Tumor suppressor genes are often considered the body’s built-in defense system against cancer. It produces proteins that help maintain and repair DNA, reducing the chance of harmful mutations accumulating. When these genes no longer function properly or are present at low levels, cancer risk can increase.
But new research suggests that having too much of one DNA repair protein can also be a problem.
Researchers at Penn State College of Medicine have discovered that excessive activity of the gene EXO1 can damage rather than protect DNA. Instead of repairing genetic material, when EXO1 is in excess, it destroys DNA and can destabilize the genome, a key hallmark of cancer.
The survey results are nature communicationsEXO1 has been shown to be overexpressed in 20% to 30% of breast and ovarian cancers, as well as melanomas that occur in the liver, gallbladder and bile ducts, testicular cancer, cervical cancer and hepatobiliary cancer.
The researchers also found that cancer cells with abnormally high levels of EXO1 behaved much like cells with BRCA mutations, which are well known to increase the risk of hereditary breast and ovarian cancers. Importantly, these BRCA-like behaviors occurred even in the absence of BRCA mutations.
EXO1 may help identify patients for targeted therapy
Researchers found that tumors with elevated EXO1 responded to treatment much like BRCA-mutated cancers.
“While EXO1 does not predict cancer risk, it may serve as a biomarker to help predict which patients are likely to respond to a particular chemotherapy, potentially leading to more personalized treatments,” said George Lucian Moldovan, professor of molecular precision medicine and senior author of the study. “The same drugs reserved for treating BRCA-mutated tumors and with fewer side effects could also be used to treat EXO1-overexpressing tumors that do not have BRCA mutations. This would expand the scope of these drugs.”
To investigate the role of EXO1, researchers analyzed tumor data from The Cancer Genome Atlas, the National Cancer Institute’s cancer genomics program. They found evidence of EXO1 overproduction in multiple types of cancer, including breast, skin, liver, and cervical tumors, consistent with previous studies. Elevated EXO1 levels were particularly associated with basal-like breast cancer, an aggressive disease.
How excess EXO1 damages DNA
The research team then conducted laboratory experiments using commercially available human cancer cells.
The researchers artificially increased EXO1 production in cells to see how excess amounts of the protein affect DNA. They also created a disabled version of EXO1 that produces protein but lacks normal biochemical activity. This allowed them to confirm that the observed DNA damage was caused by the activity of the protein, rather than simply its presence.
Under normal conditions, EXO1 acts like a pair of molecular scissors, helping to trim and repair damaged DNA. But when there is too much EXO1, those scissors start cutting DNA structures that should remain intact.
The researchers found that excess EXO1 destabilizes newly formed DNA through two main mechanisms: widening the single-stranded DNA gap and disassembling reverse replication forks. Both processes erode DNA, causing localized loss of genetic material, Moldova explained.
“We ultimately believe that, regardless of the pathway, overexpression of EXO1 causes the generation and accumulation of toxic lesions in DNA, such as double-strand breaks, that make tumors more sensitive to chemotherapy and increase cell death,” said the study’s first author, Alexandra Nusawardana, who received her doctorate in biomedical science from Penn State College of Medicine this year.
Why EXO1 mimics BRCA mutations
BRCA genes normally produce proteins that help protect vulnerable DNA structures during replication. When BRCA genes mutate, cells lose some of this protective function and can contribute to the development of cancer.
However, in this study, the researchers found that even when the BRCA gene is functioning normally and there are no mutations, excess EXO1 activity can overwhelm these protective mechanisms.
The researchers also discovered that EXO1 works with another protein called MRE11 to widen DNA gaps and generate dangerous DNA breaks.
“Mechanistically, this overexpression does exactly the same thing as loss of the BRCA pathway in BRCA-mutant tumor cells,” Professor Moldovan said.
He pointed out that EXO1 overexpression differs from BRCA mutations in important ways. It is not inherited, and researchers do not yet know whether it directly causes cancer.
Potential impact on cancer treatment
Because EXO1-overexpressing tumors behave very similarly to BRCA-mutant tumors, the researchers investigated whether they would respond similarly to treatment.
They tested olaparib, a drug commonly used against BRCA-mutant cancers that targets the cell’s DNA repair pathway. Tumors with elevated EXO1 were more sensitive to treatment and responded similarly to BRCA-mutated cancers.
This result suggests that patients whose tumors overexpress EXO1 could potentially benefit from the same reparative targeted therapies even if they do not carry BRCA mutations.
The researchers also found that tumors that overexpress EXO1 responded to cisplatin, a widely used chemotherapy drug. Their findings raise the possibility that lower doses of cisplatin can achieve comparable tumor shrinkage while reducing side effects.
Because EXO1 overexpression appears in a wider range of tumors than BRCA mutations, Professor Moldova said EXO1 overexpression could be a valuable biomarker to guide treatment decisions.
“Cancer treatment should not be based on what tissue the cancer comes from, but rather on the genetic mutations present within the tumor,” Professor Moldova said. “Then we can have highly efficient treatments. That’s the future of cancer treatment.”
The research team plans to continue their work with the long-term goal of starting clinical trials in patients with tumors that overexpress the EXO1 gene.
Claudia Nicolae, assistant professor of molecular precision medicine at Penn State College of Medicine, also contributed to the research.
This research was supported by funding from the National Institutes of Health and Four Diamonds.
