Prevalence and Prognostic Significance of FLT3 Mutations in Acute Myeloid Leukemia: Association of ITDs with Poor Outcome in Patients with Normal Cytogenetics.

Acute Myeloid Leukemia (AML) is a difficult disease to treat, and better treatments are needed. Molecular targeted therapy represents a novel therapeutic approach.

Activating mutations of FMS-like tyrosine kinase 3 (FLT3) are present in approximately one third of patients with de novo AML and have been implicated in its pathogenesis. The leukemic blasts of most AML patients have the internal tandem duplications (ITDs) in the juxtamembrane region or point mutations in Asp835 and Iso836 codons in the activation loop of the kinase domain (TKD) of the FLT3 receptor. Both mutations result in constitutive FLT3 receptor activity and may play a significant role in leukemogenesis.

In this study we have analyzed the incidence and type of FLT3 mutations in a large series of newly diagnosed AML patients. Furthermore, we have evaluated the prognostic impact of FLT3 mutations.

The FLT3/ITD was determined by polymerase chain reaction (PCR). The mutations of D835 and I836 codons were determined by PCR followed by restriction enzyme digestion (PCR-RFLP). For the estimation of the statistic significance of the differences in the clinical-biological characteristics, between the mutated patients and wild-type patients, it has been used the Student’s test t for independent data. The probabilities of overall survival (OS) and disease free survival (DFS) were analysed by Kaplan-Meier method; the differences of OS and DFS, between the mutated patients and wild-type patients, were assessed using the log-rank test.

Both FLT3/ITD and FLT3/TKD mutations were found in 15%. Dual mutations were found in 2% of 126 patients. Among the FAB subtypes of AML, the rate of FLT3 aberrations was higher in M4 (27%) and M5 (26%). FLT3/ITD was associated to leukocytosis (106.8x10(e)9/l vs 30x10(e)9/l in FLT3-wt, p=0.015) and high percentage of circulating blast cells (82% vs 42% in FLT3-wt, p<0.0001). Differently, FLT3/TKD mutations were not associated with high white blood cells count and blast cells percentage.

FLT3 mutations were more prevalent in patients with normal karyotype (51%). In this group, DFS and OS were significantly inferior for patients with FLT3/ITD than patients withouth mutations (0 vs 5, p=0.0032; 5 vs 9, p=0.049, respectively). We have identified the FLT3/ITD as an independent poor prognostic factor in AML patients with normal cytogenetics. Therefore, targeting FLT3 mutations represents a potential therapeutic target for AML. These results suggest that new treatment modalities, such as therapy with a FLT3 tyrosine kinase inhibitor, are clearly needed for this group of patients with “standard risk” profile.