Glioblastoma multiforme (GBM) are highly lethal brain tumors for which exposure to ionizing radiation is the only known risk factor. GBMs are characterized by alterations in three core signaling pathways: 1) RTK-PI3K-Akt, 2) ARF-MDM2-p53, and 3) Ink4a-RB1. In order to objectively evaluate the risk of developing malignant gliomas from exposure to ionizing radiation, we have developed mouse models with brain-specific deletions of Ink4a, Ink4b, Arf, or PTEN in logical combinations representing the progression of primary or secondary GBMs. Specifically, we examined whether complex, unrepairable DNA lesions, such as those induced by charged particles, are more tumorigenic compared to simple breaks that are efficiently repaired, such as those induced by gamma-rays. Using CDKN2A knockout murine astrocytes in which the tumor suppressors Ink4a and Arf have been deleted (abrogating RB1 and p53 pathways, respectively), we have previously shown that Fe ions are highly tumorigenic compared to gamma-rays. Importantly, we find that loss of a third tumor suppressor, Ink4b, is a critical event in tumorigenesis triggered by charged particles. Based on our in vitro results, we are now using transgenic mouse models with brain-specific deletions of these three key tumor suppressors (Ink4a/b, Arf) to evaluate radiation-induced gliomagenesis in vivo. Nestin-Cre; Ink4ab-/-; Arf f/f mice were irradiated with a single dose of 1 Gy Fe ions or 4 Gy gamma-rays. While gamma-induced damage is efficiently repaired by 1 day, unrepaired DNA lesions are evident up to 1 month following Fe irradiation. Most importantly, we find that combined loss of Ink4a/b and Arf cooperate with DNA damage by Fe ions resulting in a high incidence (25%) of malignant gliomas. These highly heterogeneous tumors arise with an average latency of 4.8 months and are classified as high grade (III and IV) glial tumors. Although tumors arising after gamma irradiation are also classified as high grade, they arise with a longer latency (5.6 months) and occur at a lower frequency (15%). To identify key genetic alterations involved in radiation-induced gliomagenesis, tumors are being analyzed by array CGH and microarray platforms. Strikingly, a 20- to 40-fold amplification of the MET proto-oncogene is observed in a high percentage of tumors. As MET is frequently amplified in human GBMs, it is likely that MET activation may be an important event in tumorigenesis triggered by charged particles in these mouse models. We are hopeful that a global and accurate picture of the genetic changes underlying radiation-induced gliomagenesis will emerge from this study. The data obtained will allow for comparison with changes recently identified in human gliomas by large scale genomic analyses and will allow us to understand whether radiation-induced gliomas are fundamentally similar to those occurring spontaneously, both in mouse models and in human patients.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2360. doi:1538-7445.AM2012-2360