STAT3 activation is essential for t(8;21) RUNX1-ETO leukemogenesis.

C68 Two genetic alterations do not always result in positive augmentative effect in tumorgenesis. A negative consequence such as a clonal disappearance by apoptosis or senescence occurs occasionally. Investigation of such positive and negative combinations in a certain tumor identifies an essential mechanistic basis for the tumorgenesis. In the screening of additional genetic changes in t(8;21) leukemia, one of the most frequent types of human leukemia and generating RUNX1-ETO chimeric gene, we and others found that FLT3 is rarely mutated, while c-Kit is frequently mutated, despite the fact that FLT3 is functionally equal to c-Kit since both belong to type III receptor type tyrosine kinase (RTK). To examine these cooperations between RUNX1-ETO and RTK mutation, we carried out experiments using murine immature hematopoietic cell line, 32Dcl3 cell. Transfection of FLT3 mutant alone inhibited differentiation and accelerated proliferation, whereas transfection of the FLT3 mutant along with RUNX1-ETO led to rapid apoptosis in 32D cells. In clear contrast, c-Kit mutant showed a cooperative role with RUNX1-ETO for continued growth, in agreement with human leukemia cases. This synergism was further confirmed in vivo. The mice transplanted with 32D cells carrying RUNX1-ETO and c-Kit mutation developed leukemia, while those carrying RUNX1-ETO and FLT3 mutation did not. Interestingly, this positive cooperation between RUNX1-ETO and c-Kit coincided with phosphorylation of Stat3, whereas the negative combination between RUNX1-ETO and FLT3 mutant resulted in a complete loss of Stat3 activation, although the FLT3 mutant alone retained phosphorylation of Stat3. Therefore, Stat3 activation appears to be essential for RUNX1-ETO leukemogenesis.