Transcriptional down-regulation of N-myc expression during B-cell development

We have used nuclear run-on and DNase I sensitivity analyses to study the activity of the N-myc genes in cell lines that represent different stages of B-cell development. Both transformed pre-B-cell lines and a nontransformed pre-B-cell clone transcribe the N- and c-myc genes at substantial levels; in the nontransformed clone, transcription of these genes is regulated by the pre-B-cell growth factor interleukin-7. In contrast, transformed cell lines that represent the more mature stages of the B-cell pathway and mitogen-stimulated normal splenic B lymphocytes express the c-myc gene but do not express the N-myc gene at detectable levels. Down-regulation of N-myc expression in these cells occurs at the level of transcriptional initiation. Correspondingly, a set of DNase I-hypersensitive sites present in the 5' region of the N-myc promoter of pre-B-cell lines are absent in B-cell lines. To further elucidate this process, we have constructed fusion cell lines between an N-myc-expressing pre-B-cell line and a nonexpressing myeloma line; the hybrid cell lines transcriptionally down-regulate the pre-B copies of the N-myc gene. Lack of N-myc expression in a number of nonlymphoid cell lines also resulted from lack of N-myc transcription. Together, our findings demonstrate that the down-regulation of N-myc expression in the later stages of B-cell development is mediated primarily at the level of transcriptional initiation. They further show that dominant, trans-acting factors present in more mature B-lineage cell lines act to down-regulate the transcription of N-myc.