Poster Presentation 27th Lorne Cancer Conference 2015

Dysregulation of Pol I transcription in ovarian cancer (#268)

Jinbae Son 1 2 , Jeannine Diesch 1 , Richard B Pearon 1 2 3 4 , Elaine Sanij 1 2 5 , Ross D Hannan 1 2 3 4 6
  1. Research division, Peter MacCallum Cancer Centre, Melbourne, VIC, Australia
  2. Sir Peter MacCallum Department of Oncology, University of Melbourne, East Melbourne, VIC, Australia
  3. Biochemistry and Molecular Biology, Monash University, Clayton, VIC, Australia
  4. Biochemistry and Molecular Biology, University of Melbourne, Parkville, VIC, Australia
  5. Department of Pathology, University of Melbourne, Parkville, VIC, Australia
  6. School of Biomedical Sciences, University of Queensland, Brisbane, QLD, Australia

The transcription of the 300 copies of ribosomal RNA (rRNA) genes (rDNA) by RNA polymerase I is the first critical step in ribosome synthesis, and accounts for 35~60% of all cellular transcription. Work from our laboratory has demonstrated that inhibition of Pol I transcription can selectively kill tumour cells in vivo (Bywater, Poortinga et al. 2012). Although rDNA transcription is growth limiting, over 50% of the rRNA genes are believed to be transcriptionally silent at any one time. The epigenetic mechanisms controlling rDNA silencing in eukaryotes remain unclear. We reported that the pool of active rDNA decrease during terminal differentiation of granulocytes (2, 3). Conversely, our preliminary data, utilising the E-Myc transgenic mouse model of B-cell lymphoma demonstrate that as B-cells progress towards malignancy there is a dramatic reactivation of previously silent rDNA, which correlates with hyperactivation of Pol I and increased rDNA transcription.

Furthermore, the repetitive nature and high transcription rates of rRNA genes render the rDNA one of the most fragile sites in the genome. Structural alterations in the rDNA loci have been reported in over 50% of solid human cancers(4), making the rDNA a recombinational hotspot. We therefore hypothesize that alterations in rDNA copy number and changes in the silent-to-active rDNA ratio are associated with cellular transformation and can possibly act as a driver of malignancy. This study examines rDNA copy number variations and epigenetic changes in rDNA chromatin in a panel of ovarian cancer cell lines that exhibit varied sensitivities to inhibition of Pol I transcription to address whether their differential sensitivities correlate with structural and epigenetics differences at rDNA loci. In addition, we examine the effect of reduced rDNA copy number in cancer cells by using genome editing tools.

  1. Bywater, M. J., G. Poortinga, E. Sanij, N. Hein, A. Peck, C. Cullinane, M. Wall, L. Cluse, D. Drygin, K. Anderes, N. Huser, C. Proffitt, J. Bliesath, M. Haddach, M. K. Schwaebe, D. M. Ryckman, W. G. Rice, C. Schmitt, S. W. Lowe, R. W. Johnstone, R. B. Pearson, G. A. McArthur and R. D. Hannan (2012). "Inhibition of RNA polymerase I as a therapeutic strategy to promote cancer-specific activation of p53." Cancer Cell 22(1): 51-65.
  2. Sanij, E., G. Poortinga, K. Sharkey, S. Hung, T. P. Holloway, J. Quin, E. Robb, L. H. Wong, W. G. Thomas, V. Stefanovsky, T. Moss, L. Rothblum, K. M. Hannan, G. A. McArthur, R. B. Pearson and R. D. Hannan (2008). "UBF levels determine the number of active ribosomal RNA genes in mammals." J Cell Biol 183(7): 1259-1274.
  3. Sanij, E. and R. D. Hannan (2009). "The role of UBF in regulating the structure and dynamics of transcriptionally active rDNA chromatin." Epigenetics 4(6): 374-382.
  4. Stults, D., M. Killen, E. Williamson, J. Hourigan, H. Vargas, S. Arnold, J. Moscow and A. Pierce (2009). "Human rRNA gene clusters are recombinational hotspots in cancer." Cancer Res 69: 9096 - 9104.