Clinical Relevance of NOTCH1 and FBXW7 Mutations in DCOG and COALL T-ALL Patient Cohorts.
Authors: Zuurbier Linda , Homminga Irene , Buijs-Gladdines Jessica , de Haas Valerie , Horstman Martin , Pieters Rob , Meijerink Jules P.P. 
Published: October 2019

Abstract
Abstract 1603
Poster Board I-629

Introduction
Ten to fifteen percent of all pediatric leukemic cancers are addressed to T-cell acute lymphoblastic leukemia (T-ALL). Despite improved therapy, about 30% of these cases relapse and ultimately die due to the persistent of disease. Many chromosomal aberrations are known to be involved in T-ALL nowadays and there is evidence for some of these factors to be associated with prognosis. Aberrant activation of the NOTCH1 pathway is a frequent phenomenon in T-ALL. NOTCH1 is a transmembrane protein that as a transcription factor promotes self-renewal, proliferation and differentiation. Hyperactivation of NOTCH1 has been linked to the presence of activating mutations in NOTCH1 itself and inactivating mutations in FBXW7. FBXW7 normally targets NOTCH1, among other proteins, for ubiquitin-mediated proteasomal degradation. Several studies investigated the clinical relevance for NOTCH1 and/or FBXW7 mutations, but results are contradictory. We attempted to clarify this by screening two cohorts for these mutations in relation to clinical and molecular cytogenetic parameters.

Patients
We performed a retrospective analysis on 146 pediatric T-ALL patients who were enrolled on Dutch DCOG ALL7, ALL8 or ALL9 protocols(n=72), or the German COALL-97 protocol (n=74). DCOG patients had a median follow-up time of 67 months whereas the median follow-up for COALL patients was 52 months. The DCOG study comprised 51 males and 21 females and the COALL study cohort contained 49 males and 25 females.

Results
We were able to screen for NOTCH1 and FBXW7 mutations in 141 out of 146 pediatric T-ALL patients. NOTCH1 and FBXW7 mutations were observed in 56% (n=79) and 16% (n=23) of the patients, respectively. Together, 63% (n=89) of our pediatric T-ALL patients have a mutation in NOTCH1 and/or FBXW7. Mutations were mainly observed as single mutations in the HD domain of NOTCH1 and less frequently in the closely associated juxtamembrane domain (n=40) or found in combination with PEST domain mutations (n=9) or FBXW7 inactivating mutations (n=13). Isolated PEST domain mutations (n=17) or FBXW7 mutations (n=10) were observed as well. Mutations in FBXW7 were located in the WD40-protein binding domain. A higher incidence of NOTCH1 with or without FBXW7 mutations was observed for TLX3 rearranged T-ALL (p=0.039 and p=0.008, respectively). NOTCH1 mutations w/wo FBXW7 mutations were observed at a lower frequency in TAL or LMO rearranged T-ALL (p=0.004 and p=0.004, correspondingly). NOTCH1 mutations w/wo FBXW7 mutations were less frequently associated with a mature T-cell development status. We did not identify any association with clinical parameters including age, gender and white blood cell count. Like the German BFM study group (see abstract submitted by first author Corinne Kox), we identified a good response for NOTCH1/FBXW7 mutant patients to prednisone in vivo for 34 patients treated on DCOG ALL-7 or ALL-8 protocols (p=0.003). In relation to outcome, NOTCH1 and/or FBXW7 mutated patients demonstrated a trend to poor outcome (DCOG EFS p=0.156 and RFS p=0.101, COALL EFS p=0.903 and RFS p=0.230).

Conclusion
We identified NOTCH1 activating mutations in two third of all pediatric T-ALL patients which enrolled in the DCOG and COALL study. Based on data for selected patients groups, these patients had a significantly better in vivo prednisone response compared to wild type patients. However, NOTCH1 and/or FBXW7 mutations do not show to have a prognostic significance within the DCOG and COALL T-ALL studies.

Disclosures
No relevant conflicts of interest to declare.