USC researchers define role of protein, discover cause of chromosome damage
Pinpointing oxygen as the cause of routine chromosome damage and defining the role of a key protein in the repair of that damage are the subjects of two recently published papers from the laboratory of USC/Norris Comprehensive Cancer Center pathologist Michael Lieber, the Rita and Edward Polusky Chair in Basic Cancer Research at the Keck School of Medicine.
The first paper was published in the March 5, 2002, issue of Current Biology; the other is slated for the March 22 issue of Cell, but was posted on the journal's Web site on March 1 as part of their "immediate early publication process."
The Cell paper, which will reportedly be on the cover of the March 22 issue, reveals that a protein previously linked to a devastating form of immunodeficiency plays a key role in a pathway by which nuclear DNA is repaired--the same system, in fact, which the immune system uses to create antibodies.
About 15 percent of the cases of human severe combined immunodeficiency syndrome (known colloquially as the "bubble boy" disease) are caused by the mutation of a specific gene and its protein product. In April of 2001, a team of French researchers tracked down that gene, and named it and its product Artemis (after the Greek goddess for the protection of children); they had no idea at all, however, what kind of protein it was, nor what its function might be.
Enter Lieber and graduate student Yunmei Ma. Lieber, Ma and their colleagues from the University of Ulm in Germany, conclusively demonstrated that Artemis is a key protein in the repair of double-stranded DNA breaks, a process called NHEJ (non-homologous DNA end joining). In the NHEJ pathway, explained Lieber, the ends of the broken DNA strands are trimmed and rejoined to one another. "What Artemis does is trim away the damaged parts of the DNA so that the strands can be joined," said Lieber.
Artemis and the NHEJ pathway are so essential, Lieber continued, that mice lacking NHEJ usually die at birth-and those that don't generally lack an immune system entirely and experience accelerated aging. And, as the previous studies have shown, humans with a defective Artemis protein also wind up without any immune defense to speak of. That, says Lieber, is because the immune system creates its defenses by cutting and then rejoining bits of nuclear DNA (the rejoining relies on NHEJ). Without Artemis, the cells can't create the antibodies necessary to go after the myriad pathogenic invaders we regularly encounter.
Of course, being unable to cut and splice DNA can sometimes actually be of benefit. "What we're going to do next," said Ma, the paper's first author, "is try to screen for drugs that inhibit Artemis, because this might be useful from a cancer therapy standpoint. If we could just give a pulse of drug inhibitor for a while, we might be able to focus the effects of radiation therapy, for instance, by not allowing the cancer cells to repair themselves after being hit with the radiation."
Still, noted Lieber, for normal cells, Artemis and the NHEJ pathway are absolutely critical for survival. And that is because of how exquisitely vulnerable our cells are to DNA damage in the first place. Indeed, he said, all you have to do is take some cells out of the organism in which they live and look at them under a microscope, and you'll find that 5 to 10 percent of them will have at least one broken chromosome.
Normally, of course, the NHEJ pathway works to fix those breaks. But the NHEJ pathway doesn't always function at full capacity. Indeed, a paper published by Lieber, M.D./Ph.D. student Zarir E. Karanjawala, and Norris Cancer Center researcher Chih-Lin Hsieh in 1999 found that in cells where the NHEJ pathway is disabled or missing, the number of cells with at least one chromosome break goes shooting up to 60 percent.
What causes all this breakage? In the March 5 issue of Current Biology, Karanjawala, Lieber, and colleagues say it's the most ubiquitous of sources: oxygen.
Originally, said Karanjawala, they had wondered if the damage might be coming from some environmental source, perhaps from background radiation. But when they began to look more closely, said Karanjawala, they found it was in the very air we breathe. "It's coming from the oxygen," Karanjawala explained. "We found that if you vary the oxygen levels in which cells are grown, the breakage levels of the chromosomes vary as well----the higher the oxygen level, the more breakage you'll see."
The oxygen causes its damage, Lieber said, through oxidative free radicals--highly reactive atoms with an unpaired electron that can rip through our cells "like a bullet."
"Our bodies are being riddled with these bullets every day," explained Lieber, "whether we like it or not. And the sorts of double-strand DNA breaks we were looking at are hard to repair. Even if you put the two ends together the best you can, you usually lose a couple of nucleotides along the way. And so every time we get an oxidative free radical hit, which happens several times per day per cell, we lose a little info. Every time it hits your DNA, you wind up with a little less genetic information than you had when you started the day."
The solution? Frankly, said Lieber, there may be none. "Oxygen--can't live with it, can't live without it," he commented. "We need it to survive, but ultimately, it's also probably what kills us."
Yunmei Ma, Ulrich Pannicke, Klaus Schwarz and Michael R. Lieber, "Hairpin Opening and Overhand Processing by an Artemis/DNA-Dependent Protein Kinase Complex in Nonhomologous End Joining and V(D)J Recombination." Cell Immediate Early Publication, March 1, 2002, http://www.cell.com
Zarir E. Karanjawala, Niamh Murphy, David R. Hinton, Chih-Lin Hsieh and Michael R. Lieber, "Oxygen Metabolism Causes Chromosome Breaks and Is Associated with the Neuronal Apoptosis Observed in DNA Double-Strand Break Repair Mutants." Current Biology, Vol. 12, pp. 397-402, March 5, 2002.