Transgenic expression of PML/RARalpha impairs myelopoiesis.

Abstract

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.

Acute leukemia with promyelocytic features in PML/RARα transgenic mice

“…Transgenic mice for the PML͞RAR␣ fusion gene under the control of the myeloid CD11b promoter did not develop leukemia over the course of a more than 2-year follow-up period and total WBC and differential counts of myeloid subpopulations did not show appreciable differences between PML͞RAR␣ and control mice (41). Appropriate timing of PML͞RAR␣ expression in early myeloid progenitors certainly plays a crucial role with respect to its leukemogenic potential and its ability to interfere with normal hematopoiesis, thus possibly explaining the difference observed with our mice.…”

The theory of APL

“…This was due to the fact that ubiquitous and unrestricted expression of the fusion gene results in embryonic lethality (He et al, 1997;our unpublished observation). The expression of PML-RARa in early hemopoietic progenitors or in mature myeloid cells did not result in leukemia either (P Greer, personal communication; Early et al, 1996). It is only when the expression of the transgene was directed and restricted to the myeloid promyelocytic compartment that it was possible to draw important conclusions concerning the role of the various X-RARa and RARa-X fusion proteins in APL pathogenesis.…”

Review: genetic models of acute myeloid leukaemia

“…The APL variant of human AML is characterized by a differentiation block leading to an accumulation of promyelocytes, coagulation abnormalities, chromosomal translocations (Grisolano et al, 1997), PLZF/RARa t(11;17) (He et al, 1998;Cheng et al, 1999), nucleolar phosphoprotein NPM/RARa t(5;17) (Cheng et al, 1999), nuclear mitotic apparatus protein NuMA/RARa t(11;17)(q13;q21) (Sukhai et al, 2004) and Cyclin A1 (Liao et al, 2001) were generated with expression of exogenous cDNA directed exclusively to the myeloid compartment under regulation of human-specific promoter sequences. In particular, the human cathepsin-G (hCG) (Grisolano et al, 1994), MRP8 (Brown et al, 1997) and CD11b (Early et al, 1996) promoters were employed resulting in explicit expression of cDNA in a number of myeloid compartments, and varying disease phenotypes. Transgenic mice with NPM/RARa (Cheng et al, 1999), PML/RARa, PLZF/RARa and NuMa/RARa targeted to the promyelocytic compartment by the hCG vector exhibited diverse inter-and intra-specific phenotypes.…”