within the first 24 weeks, when compared with initiation of MTX therapy. Also on the patients' level, initiating tocilizumab may be considered as a valuable strategy in DMARD-naïve patients with early RA. Trial registration: ClinicalTrials.gov, http://clinicaltrials.gov, NCT01034137.

Identification of differential co-expressed gene networks in early rheumatoid arthritis achieving sustained drug-free remission after treatment with a tocilizumab-based or methotrexate-based strategy.

BACKGROUND: Methotrexate is endorsed to be used as first-line treatment in rheumatoid arthritis (RA). However, a large proportion of patients need additional treatment with a biological disease-modifying anti-rheumatic drug (DMARD) to adequately suppress their disease activity. A better understanding of genotypes could help to distinguish between patients with different pathogenic mechanisms. The aim of this study was therefore to identify networks of genes within DMARD-naive early RA patients associated with achieving sustained drug-free remission (sDFR) after initiating tocilizumab plus methotrexate, tocilizumab, or methotrexate therapy. METHODS: Samples were used from 60 patients from the U-Act-Early study who received tocilizumab plus methotrexate, tocilizumab, or methotrexate therapy, and who achieved sDFR (≥3 months in drug-free remission until the end of the study, n = 37) after therapy was tapered and subsequently stopped, or who were not able to discontinue the therapy as controls (n = 23). Whole blood samples were collected and ribonucleic acid (RNA) was isolated from positive cluster of differentiation 4 (CD4+) and CD14+ cells and analysed using high-throughput sequencing. Weighted gene co-expression network analyses were performed to identify clusters (i.e. modules) of differently expressed genes associated with achieving sDFR and which were subsequently used for pathway analyses. RESULTS: Network analyses within CD4+ cells identified two significant modules in the tocilizumab plus methotrexate arm and four modules in the tocilizumab and methotrexate arms, respectively (p ≤ 0.039). Important pathways in the module best correlating with achieving sDFR were in the tocilizumab plus methotrexate arm related to processes involved with transcription and translation; in the tocilizumab arm, pathways were related to migration of white blood cells and G-protein coupled receptors, and in the methotrexate arm pathways were involved with the response to a bacterial or biotic (i.e. biological material)-related stimulus. No relevant networks could be identified in the sequenced CD14+ cells. CONCLUSIONS: Within networks of co-expressed genes, several pathways were found related to achieving sDFR after initiating therapy with tocilizumab, methotrexate, or the combination. Between the three strategy arms, we identified different networks of predisposing genes which indicates that specific gene expression profiles, depending on the treatment strategy chosen, are associated with a higher chance of achieving sDFR. TRIAL REGISTRATION: Clinicaltrials.gov, NCT01034137 . Registered on 16 December 2009.

Early rheumatoid arthritis treated with tocilizumab, methotrexate, or their combination (U-Act-Early): a multicentre, randomised, double-blind, double-dummy, strategy trial.

BACKGROUND: For patients with newly diagnosed rheumatoid arthritis, treatment aim is early, rapid, and sustained remission. We compared the efficacy and safety of strategies initiating the interleukin-6 receptor-blocking monoclonal antibody tocilizumab with or without methotrexate (a conventional synthetic disease-modifying antirheumatic drug [DMARD]), versus initiation of methotrexate monotherapy in line with international guidelines. METHODS: We did a 2-year, multicentre, randomised, double-blind, double-dummy, strategy study at 21 rheumatology outpatient departments in the Netherlands. We included patients who had been diagnosed with rheumatoid arthritis within 1 year before inclusion, were DMARD-naive, aged 18 years or older, met current rheumatoid arthritis classification criteria, and had a disease activity score assessing 28 joints (DAS28) of at least 2·6. We randomly assigned patients (1:1:1) to start tocilizumab plus methotrexate (the tocilizumab plus methotrexate arm), or tocilizumab plus placebo-methotrexate (the tocilizumab arm), or methotrexate plus placebo-tocilizumab (the methotrexate arm). Tocilizumab was given at 8 mg/kg intravenously every 4 weeks with a maximum of 800 mg per dose. Methotrexate was started at 10 mg per week orally and increased stepwise every 4 weeks by 5 mg to a maximum of 30 mg per week, until remission or dose-limiting toxicity. We did the randomisation using an interactive web response system. Masking was achieved with placebos that were similar in appearance to the active drug; the study physicians, pharmacists, monitors, and patients remained masked during the study, and all assessments were done by masked assessors. Patients not achieving remission on their initial regimen switched from placebo to active treatments; patients in the tocilizumab plus methotrexate arm switched to standard of care therapy (typically methotrexate combined with a tumour necrosis factor inhibitor). When sustained remission was achieved, methotrexate (and placebo-methotrexate) was tapered and stopped, then tocilizumab (and placebo-tocilizumab) was also tapered and stopped. The primary endpoint was the proportion of patients achieving sustained remission (defined as DAS28 <2·6 with a swollen joint count ≤four, persisting for at least 24 weeks) on the initial regimen and during the entire study duration, compared between groups with a two-sided Cochran-Mantel-Haenszel test. Analysis was based on an intention-to-treat method. This trial was registered at ClinicalTrials.gov, number NCT01034137. FINDINGS: Between Jan 13, 2010, and July 30, 2012, we recruited and assigned 317 eligible patients to treatment (106 to the tocilizumab plus methotrexate arm, 103 to the tocilizumab arm, and 108 to the methotrexate arm). The study was completed by a similar proportion of patients in the three groups (range 72-78%). The most frequent reasons for dropout were adverse events or intercurrent illness: 27 (34%) of dropouts, and insufficient response: 26 (33%) of dropouts. 91 (86%) of 106 patients in the tocilizumab plus methotrexate arm achieved sustained remission on the initial regimen, compared with 86 (84%) of 103 in the tocilizumab arm, and 48 (44%) of 108 in the methotrexate arm (relative risk [RR] 2·00, 95% CI 1·59-2·51 for tocilizumab plus methotrexate vs methotrexate, and 1·86, 1·48-2·32 for tocilizumab vs methotrexate, p<0·0001 for both comparisons). For the entire study, 91 (86%) of 106 patients in the tocilizumab plus methotrexate arm, 91 (88%) of 103 in the tocilizumab arm, and 83 (77%) of 108 in the methotrexate arm achieved sustained remission (RR 1·13, 95% CI 1·00-1·29, p=0·06 for tocilizumab plus methotrexate vs methotrexate, 1·14, 1·01-1·29, p=0·0356 for tocilizumab vs methotrexate, and p=0·59 for tocilizumab plus methotrexate vs tocilizumab). Nasopharyngitis was the most common adverse event in all three treatment groups, occurring in 38 (36%) of 106 patients in the tocilizumab plus methotrexate arm, 40 (39%) of 103 in the tocilizumab arm, and 37 (34%) of 108 in the methotrexate arm. The occurrence of serious adverse events did not differ between the treatment groups (17 [16%] of 106 patients in the tocilizumab plus methotrexate arm vs 19 [18%] of 103 in the tocilizumab arm and 13 [12%] of 108 in the methotrexate arm), and no deaths occurred during the study. INTERPRETATION: For patients with newly diagnosed rheumatoid arthritis, strategies aimed at sustained remission by immediate initiation of tocilizumab with or without methotrexate are more effective, and with a similar safety profile, compared with initiation of methotrexate in line with current standards. FUNDING: Roche Nederland BV.

A major quantitative trait locus on mouse chromosome 3 is involved in disease susceptibility in different colitis models.

BACKGROUND & AIMS: Mice with a disrupted gene for the G-protein alpha inhibitory 2 chain ( Gnai2 -/- ) develop a spontaneous colitis resembling human inflammatory bowel disease. Disease expression differs markedly between inbred strains of mice, indicating genetic control of disease susceptibility. We performed a genome-wide screen to localize the chromosomal regions regulating disease expression. METHODS: A total of 284 F2 mice derived from resistant C57BL/6J Gnai2 -/- mice and susceptible C3H/HeN Gnai2 -/- mice were analyzed in a genome-wide screen for colitis susceptibility and severity. RESULTS: A highly significant locus on chromosome 3 (Gpdc1) contributed to colitis susceptibility and severity (likelihood ratio statistics [LRS] = 32.4; LOD score = 7; P < 1.0 x 10(-5)). The peak linkage of this locus at 62 cM colocalizes exactly with a previously identified locus controlling colitis susceptibility in interleukin-10-deficient mice. In addition, evidence for linkage with a locus on chromosome 1 (Gpdc2 ; LRS = 19.7; LOD = 4.3) was found, and the 2 loci interacted epistatically (combined LRS = 68.2). A third locus (Gpdc3) was found on chromosome 9 and this locus interacted epistatically with a locus on chromosome 7, which by itself did not have an effect on the trait. CONCLUSIONS: The identification of a major locus on chromosome 3 that controls susceptibility to spontaneous colitis in 2 different gene-knockout models indicates that this locus harbors a gene(s) that plays a key role in maintaining mucosal homeostasis. Identification of this gene(s) may contribute to further understanding of the mechanisms underlying human inflammatory bowel disease.