B cell defects observed in Nod2 knockout mice are a consequence of a Dock2 mutation frequently found in inbred strains

Phenotypic differences among substrains of laboratory mice due to spontaneous mutations or pre-existing genetic variation confound the interpretation of targeted mutagenesis experiments, and contribute to challenges with reproducibility across institutions. Notably, C57BL/6NHsd mice and gene-targeted mice that have been backcrossed to this substrain have been reported to harbor a duplication in exons 28 and 29 of Dock2. Here, we demonstrate the presence of this Dock2 variant in the widely used Nod2-/- mice. NOD2 is a cytosolic innate immune receptor that has been the subject of intense investigation due to its association with inflammatory bowel disease (IBD) susceptibility. Consistent with a role of NOD2 in an immunological disorder, Nod2-/- mice bred at our institution displayed multiple B cell defects including deficiencies in recirculating B cells, marginal zone B cells and B1a cells. However, we found that these effects are due to the Dock2 variant and are independent of Nod2 deletion. Despite originating from the same gene-targeted founder mice, Nod2-/- mice from another source did not harbor the Dock2 variant or B cell defects. Finally, we show that Dock2-/- mice display the same B cell defects as mice harboring the Dock2 variant, confirming that the variant is a loss-of-function mutation and is sufficient to explain the alterations to the B cell compartment observed in Nod2-/- mice. Our findings highlight the effects of confounding mutations from widely-used inbred strains on gene-targeted mice and reveal new functions of DOCK2 in B cells.