Loss of Src-like Adaptor Protein 2 Expression Increases the Transforming Potential of Oncogenic FLT3-ITD

The receptor tyrosine kinase FLT3 is found to be a mutated oncogene in hematological malignancies including acute myeloid leukemia (AML). FLT3 inhibitors in combination with chemotherapy display promising results in a clinical setting, but patients relapse after short-term treatment due to the development of resistant disease. Therefore, targeting signaling proteins downstream of FLT3 can be an alternative approach for the treatment of patients carrying mutant FLT3. Activated FLT3 is constitutively phosphorylated on several tyrosine residues. These tyrosine residues facilitate association of SH2 domain-containing signaling proteins. By using a panel of SH2 domain-containing proteins we identified SLAP2 as a potent interaction partner of FLT3. The interaction in between FLT3 and SLAP2 occurs when FLT3 is activated and an intact SH2 domain of SLAP2 is required for the interaction. SLAP2 associates with FLT3 mainly through its SRC binding sites and expression of SLAP2 inhibited oncogenic FLT3-ITD-mediated cell proliferation and colony formation in vitro, and tumor formation in vivo. By analysis of patient expression data, we found that loss of SLAP2 expression correlates with poor prognosis of AML patients carrying FLT3-ITD. SLAP2 inhibits FLT3-mediated downstream signaling such as activation of AKT, ERK, p38 and STAT5. Inhibition is partially mediated through ubiquitination-mediated degradation of FLT3. Taken together our current study demonstrates that SLAP2 is an important regulator of FLT3-mediated oncogenic signaling and thus modulation of the SLAP2 expression levels can be an alternative approach for the treatment of FLT3-ITD positive malignancies.