Abstract Background Non-affected co-twins of twins with Inflammatory Bowel Disease (IBD) have an increased risk of developing Crohn’s disease (CD) or ulcerative colitis (UC). Twins in particular share genetic and environmental factors, especially early in life when the immune system matures. The Dutch twin cohort was established in 2017 to study the complex interplay of genetic, immune and environmental factors in the pathogenesis of IBD as well as pre-clinical signatures of disease. The present report describes the set-up of the cohort and the clinical characteristics of the twins included so far. Methods Twin pairs with IBD were recruited through clinical referral, patient collaborative parties, and promotion online. At baseline, patients and their co-twin were asked to fill-out a questionnaire on zygosity, disease characteristics, environmental factors, medication use and quality of life. Medical records of IBD twins were scrutinized to confirm diagnosis and disease classification. Blood, saliva, urine, faeces and rectal biopsies were collected, every 6 months for up to 2 years, regardless of disease status. In October 2025, all participants were contacted for an update on current disease status. Results A total of 64 twin pairs (128 individuals) in which at least 1 member had IBD were recruited (34 monozygotic, 30 dizygotic). Thirty-nine pairs included at least one individual diagnosed with CD, 26 with UC, and 2 with IBD-unclassified. Within monozygotic pairs, 9 out of 34 were concordant for CD, 2 for UC, and 1 pair combined UC in one twin with IBD-unclassified in the other. Among dizygotic twins, concordance was observed in only one pair with CD, while no pairs were concordant for UC; and one pair was discordant with one twin diagnosed with IBD-unclassified. The median age at inclusion was 38.4 years (IQR 26.2), and the median age at diagnosis was 22.6 years (IQR 13.6). At baseline, all concordant CD twins shared the same Montreal age classification. Sixty percent had identical location and behaviour classifications, and 70% shared the presence or absence of perianal disease. In contrast, the two concordant UC pairs differed in their extent of disease. During follow-up, two previously unaffected twins were diagnosed with IBD: one developed CD 5.5 years after her twin sister’s diagnosis, and the other developed UC 18 years after the diagnosis of her sister, making both pairs concordant. Across all concordant pairs, the median time between diagnoses within a twin pair was 2 years, ranging from 0 to 18 years. Conclusion This cohort is thoroughly phenotyped and provides information about the etiology of IBD, pre-clinical biomarkers and supports the stronger genetic influence in Crohn’s disease than in ulcerative colitis. Conflict of interest: Ms. Dijkstra, Sarah: My research is supported by a grant from Takeda Pharmaceutical Company Limited. Brand, Eelco: Was co-applicant on an Investigator Initiated research grant of Pfizer. van Wijk, Femke: Has been a speaker and/or consultant for Janssen, Johnson & Johnson, and Takeda and has received grants from Regeneron Pharmaceuticals, Leo Pharma, Sanofi, BMS, Galapagos, and Takeda. Oldenburg, Bas: Unrestricted grants: Abbvie, Takeda, Pfizer, Galapagos Advisory boards: Abbvie, Takeda, Pfizer, Lilly, Galapagos, Janssen
Dijkstra et al. (Thu,) studied this question.