PhD, Johns Hopkins University, 1971
Mechanism and Regulation of Antibody Class Switch Recombination
B cells respond to immunization or infection by diversifying the antibodies they produce through two processes: antibody class switch recombination and somatic hypermutation of their antigen-binding regions. Switch recombination causes a change in antibody class expression from IgM to IgG, IgA or IgE, resulting in an increased ability of the antibody to remove the pathogen. Somatic hypermutation, in conjunction with B cell selection, results in increases in the antigen binding ability of the antibody.
Both class switch recombination and somatic hypermutation are initiated by the enzyme activation induced cytidine deaminase (AID), acting on the antibody genes. After AID acts, two different DNA repair pathways (base excision repair and mismatch repair) begin to attempt to repair the resulting dU bases. However, instead of accurately repairing the lesions, these repair processes result in the introduction of DNA breaks and mutations that lead to class switch recombination and mutation of the variable region genes. The figure below diagrams the process of class switch recombination.
The AID enzyme is highly induced in activated B cells, including germinal center B cells, and only occasionally found in other cell types, and sometimes in tumor cells. Although essential for a proper immune response, it is also dangerous as it occasionally targets non-Ig genes. We have performed a genomics search to attempt to identify genes targeted by AID, as evidenced by the presence of DNA double-strand breaks. We have identified reproducible AID-dependent DNA breaks that occur at several genomic sites. We are investigating whether these sites might be important for generation of B cell lymphomas, as it is known that translocations between S regions and oncogenes occur in activated B cells, and that this can lead to B cell lymphoma. We are currently investigating several aspects of these findings, for example, how AID is normally properly targeted to S regions, and how the DNA breaks introduced in S regions are normally properly shepherded toward class switch recombination.
Another project is to investigate why the C terminal 10 amino acids of AID are required for CSR but not for SHM. Our initial results have been published (Ranjit et al, 2011), and we are continuing to investigate the hypothesis that the C terminus recruits proteins involved in recombination of DNA breaks.
Diagram of Class Switch Recombination (CSR)
Top line shows Ig heavy chain genes in a B cell that expresses IgM and IgD (by alternative RNA processing). CSR occurs after AID deaminates dC residues within S regions that are transcriptionally active. RNA transcripts from the unrearranged heavy chain genes are called germline transcripts.
Middle line illustrates the intrachromosomal deletional recombination between two switch (S) regions during CSR from IgM to IgE. The DNA in between Sm and Se is excised from the chromosome as a circle.
Bottom line illustrates the chromosome after CSR showing that the identical VH region originally expressed with the Cmgene is now expressed with Ce gene in cells expressing IgE.