Novel FLT3 Mutation Shows Dominant Negative Effects on the Wild-Type FLT3 Receptor

Background

The FMS-like tyrosine kinase-3 (FLT3) gene encodes for a receptor tyrosine kinase playing an important role in hematopoiesis. In acute leukemias it is one of the most frequently mutated genes. In this study we functionally characterized a novel frameshift deletion mutation of FLT3 found in a relapsed patient with acute myeloid leukemia (AML). The frameshift leads to a premature stop codon resulting in a truncated form of the receptor lacking most of the intracellular domains.

Material and Methods

FLT3 cDNA was expressed in the IL-3 dependent pro-B cell line Ba/F3 via a retroviral expression vector. The transduced cell lines were sorted by fluorescence-activated cell sorting (FACS). Stable expression of the receptor was confirmed on mRNA level by polymerase chain reaction (PCR) and on protein level by western blotting and cell surface expression of the receptor by flow cytometry. Cell proliferation assays were performed in presence or absence of IL-3 or FLT3-ligand. By western blotting receptor activation and its downstream signaling were analyzed. Ligand-binding of the receptor was shown via biotinylated and fluorescent FLT3-ligand-receptor-complexes by flow cytometry.

Results and Discussion

Stable expression of FLT3 Q569Vstop does not enable Ba/F3 cells to grow IL-3-independent. FLT3-ligand can still be bound by the mutated receptor but is not able to stimulate receptors´ signaling and growth of the cells. Furthermore coexpression of wild-type (WT) and mutant FLT3 receptor also abolishes the ability to stimulate the WT receptor with its ligand. This is confirmed by analyzing downstream signaling in the cells as MAPK is less phosphorylated in the FLT3-WT/Q569Vstop coexpressing cells than in FLT3-WT expressing cells alone.

Conclusion

Most of the FLT3 mutations are activating mutations leading to a constitutive activation of the receptor and ligand-independent growth. In our study we characterized a novel FLT3 mutation found in an AML patient which has not been described before. The resulting truncated receptor is still integrated in the plasma membrane and binds its ligand but its ability to be fully activated is completely lost. Furthermore coexpressed with a FLT3-WT receptor, it even prevents stimulation and activation of the WT receptor, thus acting in a dominant negative manner. How the truncated form of the receptor contributes to progression of acute leukemia is of great interest and will be further investigated.