Essential role of PU.1 in maintenance of mixed lineage leukemia‐associated leukemic stem cells

Abstract

Acute myeloid leukemia is a clonal malignant disorder derived from a small number of leukemic stem cells (LSCs). Rearrangements of the mixed lineage leukemia (MLL) gene are found in acute myeloid leukemia associated with poor prognosis. The upregulation of Hox genes is critical for LSC induction and maintenance, but is unlikely to support malignancy and the high LSC frequency observed in MLL leukemias. The present study shows that MLL fusion proteins interact with the transcription factor PU.1 to activate the transcription of CSF-1R, which is critical for LSC activity. Acute myeloid leukemia is cured by either deletion of PU.1 or ablation of cells expressing CSF-1R. Kinase inhibitors specific for CSF-1R prolong survival time. These findings indicate that PU.1-mediated upregulation of CSF-1R is a critical effector of MLL leukemogenesis.

BET Bromodomain Inhibition Suppresses the Function of Hematopoietic Transcription Factors in Acute Myeloid Leukemia

“…Knockdown of C/EBPα failed to influence RN2 cell growth (data not shown), which is consistent with a role for this TF in the initiation but not the maintenance of MLL-fusion AML (Ohlsson et al, 2014). The PU.1 and MYB requirement in MLL-fusion leukemia is also consistent with prior studies (Aikawa et al, 2014; Jin et al, 2010; Zuber et al, 2011b). These results confirm that MLL-AF9/Nras G12D AML cells are dependent on several hematopoietic TFs for their aberrant proliferation.…”

Leukemogenic nucleophosmin mutation disrupts the transcription factor hub that regulates granulomonocytic fates

“…That PU.1 partial loss of function permits activation of myeloid commitment genes but represses terminal differentiation genes has been demonstrated in murine models: Pu.1-deficient HSCs could commit into the monocytic lineage, but subsequent activation of terminal differentiation was suppressed, producing leukemic hematopoiesis -replicating lineage-committed cells impeded in their further maturation (42)(43)(44)(45)(46)(47). This suspension of cells at an intermediate, inherently replicative stage of their advance along lineage differentiation axes seems to hinge on partial but not com- genic (42)(43)(44)(45)(46)(47); and (iii) mutual exclusivity of NPM1 with RUNX1 or biallelic CEBPA mutations (37-39), since some function of this master transcription factor hub is needed for existence as a lineage progenitor (17,43,(68)(69)(70)(71)(72). Nuclear export of NPM1 is mediated by the nuclear export protein XPO1.…”

“…The selinexor-resistant cells were still sensitive to noncytotoxic concentrations of decitabine ( Figure 11C), as expected from the mechanism data shown earlier (Figures 7-9). Sim- plete loss of function in the PU.1/CEBPA/RUNX1 circuit, since experimentally, complete inactivation of PU.1, CEBPA, or RUNX1 kills AML cells, even as partial loss of function of any one of these is leukemogenic (17,43,(68)(69)(70)(71)(72); and accordingly, NPM1, RUNX1, and biallelic CEBPA mutations, though highly recurrent in AML, are mutually exclusive (37)(38)(39). PU.1, CEBPA, and RUNX1 have been shown to promote exponential replication kinetics by binding to MYC enhancers to produce high-grade activation of MYC (the master transcription factor coordinator of cell proliferation) and by co-binding with MYC at its target genes (73)(74)(75)(76)(77)(78)(79)(80)(81)(82).…”

“…We do not know for certain, but possibilities include that mutant NPM1 creates the graded PU.1 loss of function that has been shown to be necessary for leukemogenesis (17,43,(68)(69)(70)(71)(72) and/or that dislocation of proteins other than PU.1 contributes to transformation. Even though mutant NPM1 dislocates several proteins, that PU.1 is key among these is suggested by the following: (i) Pu.1 partial loss of function is sufficient to transform scoring the importance of the targeted pathways to the malignant phenotype.…”

LSD1 inhibition exerts its antileukemic effect by recommissioning PU.1- and C/EBPα-dependent enhancers in AML

“…Complete loss of PU.1 abrogates MLL-AF9 leukemogenicity, while decreased expression of PU.1 modulated by heterozygous or homozygous deletion of an upstream enhancer element (PU.1 URE KO) contributes to AML development. 30,31 Given that the dynamic changes in chromatin accessibility in response to LSD1 inhibition are PU.1 dependent, we next employed this system to assess the impact of PU.1 depletion on sensitivity to LSD1 inhibition in vitro and in vivo. We treated PU.1 URE KO, PU.1 URE heterozygous, and WT MLL-AF9 primary leukemias cells with GSK-LSD1 for 48 hours in vitro.…”