User:Ajazizian/sandbox

Definition
Replication stress is defined as the events that take place when the genome is exposed to various stresses. It typically occurs during DNA replication, and can result in a stalled replication fork. ATM and ATR are proteins that mediate replication stress. Specifically, they are kinases that are recruited and activated by DNA damage. The stalled replication fork can collapse if these regulatory proteins fail to stabilize it. When this occurs, reassembly of the fork is initiated in order to repair the damaged DNA end.

Replication Fork
The replication fork is comprised of a group of proteins that influence the activity of DNA replication. In order for the replication fork to stall, the cell must posses a certain number of stalled forks and arrest length. The replication fork is specifically paused due to the stalling of helicase and polymerase activity, which are linked together. In this situation, the fork protection complex (FPC) is recruited to help maintain this linkage.

In addition to stalling and maintaining the fork structure, protein phosphorylation can also create a signal cascade for replication restart. The protein Mrc1, which is part of the FPC, transmits the checkpoint signal by interacting with kinases throughout the cascade. When there is a loss of these kinases (from replication stress), an excess of ssDNA is produced, which is necessary for the restarting of replication.

Causation
Replication stress is induced from various endogenous and exogenous stresses, which are regularly introduced to the genome. These stresses include, but are not limited to, DNA damage, excessive compacting of chromatin (preventing replisome access), over-expression of oncogenes, or difficult-to-replicate genome structures. Replication stress can lead to genome instability, cancer, and ageing.

Specific Events
The events that lead to genome instability occur in the cell cycle prior to mitosis, specifically in the S phase. Disturbance to this phase can generate negative effects, such as inaccurate chromosomal segregation, for the upcoming mitotic phase. The two processes that are responsible for damage to the S phase are oncogenic activation and tumor suppressor inactivation. They have both been shown to speed up the transition from the G1 phase to the S phase, leading to inadequate amounts of DNA replicative components. These losses can contribute to the DNA damage response (DDR). Replication stress can be an indicative characteristic for carcinogenesis, which typically lacks DNA repair systems.

Applications in Cancer
Although replication stress has been linked to carcinogenesis, studies have been conducted which show that replication stress could be used as a treatment for cancer. One specific study examined how replication stress affected APOBEC3B activity. APOBEC3 (apolipoprotein B mRNA editing enzyme, catalytic polypeptide-like 3) has been seen to mutate the cancer genome in various cancer types. Results from this study show that weakening oncogenic signaling or intensifying DNA replication stress can alter carcinogenic potential, and can be manipulated therapeutically.

Normal replicative stress occurs at low to mild levels and induces genomic instability, which can lead to tumorigenesis and cancer progression. Researchers decided to conduct a study to determine the effects of inducing high levels of replicative stress on cancer cells. The results showed that with further loss of checkpoints, replicative stress is increased to a higher level. With this change, the DNA replication of cancer cells may be incomplete or incorrect when entering into the mitotic phase. This can eventually result in cell death through mitotic catastrophe.