The retroviral oncogene, v-fms, was acquired by genetic recombination between a feline leukemia virus (FeLV) and proto- oncogene sequences (c-fms) from normal cat cells. We demonstrated that the c-fms proto-oncogene encodes a receptor for the mononuclear phagocyte colony stimulating factor, CSF-1 (M-CSF). To date, this is the only system in which a hematopoietic growth factor and its receptor have both been cloned and characterized. Expression of c-fms at high levels in macrophages or after retroviral-mediated transfer into cultured fibroblasts does not lead to transformation, whereas v-fms transforms fibroblasts, CSF-1-dependent macrophages, and IL-3- dependent myeloid cell lines. An analysis of mutant c-fms and chimeric v-fms/c-fms genes suggested that two genetic alterations in c-fms are required to fully activate its oncogenic potential: (1) an activating mutation in the body of the gene that renders the receptor tyrosine kinase CSF-1-independent, and (2) elimination of a single C-terminal tyrosine residue (tyr969) that is likely to be a negative regulatory site of receptor phosphorylation. Additional chimeric and mutant receptor molecules will be used to pinpoint the site(s) of activating mutation(s), to identify sites of autophosphorylation, and to define putative target residues for protein kinase C phosphorylation that mediate receptor down modulation in response to phorbol esters. The structure and function of CSF-1 and its ability to transform cells by an autocrine mechanism when cotransfected with the c- fms gene will be studied. The ability of the CSF-1 receptor to program differentiative and proliferative responses will be evaluated after introducing the c-fms gene into committed myeloid precursors in vitro. Parallel approaches will assess the ability of the v-fms and CSF-1 genes to transform early myeloid progenitors in vitro and to contribute to leukemias in vivo after gene transfer into murine hematopoietic stem cells. The human c-fms and CSF-1 genes, both closely linked on human chromosome 5q, will be evaluated for specific rearrangements associated with acute myeloid leukemias. Studies at the genetic level will be complemented by biochemical approaches to identify defects in receptor function affecting CSF-1 induced kinase activity, receptor turnover, and down modulation. Our studies are likely to provide mechanistic information about normal hematopoiesis and to pinpoint defects in growth factor -- receptor interactions that contribute to leukemia.
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