Cutting Edge: A cis-Acting DNA Element Targets AID-Mediated Sequence Diversification to the Chicken Ig Light Chain Gene Locus

Abstract

Somatic hypermutation and gene conversion are two closely related processes that increase the diversity of the primary Ig repertoire. Both processes are initiated by the activation-induced cytidine deaminase that converts cytosine residues to uracils in a transcription-dependent manner; these lesions are subsequently fixed in the genome by direct replication and error-prone DNA repair. Two alternative mechanisms were proposed to explain why this mutagenic activity is targeted almost exclusively to Ig loci: 1) specific cis-acting DNA sequences; or 2) very high levels of Ig gene transcription. In this study we now identify a novel 3′ regulatory region in the chicken Ig light chain gene containing not only a classical transcriptional enhancer but also cis-acting DNA elements essential for targeting activation-induced cytidine deaminase-mediated sequence diversification to this locus.

Target DNA Sequence Directly Regulates the Frequency of Activation-Induced Deaminase-Dependent Mutations

“…However, none of the known transcriptional control elements in mouse B cells appears to be absolutely required for SHM, suggesting a redundancy or unidentified elements (48). Recent studies discovered novel cis-acting elements in the IgL locus of the DT40 chicken B cell line that are both essential and sufficient for AIDmediated sequence diversification (59,60). In addition, it was shown that CAGGTG cis elements in the context of Ig enhancers were sufficient and essential to target mutations to a nearby transcribed gene in transgenic DT40 cell lines (61).…”

AID and Somatic Hypermutation

“…Deletion of 3'E reduced transcription but did not affect SHM (Inlay et al ., 2006; van der Stoep et al ., 1998), whereas deletion of Ed reduced both transcription and SHM (Xiang and Garrard, 2008). Likewise, the chicken Ig λ locus has a defined 3'E (Bulfone-Paus et al ., 1995) and another downstream enhancer, 3'RR (Kothapalli et al ., 2008). In the DT40 cell line, deletion of the 3'E had no effect on SHM (Yang et al ., 2006), but deletion of 3'RR ablated transcription and SHM (Kothapalli et al ., 2008).…”

Balancing AID and DNA repair during somatic hypermutation

“…Thus far, germline deletion of Ig enhancers has resulted in either normal SHM, or impaired SHM accompanied by reduced transcription. Interestingly, recent studies in the chicken DT40 B-cell line identified a large DNA region 3 0 of the Igl constant region with apparent SHM targeting activity [56,57].…”