Radiation is an effective anti-cancer therapy but leads to severe late radiation toxicity in 5%–10% of patients. Assuming that genetic susceptibility impacts this risk, we hypothesized that the cellular response of normal tissue to X-rays could discriminate patients with and without late radiation toxicity.
Methods and Findings
Prostate carcinoma patients without evidence of cancer 2 y after curative radiotherapy were recruited in the study. Blood samples of 21 patients with severe late complications from radiation and 17 patients without symptoms were collected. Stimulated peripheral lymphocytes were mock-irradiated or irradiated with 2-Gy X-rays. The 24-h radiation response was analyzed by gene expression profiling and used for classification. Classification was performed either on the expression of separate genes or, to augment the classification power, on gene sets consisting of genes grouped together based on function or cellular colocalization.
X-ray irradiation altered the expression of radio-responsive genes in both groups. This response was variable across individuals, and the expression of the most significant radio-responsive genes was unlinked to radiation toxicity. The classifier based on the radiation response of separate genes correctly classified 63% of the patients. The classifier based on affected gene sets improved correct classification to 86%, although on the individual level only 21/38 (55%) patients were classified with high certainty. The majority of the discriminative genes and gene sets belonged to the ubiquitin, apoptosis, and stress signaling networks. The apoptotic response appeared more pronounced in patients that did not develop toxicity. In an independent set of 12 patients, the toxicity status of eight was predicted correctly by the gene set classifier.
Gene expression profiling succeeded to some extent in discriminating groups of patients with and without severe late radiotherapy toxicity. Moreover, the discriminative power was enhanced by assessment of functionally or structurally related gene sets. While prediction of individual response requires improvement, this study is a step forward in predicting susceptibility to late radiation toxicity.
Expression profiling can discriminate between groups of patients with and without severe late radiotherapy toxicity but not (yet) predict individual responses.
More than half the people who develop cancer receive radiotherapy as part of their treatment. That is, tumor cells are destroyed by exposing them to a source of ionizing radiation such as X-rays. Ionizing radiation damages the genetic material of cancer cells so that they can no longer divide. Unfortunately, it also damages nearby normal cells, although they are less sensitive to radiation than the cancer cells. Radiotherapists minimize how much radiation hits normal tissues by carefully aiming the X-rays at the tumor. Even so, patients often develop side effects such as sore skin or digestive problems during or soon after radiotherapy; the exact nature of the side effects depends on the part of the body exposed to the X-rays. In addition, a few patients develop severe late radiation toxicity, months or years after their treatment. Like early toxicity, late toxicity occurs in the normal tissues near the tumor site. For example, in prostate cancer—a tumor that forms in a gland in the male reproductive system that lies between the bladder and the end of the gut (the rectum)—late radiation toxicity affects rectal, bladder, and sexual function in 5%–10% of patients.
Why Was This Study Done?
It is not known why some patients develop late radiation toxicity, and it is impossible to predict before treatment which patients will have long-term health problems after radiotherapy. It would be useful to know this, because radiation levels might be reduced in those patients, while larger doses of radiation could be given to patients at low risk of late complications to ensure a complete eradication of their cancer. One theory is that some patients are genetically predisposed to develop severe late radiation toxicity. In other words, their genetic make-up makes it more likely that their tissues develop long-term complications after radiation damage. In this study, the researchers looked for markers of a genetic predisposition for late radiation toxicity by comparing radiation-induced changes in the pattern of cellular proteins in patients who had late radiation toxicity after radiotherapy with the changes seen in patients who did not develop such complications.
What Did the Researchers Do and Find?
The researchers recruited 38 patients who had been treated successfully with radiotherapy for prostate cancer two years previously. Of these, 21 had developed severe late radiation toxicity. They isolated lymphocytes (a type of immune system cell) from the patients' blood, stimulated the lymphocytes to divide, exposed them to X-rays, and analyzed the pattern of genes active in these cells—their gene expression profile—before and after irradiation. The researchers found that irradiation induced the expression of numerous genes in the lymphocytes, including many well-known radiation-responsive genes. They then used an analytical process called “random cross-validation” to look for a gene expression profile (or molecular signature) that was associated with late radiation toxicity. They report that a signature based on the radiation response of 50 individual genes correctly classified 63% of the patient population in terms of whether the patient had developed late radiation toxicity. A signature based on the radiation response of gene sets containing genes linked by function or cellular localization correctly classified 86% of the patient population.
What Do These Findings Mean?
Gene expression profiling identified groups of patients who had had severe late radiation toxicity pretty well, particularly when sets of related genes were used to classify the patients. The approach was not so good, however, at identifying individual patients who had had problems, being correct and certain only half the time. Additional studies are needed, therefore, before this promising approach can be used clinically to predict patient responses to radiotherapy. Overall, the study supports the idea that some patients are genetically predisposed to develop late radiation toxicity, and it also provides clues about which cellular pathways help to determine late radiation toxicity. Most of the genes and gene sets that discriminated between the patients with and without late radiation toxicity are involved in protein metabolism, apoptosis (a special sort of cell death), and stress signaling networks (pathways that protect cells from damage). This information, if confirmed, might help researchers to develop therapeutic interventions to minimize late radiation toxicity in vulnerable individuals.
Please access these Web sites via the online version of this summary at http://dx.doi.org/10.1371/journal.pmed.0030422.
US National Cancer Institute patient information on radiotherapy and on prostate cancer
American Cancer Society information on radiation therapy
Cancer Research UK patient information on radiotherapy
Wikipedia pages on radiotherapy (note that Wikipedia is a free online encyclopedia that anyone can edit)