Genetic rearrangements of immunoglobulin heavy chain switch regions in murine plasmacytoma P3.26Bu4 include insertions of retroviruslike element ETn.

Abstract

A number of moderately reiterated murine genetic elements have been shown to have structures like those of retroviral proviruses. These elements are thought to be transposons, although little evidence for their transposability exists. Two members of one of these families of reiterated elements, the ETn family, have inserted into separate immunoglobulin heavy chain switch regions in the IgG1-producing plasmacytoma P3.26Bu4. Switch regions are those DNA segments associated with each immunoglobulin heavy chain constant region gene in which the somatic recombinations that accompany heavy chain class switches occur. This role in somatic recombination may be relevant to the ETn insertions into P3.26Bu4 switch regions. In P3.26Bu4, the mu and gamma1 switch regions have been joined, as expected in an lgG1 producer; DNA sequences surrounding their point of recombination are presented. Also presented are sequences of the ETn elements that interrupt gamma1 and gamma2a switch regions in this cell line. Like ETn elements whose sequences have been previously reported, these elements contain no open reading frames capable of encoding gag- or pol-like functions. They share sequence homology with previously described ETn elements with the exception of a region of nonhomology near the 5$\sp\prime$ LTRs. This region of nonhomology defines two ETn subfamilies, which have roughly equal numbers of members. P3.26Bu4 and a number of other B-lineage cells contain ETn transcripts.