INSERTION OF 2 KB OF BACTERIOPHAGE DNA BETWEEN AN IMMUNOGLOBULIN PROMOTER AND LEADER EXON STOPS SOMATIC HYPERMUTATION IN A KAPPA-TRANSGENE

Somatic hypermutation in rearranged immunoglobulin variable genes occu rs in a 2 kb region of DNA that is delimited on the 5' side by the pro moter and on the 3' side by intron DNA. To identify sequence features that activate the mutation mechanism, we increased the distance betwee n the promoter and the leader region to test whether the spacing of th ese elements was important. The promoter was separated from the leader sequence by inserting a 2 kb fragment of noncoding bacteriophage lamb da DNA between the TATA box and ATG initiator codon in a kappa transge ne. Mice from three founder lines were immunized, RNA and DNA were iso lated from spleen and Peyer's patch B cells, and transcription of the transgene was confirmed. The frequency of mutation in endogenous heavy chain genes was high, indicating that some B cells underwent hypermut ation. However, no hypermutation was found in the transgenic bacteriop hage or variable region sequences. Hypermutation did occur in another kappa transgene that had a deletion of the VJ coding sequence, showing that the basic construct is functional and that the VJ exon is not ne cessary for the mutation mechanism. It is likely that the bacteriophag e sequence is a potential substrate for mutation because other heterol ogous sequences have been shown to undergo mutation if placed downstre am of the leader exon. The results suggest that the promoter should be contiguous with the leader exon for the mutation mechanism to functio n. (C) 1997 Published by Elsevier Science Ltd.