The PLZF gene was identi®ed by its fusion with the RARa locus in a therapy resistant form of acute promyelocytic leukemia (APL) associated with the t(11;17)(q23;q21) translocation. Here we describe PLZF as a negative regulator of cell cycle progression ultimately leading to growth suppression. PLZF can bind and repress the cyclin A2 promoter while expression of cyclin A2 reverts the growth suppressed phenotype of myeloid cells expressing PLZF. In contrast RARa-PLZF, a fusion protein generated in t(11;17)(q23;q21)-APL activates cyclin A2 transcription and allows expression of cyclin A in anchorage-deprived NIH3T3 cells. Therefore, cyclin A2 is a candidate target gene for PLZF and inhibition of cyclin A expression may contribute to the growth suppressive properties of PLZF. Deregulation of cyclin A2 by RARa-PLZF may represent an oncogenic mechanism of this chimeric protein and contribute to the aggressive clinical phenotype of t(11;17)(q23;q21)-associated APL.

The translocation found in acute promyelocytic leukemia rearranges the promyelocytic leukemia gene (PML) on chromosome 15 with the retinoic acid receptor a (RARa) on chromosome 17. This yields a fusion transcript, PML/RARa, a transcription factor with reported dominant negative functions in the absence of hormone. Clinical remissions induced with all-trans retinoic acid (RA) treatment in acute promyelocytic leukemia are linked to PML/RARa expression in leukemic cells. To evaluate the PML/RARa role in myelopoiesis, transgenic mice expressing PML/RARa were engineered. A full-length PML/RARa cDNA driven by the CD11b promoter was expressed in transgenic mice. Expression was confirmed in the bone marrow with a reverse transcription PCR assay. Basal total white blood cell and granulocyte counts did not appreciably differ between PML/ RARa 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 versus control mice revealed a marked reduction of myeloid progenitors, especially in those responding to granulocyte/ macrophage colony-stimulating factor. Granulocyte/ macrophage colony-stimulating factor and kit ligand cotreatment did not overcome this inhibition. Impaired myelopoiesis in vivo was shown by stressing these mice with sublethal irradiation. Following irradiation, PML/RARei transgenic mice, as compared with controls, more rapidly depressed peripheral white blod cell and granulocyte counts.As expected, nearly all control mice (94.4%) survived irradiation, yet this irradiation was lethal to 45.8% of PML/RARa transgenic mice. Lethality was associated with more severe leukopenia in transgenic versus control mice. Retinoic acid treatment of irradiated PML/RARci mice enhanced granulocyte recovery. These data suggest that abnormal myelopoiesis due to PML/RARa expression is an early event in oncogenic transformation.

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Insertional mutagenesis of c-myb by Moloney murine leukemia virus occurs in 100% of promonocytic leukemias (MMLS) induced by the virus. These leukemias, which resemble acute monocytic leukemia-M5 in humans are induced only in mice undergoing a peritoneal chronic inflammatory response. We have found that two leukemia-specific gag-myb