Retinoic acid receptor alpha (RARα) regulates myeloid differentiation and proliferation through the regulation of specific sets of genes. When unbound by a ligand, RARα is a repressive transcription factor while in its ligand-bound state it functions as a transcriptional activator. Previously, blast cells from a subset of individuals with non-APL AML were found to have a super-enhancer (SE), as revealed by H3K27 acetyl ChIP-Seq, associated with the RARA locus (hereafter called RARA-high), suggesting that tumor cell proliferation may have a dependency on RARA that can be exploited for therapeutic benefit. SEs are exceptionally large, highly active chromatin regions that are densely occupied by transcription factors and have been implicated in oncogene expression. Indeed, RARA-high non-APL AML cell lines showed >1000-fold increased sensitivity compared to RARA-low cells to the potent and selective RARα agonist SY-1425 (tamibarotene) as well as efficacy in non-APL AML patient derived xenograft models with a dependency on RARA.
Since RARα is a transcription factor and the direct target of SY-1425, we investigated the change SY-1425 had on the transcriptional program of non-APL AML cell lines and the mechanism underlying those changes. Expression profiling on a panel of AML cell lines revealed that RARA-high AML cell lines had profound transcriptional changes in response to SY-1425, with 437 genes significantly changed, while RARA-low cell lines did not show significant gene expression changes. Gene set enrichment analysis (GSEA) of three RARA-high AML cell lines revealed that the genes upregulated by SY-1425 in the RARA-high cells are associated with immune signaling, interferon induction, protein secretion, and pathways associated with complement, MHC and integrin functions, all pathways indicative of more differentiated blood cells. Signatures downregulated by SY-1425 include MYC target genes. These findings are consistent with SY-1425 increasing the expression of genes involved in differentiation and decreasing those involved in proliferation. Genome-wide ChIP-Seq analysis revealed an increase in H3K27 acetylation at loci found to have strong RARα peaks as well as increased expression of those genes upon treatment with SY-1425. Together, these data support a model in which RARα binding nucleates functional enhancers in response to SY-1425 thereby upregulating proximal target gene expression and promotion of differentiation.
The gene expression and epigenomic responses of RARA-high AML cell lines to SY-1425 were found to be similar to the responses of an APL cell line (NB-4) to retinoids or SY-1425. Gene sets identified in response to either retinoid treatment or genetic perturbation, such as forced expression or RAR-fusions or knockdown, matched the gene sets identified in RARA-high AML cell lines. Furthermore, the quantitative response of both NB-4 and RARA-high AML cell lines to SY-1425 was found to be similar. Across the genome, RARα binding was highly conserved between NB-4 and RARA-high AML cell lines with less overlap with the RARA-low cell lines. For example, the transcriptional and H3K27 acetylation alterations at the known PML-RARα target gene TGM2 following retinoid treatment was similar in NB-4 and the RARA- high cell lines. This locus also had a strong RARα binding site that is conserved among the cell lines and co-localized with a strong H3K27 acetylation peak. Consistent with the pattern of occupancy of RARα on the genome, the transcriptional response of the RARA enhancer-high cell lines to SY-1425 treatment was similar to the response of APL ex-vivo patient samples to retinoic acid treatment. These data support a model of a common biological response to retinoids between cells with the RARA-PML translocation in APL and cells with the RARA SE in AML. The mechanistic studies described here support the therapeutic potential of SY-1425 in myeloid leukemia patients who have a SE associated with RARA. A biomarker directed clinical trial of SY-1425, a potent and selective RARα agonist, in a subset of AML and MDS patients with an altered RARA locus (clinicaltrials.gov, NCT02807558) is supported by these data.
Fiore:Syros Pharmaceuticals: Employment, Equity Ownership. McKeown:Syros Pharmaceuticals: Employment, Equity Ownership. Lee:Syros Pharmaceuticals: Employment, Equity Ownership. Eaton:Syros Pharmaceuticals: Employment, Equity Ownership. Fritz:Syros Pharmaceuticals: Employment, Equity Ownership.
Asterisk with author names denotes non-ASH members.