Scientists at Scripps Research announced on Apr. 7 that they have discovered the enzyme Pol theta (Polθ) plays a direct role in repairing DNA at broken replication forks, a common form of damage in cancer cells. The findings were published in Molecular Cell on March 16, and could explain how tumors survive ongoing replication stress, while also clarifying why targeting Pol theta may be an effective approach for cancer treatment.
This discovery is significant because it challenges previous assumptions about how cancer cells repair DNA damage during cell division. Replication forks are points where DNA is unzipped and copied, but when this process encounters problems, dangerous breaks can occur that threaten cell survival. In cancer cells, such stress is constant.
Researchers previously believed that break-induced replication (BIR), an accurate but slow pathway, was the main method for repairing these breaks. However, the new study found that microhomology-mediated end joining (MMEJ), which is faster but more error-prone and relies on Pol theta, actually acts directly at broken forks. "Understanding that MMEJ is operating there directly, and through a distinct set of rules, gives us a clearer picture of why tumors are so resilient, and how we might exploit that for treatment," said study leader Wu.
The research team used advanced techniques including CRISPR nickase technology and genome sequencing to track repair events inside living cells. "When we looked closely at what was happening at these broken forks, we kept seeing mutational signatures that didn't fit the BIR model," explains Shibo Li, first author of the study. "That told us something else was going on...we found that MMEJ was there, acting early and directly at the fork." Wu added: "We expected fork-MMEJ to follow the same rules as the version we'd studied before...Finding that it didn't follow the same rules meant that we were looking at something entirely new."
The study also explored potential therapies by combining ATR inhibitors with Pol theta inhibitors—both already in clinical development—and found this combination killed more cancer cells under replication stress while sparing normal cells. Wu said: "This changes how we think about when and why to use Pol theta inhibitors...disrupting it could be far more consequential for cancer cells than we realized." Looking ahead, her lab plans to identify additional proteins involved in this pathway as possible drug targets.
"The more we understand about the factors involved in this pathway, the more potential targets we have," Wu adds. "That's ultimately what drives better treatments for patients."
