Interplay between UNG and AID governs intratumoral heterogeneity in mature B cell lymphoma

Abstract

Most B cell lymphomas originate from B cells that have germinal center (GC) experience and bear chromosome translocations and numerous point mutations. GC B cells remodel their immunoglobulin (Ig) genes by somatic hypermutation (SHM) and class switch recombination (CSR) in their immunoglobulin (Ig) genes. Activation Induced Deaminase (AID) initiates CSR and SHM by generating U:G mismatches on Ig DNA that can then be processed by Uracyl-N-glycosylase (UNG). AID promotes collateral damage in the form of chromosome translocations and off-target SHM, however, the exact contribution of AID activity to lymphoma generation and progression is not completely understood. Here we show using a conditional knock-in strategy that AID supra-activity alone is not sufficient to generate B cell transformation. In contrast, in the absence of UNG, AID supra-expression increases SHM and promotes lymphoma. Whole exome sequencing revealed that AID heavily contributes to lymphoma SHM, promoting subclonal variability and a wider range of oncogenic variants. Thus, our data provide direct evidence that UNG is a brake to AID-induced intratumoral heterogeneity and evolution of B cell lymphoma. Author summary Activation Induced Deaminase (AID) is a critical enzyme of the immune response because it gives rise to higher affinity antibodies as well as to various antibody isotypes (IgG, IgA, IgE). AID introduces deaminations (a pre-mutagenic alteration in the DNA) on the antibody genes, which are then processed by uracil-N-glycosylase (UNG). Occasionally, AID can target other genes, resulting in potentially oncogenic mutations or chromosome translocations. Here we have explored the contribution of AID and UNG to lymphoma generation by combining gain- and loss-of-function genetic mouse models. We find that increasing the activity of AID does not, by itself, increase the incidence of lymphomas in mice. In contrast, mice lacking UNG activity are more susceptible to suffer lymphoma when AID is expressed at higher levels. By sequencing the exomes of lymphomas from these mice, we have learned that AID can contribute to lymphoma development in a translocation-independent fashion, by increasing the variability of oncogenic mutations present in the tumors. Thus, UNG acts as a brake to AID-induced intratumoral heterogeneity and evolution.