Regulation of the DNA Repair Complex during Somatic Hypermutation and Class-Switch Recombination

Imatinib mesylate reduces c-MYC expression in double-hit lymphoma cells by suppressing inducible cytidine deaminase

BACKGROUND

Double-hit lymphoma (DHL) with c-MYC gene translocation is highly aggressive and has a poor prognosis. In DHL cells, activation-induced cytidine deaminase (AID) promotes antibody class switch recombination (CSR), ultimately leading to c-MYC gene translocation caused by Myc/IgH DNA double-strand breaks. However, currently there is still no method to suppress the expression of AID.

METHODS

In this study, we compared the clinical significance of AID expression in DHL, Additionally, two human double-hit lymphoma cell lines were used to analyze the effect of imatinib mesylate on c-MYC in vitro, and the therapeutic effect was also evaluated in xenograft mouse models.

RESULTS

Imatinib mesylate downregulated the AID and c-MYC proteins in patients with chronic myelogenous leukemia associated with DHL. In addition, imatinib mesylate reduced AID and c-MYC expression in SU-DHL-4 and OCI-Ly18 DHL cells. Imatinib mesylate exerted significant inhibitory effects on the proliferation and metastasis of SU-DHL-4 and OCI-Ly18 cells. Finally, imatinib mesylate reduced not only tumor burden in DHL mouse models, but also AID and c-MYC expression in vivo.

CONCLUSION

These findings reveal that imatinib mesylate effectively reduces the carcinogenic function of c-MYC in DHL, providing novel strategies for developing therapies targeting c-MYC-driven DHL.

Replication Protein A Enhances Kinetics of Uracil DNA Glycosylase on ssDNA and Across DNA Junctions: Explored with a DNA Repair Complex Produced with SpyCatcher/SpyTag Ligation

DNA repair proteins participate in extensive protein-protein interactions that promote the formation of DNA repair complexes. To understand how complex formation affects protein function during base excision repair, we used SpyCatcher/SpyTag ligation to produce a covalent complex between human uracil DNA glycosylase (UNG2) and replication protein A (RPA). Our covalent "RPA-Spy-UNG2" complex could identify and excise uracil bases in duplex areas next to ssDNA-dsDNA junctions slightly faster than the wild-type proteins, but this was highly dependent on DNA structure, as the turnover of the RPA-Spy-UNG2 complex slowed at DNA junctions where RPA tightly engaged long ssDNA sections. Conversely, the enzymes preferred uracil sites in ssDNA where RPA strongly enhanced uracil excision by UNG2 regardless of ssDNA length. Finally, RPA was found to promote UNG2 excision of two uracil sites positioned across a ssDNA-dsDNA junction, and dissociation of UNG2 from RPA enhanced this process. Our approach of ligating together RPA and UNG2 to reveal how complex formation affects enzyme function could be applied to examine other assemblies of DNA repair proteins.

Crlz-1 Controls Germinal Center Reaction by Relaying a Wnt Signal to the Bcl-6 Expression in Centroblasts during Humoral Immune Responses

Crlz-1 was expressed along with Wnt3a in the rapidly proliferating centroblasts within the dark zone of germinal center (GC) during humoral immune responses. Significantly, Crlz-1 relayed a Wnt/β-catenin signal to the expression of Bcl-6, the master regulator of centroblasts, by mobilizing the cytoplasmic CBFβ into the nucleus to allow Runx/CBFβ heterodimerization and its subsequent binding to the Bcl-6 promoter. The knockdown of Crlz-1 or β-catenin, as well as inhibition of Wnt signaling in the centroblasts, led to the decreased expression of Bcl-6 and, thereby, the altered expression of its various target genes, resulting in their diminished proliferation. Consistently, the administration of Wnt inhibitors into the immunized mice impaired or abolished GC reaction, with concomitant decreases of Crlz-1 and Bcl-6 expression and, thus, centroblastic proliferation. Our observation that Wnt/β-catenin signaling via Crlz-1 regulates GC reaction would suggest developmental strategies for vaccine adjuvants and cancer therapeutics because both immune efficacy and accidental lymphoma depend on GC reaction. Our studies of Crlz-1 were performed using human cell lines, mice, and their primary cells.

ETS1 and PAX5 transcription factors recruit AID to Igh DNA

B lymphocytes optimize antibody responses by class switch recombination (CSR), which changes the expressed constant region exon of the immunoglobulin heavy chain (IgH), and by somatic hypermutation (SH) that introduces point mutations in the variable regions of the antibody genes. Activation-induced cytidine deaminase (AID) is the key mutagenic enzyme that initiates both these antibody diversification processes by deaminating cytosine to uracil. Here we asked the question if transcription factors can mediate the specific targeting of the antibody diversification by recruiting AID. We have recently reported that AID is together with the transcription factors E2A, PAX5 and IRF4 in a complex on key sequences of the Igh locus. Here we report that also ETS1 is together with AID in this complex on key sequences of the Igh locus in splenic B cells of mice. Furthermore, we show that both ETS1 and PAX5 can directly recruit AID to DNA sequences from the Igh locus with the specific binding site for the transcription factor. Taken together, our findings support the notion of a targeting mechanism for the selective diversification of antibody genes with limited genome wide mutagenesis by recruitment of AID by PAX5 and ETS1 in a transcription factor complex.