Introduction: Gliomas are the commonest intrinsic tumor of the brain; the malignant form, glioblastoma, has a poor prognosis. The EGFRvIII mutation is found in 30-40% of primary glioblastomas. It is unclear whether this mutation occurs early or late in glioma formation and from where these mutated tumors originate.

Methods: We generated transgenic mice expressing the human EGFRvIII mutation under control of the nestin-cre recombinase (with early expression throughout the CNS). 50 mice were assessed for signs of neurological disease and their brains and spinal cords were analysed for histology, immunohistochemistry, cytogenetics and in vitro growth properties.

Results: EGFRvIII-Nes-cre mice displayed neurological signs from 4 months, with severity of signs necessitating culling at a median age of 8 months. Examination of the brains and spinal cords of these mice revealed multiple, often large, tumors that showed pathological features of glioma on H&E staining. These mice generated gliomas with 100% incidence, of which 10% were high grade gliomas. Immunohistochemistry for human glioma markers revealed these tumors were positive for stem cell markers (SOX2, OLIG2, nestin), proliferation (Ki67), and caspase 3 (an apoptotic marker). Analysis of tumors prior to onset of signs revealed the earliest changes were hyperproliferation and tumour precursor lesions (microneoplasias) in the subventricular zone, followed by early leptomeningeal metastasis. Cytogenetic analysis of high grade brain gliomas demonstrated amplification of chromosome 11 (containing EGFRvIII transgene) in 65% of tumours, and of chromosome 15 (containing c-myc) in 40%. Primary cultures of these gliomas were highly sensitive to afatinib (an EGFR inhibitor).

Conclusions: This is the first study to demonstrate EGFRvIII is sufficient to initiate gliomagenesis from the subventricular zone and spinal cord in vivo, with early leptomeningeal spread. Importantly, identification of this mutation as a driver of spinal gliomas suggests these intractable tumours may be amenable to treatment with EGFR inhibitors.

Patient Care: Substantial improvements in glioma treatment will come about through deeper understanding of the molecular biology of these tumours. This study improves understanding of the early biological processes taking place in glioma development - by suggesting EGFRvIII initiates tumourigenesis. It implies that treatment may be improved by targeting the subventricular zone and leptomeningeal space early in EGFRvIII-mutant gliomas (such as with radiation). Moreover, spinal glioma genetics are poorly understood - we show here that EGFRvIII drives spinal gliomas, which may therefore be treatable with targeted EGFR therapies.

Learning Objectives: - Understand the role of EGFRviii in gliomas. - Discuss the site of origin of gliomas. - Understand how targeted therapeutics may help treat brain and spinal tumors.

References: Holland, E.C., et al., Combined activation of Ras and Akt in neural progenitors induces glioblastoma formation in mice. Nat Genet, 2000. 25(1): p. 55-7. 28. Ceccarelli, M., et al., Molecular Profiling Reveals Biologically Discrete Subsets and Pathways of Progression in Diffuse Glioma. Cell, 2016. 164(3): p. 550-63.