Poster Presentation 29th Lorne Cancer Conference 2017

Targeting rDNA chromatin by BRD4 inhibition as a highly effective therapeutic strategy for synergistic checkpoint activation by Pol I inhibitor CX-5461 in acute myeloid leukaemia   (#272)

Jirawas Sornkom 1 2 , Nadine Hein 1 3 , Don Cameron 1 2 3 , Chun Yew Fong 1 , Neil Garton 4 , Mark Dawson 1 2 , Rick Pearson 1 2 5 6 , Carleen Cullinane 1 , Ross Hannan 1 2 3 5 6 7 , Gretchen Poortinga 1 2 8
  1. Research Division, Peter MacCallum Cancer Centre, Melbourne, VIC, Australia
  2. Sir Peter MacCallum Department of Oncology, University of Melbourne, Parkville, Victoria, Australia
  3. ACRF Department of Cancer Biology and Therapeutics, John Curtin School of Medical Research, Canberra, ACT, Australia
  4. Medicines Research Centre , GlaxoSmithKline R&D, Hertfordshire, UK
  5. Department of Biochemistry and Molecular Biology, Monash University, Clayton, Victoria, Australia
  6. Department of Biochemistry and Molecular Biology, University of Melbourne, Parkville, Victoria, Australia
  7. School of Biomedical Sciences, University of Queensland, Brisbane, Queensland, Australia
  8. Medicine, St Vincent's Hospital, University of Melbourne, Fitzroy, Victoria, Australia

The ribosomal DNA genes (rDNA) exist at approximately 300-400 copies in mammalian cells, and their transcription is highly upregulated in cancers. CX-5461 is a specific inhibitor of RNA polymerase I (Pol I) mediated rDNA transcription and is currently in phase I clinical trial for hematological malignancies. Specifically, inhibition of rDNA transcription by CX-5461 leads to activation of both p53-dependent and -independent cellular checkpoints1,2. Further, activation of a DNA damage response (DDR) via ATM and ATR signaling are essential for CX-5461’s ability to arrest leukemia cells in S and G2/M phases (Hein et al., under review). Importantly, factors involved in DDR have been shown to greatly sensitise cancer cells to CX-5461 treatment2. It is known that mechanistic interactions underlying the DDR rely on context-specific chromatin structure, including the structural rearrangement of the chromatin by histone chaperones and the bromodomain and extra-terminal family of proteins (BET)3. Due to the highly specialised transcriptional context of rDNA, we hypothesise that targeting epigenetic and transcriptional regulators in combination with CX-5461 may render cancer cells highly sensitive to ATM/ATR activation and result in enhanced anti-tumour activity.

 

In this study we utilised IBET-151, an inhibitor that blocks BET protein binding to acetylated chromatin marks in combination with CX-5461 in a disease model of acute myeloid leukemia (AML). We observed a range of synergy between CX-5461 and IBET-151 in a panel of human AML cell lines. Importantly, co-treatment with CX-5461 and IBET-151 significantly improved survival over single agents alone in a preclinical model of aggressive MLL-AF9; NrasG12D-driven AML. Studies in human AML cell lines showed an increase in rDNA accessibility to micrococcal nuclease digestion following treatment with IBET-151 or the knockdown of its target, BRD4. We propose that inhibition of BRD4 by IBET-151 results in an ‘open’ state of rDNA chromatin which leads to an enhanced CX-5461-mediated DDR. This model is supported by enhanced checkpoint activation, including a heightened γH2AX response following combined CX-5461 and IBET-151 treatment. We will present our work thus far investigating the mechanistic role of this drug combination in eliciting synergistic checkpoint activation in AML

  1. Bywater, M. J. et al. Inhibition of RNA polymerase I as a therapeutic strategy to promote cancer-specific activation of p53. Cancer Cell 22, 51–65 (2012).
  2. Quin, J. et al. Inhibition of RNA polymerase I transcription initiation by CX-5461 activates non-canonical ATM/ATR signaling. Oncotarget (2016).
  3. Floyd, S. R. et al. The bromodomain protein Brd4 insulates chromatin from DNA damage signalling. Nature 498, 246–250 (2013).