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Acute Myeloid Leukemia
RXRA DT448/9PP generates a dominant active variant capable of inducing maturation in acute myeloid leukemia cells
Orsola di Martino,1* Margaret A. Ferris,2* Gayla Hadwiger,1 Soyi Sarkar,1
Anh Vu,1 María P. Menéndez-Gutiérrez,3 Mercedes Ricote3 and John S. Welch
1Department of Internal Medicine, Washington University, St Louis, MO, USA; 2Department of Pediatrics, Washington University, St Louis, MO, USA and 3Myocardial Pathophysiology Area, Centro Nacional de Investigaciones Cardiovasculares (CNIC), Madrid, Spain
*OdM and MAF contributed equally as co-first authors.
ABSTRACT
RARA and RXRA contribute to myeloid maturation in both mice and humans, and deletion of Rxra and Rxrb augments leukemic growth in mice. While defining the domains of RXRA that are required for anti-leukemic effects in murine KMT2A-MLLT3 leukemia cells, we unexpectedly identified RXRA DT448/9PP as a constitutively active variant capable of inducing maturation and loss of their prolifera- tive phenotype. RXRA DT448/9PP was associated with ligand-indepen- dent activity in reporter assays, with enhanced co-activator interactions, reduced engraftment in vivo, and activation of myeloid maturation tran- scriptional signatures that overlapped with those of cells treated with the potent RXRA agonist bexarotene, suggestive of constitutive activity that leads to leukemic maturation. Phenotypes of RXRA DT448/9PP appear to differ from those of two other RXRA mutations with forms of consti- tutive activity (F318A and S427F), in that DT448/9PP activity was resist- ant to mutations at critical ligand-interacting amino acids (R316A/L326A) and was resistant to pharmacological antagonists, sug- gesting it may be ligand-independent. These data provide further evi- dence that activated retinoid X receptors can regulate myeloid matura- tion and provide a novel constitutively active variant that may be ger- mane for broader studies of retinoid X receptors in other settings.
Introduction
Retinoid receptors are highly conserved transcription factors that direct hematopoi- etic self-renewal and differentiation.1,2 Retinoids (vitamin A metabolites) bind directly to retinoid receptors, converting the receptors from transcriptional repressors to tran- scriptional activators. There are six types of retinoid receptor (RARA, RARB, RARG, RXRA, RXRB, RXRG) with distinct tissue expression and subtle differences in their binding and response to different ligands. The retinoid receptors RARA and RXRA undergo remarkable upregulation during myeloid maturation in both mice and humans, whereas RXRG is not detected.3-5
Retinoid treatments, both in vitro and in vivo, facilitate hematopoietic stem cell mat- uration and lineage commitment.1,2 Until now, the clinical application of retinoids in hematology has been restricted to acute promyelocytic leukemia (treated with all-trans retinoic acid, or tretinoin, a pan-RAR ligand) and cutaneous T-cell leukemia (treated with bexarotene, a pan-RXR ligand). However, recent studies have observed activity of all-trans retinoic acid when combined with chemotherapy in acute myeloid leukemias other than the promyelocytic form, and the RARA super-enhancer has emerged as a potential biomarker of retinoid sensitivity.6-10
To better understand the molecular determinants of anti-leukemic retinoid activity, we evaluated a series of RXRA truncations and mutations to determine which might rescue phenotypes observed in Rxra/Rxrb-deficient leukemia cells.5 We unexpectedly identified RXRA DT448/9PP as a constitutively active variant capable of inducing mat- uration and loss of proliferative capacity in leukemia cells. In this study, we character- ized the activity of this variant using cell culture assays, transcription reporter assays,
Ferrata Storti Foundation
Haematologica 2022 Volume 107(2):417-426
Correspondence:
JOHN S. WELCH
jwelch@wustl.edu
Received: February 17, 2021. Accepted: May 7, 2021. Pre-published: June 17, 2021.
https://doi.org/10.3324/haematol.2021.278603 ©2022 Ferrata Storti Foundation
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