Research Interests

The main focus of our research is the understanding of the molecular mechanism of hematopoietic stem cell (HSC) differentiation and transformation. Adult HSC reside in the bone marrow and are able to differentiate and generate all blood sub-lineages. The bone marrow sustains the differentiation of myeloid, erythroid and B cells but HSC that are destined to differentiate into T cells have to exit the marrow and home into the thymus. Our lab has previously demonstrated that HSC differentiation in the mammalian thymus depends on the cooperation between the pre-T cell receptor (pre-TCR), Notch, wingless and Hedgehog (Hh) signaling pathways. As HSC enter the thymus and commit to the T cell lineage in response to Notch signaling, they sense high Hh concentrations, activate the pathway and ensure cell cycle entry and survival. As the cells move away from Hh they initiate to recombine their TCR loci and express the pre-TCR that –in collaboration with Notch- supports progenitor homeostasis and differentiation. Our studies have shown that this signaling cooperation has a profound medical significance. Indeed, failure to optimally express the pre-TCR, Notch and Hh receptors lead to profound immunodeficiency. On the other hand, over-activation of some of these signaling pathways can be oncogenic. Interestingly, it was recently found that the majority of T cell leukemia patients harbor activating mutations in the Notch1 locus. Also, expression of mutated, active, Notch1 in mouse HSCs leads to fatal leukemia and lymphoma. Underlining the cooperation of these signaling pathways, HSCs that lack the ability to express the pre-TCR or induce certain pre-TCR gene targets (cyclin D3) are resistant to Notch1-induced oncogenic transformation.
The mechanisms of Notch1-induced HSC transformation is the major focus of our research. Our work attempts to characterize in detail the Notch1-induced oncogenic signaling pathway and also identify Notch1 regulators that can be used to target the disease. Initially, we have recently shown that oncogenic Notch1 can induce transformation by activating the NF-kB signaling pathway. Suppression of NF-kB signaling can inhibit the development of leukemia both in vivo and in vitro suggesting that its pharmacological targeting can be a powerful theurapeutic tool. Also, we are currently characterizing enzymes that can control Notch1 protein stability. Using genetic and biochemical screenings we identified the E3 ubiquitin ligase Fbw7 as a protein that interacts with a phospho-threonine-centered “degron” sequence in the –COOH-terminal end of Notch1. This Fbw7-Notch1 interaction is important for Notch1 ubiquitination and degradation. Further analysis demonstrated that T-ALL-inducing Notch1 mutations target this FBW7 degron and that the FBW7 gene itself is mutated and inactivated in a significant portion of T-ALL cell lines and primary leukemic samples (~20%). The mutations target three conserved arginine residues that form the binding pocket of the ligase. FBW7 inactivation stabilizes Notch1 as well as cyclin E and c-Myc, two other oncogenes regulated by FBW7-induced ubiquitination. Finally, an additional research focus in the lab is the study of the mechanisms of cell cycle regulation during hematopoiesis. We specifically focus on the function of the D-type cyclins (D1-123) and have recently shown, using gene-targeted mice, that cycln D3 is essential for cell cycle entry and differentiation of both T and B cell progenitors. Our data have also shown that cyclin D3 can be regulated both at the level of transcription and by ubiquitination and proteasomemediated degradation suggesting novel ways of targeting cyclin D3 function in leukemia as Dtype cyclins are frequently over-expressed in multiple types of hematopoietic malignancies.