Somatic diversification of S107 from an antiphosphocholine to an anti-DNA autoantibody is due to a single base change in its heavy chain variable region.

The S107 myeloma cell line expresses the germ-line sequence of the T15 antiphosphocholine (P-Cho) antibody, which is the major antibody made by BALB/c mice in response to P-Cho, either on a variety of bacterial polysaccharides or when attached to a protein carrier. We have previously reported that a somatic mutant of the S107 cell line produces an antibody that has lost the ability to bind P-Cho and has acquired binding for double-stranded DNA. This antibody has a substitution of an alanine for a glutamic acid at residue 35 in the heavy chain variable region. We now show that this amino acid substitution is due to a single A-C transversion, which is the only nucleotide change in the heavy and light chain variable regions. Further, it appears that this change is due to somatic mutation rather than to gene conversion.