Induction of Genomic Instability in a Primary Human Fibroblast Cell Line Following Low-Dose Alpha-Particle Exposure and the Potential Role of Exosomes

Simple Summary Exposure to the naturally occurring radioactive gas Radon, and the resulting alpha-particle emitting progeny, dominates human exposure and is the second largest cause of lung cancer after smoking. The work presented shows that not only can very low doses of alpha-particles produce DNA damage in normal HF19 fibroblast cells (detected using comet and micronuclei assays) in the irradiated cell population, but also results in an enhanced yield of DNA damage in the progeny of these cells out to 10 and even 20 population doublings. This persistent elevation of DNA damage is consistent with genomic instability, a well-recognised feature of many tumours with its ability to generate genetic diversity in the dividing population. Environmental exposure is normally associated with a small fraction of cells being irradiated with a single alpha-particle, while the vast majority of cells remain unirradiated. Here, we demonstrate an enhanced yield of DNA damage observed at 10 and 20 population doublings even when less than 1% of the cell population is traversed, with a response similar to that observed when essentially all cells are irradiated. The finding also highlights the potential of exosomes produced by irradiated cells contributing to DNA damage observed in unirradiated bystander cells. Purpose: To study the induction of genomic instability (GI) in the progeny of cell populations irradiated with low doses of alpha-particles and the potential role of exosome-encapsulated bystander signalling. Methods: The induction of GI in HF19 normal fibroblast cells was assessed by determining the formation of micronuclei (MN) in binucleate cells along with using the alkaline comet assay to assess DNA damage. Results: Low dose alpha-particle exposure (0.0001-1 Gy) was observed to produce a significant induction of micronuclei and DNA damage shortly after irradiation (assays performed at 5 and 1 h post exposure, respectively). This damage was not only still evident and statistically significant in all irradiated groups after 10 population doublings, but similar trends were observed after 20 population doublings. Exosomes from irradiated cells were also observed to enhance the level of DNA damage in non-irradiated bystander cells at early times. Conclusion: very low doses of alpha-particles are capable of inducing GI in the progeny of irradiated cells even at doses where <1% of the cells are traversed, where the level of response was similar to that observed at doses where 100% of the cells were traversed. This may have important implications with respect to the evaluation of cancer risk associated with very low-dose alpha-particle exposure and deviation from a linear dose response.