RNA Polymerase I Transcription Inhibition: A novel therapeutic strategy for osteosarcoma
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2020
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Kang, Changwon
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Abstract
Osteosarcoma (OS) is the most commonly occurring primary bone cancer in the young. A combination of surgery and chemotherapy is the most effective therapeutic strategy, however, the survival rate for patients with metastatic disease is less than 20% at 5 years, a statistic that has not changed for over 30 years. To support the rapid cell growth and proliferation that are characteristic of cancer cells, the rate of ribosome biogenesis must increase, and the transcriptional activity of RNA Polymerase (Pol) I plays a key role in this process. RNA Pol I transcription activity in normal cells is maintained at a lower rate compared to that observed in cancer cells, which is mediated by a wide range of regulators such as p53, RB, MYC, AKT, S6K and MAPK. In OS, however, these regulators themselves can be dysregulated, for example TP53 and RB1 are frequently mutated, whereas c-Myc and MDM2 are amplified. c-Myc can enhance RNA Pol I transcription as can MDM2 by promoting degradation of p53, albeit indirectly. This combination of molecular alterations reported in OS led to the proposal that RNA Pol I transcription is likely to be enhanced, similar to many other cancers, hence its inhibition may prove an effective therapeutic avenue in OS. CX-5461 and PMR-116 are novel RNA Pol I transcription inhibitors (RNA Pol Ii) in the early stages of clinical development. The anti-tumour effects of CX-5461 have been demonstrated in Phase I clinical trials for breast cancer, lymphoma, leukaemia and prostate cancer. PMR-116 has proven effective in murine leukaemia, prostate cancer and myeloma models, but has not yet been tested in humans. The overall aim of this study was to investigate the therapeutic potential of RNA Pol Ii, CX-5461 and PMR-116, for the treatment of OS. Experiments in Chapter 3 describe the molecular characterization of 10 human OS cell lines. The expression status of p53, c-Myc, MDM2 and RB1 in these cell lines was assessed. The results of this work suggest three potential mechanisms through which RNA Pol I transcription activity and ribosome biogenesis are enhanced, thus promoting OS cell proliferation.
In Chapter 4, the effects of CX-5461 and PMR-116 on 10 human OS cell lines were evaluated in-vitro. Both drugs proved cytotoxic demonstrating anti-proliferative effects on all the OS cell lines tested. Both drugs inhibited their primary target, rDNA transcription, however CX-5461 showed a more pronounced anti-tumour effect through G2 cell cycle arrest in comparison to PMR-116.
In Chapter 5, the in-vivo effects of RNA Pol Ii were investigated using xenograft models of OS in Rag 2 knockout mice. The maximum tolerated drug doses in this mouse strain were established as 30 mg/kg for CX-5461 thrice-weekly and 80 mg/kg of PMR-116 twice-weekly. CX-5461 demonstrated significant tumour growth inhibitory effects in the xenografts using the human OS tumour 143B-Luc and SJSA-1-Luc, that are p53 mutant and p53 wild type respectively. In conclusion, these in-vitro studies proposed three possible mechanisms contributing to enhanced ribosome biogenesis in OS. Also, it demonstrated that CX-5461 and PMR-116 can suppress rDNA transcription and exerted an anti-proliferative effect on OS cell lines via G2 cell cycle arrest. In addition, CX-5461 demonstrated a significant reduction in OS tumour growth in-vivo. This research suggests that inhibition of RNA Pol I transcription is certainly worth pursuing as an additional therapeutic weapon in the fight against this aggressive cancer.
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