NOTCH2 haploinsufficiency results in a reduced proportion of circulating MZB cells. (A) Scheme of the NOTCH2 protein with its different domains. The extracellular domain of NOTCH2 contains 36 epidermal growth factor (EGF) repeats involved in ligand binding and three Lin/Notch repeats (LNRs). The intracellular portion of the protein includes seven ankyrin repeats (ANK) required for binding and activating the RBPJ transcription factor, flanked by nuclear localization signals (NLSs). Other domains include: a transmembrane domain (TMD), an RBP-J–associated module (RAM), and a proline/glutamic acid/serine/threonine rich domain (PEST) involved in proteosomal degradation. The mutations identified in the four Alagille patients studied are depicted; two affect splicing sites at intron borders (P2 and P3) and two are a missense mutation in RAM domain (P1) or NLS (P4). (B) Blood B cells from four patients heterozygous for NOTCH2 mutations and age-matched controls (healthy donors, HDs) were analyzed by flow cytometry after IgD, CD27, and CD19 staining. The proportion of IgD⁺CD27⁺ (MZB) and switched IgD⁻CD27⁺ (as a percentage of total B cells) for each of them is indicated on dot plots. (C) Relative frequencies of blood IgD⁺CD27⁺ cells (expressed as percentages of CD19⁺ B cells) of Alagille patients (three children and one adult, same symbols as for age-matched control group) and healthy donors (HDs) that were divided into two age groups as indicated. (D) Ratios of IgD⁺CD27⁺ versus switched cells were plotted for each patient and for a control group (with ages varying from 6 to 40 yr), with mean values ± SEM indicated by error bars. **, P < 0.01; *, P < 0.05; Mann-Whitney statistical test.