Induction of the Hajdu-Cheney Syndrome Mutation in CD19 B Cells in Mice Alters B-Cell Allocation but Not Skeletal Homeostasis

Mice harboring Notch2mutations replicating Hajdu-Cheney syndrome (Notch2tm1.1ECan) have osteopenia and exhibit an increase in splenic marginal zone B cells with a decrease in follicular B cells. Whether the altered B-cell allocation is responsible for the osteopenia of Notch2tm1.1ECanmutants is unknown. To determine the effect of NOTCH2 activation in B cells on splenic B-cell allocation and skeletal phenotype, a conditional-by-inversion (COIN) Hajdu-Cheney syndrome allele of Notch2(Notch2[ΔPEST]COIN) was used. Cre recombination generates a permanent Notch2ΔPESTallele expressing a transcript for which sequences coding for the proline, glutamic acid, serine, and threonine–rich (PEST) domain are replaced by a stop codon. CD19-Credrivers were backcrossed into Notch2[ΔPEST]COIN/[ΔPEST]COINto generate CD19-specific Notch2ΔPEST/ΔPESTmutants and control Notch2[ΔPEST]COIN/[ΔPEST]COINlittermates. There was an increase in marginal zone B cells and a decrease in follicular B cells in the spleen of CD19Cre/WT;Notch2ΔPEST/ΔPESTmice, recapitulating the splenic phenotype of Notch2tm1.1ECanmice. The effect was reproduced when the NOTCH1 intracellular domain was induced in CD19-expressing cells (CD19Cre/WT;RosaNotch1/WTmice). However, neither CD19Cre/WT;Notch2ΔPEST/ΔPESTnor CD19Cre/WT;RosaNotch1/WTmice had a skeletal phenotype. Moreover, splenectomies in Notch2tm1.1ECanmice did not reverse their osteopenic phenotype. In conclusion, Notch2 activation in CD19-expressing cells determines B-cell allocation in the spleen but has no skeletal consequences.