PDTM-27. THE CRISPR-Cas9 SYSTEM-MEDIATED ENDOGENOUS GENE-REARRANGEMENT INDUCED C11orf95-RELA FUSION IN VITRO AND IN VIVO THAT LED TO DEVELOPMENT OF BRAIN TUMORS

Recent large-scale genomic studies of ependymal tumors have identified recurrent RELA and YAP1 fusion genes in supratentorial ependymomas. We have so far shown that C11orf95-RELA fusion was a potent oncogene capable of inducing human ependymoma-like tumors in the RCAS/tv-a retroviral gene transfer system. However, molecular mechanism underlying the tumor formation has not been fully investigated. Given that C11orf95-RELA fusion genes are caused by gene rearrangement between C11orf95 and RELA gene due to chromothripsis involving 11q, it has been suggested that expression of these fusion genes is regulated under the C11orf95 promoter. In this study, we developed a system to reproduce the oncogenic gene rearrangement using the CRISPR-Cas9 system and examined if consequent endogenous ependymoma fusion genes are competent to form brain tumor in mice. Initially, we designed guide RNAs on the human and mouse genomic loci corresponding to breakpoints observed in human RELA fusion positive ependymomas and introduced them into HEK293T or NIH3T3 cells. RT-PCR and immunoblot analyses revealed that a combination of multiple guide RNAs efficiently induced gene rearrangement between C11orf95 and RELA gene, resulting in C11orf95-RELA fusion protein. Subsequently, we introduced the same guide RNAs using a lentiviral gene transfer system into mouse brain. Brain tumor formation was observed around 2 months after the guide RNA delivery, indicating successful gene rearrangement followed by C11orf95-RELA fusion expression in vivo. These results thus firmly establish that a gene rearrangement is the mechanism to form the C11orf95-RELA fusion which is the direct driver of tumorigenesis. Furthermore, we found that endogenous YAP1-FAM118B fusion gene can be also induced with a similar experimental strategy to that of C11orf95-RELA fusion. Our system to simulate genomic event will provide insights into the tumorigenic mechanism of ependymomas that harbor various fusion genes.