IntroductionA fundamental hallmark of humoral immunity is the ability to produce class-switched antibodies that have enhanced affinity for antigen. This process, termed affinity maturation, allows clonally selected B cells to refine their response to theoretically target any antigen with high specificity. After activation, B cells mutate the immunoglobulin (Ig) locus in a process known as somatic hypermutation (SHM). 1 Mutated clones that have acquired increased affinity for antigen preferentially expand over their low-affinity counterparts. The selection process occurs in the germinal center (GC), a transient microenvironment that forms in secondary lymphoid tissue shortly after antigen exposure. 2 The GC provides a competitive setting for B cells whereby ineffectual clones are actively cleared from the system via apoptosis, although the processes that govern this selection still remain vague.SHM and class switch recombination (CSR) are initiated by the enzyme activation-induced cytidine deaminase (AID), 3,4 which deaminates cytidines to uridines specifically at the Ig locus. 5-10 AID-generated uridines are then engaged by various DNA repair pathways that either lead to the generation of point mutations in the antibody variable region or recombinogenic events that lead to CSR. AID Ϫ/Ϫ mice lack mutations at their Ig locus and are incapable of producing class-switched antibodies. 4 Interestingly, these mice were previously shown to harbor an abnormally high proportion of splenic GC B cells. 11 This phenotype is also recapitulated in humans with AID deficiencies. 3 Although a link between reduced gut immunity (resulting from an inability to produce mucosal IgA) and peripheral GC formation was hypothesized to account for these abnormalities, this remains to be conclusively proven, and the possibility of a B cell-intrinsic effect that could explain the profound expansion of GC B cells has not been examined.GC B cells represent a unique lymphoid compartment of actively proliferating cells where numerous apoptotic factors must synergize to induce the elimination of nonproductive clones. Factors contributing to the intrinsic pathway of apoptosis, such as Bcl-2, Bcl-x L , and Bim, 12-17 and the extrinsic pathway, such as Fas, [18][19][20][21][22] have been implicated in GC selection. The unique physiology of these cells makes them highly susceptible to disease progression, often serving as etiologic sites for autoimmune and malignant B cells. 13,[23][24][25][26] Recent evidence has implicated AID as a fundamental contributor to the genetic aberrations that lead to these disease phenotypes. 24,[27][28][29] Given the importance of apoptosis as a parameter for both GC B-cell selection and lymphomagenesis, we investigated the relationship between AID-induced DNA mutation and cell death to understand how this enzyme may impinge on survival and death within the GC niche. Here we report that many of the potentially harmful AID-induced genetic alterations may indeed lead to the death of these cells within the GC. Methods Mice...

Antibody diversification necessitates targeted mutation of regions within the immunoglobulin locus by activation-induced cytidine deaminase (AID). While AID is known to act on single-stranded DNA (ssDNA), the source, structure, and distribution of these substrates in vivo remain unclear. Using the technique of in situ bisulfite treatment, we characterized these substrates—which we found to be unique to actively transcribed genes—as short ssDNA regions, that are equally distributed on both DNA strands. We found that the frequencies of these ssDNA patches act as accurate predictors of AID activity at reporter genes in hypermutating and class switching B cells as well as in Escherichia coli. Importantly, these ssDNA patches rely on transcription, and we report that transcription-induced negative supercoiling enhances both ssDNA tract formation and AID mutagenesis. In addition, RNaseH1 expression does not impact the formation of these ssDNA tracts indicating that these structures are distinct from R-loops. These data emphasize the notion that these transcription-generated ssDNA tracts are one of many in vivo substrates for AID.

An effective immune response requires B cells to produce several classes of antibodies through the process of class switch recombination (CSR). Activation-induced cytidine deaminase initiates CSR by deaminating deoxycytidines at switch regions within the Ig locus. This activity leads to double-stranded DNA break formation at the donor and recipient switch regions that are subsequently synapsed and ligated in a 53BP1-dependent process that remains poorly understood. The DNA damage response E3 ubiquitin ligases RNF8 and RNF168 were recently shown to facilitate recruitment of 53BP1 to sites of DNA damage. Here we show that the ubiquitination pathway mediated by RNF8 and RNF168 plays an integral part in CSR. Using the CH12F3-2 mouse B cell line that undergoes CSR to IgA at high rates, we demonstrate that knockdown of RNF8, RNF168, and 53BP1 leads to a significant decrease in CSR. We also show that 53BP1-deficient CH12F3-2 cells are protected from apoptosis mediated by the MDM2 inhibitor Nutlin-3. In contrast, deficiency in either E3 ubiquitin ligase does not protect cells from Nutlin-3-mediated apoptosis, indicating that RNF8 and RNF168 do not regulate all functions of 53BP1.53BP1 | activation-induced cytidine deaminase | DNA damage response P art of an effective immune response requires the production of antibodies of different classes, each of which mediates a different effector function. This process is initiated by activationinduced cytidine deaminase (AID), which induces class switch recombination (CSR) by deaminating deoxycytidines within Ig switch regions (1, 2). The recognition and subsequent processing of the mutated residues by base excision repair and/or mismatch repair machineries generates double-stranded DNA breaks (DSBs) at switch regions (3). In an attempt to mend the ensuing breaks, B cells mount a damage response similar to that signaled by irradiated cells (1). Ultimately, AID-induced DSBs are repaired predominantly by nonhomologous end-joining (4, 5) and to a lesser extent by an alternative end-joining pathway (6).Generally, DSBs are readily recognized by sensor-kinase protein complexes, including the Mre11-Rad50-Nbs1 (MRN)/ataxiatelangiectasia mutated (ATM), Ku70/Ku80-DNA-PKcs, and ataxiatelangiectasia and Rad3-related (ATR)-ATR-interacting protein complexes (7). Upon binding to a DSB site, these factors are activated to trigger a cascade of events that culminates in cell cycle delay and/or DNA repair (7). During this process, sequential chromatin modifications around the break sites appear to be crucial for adequate resolution of the DNA breaks. Active chromatin modification is initiated by ATM-dependent phosphorylation of the histone variant H2AX to the γH2AX form (8-10). γ-H2AX then preferentially binds the tandem-BRCA1 C-terminal domain (BRCT) motifs of mediator of DNA damage checkpoint 1 (MDC1) (11, 12), which in turn recruits more MRN/ATM, thereby amplifying the γ-H2AX signal (11, 12). Recently, it was shown that ATM also phosphorylates MDC1 at a TQxF consensus, allowing for the recruitmen...

DNA breaks caused by recombination-activating gene 1 (RAG1) and activation-induced cytidine deaminase (AID) induce c-myc/immunoglobulin (Ig) heavy chain chromosomal translocations and thereby stimulate lymphomagenesis. However, constitutive expression of c-myc alone is not sufficient to induce lymphomas. Because RAG1 and AID activity occurs outside of Ig genes, we assessed whether these enzymes provide the secondary genetic lesions in El c-myc transgenic mice to promote lymphoma development. We found that the tumor incidence and tumor phenotype in El c-myc transgenic mice is similar in AID þ / þ , AID þ /À and AID À/À backgrounds in both specific pathogen-free and conventional animal facilities, indicating that AID does not contribute to lymphoma development in El c-myc transgenic mice. To examine the role of RAG proteins in El c-myc mice, we examined El c-myc transgenic mice that harbor the Ig-HEL heavy-and light-chain transgenes, and thus have reduced RAG expression in B cells. We found that tumor incidence was not affected by these Ig transgenes. However, we found that RAG1 À/À El c-myc mice exhibited accelerated tumor development compared to controls. This data combined with our finding that El c-myc mice lived longer in the conventional facility than in the specific pathogen-free facility suggest an immunemediated activity that suppresses lymphoma development.

Hepatocellular carcinoma (HCC) presentation is heterogeneous necessitating a variety of therapeutic interventions with varying efficacies and associated prognoses. Poor prognostic patients often undergo non-curative palliative interventions including transarterial chemoembolization (TACE), sorafenib, chemotherapy, or purely supportive care. The decision to pursue one of many palliative interventions for HCC is complex and an economic evaluation comparing these interventions has not been done. This study evaluates the cost-effectiveness of non-curative palliative treatment strategies such as TACE alone or TACE+sorafenib, sorafenib alone, and non-sorafenib chemotherapy compared with no treatment or best supportive care (BSC) among patients diagnosed with HCC between 2007 and 2010 in a Canadian setting. Using person-level data, we estimated effectiveness in life years and quality-adjusted life years (QALYs) along with total health care costs (2013 US dollars) from the health care payer’s perspective (3% annual discount). A net benefit regression approach accounting for baseline covariates with propensity score adjustment was used to calculate incremental net benefit to generate incremental cost-effectiveness ratio (ICER) and uncertainty measures. Among 1,172 identified patients diagnosed with HCC, 4.5%, 7.9%, and 5.6%, received TACE alone or TACE+sorafenib, sorafenib, and non-sorafenib chemotherapy clone, respectively. Compared with no treatment or BSC (81.9%), ICER estimates for TACE alone or TACE+sorafenib was $6,665/QALY (additional QALY: 0.47, additional cost: $3,120; 95% CI: -$18,800-$34,500/QALY). The cost-effectiveness acceptability curve demonstrated that if the relevant threshold was $50,000/QALY, TACE alone or TACE+sorafenib, non-sorafenib chemotherapy, and sorafenib alone, would have a cost-effectiveness probability of 99.7%, 46.6%, and 5.5%, respectively. Covariates associated with the incremental net benefit of treatments are age, sex, comorbidity, and cancer stage. Findings suggest that TACE with or without sorafenib is currently the most cost-effective active non-curative palliative treatment approach to HCC. Further research into new combination treatment strategies that afford the best tumor response is needed.