Lack of Cell Cycle Checkpoints Maintains Integrity of Embryonic Stem Cell DNA

Peter J. Stambrook, Ph.D.
University of Cincinnati College of Medicine
U01ES11038 and P30ES06096

Background: Embryonic stem cells differ markedly from somatic cells in their responses to DNA damage and cell-cycle regulation. Stem cells need extremely sensitive mechanisms to maintain the integrity of their DNA which is done partially by suppressing spontaneous mutation. The frequency of mutations in somatic cells is about one cell in one thousand whereas the frequency in stem cells is one in one million. Stem cells lack a checkpoint in the G1 (Gap 1) phase of the cell cycle. This combined with their increased sensitivity to ionizing radiation and other DNA-damaging agents facilitates programmed cell death and the removal of cells with mutations, thereby keeping the population of stem cells free of damaged cells. This characteristic is very important because a mutation in a stem cell can compromise multiple cell lineages and affect the well being of subsequent generations.

Advance: This NIEHS-supported researcher has discovered the signaling pathways that are compromised and lead to a the natural absence of G1 arrest in embryonic stem cells after DNA damage. In somatic cells, a G1 checkpoint allows time for DNA repair. The lack of this checkpoint in embryonic stem cells is due to an altered localization of a checkpoint kinase known as Chk2. In somatic cells Chk2 is found in the cell's nucleus, but in embryonic stem cells, it is located in centrosomes and is therefore unavailable for protein phosphorylation. The researcher went on to demonstrate the G1 checkpoint can be restored in stem cells by genetic modifications that cause expression of Chk2 in the stem cell nucleus. The restoration of G1 arrest protects the embryonic stem cells from apoptosis and thus would allow them to carry any mutations to subsequent generations.

Implications: This research identifies the mechanism for the natural absence of a G1 checkpoint in embryonic stem cells. It demonstrates that this absence is beneficial to the survival of the embryonic stem cell population and the species by facilitating removal of stem cells harboring damaged DNA and thus maintaining a pristine embryonic stem cell population. If embryonic stem cells did have a G1 checkpoint, they would be protected from apoptosis and could possibly carry mutations and DNA damage to subsequent generations of the species.

Citation: Hong Y, Stambrook PJ. Restoration of an absent G1 arrest and protection from apoptosis in embryonic stem cells after ionizing radiation. Proc Natl Acad Sci U S A. 2004 Oct 5;101(40):14443-8. Epub 2004 Sep 27.