DNAR-09. Telomere dysfunction and fusions in glioblastoma - defining prognosis and genome instability

BACKGROUND Glioblastoma (GBM) remains a cancer of unmet need. Identifying biomarkers for patient stratification and prediction of response to therapy is a priority. Telomeres protect chromosome-ends from aberrant DNA damage responses (DDR), and telomere attrition has been shown to drive genome instability and clonal evolution in several cancers. We examined the relationship between telomere dysfunction, fusion and clinical outcome in GBM patients. 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 coupled with long-read nanopore sequencing was used to characterise telomere fusions. 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 was more pronounced in patients with complete resection at index surgery (HR 21.55, p=0.0074). Widespread telomere fusions were observed that revealed microhomology, subtelomeric deletions and insertions of genomic loci within complex chromosomal rearrangements. Insertions often included protein-coding genes with driver mutations, long non-coding RNA, telomeric repeats, and subtelomeric and peri-centromeric regions. This mutational spectrum suggested 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 data defines patient subgroups (MGMT unmethylated, short TL) which may benefit from alternatives to TMZ, and aberrant DDR signatures could help inform stratification within adaptive platform trials of DDR inhibitors. Results from a prospective cohort of all-grade diffuse gliomas with 5-ALA and image-guided multi-site sampling will be shared at this meeting.