INTERNAL TANDEM DUPLICATION OF THE FLT3 GENE IS A NOVEL MODALITY OF ELONGATION MUTATION WHICH CAUSES CONSTITUTIVE ACTIVATION OF THE PRODUCT

An internal tandem duplication (ITD) of the FLT3 gene is found in near ly 20% of acute myeloid leukemia (AML) and 5% of myelodysplastic syndr ome cases. Our serial studies on 51 samples with the FLT3 gene mutatio n indicated that the ITD was frequently (47/51) clustered in the tyros ine-rich stretch from codon 589 to 599 and rarely (3/51) in its downst ream region, both of which are located within the juxtamembrane (JM) d omain. One remaining sample had an insertion into the JM domain of nuc leotides of unknown origin. To eludicate the biological relevance of t he ITD or the insertion, we expressed various types of mutant FLT3 in Cos 7 cells. All mutant FLT3 studied were ligand-independently dimeriz ed and their tyrosine residues were phosphorylated. The Y589 of FLT3 w as essential for the phosphorylation in the wild FLT3, but a Y589F con version did not affect the phosphorylation status of the mutant FLT3. These findings suggest that the elongation of the JM domain rather tha n increase of tyrosine residues causes gain-of-function of FLT3. Thus, ITD is a novel modality of somatic mutation which activates its produ ct. Since the DNA corresponding to codon 593 to 602 potentially forms a palindromic intermediate, we propose that a DNA-replication error mi ght be associated with generating the ITD of the FLT3 gene.