{MEDB}-68. Analysis of telomeres length and Alternative Lengthening of Telomeres ({ALT}) in molecular subgroups of infant medulloblastoma

We investigated the association between the molecular profile and telomere length in a infant medulloblastoma (iMB) cohort, retrospectively studied. Activation of telomeres maintenance mechanisms was analyzed to determine whether the senescence escape triggered by telomere-elongation mechanisms could explain the aggressivity of some iMB belonging to the same molecular subgroup. Interestingly, several telomerase- and ALT-targeted therapies have recently been tested on pediatric cancers and might represent a promising strategy for the future treatment of aggressive telomerase- or ALT-positive iMB. We analyzed a cohort of 50 FFPE tissues from young MB patients (age ≤ 3); IHC, FISH, and an Illumina 850K methylation profile were used to identify molecular subgroups. Telomere length was measured using Telo-quantitative FISH, and image analysis was performed using TFL-Telo software. Three distinct telomere intensity categories (low (L), medium (M), and high (H)) were identified by comparing neoplastic- to endothelial-cell signals in each sample. ATRX loss and TERTp mutation/methylation were investigated using IHC and Sanger sequencing/methylation-specific PCR. SHH-MBs accounted for 59% of our cohort, while Group3/4-MBs accounted for 41%; no WNT-MBs were detected. ALT was found to be activated in 10% of iMBs and was not exclusive to any molecular subgroup, implying that it could be a potential mechanism associated with aggressive behaviour in a subset of iMBs. Promising results have been found in the telomere length distribution among the iMB molecular subgroups: SHH iMBs had a higher frequency of High (H) telomeres length (85%) than NON-SHH/NON-WNT iMBs (p=0.046), which were more frequently associated with Medium (M) telomeres length. CONCLUSIONS: ALT activation in infant MBs (10%) could be a novel target for risk-stratification and personalized therapy. It may be useful to examine ALT as a potential predictor of aggressive behaviour and as a promising novel therapeutic approach for a subset of these tumors in the diagnostic workup.