When a Good Gene That Repairs DNA Goes Out of Control, It Exposes a Weakness in Cancer Cells

DNA repair is usually seen as a line of defense that helps cells resist becoming cancerous, but new research warns that the defense itself may become a risk when its dosage falls out of balance. A team at Penn State College of Medicine and collaborators found that when a DNA repair-related gene called EXO1 is expressed at excessively high levels in cells, its normal DNA-trimming function may overstep its bounds, damaging newly formed DNA that is being replicated and thereby increasing genomic instability.

The work has been published in Nature Communications. Researchers including Alexandra Nusawardhana, Claudia M. Nicolae, and George-Lucian Moldovan noted that EXO1 is a nuclease that normally participates in DNA repair and in processing under replication stress; the problem is not that it exists, but that there is too much of it. When EXO1 is excessive, it degrades single-stranded DNA gaps and newly formed DNA at reversed replication forks in cells whose BRCA function remains intact, making the cells more likely to accumulate double-strand breaks.

The research team began with cancer genomics databases. After analyzing resources including The Cancer Genome Atlas and cBioPortal, they found that in cancer samples, the more common abnormality involving EXO1 was not deletion but overexpression. A Penn State news release said this overexpression can be seen in about 20% to 30% of breast and ovarian cancers, and also appears in melanoma, testicular cancer, cervical cancer, and hepatobiliary cancers; the paper’s analysis of TCGA PanCancer Atlas data showed that about 22% of breast invasive carcinoma samples had EXO1 overexpression.

Next, the researchers tested this phenomenon in commercially obtained human cancer cells, seeking to move from database correlations toward a cell-level mechanism. The results showed that EXO1 does not cause all the damage on its own; it works together with another DNA repair-related protein, MRE11, to promote the resection of newly formed DNA and increase the formation of double-strand breaks. In other words, although the cells do not have BRCA mutations, excessive EXO1 may put them into a vulnerable state with some “BRCA-defect-like” features.

This point also gives the research therapeutic possibilities. The team found that tumor cells with EXO1 overexpression were more sensitive to olaparib and cisplatin; the former is a PARP inhibitor already used in some BRCA-related cancers, while the latter is a platinum chemotherapy drug that causes DNA damage. If subsequent research holds up, EXO1 expression levels could become a biomarker for determining whether some tumors may respond to these treatments, rather than being limited to traditional BRCA mutation testing.

However, this is not yet clinical medication guidance. The current evidence comes mainly from analyses of public cancer genomics data and cell experiments, and it has not yet shown that using EXO1 as a marker can reliably predict treatment efficacy in patients. Nor has it clarified the boundaries among different cancer types, treatment combinations, and safety. In particular, DNA repair networks are interconnected, and overexpression of a single gene may not be enough to fully describe a tumor’s vulnerability.

The significance of this study lies in slightly overturning the intuition that “repair genes must be good actors”: the same molecular scissors maintain order when they are in the right place and present in the right amount, but in excess they may create breaks. For cancer treatment, these breaks are not only a source of danger; they may also become new clues for identifying tumor weaknesses.