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

Mice harboring Notch2 mutations replicating Hajdu-Cheney syndrome (Notch2 ^tm1.1ECan ) 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 Notch2 ^tm1.1ECan mutants 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 Notch2 ^ΔPEST allele 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-Cre drivers were backcrossed into Notch2 ^{[ΔPEST]COIN/[ΔPEST]COIN} to generate CD19-specific Notch2 ^{ΔPEST/ΔPEST} mutants and control Notch2 ^{[ΔPEST]COIN/[ΔPEST]COIN} littermates. There was an increase in marginal zone B cells and a decrease in follicular B cells in the spleen of CD19 ^Cre/WT ;Notch2 ^{ΔPEST/ΔPEST} mice, recapitulating the splenic phenotype of Notch2 ^tm1.1ECan mice. The effect was reproduced when the NOTCH1 intracellular domain was induced in CD19-expressing cells (CD19 ^Cre/WT ;Rosa ^Notch1/WT mice). However, neither CD19 ^Cre/WT ;Notch2 ^{ΔPEST/ΔPEST} nor CD19 ^Cre/WT ;Rosa ^Notch1/WT mice had a skeletal phenotype. Moreover, splenectomies in Notch2 ^tm1.1ECan mice 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.
(Copyright © 2018 American Society for Investigative Pathology. Published by Elsevier Inc. All rights reserved.)