DNAR-10. Concurrent levels of TERT promoter variants in pediatric medulloblastomas: rare occurance of intratumoral heteogenetity?

Abstract BACKGROUND TERT promoter variants (TERTp) are recognized as diagnostic and prognostic biomarkers in CNS tumors. The prognostic significance of TERT promoter variants in medulloblastomas (MB) remains unclear. It appears to be subgroup-dependent: tumors with TERT variants demonstrate improved 5-year overall survival in the SHH subgroup, whereas a poorer prognosis is associated with TERT variant tumors in Group 4 MB. METHODS tumor samples were obtained from 9 patients, (age range: 5 months-14 years; mean age: 55 months) who underwent surgical treatment at Meyer Children’s Hospital IRCCS (Florence). Histological diagnoses included desmoplastic/nodular MB (6 cases), MB with extensive nodularity (1 case), and classical MB with focal desmoplastic features (2 cases). Workflow procedures for Droplet Digital PCR (ddPCR) were performed using The QX200 Droplet Digital PCR System (Biorad). RESULTS DNA extracted from 9 FFPE was analysed for TERTp C228T and C250T hotspots using ddPCR approach. All patients contained both hotspots (range 0.14 - 61.7% for C228T and 0.06 - 44.6% for C250T). Only two patients (3 and 7) presented high variant allele frequencies (VAF): 61.7% (C228T) and 44.6% (C250T), respectively. CONCLUSIONS the two variants (C228T and C250T) in the TERT promoter are mostly mutually exclusive. In our cases, the use of high-sensitivity methods as ddPCR has shown that the two variants are present a low level and not mutually exclusive. Would the data of literature be different if the studies had been performed using high-sensitivity methods such as NGS or ddPCR? Our study confirms the correlation between the presence of these variants and increased age. Indeed, both patients with the heterozygous variant are over 10 years old. We should definitely extend this study to other MB types using a highly sensitive ddPCR approach. This is crucial because TERT promoter variants are crucial in MB development and influence tumour behavior and clinical outcome.