Somatic hypermutation of an immunoglobulin transgene in K transgenic mice

Abstract

Initial studies of somatically acquired mutations in immunoglobulin V regions from hybridomas and myelomas that are not derived from joining aberrations, suggested a controlled and specific hypermutation process^1–9, because spontaneous mutation rates observed for other genes are extremely low¹⁰. Some evidence for the idea that mutations are introduced during V-gene rearrangement came from the clustering of mutations at the joining sites^6,8, from the absence of mutations in unrearranged V genes^4,5,11 and from the low level of mutations in only partially (D-J) rearranged nonproductive heavy-chain alleles¹². Another model in which mutations accumulate with each cell division, rather than being introduced all at once⁸, was supported by the finding that immunoglobulin genes of hybridomas derived from a single mouse frequently had several mutations in common, and so might be derived from the same precursor cell whose daughters then accumulated additional mutations^13–17. But the common mutations in some cases could be due to as yet unidentified related germline genes, or could represent the effect of antigen selection for certain amino acids. To try to detect hypermutation in the absence of V-gene rearrangement, we isolated B lymphocytes with endogenous heavy-chain gene mutations from transgenic mice carrying pre-rearranged K-transgenes. We found that these K-transgenes were also somatically mutated. This and other observations indicated that: (1) ongoing rearrangement is not required for mutation; (2) there are signals for hypermutation in the trans-genes; (3) the mutations are found only in the variable region, so the constant region may not be a target; (4) different transgene insertion sites are compatible with hypermutations and (5) more than one transgene is expressed in the same cell.