Targeted cytoplasmic irradiation with alpha particles induces mutations inmammalian cells

Ever since x-rays were Shown to induce mutation in Drosophila more than 70 years ago, prevailing dogma considered the genotoxic effects of ionizing ra diation, such as mutations and carcinogenesis, as being due mostly to direc t damage to the nucleus. Although there was indication that alpha particle traversal through cellular cytoplasm was innocuous, the full impact remaine d unknown, The availability of the microbeam at the Radiological Research A ccelerator Facility of Columbia University made it possible to target and i rradiate the cytoplasm of individual cells in a highly localized spatial re gion. By using dual fluorochrome dyes (Hoechst and Nile Red) to locate nucl eus and cellular cytoplasm, respectively, thereby avoiding inadvertent trav ersal of nuclei, we show here that cytoplasmic irradiation is mutagenic at the CD59 (S1) locus of human-hamster hybrid (AL) cells, while inflicting mi nimal cytotoxicity. The principal class of mutations induced are similar to those of spontaneous origin and are entirely different from those of nucle ar irradiation. Furthermore, experiments with radical scavenger and inhibit or of intracellular glutathione indicated that the mutagenicity of cytoplas mic irradiation depends on generation of reactive oxygen species. These fin dings suggest that cytoplasm is an important target for genotoxic effects o f ionizing radiation, particularly radon, the second leading cause of lung cancer in the United States. In addition, cytoplasmic traversal by alpha pa rticles may be more dangerous than nuclear traversal, because the mutagenic ity is accomplished by little or no killing of the target cells.