Impact of ionizing radiation on human embryonic stem cells

The radiosensitivity of the early developing human embryo is not well characterized. Radiation protection guidelines in case of an in utero exposure are mainly based on animal experiments and on epidemiological data. The mechanisms behind a radiation response of the embryo especially of high LET radiation is not completely understood. Therefore, there is a need to elucidate the effects of low and high LET ionizing radiation on early embryonic development. To contribute to a better understanding of this topic, H9 human embryonic stem cells were used as an in vitro tool to investigate the early blastocyst-like stage of the human embryonic development. As the latter give rise to an entire functional organism, maintaining their structural and genomic integrity is mandatory to avoid passing DNA damage on to the progeny. In the literature, it is reported that embryonic stem cells indeed exhibit a lower mutation frequency and that they show a higher apoptotic activity upon DNA damage, compared to differentiated somatic cells. A previous study on mouse embryonic stem cells performed at GSI indicates that even though eighth generation daughter cells still harbor chromosomal aberrations after exposure to high LET radiation, they are capable to maintain a pluripotent cell population. This study elucidates the capacity of human embryonic stem cells to efficiently eliminate damaged cells upon ionizing radiation exposure and to proceed normally into endoderm differentiation. Thus, human embryonic stem cells were exposed to 1 and 3 Gy of X-rays, as well as to 1 Gy and 3 Gy of high LET radiation i.e. C, Ca, Ni and Ti ions. Human embryonic stem cells were observed up to 14 days after exposure in pluripotency maintaining culture conditions, and up to 11 days in conditions promoting differentiation into definitive endoderm. Endpoints such as cell cycle block, apoptosis, chromosomal aberrations, and gene expression related to embryonic signaling pathways and pluripotency as well as differentiation were investigated. It was found that isodoses of high LET radiation were more effective than X-rays in inducing apoptosis and chromosomal aberrations in human embryonic stem cells. After a radiation induced G2 block, a fraction of cells undergoes apoptosis. However, few stable chromosomal aberrations are still found in the progeny of exposed human embryonic stem cells. Gene expression studies of embryonic signaling pathways showed a majority of downregulated genes related to WNT, FGF, Hedgehog, TGF-ß and Notch signaling pathways. These signaling alterations presumably cause the observed downregulation of core pluripotency markers like OCT4A and NANOG. Consequently, differentiation capability into definitive endoderm was impaired. Together, the results reported in this work might indicate several consequences for the early embryo in case of an in utero exposure, like implantation failure due to cell cycle arrest and apoptosis, or malformations due to alterations in cell signaling and differentiation processes