A key way radiation therapy and chemotherapy work is by making highly lethal double-strand breaks in the DNA of cancer cells. A Georgia Cancer Center scientist wants to help those therapies work better by better understanding the complex DNA damage repair process, because sometimes these therapies can inadvertently contribute to cancer.
We are trying to identify a repair protein that can help healthy cells avoid dying or becoming cancerous."
Dr. Chunhong Yan, Molecular Biologist, Cancer center, Department of Biochemistry and Molecular Biology, Medical College of Georgia, Augusta University
ATF3, a sensor of cell stress which Yan's team has shown is an early and important player in DNA damage repair, may be that protein. A new $1.7 million grant (R01CA240966) from the National Cancer Institute is helping them find out.
Our hereditary material is contained in the nucleus of our cells, and is constantly bombarded by factors like sunlight and oxidative stress, even chemicals in our food. Our healthy cells are mostly adept at DNA damage repair, but cancer cells have a defect in their DNA damage repair mechanism that should leave them more vulnerable to chemotherapy and radiation. In fact, our healthy cells' natural, rapid ability to repair DNA damage is considered a natural cancer barrier because incomplete repairs can accumulate and become cancer, Yan says. That's one of the reasons cancer risk generally increases with age.
One of the problems with radiation and chemotherapy is the collateral damage it does to healthy cells. Despite efforts at more targeted delivery, the treatments also can produce serious, double-strand breaks in the DNA of healthy cells, putting them at risk of dying or becoming part of the tumor, one of the unfortunate side effects of these therapies and key reasons for Yan's interest. "If we can find something that specifically only kills cancer cells, but keeps normal cells healthy, that could be very beneficial to patients," he says.
So Yan and his lab are dissecting this important "genome maintenance" of DNA repair. If their findings continue to hold, their ultimate goal is new cancer therapies that make increased use of the ATF3's skill at stopping spontaneous tumor production.
They have already shown that the protein ATF3 is essential to efficient, complete access to DNA and its repair. That without it mice get more tumors, which suggests that ATF3 is important in suppressing tumor formation, he says. That includes establishing a direct link between ATF3 and the established tumor suppressor p53. They found ATF3 can bind to p53 and increase expression of this protein which also has a role in DNA damage response, including going to the scene and putting the cell in a state of rest to ease repair. The other side of the coin is that when a cell can't repair, p53 enables it to commit suicide. Without ATF3, there is a better chance the cell will just become cancer, Yan says.
But good repair first requires access. To get our long DNA to fit inside our compact cells, proteins called histones provide a sort of spool, called a nucleosome, around which the iconic double-stranded DNA is wound. Chromatin is the biological packaging. In the snug confines of chromatin, the familiar classic double helix that resembles a twisted ladder is more of an X-shape resembling a clothespin.
When a cell senses DNA damage, the histones need to modify the chromatin so repair proteins can get inside and do their work and the DNA needs to relax its grip on the nucleosome. One they gain access to the damage, repair proteins enable what is called non-homologous end-joining by essentially trimming the broken ends of the damaged DNA and patching them back together.
Yan is learning more about those modifications to the histones and the ones needed to recruit those repair proteins, which already are in the nucleus, right to the damage site.
Their goals including learning more about how ATF3, also already present in the cell nucleus, gets to the actual DNA damage site. They have evidence that yet another histone, called H2AX, may be part of that.
H2AX is in the chromatin, and when there is a double-strand break in the DNA, it gets modified within seconds into? H2AX, which Yan's lab has evidence recruits ATF3 to the damage site. Yan notes that while he cannot yet say that these are the first changes, he can say they are very early ones.
"What we have found is ATF3 can come to the damage site pretty quickly, and promote the chromatin change," Yan says. They've found ATF3 binding to and stabilizing the enzymes Tip60 and p300/CBP can help provide direct access to the DNA damage site so repair proteins can move in.
This so-called histone acetylation is considered a principal way DNA damage repair happens, so identifying the genes that regulate this important intersection so that cells can be properly repaired and avoid becoming cancerous is important, Yan says.
Yan's lab has shown that ATF3 can activate the natural tumor suppressor p53 while getting Tip60 to activate the major DNA damage response kinase ATM, which provides a sort of framework for the team of repair proteins that will be recruited. p53 also is an early arriver, like a master engineer, helping make decisions on whether or not the DNA is a loss or can be repaired.
Now they want to learn more about how ATF3 promotes p300/CBP that ultimately brings on multiple repair proteins. That includes learning more about how ATF3 alters chromatin's structure to help recruit these repair proteins. A mouse missing ATF3 is enabling them to better see the roles of ATF3 including exploring further whether cancer increases when it is MIA.
Yan has documented lower ATF3 levels in people with cancer; and how taking down ATF3 levels decreases DNA repair and increases susceptibility to radiation. His research team also has found ATF3 is important in stopping damaged cells from becoming cancer. Yan and others additionally have shown that ATF3 can suppress spread of lung, colon and bladder cancers.
DNA damage is one of the most common sources of cell stress.
Like many body functions, the DNA repair mechanism tends to get less efficient with age. DNA damage, unrepaired or incompletely repaired, can lead to mutations, which increase the risk of the cell becoming cancer; or, with the help of p53, cell death from apoptosis, the innate ability of a cell to kill itself, when an injury likely cannot be repaired.