Genetics and genomics of radiotherapy toxicity: towards prediction

Abstract

Radiotherapy is involved in many curative treatments of cancer; millions of survivors live with the consequences of treatment, and toxicity in a minority limits the radiation doses that can be safely prescribed to the majority. Radiogenomics is the whole genome application of radiogenetics, which studies the influence of genetic variation on radiation response. Work in the area focuses on uncovering the underlying genetic causes of individual variation in sensitivity to radiation, which is important for effective, safe treatment. In this review, we highlight recent advances in radiotherapy and discuss results from four genome-wide studies of radiotoxicity.

Figure 1

Radiotherapy for cancer. (a) Treatment plan for a cervix tumor from 1990 with radiation delivered as two fields from the front and back - parallel opposed pair - with the gantry of the linear accelerator moving to deliver high energy X-rays from different directions. A large rectangular volume, including the central uterus containing the tumor, parts of the bowel (top) and the base of the spine (bottom), received the maximum planned dose. (b) Treatment plan for a cervix tumor from 2011 with radiation delivered as four fields (front, back, both sides) and multileaf collimators (metal leaves that move independently to block the path of the beam) to shape (conform) the maximum radiation dose not only around the large tumor but also to follow the lymph node chains where the disease had spread, while sparing as much normal tissue as possible. (c) Treatment plan from 2011 to treat pelvic sidewall disease following surgical resection for a cervix tumor. The intensity modulated radiotherapy was given as eight fields, with radiation intensity modulated along the beams using multileaf collimators to deliver the maximum dose to the tumor and to spare normal tissue. (d) Intensity modulated radiotherapy for a breast tumor. Examples of uneven radiation dose distribution using standard two-dimensional radiotherapy (left). The orange color depicts regions of unwanted high dose, superiorly and inferiorly. There is also an unwanted low-dose region depicted in green. Changing to intensity-modulated radiotherapy evens the dose distribution across the breast, as shown by the more homogeneous yellow color (right).

Figure 2

Summary of the pathways and mechanisms involved in cell and tissue response to radiotherapy. The interaction of ionizing radiation with tissues leads to multiple types of DNA damage (for example, base damage, single-strand breaks, double-strand breaks). Double-strand breaks are harder to repair and are the most important DNA lesion induced by radiation. Radiation also produces reaction oxygen (ROS) and nitrogen (RNOS) species that stimulate cytokine, growth factor and chemokine responses. There are multiple interconnected signaling networks that respond to radiation damage that can lead to cell death, cell senescence, genomic instability, mutations and inflammatory response. Some of the key genes involved in the processes are shown. The information taken from Bentzen (2006) [28], Jeggo and Lavin (2009) [80] and Bhatti et al. [132]. HR, homologous recombination; NHEJ, non-homologous end joining; NOS, nitric oxide synthase; SOD, superoxide dismutase.

Figure 3

Measuring radiosensitivity. There are many assays for measuring radiosensitivity. The gold standard is a clonogenic assay where single cells are plated and allowed to grow for 1 to 4 weeks to assess ability to form colonies. (a) Colonies from fibroblasts cultured from a human skin sample. As it takes several weeks to culture fibroblasts and carry out a clonogenic assay, more rapid assays are often used. (b) An example of a more rapid assay is the G2 assay: a peripheral blood sample is taken, lymphocytes are stimulated to proliferate with the mitogen phytohemagglutinin, after 72 hours the cells are irradiated with 0.5 gray (Gy), and after 30 minutes colcemid is added for 60 minutes to arrest cells at metaphase that were in G2 when irradiated. The number of chromosome aberrations (arrows) is scored relative to unirradiated controls [41]. (c) Another example is the micronucleus assay: peripheral blood lymphocytes are irradiated with approximately 2 Gy and incubated for 2 days, cytochalasin B is added to prevent cytoplasm division after mitosis, and cells are harvested after 1 day and the number of micronuclei per 100 to 1,000 cells is scored [42]. Demonstration of cellular radiosensitivity in individuals with life-threatening radiotherapy toxicity or cancer-predisposing syndromes usually involves fibroblasts and derivation of radiation survival curves. (d) Survival curves for a number of individuals, including one (blue line) with ataxia telangiectasia, showing extreme cellular radiosensitivity. Parameters can be obtained from fitting curves to the data and parameters that reflect the initial slope, such as alpha and surviving fraction at 2 (SF2) or 3 Gy, are better at showing differences in radiosensitivity between people [133]. (e) Normal (that is, non-syndromic) individuals vary in radiosensitivity with a distribution that is approximately normal [134].