Mechanistic Analysis of Radiation-Induced Gliomagenesis

Glioblastomas (GBM) are devastating brain tumors refractory to any available treatment. Exposure to ionizing radiation (IR) is the only known GBM risk factor. The link between low-linear energy transfer (LET) IR and gliomagenesis has been clearly demonstrated by epidemiological studies of human patients receiving diagnostic or therapeutic radiation. Whether such risk exists with particle radiation exposure, which is more densely ionizing, has not been evaluated. Particle radiation is increasingly used in radiotherapy and is also an occupational hazard for astronauts in space. With no human data available, animal models mimicking the process of radiation carcinogenesis are essential for risk assessment. Through a large scale systematic interrogation of multi-allele transgenic mice with brain-restricted deletions of GBM-relevant tumor suppressor genes we identified two complementary genotypes (NesCreInk4ab-/-ArfF/F and NesCrep53+/F;Pten+/F). We irradiated both models intra-cranially with equal doses of a range of charged particles with different LETs. Interestingly, we found an increase in gliomagenesis with LET until a peak frequency was reached with silicon ions (LET of 79.3 KeV/μm) following which tumor frequencies declined with heavier particles with higher LETs. These radiation-induced mouse tumors phenocopy the histopathological features of human GBM, including infiltrative growth, pseudopalisading necrosis, high mitotic index, and positivity for glial (Gfap, Olig2) and stem/progenitor markers (Sox2). Ex-vivo cultures derived from these tumors showed features of glioma stem-like cells underscoring the undifferentiated nature of the parental tumors. Integrated genomic and functional analyses revealed the driving oncogenic changes in tumors from the NesCrep53+/F;Pten+/F model. Regardless of radiation quality, all tumors had genomic deletions of the wild-type alleles of both p53 and Pten. Such concomitant loss signifies the crucial roles that p53 and Pten together play as barriers to radiation-induced transformation. Over-expression of the receptor tyrosine kinase Met following a genomic amplification event harbored by 40% of tumors was similarly observed across all radiation qualities. Met overexpression enhanced the stemness phenotype in the context of p53 loss, and additionally conferred radioresistance. These combinatorial effects illustrate the importance of evaluating GBM drivers as integrated nodes in an oncogenic signaling network. In sum, the identification of two mouse models carrying deletions of independent TSGs has allowed us to establish the universal role of radiation as a genotoxic agent capable of inducing high grade gliomas. These models and the identified key molecular changes accompanying radiation-induced gliomagenesis can be used in the design of therapeutic strategies for patients with secondary glioma who are currently limited in their options.