Poster Presentation 25th Lorne Cancer Conference 2013

A Novel Inhibitor of Ribosomal RNA Gene Transcription Identifies both p53-Dependent and p53-Independent Checkpoint Responses to Perturbation of Pol I Transcription (#321)

Jaclyn Quin 1 2 , Elaine Sanij 2 , Denis Drygin 3 , Richard Pearson 1 2 , Grant McArthur 2 , Ross Hannan 1 2
  1. Department of Biochemistry and Molecular Biology, University of Melbourne, Melbourne, VIC, Australia
  2. Peter MacCallum Cancer Centre, East Melbourne, VIC, Australia
  3. Cylene Pharmaceuticals Inc, San Diego, CA, USA

The transcription of the 45S ribosomal RNA (rRNA) genes by RNA Polymerase I (Pol I) is a major rate limiting step for ribosomal biogenesis, and consequently for cell growth and proliferation. Dysregulation of rDNA transcription is an almost universal feature of cell transformation and cancer, and work from our laboratory has shown that targeting this process using a novel small molecule inhibitor of Pol I transcription, CX-54611 , is a promising new approach for the treatment of hematologic malignancies2 . CX-5461 also provides an opportunity to investigate acute checkpoint responses to the selective perturbation of rDNA transcription.  We have utilized CX-5461 to examine the response of BJ (hTert) immortalized primary human fibroblasts, and isogenically matched BJ cell lines at defined stages of transformation, to inhibition of Pol I transcription.

Following Pol I transcription inhibition, BJ (hTert) cells display a robust proliferation defect and eventually undergo senescence. Pre-malignant BJ (Large-T, hTert) cells display increased sensitivity to CX-5461, and undergo cell death; Transformed BJ (Large-T, Small-T, hTert, Ras) cells are more resistant to CX-5461, but exhibit increased ploidy. The proliferative defect following CX-5461 treatment in BJ (hTert) cells is associated with both p53-dependent G1 cell cycle arrest, and p53-independent S-phase delay and G2 cell cycle arrest. While the mechanisms of activation of p53 in response to Pol I transcription inhibition have been well described, the p53-independent checkpoints are less well characterized. We have identified acute activation of ATM–dependent signaling following CX-5461 treatment, in the absence of DNA damage. Understanding these novel p53-independent mechanisms of response to CX-5461 may provide the opportunity to utilize inhibitors of Pol I transcription to more efficiently target p53 mutant tumors.

  1. Drygin, D., et al., Targeting RNA Polymerase I with an Oral Small Molecule CX-5461 Inhibits Ribosomal RNA Synthesis and Solid Tumor Growth. Cancer Research. 71(4): p. 1418-1430.
  2. Bywater, M.J., et al., Inhibition of RNA Polymerase I as a Therapeutic Strategy to Promote Cancer-Specific Activation of p53. Cancer Cell. 22(1): p. 51-65.