Scientists at the UNC School of Medicine have made the surprising discovery that a molecule called EdU, commonly used in lab experiments to label DNA, is actually recognized by human cells as DNA damage, triggering an uncontrolled DNA repair process that ultimately considered fatal to affected cells, including cancer cells.
The discovery, published in Proceedings of the National Academy of Sciencespoints out the possibility of using EdU as a basis for cancer treatment, given its toxicity and its selectivity for rapidly dividing cells.
“The unexpected properties of EdU suggest that further studies of its potential, particularly against brain cancer, would be worth pursuing,” said lead study author Aziz Sankar, MD, PhD, Sarah Graham Kenan Professor in biochemistry and biophysics at the UNC School of Medicine and a member of the UNC Lineberger Comprehensive Cancer Center. “We want to emphasize that this is a basic but important scientific discovery. The scientific community has a lot of work to do to find out if EdU can really become a weapon against cancer.”
EdU (5-ethynyl-2′-deoxyuridine) is essentially a popular science tool first synthesized in 2008 as an analog or chemical mimic of the DNA building block thymidine — which represents the letter “T” in the DNA code for adenine ( A), cytosine (C), guanine (G) and thymine (T). Scientists add EdU to cells in lab experiments to replace thymidine in DNA. Unlike other thymidine analogs, it has a convenient chemical “handle” to which fluorescent probe molecules will bind tightly. Thus, it can be used relatively easily and efficiently to label and track DNA, for example in studies of the DNA replication process during cell division.
Since 2008, scholars have used EdU as a tool in this way, as published in thousands of studies. Sankar, who won the 2015 Nobel Prize in Chemistry for his seminal work on DNA repair, is one such scientist. When his lab began using EdU, his team unexpectedly noticed that EdU-labeled DNA triggered a DNA repair response even when it was not exposed to DNA-damaging agents such as ultraviolet light.
“It was a big shock,” Sankar said. “So we decided to investigate it further.”
Following up on the strange observation, the team discovered that EdU, for reasons that are not yet clear, changes DNA in a way that provokes a repair reaction called nucleotide excision repair. This process involves removing a short stretch of damaged DNA and re-synthesizing a replacement strand. This is the mechanism that repairs most damage from UV light, cigarette smoke, and DNA-altering chemicals. The researchers mapped EdU-induced excision repair at high resolution and found that it occurs throughout the genome and apparently occurs over and over again, as each new repair strand incorporates EdU and thus triggers the repair reaction again.
EdU was known to be moderately toxic to cells, although the mechanism of its toxicity was a mystery. The team’s findings strongly suggest that EdU kills cells by inducing a useless process of excision repair, which ultimately causes the cell to self-destruct through a programmed cell death process called apoptosis.
This finding was interesting in itself, Sankar said, because it suggested that researchers using EdU to label DNA should take into account its triggering of failed excision repair.
“As we speak, hundreds and perhaps thousands of researchers are using EdU to study DNA replication and cell proliferation in laboratory experiments, unaware that human cells detect it as DNA damage,” Sankar said.
Sankar and his colleagues also realized that the properties of EdU could make it the basis for an effective drug for brain cancer, because EdU is incorporated into DNA only in cells that are actively dividing, whereas in the brain most healthy cells are not dividing. Thus, in principle, EdU can kill rapidly dividing cancer brain cells while sparing non-dividing, healthy brain cells.
Sankar and his team hope to continue collaborating with other researchers to investigate the properties of EdU as an anti-cancer agent.
“Previous studies have already found evidence that EdU kills cancer cells, including brain cancer cells, but strangely, no one has ever followed up on these results,” Sankar said.