Myb expression is critical for myeloid leukemia development induced by Setbp1 activation

Abstract

SETBP1 missense mutations have been frequently identified in multiple myeloid neoplasms; however, their oncogenic potential remains unclear. Here we show that expression of Setbp1 mutants carrying two such mutations in mouse bone marrow progenitors efficiently induced development of acute myeloid leukemias (AMLs) in irradiated recipient mice with significantly shorter latencies and greater penetrance than expression of wild-type Setbp1, suggesting that these mutations are highly oncogenic. The increased oncogenicity of Setbp1 missense mutants could be due in part to their capability to drive significantly higher target gene transcription. We further identify Myb as a critical mediator of Setbp1-induced self-renewal as its knockdown caused efficient differentiation of myeloid progenitors immortalized by wild-type Setbp1 and Setbp1 missense mutants. Interestingly, Myb is also a direct transcriptional target of Setbp1 and Setbp1 missense mutants as they directly bind to the Myb locus in immortalized cells and dramatically activate a critical enhancer/promoter region of Myb in luciferase reporter assays. Furthermore, Myb knockdown in Setbp1 and Setbp1 missense mutations-induced AML cells also efficiently induced their differentiation in culture and significantly prolonged the survival of their secondary recipient mice, suggesting that targeting MYB pathway could be a promising strategy for treating human myeloid neoplasms with SETBP1 activation.

Somatic SETBP1 mutations in myeloid neoplasms

“…Moreover, by comparison between the same amount of wild-type and mutant SETBP1 proteins, more proliferative potential was observed in the mutant experiments [30]. Mutated proteins also bind more efficiently to DNA at promotor sites of target genes [68,69]. Altogether, SETBP1 mutations were supposed to have both quantitatively and qualitatively activating effects on SETBP1 functions.…”

“…An additional target of activated SETBP1 is a well-known oncogene MYB. Mutant forms of SETBP1 bind to MYB promotor sites and cause overexpression of this gene [69]. For these HOXA9/10, RUNX1, and MYB studies, transplantation experiments in mouse models were performed to confirm biological significance of mutated SETBP1 in myeloid leukemogenesis.…”

SETBP1 dysregulation in congenital disorders and myeloid neoplasms

“…More recently, both the overexpression of wild-type SETBP1 and the presence of a mutant SETBP1 were shown to be capable, alone, of inducing AML in a murine model [ 73 , 74 ]. Again it was clear that SETBP1 mutations have a significantly higher oncogenic potential than wild-type SETBP1 , triggering leukemia with a shorter latency and greater penetrance.…”

“…Intriguingly, both wild type and mutant SETBP1 proteins were found directly bound to MYB , in the promoter regions but also introns 2 and 3, suggesting that SETBP1 regulates both transcriptional activation and elongation (Figure 2 ). As mutant SETBP1 proteins showed a higher transcriptional ability, Nguyen et al suggested that, besides the increased stability of the protein, mutants could have an enhanced DNA-binding activity and/or that mutations could affect the interaction of SETBP1 with unknown key transcriptional co-factors or repressors [ 74 ]. Moreover, a novel function of SETBP1 as a transcriptional repressor through the recruitment of the Nucleosome Remodeling Deacetylase (NuRD) complex was proposed.…”

“…Furthermore, the finding of interplay between MYB and mutant or wild type SETBP1 suggests that MYB inhibition could be a promising approach for treating myeloid neoplasms with SETBP1 activation. In vitro experiments with primary cultures from cells of a CMML patient with SETBP1 mutations showed that MYB gene knockdown dramatically inhibited colony-forming capability [ 74 ]; indeed, it seems that leukemia cells are more sensitive to a reduction of MYB activity than normal hematopoietic progenitors [ 117 , 118 ] and that interaction of MYB with P300 is required for MYB-mediated leukemia transformation, but is less critical for normal hematopoiesis [ 119 ]. In the light of this observation, the triterpenoid Celastrol, a recently identified inhibitor of this interaction, offers a treatment opportunity, as it has been demonstrated in vitro and in vivo to be efficient in inhibiting the growth of mouse AML cells, while sparing the expansion of normal bone marrow progenitors [ 120 ].…”