Because mutations in receptor tyrosine kinases may contribute to cellular transformation, studies were undertaken to examine c-kit in human leukemia. Isoforms of c-kit have been characterized in the human megakaryoblastic leukemia cell line M-07. Deletion of the four amino acids Gly-Asn-Asn-Lys in the extracellular domain represents an alternatively spliced isoform that has been shown by others, in mice, to be associated with constitutive receptor autophosphorylation (Reith et al, EMBO J 10:2451, 1991). Additional isoforms differ in the inclusion or exclusion of a serine residue in the interkinase domain, a region that contains the binding site for phosphatidylinositol 3- kinase. By RNase protection analysis, we have shown coexpression of the Gly-Asn-Asn-Lys+ and Gly-Asn-Asn-Lys- isoforms, with dominance of the Gly-Asn-Asn-Lys- transcript, in normal human bone marrow, normal melanocytes, a range of tumor cell lines, and the blasts of 23 patients with acute myeloid leukemia. Analysis of transcripts for the Ser+ and Ser- isoforms also showed coexpression in all normal and leukemic cells examined. The ratios of isoform expression for both the Gly-Asn-Asn-Lys and Ser variants were relatively constant, providing no evidence in the tumors examined that upregulation of one isoform contributes to the neoplastic process.

We used functional expression cloning in mammalian cells to identify a cDNA clone encoding a hematopoietic growth factor that is mitogenic for the factor-dependent human megakaryoblastic leukemic cell line, MO7E. Analysis of the sequence of this cDNA revealed striking similarity to that of a recently reported novel murine growth factor for helper T- cell clones designated T-cell growth factor P40. The mRNA for the human P40 protein is expressed by several different human T-cell lines and by mitogen-stimulated peripheral blood lymphocytes. The recombinant protein displays substantial size heterogeneity typical of other glycoprotein cytokines. These properties plus the observation that this cytokine may well act within both the lymphoid and myeloid lineages warrant the designation of P40 as interleukin-9.