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Radiation inflicts extensive damage on cells’ DNA. For many cells, the damage is too complex to repair, and the cells die–making radiotherapy a frontline cancer treatment. However, some tumor cells are resistant to radiation: They repair the inflicted damage and survive. New research, published today in Science, finds that tumor cells buy themselves time for these repairs by self-inflicting smaller, easier-to-repair damage to their DNA, widening the window in which they can repair the more extensive radiation-inflicted damage. 

The study “presents an intriguing and essentially unanticipated finding that tumor cells have the capacity to promote a ‘secondary wave of strand breaks’ in response to stress such as ionizing radiation,” David Gewirtz, a pharmacologist at Virginia Commonwealth University who was not involved in the study, tells The Scientist in an email. Members of his own laboratory had also observed such breaks, he writes, but were unable to determine their origin. “The authors identify the ‘culprit’ as being caspase activated DNAse (CAD). . . . Whether CAD ‘self-inflicted DNA breaks’ might contribute to various resistance phenotypes clearly represents a fundamental question that could ultimately yield novel therapeutic strategies.”

Normal, noncancerous cells protect themselves from radiation damage through a mechanism that helps them avoid cell division when chromosomes are entangled or broken. “If the mother cell is damaged, the two daughter cells will inherit the damage, and that’s not sustainable in the long run—this is something also cancer cells need to avoid,” says Claus Storgaard Sørensen, a cancer biologist at the University of Copenhagen and coauthor of the current study. In cancer cells, however, this mechanism is often dysfunctional. The researchers wanted to understand how cancer cells avoid cell division after radiation. 

From a screen using human bone cancer cells, CAD emerged as a potential player in this process. CAD had already been known to break up DNA during apoptotic cell death. After irradiating human cancer cell lines, the researchers observed “mysterious nicks” in the DNA of treated cells, damage that had been written about in some “under the radar studies” in the field, says Sørensen. These were not full breaks or other serious damage, but single-strand breaks arising 12 to 18 hours after radiation. 

Sørensen and colleagues found that these nicks were dependent on CAD activity. When the team knocked out the enzymatic activity of CAD, the cells became more sensitive to radiation. Cells lacking CAD also entered mitosis prematurely. Also, in mice into which human tumors had been implanted, CAD wildtype cells continue to proliferate after radiation, while CAD-deficient tumors “undergo what is largely a prolonged growth arrested state in response to radiation,” writes Gewirtz. “This is consistent with the premise that CAD-induced breaks provide a survival advantage.” 

Sørensen suggests that CAD gives the cells time to repair radiation damage before dividing. “Maybe CAD is making these easy-to-repair lesions that might operate as a stop signal for cancer cells—that cancer cells enforce the stop right before mitosis, so that the mother cell is protected from dividing with difficult-to-repair radiation damage.” These simple lesions are repaired continuously and rapidly while the mother cell is finishing repair of complicated lesions.

CAD’s activity is usually controlled by caspases, enzymes that facilitate apoptosis. However, the authors find that chemical inhibition of caspase activity does not stop the DNA nicks from forming. Instead, CAD is still physically associated with its inhibitor ICAD. “CAD, as far as we can say, moves around with the modulator that keeps it in check. That’s how probably cancer cells can control it and exploit it,” says Sørensen. “A full-blown apoptotic pathway . . . that’s not really necessary for this response.” 

“What’s novel is that they found that instead of the caspase-mediated cleavage of ICAD, which leads to release and activation of CAD, the binary complex itself is recruited to the sites of DNA damage and then causes secondary damage,” says Chuan-Yuan Li, a cancer biologist at Duke University Medical Center who was not involved in the study“Mechanistically, it’s unexpected and novel for this complex itself to have this function.”

Li says he would have liked to see more stringent tests that single-strand breaks induced by CAD are responsible for radiation protection. In the paper, the researchers report finding that cancer cells lacking CAD show increased radiosensitivity and have reduced colony outgrowth after radiation. “Colony outgrowth can be explained by alternative effects,” as it takes a week to 10 days for outgrowth to occur, Li says. “We don’t know whether it’s longer-term genetic instability or the short-term strand breaks.” 

The authors have developed CAD inhibitors and are testing what inhibiting CAD might mean for cancer therapy. While cancer cells become more sensitive to radiation damage after CAD inhibition, this effect is not seen in normal cells. “If we inhibit CAD in the cancer cells that have hijacked the enzyme and use it to a different purpose, we can make cancer cells selectively more sensitive to radiation,” says Sørensen. In addition, Sørensen thinks that CAD inhibition can be used to make tumors more visible to the immune system. “Pushing irradiated cells into division helps evoke and display new antigens and strengthen immune signaling,” he says, giving the immune cells new targets for attack.