Repression of Gadd45alpha by activated FLT3 and GM-CSF receptor mutants contributes to growth, survival and blocked differentiation

The tumor suppressor Gadd45 alpha was earlier shown to be a repressed target of sustained receptor-mediated ERK1/2 signaling. We have identified Gadd45 alpha as a downregulated gene in response to constitutive signaling from two FLT3 mutants (FLT3-ITD and FLT3-TKD) commonly found in AML, and a leukemogenic GM-CSF receptor trans-membrane mutant (GMR-V449E). GADD45A mRNA downregulation is also associated with FLT3-ITD+ AML. Sustained ERK1/2 signaling contributes significantly to receptor-mediated downregulation of Gadd45 alpha mRNA in FDB1 cells expressing activated receptor mutants, and in the FLT3-ITD+ cell line MV4;11. Knockdown of Gadd45 alpha with shRNA led to increased growth and survival of FDB1 cells and enforced expression of Gadd45 alpha in FDB1 cells expressing FLT3-ITD or GMR-V449E resulted in reduced growth and viability. Gadd45 alpha overexpression in FLT3-ITD+ AML cell lines also resulted in reduced growth associated with increased apoptosis and G(1)/S cell cycle arrest. Overexpression of Gadd45 alpha in FDB1 cells expressing GMR-V449E was sufficient to induce changes associated with myeloid differentiation suggesting Gadd45 alpha downregulation contributes to the maintenance of receptor-induced myeloid differentiation block. Thus, we show that ERK1/2-mediated downregulation of Gadd45 alpha by sustained receptor signaling contributes to growth, survival and arrested differentiation in AML