Activation Induced Cytidine Deaminase-Associated DNA Repair Pathways Influence Germinal Center B Cell Lymphomagenesis

Somatic hypermutation and class switch recombination of immunoglobulin (Ig) genes occur in germinal center (GC) B cells and are initiated through deamination of cytidine to uracil by activation induced cytidine deaminase (AID). Resulting uracil-guanine (U-G) mismatches are processed by UNG-dependent base-excision repair (BER) and MSH2-dependent mismatch repair (MMR) pathways to yield mutations and DNA strand lesions. Although off-target AID activity also contributes to oncogenic point mutations and chromosome translocations associated with B cell lymphomas, the role of downstream AID-associated DNA repair pathways in lymphomagenesis is not defined. Through deregulated expression of BCL6, IµHABcl6 mice develop an AID-dependent GC-derived lymphoma that resembles human diffuse large B cell lymphoma (DLBCL). We have previously demonstrated that IµHABcl6 Ung^-/-Msh2^-/- mice have a similar incidence (35% vs 27%) but a 2.5-fold shorter median time to development of B220⁺ IgM⁺ PNA^hi CD138^- DLBCL compared with IµHABcl6 mice (6.5 months vs. 16.2 months; P = 0.0003). This suggests that AID-associated DNA repair pathways serve to protect the GC B cell and delay BCL6-driven lymphomagenesis. To investigate the individual contribution of BER and MMR in the pathogenesis of GC-derived lymphoma, we have now generated IµHABcl6 Ung^-/- and IµHABcl6 Msh2^-/- single-deficient mice. The majority of IµHABcl6 Ung^-/- mice remained healthy beyond 20 months with only 3 of 22 (13.6%) mice becoming sick starting at ∼16 months. Sick mice were found to have splenic lymphomas comprised of mature B220⁺ IgM⁺ PNA^low CD138^- B cells. Histological examination revealed expanded follicles with a population of small lymphocytes, consistent with a follicular B cell lymphoma which has been shown to arise in Ung^-/- mice. In contrast, 18 of 22 (81.8%) IµHABcl6 Msh2^-/- mice rapidly succumbed to malignancy starting at ∼3 months and had a median survival of 6 months. Of 15 tumors available for analysis, there was 1 histiocytic sarcoma, 1 squamous cell carcinoma, 4 T cell lymphomas, and 9 B220⁺ IgM^- PNA^- CD138^- pre-B cell lymphomas (determined by histology, immunophenotyping and gene expression profiling). None of the IµHABcl6 Ung^-/- or IµHABcl6 Msh2^-/- mice developed DLBCL. Since lack of UNG is strongly protective when MSH2 is present, we conclude that in the setting of deregulated BCL6, UNG promotes the development of DLBCL. In contrast, MSH2 is protective against the development of tumors in general and does not facilitate DLBCL in the absence of UNG. Combined with the observation that IµHABcl6 Ung^-/-Msh2^-/- mice develop DLBCL with a significantly shorter latency than IµHABcl6 mice, this data indicates that a complex interplay between AID-associated BER and MMR produces a net protective effect against lymphomagenesis. In the absence of UNG and MSH2, AID-generated U-G mismatches are not processed into strand lesions and are simply replicated, yielding C/G to T/A transition mutations. Thus, to assess how combined lack of UNG and MSH2 might promote the accelerated development of BCL6-driven lymphoma, we carried out spectral karyotyping and sequence analysis of AID target genes (IgJH4, cMyc, Pim1, RhoH, Cd79a, CD79b, H2afx, Pax5, and Cd83) in lymphomas from the different genotypes. IµHABcl6 DLBCLs (3/3) harbored various complex chromosome abnormalities, consistent with previous findings. Numerous clonal and sub-clonal chromosome abnormalities including translocations, duplications, deletions, and aneuploidies were also detected in IµHABcl6 Ung^-/-Msh2^-/- (4/4) and IµHABcl6 Ung^-/- (2/2) lymphomas. Pre-B cell tumors from IµHABcl6 Msh2^-/- mice could not be stimulated to produce metaphase chromosomes. Clonal and non-clonal mutations of the IgJH4 intronic region were identified in lymphomas from IµHABcl6 (2/3), IµHABcl6 Ung^-/-Msh2^-/- (4/4), and IµHABcl6 Ung^-/- (2/3) mice, consistent with ongoing AID activity. No mutations were detected in 3 pre-B cell lymphomas, consistent with their pre-GC origin. Six clonal mutations within AID hotspots (all C/G to T/A) were identified in Pim1, RhoH, and Pax5 in 2 of 4 IµHABcl6 Ung^-/-Msh2^-/- DLBCLs. None of the other genotypes carried any clonal mutations of non-Ig genes. Thus, chromosome abnormalities in GC B cell lymphomas can arise through mechanisms independent of BER and MMR but may be due to off-target effects of AID on other genes that regulate cell cycle, apoptosis, or genomic stability.