DOCK2 Interacts with FLT3 and Modulates the Survival of FLT3-Expressing Leukemia Cells

The FMS-like tyrosine kinase-3 (FLT3) receptor gene is the most commonly mutated gene in acute myeloid leukemia (AML), and patients carrying FLT3/ITD mutations have a poor prognosis. Despite continuing progress in the development of more effective FLT3 inhibitors, long-term success in inhibition of FLT3 activity in AML patients is still elusive. In order to achieve a better understanding of FLT3 biology and more effective strategies for the inhibition of FLT3 activity, a screen was performed on leukemia cell lines to search for FLT3-interacting proteins. One of the proteins identified in the screen was dedicator of cytokinesis 2 (DOCK2).

The DOCK family of proteins acts as guanine nucleotide exchange factors (GEFs) for Rho family of GTPases, which includes Rac GTPases. DOCK2 expression is limited to hematopoietic cells, and is known to regulate several crucial processes, including lymphocyte migration, activation and differentiation of T cells, and cell-cell adhesion and bone marrow homing of various immune cells.

We first verified that DOCK2 is expressed in primary AML samples from patients, and co-immunoprecipitation experiments showed that DOCK2 interacts with both wild-type FLT3 and FTL3/ITD in these cells. Co-immunoprecipitation experiments using leukemia cell lines demonstrated that DOCK2 interacts with FLT3, FLT3/ITD, FLT3/D835Y and FLT3/D835H, and that it predominantly interacts with the unphosphorylated form of FLT3. Knock-down of DOCK2 by shRNA did not significantly affect the growth of cell lines that lack expression of FLT3, but greatly reduced growth of cell lines expressing amplified wild type FLT3 (Sem K2), FLT3/D835H (HB11;19) and FLT3/ITD (MV4;11). Accordingly, colony formation assays revealed that cell lines with elevated expression of wild type or mutant FLT3 produced fewer, smaller and more compact colonies when the expression of DOCK2 was decreased, while colonies from cell lines lacking FLT3 expression showed no significant difference in response to the knock-down of DOCK2. Furthermore, an Annexin V binding assay indicated that reduction in DOCK2 expression level greatly sensitized cells with elevated FLT3 activity (MV4;11 and Sem K2) to cytarabine, resulting in increased apoptosis, but no significant sensitization was observed in cell lines that lack FLT3 expression.

These findings demonstrate that DOCK2 interacts with FLT3 in leukemia cell lines, and suggest that this interaction has important roles in regulating the survival of leukemia cells with elevated FLT3 activity, both alone and in combination with conventional anti-leukemic agents. Therefore, DOCK2 is a potential therapeutic target for AML treatment, and better understanding of the interaction between DOCK2 and FLT3 may contribute to the development of novel strategies to effectively inhibit FLT3 activity in AML patients.