Disease-associated CTNNBL1 mutation impairs somatic hypermutation by decreasing nuclear AID.

Patients with common variable immunodeficiency associated with autoimmune cytopenias (CVID+AIC) generate few isotype-switched B cells with severely decreased frequencies of somatic hypermutations (SHM) but their underlying molecular defects remain poorly characterized. We identified a CVID+AIC patient who displays a rare homozygous missense M466V mutation in the beta catenin-like protein 1 (CTNNBL1). Since CTNNBL1 binds activation-induced cytidine deaminase (AID) that catalyzes SHM, we tested AID interactions with the CTNNBL1 M466V variant. We found that the M466V mutation interfered with the association of CTNNBL1 with AID, resulting in decreased AID in the nucleus of patient EBV-transformed B cell lines and of CTNNBL1 466V/V Ramos B cells engineered to only express M466V CTNNBL1 using CRISPR/Cas9 technology. As a consequence, the scarce IgG+ memory B cells from the CTNNBL1 466V/V patient showed a low SHM frequency that averaged 6.7 mutations compared to about 18 mutations per clone in healthy donor counterparts. In addition, CTNNBL1 466V/V Ramos B cells displayed a decreased incidence of SHM that was reduced by half compared to parental wild-type Ramos B cells, demonstrating that the CTNNBL1 M466V mutation is responsible for defective SHM induction. We conclude that CTNNBL1 plays an important role in regulating AID-dependent antibody diversification in humans.