Unexpected sensitivity to radiation of fibroblasts from unaffected parents of children with hereditary retinoblastoma

Comparative study of radioresistance between feline cells and human cells

Radioresistance of cats has been seen in animal radiotherapy. Feline radioresistance and its underlying mechanism(s) were investigated in fibroblast cells and lymphocytes. We hypothesized that radioresistance was attributable to an increase in the cells ability to repair DNA damage. To investigate this hypothesis, fibroblast cells were exposed to various doses of X rays and then colony formation assays were performed. Survival curves showed that potential lethal damage repair (PLDR) for feline cells were greater than that for human cells. γ-H2AX foci assays were performed to evaluate DNA double-strand breaks (DSBs) formation and repair kinetics. After PLDR, feline cells displayed a decreased residual amount of γ-H2AX foci. Formation of chromosome aberrations (dicentrics) after PLDR as an indicator of radiation-induced DNA damage and repair; human, feline and canine lymphocytes were evaluated. Human and canine lymphocytes showed two to three times the number of dicentrics compared to feline lymphocytes. Finally, micronuclei assays were performed to further confirm the radioresistant nature of feline lymphocytes. In concordance with the results of the chromosome aberration assay, the number of micronuclei in feline lymphocytes was less than observed in human and canine lymphocytes. Taken together, these results show that DNA and chromosome damage induced by X irradiation is more effectively repaired in feline cells, resulting in less residual damage. Our results suggest that both feline fibroblasts and lymphocytes are more radioresistant compared to human cells of similar tissues, and this resistance can be contributed, at least in part, to greater ability for PLDR.

Genetic susceptibility: radiation effects relevant to space travel

Genetic variation in the capacity to repair radiation damage is an important factor influencing both cellular and tissue radiosensitivity variation among individuals as well as dose rate effects associated with such damage. This paper consists of two parts. The first part reviews some of the available data relating to genetic components governing such variability among individuals in susceptibility to radiation damage relevant for radiation protection and discusses the possibility and extent to which these may also apply for space radiations. The second part focuses on the importance of dose rate effects and genetic-based variations that influence them. Very few dose rate effect studies have been carried out for the kinds of radiations encountered in space. The authors present here new data on the production of chromosomal aberrations in noncycling low passage human ATM+/+ or ATM+/- cells following irradiations with protons (50 MeV or 1 GeV), 1 GeV(-1) n iron ions and gamma rays, where doses were delivered at a high dose rate of 700 mGy(-1) min, or a lower dose rate of 5 mGy min(-1). Dose responses were essentially linear over the dose ranges tested and not significantly different for the two cell strains. Values of the dose rate effectiveness factor (DREF) were expressed as the ratio of the slopes of the dose-response curves for the high versus the lower (5 mGy min(-1)) dose rate exposures. The authors refer to this as the DREF5. For the gamma ray standard, DREF5 values of approximately two were observed. Similar dose rate effects were seen for both energies of protons (DREF5 ≈ 2.2 in both cases). For 1 GeV(-1) n iron ions [linear energy transfer (LET) ≈ 150 keV μ(-1)], the DREF5 was not 1 as might have been expected on the basis of LET alone but was approximately 1.3. From these results and conditions, the authors estimate that the relative biological effectiveness for 1 GeV(-1) n iron ions for high and low dose rates, respectively, were about 10 and 15 rather than around 20 for low dose rates, as has been assumed by most recommendations from radiation protection organizations for charged particles of this LET. The authors suggest that similar studies using appropriate animal models of carcinogenesis would be valuable.

G2-phase chromosomal radiosensitivity of primary fibroblasts from hereditary retinoblastoma family members and some apparently normal controls

We previously described an enhanced sensitivity for cell killing and gamma-H2AX focus induction after both high-dose-rate and continuous low-dose-rate gamma irradiation in 14 primary fibroblast strains derived from hereditary-type retinoblastoma family members (both affected RB1(+/-) probands and unaffected RB1(+/+) parents). Here we present G(2)-phase chromosomal radiosensitivity assay data for primary fibroblasts derived from these RB family members and five Coriell cell bank controls (four apparently normal individuals and one bilateral RB patient). The RB family members and two normal Coriell strains had significantly higher ( approximately 1.5-fold, P < 0.05) chromatid-type aberration frequencies in the first postirradiation mitosis after doses of 50 cGy and 1 Gy of (137)Cs gamma radiation compared to the remaining Coriell strains. The induction of chromatid-type aberrations by high-dose-rate G(2)-phase gamma irradiation is significantly correlated to the proliferative ability of these cells exposed to continuous low-dose-rate gamma irradiation (reported in Wilson et al., Radiat. Res. 169, 483-494, 2008). Our results suggest that these moderately radiosensitive individuals may harbor hypomorphic genetic variants in genomic maintenance and/or DNA repair genes or may carry epigenetic changes involving genes that more broadly modulate such systems, including G(2)-phase-specific DNA damage responses.

Retinoblastoma

Retinoblastoma (Rb) is a malignant tumor that originates from developing retina. Diagnosis based on clinical signs and symptoms and is usually made in children under the age of five years. Mutations in both alleles of the RB1 gene are a prerequisite for this tumor to develop. In most patients with sporadic unilateral Rb, both RB1 gene mutations occur in somatic cells and are not passed over to offspring (nonhereditary Rb). Almost all patients with sporadic bilateral and virtually all patients with familial Rb are heterozygous for RB1 gene mutations that cause predisposition to Rb (hereditary Rb). In families, Rb predisposition is transmitted as an autosomal dominant trait (familial Rb). In addition to Rb, patients with hereditary disease also have an increased risk of tumors outside the eye (second cancer). This risk is enhanced in patients who have received external beam radiotherapy. Analysis of genotype-phenotype associations has shown that the mean number of tumor foci that develop in carriers of mutant RB1 alleles is variable depending on which functions of the normal allele are retained and to what extent. Moreover, phenotypic expression of hereditary retinoblastoma is subject to genetic modification. Identification of the genetic factors that underlie these effects will not only help to arrive at a more precise prognosis but may also point to mechanisms that can be used to reduce the risk of tumor development.

Variations in radiosensitivity among individuals: a potential impact on risk assessment?

To have an impact on risk assessment for purposes of radiation protection recommendations, significantly broad variations in carcinogenic radiosensitivity would have to exist in significant proportions in the human population. Even if we knew all the genes where mutations would have major effects, individual genome sequencing does not seem useful, since we do not know all these genes, nor can we be certain of the phenotypic effect of polymorphisms discovered. Further, sequencing would not reveal epigenetic changes in gene expression. Another approach to develop phenotypic biomarkers for cells or tissues for which variations in radiation response may reflect the variations in carcinogenic sensitivity. To be useful, experimental evidence for such a correlation would be crucial, and it is also evident that correlations may be tissue or tumor specific. Some cellular markers are discussed that have shown promise in this regard. They include chromosome aberration induction and DNA repair assays that are sufficiently sensitive to measure after modest or low doses or dose rates. To this end we summarize here some of these assays and review the results of a number of experiments from our laboratory that show clear differences in DNA repair capacity reflected by gamma-H2AX foci formation in cells from a high proportion (perhaps 1/3) of apparently normal individuals. A low dose-rate assay was used to amplify such differences. Another promising assay combines G(2) chromosomal radiosensitivity with the above gamma-H2AX foci on mitotic chromosomes. There are other potentially useful assays as well.

A defect in DNA double strand break processing in cells from unaffected parents of retinoblastoma patients and other apparently normal humans

Cells from unaffected parents of retinoblastoma (RB) patients were previously shown to be hypersensitive to radiation induced G(1) arrest and cell killing [1]. The hypersensitivity was similar to that reported for cells from ATM heterozygotes. The latter was consistent with a mild DNA DSB rejoining defect which we demonstrated using a gamma-H2AX focus assay after low dose-rate (LDR) irradiation of non-cycling G(0) cells [2,3]. Since neither parent carried the mutant RB allele of the RB heterozygous probands, these results suggested the possibility of an enhanced germline mutation rate, perhaps resulting from some mild defect in genome maintenance. We therefore examined levels of gamma-H2AX foci for cells from these RB parents in this G(0) LDR assay, which reflects the non-homologous end joining (NHEJ) capacity of cells and in a G(2)/M assay, which reflects additional contributions from other G(2)-related damage processing systems. For several of the cell strains parallel radiosensitivity comparisons were made for cell killing and for G(2) chromosomal radiosensitivities. G(0) cells from the RB parents were clearly hypersensitive both in the LDR gamma-H2AX assay, and for cell killing. In addition, cultured fibroblasts from 6 of 15 apparently normal individuals in this study (and one of six in a previous study) were also hypersensitive in the same assays. In the G(2)/M gamma-H2AX assay, the relative sensitivities were similar to those seen in the low dose-rate G(0) assay and tracked with chromosomal radiosensitivity, but some differences were observed.

Levels of gamma-H2AX Foci after low-dose-rate irradiation reveal a DNA DSB rejoining defect in cells from human ATM heterozygotes in two at families and in another apparently normal individual

We have investigated the use of the gamma-H2AX assay, reflecting the presence of DNA double-strand breaks, as a possible means for identifying individuals who are mildly hypersensitive to ionizing radiation, such as some ATM heterozygotes. We compared levels of gamma-H2AX foci after irradiation in cells from six apparently normal individuals as well as from individuals from two separate AT families including the proband, mother, father and three unaffected siblings in each family. After a 1-Gy single acute (high-dose-rate) gamma-ray dose delivered to noncycling contact-inhibited monolayers of cells, clear differences were seen between samples from normal individuals (ATM(+/+)) and probands (ATM(-/-)) at nearly all sampling times after irradiation, but no clear distinctions were seen for cells from normal compared to obligate heterozygotes (ATM(+/-)). In contrast, after 24 h of continuous irradiation at a dose rate of 10 cGy/h, appreciable differences in numbers of foci per cell were observed for cells from individuals for all the known ATM genotypes compared with controls. Four unaffected siblings had mean numbers of foci per cell similar to that for the obligate heterozygotes, whereas the other two had mean values similar to that for normal controls. We determined independently that those siblings with mean numbers of foci per cell in the range of ATM heterozygotes carried the mutant allele, while both siblings with a normal number of foci per cell after irradiation had normal alleles. A more limited set of experiments using lymphoblastoid cell strains in the low-dose-rate assay also revealed distinct differences for normal compared to ATM heterozygotes from the same families and opens the possibility of using peripheral blood lymphocytes as a more suitable material for an assay to detect mild hypersensitivities to radiation among individuals.

Abnormal Gene Expression Profiles in Unaffected Parents of Patients with Hereditary-Type Retinoblastoma

The hereditary form of retinoblastoma (Rb) is associated with a germ line mutation in one RB allele and is characterized by the occurrence of multiple, bilateral Rb tumors and a predisposition to the development of second cancers. In an earlier study, we observed an unexpected hypersensitivity to ionizing radiation in skin fibroblasts derived from unaffected parents of children with hereditary Rb. In at least four of these five families, there was no family history of Rb, indicating a new germ line mutation. We hypothesize that the increased parental cell sensitivity to radiation may reflect the presence of an as yet unrecognized genetic abnormality occurring in one or both parents of children with Rb. In the present study, we use DNA microarray technology to determine whether differences in gene expression profiles occurred in the unaffected parents of patients with hereditary Rb relative to normal individuals. Microarray analyses were validated by quantitative reverse transcription-PCR measurements. A distinct difference was observed in the patterns of gene expression between unaffected Rb parents and normal controls. By use of the prediction analysis for microarrays and principal component analysis methodologies, significant differences between the two groups were identified when as few as nine genes were analyzed. Further study of this phenomenon may offer a new insight into the genetic mechanisms of Rb and perhaps more broadly in cancer biology.

Radiosensitivity of peripheral blood lymphocytes obtained from patients with cancers of the breast, head and neck or cervix as determined with a micronucleus assay

The in vitro radiation sensitivities of peripheral blood lymphocytes obtained from 48 normal females and 168 female cancer patients were measured with the cytokinesis-blocking micronucleus assay. Cancer patients group had significantly higher mean baseline micronucleus frequency than normal healthy controls. Breast cancer patients were more radiosensitive than normal individuals. Cervical cancer cases were less radiation sensitive than normal subjects. The relative lack of radiation sensitivity in cervical-cancer cases could be due to modification of the radiosensitivity of patients' immune-responsible cells by human papillomaviruses infection. Normal individuals and cancer patients were classified according to their radiation sensitivity which was evaluated with the radiation-induced micronucleus frequencies. Such a classification will be an important initial step to characterize the radiosensitive, radioresistant, or cancer-prone individuals using specific SNP typing.