The Focus of Our Work Everyday!
Diversification of the antibody response:
Mutational mechanisms and contributions to oncogenesis.
Generation of antibody diversity in mature B cells
The antigen binding site of antibodies are created in immature B cells by the random assembly of variable (V), diversity (D), and joining (J) segments into one coding exon by a process termed V(D)J recombination. This process creates a very large repertoire of antibodies with different specificities. However, most antibodies that are generated bind to antigens such as viruses and bacteria with low affinity. In order to neutralize and clear pathogens and toxins from the circulation, B-cells must produce and secrete antibodies of higher affinity and of different classes. Following exposure to antigen, the V regions of the antibody genes acquire many base changes that result in antibodies that bind with higher affinities to their respective antigens. This phenomenon is achieved by the somatic hypermutation process. The constant region of the antibody gene encodes the remaining part of the antibody molecule and is responsible for carrying out the effector functions of the antibody. Constant regions, which define the antibody class, can be replaced with other constant regions by the class switch recombination process, thereby changing the antibody effector functions without changing the antigen binding site.
The discovery of the B cell-specific activation-induced cytidine deaminase (AID) gene has resulted in a dramatic leap forward in our understanding of the processes of somatic hypermutation and class switch recombination. Mice and humans deficient in AID are incapable of somatic hypermutation and class switch recombination, while overexpression of AID can induce somatic hypermutation and class switch recombination in most cell types. Thus, AID is the only B cell specific protein that is required for both of these processes. AID initiates the processes of antibody diversification by deaminating cytidines within the V regions for somatic hypermutation and switch regions for class switch recombination. Current work in the laboratory is centered on delineating the molecular mechanisms of the somatic hypermutation and class switch recombination processes including the DNA repair proteins that repair the AID-induced DNA lesions.
The molecular basis for germinal center selection
In spite of intensive study of the germinal center, we still do not understand how B cells that acquire deleterious mutations in their antibody genes are dealt with, whether there is truly a survival advantage for B cells that harbour high-affinity antibodies, and what molecules would influence life versus death decisions within the germinal center, the site where high-affinity B cells are produced. Using unique systems combined with various gene-targeted mice, we are addressing these fundamental questions which will provide new insights into the affinity maturation process.
The molecular mechanisms of cancer development
We have two main cancer projects in the laboratory. In one project, we are investigating the etiology of lymphomas which encompass a variety of cancers specific to the lymphatic system, most of which are of B-cell origin. The etiology of these cancers is not known, but are likely driven by genomic instabilities in B lymphocytes that result in chromosomal translocations or other mutagenic DNA lesions. The most common lymphomas (i.e. diffuse large cell lymphoma, chronic lymphocytic leukemia, and Burkitt’s lymphoma) resemble either centroblasts or post-centroblasts. This and the fact many of these lymphomas have chromosomal translocations involving antibody genes suggests that antibody diversification processes are central to the development of these types of lymphomas. Our current work is investigating the role of AID and the oncogene c-myc in the transformation process, and the role of the mismatch repair pathway in suppressing the oncogenic transformation of lymphocytes.
The second cancer project in the laboratory is focused on uncovering the etiology of colorectal cancer, which is the third most common type of cancer and leading cause of cancer related deaths. Specific genetic mutations are linked to colorectal cancer development, and mutations in genes involved in mismatch repair are one of the most common types of genetic deficiencies that predispose to this type of cancer. The normal function of mismatch repair is to repair mutations produced during DNA replication. In the absence of mismatch repair, mutations accumulate throughout the genome. However, it is not clear why mismatch repair deficiency leads specifically to an increased risk in developing colon cancer, and our laboratories research focus is aimed at uncovering this mystery.
Gut microbiota and disease
Although the gut microbiota has beneficial effects to the host, it has also been linked to the development of certain pathological disorders, including inflammatory bowel diseases, and possibly colorectal cancer. Using an animal model of colorectal cancer (i.e. mismatch repair deficiency), we showed that the gut microbiota promotes this disease. While this work seems quite distinct to the work outlined above, the common theme is the mismatch repair system since this DNA repair pathway plays an essential role in somatic hypermutation and class switch recombination as well as in colon cancer. We currently have two questions that are designed to characterize the mouse model of colorectal cancer: (1) which bacterial species and what metabolite is/are promoting the development of colorectal cancer? (2) What is the underlying mechanism by which this occurs?
We are also carrying out research on gut microbiota and the etiology of inflammatory bowel disease (IBD). Our work centers on characterizing specific microbes and their role in IBD. As IBD patients are prone to develop colon cancer, our research on these microbes also encompass uncovering the mechanism of colitis associated colon cancer.