Retroviral enhancer insertion 5' of c-myc in two translocation-negative mouse plasmacytomas upregulates c-myc expression to different extents.

Essentially all murine plasmacytomas have deregulated c-myc expression that is typically brought about by chromosomal translocations between the c-myc/Pvt-1 locus and one of the immunoglobulin loci. ABPC 22 and RFPC 2782 are BALB/c plasmacytomas that lack chromosomal translocations yet have Southern blot evidence of c-myc gene rearrangements. In this report we show that proviral integrations 5' of the c-myc gene can deregulate c-myc expression in mouse plasmacytomas. Analysis of DNA sequences 5' of the c-myc genes from both tumors demonstrated that rearrangements were caused by retroviral integrations 5' of c-myc exon 1. The proviral insertion in RFPC 2782 was associated with a high steady-state c-myc mRNA level comparable to that seen in plasmacytomas with typical translocations. An analogous proviral insertion in ABPC 22 was associated with a c-myc RNA level that was only 38% of that of RFPC 2782. Nuclear run-on studies of c-myc transcription showed that ABPC 22 has both a lower rate of transcription and a greater degree of transcriptional attenuation than RFPC 2782. DNA sequencing of the long terminal repeat of each tumor provirus showed that the ABPC 22 provirus harbors a deletion of one of the two direct repeats in the viral enhancer, whereas both repeats are present in the RFPC 2782 provirus. These data indicate that maximum LTR enhancer effectiveness in plasmacytomas in vivo requires the presence of both LTR direct repeats. The documentation of the low level of steady-state c-myc mRNA in ABPC 22 supports the notion that deregulated c-myc expression, even at low steady state levels, is effective in supporting the development of plasmacytomas.