OP05FUNCTIONAL ANALYSIS OF MULTI-DRUG RESISTANCE TRANSPORTER ABCB1 WHICH CORRELATES WITH POOR PROGNOSIS IN PAEDIATRIC EPENDYMOMA

Abstract

INTRODUCTION: Paediatric ependymomas are aggressive drug-resistant tumours with a tendency to recur locally. This tendency is consistent with the Cancer Stem Cell hypothesis (CSCs) which states that there's a sub-population of therapy resistant cancer stem cells which remain refractory to treatment and recapitulate tumour growth. We hypothesised that drug efflux by the multi-drug resistance transporter ABCB1 is one of the resistance mechanisms upregulated in ependymoma CSCs. METHOD: ABCB1 immunohistochemistry (IHC) staining was performed on patient samples obtained from two trial cohorts; a chemotherapy-led infant trial (CNS9204) and a radiotherapy-led trial (CNS9904). Expression of ABCB1 in ependymoma derived cell lines and stem cell enriched neurospheres was assessed using RT-PCR. MTT assays and clonogenic assays were performed to assess potentiation of response to chemotherapy by ABCB1 inhibition using Verapamil and a novel agent, Vardenafil. The role of ABCB1 in tumour invasion was assessed by performing a 3-D migration assay. RESULTS: ABCB1 expression was significantly associated with reduced event -free survival (EFS) and overall survival (OS) in patients treated with chemotherapy (p-value-0.009 and 0.007 respectively). ABCB1 expression was restricted to a sub-population of cells and elevated in stem cell enriched neurospheres. ABCB1 inhibition potentiated response to chemotherapy (p = 0.0089) and reduced migration in the ependymoma derived BXD-1425 cell line. CONCLUSION: These findings constitute the first conclusive evidence that ABCB1 is capable of predicting clinical outcome in paediatric ependymoma patients where it plays an essential role in drug resistance and tumour invasion.