Ricerc@Sapienza

Purpose: Elongation of telomeres is necessary for tumor cell immortalization and senescence escape; neoplastic cells use to
alternative pathways to elongate telomeres: telomerase reactivation or a telomerase-independent mechanism termed alternative
lengthening of telomeres (ALT). Telomerase and ALT pathway has been explored in adult and pediatric gliomas and
medulloblastomas (MDBs); however, these mechanisms were not previously investigated in MDBs metastatic at the onset.
Therefore, we analyzed the activation of telomerase and ALT pathway in a homogenous cohort of 43 pediatric metastatic
medulloblastomas, to investigate whether telomere elongation could play a role in the biology of metastatic MDB.
Methods: We evaluated telomeres length via telomere-specific fluorescence in situ hybridization (Telo-FISH); we assessed
nuclear expression of ATRX by immunohistochemistry (IHC). H3F3A and TERT promoter mutations were analyzed by
pyrosequencing, while UTSS methylation status was analyzed via methylation-specific-PCR (MS-PCR).
Results: H3F3A mutations were absent in all MDBs, 30% of samples showed ATRX nuclear loss, 18.2% of cases were
characterized by TERT promoter mutations, while 60.9% harboured TERT promoter hyper-methylation in the UTSS region.
Elongation of telomeres was found in 42.8% of cases. Metastatic MDBs control telomere elongation via telomerase activation
(10.7%), induced by TERT promoter mutations in association with UTSS hyper-methylation, and ALT mechanism
(32.1%), triggered by ATRX inactivation. Among non-metastatic MDBs, only 5.9% (1/17) showed ATRX nuclear loss with
activation of ALT.
Conclusions: Our metastatic cases frequently activate ALT pathway, suggesting that it is a common process for senescence
escape in primary metastatic medulloblastomas. Furthermore, the activation of mechanisms for telomere elongation is not
restricted to certain molecular subgroups in this high-risk group of MDBs.