Ionizing radiation alters human embryonic stem cell properties and differentiation capacity by diminishing the expression of activin receptors.

Exposure of the embryo to ionizing radiation (IR) is detrimental as it can cause genotoxic stress leading to immediate and latent consequences such as functional defects, malformations, or cancer. Human embryonic stem (hES) cells can mimic the preimplantation embryo and help to assess the biological effects of IR during early development. Here we describe the alterations H9 hES cells exhibit after X-ray irradiation in respect to cell cycle progression, apoptosis, genomic stability, stem cell signaling, and their capacity to differentiate into definitive endoderm. Early post irradiation, human embryonic stem cells responded with an arrest in G2/M phase, elevated apoptosis and increased chromosomal aberrations. Significant downregulation of stem cell signaling markers of the TGF beta-, Wnt-, and Hedgehog-pathways were observed. Most prominent were alterations in the expression of activin receptors. However, hES cells responded differently depending on the culture conditions chosen for maintenance. Enzymatically passaged cells were less sensitive to IR than mechanically passaged ones showing fewer apoptotic cells and fewer changes in the stem cell signaling 24h after irradiation, but higher levels of chromosomal aberrations. Even though many of the observed changes were transient, surviving hES cells differentiated 4d post-irradiation showed a lower efficiency to form definitive endoderm than their mock-irradiated counterparts as demonstrated by lower expression levels of SOX17 and microRNA miR-375. In conclusion, hES cells are a suitable tool for the IR risk assessment during early human development. However, the careful choice of the culture methods and a vigorous monitoring of the stem cell quality are mandatory for the use of these cells. Exposure to IR influences the stem cell properties of hES cells even when immediate radiation effects are overcome. This warrants consideration in the risk assessment of radiation effects during the earliest stages of human development.