Guardian Of The Genome, Role For ATR Revealed

ATR, and a similar protein ATM, have previously been shown to be involved in the response to DNA damage. However previous experiments to determine the role of ATR in preventing cells with damaged DNA from dividing have been contradictory and the precise roles of these proteins have remained obscure. The previous attempts to determine the role of ATR were hindered by the inviability of ATR deficient mice. In this report, the authors use a clever modification of the mouse knockout technology to create cells that can be forced to lose the ATR gene at will.

Cells lacking ATR and ATM did not properly halt the cell division cycle in response to ionizing radiation, a potent DNA damage-inducing agent. Both ATR and ATM contributed to the checkpoint control soon after DNA damage, but ATR was responsible for regulating the control later in the cell cycle. ATR was also important for regulating a checkpoint signaling pathway previously described in yeast that is initiated by stalled DNA replication. Surprisingly though, ATR was not essential for cell cycle arrest in response to incomplete DNA replication, implying that an additional mechanism must be a work. Brown & Baltimore go on to show that when ATR is absent, inhibited DNA replication causes the formation of a very serious form of damage known as double strand breaks. This suggests that while ATR is dispensable for the cell cycle delay in response to incomplete DNA replication, it is essential for ensuring the cells leaving this delay are free of DNA damage.

This study shows that ATR plays an important role in the maintenance of genome integrity. Without this important guardian, cells ignore DNA damage, replicate the unrepaired chromosomes and pass on damaged DNA. Ultimately, this DNA damage could lead to a loss of cell function, cellular death and diseases such as cancer. Consistent with the later, previous work from Brown and Baltimore (2000), showed that even partial loss of ATR function can lead to increased incidence of late-onset cancer in mice.

"It is a very exciting time for the DNA damage response field. Everywhere you look in these pathways, connections can be made to how cancer is normally prevented by maintaining the integrity of the genome. Subtle, yet-to-be-determined deficiencies in any of a number of these DNA damage response molecules may broadly influence cancer risk in humans," explains Dr. Brown.