Differential Programming of B Cells in AID Deficient Mice

The Aicda locus encodes the activation induced cytidine deaminase (AID) and is highly expressed in germinal center (GC) B cells to initiate somatic hypermutation (SHM) and class switch recombination (CSR) of immunoglobulin (Ig) genes. Besides these Ig specific activities in B cells, AID has been implicated in active DNA demethylation in non-B cell systems. We here determined a potential role of AID as an epigenetic eraser and transcriptional regulator in B cells. RNA-Seq on different B cell subsets revealed that Aicda−/− B cells are developmentally affected. However as shown by RNA-Seq, MethylCap-Seq, and SNP analysis these transcriptome alterations may not relate to AID, but alternatively to a CBA mouse strain derived region around the targeted Aicda locus. These unexpected confounding parameters provide alternative, AID-independent interpretations on genotype-phenotype correlations previously reported in numerous studies on AID using the Aicda−/− mouse strain.Aicda基因座（Aicda locus）编码活化诱导胞苷脱氨酶（activation induced cytidine deaminase, AID），在生发中心（germinal center, GC）B细胞中呈高表达，用于启动免疫球蛋白（immunoglobulin, Ig）基因的体细胞超突变（somatic hypermutation, SHM）与类别转换重组（class switch recombination, CSR）。除上述在B细胞中针对免疫球蛋白的特异性功能外，AID还被证实参与非B细胞系统的活性DNA去甲基化过程。本研究旨在明确AID在B细胞中作为表观遗传擦除因子与转录调控因子的潜在作用。对不同B细胞亚群开展RNA测序（RNA-Seq）分析显示，Aicda基因敲除型（Aicda−/−）B细胞存在发育异常。然而通过RNA测序、甲基化捕获测序（MethylCap-Seq）及单核苷酸多态性（single nucleotide polymorphism, SNP）分析结果表明，此类转录组改变并非由AID缺失导致，而是与靶向敲除的Aicda基因座附近的CBA小鼠品系（CBA mouse strain）来源区域相关。这些意外发现的混杂因素，为既往诸多使用Aicda−/−小鼠品系开展的AID相关研究中所报道的基因型-表型关联，提供了非AID依赖的替代性解释。