Looping out and deletion mechanism for the immunoglobulin heavy-chain class switch.

Abstract

In the mouse pre-B-cell line 18-81, cells can switch production in vitro from immunoglobulin p chain to y2b chain. The gene encoding the y2b chain is created by a rearrangement of the I gene. This rearrangement always takes place within a homolog. In cells with a y2b gene, most of the time the gene segment encoding the constant region of the ,u chain is deleted, but often the rearrangement leads to cells that produce no immunoglobulin, and all DNA sequences are retained. The latter result is due to an inversion. Inversions exclude the unequal sister chromatid exchange model of the heavy-chain class switch. Looping out is an intermediate step in the process of generating an inversion. Our findings demonstrate that the switch rearrangement occurs by looping out and deletion.When injected with an antigen, higher vertebrates can respond by producing antibodies that are immunoglobulins of various classes. These classes differ in the heavy (H) chain that combines with the light chain to form the complete immunoglobulin molecule. In the mouse, the early immune response is dominated by IgM, which contains the H chain ,, and the later immune response is dominated by IgG3, IgGl, IgG2b, IgG2a, IgA, and (rarely) IgE, which contain the H chains y3, yl, y2b, y2a, a, and E, respectively. At the genetic level, the gene encoding the ,u chain consists of a variable (V)-region gene segment, which has been somatically assembled from one each of the VH, diversity (D), and joining (JH) segments, and the nearby C,,, gene segment, which encodes the constant (C) region of the ,u chain. Upon H-chain switching, which occurs within a clone derived from a committed lymphocyte (1, 2), the V region remains the same, but the C region changes. C,, is replaced by C,3, C 11 C,,2b, CE, or C., which are the C-region gene segments of the respective chains and are closely linked to C,,, in that order (reviewed in refs. 3 and 4).This rearrangement event generally results in deletion of the DNA sequences between a site within the intron 5' to C,, and a site within the intron 5' to the particular C-region gene segment that is to be expressed.

Anomalous rearrangements of the immunoglobulin heavy chain genes in human leukemias support the loop-out mechanism of class switch.

“…We have isolated from human B cells a structure that could only arise through an event bringing together two CH regions from the same chromosome. This is exactly in agreement with the loop-out model proven in the mouse when Jack et al ( 18) reported the finding of an inversion, similar to the one described here, in the mouse pre-B cell line 18-81. As shown in Fig.…”

Nonhomologous recombination at sites within the mouse JH-C delta locus accompanies C mu deletion and switch to immunoglobulin D secretion.

“…In response to stimulation of mature B cells by interleukins and/or other T-cell-or macrophageproduced factors, nuclear proteins (73) would bind to the tandem repeat area of the ,u locus. In response to other interleukins and other factors, the second portion of the switch complex would form near a downstream tandem repeat (switch) region (68), perhaps after the region is made accessible due to germ line (sterile) transcription (32,36,53 (20). The IgH locus inversions and duplications that are attributed to sister chromatid exchange might also be mediated by this mechanism, since switch complexes can assemble on the C of one chromosome and on a CH on another chromosome (16).…”

“…Random integration is one form of illegitimate recombination which occurs so readily in mammalian cells that transfected DNA, even with a very long (3-to 10-kbp) homology to its target, is 100 to 10,000 times more likely to be randomly integrated than to undergo homologous integration (29,58 A switch recombination model. We favor a modified version of the looping-out and deletion model of switch recombination originally described by Jack et al (20) and extended by Petrini and Dunnick (47). In our working hypothesis, switching would be mediated by a switch complex which is 3 4 assembled in stages.…”

Inversions Produced during V(D)J Rearrangement at IgH, the Immunoglobulin Heavy-Chain Locus

“…It retains the original germ line orientation for the coding sequences of both gene elements. Figure 1C shows the results of an intrinsic side reaction of the V(D)J rearrangement process (24,30). This side reaction involves cutting and recognition at the same D H RSS that is used for formation of the uninverted DJ H coding joint.…”

“…Following recognition and cutting at either the D H RSS or the J H RSS, rejoining in the original configuration can occur. These rearrangements are open-and-shut junctions (24,30), which are distinguishable from unrear-ranged gene elements only if they have sustained nucleotide deletions or additions at the junction.…”