Cell reprogramming by a novel PAX5 mutant: To B-cells or not to B-cells? – Bcell-ID

The oncogenic alterations that arise in somatic cells lead to a whole reprogramming of the normal cellular fate and open a new pathologic developmental program. Cellular plasticity, reprogramming and cancer initiation are therefore tightly intertwined. The development of acute leukemia is a multistep process characterized by the acquisition of diverse genetic alterations contributing to the clonal transformation of normal hematopoietic progenitor cells. Illustrated by the exceptional situation of pre-leukemic and leukemic twins, it was shown that TEL-AML1 oncogene acts as a first-hit mutation in committed cells to establish a pre-leukemic state in human. Therefore, the development of transgenic mouse models in which are activated specific human oncogenes in specific tissues opens new challenges and has a dual perspective: It helps to understand the early steps of leukemia initiation before malignant transformation. On the other hand, it also allows understanding the cellular and molecular mechanisms by which a primary oncogene reprogram a committed progenitor and perturb the normal hematopoiesis.

The transcription factor PAX5 has been described as the guardian of B-cell identity but is also the main target of somatic alterations in B-acute lymphoblastic leukemia (B-ALL). These different genetic alterations have not the same impact on the leukemic process. While PAX5 deletion seems to be a secondary oncogenic event, PAX5 translocations are considered as primary oncogenes altering normal B-cell development in the early steps of the disease. The general objective of this research proposal is to decipher the cellular and molecular mechanisms by which PAX5 mutant proteins reprogram normal B-cell progenitors and perturb B-cell identity and function. In this purpose, we generated a new genetically engineered mouse model expressing PAX5-ELN, a fusion protein that we previously identified in B-ALL patients. This transgenic mouse model represents a rare and accurate B-ALL modelization that reveals a pre-leukemic phase and recapitulates the key features of the human disease. Therefore, this in vivo model gives us the opportunity to address the deregulation of normal B-cell development by a novel PAX5-fusion protein.

Through a multi-disciplinary approach including genetic mouse models, computational analysis of gene expression profiles, CRISPR-Cas9 technology, chemical high-throughput screening, functional in vitro and in vivo experiments, the present fundamental project aims at i) identifying the functional properties reprogrammed by PAX5-ELN in normal B-cell progenitors, ii) defining the common molecular mechanisms between different PAX5 mutant proteins to control the B-cell reprogramming, iii) developing mechanism-based approaches to reverse B-cell reprogramming induced by PAX5 mutant proteins. Together, this research proposal should provide novel insight on the general biological mechanisms required for normal and pathological B-cell development.