NOTCH3: A MORE PROMINENT PLAYER THAN NOTCH1 IN THE PATHOGENESIS OF T-ALL?

Background. T-acute lymphoblastic leukemia (T-ALL) includes several subtypes that correlate with genetic aberrations at different stages of thymocyte differentiation. NOTCH family controls various steps of T cell development; hence dysregulated NOTCH signaling could be involved in the development of T-ALL. NOTCH3 gene was previously shown to be expressed in all T-ALL patients, whereas its expression was reduced in remission; it is associated with the expression of HES1 and the invariant chain of pre-TCR (p-Talpha). Thus, a T-ALL signature characterizes the active or relapsing disease. NOTCH1 was discovered as a partner gene in the rare t(7;9), representing <1% of T-ALLs, yet about 50% of human T-ALLs were noted to harbor activating point mutations in NOTCH1 that lead to aberrant activation of NOTCH signaling, placing it at the center of T-ALL pathogenesis. Other studies reported that NOTCH1 expression was not pathognomonic for T-ALL, as it was detected not only in normal peripheral blood T lymphocytes but also in non-T cell leukemias. More recently NOTCH1 activating mutations were reported as secondary events in T-ALL. Aims. The aim was to study the expression of the main genes involved in the NOTCH pathway, to verify the possible specific role of NOTCH1 versus NOTCH3 in the pathogenesis of T-ALL and to correlate the level of the expression of such genes to other prognostic parameters. Methods. Under informed consent the study was performed on 46 T-ALL patients (38 children/8 adults, 33 males/13 females); 12 cases of precursor BALL and 13 healthy subjects served as control groups. Gene expression was evaluated using Real Time PCR. Results. NOTCH1, NOTCH3, pTalpha and HES1 expression was increased in T-ALL compared to precursor B-ALL (P=0.015-0.044) and healthy subjects (P=0.0001-0.022). Genes expression was higher in children compared to adults; the difference was significant for NOTCH3 (P=0.02) and pTalpha (P=0.005). The genes level in early, intermediate and late T-ALL was examined. Due to the comparable level of gene expression in both intermediate and late TALL they were considered as one group in comparison with early TALL. Gene expression was higher in intermediate and late T-ALL group compared to early T-ALL for all the studied genes (P=0.006-0.016) except for HES1 (P=0.11). A correlation was found between NOTCH1 and NOTCH3 (r=0.508/P=0.0001) and between each of them and pTalpha (r=0.481, P=0.0001 and r=0.871,P=0.0001 respectively). HES1 moderately correlated with NOTCH1 (r=0.48, P=0.021). No association was encountered between genes expression and any of the prognostic parameters: age, TLC, mediastinal involvement, CNS involvement, hepatosplenomegaly, or lymphadenopathy except for a high expression of NOTCH1 in association with hemoglobin level <10 g/dL (P=0.049) and a negative correlation between them (r=-0.476/P=0.003). NOTCH3/NOTCH1 ratio was calculated for the different groups. Both T-ALL and precursor B-ALL showed comparable ratios to each other (P=0.312) while both showed a higher ratio in comparison to healthy subjects (P=0.0001 and 0.013 respectively) Conclusions. Our study confirms a pivotal role of NOTCH pathway in the pathogenesis of T-ALL together with pTalpha and HES1. The higher NOTCH3/NOTCH1 ratio suggests that NOTCH3 dysregulation may play a more central role than NOTCH1 in ALL pathogenesis.