Office of Biological and Environmental Research
DOE Low Dose Radiation Research Program Workshop III

Abstract

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Title: Genetic Factors Affecting Susceptibility to Low Dose and Low Dose-Rate
Radiation Exposure

Authors: J. S. Bedford, H. Nagasawa, and J.B. Little

Institutions: Department of Radiological Health Sciences, Colorado State University
Laboratory of Radiobiology, Harvard School of Public Health

The goals of this project are to identify new genes controlling radiosensitivity, obtain information regarding the frequencies of polymorphisms of these and other such genes, and to assess their biological significance with respect to risk assessment for low doses of low LET ionizing radiation. The criterion for assessing differences in radiosensitivity involves the measurement of radiation-induced chromosomal aberrations, since these figure prominently in carcinogenesis; the main hazard of interest for radiation protection. The particular focus is on chromosomal radiosensitivity following low-doses of gamma radiation, in the 0 to 10 cGy range, and for continuous low dose-rate exposures.

We already have human cell cultures available from some 33 different individuals that differ by a factor of about two in sensitivity for cell killing after high-dose, high dose-rate irradiation. Among these individuals there is no known defect in any currently known DNA repair or damage processing genes. We will attempt to correct or partially correct the unknown defect to restore a more "normal" radioresistant phenotype by well-established procedures of DNA library gene transfection.

For acute high dose-rate exposures the difference in killing for hypersensitive and normal cells is unlikely to be great enough for the very high efficiency selection necessary to isolate the gene-corrected cells, we are employing a low dose-rate (LDR) strategy we previously used to isolate a radiosensitive CHO mutant and to map and identify the gene and the mutation involved. Using this approach, we will isolate DNA from partially or fully corrected human clones by selective PCR amplification of the unique primer-flanked sequence present in all the sequences in the original human library, then we will sequence and compare the correcting DNA with sequences of known or unknown function.

Radiation hypersensitivity for cell killing correlates with hypersensitivity for chromosomal aberration induction in every instance where it has been examined, but we will examine this for the cells from this study using whole chromosome painting and mFISH techniques, to compare chromosomal radiosensitivities at low doses and dose-rates. The focus here would be on stable aberrations of the general kinds known to be relevant to cancer. A few similar comparisons of low dose and low dose-rate chromosomal radiosensitivities will be made using cells from individuals who are heterozygous or homozygous for genes known to involve "repair defects" for ionizing radiation such as Nijmegen Breakage Syndrome (NBS), BRCA1 and 2, or Ataxia-Telangiectasia. In some instances, genetic complementation studies with cell hybrids between these and cells of unknown defects may be necessary.
Current estimates of the risks of radiation exposure to humans are based largely on the probability of an effect to the "average" individual in an irradiated population. By better identification of genetic factors underlying the control of radiosensitivity it may be possible to tailor risk estimates to individuals based on their genotype.