PAX5 is a well-known haploinsufficient tumor suppressor gene in human B-cell precursor acute lymphoblastic leukemia (B-ALL) and is involved in various chromosomal translocations that fuse a part of PAX5 with other partners. However, the role of PAX5 fusion proteins in B-ALL initiation and transformation is ill-known. We previously reported a new recurrent t(7;9)(q11;p13) chromosomal translocation in human B-ALL that juxtaposed PAX5 to the coding sequence of elastin (ELN). To study the function of the resulting PAX5-ELN fusion protein in B-ALL development, we generated a knockin mouse model in which the PAX5-ELN transgene is expressed specifically in B cells. PAX5-ELN–expressing mice efficiently developed B-ALL with an incidence of 80%. Leukemic transformation was associated with recurrent secondary mutations on Ptpn11, Kras, Pax5, and Jak3 genes affecting key signaling pathways required for cell proliferation. Our functional studies demonstrate that PAX5-ELN affected B-cell development in vitro and in vivo featuring an aberrant expansion of the pro-B cell compartment at the preleukemic stage. Finally, our molecular and computational approaches identified PAX5-ELN–regulated gene candidates that establish the molecular bases of the preleukemic state to drive B-ALL initiation. Hence, our study provides a new in vivo model of human B-ALL and strongly implicates PAX5 fusion proteins as potent oncoproteins in leukemia development.

In recent years, through whole genome analyses, convincing evidence for the contribution of genetic predisposition to childhood B-cell precursor - acute lymphoblastic leukemia (BCP-ALL) due to altered PAX5 has been provided. A recurrent mutation p.Gly183Ser affecting the octapeptide domain has been described in three unrelated families and a p.Arg38His mutation affecting the DNA-binding paired domain reported in another one. We strengthen here the assumption of the inherited character of familial BCP-ALL by identifying the PAX5 p.Arg38His mutation in a family in which the three children developed BCP-ALL. One relapsed two years after his initial diagnosis and was allografted with his brother's cells before the latter developed BCP-ALL. The patient allografted relapsed later from donor-related cells. By syngeneic transplantations in mice, we showed that p.Arg38His expression does not abrogate the engraftment capacity of transduced Pax5-/- pro-B cells unlike wild type PAX5-rescued Pax5-/- pro-B cells and can predispose to BCP-ALL. Through functional and molecular analyses, we demonstrated that p.Arg38His acts as a hypomorphic variant altering the pattern of expression of PAX5 target genes. Our data highlight the importance of transcriptional deregulation, particularly of genes involved in B cell differentiation in familial BCP-ALL. We demonstrated that inherited genetic basis of susceptibility to BCP-ALL has been underestimated and should be considered before any familial allograft.