DNA polymerases beta and lambda do not directly affect Ig variable region somatic hypermutation although their absence reduces the frequency of mutations
Department of Microbiology and Physiological Systems
Animals; B-Lymphocytes; Cells, Cultured; DNA Breaks, Double-Stranded; DNA Polymerase beta; Embryo, Mammalian; Female; Gene Deletion; Immunoglobulin Class Switching; Immunoglobulin G; Immunoglobulin Variable Region; Mice; Mice, Inbred C57BL; Mice, Knockout; *Mutation Rate; Point Mutation; Somatic Hypermutation, Immunoglobulin
Cellular and Molecular Physiology | Genetics and Genomics
During somatic hypermutation (SHM) of antibody variable (V) region genes, activation-induced cytidine deaminase (AID) converts dC to dU, and dUs can either be excised by uracil DNA glycosylase (UNG), by mismatch repair, or replicated over. If UNG excises the dU, the abasic site could be cleaved by AP-endonuclease (APE), introducing the single-strand DNA breaks (SSBs) required for generating mutations at A:T bp, which are known to depend upon mismatch repair and DNA Pol eta. DNA Pol beta or lambda could instead repair the lesion correctly. To assess the involvement of Pols beta and lambda in SHM of antibody genes, we analyzed mutations in the VDJh4 3' flanking region in Peyer's patch germinal center (GC) B cells from polbeta(-/-)pollambda(-/-), pollambda(-/-), and polbeta(-/-) mice. We find that deficiency of either or both polymerases results in a modest but significant decrease in V region SHM, with Pol beta having a greater effect, but there is no effect on mutation specificity, suggesting they have no direct role in SHM. Instead, the effect on SHM appears to be due to a role for these enzymes in GC B cell proliferation or viability. The results suggest that the BER pathway is not important during V region SHM for generating mutations at A:T bp. Furthermore, this implies that most of the SSBs required for Pol eta to enter and create A:T mutations are likely generated during replication instead. These results contrast with the inhibitory effect of Pol beta on mutations at the Ig Smu locus, Smu DSBs and class switch recombination (CSR) reported previously. We show here that B cells deficient in Pol lambda or both Pol beta and lambda proliferate normally in culture and undergo slightly elevated CSR, as shown previously for Pol beta-deficient B cells.
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Citation: DNA Repair (Amst). 2013 Dec;12(12):1087-93. doi: 10.1016/j.dnarep.2013.09.002. Epub 2013 Sep 29. Link to article on publisher's site
Schrader, Carol E.; Linehan, Erin K.; Ucher, Anna J.; Bertocci, Barbara; and Stavnezer, Janet, "DNA polymerases beta and lambda do not directly affect Ig variable region somatic hypermutation although their absence reduces the frequency of mutations" (2013). University of Massachusetts Medical School Faculty Publications. 721.