A pivotal role for DNase I-sensitive regions 3b and/or 4 in the induction of somatic hypermutation of IgH genes

Chimeric mice were prepared from embryonic stem cells transfected with IgH genes as transgenes and RAG-2-deficient blastocysts for the purpose of identifying the cis-acting elements responsible for the induction of somatic hypermutation. Among the three transgene constructs used, the V-H promoter, the rearranged V-H-D-J(H), an intron enhancer/matrix attachment region, and human C mu were common to all, but the 3 ' -untranslated region in each construct was different. After immunization of mice with a T cell-dependent Ag, the distribution and frequency of hypermutation in transgenes were analyzed. The transgene lacking the 3 ' untranslated region showed a marginal degree of hypermutation. Addition of the 3 ' enhancer resulted in a slight increase in the number of mutations. However, the transgene containing DNase I-sensitive regions 3b and 4 in addition to the 3 ' enhancer showed more than a 10-fold increase in hypermutation, reaching levels comparable to those observed in endogenous V(H)186.2 genes of C57BL/6 mice.