Association Of FLT3 Mutations With Unfavorable Factors In AML and APL Patients

Mutations in the fms-like tyrosine kinase 3 (FLT3) gene have been considered to predict a poor prognosis in acute myeloid leukemia (AML). Internal tandem duplication (ITD) of the FLT3 is one of the most frequent mutations activating aberrant signal-transduction in AML. The impact of D835 point mutations in tyrosine kinase domain (TKD) on prognosis is less clear.

Characterization of AML and APL (acute promyelocytic leukemia) patients with FLT3 mutations.

FLT3 status in bone marrow samples of adult patients (pts) with AML (63 pts) (m\28; f\35; median 53 yrs, 19-73 yrs) and APL (14 pts) (m\3; f\11; median 43 yrs, 25-62 yrs) at diagnosis was examined. Genomic PCR amplification of ITD and TKD containing regions of the FLT3 gene was performed. The TKD amplicon product was restricted with the EcoRV enzyme. Results were detected by gel electrophoresis.

FLT3\ITD was found in 8 AML pts (12.7%) and in 5 APL pts (35.7%), p=0.053 (Fisher Exact test). FLT3\TKD was found in 4 AML pts (6.4%) and in 1 APL pt (7.1%).

In AML group there were 1 pt with t(6;9) in FLT3\ITD+ subgroup and 1 pt with inv16 in FLT3\TKD+. There were 1 pt with t(6;11), 1 with dupMLL, 3 with t(8;21), 7 with inv16 in AML pts without FLT3 mutations (FLT3-). There were no significant differences in sex, age, WT1 expression at diagnosis between AML pts with FLT3\ITD, FLT3\TKD and without FLT3 mutations. But the presence of FLT3 mutations was related to high peripheral white blood cell (WBC) count in AML pts at diagnosis. The median value (m) of WBC was 49.2 x10⁹\L (range 2.75-191) in FLT3\ITD+; m= 81 x10⁹\L (range 7.2-153) in FLT3\TKD+; m= 5.4 x10⁹\L (range 0.2-230) in FLT3-. The differences are statistically significant between subgroups with FLT3\ITD+ and FLT3- (p<0.05, Mann–Whitney U test); FLT3\ITD+TKD and FLT3- (p<0.01, Mann–Whitney U test). In FLT3\ITD+TKD subgroup 66.7% of pts had WBC over 30 x10⁹\L, whereas in subgroup without FLT3 mutations only 23.4% of pts had the same unfavorable factor, p=0.012 (Fisher Exact test).

In APL group the t(15;17) translocation was detected in 11 pts. 3 pts were without PML\RARa, NPM1\RARa, PLZF\RARa fusions. One FLT3\TKD+ pt had bcr1 PML breakpoint. In FLT3\ITD+ subgroup 4 pts (80%) had bcr3 PML breakpoint, whereas in FLT3- subgroup only 1 pt (12.5%) had bcr3, p=0.032 (Fisher Exact test). In FLT3\ITD+ subgroup all pts had WBC >10 x10⁹\L; moreover, 3 pts (60%) had two unfavorable factors in combination (WBC >10 x10⁹\L plus platelet count < 40 x10⁹\L) at diagnosis, and in FLT3- subgroup nobody had the same combination, p=0.035, (Fisher Exact test). Out of 5 pts with FLT3\ITD 2 ones had death in induction, 1 pt had late molecular remission, 1 had molecular persistence and 1 pt without RARa fusion had failure in WT1 reduction after induction (under 2 log). One FLT3\TKD+ pt had the early relapse.

The results confirm previous reports. FLT3\ITD mutations are seen more frequently in APL than in other subtypes of AML. FLT3 mutations in AML have association with such unfavorable prognostic factor as high WBC count at diagnosis. FLT3\ITD in APL associates with bcr3 and with combination of high WBC count plus low platelet count. The presence of FLT3 mutations is related to unfavorable events in APL pts.

No relevant conflicts of interest to declare.