Expression of the Hematopoietic Growth Factor Receptor FLT3 (STK-UFIk2) in Human Leukemias

Normal expression of the hematopoietic growth factor receptor FLT3 (STK-lIIFIkZ) is limited t o CD34+ stem/progenitor cells. We have evaluated the expression of FLTB by RNase protection assay and Western blotting in 161 primary bone marrow (BM) samples from patients with leukemia. FLTB RNA was found t o be expressed at a higher level than in normal BM controls in 33 of 33 B-lineage acute leukemias, 11 of 12 acute myeloid leukemias (AMLs), and 3 of 11 T-cell acute leukemias (T-ALLs). Expression of FLT3 RNA was also observed in some cases of blast crisis CML. The FLT3 signal resulted from expression on the leukemic blasts, and was not caused by increased FLT3 expression on normal CD34+ stem/progenitor cells in the leukemic samples. To determine if FLTB protein was also overexpressed, proteins were ex-