Scientists discover how a well-known protein repairs broken DNA ends

During the life cycle of our cells, a minefield of environmental and biological assaults can lead to double-stranded DNA breaks, the most lethal and dangerous form of DNA damage. Now, in research published online this week in Nature, Rockefeller University scientists reveal that when these breaks occur, a protein called 53BP1 helps repair them by mobilizing their dangly DNA ends - findings that uncover a previously unknown aspect of how double-stranded breaks can get fixed.

"We have learned something fundamentally new about double-stranded DNA repair," says Titia de Lange, head of the Laboratory of Cell Biology and Genetics

and Leon Hess Professor. "We already knew that when cells encounter a double-stranded break in their chromosomes, they can repair it by a well-studied fusion pathway called nonhomologous end joining. What we didn't know is that cells facilitate this repair by increasing the motility of chromatin surrounding the break. That way, breaks that are far apart in the nucleus can be brought close together and then mended."

To study the repair of double-stranded DNA breaks, de Lange and her colleagues, including David Spector at Cold Spring Harbor Laboratory, worked with telomeres, the relatively stationary ends of linear chromosomes that are capped by a protein complex called shelterin. When this cap is removed, the cell perceives the ends of the chromosomes as double-stranded breaks in need of repair - that is, they engage the end-to-end fusion pathway. Using digital cell imaging, the team has now become the first to create live video footage of how telomeres move around in the nucleus when uncapped.

In their work, de Lange and her team genetically deleted a protein from the shelterin complex, thus exposing the ends of the linear chromosomes. By marking the telomeres with a fluorescent protein and recording their movement in live mouse cells, the researchers saw that the telomeres became more mobile when they were unprotected, moving faster and exploring a larger region of the nucleus. "You go from a rather dull party to a wild dance where the telomeres start meeting one another," says de Lange. "The party really gets going."

But when the researchers removed 53BP1 from the cells, the movies revealed that the telomeres no longer exhibited this ramped up mobility. "The effect is quite striking," says first author Nadya Dimitrova, a graduate student in the lab. "Without 53BP1, we don't see this dynamic behavior anymore. The ends are much more restricted in their mobility. It's not that the ends can't fuse, the fusion machinery is intact. It's just that the ends don't get close enough to each other." The findings not only point to 53BP1 as the main driver of telomere mobility but show that this mobility is important for the repair of uncapped telomeres.

The researchers are quick to point out that the repair process of dysfunctional telomeres is not the same as that of internal chromosomal double-stranded breaks. At dysfunctional telomeres, there is only a single DNA end, whereas internal breaks lead to two ends that are close together and do not need to search for a fusion partner. But one case that does require this long-distance repair occurs in immune cells, which intentionally create breaks in their antibody genes and rearrange them. This way they can create a virtually unlimited repertoire of antibodies with a limited number of genes.

"So we think that our findings may apply to this kind of long-distance, rather than short-distance, repair," says Dimitrova. "It will be interesting to see how this plays out."

In a related work, published in the same issue of Nature, de Lange's colleague Michel C. Nussenzweig, head of the Laboratory of Molecular Immunology and Sherman Fairchild Professor at Rockefeller, found that 53BP1 is also required for long-distance gene rearrangement and fusions that help fortify a healthy immune system. "This is a nice surprise," says de Lange. "It looks like there are a lot of questions that now need to be answered about DNA repair - questions we never knew existed."

Nature online: October 19, 2008

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