Abstract
High grade astrocytomas are brain tumors that currently have no cure. Understanding mechanisms that sustain and help them to resist therapy are urgently necessary. The capicua (CIC) transcriptional repressor is a target of receptor tyrosine kinase (RTK) signaling. Upon phosphorylation by RTK signaling, CIC loses its repressor activity. We hypothesize that loss of CIC activity and the subsequent activation of the genes it normally represses plays an important role in astrocytoma biology. In humans, CIC inactivating mutations are found in a subset of gliomas, specifically oligodendrogliomas associated with 1p/19q chromosomal losses. In astrocytoma and oligodendroglioma without 1p/19q loss, CIC mutations are rare. Given the genetic heterogeneity of gliomas, it is conceivable that subpopulations within astrocytomas may utilize different mechanisms to ablate CIC activity, for example through RTK signaling. Using a novel mouse model of astrocytoma with enhanced RTK signaling, we identified that CIC expression was decreased by 50–70% compared to normal cortex and correlated with significant increased RNA and protein levels of known oncogenic signals including E-twenty six transformation specific (ETS)-related transcription factor 4 (ETV4) and matrix metalloproteinase 2 (MMP2). Mouse astrocytoma tumor spheres also had lower CIC protein levels and higher protein levels of ETV4 and MMP2 compared to normal neurospheres. Low CIC-expressing human astrocytoma stem cells were also highly expressing ETV4 and MMP2. Functional effects of CIC knockdown in mouse astrocytoma tumor spheres included enhanced self-renewal and invasiveness which also correlated with increased ETV4 and MMP2. Intracranial tumors generated from injection of control or CIC knockdown glioma stem cells into mouse brain led to no difference in Kaplan-Meier survival curves however, histological analysis suggested that the CIC knockdown tumors displayed evidence of enhanced pleomorphism, spindle shaped cells and staghorn vasculature. These studies highlight that even when not mutated, CIC activity may be dynamically regulated to modulate astrocytoma biology.