ROLE OF THE CHROMATIN REGULATOR, MLL, IN T CELL DEVELOPMENT

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. To develop model systems in which the role of MLL can be assessed during adult hematopoiesis and lymphopoiesis, we have developed a loxP-flanked version of the murine Mll gene, backcrossed this strain, and crossed it to the Mx1-Cre transgenic strain. The Mx1 transgene is induced by the injection of polyI:polyC, a double stranded RNA analog that can be injected into the peritoneal cavity of adult animals. This approach allowed us to study the effect of Mll deletion in an acute setting. Using this model, we demonstrated that the maintenance of hematopoietic stem and progenitor populations depend on Mll for their homoestasis. We demonstrated that this was likely due to two different cell-context dependent roles of Mll in these primitive cell types. First, Mll deletion in hematopoietic stem cells resulted in their escape from quiescence and an increase in proliferation, coupled to symmetric differentiation of progeny cells. This resulted in the rapid exhaustion of stem cell activity from the bone marrow. Second, we demonstrated that in myelo-erythroid progenitors, Mll deficiency resulted in the reduction in proliferation resulting in reduced progenitor pool sizes. Together, these alterations resulted in the rapid decline in bone marrow cell numbers upon Mll deletion and ultimately bone marrow failure and animal death. Surprisingly, when we crossed the Mll loxP flanked strain to Cre transgenic animals that express Cre only in differentiating lineages (T cells [lck-Cre], B cells [CD19-Cre] or myelomonocytic precursors [lysozyme M-Cre]), we found that there was no effect on steady state cell numbers, demonstrating that Mll is only essential in the early stem and progenitor populations. Progress toward the main goal of this Project has been great in the area of lymphopoiesis. We are currently finishing experiments that will form the bulk of a manuscript that defines a block in B lymphopoiesis in the absence of Mll. To perform these studies, we obtained a Vav-Cre strain of mice from Dr. T. Graf (via Dr. H. Mikkola). This strain expresses the Cre recombinase within the hematopoietic system starting at embryonic day 13 in the fetal liver. Using this approach, we are able to study the early steps in commitment to the lymphoid lineages (T and B cells) which was not possible using the Mx1-Cre strain used in the studies described above, due to the confounding effects of polyI:polyC injection. With the Vav-cre model, we observe a stringent requirement for MLL in B lymphopoiesis but not T lymphopoiesis. This is interesting since patients with MLL translocations, particularly infants, have acute lymphocytic leukemia (ALL) of an early B cell phenotype and T cell ALL is very rare.