A joint physics and radiobiology DREAM team vision - Towards better response prediction models to advance radiotherapy

Radiotherapy developed empirically through experience balancing tumour control and normal tissue toxicities. Early simple mathematical models formalized this practical knowledge and enabled effective cancer treatment to date. Remarkable advances in technology, computing, and experimental biology now create opportunities to incorporate this knowledge into enhanced computational models. The ESTRO DREAM (Dose Response, Experiment, Analysis, Modelling) workshop brought together experts across disciplines to pursue the vision of personalized radiotherapy for optimal outcomes through advanced modelling. The ultimate vision is leveraging quantitative models dynamically during therapy to ultimately achieve truly adaptive and biologically guided radiotherapy at the population as well as individual patient-based levels. This requires the generation of models that inform response-based adaptations, individually optimized delivery and enable biological monitoring to provide decision support to clinicians. The goal is expanding to models that can drive the realization of personalized therapy for optimal outcomes. This position paper provides their propositions that describe how innovations in biology, physics, mathematics, and data science including AI could inform models and improve predictions. It consolidates the DREAM team's consensus on scientific priorities and organizational requirements. Scientifically, it stresses the need for rigorous, multifaceted model development, comprehensive validation and clinical applicability and significance. Organizationally, it reinforces the prerequisites of interdisciplinary research and collaboration between physicians, medical physicists, radiobiologists, and computational scientists throughout model development. Solely by a shared understanding of clinical needs, biological mechanisms, and computational methods, more informed models can be created. Future research environment and support must facilitate this integrative method of operation across multiple disciplines.

ClonoScreen3D - A Novel 3-Dimensional Clonogenic Screening Platform for Identification of Radiosensitizers for Glioblastoma

PURPOSE

Glioblastoma (GBM) is a lethal brain tumor. Standard-of-care treatment comprising surgery, radiation, and chemotherapy results in median survival rates of 12 to 15 months. Molecular-targeted agents identified using conventional 2-dimensional (2D) in vitro models of GBM have failed to improve outcome in patients, rendering such models inadequate for therapeutic target identification. A previously developed 3D GBM in vitro model that recapitulates key GBM clinical features and responses to molecular therapies was investigated for utility for screening novel radiation-drug combinations using gold-standard clonogenic survival as readout.

METHODS AND MATERIALS

Patient-derived GBM cell lines were optimized for inclusion in a 96-well plate 3D clonogenic screening platform, ClonoScreen3D. Radiation responses of GBM cells in this system were highly reproducible and comparable to those observed in low-throughout 3D assays. The screen methodology provided quantification of candidate drug single agent activity (half maximal effective concentration or EC50) and the interaction between drug and radiation (radiation interaction ratio).

RESULTS

The poly(ADP-ribose) polymerase inhibitors talazoparib, rucaparib, and olaparib each showed a significant interaction with radiation by ClonoScreen3D and were subsequently confirmed as true radiosensitizers by full clonogenic assay. Screening a panel of DNA damage response inhibitors revealed the expected propensity of these compounds to interact significantly with radiation (13/15 compounds). A second screen assessed a panel of compounds targeting pathways identified by transcriptomic analysis and demonstrated single agent activity and a previously unreported interaction with radiation of dinaciclib and cytarabine (radiation interaction ratio 1.28 and 1.90, respectively). These compounds were validated as radiosensitizers in full clonogenic assays (sensitizer enhancement ratio 1.47 and 1.35, respectively).

CONCLUSIONS

The ClonoScreen3D platform was demonstrated to be a robust method to screen for single agent and radiation-drug combination activity. Using gold-standard clonogenicity, this assay is a tool for identification of radiosensitizers. We anticipate this technology will accelerate identification of novel radiation-drug combinations with genuine translational value.

O2: TOWARDS A LIVING BIOBANK OF SURGICALLY-RELEVANT 3-DIMENSIONAL GLIOBLASTOMA STEM CELL MODELS TO EVALUATE NOVEL THERAPEUTICS AND INTERROGATE INTRATUMOURAL HETEROGENEITY

Introduction

Glioblastoma is the most common cancer arising within the brain. Despite surgery, followed by DNA-damaging chemoradiotherapy, average survival remains between 12-15 months. Unacceptable survival rates underline the need to develop preclinical research models which recapitulate features underpinning therapeutic resistance in patients, such as intratumoural heterogeneity and treatment resistant glioblastoma stem cell (GSC) subpopulations which demonstrate elevated DNA damage response (DDR) activity.

Method

Tumour specimens from patients were used to generate 2D and 3D scaffold-based GSC models, with a range of preclinical survival and molecular assays used to interrogate cancer biology and assess therapeutic responses.

Result

We have developed a ‘living biobank’ of 20+ ex-vivo GSC models which reflect key clinicopathological diversity. These models include residual disease models based on careful macrodissection of rare en-blocpartial lobectomy specimens to liberate parallel GSC lines from the tumour core and adjacent infiltrated brain, to represent cells typically left behind after surgery. Therapeutic strategies targeting fundamental DDR processes demonstrate preclinical efficacy, for example dual inhibition of ATR and the FA DNA damage repair pathways elicits profound radiosensitisation (sensitiser enhancement ratio of 3.23 (3.03-3.49, 95%-CI)) with evidence of delayed DNA damage repair on single-cell gel electrophoresis. Finally, characterisation of our surgically-relevant resected and residual models reveals numerous divergent properties including elevated stem cell marker expression in residual models (p=0.0021), which may partially explain treatment resistance in disease left behind after surgery.

Conclusion

Our living biobank represents a useful resource for preclinical glioblastoma research and demonstrates the value of partnership between surgeons and laboratory-based scientists.

Take-home message

Our living biobank represents a useful resource for preclinical glioblastoma research and demonstrates the value of partnership between surgeons and laboratory-based scientists.

Abstract PO-017: Radiotherapy in combination with the brain penetrant ATM inhibitor AZD1390 does not exacerbate radiation toxicity of neural stem cells in vitro or in vivo

While radiotherapy (RT) is fundamental for the treatment of brain tumors, irradiation of the brain frequently causes devastating effects on cognitive function and quality of life. DNA damage within neural stem cells (NSC) is a key factor in the pathogenesis of radiation-induced cognitive dysfunction. The ataxia telangiectasia mutated (ATM) kinase is a central protein in the DNA damage response and a critical determinant of tumor cell survival after radiation. ATM inhibition potently radiosensitizes preclinical models of GBM in vitro and in vivo. A novel, brain penetrant ATM inhibitor AZD1390, which is predicted to achieve brain tumor concentrations in the range of 1-5nM, is currently in early phase clinical evaluation in combination with RT. In marked contrast to observations in tumor models, genetic knockdown of ATM has radioprotective effects on NSC in vitro; the proposed mechanism is via suppression of p53 mediated apoptosis. The purpose of this study was to investigate the impact of AZD1390 on survival responses and mode of death in NSCs exposed to RT in vitro and in vivo. NSCs were derived from the telencephalon of E13 mouse embryos. Cells were treated with AZD1390 (0.1-10nM) 1 hour prior to ionizing radiation (IR; 0-5 Gy). Mode and timing of cell death was interrogated using IncuCyte live cell analysis to measure proliferation, cytotoxicity and apoptosis up to 72 hours post-IR. Cell viability and neurosphere formation assays were also used to measure radiation sensitivity in vitro. C57BL/6 mice received 20Gy hemibrain irradiation +/- 7-day treatment with AZD1390 (10mg/kg). Immunohistochemistry for Ki67 and Sox2 was used to assess effects on NSC in the subventricular zone (SVZ) 50 days post-irradiation. In vitro AZD1390 (1-10nM) inhibited ATM kinase function within 1 hour, evidenced by abrogation of KAP1 and p53 phosphorylation. NSCs primarily undergo apoptosis in response to IR. AZD1390 at 1 and 3nM significantly reduced apoptosis in irradiated NSCs (ratios of annexin V area under the curve 1.95 and 2 respectively); 10 nM had no effect on this parameter. Proliferation rates and cell viability after radiation were preserved at all drug concentrations. AZD1390 at 1nM did not modulate radiation effects on neurosphere formation whereas at 10nM a radiosensitizing effect was observed (ratio of SF[3Gy]=0.25). In vivo, IR decreased the number of Ki67 positive proliferating cells (92% reduction) and Sox2-positive cells (24% reduction) in the SVZ after 50 days; these effects were not exacerbated by addition of AZD1390. Acute effects (24 hours post-IR) are under investigation. We demonstrate in vitro that AZD1390 has radioprotective effects on NSCs at clinically achievable concentrations. In vivo, treatment with AZD1390 did not enhance the effects of radiation on NSCs in the SVZ. In the context of its profound radiosensitizing effects on GBM models, the absence of radiosensitization of NSCs both in vitro and in vivo strengthens the rationale for evaluating AZD1390 in combination with RT in GBM patients.

Citation Format: Rodrigo Guttierez-Quintana, David J. Walker, Mark R. Jackson, Natividad Gomez-Roman, Sandeep Chahal, Stephen T. Durant, Anthony J. Chalmers. Radiotherapy in combination with the brain penetrant ATM inhibitor AZD1390 does not exacerbate radiation toxicity of neural stem cells in vitro or in vivo [abstract]. In: Proceedings of the AACR Virtual Special Conference on Radiation Science and Medicine; 2021 Mar 2-3. Philadelphia (PA): AACR; Clin Cancer Res 2021;27(8_Suppl):Abstract nr PO-017.

Quantitative in vivo bioluminescence imaging of orthotopic patient-derived glioblastoma xenografts

Despite extensive research, little progress has been made in glioblastoma therapy, owing in part to a lack of adequate preclinical in vivo models to study this disease. To mitigate this, primary patient-derived cell lines, which maintain their specific stem-like phenotypes, have replaced established glioblastoma cell lines. However, due to heterogenous tumour growth inherent in glioblastoma, the use of primary cells for orthotopic in vivo studies often requires large experimental group sizes. Therefore, when using intracranial patient-derived xenograft (PDX) approaches, it is advantageous to deploy imaging techniques to monitor tumour growth and allow stratification of mice. Here we show that stable expression of near-infrared fluorescent protein (iRFP) in patient-derived glioblastoma cells enables rapid, direct non-invasive monitoring of tumour development without compromising tumour stemness or tumorigenicity. Moreover, as this approach does not depend on the use of agents like luciferin, which can cause variability due to changing bioavailability, it can be used for quantitative longitudinal monitoring of tumour growth. Notably, we show that this technique also allows quantitative assessment of tumour burden in highly invasive models spreading throughout the brain. Thus, iRFP transduction of primary patient-derived glioblastoma cells is a reliable, cost- and time-effective way to monitor heterogenous orthotopic PDX growth.

Radiation Responses of 2D and 3D Glioblastoma Cells: A Novel, 3D-specific Radioprotective Role of VEGF/Akt Signaling through Functional Activation of NHEJ

Glioblastoma is resistant to conventional treatments and has dismal prognosis. Despite promising in vitro data, molecular targeted agents have failed to improve outcomes in patients, indicating that conventional two-dimensional (2D) in vitro models of GBM do not recapitulate the clinical scenario. Responses of primary glioblastoma stem-like cells (GSC) to radiation in combination with EGFR, VEGF, and Akt inhibition were investigated in conventional 2D cultures and a three-dimensional (3D) in vitro model of GBM that recapitulates key GBM clinical features. VEGF deprivation had no effect on radiation responses of 2D GSCs, but enhanced radiosensitivity of GSC cultures in 3D. The opposite effects were observed for EGFR inhibition. Detailed analysis of VEGF and EGF signaling demonstrated a radioprotective role of Akt that correlates with VEGF in 3D and with EGFR in 2D. In all cases, positive correlations were observed between increased radiosensitivity, markers of unrepaired DNA damage and persistent phospho-DNA-PK nuclear foci. Conversely, increased numbers of Rad51 foci were observed in radioresistant populations, indicating a novel role for VEGF/Akt signaling in influencing radiosensitivity by regulating the balance between nonhomologous end-joining and homologous recombination-mediated DNA repair. Differential activation of tyrosine kinase receptors in 2D and 3D models of GBM explains the well documented discrepancy between preclinical and clinical effects of EGFR inhibitors. Data obtained from our 3D model identify novel determinants and mechanisms of DNA repair and radiosensitivity in GBM, and confirm Akt as a promising therapeutic target in this cancer of unmet need.

TMOD-39. EX-VIVO 3-DIMENSIONAL MODELS OF POST-SURGICAL RESIDUAL DISEASE IN HUMAN GLIOBLASTOMA

Translation of novel therapies for glioblastoma from laboratory to clinical trials has relied heavily on cells derived from within patient tumours. However, it is clear from over 40 years of limited improvement in survival rates, that the traditional paradigm of 2-dimensional in-vitro studies using these cells followed by implantation into mouse models poorly predicts the treatment response of glioblastoma cells left behind after surgery in patients. Additionally, recent increases in the number of new therapies requiring pre-clinical evaluation emphasise a need to apply disease models that faithfully and efficiently reproduce biological features of post-surgical residual glioblastoma in patients. Dependent on the anatomical considerations of individual cases, surgical resection occasionally provides an en-bloc specimen (e.g. partial lobectomy), which includes portions of infiltrated brain that would not otherwise be resected. Such specimens provide crucial material to model and understand residual (typically left behind) disease in glioblastoma. Using such specimens, we have developed a pragmatic methodology to liberate glioblastoma stem cells (GSC) from the tumour core and adjacent infiltrated brain to model resected and residual disease, respectively. Imperative considerations include careful patient selection and intraoperative clinical-academic collaboration, with immediate ex-vivo hemi-section and processing of suitable specimens. Initial biological characterisation of our resected and residual models reveals numerous divergent properties including morphological differences and elevated stem cell marker expression in residual models (p=0.0021). Importantly, although GSC within recently derived 2-dimensional residual disease models are frequently too migratory to reliably form discrete colonies, we are able to perform robust clonogenic survival studies using the same GSC within a novel customised 3-dimensional scaffold-based architecture (3.4-fold greater clonogenic potential, p=0.0010). In conclusion, we believe that our models provide a biologically relevant and cost-effective tool to investigate the nuances of residual glioblastoma and assess the potential of therapies to tackle disease normally left behind following surgery in patients.

RDNA-12. THE FANCONI ANAEMIA (FA) PATHWAY AND GLIOBLASTOMA: A NEW FOUNDATION FOR DNA DAMAGE RESPONSE TARGETED COMBINATIONS

Treatment resistance in glioblastoma is underpinned by highly interconnected DNA damage response (DDR) processes. The FA-pathway is a fundamental DDR process required for the resolution of replication fork impeding lesions, and we have previously shown that it is inactive in normal brain, but is re-activated in glioblastoma, providing a cancer-specific target for combination DDR therapies. Here, we find that elevated FA-pathway gene expression in gliomas is associated with poor survival (-17.1% 5-year OS, p< 0.0001, n=329–REMBRANT). Furthermore, patient-derived glioblastoma stem cell (GSC) populations, which drive therapeutic resistance, display high FA-pathway expression relative to paired bulk tumour cell populations (mean 2.3-fold higher across genes, p=0.0073). We further show that inhibition of a single DDR process (FA-pathway, PARP, ATR or ATM) increases the susceptibility of glioblastoma cell lines and patient-derived GSCs to current adjuvant therapy. Importantly, clinically approved PARP inhibitor (PARPi) monotherapy stimulates robust FANCD2 mono-ubiquitination, supporting a role of FA-pathway activation in response to current DDR-targeted therapy. In clinically-relevant 3D GSC models, simultaneous inhibition of the FA-pathway (FAPi) and PARP or ATR enhanced temozolomide sensitisation compared to a single DDR inhibitor (DDRi). Furthermore, combined FAPi+PARPi consistently conferred radiosensitisation whilst combined FAPi+ATRi led to a profoundly radiosensitising effect; e.g. sensitizer enhancement ratio (SER0.37) of 3.23 (3.03–3.49, 95% CI). Furthermore, comparison of α/β ratio enhancement suggests dual-DDRi strategies fundamentally alter the response of GSCs, whilst single cell gel electrophoresis & immunofluorescence studies suggest FA-pathway based DDRi combinations profoundly delay the resolution of IR-induced DNA strand breaks at 6 hours post-treatment, with increased persistent DNA double strand breaks at 24 hours. In conclusion, simultaneously targeting the FA-pathway and interconnected DDR processes represents an appealing therapeutic strategy. Additionally, constitutive lack of FA pathway function in some tumours, could serve as a novel predictive biomarker for patient response to PARPi and ATRi currently in clinical trials.

Inhibitors of poly(ADP-ribose) glycohydrolase (PARG) exhibit single agent therapeutic activity and sensitize glioblastoma cells to ionizing radiation

Objective

The lack of an effective therapy for glioblastoma (GBM) largely results from the intrinsic resistance of GBM cells. The radiosensitizing activity of inhibitors of poly(ADP-ribose) polymerases (PARPs) highlights the important role of poly(ADP-ribose) (PAR) in the DNA damage response. In contrast to PARPs, inhibition of poly(ADP-ribose) glycohydrolase (PARG), the enzyme responsible for degrading PAR chains, has shown single agent therapeutic activity in non-glioma cancer cells. This work aims to validate the therapeutic potential of PARG inhibitors (PARGi) in GBM.

Results

Baseline PAR levels were found to vary between different primary and commercial GBM cells, with PARylation increasing upon exposure of cells to ionizing radiation (IR), as expected. Target engagement of a novel PARGi, PDD00017273, was confirmed by the accumulation of nuclear PAR in treated cells. Inhibitor specificity was demonstrated using an inactive control compound and by combining PARGi with the PARP inhibitor olaparib, which blocked the effect. Single agent treatment with PARGi reduced the clonogenic survival of GBM cells in a concentration-dependent manner. Importantly, PARGi also sensitized GBM cells to IR (sensitizer enhancement ratios, SER, ≥ 1.40)

Conclusion

In contrast to PARP inhibitors, novel PARGi exhibit single agent activity against a panel of GBM cell lines, and also show robust radiosensitizing activity. PARGi therefore have therapeutic potential in this cancer of unmet need.

Replication Stress Drives Constitutive Activation of the DNA Damage Response and Radioresistance in Glioblastoma Stem-like Cells

Glioblastoma (GBM) is a lethal primary brain tumor characterized by treatment resistance and inevitable tumor recurrence, both of which are driven by a subpopulation of GBM cancer stem-like cells (GSC) with tumorigenic and self-renewal properties. Despite having broad implications for understanding GSC phenotype, the determinants of upregulated DNA-damage response (DDR) and subsequent radiation resistance in GSC are unknown and represent a significant barrier to developing effective GBM treatments. In this study, we show that constitutive DDR activation and radiation resistance are driven by high levels of DNA replication stress (RS). CD133+ GSC exhibited reduced DNA replication velocity and a higher frequency of stalled replication forks than CD133- non-GSC in vitro; immunofluorescence studies confirmed these observations in a panel of orthotopic xenografts and human GBM specimens. Exposure of non-GSC to low-level exogenous RS generated radiation resistance in vitro, confirming RS as a novel determinant of radiation resistance in tumor cells. GSC exhibited DNA double-strand breaks, which colocalized with "replication factories" and RNA: DNA hybrids. GSC also demonstrated increased expression of long neural genes (>1 Mbp) containing common fragile sites, supporting the hypothesis that replication/transcription collisions are the likely cause of RS in GSC. Targeting RS by combined inhibition of ATR and PARP (CAiPi) provided GSC-specific cytotoxicity and complete abrogation of GSC radiation resistance in vitro These data identify RS as a cancer stem cell-specific target with significant clinical potential.Significance: These findings shed new light on cancer stem cell biology and reveal novel therapeutics with the potential to improve clinical outcomes by overcoming inherent radioresistance in GBM. Cancer Res; 78(17); 5060-71. ©2018 AACR.

Hypoxia-inducible factor 1 alpha is required for the tumourigenic and aggressive phenotype associated with Rab25 expression in ovarian cancer

The small GTPase Rab25 has been functionally linked to tumour progression and aggressiveness in ovarian cancer and promotes invasion in three-dimensional environments. This type of migration has been shown to require the expression of the hypoxia-inducible factor 1 alpha (HIF-1α). In this report we demonstrate that Rab25 regulates HIF-1α protein expression in an oxygen independent manner in a panel of cancer cell lines. Regulation of HIF-1α protein expression by Rab25 did not require transcriptional upregulation, but was dependent on de novo protein synthesis through the Erbb2/ERK1/2 and p70S6K/mTOR pathways. Rab25 expression induced HIF-1 transcriptional activity, increased cisplatin resistance, and conferred intraperitoneal growth to the A2780 cell line in immunocompromised mice. Targeting HIF1 activity by silencing HIF-1β re-sensitised cells to cisplatin in vitro and reduced tumour formation of A2780-Rab25 expressing cells in vivo in a mouse ovarian peritoneal carcinomatosis model. Similar effects on cisplatin resistance in vitro and intraperitoneal tumourigenesis in vivo were obtained after HIF1b knockdown in the ovarian cancer cell line SKOV3, which expresses endogenous Rab25 and HIF-1α at atmospheric oxygen concentrations. Our results suggest that Rab25 tumourigenic potential and chemoresistance relies on HIF1 activity in aggressive and metastatic ovarian cancer. Targeting HIF-1 activity may potentially be effective either alone or in combination with standard chemotherapy for aggressive metastatic ovarian cancer.

A novel 3D human glioblastoma cell culture system for modeling drug and radiation responses

Background

Glioblastoma (GBM) is the most common primary brain tumor, with dismal prognosis. The failure of drug-radiation combinations with promising preclinical data to translate into effective clinical treatments may relate to the use of simplified 2-dimensional in vitro GBM cultures.

Methods

We developed a customized 3D GBM culture system based on a polystyrene scaffold (Alvetex) that recapitulates key histological features of GBM and compared it with conventional 2D cultures with respect to their response to radiation and to molecular targeted agents for which clinical data are available.

Results

In 3 patient-derived GBM lines, no difference in radiation sensitivity was observed between 2D and 3D cultures, as measured by clonogenic survival. Three different molecular targeted agents, for which robust clinical data are available were evaluated in 2D and 3D conditions: (i) temozolomide, which improves overall survival and is standard of care for GBM, exhibited statistically significant effects on clonogenic survival in both patient-derived cell lines when evaluated in the 3D model compared with only one cell line in 2D cells; (ii) bevacizumab, which has been shown to increase progression-free survival when added to standard chemoradiation in phase III clinical trials, exhibited marked radiosensitizing activity in our 3D model but had no effect on 2D cells; and (iii) erlotinib, which had no efficacy in clinical trials, displayed no activity in our 3D GBM model, but radiosensitized 2D cells.

Conclusions

Our 3D model reliably predicted clinical efficacy, strongly supporting its clinical relevance and potential value in preclinical evaluation of drug-radiation combinations for GBM.

Cucurbit [7] uril encapsulated cisplatin overcomes resistance to cisplatin induced by Rab25 overexpression in an intraperitoneal ovarian cancer model

Background

Ovarian cancer is the most fatal of gynaecological malignancies, usually detected at a late stage with intraperitoneal dissemination. Appropriate preclinical models are needed that recapitulate both the histopathological and molecular features of human ovarian cancer for drug-efficacy analysis.

Methods

Longitudinal studies comparing cisplatin performance either alone or in a novel cisplatin-based delivery-system, cucurbit[7]uril-encapsulated cisplatin (cisplatin@CB[7]) were performed on subcutaneous (s.c.) and intraperitoneal (i.p.) xenografts using the human ovarian cancer cell line A2780 stably expressing the small GTPase Rab25, which allows A2780 intraperitoneal growth; and luciferase, to allow tumour load measurement by non-invasive bioluminescent imaging.

Results

Rab25 expression induced cisplatin resistance compared to the parental cell line as assessed by the MTT assay in vitro. These findings did not translate in vivo, where cisplatin resistance was determined by the microenvironment. Subcutaneous xenografts of either parental A2780 or cisplatin-resistant Rab25-expressing A2780 cells presented similar responses to cisplatin treatment. In contrast, increased cisplatin resistance was only detected in i.p. tumours. Treatment of the cisplatin-resistant i.p. model with the novel cisplatin@CB[7] delivery system resulted in a substantial reduction of i.p. tumour load and increased necrosis.

Conclusions

Poor clinical performance of novel chemotherapeutics might reflect inappropriate preclinical models. Here we present an ovarian i.p. model that recapitulates the histopathological and chemoresistant features of the clinical disease. In addition, we demonstrate that the novel cisplatin-delivery system, cisplatin@CB[7] may have utility in the treatment of drug-resistant ovarian human cancers.

Abrogation of radioresistance in glioblastoma stem-like cells by inhibition of ATM kinase

Resistance to radiotherapy in glioblastoma (GBM) is an important clinical problem and several authors have attributed this to a subpopulation of GBM cancer stem cells (CSCs) which may be responsible for tumour recurrence following treatment. It is hypothesised that GBM CSCs exhibit upregulated DNA damage responses and are resistant to radiation but the current literature is conflicting. We investigated radioresistance of primary GBM cells grown in stem cell conditions (CSC) compared to paired differentiated tumour cell populations and explored the radiosensitising effects of the ATM inhibitor KU-55933. We report that GBM CSCs are radioresistant compared to paired differentiated tumour cells as measured by clonogenic assay. GBM CSC's display upregulated phosphorylated DNA damage response proteins and enhanced activation of the G2/M checkpoint following irradiation and repair DNA double strand breaks (DSBs) more efficiently than their differentiated tumour cell counterparts following radiation. Inhibition of ATM kinase by KU-55933 produced potent radiosensitisation of GBM CSCs (sensitiser enhancement ratios 2.6-3.5) and effectively abrogated the enhanced DSB repair proficiency observed in GBM CSCs at 24 h post irradiation. G2/M checkpoint activation was reduced but not abolished by KU-55933 in GBM CSCs. ATM kinase inhibition overcomes radioresistance of GBM CSCs and, in combination with conventional therapy, has potential to improve outcomes for patients with GBM.

Differential sensitivity of Glioma stem cells to Aurora kinase A inhibitors: implications for stem cell mitosis and centrosome dynamics

Glioma stem-cell-like cells are considered to be responsible for treatment resistance and tumour recurrence following chemo-radiation in glioblastoma patients, but specific targets by which to kill the cancer stem cell population remain elusive. A characteristic feature of stem cells is their ability to undergo both symmetric and asymmetric cell divisions. In this study we have analysed specific features of glioma stem cell mitosis. We found that glioma stem cells appear to be highly prone to undergo aberrant cell division and polyploidization. Moreover, we discovered a pronounced change in the dynamic of mitotic centrosome maturation in these cells. Accordingly, glioma stem cell survival appeared to be strongly dependent on Aurora A activity. Unlike differentiated cells, glioma stem cells responded to moderate Aurora A inhibition with spindle defects, polyploidization and a dramatic increase in cellular senescence, and were selectively sensitive to Aurora A and Plk1 inhibitor treatment. Our study proposes inhibition of centrosomal kinases as a novel strategy to selectively target glioma stem cells.

Cucurbit[7]uril encapsulated cisplatin overcomes cisplatin resistance via a pharmacokinetic effect

The cucurbit[n]uril (CB[n]) family of macrocycles has been shown to have potential in drug delivery where they are able to provide physical and chemical stability to drugs, improve drug solubility, control drug release and mask the taste of drugs. Cisplatin is a small molecule platinum-based anticancer drug that has severe dose-limiting side-effects. Cisplatin forms a host-guest complex with cucurbit[7]uril (cisplatin@CB[7]) with the platinum atom and both chlorido ligands located inside the macrocycle, with binding stabilised by four hydrogen bonds (2.15-2.44 Å). Whilst CB[7] has no effect on the in vitro cytotoxicity of cisplatin in the human ovarian carcinoma cell line A2780 and its cisplatin-resistant sub-lines A2780/cp70 and MCP1, there is a significant effect on in vivo cytotoxicity using human tumour xenografts. Cisplatin@CB[7] is just as effective on A2780 tumours compared with free cisplatin, and in the cisplatin-resistant A2780/cp70 tumours cisplatin@CB[7] markedly slows tumour growth. The ability of cisplatin@CB[7] to overcome resistance in vivo appears to be a pharmacokinetic effect. Whilst the peak plasma level and tissue distribution are the same for cisplatin@CB[7] and free cisplatin, the total concentration of circulating cisplatin@CB[7] over a period of 24 hours is significantly higher than for free cisplatin when administered at the equivalent dose. The results provide the first example of overcoming drug resistance via a purely pharmacokinetic effect rather than drug design or better tumour targeting, and demonstrate that in vitro assays are no longer as important in screening advanced systems of drug delivery.

TRRAP and GCN5 are used by c-Myc to activate RNA polymerase III transcription

Activation of RNA polymerase (pol) II transcription by c-Myc generally involves recruitment of histone acetyltransferases and acetylation of histones H3 and H4. Here, we describe the mechanism used by c-Myc to activate pol III transcription of tRNA and 5S rRNA genes. Within 2 h of its induction, c-Myc appears at these genes along with the histone acetyltransferase GCN5 and the cofactor TRRAP. At the same time, occupancy of the pol III-specific factor TFIIIB increases and histone H3 becomes hyperacetylated, but increased histone H4 acetylation is not detected at these genes. The rapid acetylation of histone H3 and promoter assembly of TFIIIB, c-Myc, GCN5, and TRRAP are followed by recruitment of pol III and transcriptional induction. The selective acetylation of histone H3 distinguishes pol III activation by c-Myc from mechanisms observed in other systems.

Activation by c-Myc of transcription by RNA polymerases I, II and III

The proto-oncogene product c-Myc can induce cell growth and proliferation. It regulates a large number of RNA polymerase II-transcribed genes, many of which encode ribosomal proteins, translation factors and other components of the biosynthetic apparatus. We have found that c-Myc can also activate transcription by RNA polymerases I and III, thereby stimulating production of rRNA and tRNA. As such, c-Myc may possess the unprecedented capacity to induce expression of all ribosomal components. This may explain its potent ability to drive cell growth, which depends on the accumulation of ribosomes. The activation of RNA polymerase II transcription by c-Myc is often inefficient, but its induction of rRNA and tRNA genes can be very strong in comparison. We will describe what is known about the mechanisms used by c-Myc to activate transcription by RNA polymerases I and II.

Deregulation of RNA polymerase III transcription in cervical epithelium in response to high-risk human papillomavirus

RNA polymerase (pol) III transcription is a major determinant of biosynthetic capacity, providing essential products such as tRNA and 5S rRNA. It is controlled directly by the tumour suppressors RB and p53. High-risk types of human papillomavirus (HPV), such as HPV16, express the oncoproteins E6 and E7 that can inactivate p53 and RB, respectively. Accordingly, both E6 and E7 stimulate pol III transcription in cultured cells. HPV16-positive cervical biopsies express elevated levels of tRNA and 5S rRNA when compared to biopsies that test negative for HPV or are infected with the lower risk HPV11. Integration of viral DNA into the host cell genome stimulates expression of E6 and E7 and correlates with induction of tRNA and 5S rRNA. Expression of mRNA encoding the pol III-specific transcription factor Brf1 also correlates with the presence of integrated HPV16. Brf1 levels are limiting for tRNA and 5S rRNA synthesis in cervical cells. Furthermore, pol III-transcribed genes that do not use Brf1 are not induced in HPV16-positive biopsies. Three complementary mechanisms may therefore allow high-risk HPV to stimulate production of tRNA and 5S rRNA: E6-mediated removal of p53; E7-mediated neutralization of RB; and induction of Brf1. The resultant increase in biosynthetic capacity may contribute to deregulated cell growth.

c-Myc binds to human ribosomal DNA and stimulates transcription of rRNA genes by RNA polymerase I

c-Myc coordinates cell growth and division through a transcriptional programme that involves both RNA polymerase (Pol) II- and Pol III-transcribed genes. Here, we demonstrate that human c-Myc also directly enhances Pol I transcription of ribosomal RNA (rRNA) genes. rRNA synthesis and accumulation occurs rapidly following activation of a conditional MYC-ER allele (coding for a Myc-oestrogen-receptor fusion protein), is resistant to inhibition of Pol II transcription and is markedly reduced by c-MYC RNA interference. Furthermore, by using combined immunofluorescence and rRNA-FISH, we have detected endogenous c-Myc in nucleoli at sites of active ribosomal DNA (rDNA) transcription. Our data also show that c-Myc binds to specific consensus elements located in human rDNA and associates with the Pol I-specific factor SL1. The presence of c-Myc at specific sites on rDNA coincides with the recruitment of SL1 to the rDNA promoter and with increased histone acetylation. We propose that stimulation of rRNA synthesis by c-Myc is a key pathway driving cell growth and tumorigenesis.

Direct regulation of RNA polymerase III transcription by RB, p53 and c-Myc

The synthesis of tRNA and 5S rRNA by RNA polymerase (pol) III is cell cycle regulated in higher organisms. Overexpression of pol III products is a general feature of transformed cells. These observations may be explained by the fact that a pol III-specific transcription factor, TFIIIB, is strongly regulated by the tumor suppressors RB and p53, as well as the proto-oncogene product c-Myc. RB and p53 repress TFIIIB, but this restraint can be lost in tumors through a variety of mechanisms. In contrast, c-Myc binds and activates TFIIIB, causing potent induction of pol III transcription. Using chromatin immunoprecipitation and RNA interference, we show that c-Myc interacts with tRNA and 5S rRNA genes in transformed cervical cells, stimulating their expression. Availability of pol III products may be an important determinant of a cell's capacity to grow. The ability to regulate pol III output may therefore be integral to the growth control functions of RB, p53 and c-Myc.

Direct activation of RNA polymerase III transcription by c-Myc

The proto-oncogene product c-Myc has a direct role in both metazoan cell growth and division. RNA polymerase III (pol III) is involved in the generation of transfer RNA and 5S ribosomal RNA, and these molecules must be produced in bulk to meet the need for protein synthesis in growing cells. We demonstrate here that c-Myc binds to TFIIIB, a pol III-specific general transcription factor, and directly activates pol III transcription. Chromatin immunoprecipitation reveals that endogenous c-Myc is present at tRNA and 5S rRNA genes in cultured mammalian cells. These results suggest that activation of pol III may have a role in the ability of c-Myc to stimulate cell growth.