Janice Telfer

Role of RUNX Family Transcription Factors in Immune System Development

All of the cells of the blood descend from the progeny of hematopoietic stem cells resident in the bone marrow. Hematopoietic stem cells are very long-lived: they survive and divide for the life of the organism. Their daughter cells go through a period of development in which they become more and more specialized, turning on and off the expression of specific genes, until they are fully differentiated. The subversion of this process of normal development often results in cancer. My laboratory is interested in how the regulation of gene expression by the RUNX family of transcription factors influences the development and cancerous transformation of cells of the immune system.

RUNX family transcription factors (RUNX1, RUNX2, and RUNX3) bind to DNA and other proteins to activate or suppress transcription of specific genes. We have previously found that expression of RUNX1 has profound effects on the development of T cells and the myeloid cells known as neutrophils. We use a retroviral expression system that allows us to express normal and mutated RUNX proteins in both primary cells and cell lines, which can then be cultured. The projects currently pursued by my laboratory include:

1. We have found that the expression of RUNX1 in the myeloid cell line 32Dcl3 promotes the cells’ continued proliferation, which is reminiscent of the high proportion of immature proliferating myeloid cells seen in patients with the cancer acute myelogenous leukemia. The fact that increased expression of RUNX1 in humans is associated with a predisposition to develop acute myelogenous leukemia supports our hypothesis that an increase of RUNX1 expression in immature myeloid cells leads to a pre-cancerous state of extended proliferative capacity. We will express mutated forms of RUNX1 in 32Dcl3 cells and primary myeloid cells to discover the mechanism by which RUNX1 works.
2. We have found that expression of the transcription factor RUNX1 suppresses the development of gamma delta T cells (Fig. 1 gd TCR) and silences the expression of CD4 specifically at the immature alpha beta TCR double-positive thymocyte stage (Fig. 2 ab TCR DP). This leads to a bias first towards the production of ab T cells and later, to a bias towards the production of CD8 cytotoxic T cells (Fig. 1 CD8+ TCRbhi). RUNX1 is the first transcription factor known to influence production of either of these types of T cells. It is important to understand how production of different kinds of T cells is regulated: for instance, one can see from AIDS that neither gd T cells nor CD8+ ab T cells can make up for the lack of CD4+ ab T cells. We are characterizing the nature of RUNX family member regulation of CD4 expression, seeking to identify RUNX gene targets in early thymocytes, and investigating what signals normally control the level of RUNX family members in thymocytes.
3. RUNX1 is associated with the development of pediatric acute lymphoblastic leukemia (ALL). We have discovered truncated splice isoforms of RUNX1 that, when expressed via retroviral transduction in immature thymocytes, cause a proliferative expansion or give a survival advantage to these cells. We have localized this effect to a short domain in the N-terminus of RUNX1 and are working on identifying specific amino acids and potential interacting proteins responsible for this effect.

Thymocyte Development: Arrows indicate the direction and timing of branches in the thymocyte maturation pathway, which involves natural killer cells (NK, orange), gamma delta T cells (gd TCR, green), alpha beta T cells (ab TCR, blue and purple), immature "double-negative" thymocytes (DN1-DN4), "double-positive" thymocytes expressing both the CD4 and CD8 proteins (DP, light blue), and "single-positive" thymocytes expressing either CD4 or CD8 proteins (SP, blue or purple).