Checkpoint Charlie needs no introduction to Cold War history aficionados and James Bond movie buffs. Allied forces nicknamed the two-way political-military crossing between East and West Berlin before the Berlin Wall came down. Checkpoint Charlie itself soon came down, and was removed intact to a museum.
Now the "C" in Charlie might well label the biology term "cell cycle checkpoint," which marks the stopping and starting places in a cell's division. "This is a very fundamental property of the cell," observed biochemist and DNA researcher Aziz Sancar. "When a cell's DNA is damaged, it stops cell cycle progression. Of course," he added, "when this process is not working, cells proliferate without benefit of checkpoint - and that is the problem in all cancer cells. So the checkpoint is important in preventing cells from dividing while there is DNA damage, and therefore becoming cancerous.
"So it is important from a basic science point of view," Sancar continued, "but also from the viewpoint of cancer research. This checkpoint is an important target for cancer chemotherapy, because in most if not all cancers there is abnormality in the cell-division signaling pathway. This whole reaction," he went on, "is called DNA damage checkpoint. Yeast biologist Leland Hartwell, at the University of Washington in Seattle, who discovered this phenomenon in 1988 got the Nobel Prize for it last year. This is a very important phenomenon in both normal and cancerous cells, and there has been a great deal of research about it," Sancar observed.
A distinguished professor of biochemistry and biophysics at the University of North Carolina at Chapel Hill, Sancar is senior author of an article dated May 14, 2002, in the Proceedings of the National Academy of Sciences (PNAS). It bears the title: "Preferential binding of ATR protein to UV-damaged DNA."
New DNA Repair Gig For Known ATR Protein
"The main finding in our paper," Sancar told BioWorld Today, "is identification of a protein - ATR - that senses DNA damage, and starts a cascade of cell cycle progression. Its novelty is the demonstration that this protein recognizes DNA damage. It has been known for many years that when DNA is damaged, the cells stop replicating. These DNA lesions must be repaired before the cell divides and propagates the mutation. Little is known, however, about how this molecular fix-it team recognizes the problem. Obviously, it has to assess the damage, and alert which proteins recognize it. And here," Sancar said, "we identify a protein that does recognize the damage, and initiates the DNA and checkpoint repair signaling.
"ATR appears to act like a switch that starts the repair process, and also stops cells from proliferating while they are being repaired. This new work is not going to cure cancer by itself," he added, "but it is a significant step forward."
Sancar explained: "DNA has four normal bases: two purines - adenine and guanine - and two pyrimidines - cytosine and thymine. When these bases are damaged, and there is something abnormal in their structure, the question arises: How does the DNA repair process detect that abnormality? In the PNAS paper we showed that it is detected by the ATR protein we identified, so when there is an abnormality in the form of pryimidine dimers, the protein binds to it and becomes activated to phosphorylation. It stops the cells from proliferating - from dividing - and becoming cancerous.
"The DNA damage is called pyrimidine dimer," Sancar went on. "Two adjacent pyrimidines are linked to one another by ultraviolet damage, and that blocks cell replication. It interferes with DNA function, and can promote malignancy. We use UV lesions just as a convenient, experimental source of DNA damage, but this will work equally well on cisplatin damage. Cisplatin is the commonest anticancer drug. And cancer chemotherapy is also a major source of DNA impairment.
"In the first approach, we damage DNA in cancer cells. They may have normal checkpoints, but if you interfere with them, they will die. The second way is simply to interfere with the checkpoint mechanism without any damage, and the cell will die.
"We know that when there is damage, it initiates a signaling cascade to stop cells from replicating. Lots of the molecular players along this pathway has been known for a number of years, but what signals the damage to initiate the entire process hasn't been known. So our contribution is to report that this ATR protein is involved in initiating that cascade signal upon sensing damage by UV, cisplatin or other cancer-causing DNA mutations."
To find out if ATR directly sensed damaged DNA, Sancar and his co-authors purified and tagged the ATR protein from human fibroblast cells They incubated the tagged protein with bits of normal DNA or fragments mutated by UV radiation. ATR bound more often to damaged than to undamaged DNA, and its activity increased.
"Now since submitting our PNAS paper," he continued, "we are investigating the interaction of this protein with other DNA damages, and its interaction with other proteins - to define the signaling pathway better. And recently we have identified novel proteins that are interacting with ATR."
Anticancer Chemotherapy Vs. Biotherapy
"But I think from a pharmaceutical standpoint," he pointed out, "the importance is that this signaling pathway is considered by many pharma companies a major anticancer target for either chemotherapy or biotherapy." In the latter category, Sancar included "prodrugs identified as not interfering with the signaling pathway that's initiated by ATR." Sancar said those don't strictly qualify as chemotherapy and neither would the miracle drug Genvec. "It's a drug that specifically binds to cancer protein, and kills cancer cells. Those kinds of approaches, as well as treating cancers with interferon or antibodies, are called biotherapy."
Sancar revealed, "Actually I have just become a collaborating consultant to a biotech company myself, which encourages me to go into this field - the way that cancer treatment is going. They really haven't started doing anything on this. The company has its own specialties, and is looking for new research areas or product targets. DNA damage recognition," Sancar concluded, "is one target I suggested."