Abstract 3110: RNA Polymerase I: A novel target in the treatment of Myc-driven malignancy
Authors: Bywater Megan J. , Poortinga Gretchen , Cullinane Carleen , Stanley Kym , Walker Rachael , Drygin Denis , Anderes Kenna , Johnstone Ricky , McArthur Grant A. , Hannan Ross D. 
Published: February 2011

Abstract
c-MYC plays a prominent role in cancer. Intriguingly, many of the genes regulated by this oncoprotein are associated with ribosome biogenesis and we have previously demonstrated that Myc regulates a major rate limiting step in this pathway, transcription of the 45S rRNA genes by RNA Polymerase I (Pol I). These observations lead us to hypothesize that cMyc's regulation of rRNA synthesis may contribute to its oncogenic properties. We have tested this hypothesis in a mouse model of Myc-driven lymphoma, the EμMyc transgenic mouse.
B-cells purified from EμMyc mice display an increased growth rate in comparison to their wild-type littermates, with increased cell volume, total RNA and protein per cell. This phenotype is characterized by higher rates of 45S rRNA transcription and increased expression of factors specific for Pol I transcription. Knockdown of one of these factors, UBF, by RNAi in EμMyc lymphoma cell lines results in a selective proliferative disadvantage of cells in vitro, in a competition assay, and in vivo, in a transplant model. This phenotype is driven by an increased rate of apoptosis associated with a reduction in 45S rRNA transcription.
Based on these findings we explored the potential therapeutic effectiveness in this model of a novel specific small molecule inhibitor of Pol I, CX-5461, currently in preclinical development. Transplanted EμMyc tumors showed marked sensitivity to CX-5461 in vivo, with a dramatic reduction in tumor burden in the peripheral blood (97.54%±0.56) and lymph nodes (94.96%±0.90) due to induction of apoptosis 24hrs following a single oral dose at 75mg/kg. Importantly a normal B-cell population was preferentially maintained in treated mice (13%±1.39 wt B220+ cells versus 1.04%±0.24 tumor B220+ cells, as a percentage of total WBC) indicating specificity of the compound for tumor cells. Four doses of CX-5461, 75mg/kg orally every third day, significantly delayed time to endpoint by 9.5 days (P&lt;0.0001) compared to untreated animals. This delay was accompanied by a period of complete remission with normal white blood cell counts (6.76±0.48 ×10^9cells/L) and no identifiable tumor cells in the peripheral blood. Interestingly, in vitro dose curves indicate a dependence of CX-5461 sensitivity on wild-type p53 function (p53 wt and ARF−/− cell line IC50=9.28nM±1.53 in comparison to p53 mutant and p53−/− IC50=1.70uM±0.03), which can be reduced with over expression of Bcl2 (Bcl2 IC50=2.33uM±1.3). Notably even in the more resistant p53 mutant and p53−/− cell lines, cell death also occurred via apoptosis, suggesting p53-dependent and independent mechanisms to be involved in CX-5461 mediated cell death.
In summary, this work with UBF RNAi and CX-5461 identifies inhibition of RNA Pol I transcription as a novel and effective target in the treatment of cMyc-driven malignancies, and for the first time establishes that dysregulation of rDNA transcription can directly contribute to malignant transformation.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3110.