Down-regulation of DNA polymerase β accompanies somatic hypermutation in human BL2 cell lines

Somatic hypermutation (SHM) is a fundamental process in immunoglobulin gene maturation that results in increased affinity of antibodies toward antigens. In one hypothesis explaining SHM in human B cells, the process is initiated by enzymatic deamination of cytosine to uracil in the immunoglobulin gene V-region and this in turn triggers mutation-prone forms of uracil-DNA base excision repair (BER). Yet, an uncertainty with this model is that BER of uracil-DNA in mammalian cells is generally error-free, wherein DNA polymerase β (pol β) conducts gap-filling synthesis by insertion of bases according to Watson-Crick rules. To evaluate this inconsistency, we examined pol β expression in various SHM proficient human BL2 cell line subclones. We report that expression of pol β in SHM proficient cell lines was strongly down-regulated. In contrast, in other BL2 subclones, we found that SHM was deficient and that pol β expression was much higher than in the SHM proficient subclones. We also found that overexpression of recombinant human pol β in a SHM proficient subclone abrogated its capacity for SHM. These results suggest that down-regulation of the normal BER gap-filling DNA polymerase, pol β, accompanies induced SHM in BL2 cells. This is consistent with the hypothesis that normal error-free BER must be silenced to make way for an error-prone BER process that may be required during somatic hypermutation.