FLT3-ITD Mutations in AML: Structural and Numerical Variation of Mutations and Identification of Subgroup with Poor Prognosis

Abstract 4868

FLT3-ITD occurs with a frequency of 35%–45% in normal karyotype AML and has an adverse impact on prognosis. FLT3-ITD mutations are very variable in length and position of the tandem duplication. We have analyzed 211 AML patients with a median age of 54 years (range, 17–75). For all patients, intensive double-induction and consolidation therapy were intended. Patients with a matched related (MRD) or unrelated donor (MUD) were allocated to allogeneic stem cell transplantation (alloSCT). To investigate ITD insertion site and length, as well as their clinical impact in AML patients, we have performed sequencing analysis in diagnostic samples from 46 FT3- ITD–mutated patients. In 39 (87%) patients a single sequence was found, 6 patients displayed 2 and more different mutations. The median size of the inserted sequences was 63 nucleotides (range, 21–203 nucleotides). ITD integration sites were categorized according to functional regions of the FLT3 receptor: juxtamembrane domain (JMD), n =29 (JMD switch motif, n=3; JMD zipper motif, n=20; and JMD hinge region, n=6) and beta1-sheet of the tyrosine kinase domain-1 (TKD1), n=17. In the majority of the cases insertion sites were localized in JMD between amino acid (AA) 593 and 609 (n=20; 43%) and in TKD1 between AAs 610 and 615 (n=17; 37%). Integration site was strongly correlated with the ITD size (p=0.001) and with white blood cells (WBC) count (p=0.05). The prognostic significance of FLT3-ITD size is discussed controversial. Several studies reported conflicting results. We have found that CR durations was significant better (p=0.05) in patients with bigger ITD size, and there were no statistical differences in OS and DFS. There was no significant association between the number of mutations and OS or DFS. Presence of the any type of mutations was more common in M5 (16 of 54, 29%, P 0.01). It was associated with normal karyotype (72% vs 43%), p=0,0001; higher white blood cells (WBC) count (98×10⁹/l vs 21×10⁹/l), p=0,0001; higher BM blasts (85% vs 71%, p=0,0001), de novo AML (91% vs 69%), p=0,0001); and inferior OS (p=0,008) and DFS (p=0,01), when we excluded AML M3 patients. The prevalence of ITD allele on the DNA level was heterogeneous. Based on quantitative analysis, the mutant/wild-type (wt) ratio ranged from 0,1 to 11,5. Patients with a mutant/wt ratio above 0,3 had significantly shorter CR duration (p=0,02), OS (p=0,03), and DFS (p=0,01). Taken together, our data confirm that FLT3 mutation represent a common aberration in adult AML and associated with inferior outcome. A high mutant/wt ratio appears to have a major impact on the prognostic relevance.