RAS, KIT and FLT3 Gene Mutations in inv(16)/t(16;16)-Positive Acute Myeloid Leukemia (AML): Incidence and Relevance on Clinical Outcome.

Inversion or translocation of chromosome 16 inv(16)/t(16;16) [hereafter abbreviated inv(16)] represent common cytogenetic abnormalities in adult acute myeloid leukemia (AML). At the molecular level inv(16) result in the generation of the CBFb-MYH11 fusion protein that is known to interfere with the heterodimeric transcription factor RUNX1/CBFb and thereby contributes to impaired differentiation of hematopoietic cells. Patients (pts.) with inv(16) are considered to have a favorable outcome, in particular when treated with cytarabine-based consolidation regimens. However, a significant proportion of these pts. relapse and survival after 5 years is about 60%. These findings together with studies from murine models suggest that additional genetic lesions are underlying the clinical heterogeneity of inv(16)-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 constitutive activation.

In this study we determined the incidence of KIT (exons 8, 10, 11, and 17), FLT3 (ITD; TKD at D835/I836,) and RAS (NRAS/KRAS exon1, exon2) mutations in 94 adult AML pts. (16 to 60 years; median age 41 years) with inv(16) 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. Pts. were entered on 3 AMLSG treatment trials [AML HD93, AML HD98A, AMLSG 07–04]. Postremission therapy implied cytarabine-based (HiDAC n=57) regimens as well as autologous (n=23) or allogeneic (n=13) stem cell transplantation (SCT) in first CR.

Mutations were identified in 84% of inv(16) AML with highest frequencies in NRAS (47%) followed by KIT (26%) and FLT3-TKD (15%); 10/24 KIT mutations affected exon17. KRAS and FLT3-ITD mutations were detected in 10% and 3%, respectively. Complete remission (CR) rate was 90% for the whole group. In univariable analyses, FLT3-TKD mutations were associated with a significant inferior relapse-free survival (RFS) (p=0.01). For the other mutations there was no significant difference in RFS when comparing mutated and unmutated pts. Multivariable analysis adjusted for postremission therapy revealed FLT3-TKD (HR 2.39, p=0.04) and in trend KIT exon17 mutations (HR 2.8, p=0.06) as adverse prognostic factors. Therefore, an explorative subgroup analysis was performed for KIT exon17 mutations for the different postremission strategies. In pts. treated with HiDAC, KIT exon17 mutations were associated with a significant inferior RFS (p<0.0001), in contrast to pts. receiving SCT (p=0.70). For overall survival (OS) none of the tested variables were significantly associated with prognosis.

KIT, FLT3, or RAS gene mutations can be detected in 84% of inv(16)-positive AML further sustaining the model of cooperating gene mutations. Although the numbers are still quite small, FLT3-TKD and KIT exon17 mutations are of prognostic relevance; the prognostic impact of KIT exon17 mutations seems to be abrogated by SCT strategies. Thus, KIT and FLT3 mutation status might reach clinical importance with regard to the availability of specific inhibitors and the type of postremission therapy.