Detection of RAS, KIT and FLT3 Gene Mutations in t(8;21)-Positive Acute Myeloid Leukemia (AML): Evaluation of the Clinical Relevance.

Acute myeloid leukemia (AML) with translocation t(8;21)(q22;q22) creating the AML1-ETO fusion gene is a distinct type of AML generally associated with a favorable prognosis. However, a significant proportion of these patients (pts.) relapse and survival after 5 years is approximately 50%. These findings together with studies from murine models suggest that additional genetic lesions are underlying the clinical heterogeneity of t(8;21)-positive AML. The recently described mutations in the signaling molecules FLT3, KIT and RAS represent potential secondary genetic lesions that might contribute to leukemic transformation through their constitutive activation.

In this study we determined the incidence of KIT (exons 8, 10, 11, and 17), FLT3 (internal tandem duplications, ITD; tyrosine kinase mutations at D835/I836, TKD) and RAS (NRAS and KRAS exon1 and exon2) mutations in 65 adult pts. (median age 49 years) with t(8;21)-positive AML and evaluated their prognostic impact on clinical outcome.

KIT and RAS mutation screening was performed using a sensitive DHPLC-based assay; samples with abnormal profile were confirmed by direct sequencing. FLT3 mutations were identified as previously described (Fröhling et al., Blood 2002). Pts. were entered on 3 AMLSG treatment trials [AML HD93, AML HD98A, AMLSG 07–04]. Postremission therapy was high-dose cytarabine-based (Ara-C) in all trials.

Mutations were identified in 51% of the t(8;21)-positive AML, with highest frequencies in NRAS (18%) followed by KIT (17%) and FLT3 (ITD; 9%;); 10/11 KIT mutations affected exon17. FLT3-TKD and KRAS mutations were detected in 3% and 1.5%, respectively. Complete remission (CR) rate was 88% for the entire group. In univariable analyses, KIT exon 17 mutated pts. had a significantly inferior event-free (EFS) (p=0.04), overall (OS) (p=0.05), but not relapse-free survival (RFS) (p=0.43). In addition, the presence of FLT3-ITD mutations was in trend associated with a shorter EFS (p=0.09) and OS (p=0.07), and significantly with a shorter RFS (p=0.04). For the other mutations there was no significant or in trend difference in EFS, OS and RFS when comparing mutated and unmutated pts. Multivariable analysis for OS and RFS revealed FLT3-ITD (HR 3.16, p=0.04) and KIT exon17 mutations (HR 2.89, p=0.03) as adverse prognostic factors for OS, whereas only FLT3-ITD showed a significant prognostic impact on RFS (HR 3.32, p=0.04). Since FLT3-ITD and KIT exon 17 mutations both represent potential targets for tyrosine kinase inhibitors, an explorative analysis combining both gene mutations versus unmutated patients was performed revealing a significant inferior RFS (p=0.04) and OS (p=0.006).

Mutations in the genes KIT, FLT3 or RAS were detected in 51% of t(8;21)-positive AML. Univariable and multivariable analyses showed a significant or in trend negative prognostic impact for FLT3 and KIT mutations on clinical outcome. Thus, information on the mutation status might reach clinical importance for identifying pts. who are eligible for molecular targeted therapies.