Multiple Mixed Lineage Leukemia (MLL) Fusion Proteins Suppress p53-mediated Response to DNA Damage

Abstract

Chromosomal translocations involving the mixed lineage leukemia (MLL) gene are often observed in acute leukemias of both myeloid and lymphocytic origin. Expression of MLL fusion proteins is known to induce malignant transformation of normal blood progenitors; however, molecular mechanisms of this process are still poorly understood. In this study we investigated the effect of several frequently detected MLL fusion proteins on p53 transcriptional activity. Our data show that MLL-AF9, MLL-AF10, MLL-ENL, and MLL-ELL substantially down-regulate p53-mediated induction of p21, MDM2, and Bax in response to DNA damage. Furthermore, we identify the reduction in p53 acetylation by p300 as a major mechanism of the inhibitory effect of MLL leukemic fusions. Our data suggest that abrogation of p53 functional activity can be a common feature of MLL fusion-mediated leukemogenesis.Reciprocal chromosomal rearrangements involving the mixed lineage leukemia (MLL) 1 gene at the 11q23 locus are frequently detected in patients diagnosed with acute forms of both myeloid and lymphocytic leukemias (1, 2). As a result of these chromosomal aberrations, the N terminus of MLL is consistently fused in-frame to a number of partner proteins. Although more than 30 genes have been identified as fused to MLL in human leukemias, AF4, AF9, ENL, AF10, and ELL account for the absolute majority of recurrent MLL partners (3). The precise molecular mechanisms underlying the oncogenic function of MLL fusions are still poorly understood. We and others have recently shown that transient expression of MLL-ELL results in a potent and specific inhibition of p53 (4, 5), a critical tumor suppressor protein that mediates expression of multiple cell cycle regulatory and pro-apoptotic genes in response to stress (6). The disruption of p53 interactions with its co-activator p300 has been demonstrated to contribute to the MLL-ELL inhibitory effect on p53 (5).In the present study, we systematically analyzed the effect of additional frequently detected MLL fusion proteins on p53 activity. Our results show that MLL-AF9, MLL-AF10, and MLL-ENL associate with p53 through their partner proteins and significantly down-regulate the transcriptional activity of p53 in reporter assays. Furthermore, stable cell lines expressing low levels of MLL-AF9, MLL-AF10, MLL-ENL, and MLL-ELL exhibit impaired endogenous p53 response to both ionizing radiation and adriamycin treatment. Although having no discernible effect on p53 protein levels, MLL fusions significantly suppress p53-mediated induction of p21, MDM2, and Bax in response to various types of DNA damage. We also find that MLL fusions inhibit stress-induced p300-mediated p53 acetylation, which could explain their inhibitory effect on p53. Collectively, our data identify p53 functional inactivation as a common characteristic of multiple MLL fusions. EXPERIMENTAL PROCEDURESPlasmid Design-pcDNA-FLAG-p53, Myc-p53, pcDNA-FLAG-p53(6KR), pcDNA-MLL-ELL and pcDNA-MLL-ELL⌬CT have been described previously (5). MLL-AF9, MLL-AF10, and MLL-EN...

G1 cell-cycle arrest and apoptosis by histone deacetylase inhibition in MLL-AF9 acute myeloid leukemia cells is p21 dependent and MLL-AF9 independent

“…3 Induction of p21 by VPA was also p53-independent as THP-1 is p53-absent. 7 As in AML with MLL fusion proteins (including MLL-AF9) p53 is functionally inactivated leading to a lost response to DNA damage, 8 our data indicate that VPA could activate a p53-independent G1 cell-cycle arrest and apoptosis in MLL-mut AML.…”

Oncogenic stress sensitizes murine cancers to hypomorphic suppression of ATR

“…Within 20 days of ATR suppression, the abundance of ATR Δ/seckel AML cells was rapidly reduced to only a minor fraction of total AML cells in treated animals ( Figure 3C). This potent selection against ATR Δ/seckel AML cells was observed both in p53 +/+ AMLs, in which p53 function is partially inhibited through MLL-ENL expression (34), and in p53 -/-AMLs. Due to the robust expansion of residual AML cells that retained ATR expression through an unrecombined ATR fl allele, no survival benefit was readily evident, even with continuous tamoxifen treatment (data not shown).…”

Covert preleukemia driven by MLL gene fusion

“…This explanation accords with the current case. The question unanswered thus far is whether the additional mutational event, necessary for progression to overt leukemia, is stochastic or whether MLL fusion itself induces a particular type of aberration (e.g., through affecting of TP53 signaling (Wiederschain et al, 2005)). …”