Ink4a-Arf loss cooperates with KRas activation in astrocytes and neural progenitors to generate glioblastomas of various morphologies depending on activated Akt.

Deletion of the INK4a-ARF locus is found in the majority of human malignant gliomas. However, the role of INK4a-ARF loss in gliomagenesis is unclear. Animal modeling has shown that mice with targeted deletions in the Ink4a-Arf gene do not develop spontaneous gliomas. We have previously reported that combined KRas and Akt signaling could induce glioblastoma (GBM) formation from neural progenitor cells but had no effect in differentiated astrocytes. In this investigation, we have studied the effects of Ink4a-Arf loss on the formation of GBM induced by KRas and Akt gene transfer into neural progenitor cells and astrocytes. We show here that Ink4a-Arf deficiency allows for GBM formation from astrocytes and that it enhances tumor incidence in neural progenitor cells. Furthermore, KRas alone can cooperate with deletion of the Ink4a-Arf locus in tumor formation from both neural progenitor cells and astrocytes. The resulting tumors were nestin positive and resembled a spectrum of glioma morphologies ranging in astrocytic character depending on cell-of-origin and presence of activated Akt. Our data strongly supports the view that one role of loss of Ink4a-Arf in gliomagenesis could be to sensitize astrocytes to transformation through dedifferentiation in response to the appropriate oncogenic stimuli.