Is Crohn's Disease the Price to Pay Today for Having Survived the Black Death?

BACKGROUND AND AIMS: Nucleotide Oligomerisation Domain 2 [NOD2] is a key gene of innate immunity which participates in the host defence against pathogens. Several loss-of-function NOD2 mutations are associated with Crohn's disease [CD]. Their high frequencies in populations of European ancestry suggest a model of balancing selection. Because NOD2 deficiency has been associated with a resistance to Yersinia pseudotuberculosis in mice, we hypothesised that NOD2 mutations have been selected during past plague outbreaks due to the closely related bacterium Yersinia pestis.

METHODS: Contemporary frequencies of the main CD-associated NOD2 mutations [R702W, G908R, and 1007fs], measured in healthy people from European and Mediterranean countries, were collected from 60 studies via a PubMed search. Plague exposure was calculated from a dataset providing outbreaks from 1346 to 1860 in Europe and the Mediterranean Bassin. A plague index was built to capture the intensity of plague exposure in the studied geographical areas.

RESULTS: NOD2 mutation frequencies were associated with the past exposure to plague. Statistical significance was obtained for the most frequent mutation [R702W, p = 0.03] and for the pooled three mutations [p = 0.023]. The association remained significant when putative demographic biases were considered.

CONCLUSIONS: This result argues for a selection of CD-associated NOD2 mutations by plague outbreaks and further questioned the role of exposure to enteropathogenic Yersinia species in CD.