The role of activation-induced cytidine deaminase (AID) mediated DNA damage in the pathogenesis of Burkitt's Lymphoma

通过体细胞超突变(SHM)和类开关重组(CSR)实现免疫球蛋白谱的二次多样化是产生高亲和力抗体反应的关键。在SHM过程中，点突变被引入抗体的可变区，而在CSR过程中的缺失重组反应有助于将特定的可变区与新的恒定区配对，从而产生不同的抗体类别。SHM和CSR发生在生发中心(GC)的B细胞，都是由激活诱导的胞苷脱氨酶(AID)介导的，AID是一种在GC中的B细胞激活时特异性表达的酶。AID酶的工作原理是使活跃转录基因中的胞嘧啶残基脱氨基，并已被证明推动免疫球蛋白(Ig)基因以及一系列非Ig基因的突变，包括c-myc等癌基因。辅助诱导的突变可以通过不同的修复途径进行处理，有时会产生DNA双链断裂(DSB)，它是CSR过程中的必备中间产物。这些DSB可以在局部修复，但也可以重组以产生染色体易位，这在白血病和淋巴瘤中是常见的发现。这种易位的发生与体外AID有关，其中之一是t(8；14)，它在80%的Burkitt淋巴瘤病例中被发现。Burkitt淋巴瘤的发生与慢性感染性刺激密切相关，特别是与EB病毒(EBV)和疟疾(疟疾)有关。感染。众所周知，这些慢性感染性刺激会导致慢性B细胞刺激和GCs中AID的慢性表达。潜在地，这种慢性的AID表达可能通过介导致病易位而导致淋巴肿大。然而，到目前为止，对体内AID表达引起的突变知之甚少。因此，在这个项目的过程中，我们想要研究AID介导的突变是否会在体内导致B细胞肿瘤的形成。为此，最近建立了使用查鲍迪疟原虫的慢性疟疾感染的小鼠模型。在p53基因缺失的情况下，这些小鼠会患上成熟的B细胞淋巴瘤。利用这个小鼠模型和新的深度测序方法，我们将描述慢性感染疟疾的小鼠的初级B细胞中AID介导的突变情况。通过将这一图景与在这些慢性感染的小鼠中发现的成熟淋巴瘤的突变图景进行比较，我们将确定AID对体内淋巴瘤形成的贡献。

Secondary diversification of the immunoglobulin repertoire through somatic hypermutation (SHM) and class switch recombination (CSR) is crucial for generating high affinity antibody responses. During SHM point mutations are introduced into the variable region of the antibody, while a deletional recombination reaction during CSR helps to pair a particular variable region to a new constant region thereby generating a different antibody class. SHM and CSR occur in germinal center (GC) B-cells and are both mediated by activation-induced cytidine deaminase (AID), an enzyme that is specifically expressed upon activation of B-cells in the GC. The enzyme AID works by deaminating cytosine residues in actively transcribed genes and has been shown to drive mutations in immunoglobulin (Ig) genes as well as in a spectrum of non-Ig genes, including oncogenes such as c-myc. AID-induced mutations can be processed by different repair pathways, sometimes yielding DNA double-strand breaks (DSBs) which are an obligate intermediate during the process of CSR. These DSBs can be repaired locally, but can also be recombined to yield chromosome translocations, which are common findings in leukemias and lymphomas.One such translocation whose occurrence has been linked to AID in vitro is the t(8;14) which is found in 80% of cases of Burkitt's lymphoma. The incidence of Burkitt's lymphoma is strongly associated with chronic infectious stimuli, in particular with Epstein-Barr-Virus (EBV) and malaria (Plasmodium spp.) infections. These chronic infectious stimuli are known to result in chronic B-cell stimulation and chronic expression of AID in GCs. Potentially, this chronic expression of AID might be responsible for lymphomagenesis by mediating disease-initiating translocations. However, little is so far known about the mutations caused by AID expression in vivo. In the course of this project, we, therefore, want to investigate whether AID mediated mutations drive tumor formation in B-cells in vivo. For this purpose, a mouse model of chronic malaria infection using Plasmodium chabaudi has recently been established. In the context of genetic loss of p53, these mice develop mature B-cell lymphomas. Using this mouse model and novel deep sequencing approaches, we are going to characterize the AID-mediated mutational landscape in primary B-cells of mice that are chronically infected with malaria. In comparing this landscape with the mutational landscape found in mature lymphomas arising in these chronically infected mice, we are going to determine the contribution of AID towards lymphoma formation in vivo.