When Alexandra Nusawardhana first saw the DNA in her lab samples crumbling like over-trimmed hedges, she knew something fundamental was shifting. At Penn State College of Medicine, her team had turned up the activity of a single gene—EXO1—in human cancer cells, expecting to see stronger DNA repair. Instead, they watched as the genetic blueprint unraveled. "EXO1 doesn’t predict cancer risk, but it could potentially serve as a biomarker to help predict which patients are more likely to respond to certain chemotherapy treatments," said George-Lucian Moldovan, professor of molecular and precision medicine and senior author of the study. This unexpected twist could open a new front in the fight against cancer—one that doesn’t rely on inherited mutations but on a rogue repair mechanism hiding in plain sight.
For decades, BRCA mutations have defined a key vulnerability in cancers, particularly in breast and ovarian tumors. These mutations impair the cell’s ability to protect fragile DNA structures during replication, making cancer cells susceptible to drugs like olaparib, which exploit that weakness. But what if tumors without BRCA mutations could mimic that same vulnerability? The Penn State team’s findings, published in Nature Communications, suggest that overexpression of EXO1—present in 20% to 30% of breast and ovarian cancers—does exactly that. In lab tests, cancer cells with excessive EXO1 activity showed the same genomic instability and treatment sensitivity as BRCA-mutant cells, even when BRCA genes were fully intact.
The researchers analyzed tumor data from The Cancer Genome Atlas and confirmed EXO1 overexpression across multiple cancer types: melanoma, testicular, cervical, and hepatobiliary cancers, with particularly high levels in aggressive basal-like breast cancer. Using engineered human cancer cells, they demonstrated that too much EXO1 acts like molecular scissors gone rogue—expanding single-stranded DNA gaps and degrading reversed replication forks. These actions generate toxic double-strand breaks, the very lesions that make cancer cells vulnerable to targeted therapies. Crucially, a disabled version of EXO1 that lacked enzymatic activity caused no such damage, proving it’s the protein’s function, not just its presence, that drives genomic chaos.
The discovery hinges on a powerful irony: a gene meant to protect DNA ends up destroying it when overactive. And while EXO1 overexpression isn’t inherited like BRCA mutations, it creates a therapeutic window just as valuable. When treated with olaparib, EXO1-overexpressing tumors responded dramatically—just as BRCA-mutant cancers do. This means patients who’ve long been excluded from precision therapies could now benefit from them.
"Mechanistically, this overexpression does exactly what the loss of the BRCA pathway does," Moldovan said. As the team moves toward clinical validation, the hope is clear: a new biomarker could unlock life-extending treatments for tens of thousands of patients who never had a BRCA mutation but whose cancers behave as if they did.