Epigenetic and genetic influence on radiation responce.

Clinical experience has demonstrated that some tumors are more resistant towards ionizing radiation than others. There is strong evidence that both genetic and epigenetic characteristics of the tumor and surrounding tissue have impact on the response and outcome of treatment. As of today, there are no clinical tests that can identify patients with high risk for development of severe late adverse effects after radiotherapy, or those who are likely to respond poorly to the treatment. These tests would have provided the opportunity to optimize the radiotherapy, but in order to develop such tests we need better knowledge about the molecular mechanisms behind radiation response. We have studied 19 pairs of tumors sampled before and after radiation treatment for molecular markers of radiation response. The low number of cases and events increases the probability of false positive findings in microarray analysis. The results from this study should therefore be validated in independent datasets. The overall aim of this study was to investigate whole genome epigenetic effects for their impact on response to radiotherapy as well as selected genetic variations in TP53, MDM2, MDM4 and PIK3CA. Mutations in TP53 and SNP’s in MDM2 and MDM4 were selected because of the pivotal role of the p53 pathway in stress response. Two exons in PIK3CA were sequenced because mutations in these regions are reported to contribute to radiation resistance. DNA methylation was investigated since this epigenetic alteration is thought to have great impact on tumorigenesis and response to cancer treatment, because of the ability to regulate gene expression. Based on the methylation level of 84 genes we were able to separate the irradiated tumor samples from the non‐irradiated tumor samples with an overall error rate of 12,8%. Some of these genes, such as the inflammatory cytokine IL1A and H2AFY involved in DNA‐repair were significantly differentially methylated in the irradiated compared to the non‐irradiated tissue. For some genes such as IL1A, PPGB and H2AFY the alteration in methylation levels showed evidence for a dose dependency. Based on their clinical evaluation, samples were then divided into two major groups, good and poor responders. With an overall error rate of zero, samples from the two response groups were separated based on the methylation levels in 342 genes. The gene ESRRB was highly methylated in those with good response, while the gene EGR2, which is reported to be proapoptotic, was highly methylated in those reported with poor response. Overall, genes involved in the pathways connected to the immune response were found differentially methylated in tumor tissue exposed to irradiation compared with non‐irradiated tumor tissue, as well as between the good and reduced responders. A tendency towards lower response to treatment was seen in those tumors harboring TP53 mutations together with the unfavorable variant SNP’s in MDM2 or MDM4, or in those tumors carrying alterations PIK3CA. These results suggest that both genetic and epigenetic alterations have impact on the response to radiotherapy, and that methylation levels in both genes and pathways, are changed as a result of irradiation