Deregulation of RARγ and RARγ-Specific miRNAs in AML.

All-trans-retinoic acid (ATRA) plays important regulatory roles in hematopoiesis and is successfully used in differentiation therapy of acute promyelocytic leukemia (APL). Although ATRA effectively inhibits growth and stimulates myelomonocytic differentiation of myeloid progenitors, it is equally potent in causing expansion of multipotent hematopoietic stem cells. Results of studies utilizing mice that lack expression of a specific RAR and/or RAR subtype specific retinoids, as well as work addressing the molecular pathogenesis of acute promyelocytic leukemia (APL), indicate that the effects of ATRA on hematopoietic stem cells and myeloid progenitors are differentially mediated via the RARγ and RARα, respectively. Using qPCR we have now shown that RARγ is expressed in human cord blood derived stem cells but not in more mature myeloid progenitors or myelomonocytic cells. This change in the RARγ expression levels is paralleled by a reciprocal change in expression of RARγ specific miRNA, which we have identified and validated using multiple experimental strategies, including RARγ 3′UTR based reporter assays. RARγ is also expressed in blasts derived from non-APL AML patients (over 80% of cases examined) and AML cell lines, but is not expressed in ATRA responsive APL cell lines. The expression of RARγ miRNA, on the other hand, is markedly decreased in AML blasts when compared to the levels detected in cord blood derived CD34⁺ myeloid progenitor cells. Taken together our results suggest that finely tuned and miRNA mediated down-regulation of RARγ expression in the myelomonocytic lineage provides a switch from pro-proliferation to RARα mediated pro-differentiation effects of ATRA. We predict that use of a RARα specific agonist, possibly in conjunction with a strategy that negatively targets RARγ (as with RARγ selective antagonist or siRNA), would be most effective in retinoid based differentiation therapy of non APL-AML.