The 5′ hypermutation boundary of x chains is independent of local and neighbouring sequences and related to the distance from the initiation of transcription

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Abstract
The hypermutation of antibody genes targets 1–2 kb of DNA which includes the rearranged V(D)J gene segments. The precise nature, location and limits of the targeted region are of considerable interest in terms of the mechanism of hypermutation. We have analyzed the frequency and distribution of mutations in the 5′ region of immunoglobulins using several modified x transgenes. We found that the position of the boundary, relative to the transcription initiation site, is not affected by the sequence of the V segment or by substituting the x chain promoter for a β‐globin promoter. Furthermore, the deletion of the leader intron (containing the hypermutation boundary) does not affect hypermutation per se, but shifts the boundary from the leader intron to the V region such that the distance between the boundary and the site of initiation of transcription remains constant. These results show that the position of the hypermutation boundary (about 185 bases downstream of the site of initiation of transcription) is not defined by the nucleotide sequence but rather by the distance to a fixed upstream position. Although mutations are also observed in the region upstream of the boundary, the frequency at which they occur is one order of magnitude lower relative to the frequency observed in the V segment. Nonetheless this upstream mutation rate remains more than two orders of magnitude higher than that of somatic genes. We discuss possible mechanisms explaining the nature and position of the boundary in the context of an error‐prone DNA repair model.