Alexandra Nusawardhana was analyzing tumor cells when she noticed something unexpected: a DNA repair protein gone haywire. The EXO1 gene, which normally makes a protein that acts like molecular scissors to trim and fix damaged DNA, was being overproduced in cancer cells—and when there's too much of it, the scissors start cutting things they shouldn't. This discovery, published in Nature Communications by researchers at Penn State College of Medicine, reveals a hidden vulnerability in 20 to 30 percent of breast, ovarian, melanoma, testicular, cervical, and hepatobiliary cancers that could transform how these tumors are treated.
The finding matters because it upends a basic assumption about tumor suppressor genes. These are typically the "good guys" of cancer biology—they protect and repair DNA, and when they fail, cancer risk climbs. But EXO1 shows that too much of a good thing becomes dangerous. When cells overexpress the gene, the resulting protein destabilizes newly synthesized DNA in two primary ways: by expanding single-stranded DNA gaps and degrading reversed replication forks. Both processes chip away at DNA sequences and create toxic lesions, including double-strand breaks that accumulate in the tumor. This sounds destructive—and it is—but it also reveals an unexpected opportunity.
George-Lucian Moldovan, the senior author and a professor of molecular and precision medicine, and his team made a striking connection. Tumor cells with high levels of EXO1 protein behave like cells with a BRCA mutation, the genetic change famous for its link to hereditary breast and ovarian cancers. They respond to chemotherapy the same way BRCA-mutant cells do, even when there is no BRCA mutation present. This hadn't been established before. "The same drugs that are reserved for treating BRCA-mutant tumors and that have fewer side effects could potentially be used to treat EXO1 overexpressing tumors, which don't have BRCA mutations," Moldovan said. "It would expand the applicability of those drugs."
To reach this conclusion, the research team analyzed The Cancer Genome Atlas, a cancer genomic program of the National Cancer Institute, for EXO1 alterations across tumor samples. They then conducted laboratory studies with human cancer cells, overexpressing the EXO1 gene to watch how an excess affected DNA. They also tested a biochemically disabled version of the protein to confirm that any DNA damage was specifically caused by protein activity, not merely its presence.
The implications ripple outward immediately. High EXO1 levels were particularly linked to basal-like breast cancers, an aggressive subtype. But beyond prognosis, the research opens a path toward precision medicine. "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, leading to more personalized therapies," Moldovan explained. Rather than giving all patients with these cancers the same treatment, doctors could eventually use EXO1 status to match tumors with drugs most likely to work—and with fewer side effects.
For patients carrying these cancers, the path forward means better options. For researchers, it's a reminder that sometimes understanding how a good system breaks down leads to better ways to break down cancer itself.