Homodimerization of Retinoic Acid Receptor alpha through its Fusion Partners Underlies Pathogenesis of Acute Promyelocytic Leukemia.

In almost all cases of acute promyelocytic leukemia (APL), t(15;17)(q22;q12) fuses the promyelocytic leukemia ( PML ) gene with the retinoic acid receptor alpha ( RARA ) gene. In rare cases of APL, other genes are fused to RARA including PLZF , NPM , NuMA , and STAT5b . These chimeras are collectively referred to as X-RARA fusions. Common features of the X-RARα fusions include their ability to form dimers through the X domain proteins while retaining the ability to interact with rexinoid receptors (RXRs, the proteins with which RARα normally heterodimerizes). We previously created an artificial X-RARα: p50-RARα (a fusion of the dimerization interface of p50 NFκB with the portion of RARα found in naturally occurring X-RARα proteins). p50-RARα has effects similar to PML-RARα including (i) enhancing repression at retinoic acid response elements, due to impaired release of co-repressors at low levels of all- trans retinoic acid (ATRA), and (ii) inhibiting differentiation of myeloid cell lines. We generated transgenic mice expressing p50-RARα under the control of the MRP8 promoter. p50-RARα had a minimal effect on myelopoiesis and initiated myeloid leukemias at a rate of p50-RARA transgenic bone marrow with a retrovirus expressing an activated cytokine receptor (β c V449E ) generated leukemias with features of APL including therapeutic responsiveness to ATRA. Complementing our work with the p50-RARA transgenic mice, retroviral co-transduction of normal bone marrow with β c V449E plus either p50-RARA or PML-RARA generated APL-like myeloid leukemias. Although retrovirally generated β c /p50-RARα and β c /PML-RARα leukemias were nearly identical, the PML fusion appeared associated with a subtle decrease in differentiation relative to the p50 fusion. In contrast to these X-RARα fusions, preliminary experiments suggest that RCRα (a homodimeric form of RARα in which the dimerization interface of RARα has been replaced by the dimerization domain of the homodimeric COUP-TF transcription factor) does not readily cooperate with β c V449E to cause leukemia. This finding lends support to the hypothesis that the decreased binding site selectivity of X-RARα fusions contributes to APL pathogenesis. Furthermore, we have generated a novel X-RARα fusion, F3-RARα, in which three copies of the F36M mutant of the Rapamycin-binding protein have been fused to RARα. This protein mimics the in vitro effects of PML-RARα on transcription & differentiation, and de-dimerization of F3-RARα by rapamycin reverses these effects. Additional experiments in vivo with F3-RARα are expected to further confirm the importance of abnormal transcription factor dimerization in leukemia pathogenesis.