Frequent c-fms activation by proviral insertion in mouse myeloblastic leukaemias

Retroviruses lacking oncogenes can induce tumours in animals, and the tumour cells are frequently found to contain proviral DNA inserted next to a proto-oncogene, which is thus placed under the regulatory control of the retroviral long terminal repeat (LTR). This altered regulation leads to overexpression of the protooncogene, which presumably contributes to the growth properties of the tumour cells 1 , fim -2 has been described as a retroviral integration site frequently and specifically involved in murine myeloblastic leukaemias induced in vivo or in vitro by the replication-competent Friend murine leukaemia virus (F-MuLV) 2 . Here we report that fim -2 spans the 5′-end of the murine proto-oncogene c- fms , known to code for a transmembrane glycoprotein with tyrosine kinase activity probably identical to the receptor of the haemopoietic growth factor, monocyte-macrophage colony-stimulating factor (M-CSF or CSF-1) 3 . Proviral integration in the fim -2 region results in a high expression of a normal sized c- fms messenger RNA. We also observe that some tumours have lost the fim -2/c- fms germ line allele. These results provide the first evidence for the presumed involvement of c- fms in myelomonocytic leukaemias.