Pol Theta Enzyme Drives DNA Repair at Broken Replication Forks, Revealing New Cancer Vulnerability

Scientists at Scripps Research (搜索)搜索该词条 have discovered that the enzyme Pol theta (搜索)搜索该靶点 (Polθ) drives a DNA repair mechanism directly at broken replication forks—one of the most frequent forms of DNA damage in cancer cells. The findings, published in Molecular Cell on March 16, 2026, help explain how tumors survive relentless replication stress and clarify why Pol theta inhibitors may be an effective strategy to selectively target cancer.

"We've uncovered a whole new dimension of how cancer cells cope with DNA damage at replication forks," said Xiaohua Wu, professor at Scripps Research (搜索)搜索该词条 and senior author of the study.

A New Understanding of Fork Repair

Every time a cell divides, it must make an exact copy of its entire genome. The point where the DNA double helix is unzipped and copied is called a replication fork. When the replication machinery encounters damage, forks can stall or collapse, leaving behind dangerous one-ended DNA breaks that are particularly difficult to repair and, if left unresolved, can kill the cell. This is especially true in cancer cells, where replication stress is constant.

Scientists previously believed that break-induced replication (BIR) was the primary first responder to this type of damage. BIR uses an intact DNA template to restart replication, making it relatively accurate but slow. In contrast, microhomology-mediated end joining (MMEJ) is a faster, more error-prone process that repairs breaks by aligning short matching DNA sequences. The prevailing view held that BIR was the active, front-line mechanism, while MMEJ was predominantly used to repair replication-independent double-ended double-strand breaks. The Scripps Research (搜索)搜索该词条 discovery overturns that view.

"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," Wu noted.

How the Discovery Was Made

Because microhomology sequences are frequently found in cancer genomes, Wu and her lab investigated whether MMEJ also operates to repair broken replication forks. The team combined key emerging technologies to study this process in unprecedented detail: CRISPR nickase technology to mimic the damage that triggers replication fork collapse in real cells; specialized reporter systems, molecular tools that signal when a specific repair event occurs inside living cells; and genome sequencing to track the deletion patterns each pathway leaves behind.

"When we looked closely at what was happening at these broken forks, we kept seeing mutational signatures that didn't fit the BIR model," explained Shibo Li, first author of the study and former postdoctoral researcher in the Wu lab. "That told us something else was going on, and when we started pulling on that thread, we found that MMEJ was there, acting early and directly at the fork."

The finding pointed to Pol theta (搜索)搜索该靶点 as the engine driving MMEJ activity right at the moment of fork collapse, before BIR—which had been thought of as the primary mechanism to repair broken forks.

Fork-MMEJ Follows Distinct Rules

The team also discovered that fork-MMEJ behaves differently from its standard canonical form, which was observed at replication-independent breaks. While both rely on Pol theta (搜索)搜索该靶点, fork-MMEJ is initiated by the protein RPA (搜索)搜索该靶点 and produces uneven deletion patterns on either side of the break—a distinctive fingerprint that sets it apart from its canonical form. These error-prone repair signatures are frequently observed in cancer genomes, suggesting the MMEJ mechanism may enable tumor cells to survive otherwise lethal DNA damage.

"We expected fork-MMEJ to follow the same rules as the version we'd studied before," said Wu. "Finding that it didn't follow the same rules meant that we were looking at something entirely new."

Clinical Implications and Combination Strategy

Several Pol theta (搜索)搜索该靶点 inhibitor drugs are already in clinical development, with early promise in cancers harboring BRCA1 (搜索)搜索该靶点 or BRCA2 (搜索)搜索该靶点 mutations. These mutations, linked to hereditary breast and ovarian cancer (搜索)搜索该疾病, disable a major DNA repair pathway that leaves tumor cells unusually dependent on MMEJ for survival. By showing that MMEJ is an active front-line responder at broken forks rather than a backup, the study suggests that blocking Pol theta could be more disruptive to cancer cell survival than previously understood.

The study also identifies a promising combination strategy. The team found that ATR查看药品详情, a cellular protein that senses DNA damage, acts as a pivotal switch at broken forks, suppressing fork-MMEJ while steering cells toward BIR. When the researchers blocked both ATR and Pol theta (搜索)搜索该靶点 together, cancer cells under replication stress died at far higher rates, while normal cells were largely unaffected. Because ATR inhibitors are already in clinical development, this synergistic effect points toward a potential combination therapy approach.

"This changes how we think about when and why to use Pol theta (搜索)搜索该靶点 inhibitors," said Wu. "If this pathway is acting at the very moment a fork breaks, not just at replication-independent breaks, then disrupting it could be far more consequential for cancer cells than we realized, as cancer cells are constantly under replication stress that causes replication fork breakage."

Future Directions

The lab's next steps include identifying additional proteins in the fork-MMEJ pathway—each representing a potential new drug target—and further characterizing how ATR查看药品详情 orchestrates the balance between fork-MMEJ and BIR.

"The more we understand about the factors involved in this pathway, the more potential targets we have," Wu added. "That's ultimately what drives better treatments for patients."