Matthew Strout MD, PhD
Assistant Professor of Medicine (Hematology)
Cancer genetics; Lymphomagenesis; Leukemogenesis; Immune diversification; Epigenetics; Tissue banking; Targeted therapy
In order to cope with an unpredictable variety of invading antigenic insults, the humoral immune system is able to generate an enormously diverse antibody repertoire. This diversity is achieved through a complex series of molecular events that result in direct modification of Ig gene sequences with consequent changes in the specificity of the encoding antibodies. In the first step of immune diversification, variable (V), diversity (D), and joining (J) segments of the Ig genes are assembled in immature B cells by a site-specific reaction known as V(D)J recombination. This process occurs in the bone marrow early in B cell development and results in the production of a pool of mature B cells, each expressing a unique immunoglobulin B cell receptor.
Upon antigenic stimulation of mature B cells, germinal centers are formed where, in mice and humans, two additional DNA modification reactions serve to further diversify the immune response: somatic hypermutation (SHM) and class switch recombination (CSR). SHM introduces point mutations into the variable region of Ig heavy and light chain genes. This process underlies affinity maturation and results in the selection of a B cell that produces an antibody with high affinity for its target antigen. CSR is a deletion event that combines the variable region with different constant regions to result in the production of different immunoglobulin isotypes (IgG, IgE, or IgA). The enzyme activation induced cytidine deaminase (AID) is a cytidine deaminase that initiates these processes by introducing point mutations into the Ig variable regions (for SHM) and double-stranded DNA breaks into the Ig class switch regions (for CSR).
During the first phase of SHM and CSR, AID converts cytosine to uracil to result in a uracil-guanine (U-G) mismatch. Spontaneous U-G mismatches are normally corrected by high-fidelity base excision repair (BER) and mismatch repair (MMR) pathways. However, during the second phase of SHM and CSR, U-G mismatches are repaired by error-prone, low-fidelity BER and MMR pathways to yield mutations (for SHM) and DNA strand lesions (for CSR).
The intrinsic activity of AID is that of a DNA mutator and tight regulation is required to restrict this activity to the appropriate cell type and target loci in order to avoid mutations throughout the genome. However, aberrant targeting of AID activity contributes to translocations and point mutations of proto-oncogenes associated with many malignancies of B cell origin. Thus, the mechanisms that determine which genes will be targeted by AID and the factors that regulate how mutations are subsequently processed are of central importance to our understanding of the etiology of malignant transformation. Our lab currently has several projects underway that are directed at understanding the role of AID and associated factors in the pathogenesis of B cell malignancy:
1) The role of AID-associated DNA repair pathways in the pathogenesis of B cell lymphoma.
2) AID-induced epigenetic reprogramming in lymphoma through deamination of methylated CpG sites.
3) Somatic hypermutation of the mitochondrial genome by AID.
4) The role of AID in the pathogenesis of chronic lymphocytic leukemia.
5) Dysregulated expression of AID in acute lymphoblastic leukemia.
6) Regulation of the genotoxic threshold in B cells.
7) Differential targeting of the Ig loci for SHM versus CSR.
Antibodies are produced by the immune system’s B cells and possess the ability to recognize and clear virtually any invading pathogen that the human body encounters. This is made possible by the enzyme activation induced cytidine deaminase (AID). The natural function of AID is to introduce mutations into genes that encode antibodies. The end result is production of an antibody with enhanced recognition of the invading pathogen. Under normal circumstances, AID should only mutate genes encoding antibodies. However, some B cell malignancies (non-Hodgkin lymphoma, multiple myeloma and chronic lymphocytic leukemia) harbor genetic mutations of non-antibody genes and there is evidence that AID causes these mutations. Research efforts in our lab are directed at:
- Establishing the role of AID in the pathogenesis of B cell malignancies;
- Characterizing AID-associated DNA repair pathways; and
- Identifying novel mechanisms of lymphomagenesis.