Research Summary

Our research program is focused on a family of transcription factors called core-binding factors (CBFs). CBFs are heterodimers consisting of a DNA-binding CBFα subunit (Runx1/AML1, Runx2, or Runx3) and a non-DNA-binding CBFβ subunit. All four genes are required for normal development, and RUNX1 and CBFB are the most frequently rearranged genes in human leukemia. RUNX1 (AML1 ) is disrupted in de novo acute myeloid and lymphocytic leukemias (AML and ALL) and in therapy-related myelodysplasias and leukemias by many chromosomal translocations, the most frequent of which are the t(8;21) and t(12:21) in de novo AML (M2 subtype) and ALL, respectively. Biallelic inactivating mutations in RUNX1 have been found in 20% of AML of the M0 subtype. The CBFB gene is disrupted in AML (M4) by inv(16).

One of our goals is to delineate the CBF's roles during normal hematopoiesis. We (Wang et al., Proc. Natl. Acad. Sci. USA 93: 3444, 1996; Wang et al., Cell 87: 697, 1996) and others demonstrated that homozygous germline disruption of either Runx1 or Cbfb completely eliminates hematopoiesis in vivo . One important conclusion from these studies was that both the DNA-binding and non-DNA-binding subunits of this CBF complex are essential for its in vivo function. We determined that Runx1 is expressed in a small population of endothelial cells early in mouse development, and appears to be required for the differentiation of hematopoietic stem cells (HSCs) from this "hemogenic" endothelium (North et al. Development 126: 2563, 1999; North et al. Immunity 16: 661-672). Our more recent studies

indicate that Runx1 is required in the endothelium per se for HSCs to form. We are also delineating the role of Runx1 and Cbfb during later stages of hematopoiesis by utilizing conditional and hypomorphic alleles. Using a hypomorphic Cbfb allele, we recently identified a requirement for CBFs at the earliest states of T and natural killer cell development, and are in the process of placing the CBFs in the genetic hierarchy that controls the differentiation of these two lineages. We are also studying the effect of Runx1 deletion in HSCs, in order to better understand how acquired Runx1 deficiency in adult HSCs contributes to leukemogenesis.

A second research goal has been to characterize the biophysical properties of the CBF subunits, which we have done in the context of a long-standing collaboration with a structural biologist, John Bushweller, at the University of Virginia. For the first 10 years we focused on the normal CBF proteins (the DNA binding domain of Runx1, and CBFβ). More recently we shifted our emphasis to the oncogenic Runx1 and CBFβ fusion proteins found in leukemia, and have launched a translational effort to develop small molecule inhibitors of these proteins. We are currently determining which protein-protein interactions mediated by the AML1-ETO protein [which is formed as a result of the t(8;21)] are necessary for its leukemogenic properties, and are developing small molecules to inhibit these interactions. The drug development project is funded by a Specialized Center of Research (SCOR) grant from the Leukemia and Lymphoma Society.