The DNA binding property of PML/RARA but not the integrity of PML nuclear bodies is indispensable for leukemic transformation

PML::RARA and GATA2 proteins interact via DNA templates to induce aberrant self-renewal in mouse and human hematopoietic cells

The underlying mechanism(s) by which the PML::RARA fusion protein initiates acute promyelocytic leukemia is not yet clear. We defined the genomic binding sites of PML::RARA in primary mouse and human hematopoietic progenitor cells with V5-tagged PML::RARA, using anti-V5-PML::RARA chromatin immunoprecipitation sequencing and CUT&RUN approaches. Most genomic PML::RARA binding sites were found in regions that were already chromatin-accessible (defined by ATAC-seq) in unmanipulated, wild-type promyelocytes, suggesting that these regions are "open" prior to PML::RARA expression. We found that GATA binding motifs, and the direct binding of the chromatin "pioneering factor" GATA2, were significantly enriched near PML::RARA binding sites. Proximity labeling studies revealed that PML::RARA interacts with ~250 proteins in primary mouse hematopoietic cells; GATA2 and 33 others require PML::RARA binding to DNA for the interaction to occur, suggesting that binding to their cognate DNA target motifs may stabilize their interactions. In the absence of PML::RARA, Gata2 overexpression induces many of the same epigenetic and transcriptional changes as PML::RARA. These findings suggested that PML::RARA may indirectly initiate its transcriptional program by activating Gata2 expression: Indeed, we demonstrated that inactivation of Gata2 prior to PML::RARA expression prevented its ability to induce self-renewal. These data suggested that GATA2 binding creates accessible chromatin regions enriched for both GATA and Retinoic Acid Receptor Element motifs, where GATA2 and PML::RARA can potentially bind and interact with each other. In turn, PML::RARA binding to DNA promotes a feed-forward transcriptional program by positively regulating Gata2 expression. Gata2 may therefore be required for PML::RARA to establish its transcriptional program.

Tumor suppressor function of Gata2 in Acute Promyelocytic Leukemia

Most patients with acute promyelocytic leukemia (APL) can be cured with combined All Trans Retinoic Acid (ATRA) and Arsenic Trioxide therapy, which induce the destruction of PML-RARA, the initiating fusion protein for this disease1. However, the underlying mechanisms by which PML-RARA initiates and maintains APL cells are still not clear. We therefore identified genes that are dysregulated by PML-RARA in mouse and human APL cells, and prioritized GATA2 for functional studies because 1) it is highly expressed in pre-leukemic cells expressing PML-RARA, 2) its high expression persists in transformed APL cells, and 3) spontaneous somatic mutations of GATA2 occur during APL progression in both mice and humans. These and other findings suggested that GATA2 may be upregulated to thwart the proliferative signal generated by PML-RARA, and that its inactivation by mutation (and/or epigenetic silencing) may accelerate disease progression in APL and other forms of AML. Indeed, biallelic knockout of Gata2 with CRISPR/Cas9-mediated gene editing increased the serial replating efficiency of PML-RARA-expressing myeloid progenitors (and also progenitors expressing RUNX1-RUNX1T1, or deficient for Cebpa), increased mouse APL penetrance, and decreased latency. Restoration of Gata2 expression suppressed PML-RARA-driven aberrant self-renewal and leukemogenesis. Conversely, addback of a mutant GATA2R362G protein associated with APL and AML minimally suppressed PML-RARA-induced aberrant self-renewal, suggesting that it is a loss-of-function mutation. These studies reveal a potential role for Gata2 as a tumor suppressor in AML, and suggest that restoration of its function (when inactivated) may provide benefit for AML patients.

Location of NLS-RARα protein in NB4 cell and nude mice

In the majority of acute promyelocytic leukemia (APL) cases, translocons produce a promyelocytic leukemia protein-retinoic acid receptor α (PML-RARα) fusion gene. Studies have reported that neutrophil elastase (NE) cleaves bcr-1-derived PML-RAα in early myeloid cells, leaving only the nuclear localization signal (NLS) of PML attached to RARα. NLS-RARα promotes cell growth and inhibits differentiation in response to ATRA. However, the mechanisms by which NLS-RARα affects cell biological characteristics are yet to be fully elucidated. The present study found that the location of RARαwas altered after it was cleaved by NE. Firstly, NE was overexpressed during the preparation of recombinant plasmid NB-4/pCMV6-NE-Myc to cleave PML-RARα. The total protein expression levels of myc and NE and expression levels of NLS-RARα in nucleoprotein were detected by western blotting. Location of NLS-RARα protein was detected by immunofluorescence and confocal laser scanning. Secondly, a nude mice model was constructed and NE protein, NLS-RARα and RARα protein assays, and the location of NLS-RARα and RARα proteins were assessed as described. The present results showed that, compared with the control groups, the location of NLS-RARα protein was predominantly detected in the nucleus, whereas RARα was mainly distributed in the cytoplasm. These findings were consistent with those of the nude mice model, and these may be used as a foundation to explain the occurrence mechanism of APL.