CIRCULAR DNA IS A PRODUCT OF THE IMMUNOGLOBULIN CLASS SWITCH REARRANGEMENT

THE class of immunoglobulin is defined by the constant region of its heavy chain. When a B lymphocyte switches the class of heavy chain it produces, the constant region of μ-type heavy chain is replaced (reviewed in refs 1 and 2, and see ref. 3); this occurs through a DNA rearrangement that brings the gene segment encoding the new constant region close to the VDJ segment encoding the variable region. The pre-B-cell line 18-81, which switches from heavy chain μ to γ2b production in culture4,5, occasionally abnormally rearranges the heavy chain locus so that DNA sequences between the switch regions of μ and γ2b are inverted6. Because looping-out is an intermediate step in generating an inversion, the switch rearrangement could occur by looping-out and deletion. Provided that recombination is reciprocal, this would produce a circle of DNA. Indeed, circular DNA molecules have been isolated as products of rearrangement among gene segments encoding the variable regions of the T-cell receptor7-9 and of the immunoglobulin heavy chain10,11 and light chain11,12. But whereas the breakpoints for the variable region rearrangement are precisely defined, the breakpoints for any given heavy chain class switch are scattered over a length of >6 kilobases, including both switch regions. We have now isolated circular DNA containing the sequences deleted by class-switching, thereby showing that the immunoglobulin heavy chain class switch occurs through looping-out and deletion. © 1990 Nature Publishing Group.