Whole exome sequencing analysis of radiation therapy treated orthotopic xenografts initiated from glioblastoma stem-like cells

A consequence of the intratumor heterogeneity (ITH) of glioblastoma (GB) is the susceptibility to treatment driven evolution. To determine the potential of radiotherapy to influence GB evolution we used orthotopic xenografts initiated from CD133+ GB stem-like cells (GSCs).Towards this end, orthotopic xenografts grown in nude mice were exposed to a fractionated radiation protocol that resulted in a significant increase in animal survival. Brain tumors from control and irradiated mice were then collected at morbidity and compared in terms of growth pattern, clonal diversity and genomic architecture. In mice that received fractionated radiation, tumors were less invasive with more clearly demarcated borders and tumor core hypercellularity as compared to controls suggesting a fundamental change in tumor biology. Viral integration site analysis indicated a reduction in clonal diversity in the irradiated tumors implying a decrease in intratumor heterogeneity. Changes in clonal diversity were not detected after irradiation of GSCs in vitro suggesting that the radiation-induced reduction in intratumor heterogeneity was dependent on the brain microenvironment. Whole exome sequencing revealed differences in mutation patterns between control and irradiated tumors, which included modifications in the presence and clonality of driver mutations associated with GB. Moreover, changes in the distribution of mutations as a function of subpopulation size between control and irradiated tumors were consistent with subclone expansion and contraction, i.e. subpopulation evolution.Taken together, these results indicate that radiation drives the evolution of the GSC-initiated orthotopic xenografts and suggest that radiation-driven evolution may have therapeutic implications for recurrent GB.