Cells moving through small gaps can damage their DNA even without rupturing their nuclei by causing problems with DNA replication, a possible outcome for cancer cells as they move throughout the body during metastasis.
The authors of a study in Current Biology discovered that squeezing some kinds of cells while preserving their nuclear membranes can interrupt the duplication of DNA or cause a break in the genetic material. The findings uncovered a new mechanism for damaging DNA, which can make cancer cells more dangerous by making them multiply more rapidly or resist treatments.
In nearly all cells, the nucleus contains DNA as well as compounds that maintain and express the genetic code. Ripping its membrane and exposing its contents to the rest of the cell is a well-known source of DNA damage, but whether and how problems can arise without rupture had been an open question, according to Jan Lammerding, a professor of biomedical engineering at Cornell University and the senior author of the study.
“We've been studying the nucleus for a long, long time, and we'd recognized that the nucleus is a much stiffer organelle than the rest of the cell,” Lammerding said. He said his lab wondered, “How does that affect the ability of cells when they have to migrate through small spaces … that are much, much smaller than the size of the nucleus?”
The researchers sent cells through tight spaces and manually compressed others to see when and where DNA damage occurred, engineering the cells to glow during damage or nucleus rupture. The trials were intended to simulate the conditions of metastasizing cancer, during which cancer cells squeeze through spaces much narrower than the width of their nuclei.
Both cancerous and non-cancerous cell lines experienced non-rupture damage during the tests, and the outcomes were even worse than when their nuclear membranes broke. The results were the same whether the cells were migrating through small spaces or being flattened by the scientists, demonstrating that the effect is entirely mechanical.
“The reason why it's so exciting is because this is the first time that we’re showing that just the deformation of the nucleus is sufficient to cause DNA damage, that you do not need rupture,” said Pragya Shah, the study’s lead author and a former Ph.D. student and member of Lammerding’s lab at Cornell.
Shah and her team observed that the non-rupture DNA damage occurred during only some parts of the cell cycle — namely the cells’ G2 and S stages, when they replicate their DNA and prepare to divide. This prompted the researchers to further investigate replication stress as the nature of the damage, and their later trials confirmed that it occurred at the replication fork, where the two DNA helices are separated into two prongs during replication.
Non-rupture damage was found in only some of the cell lines tested in the study, while others were only harmed when the nuclear membrane broke. This occurred in both cancerous and non-cancerous cells.
The difference between the two groups may lie in their genetic codes. The researchers found that the cells with non-rupture damage all had a mutated p53 gene — the most common mutation in cancer cells that also reduces replication stress — leading them to speculate that the gene could protect against non-rupture DNA damage when functioning normally.
The team’s findings contribute to understanding the source of dangerous mutations in migrating cancer cells. Numerous studies have shown that causing DNA damage creates genomic instability, a “hallmark of cancer” that speeds up metastasis by creating more mutations and causing cancer cells to become more aggressive and resilient.
In addition, squishing stationary cells and causing damage without nucleus rupture resembles conditions found in tumors and other parts of the body, according to Lammerding.
“It could also happen, for example, in the brain and your organs and other cells as well, where it’s confined,” Lammerding said, “and so that makes the relevance much, much broader than just looking at migrating cancer cells.”
The Cornell professor said the paper raises new questions about how to determine which cells can experience this kind of DNA damage and whether there are other mechanisms in which to harm DNA. He said diagnostic or therapeutic applications that employ these could be farther down the line.
The article, “Nuclear Deformation Causes DNA Damage by Increasing Replication Stress” was published Dec. 15 in Current Biology. The authors of the study were Pragya Shah, Svea Cheng, Marshall Colville, Matthew Paszek and Jan Lammerding, Cornell University; and Chad Hobson and Richard Superfine, University of North Carolina at Chapel Hill. The lead author was Pragya Shah.