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.

Arsenic is an important environmental carcinogen that affects millions of people worldwide through contaminated water supplies. For decades, arsenic was considered a nongenotoxic carcinogen. Using the highly sensitive A L mutation assay, we previously showed that arsenic is, indeed, a potent gene and chromosomal mutagen and that its effects are mediated through the induction of reactive oxygen species. However, the origin of these radicals and the pathways involved are not known. Here we show that mitochondrial damage plays a crucial role in arsenic mutagenicity. Treatment of enucleated cells with arsenic followed by rescue fusion with karyoplasts from controls resulted in significant mutant induction. In contrast, treatment of mitochondrial DNA-depleted (R 0 ) cells produced few or no mutations. Mitochondrial damage can lead to the release of superoxide anions, which then react with nitric oxide to produce the highly reactive peroxynitrites. The mutagenic damage was dampened by the nitric oxide synthase inhibitor, N G -methyl-L-arginine. These data illustrate that mitochondria are a primary target in arsenic-induced genotoxic response and that a better understanding of the mutagenic/carcinogenic mechanism of arsenic should provide a basis for better interventional approach in both treatment and prevention of arsenic-induced cancer. (Cancer Res 2005; 65(8): 3236-42)

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