Sequence dependent hypermutation of the immunoglobulin heavy chain in cultured B cells

The variable (V) regions of immunoglobulin heavy and light chains undergo high rates of somatic mutation during the immune response. Although point mutations accumulate throughout the V regions and their immediate flanking sequences, analysis of large numbers of mutations that have arisen in vivo reveal that the triplet AGC appears to be most susceptible to mutation. We have stably transfected B cell lines with γ2a heavy chain constructs containing TAG nonsense codons in their V regions that are part of either a putative (T)AGC hot spot or a (T)AGA non-hot spot motif. Using an ELISA spot assay to detect revertants and fluctuation analysis to determine rates of mutation, the rate of reversion of the TAG nonsense codon has been determined for different motifs in different parts of the V region. In the NSO plasma cell line, the (T)AGC hot spot motif mutates at rates of ≃6 x 10^-4/bp per generation and ≃3 x 10^-5/bp per generation at residues 38 and 94 in the V region. At each of these locations, the (T)AGC hot spot motif is 20-30 times more likely to undergo mutation than the (T)AGA non-hot spot motif. Moreover, the AGA non-hot spot motif mutates at as high a rate as the hot spot motif when it is located adjacent to hot spot motifs, suggesting that more extended sequences influence susceptibility to mutation.