Role of epigenetic effectors in maintenance of the long-term persistent bystander effect in spleen in vivo.

Radiation therapy is a primary treatment modality for brain tumors, yet it has been linked to the increased incidence of secondary, post-radiation therapy cancers. These cancers are thought to be linked to indirect radiation-induced bystander effect. Bystander effect occurs when irradiated cells communicate damage to nearby, non-irradiated 'bystander' cells, ultimately contributing to genome destabilization in the non-exposed cells. Recent evidence suggests that bystander effect may be epigenetic in nature; however, characterization of epigenetic mechanisms involved in bystander effect generation and its long-term persistence has yet to be defined. To investigate the possibility that localized X-ray irradiation induces persistent bystander effects in distant tissue, we monitored the induction of epigenetic changes (i.e. alterations in DNA methylation, histone methylation and microRNA (miRNA) expression) in the rat spleen tissue 24 h and 7 months after localized cranial exposure to 20 Gy of X-rays. We found that localized cranial radiation exposure led to the induction of bystander effect in lead-shielded, distant spleen tissue. Specifically, this exposure caused the profound epigenetic dysregulation in the bystander spleen tissue that manifested as a significant loss of global DNA methylation, alterations in methylation of long interspersed nucleotide element-1 (LINE-1) retrotransposable elements and down-regulation of DNA methyltransferases and methyl-binding protein methyl CpG binding protein 2 (MeCP2). Further, irradiation significantly altered expression of miR-194, a miRNA putatively targeting both DNA methyltransferase-3a and MeCP2. This study is the first to report conclusive evidence of the long-term persistence of bystander effects in radiation carcinogenesis target organ (spleen) upon localized distant exposure using the doses comparable with those used for clinical brain tumor treatments.