Modeling PAX5P80R mutation reveals Hif2a as a common oncogenic relay in B-cell acute lymphoblastic leukemia

The transcription factor PAX5 is a major target of genetic alterations in human B-cell precursor acute lymphoblastic leukemia (B-ALL). Among the alterations, the P80R mutation affecting the DNA-binding domain represents the most frequent PAX5 point mutation in B-ALL. In contrast to other somatic PAX5 mutations, PAX5P80R defines a distinct B-ALL subtype characterized by a unique transcriptional program. Here, we aimed to develop a model to elucidate the mechanism by which PAX5P80R perturbs normal B-cell differentiation and the oncogenic relays involved in PAX5P80R-driven malignant progression. A retroviral complementation approach of Pax5-deficient murine fetal liver cells demonstrated at the functional and molecular levels that PAX5P80R failed to rescue definitive B-cell commitment but maintained the repression of T-cell development. Moreover, PAX5P80R eventually led to clonal B-ALL transformation after transplantation through the acquisition of secondary mutations in genes involved in the JAK/STAT and RAS/MAPK pathways. Finally, transcriptomic analyses combined with pharmacological investigation revealed ectopic activation of HIF2a as a common feature of B-ALL and identified acriflavine as a potent drug against B-ALL. Hence, this study provides a strategy to model the multistep process of B-ALL and sheds light on the biological mechanism by which PAX5P80R mutation leads to leukemia. Overall design: To characteize PAX5P80R-induced B-ALLs in vivo, we compared by RNA-sequencing the gene expression profiles of purified PAX5 P80R GFP+ B-ALL cells from four primary recipients (#1300, #1302, #1301, #1308) with purified CLP/pre-pro-B cells from Pax5-/- FLs (n=4) and with purified pro-B/pre-BI cells from Pax5+/+ FLs (n=4)