Characterization of Ig Gene Somatic Hypermutation in the Absence of Activation-Induced Cytidine Deaminase

Abstract

Somatic hypermutation (SHM) of Ig genes depends upon the deamination of C nucleotides in WRCY (W = A/T, R = A/G, Y = C/T) motifs by activation-induced cytidine deaminase (AICDA). Despite this, a large number of mutations occur in WA motifs that can be accounted for by the activity of polymerase η (POL η). To determine whether there are AICDA-independent mutations and to characterize the relationship between AICDA- and POL η-mediated mutations, 1470 H chain and 1313 κ- and λ-chain rearrangements from three AICDA−/− patients were analyzed. The Ig mutation frequency of all VH genes from AICDA−/− patients was 40-fold less than that of normal donors, whereas the mutation frequency of mutated VH sequences from AICDA−/− patients was 6.8-fold less than that of normal donors. AICDA−/− B cells lack mutations in WRCY/RGYW motifs as well as replacement mutations and mutational targeting in complementarity-determining regions. A significantly reduced mutation frequency in WA motifs compared with normal donors and an increased percentage of transitions, which may relate to reduced uracil DNA-glycosylase activity, suggest a role for AICDA in regulating POL η and uracil DNA-glycosylase activity. Similar results were observed in VL rearrangements. The residual mutations were predominantly G:C substitutions, indicating that AICDA-independent cytidine deamination was a likely, yet inefficient, mechanism for mutating Ig genes.