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From the Department of Microbiology and Immunology, Kimmel Cancer Institute, Thomas Jefferson
Medical College, Philadelphia, Pennsylvania 19107
A somatic process introduces mutations into antibody variable (V) region genes at a high rate in
many vertebrates, and is a major source of antibody diversity. The mechanism of this hypermutation process remains enigmatic, although retrospective studies and transgenic experiments
have recently suggested a role for transcriptional regulatory elements. Here, we demonstrate
that mouse heavy (H) chain loci in which the natural VH promoter has been replaced by a heterologous promoter undergo hypermutation. However, while the distribution of mutation in
such loci appears normal, the frequency of mutation does not. Conversely, moving the VH
promoter 750 bp upstream of its normal location results in a commensurate change in the site
specificity of hypermutation in H chain loci, and the foreign DNA inserted into the VH leader
intron to produce this promoter displacement is hypermutated in a manner indistinguishable from natural Ig DNA. These data establish a direct mechanistic link between the IgH transcription and hypermutation processes.
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