IgH enhancer‐mediated deregulation of N‐myc gene expression in transgenic mice: generation of lymphoid neoplasias that lack c‐myc expression.

We have generated transgenic mouse lines that carry one of three different constructs in which the murine N‐myc gene is expressed under the control of the immunoglobulin heavy chain transcriptional enhancer element (E mu‐N‐myc genes). High‐level expression of the E mu‐N‐myc transgenes occurred in lymphoid tissues; correspondingly, many of these E mu‐N‐myc lines reproducibly developed pre‐B‐ and B‐lymphoid malignancies. The E mu‐N‐myc transgene also appeared to participate in the generation of a T cell malignancy that developed in one E mu‐N‐myc mouse. These tumors and cell lines adapted from them expressed exceptionally high levels of the E mu‐N‐myc transgene; the levels were comparable to those observed in human neuroblastomas with highly amplified N‐myc genes. In contrast, all of the E mu‐N‐myc cell lines had exceptionally low or undetectable levels of the c‐myc RNA sequences, consistent with the possibility that high‐level N‐myc expression can participate in the negative ‘cross‐regulation’ of c‐myc gene expression. Our findings demonstrate that deregulated expression of the N‐myc gene has potent oncogenic potential within the B‐lymphoid lineage despite the fact that the N‐myc gene has never been implicated in naturally occurring B‐lymphoid malignancies. Our results also are discussed in the context of differential myc gene activity in normal and transformed cells.