Stress Response Transcription Factor Egr-1 As Tumor Suppressor Of 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 treatment derived AMLs; (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 different leukemias. CML is a disease resulting from the neoplastic transformation of hematopoietic stem cells (HSC) with the BCR-ABL oncogene. The BCR-ABL protein is a constitutively active tyrosine kinase, which promotes cell survival and proliferation by means of diverse intracellular signaling pathways, thereby being the culprit for malignant transformation. Although the Tyrosine Kinase Inhibitor (TKI) imatinib mesylate (Gleevec, Novartis) is effectively used on CML patients, resistance to imatinib has been described. Thus there is a high priority to enhance our understanding of how BCR/ABL subverts normal hematopoiesis and to identify novel targets for therapy. It was observed that Egr-1 expression is reduced in bone marrow (BM) of CML patients, and its expression is further reduced in more advanced stages of CML. Consistent with this data, Egr-1 expression is reduced in BCR-ABL-expressing murine BM. The tumor suppressor role of Egr-1 in CML was validated using mouse models. 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-retrovirus (bone marrow transplantation {BMT}). Loss of Egr-1 accelerated the development of BCR-ABL driven leukemia in recipient mice. Furthermore, no statistically significant difference in the percentage of stem cells (Lin^-Sca⁺c-Kit⁺, LSK) was observed between Egr-1 WT and Egr-1^-/- BM. Thus, the BM stem cell compartment of the Egr-1^-/- mice does not offer a quantitative advantage to justify the faster development of leukemia compared to Egr-1 WT mice. An increased population of lineage negative BM cells was observed in Egr-1^-/- BCR-ABL recipients when compared to animals transplanted with WT BCR-ABL BM, consistent with more rapid development of disease. Preliminary results from serial BMT has shown that Egr-1^-/- BCR-ABL BM has an increased leukemic burden when compared to the WT counterpart. Data from serial colony transfer and studies using spleens from diseased mice as well as BCR-ABL-expressing BM will be presented. These data 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.