P05.18.B TELOMERE DYSFUNCTION AND FUSIONS IN GLIOBLASTOMA - DEFINING PROGNOSIS AND GENOME INSTABILITY

Abstract BACKGROUND Glioblastoma (GBM) remains a devastating disease with a median survival of 15 months. Its molecular definition illustrates underlying unstable genomes with copy number variation (chr 7+/10-), epigenetic modification in DNA repair enzymes (MGMT methylation), and changes to telomere maintenance genes (TERT or ATRX), all recognised as key factors in gliomagenesis and patient outcomes. Telomeres protect chromosome-ends from aberrant DNA damage responses (DDR), and telomere erosion has been shown to drive such genome instability and clonal evolution in several cancers. As more innovative platform trial designs emerge, identifying novel markers to better stratify patients and predict response to targeted therapies is a priority. We conducted single molecule telomere length (TL) analysis and long-read fusion sequencing in GBM patients and related this to clinical outcome. MATERIAL AND METHODS 146 adults with molecular GBMs had tumour TL assessed with high-throughput single telomere length analysis. Assays for extrachromosomal c-circles and TERT promoter mutation status were performed, to assess underlying telomere maintenance mechanisms. Uni- and multi-variable analysis was conducted to assess tumour TL impact on OS and PFS. Single-molecule PCR was used to identify telomere fusions which were further characterised with long-read nanopore sequencing, including in patients with paired samples from index surgery and at recurrence. RESULTS GBMs displayed shortened telomeres (median TL 4.0kb), unrelated to age at diagnosis or MGMT methylation status. Short TL was negatively predictive for OS on multivariable analysis (HR 4.81, p=0.009). This impact was more pronounced in patients with gross total resection at index surgery (HR 21.55, p=0.0074). Long read Fusion-Seq demonstrated widespread telomere fusions in GBMs and revealed microhomology, telomeric deletions and insertions of genomic loci, including from known protein-coding genes with driver mutations, long non-coding RNA, canonical telomeric repeats, and subtelomeric regions. This mutational spectrum suggests utilisation of specific DNA repair pathways in mediating telomere fusion. CONCLUSION This study provides mechanistic evidence of dysfunctional telomeres in GBMs, with high resolution TL analysis together with MGMT methylation status providing powerful independent prognostic information. The presence of short telomeres in patients with unmethylated MGMT defines a subgroup of GBM patients that may benefit from alternative treatment regimens. The presence of telomere fusions offers direct evidence for significant genomic instability, potentially favouring targeted DDR inhibitors for patients exhibiting this signature. HB is supported by a WCAT fellowship and Brain Research UK. Work in the Baird Lab is supported by Cancer Research UK and US NIH. Thanks go to BRAIN UK and all our patients.