Expression et fonction des genes du groupe Polycomb (PcG) dans l'hematopoiese normale et leucemique

The studies presented in this thesis establish that the Polycomb Group (PcG) gene Bmi-1 is a key genetic determinant of the proliferative capacity of fetal and adult hemopoietic stem cells (HSCs). Consistent with this, homozygosity for a null allele of Bmi-1 in mice leads to progressive and lethal aplastic anemia by early adulthood. This work also shows that the function of Bmi-1 is preserved in leukemic stem cells (LHSCs), providing the first molecular basis for the concept that stem cell function (whether normal or neoplastic) is regulated by common regulatory genes. These findings reinforce the notion of a structure in the leukemic hierarchy where Bmi-1 defines "sternness". Determining whether this function of Bmi-1 extends to other types of normal and "neoplastic stem cells" is eagerly awaited. A detailed expression analysis of selected members of the PcG gene family in purified subpopulations of human bone marrow cells revealed a curious dissociation of the expression profile of Bmi-1 (mostly restricted to the HSC compartment) and that of the other known PcG proteins which physically interact with Bmi-1. In an attempt to define the biochemical nature of the Bmi-1-containing complex in HSCs, a yeast-two-hybrid screen was performed using an expression library enriched for primitive hemopoietic cells. This approach led to the identification of eight novel stem cell-specific co-factors of Bmi-1, including a zinc-finger molecule which appears to represent a key mediator of Bmi-1-induced HSC proliferation. Conversely, it was established that the PcG gene eed performs an antagonistic function to Bmi-1 in the regulation of HSC proliferation. Heterozygosity for a null allele of eed (eed3354/+) leads to severe myelo- and lympho-proliferative defects and lymphoid tumor development in mice, indicating that Eed is involved in the negative regulation of the pool size of early bone marrow progenitor cells. Together, the work presented in this thesis reveals that the proliferative tone of the HSC is intrinsically regulated by the relative contribution of a pro-proliferative (Bmi-1-containing) and an anti-proliferative (Eed-containing) PcG gene complex.