PAX5-ELN oncoprotein promotes multistep B-cell acute lymphoblastic leukemia in mice.

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.

Combined deficiency of MSH2 and Sµ region abolishes class switch recombination.

Class switch recombination (CSR) is mediated by G-rich tandem repeated sequences termed switch regions. Transcription of switch regions generates single-stranded R loops that provide substrates for activation-induced cytidine deaminase. Mice deficient in MSH2 have a mild defect in CSR and analysis of their switch junctions has led to a model in which MSH2 is more critical for switch recombination events outside than within the tandem repeats. It is also known that deletion of the whole Sµ region severely impairs but does not abrogate CSR despite the lack of detectable R loops. Here, we demonstrate that deficiency of both MSH2 and the Sµ region completely abolishes CSR and that the abrogation occurs at the genomic level. This finding further supports the crucial role of MSH2 outside the tandem repeats. It also indicates that during CSR, MSH2 has access to activation-induced cytidine deaminase targets in R-loop-deficient Iµ-Cµ sequences rarely used in CSR, suggesting an MSH2-dependent DNA processing activity at the Iµ exon that may decrease with transcription elongation across the Sµ region.

Regulation of the nuclear localization of the human Nedd4-related WWP1 protein by Notch.

Nedd4 family ubiquitin ligases regulate trafficking and degradation of numerous target substrates in different cellular compartments, including at the plasma membrane, in endosomes, in the secretory pathway and in the nucleus. WWP1 is a Nedd4 family protein closely related to mouse Itch and Drosophila Su(dx), both of which have been shown to regulate the Notch receptor. To investigate the possibility that WWP1 is also associated with Notch signalling we coexpressed human Notch1 and WWP1 in mouse myoblast cells. We found that WWP1 could localize to both the nucleus and cytoplasm in a context dependent manner. Coexpression of human Notch1 (hN1) depleted WWP1 from the nucleus to colocalise with hN1 in early endosomes, dependent on the presence of the C-terminal HECT domain. Furthermore we found that full-length expressed WWP1 could interact in vitro with the cytoplasmic domain of human Notch1. The Notch receptor has multiple roles in development, mediating a short-range signal that controls cell fate and pattern formation. The canonical Notch signal involves proteolytic release of the soluble Notch intracellular domain and the activation by the latter of the transcription factor Suppressor of Hairless/CBF-1 in the nucleus. This pathway does not however account for all of the activity of Notch. The ability of Notch to regulate the nuclear localization of WWP1 suggests a possible alternative mechanism by which Notch may communicate a signal to the nucleus. Drosophila Notch similarly regulated the nuclear localization of the Drosophila Nedd4 family protein, Suppressor of deltex, implying conservation of this mechanism during evolution.