Abstract 3466: ALT-positive pediatric high grade glioma cells display chemosensitivity to ATR pathway inhibition

Overcoming the end-replication problem is a major hurdle for cancer cells due to progressive telomere shortening that results from excessive cell division during tumorigenesis. While most cancers activate telomerase to maintain their telomere length, about five percent of cancers use a telomerase-independent telomere maintenance mechanism, termed alternative lengthening of telomeres (ALT). Despite its low overall frequency in cancer, ALT is enriched in several cancer subtypes, including pediatric high grade glioma (HGG). Previous work in our laboratory suggests that nearly half of pediatric HGG utilize the ALT mechanism. To date, therapeutic options are largely ineffective for children with HGG, reflected in the dismal five-year survival rate, which is less than 33 percent. Our goal is to better harness ALT as a clinical marker in pediatric HGG, specifically by identifying drugs that target ALT-positive cancers. In order to study ALT in this context, we obtained and characterized a panel of six pediatric HGG cell lines. Two of the six cell lines in this panel display hallmarks of the ALT pathway. To assess the potential of ALT as a therapeutic biomarker in pediatric HGG, we measured cell viability in the presence of inhibitors, stratifying our analysis by ALT status. ALT is predicted to occur via a homologous-recombination-based mechanism. As such, we focused our attention on inhibitors of the DNA damage response, as well as agents that induce DNA damage. Treatment with inhibitors of DNA-PK, RAD51, and MRE11 did not result in significant differences in cell viability when stratified by ALT status, nor did treatment with temozolomide or hydroxyurea. However, inhibitors of ATR (VE-821, VE-822/VX-970, and AZD-6738) and CHK1 (MK-8776) led to significantly greater reductions in cell viability in ALT-positive cells. In order to further our understanding of the mechanism through which ATR inhibition preferentially targets ALT-positive cell lines, we examined the effect of these inhibitors on ALT-specific properties, such as the presence of ultrabright telomeric foci and extrachromosomal telomeric DNA (c-circles). Preliminary results indicate that ATR inhibition does not diminish these ALT-associated properties and, therefore, the differential sensitivity to these agents is unlikely to be due to directly blocking the ALT telomere maintenance mechanism. Continued study of the effect of ATR/CHK1 inhibition on ALT-positive cells will yield further insight into the mechanism of ALT-specific toxicity. Overall, we have identified ATR/CHK1 as a promising pathway to target for ALT-positive pediatric HGG. Our goal is to better understand the relationship between ATR/CHK1 signaling and ALT in order to effectively translate this observation to the clinic, both for pediatric HGG and for other ALT-positive cancers. Citation Format: Jacqueline A. Brosnan-Cashman, Anthony J. Rizzo, Ezgi Goger, Kaylar M. Myers, Reza Zarinshenas, Christine Davis, Ming Yuan, Dinesh Rakheja, Eric H. Raabe, Charles G. Eberhart, Christopher M. Heaphy, Alan K. Meeker. ALT-positive pediatric high grade glioma cells display chemosensitivity to ATR pathway inhibition [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 3466. doi:10.1158/1538-7445.AM2017-3466