Circular DNA is a product of the immunoglobulin class switch rearrangement

Abstract

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 culture^4,5, 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 receptor^7,9 and of the immunoglobulin heavy chain^10,11 and light chain^11,12. 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.