DNA damage responses in human pluripotent stem cells

Pluripotent stem cells have the capability to undergo unlimited self-renewal and differentiation into all somatic cell types. They have acquired specific adjustments in the cell cycle structure that allow them to rapidly proliferate, including cell cycle independent expression of cell cycle regulators and lax G1 to S phase transition. However, due to the developmental role of embryonic stem cells (ES) it is essential to maintain genomic integrity and prevent acquisition of mutations that would be transmitted to multiple cell lineages. Here we show that several modifications in DNA damage response of ES cells accommodate dynamic cycling and preservation of genetic information. ATM-dependent checkpoint signaling cascade is activated after irradiation of ES cells, and induces G2/M, but not G1/S cell cycle arrest. The absence of a G1/S cell cycle arrest promotes apoptotic response of damaged cells before DNA changes can be fixed in the form of mutation during the S phase, while G2/M cell cycle arrest allows repair of damaged DNA following replication. Human ES cells express higher level of DNA repair proteins, and rely on homologous recombination to repair double strand breaks. Radiation does not lead to long-term loss of pluripotency, since irradiated ES cells show transient decrease in the level of pluripotency factor transcripts, while protein levels remains stable. One week after irradiation, ES cells retain capacity to differentiate into three germ layers and form teratomas in immunocompromised mice.Similarly to ES cells, induced pluripotent stem (iPS) cells are poised to proliferate and exhibit extreme sensitivity to DNA damage, lack of G1/S cell cycle arrest, and express high level of DNA repair genes, suggesting that DNA damage responses are controlled by developmental state of the cell.Public health significance of this study originates in great promise that human ES and iPS cells hold in cell replacement therapies. Since human ES, and particularly iPS, cells represent potential source of cells for clinical and pharmaceutical applications, the DNA damage response pathways that maintain genomic integrity need to be studied in greater detail