A targeted κ immunoglobulin gene containing a deletion of the nuclear matrix association region exhibits spontaneous hyper-recombination in pre-B cells

Previous studies employing ectopic integration of reporter genes have shown that the nuclear matrix association region (MAR) adjacent to the intronic enhancer of the mouse κ immunoglobulin (Ig) gene is required for high level transcription of rearranged genes, demethylation, reduction of position effects and maximal somatic hypermutation in B cells. To test for the function of this MAR in its natural chromosomal environment, we pursued the HIT-and-RUN procedure with the mouse pre-B cell line 103 to create a targeted MAR deletion. We observed a HIT targeting frequency of 1\684 but 0\2100 RUN clones maintained the MAR-deleted germline locus because of an unexpected hyper-recombination for Vκ–Jκ joining, specifically to the MAR-deleted allele, and primarily at Jκ4 and Jκ5. This hyper-recombination was correlated with undermethylation of the Jκ–Cκ region but not with the level of local transcription. These results are consistent with the possibility that the MAR and\or DNA methylation negatively regulate(s) Vκ–Jκ joining during the pre-B cell stage of development.