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 Cμ 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_H186.2 genes of C57BL/6 mice.