Intron sequences determine the expression of kappa light chain genes.

Alterations in kappa light chain expression were demonstrated to originate from genomic changes in the L-V intron (L-IVS) which changed the splicing pattern of the kappa mRNA. In R15, a mutant of mouse myeloma W3129 which produces no kappa light chain, a 358 bp novel sequence element (R15ns) of unknown origin replaced 19 bases of wild-type L-IVS, both altering the normal splicing pattern and activating a cryptic polyadenylation site. Subclones of R15 which reverted to kappa light chain production contained genomic deletions of R15ns and/or the surrounding intron. These deletions led to partial or full restoration of wild type kappa mRNA levels due to further changes in the pattern of mRNA processing. Two cryptic splice acceptor sites and a polyadenylation signal exist in the L-IVS; a cryptic splice acceptor sequence also exists in V kappa. These cryptic sites can be activated by changing the genomic context. It is thus possible to influence light chain expression without altering either the exon sequences or the known regulatory elements. Alterations in splicing patterns also serve to produce kappa light chains with novel variable region sequences and thereby could contribute to antibody diversity. Surprisingly, in the cell line producing this novel kappa light chain, intact alpha heavy chains were secreted in the absence of an apparent association with light chain. These studies also demonstrate that it is not possible to distinguish functional from non-functional genes solely by sequence analysis and that genes can both be inactivated and activated by changes in intron sequences.