Egr-1, a Stress Response Transcription Factor and Myeloid Differentiation Primary Response Gene, Behaves As Tumor Suppressor in CML

The transcription factor early growth response 1 (Egr-1) gene was identified as a macrophage differentiation primary response gene, shown to be essential for and to restrict differentiation along the macrophage lineage. There’s evidence consistent with Egr-1 behaving as a tumor suppressor of leukemia, both in vivo and in vitro, including (1) loss of Egr-1 associated with therapy derived MDS and AML; (2) deregulated Egr-1 overriding blocks in myeloid differentiation, and (3) haplo-insufficiency of Egr-1 in mice leading to increased development of myeloid disorders following treatment with the potent DNA alkylating agent, N- ethyl-nitrosourea (ENU). BCR-ABL driven leukemia (Chronic Myelogenous Leukemia [CML]) was chosen as a model system to investigate the role of Egr-1 as a tumor suppressor for leukemia. CML is a disease resulting from the neoplastic transformation of hematopoietic stem cells (HSC) with the BCR-ABL oncogene. To assess the effect of Egr-1 on BCR-ABL driven leukemia, lethally irradiated syngeneic wild type mice were reconstituted with bone marrow (BM) from either wild type or Egr-1 null mice transduced with a 210-kD BCR-ABL-expressing MSCV-GFP retrovirus (bone marrow transplantation {BMT}). It was observed that loss of Egr-1 accelerated the development of BCR-ABL driven leukemia in recipient mice. Analysis of hematopoietic organs is consistent with loss of Egr-1 accelerating development of CML but not altering the nature of the disease. Cells expressing BCR-ABL that are null for Egr-1 exhibit increased survival and proliferation, which can account for some of the above observations. Substantial evidence is available that loss of Egr1 increases the leukemia initiating cell population, which can account for the more rapid development of disease. An increased population of lineage negative BM cells was observed in Egr-1^-/- BCR-ABL recipient mice compared to animals transplanted with WT BCR-ABL BM. Serial BMT has shown that Egr-1^-/- BCR-ABL BM has an increased leukemic burden compared to the WT counterpart. Furthermore, data from serial colony transfer experiments confirmed that loss of Egr-1 increases leukemia initiating cells. Preliminary studies on the effect of gain of Egr-1 function are consistent with the tumor suppressor behavior of Egr-1. These data as well as analysis of human CML samples will be presented. Further investigation could result in novel targets for diagnosis, prognosis, and targeted therapeutics, including strategies for activating Egr-1 expression that can be used to treat CML, as well as other leukemic diseases.