IMMUNOGLOBULIN TRANSGENES AS TARGETS FOR SOMATIC HYPERMUTATION

This review describes studies on somatic hypermutation of immunoglobul in genes that were started in the mid-80s in collaboration with Ralph Brinster. Almost all of the experiments were carried out using lg tran sgenes as targets for the somatic mutation mechanism. Ig transgenes ca n be very good targets of somatic mutation, despite many different tra nsgene integration sites. Thus, the required cis-acting elements must be present within the approximately 10 kb of the transgene. Only the l g variable region and its proximate flanks are mutated, not the consta nt region in unmanipulated sequences. Several lg gene enhancers are pe rmissive for somatic mutation and they do not have to be associated wi th the Ig promoter they normally interact with. However, the mutation process does seem to be specific for Ig genes. No mutations were found in several housekeeping genes isolated from cells that had very high levels of somatic hypermutation of their lg genes. This suggests that the Ig enhancers provide the lg gene specificity, An exception is the Bcl- 6 gene, encoding a transcription factor, which was found to be mu tated in normal human memory B cells. When the transcriptional promote r that is located upstream of the variable region is duplicated upstre am of the constant region, this region is mutated as well. This sugges ts a transcription coupled model in which a mutator factor associates with the RNA polymerase at the initiation of transcription, travels wi th the polymerase during elongation, and causes mutations during polym erase pausing. Our recent data with an artificial substrate for somati c mutation suggest that the mutations are increased by increased stabi lity of the secondary structures in the nascent RNA, and the specific nucleotides that are mutated are due to preferences of a mutator facto r.