Low doses of IR have unknown biological consequences. In clinical radiation therapy, high doses of radiation are delivered to the target (tumor) tissue, whereas adjacent tissues receive lower doses. As more radiation therapy is delivered using highly conformal beam arrangements, such as IMRT, there are increases in the scattered dose of radiation to surrounding healthy tissue. With the surge in the number of cancer survivors, there comes a population of people who are living with the long-term effects of such radiation exposure. They are both a population of concern and one in whom biological studies can be undertaken to help broaden our understanding of the effects of low-dose radiation.
We have developed a model system that allows direct evaluation of radiation effects on healthy tissue by using techniques to confirm accurate dosimetry on individuals who are receiving localized therapeutic radiation for early-stage prostate cancer (4
). This model allows for real-time sampling of human tissue after in vivo
radiation exposures. This model is therefore unique in allowing the evaluation of whole tissue effects when exposure is under normal physiologic conditions. The full-thickness biopsy samples examined in these studies were evaluated as a whole tissue containing both epithelial and stromal cells. Therefore, the transcriptional profile is a comprehensive assessment of the responses of thousands of cells of multiple lineages, some classically “radiation sensitive,” whereas others would be “radiation resistant. ” Although specific cell-line outcomes cannot be identified, the tissue-level response is evaluated. Because the biomedical community is ultimately interested in defining risk to human tissues, the whole-tissue evaluation is a necessary level of study to begin to define human response to LDIR.
To study tissue-specific genomic responses to LDIR, gene groups and pathways known to be radiation sensitive were selected. These groupings are discussed in detail in Goldberg et al
), but in brief, they are involved in DNA repair, damage and remodeling (topoisomerase, zinc finger proteins), nuclear signaling, cell-cycle and associated check-points (cyclins, chk-1, chk-2, GRAP2, GPR51), inflammatory mediators (prostaglandin E2, cyclooxygenase-2, and interleukins), growth factors (epidermal growth factor receptor, tumor necrosis factor, vascular endothelial growth factor), apoptosis/survival signaling (Akt/phosphoinositide-3-kinase), and tissue structure and reorganization (keratins and ANLN).
Of the nine statistically significant groups, the zinc finger family of genes, the keratin gene group, and the cyclins were transiently up-regulated. Keratins are clearly tissue specific and suggest that there is repair or remodeling that is completed at the transcript level within 24 hours. Similarly, the cyclins and zinc finger proteins suggest that alterations in cell cycling and DNA remodeling are acute, transient responses to IR. Given that the doses examined are far below those that are frankly cytotoxic, this pattern of responses suggests that the tissue is actively undergoing some repair, even to such a low dose. The transient decrease in transcription of heat shock protein, TNF, and interleukin genes raises the possibility that the tissue attempts to diminish the acute stress response while it up-regulates transcription of DNA and tissue remodeling genes. Although this has not yet been evaluated in other human tissues, our results suggest that at least the skin response to LDIR may be substantially different from those seen following higher dose exposure. A response pattern of this type would be consistent with the emerging data from in vitro studies. The clinical implications of such a differing pattern have not yet been determined and will likely not be fully appreciated for many years to come.
The design of the study allowed for detection of transient
up-regulation or down-regulation but not for detection of responses that are sustained beyond 24 hours. Given the positive gene group findings, as well as our previously reported positive low-dose radiation human skin biosignature data, the sustained nature of the tissue response seems a likely explanation for the observation that none of the pathways were differentially expressed (18
). The temporal dynamics of the transcriptional pathways we examined likely last longer than the 24-hour period of this data set. We are currently engaged in additional studies that will allow for detection of more sustained responses.
These data represent the first whole tissue, human temporal response data examining the effects of a single exposure to LDIR using precise dosimetry and statistically principled analyses. We have shown that it is possible to detect transient response to LDIR in vivo in humans and have identified nine gene groups that are either significantly up-regulated or significantly down-regulated. These data represent a reference library for genomic analysis of the temporal response of human skin exposed to a single dose of LDIR.