Study population and sample collection

This study population consisted of 3619 participants from 2 observational studies for whom genetic data were available: Colorectal Cancer Genetics & Genomics (CRCGEN; n = 1801) and Colorectal Cancer Screening (COLSCREEN; n = 1818). CRCGEN combines data of two Spanish case-control studies. The first one, conducted in the University Hospital of Bellvitge, L’Hospitalet del Llobregat, (Barcelona), recruited incident pathology-confirmed CRC cases (n = 304) and age and sex frequency-matched hospital controls (n = 293) during the period 1996–1998. The control group was randomly selected among patients without previous CRC admitted to the same hospital during the same period. To avoid selection bias, the criterion of inclusion in the control group was a new diagnosis. The second study was conducted in parallel in the University Hospital of Bellvitge and in the Hospital of León, León, during 2007–2015 and recruited a total of 633 incident CRC cases (313 Bellvitge and 320 León) and 571 population controls (free of CRC, 164 Bellvitge and 407 León). The control group was recruited by inviting to participate subjects selected from the primary health care lists of the hospitals’ referral areas, frequency matched by age and sex. COLSCREEN is a cross-sectional screening cohort study designed to recruit participants from the ongoing population-based CRC screening program conducted by the Catalan Institute of Oncology, L’Hospitalet del Llobregat (Barcelona), from 2011 to 2020. The design of the CRC screening program, based on FOBT, has been published elsewhere [12, 13]. Exclusion criteria to participate at the biennial screening program were as follows: gastrointestinal symptoms; family history of hereditary or familial colorectal cancer, personal history of CRC, adenomas or inflammatory bowel disease; and colonoscopy in the previous 5 years or a FIT within the last 2 years, terminal disease, and severe disabling conditions. Most of the participants of the COLSCREEN study were invited to participate after a positive FIT result (n = 1242, 77%; ≥ 20 μg Hb/g feces), but we also invited to undergo a colonoscopy to a sample of 362 subjects with a negative FIT result (< 20 μg Hb/g feces). To increase CRC sample size, we further included in the COLSCREEN study 70 newly diagnosed CRC identified by the hospital CRC Functional Unit. These clinically diagnosed CRC cases, though labeled under COLSCREEN because they were recruited simultaneously to that study, for analyses were combined with others of CRCGEN and were excluded when the analyses were restricted to participants in screening.

Colonoscopy and/or CRC histological reports were examined and used to classify COLSCREEN participants into different categories following the proposal by Castells et al. for risk stratification of patients with CRC and/or serrated polyps [14]: low-risk lesions (LRL; n = 286), intermediate-risk lesions (IRL; n = 352), high-risk lesions (HRL; n = 226), and CRC screening program cases (n = 70). Controls (free of CRC) were classified into population-based controls and screening controls (normal colonoscopy or no-risk lesions).

All participants who agreed to take part of the CRCGEN and COLSCREEN studies provided written informed consent and donated a blood sample at recruitment. Each hospital’s ethics committees (University Hospital of Bellvitge and Hospital of León) approved the protocols of the studies (PR148/08, PR073/11, PR084/16).