Role of TIF1gamma in hematopoiesis – TIF1GH

Transcription Intermediary Factor 1 gamma (TIF1g) is a transcription and elongation co-regulator interacting with Smads and displaying an E3 ubiquitin ligase activity. In zebrafish and human CD34+ cells, TIF1g links positive elongation factors and blood specific transcription complexes to regulate transcription during erythropoiesis.
The two teams involved in the project have studied the role of TIF1g in adult hematopoiesis by crossing cFES-Cre or Mxe-Cre mice with floxed TIF1g mice. These studies have shown that TIF1g is a key modulator of hematopoietic stem and progenitor cells (HSPCs) fate and that TIF1g deficiency impaired myelomonocytic differentiation and favors the appearance of a chronic myelomonocytic leukemia (CMML). Studies of TIF1g expression in patients showed that a low level of TIF1g in 35% of them could result from the TIF1g promoter hypermethylation. Hence, TIF1g is an epigenetically regulated tumor suppressor gene in hematopoietic cells.
This project aims at understanding:
1- Consequences of the timing of TIF1g gene deletion on the fate of HSCs as different hematopoietic or leukemic phenotypes have been described depending on the regulatory regions that drive the expression of the Cre recombinase in mouse.
2- The functions of the different domains of TIF1g in the hematopoietic stem cells fate. TIF1g consists of an N-terminal tripartite motif domain containing a RING domain, two B boxes and a coiled-coil domain and displaying an E3 ubiquitin ligase activity, a specific motif (TSS) that is not conserved in the other members of the TIF1 family, a middle region that binds Smad2/3 and a C-terminal region containing one plant homeodomain adjacent to a Bromo domain region. We will infect lin-c-kit+ hematopoietic progenitor cells sorted from TIF1g-/- or TIF1g+/+ bone marrow (BM) with lentiviruses containing wild type TIF1g or TIF1g mutants, reconstitute the hematopoiesis of lethally irradiated mice with these cells and study the reconstituted hematopoiesis.
3- The role(s) of TIF1g in the TGF-b signaling pathway that is a major regulator of adult HSPCs. One of the two models is that TIF1g could antagonize Smad4 activity through its ubiquitin ligase function. Besides the analyses of TGF-b members in HSPCs from TIF1g-/- bone marrow and the injections of TGF-b or BMP ligands in the BM of TIF1g-/- mice, we will analyze Smad4-TIF1g DKO to determine the role of Smad4 in TIF1g-/- mice.
4- The role of TIF1g during monocytic differentiation. One of the striking phenotypes of the TIF1g-/- hematopoiesis is the increase GMP compartment at the expense of the CMP and MEP compartment. We have also found a large increase of MDP and mature myeloid cells in the bone marrow and in the spleen of TIF1g-/- mice and that myeloid cells derived from TIF1g-/- MDP did not express the Csf-1 receptor. Using the two mouse models, we will study the genes directly regulated by TIF1g and involved in monocytic differentiation, by transcriptomic analyses and ChiP-seq using purified populations of progenitor or mature myeloid cells. We will also study the signaling pathway(s) that compensate the Csf-1 signaling pathway to promote monocytic differentiation of TIF1g-/- myeloid precursors.
5- The contribution of TIF1g deficiency in the hematopoietic transformation by loss of proliferative control at the stem/progenitor stage or in a committed myeloid lineage. The monocytic proliferation could result from the appearance of secondary genetic or epigenetic abnormalities. We will investigate the DNA copy number gains and losses across the whole karyotype and other genomic aberrations in mice.
The two partners have published pioneering studies on the role of TIF1g in adult hematopoiesis. These partners are highly qualified in the field of HSCs fate and leukemogenesis. As indicated in the project aims, they will work on different but yet complementary aspects of novel processes involved in HSC homeostasis and control of differentiation