The molecular basis for central nervous system primitive neuroectodermal tumour development

Abstract

Central nervous system primitive neuroectodermal tumours (CNS-PNETs) are highly aggressive tumours with similar histopathological features to other intracranial PNETs (medulloblastomas). These two tumours have accordingly been treated using unified approaches, but CNS-PNETs have a dismal prognosis. Few studies have investigated the genetic features of CNS-PNETs. The molecular basis of CNS-PNET was therefore investigated in a cohort containing CNS-PNETs from children (n=33) and adults (n=5), to aid improvements in disease classification and treatment. The common medulloblastoma molecular defects were investigated in CNS-PNETs, and showed RASSF1A promoter hypermethylation is a frequent event (18/22, 82%), and MYC family gene amplification occurs in a subgroup (MYCN: 3/25 (12%), MYCC: 0/25 (0%)). In contrast and in distinction to medulloblastoma, chromosome 17p loss is not a common feature (2/23, 9%), whilst p53 pathway signalling appears to play a major role (20/22, 91%), and associated with TP53 mutations (4/22, 18%). Aberrant Wnt signalling was identified in 2 cases (2/22, 9%) and coupled with CTNNB1 mutation in a single case. IDH1 mutations (2/25, 8%) however, appear to occur in adult but not childhood CNS-PNETs or medulloblastoma. Subsequent genome-wide investigations of the CNS-PNET DNA methylome aimed at a wider characterisation of the molecular features of CNS-PNETs and its relationships to other childhood tumours identified CNS-PNETs as a heterogenous disease group without defined sub-clusters, which were predominantly distinct from medulloblastomas, but exhibited overlap with high-grade gliomas. A panel of 76 tumour-specific methylation events were identified as disease markers. The combination of either RASSF1A hypermethylation or HLA-DPB1 hypomethylation discerned normal brain from CNS-PNET in 94% of cases (64/68). In addition, hypermethylation of TAL1, MAP3K1 and IGFBP1 is associated with non-infant disease. In conclusion, this study has shown CNS-PNETs are a heterogenous group of tumours that are molecularly distinct from medulloblastomas, and has implicated developmental pathways and genetic events in their tumorigenesis. The associations between molecular events identified and clinical features warrant further investigation to aid classification and treatment advancements.