Dysregulation of rRNA gene chromatin in ovarian cancer — ASN Events
  1. Schedule
  2. Poster Session 2
  3. Dysregulation of rRNA gene chromatin in ovarian cancer

Dysregulation of rRNA gene chromatin in ovarian cancer (#275)

Jinbae Son 1 2 , Karen E Sheppard 1 2 3 , Jessica Ahern 2 , Katherine M Hannan 2 4 , Gretchen Poortinga 2 5 , Austen R.D. Ganley 6 , Richard B Pearson 1 2 3 7 , Elaine Sanij 2 8 , Ross D Hannan 2 4 7 9
  1. Sir Peter MacCallum Department of Oncology, University of Melbourne, Melbourne, VIC, Australia
  2. Research division, Peter MacCallum Cancer Centre, Melbourne, VIC, Australia
  3. Biochemistry and Molecular Biology, University of Melbourne, Parkville, VIC, Australia
  4. Department of Cancer Biology and Therapeutics, The John Curtin School of Medical Research, Acton, ACT, Australia
  5. Department of Medicine, St Vincent's hospital, University of Melbourne, Fitzroy, VIC, Australia
  6. School of Biological Sciences, University of Auckland, Auckland, New Zealand
  7. Biochemistry and Molecular Biology, Monash University, Clayton, VIC, Australia
  8. Department of Pathology, University of Melbourne, Parkville, VIC, Australia
  9. School of Biomedical Sciences, University of Queensland, Brisbane, QLD, Australia

Ribosomal RNA (rRNA) gene (rDNA) is arrayed in multiple tandem repeats on the 5 acrocentric chromosomes, and is transcribed by RNA Polymerase I (Pol I) to produce 47S pre-rRNA, the precursor of 18S, 5.8S and 28S rRNAs. rDNA transcription accounts for around 35-60% of all cellular transcription, and is highly energy consuming processes as well as a key determinant of cellular growth and proliferation. The highly repetitive and actively transcribed nature of rDNA gives rise to a high recombinogenic potential. Further, structural dynamics and variation in the rDNA loci have been reported in over 50% of solid human cancers (1).

 

Although rDNA transcription is rate limiting in ribosome biogenesis, only a subset of rRNA genes are transcribed from “active” rDNA at any given time. rDNA chromatin exists in active or silent forms. Active rDNA chromatin is “open” and bound by the upstream binding transcription factor (UBF), which is essential in governing and maintaining active rDNA states (2).

We have reported that rDNA silencing increases during terminal differentiation of granulocytes due to decreased UBF levels(3). Conversely, our studies utilising MYC-driven B-lymphoma mouse model demonstrate a reactivation of silent rDNA as B-cells progress towards malignancy. We have also demonstrated that inhibition of rDNA transcription by the novel Pol I inhibitor CX-5461 can selectively kill B-lymphoma cells in vivo, while sparing wild-type B-cells (4). However, the mechanisms underlying sensitivity to CX-5461 remain unclear.

 

We have undertaken a systematic approach across a panel of ovarian cancer (OVCA) cell lines to examine their sensitivity to CX-5461. Our preliminary data shows that OVCA cell lines display similar sensitivities to Pol I transcription inhibition however they exhibit differential cellular responses, involving either an immediate or a delayed cell cycle arrest. Furthermore, our studies suggest that increased active rDNA chromatin and not rDNA transcription rate per se is associated with a prompt growth inhibitory response to CX-5461 treatment of OVCA cells.

We will utilise CRISPR-Cas9 knock-out and UBF knock-down approaches to investigate whether rDNA dosage and/or rDNA chromatin states play a role in determining sensitivity to CX-5461. This study will demonstrate the importance of stability of rDNA in contributing to genome wide instability, also could potentially serve as a biomarker to screen for OVCA patients that may benefit from CX-5461 treatment.

  1. Stults D, Killen M, Williamson E, Hourigan J, Vargas H, Arnold S, et al. Human rRNA gene clusters are recombinational hotspots in cancer. Cancer research. 2009;69:9096 - 104.
  2. Sanij E, Poortinga G, Sharkey K, Hung S, Holloway TP, Quin J, et al. UBF levels determine the number of active ribosomal RNA genes in mammals. The Journal of cell biology. 2008;183(7):1259-74.
  3. Sanij E, Diesch J, Lesmana A, Poortinga G, Hein N, Lidgerwood G, et al. A novel role for the Pol I transcription factor UBTF in maintaining genome stability through the regulation of highly transcribed Pol II genes. Genome research. 2015;25(2):201-12.
  4. Bywater MJ, Poortinga G, Sanij E, Hein N, Peck A, Cullinane C, et al. Inhibition of RNA polymerase I as a therapeutic strategy to promote cancer-specific activation of p53. Cancer cell. 2012;22(1):51-65.
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