Understanding and ameliorating late-effects in childhood medulloblastoma survivors

Abstract

Medulloblastoma (MB) is the most common malignant paediatric brain tumour, with 5-year survival rates over 70%. Survivors frequently suffer a wide variety of late-effects due to their tumour and its multi-modal treatment: tumour resection, chemotherapy and craniospinal radiotherapy with posterior fossa boost (PFB). Treatment induces deleterious late-effects through damage to normal tissues and increases the risk of neurocognitive impairment, endocrine impairment, ototoxicity, secondary tumours, cardiotoxicity, poor physical function and premature ageing/frailty. Sadly, approaches to ameliorate treatment-induced late-effects are lacking; a paucity of appropriate model systems hinders their development. This thesis aims to develop clinically-relevant models of MB treatment to enable investigation of the biological mechanisms that underpin late-effect onset, and facilitate the appraisal of interventions. To recapitulate childhood medulloblastoma radiotherapy and late-effect profile in vivo, age-equivalent mice received CT image-guided, human-equivalent cranial radiotherapy (CRT) or CRT+PFB and were longitudinally assessed for over 1 year. Following CRT, mice were significantly more frail, had reduced physical functioning and exhibited neurocognitive deficits. Receipt of PFB did not induce a more severe late-effect profile. The biological underpinnings of radiation-induced late-effects were explored, ex vivo. Assessment of transcriptional modifications 1 year post-irradiation, via RNA-sequencing, showed CRT did not induce consistent global changes, instead pathways including interferon-α/γ and epithelial-mesenchymal-transition were downregulated. Quantification of pro-inflammatory proteins, using immunohistochemistry, showed higher abundance following CRT, though response was not dose-dependent. Altered DNA methylation patterns are associated with premature ageing. Utilising a prebuilt epigenetic-clock, predicted age increased with chronological age, though this was more accurate in DNA from peripheral blood than brain tissue. CRT did not induce a significantly accelerated epigenetic age. To understand acute response to radiation-insult, human-equivalent radiation was delivered in vitro and the Luminex assay was utilised to develop a novel, multi-analyte assessment of molecular insult response to MB-equivalent radiation. Markers of inflammation increased 1 hour post-irradiation, and typically increased further at 48 hours. Increased inflammation was expected and is thought to play a major role in the development of radiation-induced late effects. To provide a baseline model for future investigation of chemotherapy-induced late-effects, a close-MB chemotherapy regimen was developed in vivo that replicated the popular Packer-style chemotherapy. The development of clinically-relevant, treatment-induced late-effect models enables the elucidation of novel/target mechanisms underpinning MB late-effects and the development of novel interventions for their amelioration.