INCREASED EXPRESSION OF C-KIT OR ITS LIGAND STEEL FACTOR IS NOT A COMMON FEATURE OF ADULT ACUTE MYELOID-LEUKEMIA

Cell surface levels of the receptor tyrosine kinase P145(c-kit), the p roduct of the c-kit proto-oncogene, in a panel of 80 primary adult acu te myeloid leukaemia (AML) specimens collected at presentation were qu antitated by immunofluorescence and flow cytometry, and compared with levels on CD34(+) bone marrow cells from normal donors. Receptor level s on AML blast cells were extremely variable and were similar to, or l ess than, those on normal stem and progenitor cells. In general P145(c -kit) expression was higher on cells of immature phenotype (FAB M1 and M2). c-kit mRNA was quantitated by ribonuclease protection assay (RPA ) and was shown to be correlated with cell surface protein expression (r=0.76; P < 0.001). This indicates that ligand-mediated receptor inte rnalisation or other mechanisms of increased protein turnover are not responsible for variations in the level of P145(c-kit) in AML specimen s. Quantitative Southern blotting was used to examine c-kit gene copy number in 25 of these specimens and was found to be normal in all but one. Thus we have found little evidence of over-expression of c-kit in adult AML. mRNA for the c-Kit ligand, Steel Factor (SLF) was also qua ntitated by RPA in these specimens. While SLF message was detectable ( limit of detection similar to 10(4) copies per 10 mu g total RNA; equi valent to 1 copy per 100 cells) in 19% of cases, these specimens in ge neral contained low levels of c-kit mRNA. Thus, an autocrine cycle inv olving c-Kit and SLF does not appear to be a common feature of AML.