Loss of a consensus splice signal in a mutant immunoglobulin gene eliminates the CH₁ domain exon from the mRNA

A series of mouse myeloma mutants, derived from a cell line of the murine MPC-11 tumor (γ2b, kappa), resemble human heavy-chain disease in their loss of an internal domain (exon). In these mutants, most of the γ2b CH₁ exon was present in the nuclear RNA but was removed during splicing to form the mature cytoplasmic RNA. Amino acid sequence studies of one mutant (10.1) are consistent with the loss of the complete CH₁ domain. A second mutant cell line (I17) dervied from 10.1 and containing the same CH₁ alteration was shown by S1 nuclease protection experiments to have an additional mRNA deletion spanning the CH₂-CH₃ domain boundary. This second deletion was shown to result from a genomic alteration that provided a marker for the isolation of the expressed H-chain allele. To determine the basis of the CH₁ splicing defect, the I17 genome-expressed γ2b constant region DNA was cloned. Sequence studies showed a deletion of 99 nucleotides around the 3' and of the CH₁ domain, which removed the splice site and flanking DNA, apparently causing the aberrant splicing of the RNA transcript. The sequence deleted in the mutant is flanked by short repeats of the octameric sequence CCAGCCAG in the wild-type gene. In the mutant, one copy of the repeat, in addition to the sequences between the repeats, has been lost.