The tetramer structure of the Nervy homology two domain, NHR2, is critical for AML1/ETO's activity

Acute promyelocytic leukemia: New issues on pathogenesis and treatment response

Pathogenesis of acute promyelocytic leukemia appears to be one of the best understood among human malignancies. The ability of retinoic acid (RA) and arsenic trioxide to directly target the oncogenic promyelocytic leukemia-retinoic receptor A (PML-RARA) fusion protein also made this disease the first model for oncogene-targeted therapies. A set of recent data has significantly increased the complexity of our view of acute promyelocytic leukemia pathogenesis, as well as of therapeutic response. This review summarizes and discusses these findings, which yield novels questions and models.

Oncogenic pathways of AML1-ETO in acute myeloid leukemia: multifaceted manipulation of marrow maturation

The leukemic fusion protein AML1-ETO occurs frequently in human acute myeloid leukemia (AML) and has received much attention over the past decade. An initial model for its pathogenetic effects emphasized the conversion of a hematopoietic transcriptional activator, RUNX1 (or AML1), into a leukemogenic repressor which blocked myeloid differentiation at the level of target gene regulation. This view has been absorbed into a larger picture of AML1-ETO pathogenesis, encompassing dysregulation of hematopoietic stem cell homeostasis at several mechanistic levels. Recent reports have highlighted a multifaceted capacity of AML1-ETO directly to inhibit key hematopoietic transcription factors that function as tumor suppressors at several nodal points during hematopoietic differentiation. A new model is presented in which AML1-ETO coordinates expansion of the stem cell compartment with diminished lineage commitment and with genome instability.

Recruitment of RXRα by Homo-Oligomeric RARα Is a Key for RARα-Mediated Transformation

Chimeric fusion proteins involving Retinoic Acid Receptor Alpha (RARα) invariantly associate with almost 100% of Acute Promyelocytic Leukemia (APL). While we and others have recently demonstrated an essential role of aberrant self-association in RARα fusions mediated transformation1,2, forced oligomerization of truncated transcription factors including MLL3 and AML14 has emerged as an important oncogenic property shared by these most common chimeric fusion proteins involved in AML. To further characterize the underlying mechanisms and its modes of action, the present study dissects the essential molecular properties and transformation mechanisms by oligomeric RARα-oncoproteins. Firstly, we define the oncogenic threshold of self-association by showing that homo-oligomerization rather than homo-dimerization is required for Stat5b-RARα mediated transformation of primary hematopoietic cells. Although constitutive transcriptional repression is believed to be the underlying mechanism for RARα-mediated transformation, oligomeric RARα fusion proteins can bind retinoic acid response elements (RAREs) either as homo-oligomers or higher order hetero-oligomers with RXRα. To gain further insights into the transcriptional control mediated by RARα fusions, we successfully constructed various Stat5b-RARα point mutants that maintained the homo-oligomerization properties but lost their abilities to bind DNA as homo-oligomers. These resulting mutants could still efficiently transform primary hematopoietic cells in spite of their lack of homo-oligomeric DNA binding ability. To investigate if this is unique to Stat5b-RARα or is a general phenomena that can also apply to other RARα fusions, we further demonstrated that the synthetic FKBP-RARα fusion, which closely mimick both biochemical and transformation properties of bona fide RARα fusion proteins, was incapable of binding DNA as homo-oligomers, suggesting that homo-oligomeric DNA-binding is dispensable for RARα-mediated transformation. Conversely we reveal that the higher-order RARα-fusion/RXRα hetero-oligomeric complex, which aberrantly recruits transcriptional co-repressor SMRT, represents the major functional DNA-binding module. Specific knockdown of RXRα expression using shRNA approach can suppress transformation mediated by both bona fide and synthetic RARα fusion proteins, thus indicating a critical functional role of RXRα in RARα-mediated transformation of primary hematopoietic cells. Finally to assess the possible therapeutic values of targeting RXRα-mediated pathways in RARα-mediated transformation, we also demonstrated specific inhibition of myeloid transformation mediated by various RARα oncoproteins using the panRXR-selective agonist, SR11237. Taken together, these results not only highlight the functional significance of higher-order RARα-fusion/RXRα hetero-oligomeric complex, but also attest RXRα as a potential therapeutic target for APL.

Dimerization-induced corepressor binding and relaxed DNA-binding specificity are critical for PML/RARA-induced immortalization

The pathogenesis of acute promyelocytic leukemia involves the transcriptional repression of master genes of myeloid differentiation by the promyelocytic leukemia-retinoic acid receptor alpha (PML/RARA) oncogene. PML-enforced RARA homodimerization allows the tighter binding of corepressors, silencing RARA target genes. In addition, homodimerization dramatically extends the spectrum of DNA-binding sites of the fusion protein compared with those of normal RARA. Yet, any contribution of these two properties of PML/RARA to differentiation arrest and immortalization of primary mouse hematopoietic progenitors was unknown. We demonstrate that dimerization-induced silencing mediator of retinoid and thyroid receptors (SMRT)-enhanced binding and relaxed DNA-binding site specificity are both required for efficient immortalization. Thus, enforced RARA dimerization is critical not only for triggering transcriptional repression but also for extending the repertoire of target genes. Our studies exemplify how dimerization-induced gain of functions converts an unessential transcription factor into a dominant oncogenic protein.