Transient Genome-Wide Transcriptional Response to Low-Dose Ionizing Radiation In Vivo in Humans

Purpose

The in vivo effects of low-dose low linear energy transfer ionizing radiation on healthy human skin are largely unknown. Using a patient-based tissue acquisition protocol, we have performed a series of genomic analyses on the temporal dynamics over a 24-hour period to determine the radiation response after a single exposure of 10 cGy.

Methods and Materials

RNA from each patient tissue sample was hybridized to an Affymetrix Human Genome U133 Plus 2.0 array. Data analysis was performed on selected gene groups and pathways.

Results

Nineteen gene groups and seven gene pathways that had been shown to be radiation responsive were analyzed. Of these, nine gene groups showed significant transient transcriptional changes in the human tissue samples, which returned to baseline by 24 hours postexposure.

Conclusions

Low doses of ionizing radiation on full-thickness human skin produce a definable temporal response out to 24 hours postexposure. Genes involved in DNA and tissue remodeling, cell cycle transition, and inflammation show statistically significant changes in expression, despite variability between patients. These data serve as a reference for the temporal dynamics of ionizing radiation response following low-dose exposure in healthy full-thickness human skin.

Introduction

The effects of low-dose low linear energy transfer ionizing radiation (LDIR) in humans are of growing concern, especially in the context of current radiation techniques such as intensity-modulated radiation therapy (IMRT) and medical imaging. The biological response of healthy tissue to low doses of 1–10 cGy in vivo is unknown. Because of ethical considerations in vivo studies have been hindered because it is not possible to irradiate otherwise healthy individuals solely to study the human response to LDIR. There are no data examining the acute, transcriptional changes in normal tissue response at the lower doses received outside of the primary treatment field. These surrounding tissues are at risk for late normal tissue complications. The amount of tissue receiving low-dose exposures is increasing given the use of IMRT, for which a more conformational treatment of the target tissue results in additional scatter dose associated with substantially longer beam times.

Therapeutic radiation is used to treat a variety of malignancies, either as the primary treatment or in an adjuvant setting. The treatment strategies have evolved over time on the basis of clinical trials and evaluation of maximally tolerated doses of radiation on normal tissues. A standard time between doses (or fractions) of radiation therapy is 24 hours, based on workday scheduling and empiric observations. There has been an assumption that the immediate effects of each fraction would have returned to baseline before the next treatment, but this has not been evaluated outside of the repair kinetics for the spinal cord at the moderate ionizing radiation (IR) dose of approximately 2 Gy 1, 2, 3. Although the intratumoral treatment dose is usually 1.8–2.0 Gy/fraction, the surrounding normal tissues outside of the tumor receive lower doses, with some tissues receiving very low-dose exposures (4). Previous work from this group 4, 5 has detailed a methodology for using the lower dose exposure areas on the skin surface as an accessible human tissue model for biologic sampling for low-dose radiation biology studies 4, 5.

Response to LDIR has been studied in animal models, keratinocytes, fibroblasts, lymphocytes, and other cell lines, and differing, independent profiles are seen in the cellular response to exposure to either low or high doses of ionizing radiation 6, 7, 8, 9, 10, 11. Yin et al.(12) examined brain tissue derived from low- and high-dose full-body IR exposures and demonstrated that each response was qualitatively different from the other. These studies found that the transcriptional profiles could be categorized into three main groups: those that were altered by low- and high-dose exposures, those that were unique to low dose, and those unique to high-dose exposures. These data highlight the fact that low-dose and high-dose radiation responses are biologically diverse. In the era of IMRT, it is possible that late normal tissue responses from low-dose exposure only may be of a profile different from those arising in the high-dose region. These differing profiles may reflect entirely different processes, not simply a lesser degree of the same response.

Identifying the low-dose temporal response profile is of medical importance and has implications for counseling patients undergoing radiation therapy. Interest in normal tissue complications have increased because of the growing number of patients who have undergone ionizing radiation therapy and now have extended life expectancies (13). Furthermore, attempts to alter the time between fractions to increase the efficacy of treating the cancer have shown promise, but there are no data on how this might affect healthy tissue tolerance, especially for the tissue outside the targeted treatment field (14). Additionally, radiation response profiles are also now of national strategic interest with the growing concern for potential population exposure to low levels of radiation via terrorist acts involving radiation dispersal devices (as reviewed in 15, 16, 17). Thus, for both individual cancer patient counseling and public policy development, information on the temporal response to low-dose radiation exposure in human tissue is needed.

This study begins to address the information gap of human response to LDIR. We have developed a model for direct evaluation of the effects of LDIR in normal, healthy human tissue. Initial studies using this approach in conjunction with a statistically valid data analysis model have been used to evaluate the dose response profile at 3 hours postexposure 18, 19, 20. The studies presented herein describe the human responses to equal doses of LDIR over a 24-hour period following a single radiation exposure. By analyzing the response pattern over this period, we have been able to develop quantitative data on normal human skin responses that can be used as a benchmark for evaluating LDIR temporal patterns.