Shared Environment and Colorectal Cancer: A Nordic Pedigree Registry-based Cohort Study

Rahma Elmahdi, Elna C. M. Wennerström, Mikael Andersson, Jan Wohlfahrt, Mads Melbye, Eero Pukkala, Maria Hortlund, Kaisa Silander, Kyösti Sutinen, Tine Jess*, Joakim Dillner

*Corresponding author for this work

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Risk of colorectal cancer (CRC) increases in relatives of patients with CRC. The extent to which this is attributable to genetic predisposition or shared environment is unclear. We explored this question using nationwide cohorts from Denmark, Finland and Sweden. From 1977 to 2013, we identified 359 879 individuals with a CRC diagnosis and 2 258 870 of their relatives who we followed for CRC incidence. We calculated standardized incidence ratios (SIR) and 95% confidence intervals (CI) for CRC in individuals with an affected relative. We used nationwide household and pedigree data along with national SIR estimates to calculate risk ratios (RR) for the contribution of shared household environment, childhood environment and genetic relationship to CRC risk in those with an affected relative. SIR of CRC was increased for individuals with an affected relative, across all countries and ages. For those with an affected parent, the SIR was 1.65 (95% CI: 1.61-1.69), 1.98 (95% CI: 1.87-2.09), for those with an affected sibling and 2.14 (95% CI: 1.84-2.49) for those with an affected halfsibling. In those <65 years old, shared childhood (RR: 1.41, 95% CI: 1.26-1.57) and household (RR: 2.08, 95% CI: 1.25-3.46) environments were significantly greater contributors to familial risk of CRC than genetics (RR: 0.88, 95% CI: 0.53-1.46). This large-scale Nordic population-based study of excess risk of CRC among relatives of those with CRC addresses the difficult disentangling of shared environment from genetic predisposition in the heritability of CRC. We found shared environment to be the most important contributor to CRC risk.

Original languageEnglish
JournalInternational Journal of Cancer
Issue number8
Pages (from-to)1261-1269
Number of pages9
Publication statusPublished - 15 Oct 2022

Bibliographical note

© 2022 The Authors. International Journal of Cancer published by John Wiley & Sons Ltd on behalf of UICC.


  • cohort study
  • colorectal cancer
  • pedigree registries


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