| Research Abstract |
Internal tandem duplication (ITD) of the FLT3 gene (FLT3/ITD) is found in approximately 20% of acute myeloid leukemia (AML) cases, and is strongly associated with the leukocytosis and poor prognosis in AML patients. In addition, we newly identified activating mutations of D835 (D835-Mt) within a TK domain of FLT3. This mutation was found in approximately 7% of AML cases. Mutant FLT3-expressing 32D cells proliferated without IL3 or FL, and mutant FLT3 was constitutively activated. In this study, we elucidated the role of the JM domain in the activation of FLT3. Mutant FLT3 with not only ITD but also an elongating or shortening JM domain transformed 32D cells regardless of the tyrosine residues in the JM domain. These mutant FLT3s were constitutively tyrosine phosphorylated and activated signal-transduction molecules such as SHC, MAP kinase and STAT5a. Notably, co-transfection of the truncated FLT3/ITD lacking kinase and C-terminal domains with the Wt-FLT3 into 32D cells resulted in autonomous proliferation. In these cells, truncated FLT3/ITD generated a hetero-complex with Wt-FLT3, and Wt-FLT3 was constitutively tyrosine phosphorylated. These findings indicated that the FLT3 JM domain plays an important role in receptor activation, and that the length-mutated JM domain induces ligandindependent receptor activation but also activates Wt-FLT3 in a trans-manner.Furthermore, 32D cells are known to differentiate into mature neutrophils in response to G-CSF, while FLT3/ITD-expressing did not differentiate into mature neutrophils when treated with G-CSF. However, treatment with PKC modulator and inhibitors for molecular chaperon recovered the ability to differentiate into mature neutrophils by GCSF.
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