Somatic Hypermutation of Immunoglobulin Genes Is Linked to Transcription Initiation

Abstract

To identify DNA sequences that target the somatic hypermutation process, the immunoglobulin gene promoter located upstream of the variable (V) region was duplicated upstream of the constant (C) region of a kappa transgene. Normally, kappa genes are somatically mutated only in the VJ region, but not in the C region. In B cell hybridomas from mice with this kappa transgene (P5'C), both the VJ region and the C region, but not the region between them, were mutated at similar frequencies, suggesting that the mutation mechanism is related to transcription. The downstream promoter was not occluded by transcripts from the upstream promoter. In fact, the levels of transcripts originating from the two promoters were similar, supporting a mutation model based on initiation of transcripts. Several "hot-spots" of somatic mutation were noted, further demonstrating that this transgene has the hallmarks of somatic mutation of endogenous immunoglobulin genes. A model linking somatic mutation to transcription-coupled DNA repair is proposed.

The c-MYC allele that is translocated into the IgH locus undergoes constitutive hypermutation in a Burkitt's lymphoma line

“…A connection between mutability and transcription has been noted in several studies (Betz et al, 1994;Peters and Storb, 1996;Goyenechea et al, 1997;Fukita et al, 1998) and Peters and Storb (1996) have proposed that transcriptional pausing may play a role in hypermutation. Transcriptional pause sites in c-MYC are amongst the best characterized of those in mammalian RNA polymerase II transcription units (Bentley and Groudine, 1986;Chung et al, 1987;Kerppola and Kane, 1988;Strobl and Eick, 1992;Keene et al, 1999).…”

“…The V gene, whilst the physiological target of the mutation, is not needed for mutation recruitment: other sequences can be mutated in its place (Yelamos et al, 1995). Mutation recruitment depends upon the immunoglobulin transcription regulatory elements (Betz et al, 1994) with the location of the mutation domain being de®ned by the position of the transcription start site (Peters and Storb, 1996;Rada et al, 1997;Tumas-Brundage and Manser, 1997). Whilst the immunoglobulin loci are certainly the preferred targets of the hypermutation, the process is not wholly exclusive to them: BCL-6 (but not several other genes analysed) has been found to be a natural target for the hypermutation process ± albeit at a much reduced mutation rate (Migliazza et al, 1995;Pasqualucci et al, 1998;Shen et al, 1998).…”

Evolution of Ig DNA Sequence to Target Specific Base Positions Within Codons for Somatic Hypermutation

“…For example, sequencing studies have revealed a sharp 5Ј mutation boundary delimited by the promoter, with mutation frequencies being highest in the coding exon and gradually diminishing to background levels over a 2-kb range (14 -17). Inserting an Ig promoter and leader immediately upstream of the constant region targeted mutations to a previously unsusceptible location (18). Similarly, increasing the distance between the Ig promoter and the V coding region did not change the overall frequency of mutations but did alter their distribution (19).…”

The activation-induced cytidine deaminase (AID) efficiently targets DNA in nucleosomes but only during transcription

“…The track of mutations coincides roughly with that of the transcribed 1-2 kb after the start site of transcription (2). The SHM process is initiated by the activationinduced cytidine deaminase (AID), which targets deoxycytidines (Cs) in cell-free assays in singlestranded DNA (3)(4)(5) and mainly in the nontranscribed strand during transcription of linear double-stranded DNA (6-10) but targets Cs on both DNA strands when they are flipped out in supercoiled DNA (11,12).…”