GFAPδ/GFAPα ratio directs astrocytoma gene expression towards a more malignant profile.

Astrocytomas are the most common malignant brain tumours and are to date incurable. It is unclear how astrocytomas progress into higher malignant grades. The intermediate filament cytoskeleton is emerging as an important regulator of malignancy in several tumours. The majority of the astrocytomas express the intermediate filament protein Glial Fibrillary Acidic Protein (GFAP). Several splice variants have been identified and the main variants expressed in human astrocytoma are the α and δ isoforms. Here we show a significant downregulation of α in grade IV astrocytoma compared to grade II and III, resulting in an increased GFAPδ/ ratio. Mimicking this increase in ratio in astrocytoma cell lines and comparing the subsequent transcriptomic changes with the changes in the patient tumours, we have identified a set of ratio-regulated high-malignant and low-malignant genes. These genes are involved in cell proliferation and protein phosphorylation, and their expression correlated with patient survival. We additionally show that changing the ratio of , by targeting expression, affected expression of high-malignant genes. Our data imply that regulating expression and splicing are novel therapeutic targets that need to be considered as a treatment for astrocytoma.