TRANSGENIC EXPRESSION OF PML RAR-ALPHA IMPAIRS MYELOPOIESIS/

The translocation found in acute promyelocytic leukemia rearranges the promyelocytic leukemia gene (PML) on chromosome 15 with the retinoic acid receptor alpha (RAR alpha) on chromosome 17. This yields a fusion transcript, PML/RAR alpha, a transcription factor with reported domin ant negative functions in the absence of hormone. Clinical remissions induced with all-trans retinoic acid (RA) treatment in acute promyeloc ytic leukemia are linked to PML/RAR alpha expression in leukemic cells . To evaluate the PML/RAR alpha role in myelopoiesis, transgenic mice expressing PML/RAR alpha were engineered. A full-length PML/RAR alpha cDNA driven by the CD11b promoter was expressed in transgenic mice. Ex pression was confirmed in the bone marrow with a reverse transcription PCR assay. Basal total white blood cell and granulocyte counts did no t appreciably differ between PML/RAR alpha transgenic and control mice . Cell sorter analysis of Cd11b(+) bone marrow cells revealed similar CD11b(+) populations in transgenic and control mice. However, in vitro clonal growth assays performed on peripheral blood from transgenic ve rsus control mice revealed a marked reduction of myeloid progenitors, especially in those responding to granulocyte/macrophage colony-stimul ating factor. Granulocyte/macrophage colony-stimulating factor and kit ligand co-treatment did not overcome this inhibition. Impaired myelop oiesis in vivo was shown by stressing these mice with sublethal irradi ation. Following irradiation, PML/RAR alpha transgenic mice, as compar ed with controls, more rapidly depressed peripheral white blod cell an d granulocyte counts. As expected, nearly all control mice (94.4%) sur vived irradiation, yet this irradiation was lethal to 45.8% of PML/RAR alpha transgenic mice. Lethality was associated with more severe leuk openia in transgenic versus control mice. Retinoic acid treatment of i rradiated PML/RAR alpha mice enhanced granulocyte recovery. These data suggest that abnormal myelopoiesis due to PML/RAR alpha expression is an early event in oncogenic transformation.