A target selection of somatic hypermutations is regulated similarly between T and B cells upon activation-induced cytidine deaminase expression

Abstract

Activation-induced cytidine deaminase (AID) is essential for somatic hypermutations (SHM) and class switch recombination. Overexpression of AID in non-B cells can induce SHM in artificial constructs inserted in various loci in the genome. AID overexpression was thus proposed to introduce mutations in a wide variety of genes with little specificity. We previously showed that AID transgenic mice developed T cell lymphomas in which the variable region beta genes of the T cell receptor and c-myc were mutated as frequently as SHM in activated B cells. To understand the target specificity of SHM in AID-expressing T lymphomas, we sequenced six oncogenes (c-myc, pim1, p53, atm, tgfbr-2, and k-ras) and two genes (cd4 and cd5) that are actively transcribed in T lymphomas. SHM was found only in c-myc, pim1, cd4, and cd5, which share the E47 binding motif in the enhancer/promoter. The rest that are not mutated in B cells were not mutated in AID-induced T lymphomas either, although they are transcribed in T and B cells. Comparison of several features of SHM, including selection of targets and mutation distribution, suggests that the regulatory mechanism of SHM is similar between T and B cells. SHM base specificities in the CD4 and CD5 genes were biased to AT, indicating that the preference of target bases of the mutations generated by overexpression of AID is not always GC bases but variable between target genes.

Fig. 1.

Distribution of mutations in c-myc, cd4, and cd5 in lymphoma cells. The mutations described in Tables 1 and 5 are plotted on respective genomic sequences. Genomic loci are shown with untranslated and translated sequences (open and filled boxes, respectively). The bold double-headed arrows above the genes show the regions sequenced. The numbers at left and above the thin horizontal lines indicate the lymphoma numbers and the position from the transcriptional starting sites, respectively. Red vertical lines indicate mutations common to more than half of the total clones analyzed from the same individual mice. Black vertical lines indicate sporadic mutations, and arrowheads labeled with a P indicate transcriptional start sites. Primers used for amplification are shown by arrowheads above genomic loci. Data for c-myc in lymphoma 1 are taken from ref. .