Alternative lengthening of telomeres in molecular subgroups of paediatric high-grade glioma

Simone Minasi,Torsten Pietsch,Caterina Baldi,Francesca Romana Buttarelli,Manila Antonelli,Maura Massimino,Francesca Gianno,Anna Maria Buccoliero

doi:10.1007/s00381-020-04933-8

Abstract

PurposeThe maintenance of telomere length prevents cancer cell senescence and occurs via two mutually exclusive mechanisms: (a) reactivation of telomerase expression and (b) activation of alternative lengthening of telomeres (ALT). ALT is frequently related to alterations on ATRX, a chromatin-remodelling protein. Recent data have identified different molecular subgroups of paediatric high-grade glioma (pHGG) with mutations of H3F3A, TERTp and ATRX; however, differences in telomere length among these molecular subgroups were not thoroughly examined.MethodsWe investigated which genetic alterations trigger the ALT mechanism in 52 IDH-wildtype, 1p/19q-wildtype pHGG. Samples were analysed for telomere length using Tel-FISH. ATRX nuclear loss of expression was assessed by IHC, H3F3A and TERTp mutations by DNA sequencing, and TERTp methylation by MS-PCR.ResultsMutant H3.3 was found in 21 cases (40.3%): 19.2% with K27M mutation and 21.1% with G34R mutation. All H3.3G34R-mutated cases showed the ALT phenotype (100%); on the opposite, only 40% of the H3.3K27M-mutated showed ALT activation. ATRX nuclear loss was seen in 16 cases (30.7%), associated sometimes with the G34R mutation, and never with the K27M mutation. ATRX nuclear loss was always related to telomere elongation. TERTp C250T mutations were rare (5.4%) and were not associated with high intensity Tel-FISH signals, as TERTp hyper-methylation detected in 21% of the cases. H3.3/ATRX/TERTp-wildtype pHGG revealed all basal levels of telomere length.ConclusionOur results show a strong association between H3.3 mutations and ALT, and highlight the different telomeric profiles in histone-defined subgroups: H3.3-G34R mutants always trigger ALT to maintain telomere length, irrespective of ATRX status, whereas only some H3.3-K27M tumours activate ALT. These findings suggest that acquiring the gly34 mutation on H3.3 might suffice to trigger the ALT mechanism.

Highlights

Gliomas account for ~ 11% of all central nervous system (CNS) tumours in children aged 0–14 years
Telomere length can be maintained via two different mechanisms: (i) reactivation of telomerase reverse transcriptase (TERT) via promoter mutations or methylation; and (ii) a telomerase-independent mechanism known as alternative lengthening of telomeres (ALT), which relies on the homologous recombination of telomeric regions [1, 2, 5, 19, 20, 26, 38], and results in a heterogeneous length and sequence composition [19, 20, 26]
Loss of alterations of αthalassaemia/mental retardation syndrome X-linked (ATRX) expression was associated with the G34R mutation in 7/16 cases, but in none of the K27M-mutated or telomerase reverse transcriptase promoter (TERTp)-mutated samples

Summary

Introduction

Gliomas account for ~ 11% of all central nervous system (CNS) tumours in children aged 0–14 years. Paediatric highgrade glioma (pHGG) is one of the most common causes of cancer-related death under the age of 19 [42]. The recent extensive use of high-throughput molecular, genetic and epigenetic profiling has considerably increased our knowledge of the cellular origin, pathogenesis and biological features of pHGG. This has helped to classify these neoplasms by their molecular/genetic features, which correlate with age of onset, anatomical location and prognosis [5, 11, 35, 40, 45, 51]. Telomere length can be maintained via two different mechanisms: (i) reactivation of telomerase reverse transcriptase (TERT) via promoter mutations or methylation; and (ii) a telomerase-independent mechanism known as alternative lengthening of telomeres (ALT), which relies on the homologous recombination of telomeric regions [1, 2, 5, 19, 20, 26, 38], and results in a heterogeneous length and sequence composition [19, 20, 26]

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Conclusion