A Targeted Deletion of a Region Upstream from the J_κ Cluster Impairs κ Chain Rearrangement In Cis in Mice and in the 103/bcl2 Cell Line

We have shown previously that a mutation of the KI-KII site immediately 5′ to J_κ1 on the mouse immunoglobulin light chain κ locus reduces the rearrangement level in cis, although it does not affect transcription. Here we deleted by homologous recombination in mouse embryonic stem cells a 4-kb DNA fragment, located immediately upstream of the KI-KII element, which contains the promoter of the long germline transcript. Analysis of gene-targeted heterozygous mouse splenic B cells showed a strong decrease in rearrangement for the allele bearing the deletion. When both the KI-KII mutation and the 4-kb deletion were present on the same allele, the overall reduction in rearrangement was stronger than with the 4-kb deletion alone underlying the role of these two elements in the regulation of rearrangement. The same deletion was performed by homologous recombination on one allele of the rearrangement- inducible mouse 103/bcl2-hygro^R pre-B cell line, and resulted in a similar reduction in the induction of rearrangement of the mutated allele. This result validates this cell line as an in vitro model for studying the incidence of gene-targeted modifications of the κ locus on the regulation of rearrangement.