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B cells produce a diverse array of antibody specificities that are of low affinity during the initial phase of a humoral immune response. However, somatic hypermutation of the rearranged V region in the immunoglobulin locus generates new antibody affinities, accompanied by the selection of B cells that produce superior antibody affinities. Somatic hypermutation is initiated by the conversion of G:C base pairs to G:U lesions by the enzyme activation induced cytosine deaminase. Left unrepaired, G:U lesions will give rise to transition mutations at G:C base pairs, but are converted to transition and transversion mutations at G:C and A:T base pairs by the paradoxical participation of the base excision repair and mismatch repair pathways. The mismatch repair pathway, which evolved to correct errors produced during DNA replication, is co-opted by hypermutating B cells to produce A:T mutations via the processing of G:U lesions. This process requires the mismatch repair components Msh2, Msh6, and Exo1, but is additionally dependent upon the translesional DNA polymerase eta, a known A:T mutator, and on ubiquitinated PCNA, an initiator of translesion synthesis. The presence of certain types of lesions in the template strand during DNA replication leads to the activation of translesion synthesis. I propose that a similar mechanism operates during somatic hypermutation to activate translesion synthesis and recruit DNA polymerase eta. Our model suggests that mismatch repair-generated single-stranded DNA tracts contain abasic sites produced as a result of uracil excision by uracil-N-glycosylase. Synthesis opposite abasic sites activates translesion synthesis and results in the recruitment of polymerase eta and the subsequent production of A:T mutations. In this thesis, I present data from hypermutating murine B cells and the B cell line Ramos to support this model, demonstrating that the base excision repair and mismatch repair pathways cooperate during somatic hypermutation to generate A:T mutations. In addition, I explore the role of the Mre11-Rad50-Nbs1 complex in its contribution to A:T mutations in Ramos cells. Taken together, these studies demonstrate that conversion of classical DNA repair pathways into mutation-generating processes is driven by the unique environment of the V region in hypermutating B cells. |
