Myb overexpression synergizes with loss of Pten and is a dependency factor and therapeutic target in T-cell acute lymphoblastic leukemia

T-lineage acute lymphoblastic leukemia (T-ALL) is an aggressive hematological malignancy that accounts for 10%–15% of pediatric and 25% of adult ALL cases. Although the prognosis of T-ALL has improved over time, the outcome of T- ALL patients with primary resistant or relapsed leukemia remains poor. Therefore, further progress in the treatment of T-ALL requires a better understanding of its biology, and the development of more effective precision oncologic therapies. The proto-oncogene MYB is highly expressed in diverse hematologic malignancies, including T-ALLs with genomic aberrations that further potentiate its expression and activity. Previous studies have associated MYB with a malignant role in pathogenesis of several cancers. However, its role in the induction and maintenance of T-ALL remains relatively poorly understood. In this study, we found that increased copy number of MYB is associated with higher MYB levels, and might be associated with inferior event-free survival of pediatric T-ALL patients. Using our previously described conditional Myb overexpression mice, we generated two distinct MYB-driven T-ALL mouse models. We demonstrated that the overexpression of Myb synergizes with Pten deletion, but not with the overexpression of Lmo2, to accelerate the development of T-cell lymphoblastic leukemias. We also showed that MYB is a dependency factor in T-ALL, since RNA interference of Myb blocked cell cycle progression and induced apoptosis in both human and murine T-ALL cell lines. Finally, we provide preclinical evidence that targeting the transcriptional activity of MYB can be a useful therapeutic strategy for the treatment of T-ALL. To investigate the role of MYB in tumor maintenance, we performed doxycycline-inducible knockdown of Myb in vitro in two primary murine T-ALL cell line MyPL1. We crossed the conditional R26-Myb model with the spontaneous T-ALL Pten/Lck-Cre mouse model, to obtain R26-Myb/Pten/LckCre mice, that overexpress Myb and have Pten loss, specifically in the T-cell compartment. We derived a primary murine T-ALL line from the obtained tumors from these R26-Myb/Pten/LckCre mice. This cell line was named MyPL1. Next, we transduced MyPL1 with an lentiviral vector that contains an inducuble hairpin against Myb and an eGFP reporter. After transduction and GFP sorting, the parental cell line iMyb MyPL1 was established. We performed gene expression profiling analysis (RNA-seq) upon knockdown of Myb in the primary murine T-ALL cell line iMyb MyPL1 by treating it for four days with 1 µg/mL of doxycycline (DOX) or not (control). Samples were collected at day 1, 2, 3 and 4. Four replicates were used per condition.