FLT3/ITD associated with an immature immunophenotype in PML-RARα leukemia

Acute promyelocytic leukemia (APL) is characterized by the specific PML-RARα fusion gene resulting from translocation t(15;17)(q22;q12). Internal tandem duplication (ITD) of the FLT3 gene has been observed in about 35% of APLs, and large-scale studies have identified the presence of ITD as an adverse prognostic factor for AML patients. Aberrant expressions of surface antigens such as CD2, CD34, and CD56 have been found in APL, but their implication is not well understood. We investigated the incidence of the FLT3/ITD mutation and FLT3/D835 (I836) point mutation in 25 APL patients. Incidence ratios of FLT3/ITD, D835 (I836), and both FLT3/ITD and D835 (I836) were 36, 36, and 8%, respectively. FLT3/ITD+ cases showed a predominance of the bcr3 isoform (p=0.008) and M3v morphology (p<0.001). We found that all FLT3/ITD+ cases expressed CD2 (9/9) more frequently than that of FLT3/ITD (1/16) (p<0.001), while only one of the CD2+ cases (1/10, 10%) did not harbor FLT3/ITD, and all CD2+CD34+ cases (5/5) harbored FLT3/ITD. In addition, quantitative PCR analysis showed that FLT3 mRNA was more abundantly expressed in FLT3/ITD+ than that in FLT3/ITD (p=0.025), while there was no difference between D835(I836)+ and D835(I836) with regards to aberrant surface-antigen expression, expression levels of FLT3 mRNA, M3v morphology, and the bcr3 isoform of PML-RARα mRΝΑ. Here, our study demonstrates that the presence of FLT3/ITD, but not D835 (I836), is closely related to aberrant CD2 expression and high expression levels of FLT3 mRNA. Our findings also suggest that FLT3/ITD as a secondary genetic event may block differentiation at the immature stage of APL.