Mutagenic effects of a single and an exact number of α particles in mammalian cells

Abstract

One of the main uncertainties in risk estimation for environmental radon exposure using lung cancer data from underground miners is the extrapolation from high-to low-dose exposure where multiple traversal is extremely rare. The biological effects of a single ␣ particle are currently unknown. Using the recently available microbeam source at the Radiological Research Accelerator Facility at Columbia University, we examined the frequencies and molecular spectrum of S1؊ mutants induced in human-hamster hybrid (A L ) cells by either a single or an exact number of ␣ particles. Exponentially growing cells were stained brief ly with a nontoxic concentration of Hoechst dye for image analysis, and the location of individual cells was computermonitored. The nucleus of each cell was irradiated with either 1, 2, 4, or 8 ␣ particles at a linear energy transfer of 90 keV͞m consistent with the energy spectrum of domestic radon exposure. Although single-particle traversal was only slightly cytotoxic to A L cells (survival fraction Ϸ 0.82), it was highly mutagenic, and the induced mutant fraction averaged 110 mutants per 10 5 survivors. In addition, both toxicity and mutant induction were dose-dependent. Multiplex PCR analysis of mutant DNA showed that the proportion of mutants with multilocus deletions increased with the number of particle traversals. These data provide direct evidence that a single ␣ particle traversing a nucleus will have a high probability of resulting in a mutation and highlight the need for radiation protection at low doses.

Comparison of base substitutions in response to nitrogen ion implantation and 60Co-gamma ray irradiation in Escherichia coli

“…In the present study, 10 keV nitrogen ions at the fluence rank of 10 14 to 10 15 were used to implant the E. coli cells. We presumed that the cytoplasmic effect might play the most important role in the induction of the mutation, as in the study of Hei et al (1997) in a mammalian system, using exact numbers of alpha particles for the irradiation of cytoplasm. Since mutational spectra convey only the end point of a complex cascade of events, which includes formation of multiple adducts, repair processing, and polymerase errors, it is difficult to assess the mutational specificity of mutagens directly from them.…”

“…Southern blotting analysis permits the partitioning of ionizing radiation-induced mutagenesis into detectable deletions and major genomic rearrangements and into point mutations (Grosowsky et al, 1988;(Grosowski et al, 1986). Methods based on specific locus PCR were established to determine the mutational spectrum of fairly large fragments (Hei et al, 1997;Wu et al, 1999). The molecular nature of the point mutations, however, has been left unresolved (Grosowsky et al, 1988).…”

Genomic instability and bystander effects induced by high-LET radiation

“…Although these cells are immortalized Figure 8 Mutation frequency as a function of the number of aparticles per nucleus (data for the average number of particle traversals were calculated from cell population in which defined proportion of cells were exposed to a single a-particle). Due to the bystander effect, which is evident when only a proportion of the population is exposed, the risk at low doses is higher than predicted by a linear extrapolation from high doses (based on the data of Hei et al, 1997;Zhou et al, 2001) either spontaneously (MCF-10F human breast) or though viral transduction (BEP2D human bronchial), they are phenotypically normal, and do not express any transformed characteristics such as anchorage-independent growth and tumorigenicity in nude mice. After exposure to either 140 keV/mm a-particles or 1 GeV nucleon 56 Fe ions, transformed cells arise through a series of sequential stages including altered growth pattern, resistance to serum-induced terminal differentiation, agar-positive growth, tumorigenicity and metastasis ( Figure 9, Hei et al, 1994;Piao et al, 1999;Calaf and Hei, 2000).…”

“…Thus, a simple linear extrapolation of radiation risk from intermediate (where they can be measured) to lower doses (where they must be inferred) would be of questionable validity, at least at high LET. This is illustrated in Figure 8, which combines the data of Zhou et al (2001) where only a proportion of cells are irradiated with a single particle (allowing the bystander effect to be manifest), together with a previous compilation of data by Hei et al (1997) where all cells were exposed to various numbers of particles from 1 to 4. Under these experimental conditions, it is evident that a linear extrapolation of risks from high to low doses (which average less than one particle per cell) would underestimate the risks at low doses.…”

Radiation-induced genomic instability and bystander effects: implications for radiation protection

“…By use of a precision microbeam, for example, it has been reported that mutations can anse as a consequence of cytoplasmic irradiation , though at a significantly lower frequency than is found in cells receiving direct nuclear exposure (Hei et al, 1997). Two phenomenon of considerable recent interest will be described in this review as they may have particular relevance to the assessment of the risk of lowlevel radiation exposure.…”