Processing of Switch Transcripts Is Required for Targeting of Antibody Class Switch Recombination

Abstract

Antibody class switching is mediated by somatic recombination between switch regions of the immunoglobulin heavy chain gene locus. Targeting of recombination to particular switch regions is strictly regulated by cytokines through the induction of switch transcripts starting 5′ of the repetitive switch regions. However, switch transcription as such is not sufficient to target switch recombination. This has been shown in mutant mice, in which the I-exon and its promoter upstream of the switch region were replaced with heterologous promoters. Here we show that, in the murine germline targeted replacement of the endogenous γ1 promoter, I-exon, and I-exon splice donor site by heterologous promoter and splice donor sites directs switch recombination in activated B lymphocytes constitutively to the γ1 switch region. In contrast, switch recombination to IgG1 is inhibited in mutant mice, in which the replacement does not include the heterologous splice donor site. Our data unequivocally demonstrate that targeting of switch recombination to IgG1 in vivo requires processing of the Iγ1 switch transcripts. Either the processing machinery or the processed transcripts are involved in class switch recombination.

Nibrin functions in Ig class-switch recombination

“…It acts downstream of switch transcript-induced targeting of switch regions (26,27) and activation-induced cytidine deaminase (AID)-induced DNA breaks in the targeted switch regions (28). Id., identification.…”

In vivoandin vitrostudies of immunoglobulin gene somatic hypermutation

“…It has been recently shown that the germline switch transcripts, generated from an exon 5' of the switch region (I H ) and the C H exons (Lutzker et al 1988;Stavnezer-Nordgren & Sirlin 1986) are themselves required (Hein et al 1998). Switch transcripts are able to form stable RNA:DNA triplex structures with the switch region in vitro (Reaban & Gri¤n 1990).…”

Activation-induced cytidine deaminase: structure–function relationship as based on the study of mutants

“…During CSR, activation of B cells by cytokines activates the specific I promoter located 5 0 to the target C m or C x gene, and induces the synthesis and splicing of so-called germ line or sterile transcripts [Hein et al, 1998;Nagaoka et al, 2002]. This transcription step leads to the formation of RNA/ DNA hybrids on the template DNA strand, leaving the nontemplate DNA strand accessible to modification and cleavage [Chaudhuri et al, 2003;Yu et al, 2003;Ramiro et al, 2003;Dickerson et al, 2003].…”