Evolution of the AID/APOBEC family of cytidine deaminases in lampreys

Abstract Jawless vertebrates, lampreys and hagfish, have antigen receptors termed variable lymphocyte receptors (VLRs), which are composed of leucine-rich repeat (LRR) segments. Three types of VLR genes, VLRA, VLRB and VLRC, are somatically assembled in lymphocytes by a gene conversion-like process. In this process, flanking LRR gene cassettes are randomly and sequentially incorporated in a piece-wise fashion into an incomplete germline VLR gene to generate mature VLR genes. Activation-induced cytidine deaminase (AID), a member of AID/APOBEC family of cytidine deaminases (CDAs), is a well-known contributor to antigen receptor diversification in jawed vertebrates. We have only limited information about the molecular basis for the assembly and diversification of VLR genes in jawless vertebrates. Two AID/APOBEC-type CDAs, CDA1 and CDA2, have been reported in lampreys. We have previously shown that CDA1 is expressed in VLRA+ and VLRC+ lymphocytes, whereas the CDA2 is expressed in VLRB+ lymphocytes. VLRA and VLRC gene assembly coincide with expression of CDA1 in the gill tip region displaying thymus-equivalent features. Although CDA1 has been shown to induce C to U mutations in bacterial and yeast DNA, mutational activity of CDA2 has not been reported. To find molecules for VLRB gene assembly, we have searched the lamprey genomic database and our transcriptome data from lamprey lymphocytes, and have identified four splice variants of CDA2. Gene expression analysis indicated that all CDA2 splice variants are expressed only in VLRB+ cells. We also found that some of the CDA2 splice variants have AID-like mutational activity in the in vitro system. These results suggest the function and regulation of CDA2 splice variants in VLRB+ cells.