Research

My research focus has been on the DNA repair pathways used by B cells during CSR and SHM and their role in the balance between the error-prone repair necessary for antibody diversification and cell survival. My papers collectively describe the mechanism by which DNA breaks and mutations are made in DNA during CSR and SHM, the details of which are key to understanding how translocations occur and lead to B cell lymphoma. These pathways include DNA Mismatch Repair (MMR), non-homologous end-joining (NHEJ), and Base Excision Repair (BER). My current focus is on AP endonucleases (APE), the BER enzymes that make single-strand breaks in DNA, and how their differential expression, which changes during the course of B cell activation, promotes CSR and SHM. SHM occurs exclusively in germinal center (GC) B cells, and has not been detectable in activated, cultured B cells. We previously published that rapidly dividing GC B cells express low levels of the DNA repair protein APE1 and high levels of its homolog APE2, a less efficient endonuclease. We have now analyzed the effect of this differential expression on SHM and class switch recombination (CSR) in primary mouse B-cell cultures. We find that although APE1 promotes CSR, it suppresses SHM. APE1 expression decreases with cell division during B cell activation and by coupling reduced APE1 with APE2 over-expression, we were able to achieve AID-dependent somatic hypermutation of immunoglobulin variable region genes in primary mouse B cell cultures. Unique protein domains of APE2 that promote SHM and CSR were identified, giving important clues to protein-protein interaction and mechanism. However, APE2 does not increase mutations unless APE1 is reduced, suggesting down-regulation of APE1 in the GC is required for SHM. Genome-wide expression data comparing GC B cells to activated cultured B cells at key time points has identified transcription factors that may be involved in the regulation of APE1 and APE2 expression during B cell activation, which remains to be studied. A new model was developed depicting APE1 and APE2 expression changes during B cell activation and how these proteins affect the balance between accurate and error-prone repair during CSR and SHM.