Careful now, guys (Image: Vassilis Roukos)

They could be the most dangerous liaisons ever caught on camera. Time-lapse microscopy has captured severed DNA strands in the act of pairing up with partners from the wrong chromosomes – a process with links to cancer.

Our cells have repair systems in place to make sure that any genetic damage is quickly fixed. But these repairs can go wrong, splicing together broken DNA strands from different chromosomes. These mix-ups – known as chromosome translocations – can lead to harmful mutations and are a hallmark of cancer cells.

What isn’t clear is exactly how and why the translocations occur in the first place. Tom Misteli at the National Cancer Institute in Bethesda, Maryland, and colleagues decided to take a closer look. They engineered the DNA in mouse cells so that it would split apart when exposed to a yeast enzyme, and they then watched the cells repair themselves.


When DNA breaks across both strands of the double helix, it is cut into two pieces that drift apart. To see what ensues, the team tagged the break points on the DNA with fluorescent proteins that made them visible. “Up until recently there was simply no technology to do this at the required image resolution,” says Misteli.

They then took time-lapse images of several thousand cells over the course of 24 hours to identify those that developed a chromosome translocation. They found that broken DNA strands appeared to move about randomly, trying to reconnect with their correct partner. In most cases, they succeeded. “The most frequent fate of a double-strand break is to be repaired immediately,” says Misteli.

Rare events

But every so often, two severed strands from different chromosomes joined up by mistake. “These translocations are very rare events,” says Misteli. “One needs to follow at least 300 cells to see one.”

Job Dekker at the University of Massachusetts Medical School in Worcester is impressed with the study. “It essentially settles a long-term debate about what’s important in translocation,” he says. “The study shows that the broken DNA strands do move around a little, but the movement is limited.”

Misteli’s team is now investigating why DNA repair goes wrong in these rare instances. Each cell is naturally equipped with an array of enzymes that help fix broken DNA strands. The researchers found that knocking out one repair enzyme in particular – DNA-dependent protein kinase (DNAPK) – made the affected cells almost ten times as likely to experience a chromosome translocation as cells with a functioning version of the enzyme.

“DNAPK has previously been implicated in translocation formation,” says Misteli. “But until now one could only look at the end product and ask how many translocations form in the presence or absence of DNAPK.”

The new results make it possible to probe the enzyme’s role in more detail. For instance, Misteli says the study shows that DNAPK does not prevent mismatched DNA strands from lining up – they do so whether or not the enzyme is present. But it does discourage the strands from actually joining together.

The researchers plan to look even more closely at the translocation process and hope to identify strategies to prevent DNA repair going wrong, potentially giving an insight into certain cancers.

Journal reference: Science, DOI: 10.1126/science.1237150