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.