Constitutively active STAT5A and STAT5B in vitro and in vivo: Mutation of STAT5 is not a frequent cause of leukemogenesis

We recently identified several constitutively active forms of signal transd ucers and activators of transcription 5 (STAT5) using polymerase chain reac tion-driven random mutagenesis followed by retrovirus-mediated expression s creening. All constitutively active STAT5 showed constitutive phosphorylati on on their tyrosine residues and induced factor-independent growth in a mo use interleukin-3-dependent cell line, Ba/F3. Sequence analysis of these ac tive STAT5 revealed two important mutations: S710F and N642H. The N642H mut ation localized in the SH2 domain was able to induce autonomous growth of B a/F3 cells by itself, whereas S710F in the effector domain was able to indu ce autonomous growth of Ba/F3 cells in concert with a second mutation inclu ding H298R and E150G. Recently, constitutive activation of STAT5 has been r eported in patients' leukemic cells and is implicated in leukemogenesis. We attempted to clarify whether leukemic cells harbored activating mutations primarily in STAT5 proteins, and analyzed the sequence of STAT5 derived fro m 49 leukemic patients. No mutations were found, however, in the regions su rrounding S710 and N642 of STAT5A and corresponding residues of STAT5B. We also cloned full-length cDNAs for STAT5s from three patients whose leukemic cells exhibited constitutive tyrosine phosphorylation of the STAT5 protein and expressed the derived STAT5 proteins in Ba/F3 cells. However, none of these clones exhibited constitutive tyrosine phosphorylation or gave rise t o FI proliferation of Ba/F3 cells. These results indicate that constitutive activation of STAT5 is a secondary event in most leukemias. (C) 2000 The J apanese Society of Hematology.