Integration of the BALB/c ecotropic provirus into the colony-stimulating factor-1 growth factor locus in a myc retrovirus-induced murine monocyte tumor.

The development of tumors is thought to be a multistage process that requires an unknown number of genetic or epigenetic changes in a single cell. We previously described a murine monocyte tumor which was induced by a helper-free c-myc retrovirus and which also contained a DNA rearrangement at the colony-stimulating factor-1 (CSF-1) locus. The CSF-1 gene rearrangement gave rise to high levels of growth factor production and autocrine growth, implicating this secondary event in tumorigenesis. This CSF-1 gene rearrangement was found to be the result of integration of the BALB/c ecotropic retrovirus. Restriction enzyme mapping and DNA sequence analysis demonstrated that the novel provirus is identical to the BALB/c endogenous ecotropic provirus, indicating that infection was probably not due to the creation of a recombinant virus in vivo. The proviral integration site was mapped 3 kilobases 5' of the CSF-1 promoter and in an opposite transcriptional orientation, indicating that activation of CSF-1 expression was the result of the presence of the retroviral enhancer element.