Stepwise generation of AID knock-in and conditional knockout mice from a single gene-targeting event.

Activation-induced cytidine deaminase (AID) encoded by the Aicda gene initiates class-switch recombination and somatic hypermutation of immunoglobulin genes. In addition to this function, AID is also implicated in the epigenetic regulation in pluripotent stem cells and in the oncogenesis of lymphoid and non-lymphoid origins. To examine AID's role in specific cell types, we developed mouse strains of conditional knockout (Aicda-FL) and knock-in with a red fluorescent protein gene (RFP) inserted into the Aicda locus (Aicda-RFP). These two strains were obtained from a single targeting event in embryonic stem cells by a three-loxP or tri-lox strategy. Partial and complete recombination among the three loxP sites in the Aicda-RFP locus gave rise to Aicda-FL and AID-deficient loci (Aicda-KO), respectively, after mating Aicda-RFP mice with Cre-expressing mice driven by tissue-non-specific alkaline phosphate promoter. We confirmed RFP expression in B cells of germinal centers of intestine-associated lymphoid tissue. Mice homozygous for each allele were obtained and were checked for AID activity by class-switch and hypermutation assays. AID activity was normal for Aicda-FL but partially and completely absent for Aicda-RFP and Aicda-KO, respectively. Aicda-FL and Aicda-RFP mice would be useful for studying AID function in subpopulations of B cells and in non-lymphoid cells.