Abstract 4881: Dissecting telomere maintenance mechanisms in neuroblastoma

Background. Telomere maintenance is a major cancer hallmark and has recently attracted attention as an oncogenic mechanism in the pediatric malignancy, neuroblastoma (NBL), due to detection of frequent structural rearrangements near the telomerase (TERT) gene. NBLs display substantial clinical and molecular heterogeneity; tumors with MYCN amplification and TERT-associated rearrangements define separate groups of high-risk patients with active TERT. ATRX mutations are frequent in non-TERT active high-risk tumors, indicating use of alternative lengthening of telomeres (ALT) as a maintenance mechanism. Furthermore, the 4S low-risk NBLs display an elevated rate of spontaneous regression and overall good prognosis; it has been hypothesized that lacking a telomere maintenance mechanism (“telomere crisis”) may be behind this phenomenon. Methods. To gain mechanistic insight behind these phenomena, we introduce an integrative analysis of a cohort comprising 104 high-risk (32 MYCN-amplified) and 23 4S tumors with matched blood/tumor whole genome sequencing, CpG methylation, and RNA-seq data from the TARGET consortium. Here, we preformed structural variant (SV), allelic specific expression (ASE), and differential CpG methylation analyses. Additionally, we introduce a novel approach to measure relative telomere DNA abundance from short-read WGS measured as the ratio between tumor and blood telomeric reads (containing canonical repeats TTAGGG/CCCTAA) per million (TBrpm). Results. We identified 26 tumors harboring TERT rearrangements (25% of all high risk tumors) and 3 tumors with ATRX mutations. We define 4 groups of high-risk NBL with different telomere maintenance mechanisms: 1) Tumors with MYCN amplification or elevated MYC/N activity, TERT gene body hyper-methylation (β-value≅0.8), and expression of biallelic TERT; 2) Tumors harboring TERT associated SVs, hemi-methylation (β-value≅0.5), and expression of mono-allelic TERT, 3) Tumors without TERT expression, including ATRX mutants with hypo-methylated TERT (β-value≅0.3), 4) tumors with abnormally high telomeric DNA abundance (TBrpm > 1.5), hypo-methylation (β-value≅0.3), and no expression of TERT. Intriguingly, while groups 1-3 show telomeric loss (TBrpm ≅ 0.7-0.9), 4S tumors show conservation of telomere abundance between tumor and blood (TBrpm ≅ 1); hence no indication of telomere crisis, perhaps due to early timing of biopsy at diagnosis. Conclusion. We have used ASE status and gene body methylation of TERT in order to understand and validate mechanisms underlying TERT activation; we extend MYCN-driven TERT activation to tumors with high MYC/MYCN activity in the absence of MYCN amplification. Our telomere analysis suggests that different ALT mechanisms might take place in NBLs. Defining the mechanism of 4S NBL spontaneous regression requires further investigation. Citation Format: Gonzalo Lopez, Karina Conkrite, Kendra Hong, Jo Lynne Harenza, John M. Maris, Sharon Diskin. Dissecting telomere maintenance mechanisms in neuroblastoma [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 4881. doi:10.1158/1538-7445.AM2017-4881