PML/RARa, a Fusion Protein in Acute Promyelocytic Leukemia, Prevents Growth Factor Withdrawal-induced Apoptosis in TF-1 Cells1

A unique mRNA produced by the t(15;17)(q22-24;qll21) translocation in the leukemic cells of acute promyelocytic leukemia patients encodes a chimeric protein, PML/ RARa, which is formed by the fusion of the retinoic acid receptor a (RARa) and the promyelocytic locus gene (PML). This translocation is often the only visible karyotypic aberration present which is detected in almost 100% of acute promyelocytic leukemia patients. As an initial step to study the role of PMLIRARa in leukemogenesis, we attempted to express the fusion protein in hematopoietic cells through retrovirus-mediated gene transfer of the retroviral vector, pGPRCHT, which contains the PML/RARa cDNA. Transduction of the PMIJRARa cDNA fragment used in this vector, which extends from the position 31 bp to the position 2638 bp in a transcription unit driven by the Moloney murine sarcoma virus LTR, was found to abrogate the growth factor dependence of TF-1 cells. In addition, introduction of PMLIRARa into iT-i cells can protect these cells from the apoptosis usually induced in TF-1 cells by growth factor withdrawal, as measured by three assays for apoptosis: morphology, DNA ladder formation, and end labeling of nicked DNA with fluorescent-conjugated nucleotide precursors followed by a fluorescence-activated cell sorting assay. These data suggest that the PML/RARa fusion protein may inhibit programmed cell death in myeloid cells.