ALT in Pediatric High-Grade Gliomas Can Occur withoutATRXMutation and is Enriched in Patients with Pathogenic Germline MMR Variants

BackgroundTo achieve replicative immortality, most cancers develop a telomere maintenance mechanism, such as reactivation of telomerase or alternative lengthening of telomeres (ALT). There are limited data on the prevalence and clinical significance of ALT in pediatric brain tumors, and ALT-directed therapy is not available.MethodsWe performed C-circle analysis (CCA) on 579 pediatric brain tumors that had corresponding tumor/normal whole genome sequencing through the Open Pediatric Brain Tumor Atlas (OpenPBTA). We detected ALT in 6.9% (n=40/579) of these tumors and completed additional validation by ultrabright telomeric fociin situon a subset of these tumors. We used CCA to validateTelomereHunterfor computational prediction of ALT status and focus subsequent analyses on pediatric high-grade glioma (pHGG) Finally, we examined whether ALT is associated with recurrent somatic or germline alterations.ResultsALT is common in pHGG (n=24/63, 38.1%), but occurs infrequently in other pediatric brain tumors (<3%). SomaticATRXmutations occur in 50% of ALT+ pHGG and in 30% of ALT-pHGG. Rare pathogenic germline variants in mismatch repair (MMR) genes are significantly associated with an increased occurrence of ALT. Conclusions: We demonstrate thatATRXis mutated in only a subset of ALT+ pHGG, suggesting other mechanisms ofATRXloss of function or alterations in other genes may be associated with the development of ALT in these patients. We show that germline variants in MMR are associated with development of ALT in patients with pHGG.Key PointsATRX alterations are frequent, but not required, for an ALT phenotype in pHGGspHGG patients with germline mismatch repair variants have higher rate of ALT + tumorsTelomereHunteris validated to predict ALT in pHGGsImportance of the StudyWe performed orthogonal molecular and computational analyses to detect the presence of alternative lengthening of telomeres in a highly characterized cohort of pediatric brain tumors. We demonstrate that many pHGG utilize ALT without a mutation in ATRX, suggesting either loss of function of ATRX via an alternative mechanism or an alternate means of development of ALT. We show that germline variants in MMR genes are significantly associated with ALT in pHGG. Our work adds to the biological understanding of the development of ALT and provides an approach to stratify patients who may benefit from future ALT-directed therapies in this patient population.