HGG-48. THE BLOOD-BASED TELOMERIC DNA C-CIRCLE ASSAY CAN DETECT ALTERNATIVE LENGTHENING OF TELOMERES (ALT) IN PEDIATRIC HIGH-GRADE GLIOMAS

Abstract

Abstract BACKGROUND High-grade gliomas (HGG) comprise ~10% of pediatric brain tumors; 40% of pediatric HGG use the alternative lengthening of telomeres (ALT) mechanism to maintain replicative immortality. ALT cancers share a unique biology providing potential therapeutic targets. Extrachromosomal telomere DNA repeats, termed C-circles, can be detected by a unique PCR assay, providing a sensitive and specific biomarker for ALT cancers. C-circles have been detected in ALT tumors and serum of ALT patients. We have adapted the C-circle assay (CCA) to provide a sensitive and specific assay with cfDNA in patient plasma. Here we determined if cfDNA in plasma can be used to detect patients with ALT-positive HGG. METHODS Frozen plasma samples were collected in Streck tubes at time of surgery prior to resection from patients with HGG. DNA was extracted and quantified. Isothermal rolling C-circle amplification and real-time telomere PCR was performed followed by analysis. The C-circle positive neuroblastoma cell line, CHLA-90, and C-circle negative neuroblastoma cell line, CHLA-20, served as the positive and negative controls respectively. CCA values in plasma of 2 and above indicate positivity for C-circles. RESULTS Plasma samples were analyzed by CCA from three pediatric patients whose brain tumors were previously biopsied and identified as ATRX mutation positive. ATRX mutation indicates ALT however, not all ALT cancers have ATRX mutations. All plasma samples were above the 2.0 cutoff at 3.37, 3.13, and 4.59 and thus classified as C-circle positive. CONCLUSIONS This preliminary cohort shows that the CCA has potential for identifying ALT pediatric brain tumors using cfDNA from plasma. As novel therapies effective against ALT HGG are developed, the plasma CCA may allow accelerated initiation of neoadjuvant therapy to shrink tumor prior to surgical resection and lessen or prevent the need for tumor biopsy. Expansion of the initial test cohort is underway. Supported by NCI UO1 CA63988.