High Rate of FLT3 Mutations In Adult ETP-ALL.
Authors: Neumann Martin , Coskun Ebru , Benlasfer Ouidad , Heesch Sandra , Schwartz Stefan , Goekbuget Nicola , Hoelzer Dieter , Hofmann Wolf-Karsten , Thiel Eckhard , Baldus Claudia 
Published: October 2019

Abstract
Abstract 1031

Introduction:
Mutations of FLT3 are recognized as important prognostic factor in acute myeloid leukemia (AML) with an impact on new therapeutic strategies. However, FLT3 mutations are rarely (1-3%) found in acute T-lymphoblastic leukemia (T-ALL). Recently, a new T-ALL subgroup defined by a specific gene expression profile and a characteristic immunophenotype (CD1a-, CD8-, CD5weak with expression of stem cell or myeloid markers) and with poor outcome has been described in pediatric T-ALL patients. This subgroup likely originates from early thymic progenitors (ETP) and has therefore been termed ETP-ALL. To unravel the molecular alterations of this distinct subgroup with myeloid characteristics, we studied adult ETP-ALL patients for FLT3 mutations. Moreover, we analyzed miRNA expression to explore their potential involvement in the specific gene expression signature observed in ETP-ALL.

Patients and methods:
We screened 1241 peripheral blood and bone marrow samples of T-ALL patients that were sent to the central diagnostic reference laboratory of the German Acute Lymphoblastic Leukemia Multicenter Study Group (GMALL). Of the immunophenotypically identified ETP-ALL patients (n=142), sufficient material was available in 70 cases. FLT3 mutations (ITD and TKD) were assessed using the FLT3 mutation assay (InVivoScribe Technologies, San Diego, USA). Expression of miRNAs was first screened by TaqMan low density arrays including 667 miRNAs in ETP-ALL (n=8) and compared to non-ETP T-ALL (n=6). Subsequently, expression of miR-221 and miR-222 was examined by real-time PCR in all 70 ETP-ALL.

Results:
In our cohort of 1241 T-ALL, 142 (11.4 %) patients showed an immunophenotype of an ETP-ALL, which is in the range of reported frequencies in pediatric patients. Among the 70 ETP-ALL samples analyzed, we identified 9 patients with a FLT3 D835 mutation and 15 patients with a FLT3 ITD. In total, 34 % (24 of the 70) patients carried a FLT3 mutation, which represents approximately 4 % of all T-ALL patients. In contrast, only one patient was identified with a FLT3 mutation within an arbitrarily selected control group of 107 T-ALL patients with a non-ETP immunophenotype. Interestingly, ETP-ALL with FLT3 mutations showed a distinct immunophentype compared to ETP-ALL with a wild type FLT3: ETP-ALL with FLT3 mutations had a higher rate of positivity for CD2 (88% vs. 30%, P<0.001), CD117 (83% vs. 28%, P<0.001), and CD13 (100 % vs. 37%, P<0.001). On the other hand, ETP-ALL with wild type FLT3 were characterized by a more frequent positivity of CD5 (54% vs. 4%, P<0.001) and CD33 (54% vs. 4%, P<0.001). Based on these findings a specific immunophenotype may be linked to FLT3 mutated ETP-ALL: in 73% (21/29) of patients with CD2/CD13 positivity a FLT3 mutation was present, compared to only 7% (3/41) of the remaining patients with ETP-ALL. In addition, we generated miRNA expression profiles in ETP-ALL and identified 7 miRNAs that were differentially expressed compared to non-ETP T-ALL. Among these miR-221 and miR-222, which were significantly overexpressed in ETP-ALL, are targeting genes characterizing the ETP-ALL phenotype (e.g. ETS1, KIT). Examination of miR-221 and miR-222 in 70 ETP-ALL revealed their particular overexpression in FLT3 mutated samples (miR-221 2.45-fold, P=0.012; miR-222: 2.05-fold, P=0.008) compared to ETP-ALL with wild type FLT3.

Conclusion:
We identified a yet unreported high frequency of FLT3 mutations in T-ALL, which are nearly exclusively found within the subgroup of ETP-ALL. In this group the rate is comparable high to the rate of FLT3 mutations in AML. Therefore, on the basis of established flow cytometry analysis we identified T-ALL patients that should be now prospectively and routinely screened for FLT3 mutations. Moreover, the distinct miRNA expression pattern may impact on the specific gene expression pattern of ETP-ALL. Thus, patients of this molecular distinct ETP-ALL entity may benefit from tyrosine kinase inhibitors in relapse situations or with presence of minimal residual disease as a bridging therapy to allogeneic stem cell transplantation.

Disclosures:
Baldus: Novartis: Research Funding.