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J.N. Fisher et al.
cient to provide the cells the signals to escape apoptosis after crisis.
Among the genes downregulated in late passage iNUP98-KMT2A relative to control MEF, many have been implicated in cell cycle- or senescence-regulatory capacities. Previously, sirtuin1 (sirt1)-deficient MEF were shown to be resistant to replicative senescence through a p53-dependent mechanism;39 there is also evidence to suggest that Sirt1 plays a crucial role in Foxo3-activated cell cycle arrest.40 It has been shown that the insulin-like growth factor 1 receptor (IGF1R) ligand, insulin-like growth factor (IGF1), is involved in cellular senescence control through the Sirt1-p53 axis,41 in line with a pro- posed model whereby p53-dependent cellular senes- cence is counteracted by inhibition of IGF1R signaling.42 In a lung fibroblast cell model, Rbl2 (p130) expression was increased along with E2F-4 and markers of cellular senescence following heat shock protein-27 (HSP27) knock-down. Inhibition of Rbl2 counteracted the effects of HSP27 knock-down and significantly reduced senes- cence-associated β-galactosidase staining in a p53-inde- pendent manner.43 Additionally, the ribonuclease poly- merase Tert (telomerase reverse transcriptase), which maintains telomeric ends, has a well-demonstrated role in resisting senescence; however, some primary tumor samples have tested negative for telomerase activity44 and alternative mechanisms to overcome replication- associated telomeric shortening have been proposed with evidence for alternative lengthening of telomeres in 10-15% of cancers.45 Finally, the role of protein kinase C delta (Prkcd) in senescence is currently poorly under- stood, but studies have suggested that Prkcd is an impor- tant mediator of transforming growth factor-β-induced senescence.46
Peptides and small molecule antagonists of KMT2A- menin/LEDGF interactions have been shown to reduce the transforming activity of KMT2A fusions by interfering with
binding to targets, including the HOX-A gene cluster.5-10 It has been shown that leukemic transformation by NUP98 fusions is KMT2A-dependent;14 this would support the idea that iNUP98-KMT2A AML cells are susceptible to small molecules targeting the N-terminus of WT KMT2A. Conversely, the lack of the menin/LEDGF interaction site would predict poor sensitivity of iNUP98-KMT2A AML cells to these compounds. Indeed, compared to KMT2A- AF9-driven cells, iNUP98-KMT2A leukemic cells were resistant to blockade of the KMT2A-menin interaction by the small molecule MI-2-2 at concentrations previously demonstrated to inhibit growth of human KMT2A-AF4 and murine KMT2A-AF9 transformed cells.12,47 iNUP98- KMT2A AML cells also showed a reduced sensitivity to the BET-bromodomain inhibitor JQ1, which interferes with active transcription and elongation through displacement of BRD4 from chromatin,48 while challenge of KMT2A-AF9 cells recapitulated published growth inhibition.26 This sug- gests that targeting KMT2A might not be suitable for effi- cient therapeutic interference with NUP98-KMT2A AML.
Acknowledgments
The authors thank Danny Labes, Telma Lopes, Emmanuel Traunecker, and Lorenzo Raeli from the University of Basel Flow Cytometry Facility; Nicole Meier and the members of the Animal Care Facility at the University of Basel; Masao Seto for the full-length human KMT2A mRNA; Michael Kyba for pro- viding the A2Lox-Cre ES cells; and Patrick Kopp and Jean- Françoise Spetz for their assistance generating the iNUP98- KMT2A mice.
Funding
JS’s laboratory was supported by: grants from Swiss Cancer Research (KFS-4258-08-2017, KFS-3487-08-2014) and the Swiss National Science Foundation (SNF, 31003_A_173224/1). AP was supported by the Novartis Research Foundation. Basel, Switzerland.
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