High FLT3 mRNA Expression in Acute Myeloid Leukemia May Be Functionally an Alternative to Mutational Activation of the Receptor.

Mutations of the FLT3 gene are detectable in approximately 30% of all adult AML. These mutations lead to an autoactivation of the receptor inducing increased proliferation of the leukemic clone. An alternative mechanism of FLT3 activation might be mRNA overexpression. In the presented study the FLT3 expression level in 208 adult AML patients and 8 healthy donors was assessed by real time PCR and correlated to several parameters. In all patients cytomorphology, cytogenetics, and FLT3 mutation status was assessed. Significant differences of FLT3-expression levels were found in certain AML subgroups. The highest expression levels were found in FAB subtypes M5 and the lowest in M3. In total, increasing levels were shown in the following order: M3 <M3v <M6 <M2 <M4eo <M4 <M0 <M1 <M5a <M5b. Independent analysis of FLT3 expression in different cytogenetic AML subgroups showed the lowest median in the t(15;17) group, followed by t(8;21), inv(16), normal, complex karyotype and the highest median in the t(11q23) group. No difference was observed between the group of secondary AML following MDS, therapy related AML and the de novo AML (p=0.868, p=0.562, and p=0.570, respectively). Compared to clinical parameters, FLT3 expression correlated with high percentage of bone marrow blasts (p=0.0005) and high leukocyte count (p<0.001). In contrast to previous studies no difference in FLT3 expression levels was detected between AML with (n=74) and without (n=130) any FLT3 mutation. Assessment of FLT3 RNA by microarray analysis and FLT3 receptor surface expression (CD135) detected by flow cytometry correlated significantly with FLT3-expression as assessed by real time PCR (p<0.001, each). To analyze whether high FLT3 expression is a prognostic parameter 118 AML cases with normal karyotype were devided into two groups. Group 1 (n=75) was defined to have less and group 2 (n=43) more than the median of the FLT3 expression level found in the total group. No impact on OS and EFS could be shown (608 vs. 311 days, p=0.1283 and 398 vs. 208 day, p=0.3056). In conclusion, these data support the hypothesis that FLT3 activation through mRNA overexpression is an alternative mechanism to FLT3 mutations. Especially as it was found extremely high in 11q23 AML, that rarely reveal FLT3 mutations.