Targeted cytoplasmic irradiation with alpha particles induces mutations in mammalian cells

TLDR

Ionizing radiation has long been thought to cause mutations mainly through direct nuclear damage, but the effect of alpha particles traversing the cytoplasm remained unclear. Using a microbeam at Columbia University’s Radiological Research Accelerator Facility, the authors precisely irradiated the cytoplasm of single cells while sparing nuclei. Cytoplasmic alpha‑particle irradiation induced mutations at the CD59 locus with minimal cell killing, producing mutation spectra distinct from nuclear exposure and dependent on reactive oxygen species, indicating that the cytoplasm is a significant, potentially more hazardous target for radiation‑induced genotoxicity.

Abstract

Ever since x-rays were shown to induce mutation in Drosophila more than 70 years ago, prevailing dogma considered the genotoxic effects of ionizing radiation, such as mutations and carcinogenesis, as being due mostly to direct damage to the nucleus. Although there was indication that alpha particle traversal through cellular cytoplasm was innocuous, the full impact remained unknown. The availability of the microbeam at the Radiological Research Accelerator Facility of Columbia University made it possible to target and irradiate the cytoplasm of individual cells in a highly localized spatial region. By using dual fluorochrome dyes (Hoechst and Nile Red) to locate nucleus and cellular cytoplasm, respectively, thereby avoiding inadvertent traversal of nuclei, we show here that cytoplasmic irradiation is mutagenic at the CD59 (S1) locus of human–hamster hybrid (A L ) cells, while inflicting minimal cytotoxicity. The principal class of mutations induced are similar to those of spontaneous origin and are entirely different from those of nuclear irradiation. Furthermore, experiments with radical scavenger and inhibitor of intracellular glutathione indicated that the mutagenicity of cytoplasmic irradiation depends on generation of reactive oxygen species. These findings suggest that cytoplasm is an important target for genotoxic effects of ionizing radiation, particularly radon, the second leading cause of lung cancer in the United States. In addition, cytoplasmic traversal by alpha particles may be more dangerous than nuclear traversal, because the mutagenicity is accomplished by little or no killing of the target cells.