Key regulator protein controls when and where DNA is copied
In cells that cannot produce the protein Rif1 (right), the pattern of spots that shows DNA being copied (green) becomes fuzzy compared to normal cells (left, red).
Scientists at EMBL Monterotondo have identified for the first time a protein responsible for regulating the specific timing of DNA replication in mammalian cells. The study, published online in the EMBO Journal in July, also indicates that this protein might play a role in breast cancer.
Every time a cell divides it must copy – or replicate – its DNA in its entirety. With around 22000 genes to copy, multiple start sites, known as replication origins, are required. Scientists have long known that replication begins at different sites at different times. This ordering is thought to be crucial for the daughter cell’s DNA to be packaged correctly. The factors that control when and where each replication origin is activated have, however, remained a mystery, until now.
Researchers led by Sara Buonomo used special staining techniques to visualize the patterns of spots in the cell’s nucleus which represent regions that are being copied. As expected, they saw that during normal cell division the spots form a precise sequence of patterns. But when Buonomo and colleagues prevented the cells from producing a particular protein, Rif1, this sequence was disrupted and the patterns became mixed up and poorly defined. Furthermore, regions that are normally copied late in the cycle were replicated much earlier than normal and vice versa. Consequently, in the absence of Rif1 the normal packaging of the DNA in the daughter cells was lost.
These findings raised an interesting connection. “Rif1 has been found mutated in breast cancer cells,” says Buonomo “We don’t yet know whether these Rif1 mutations cause cancer or if they are just passenger mutations, but our findings give an explanation, at multiple levels, as to how a change in Rif1 function could really be a causative event in transformation into a cancerous cell.” Scientists now know that certain breast cancer-causing mutations disrupt DNA packaging. Since Rif1 controls how DNA packaging is passed from a cell to its daughters, faulty versions of this protein may cause – or contribute to – cancer in a similar way. Rif1 mutations could also, the scientists believe, affect the cell’s ability to detect and react to problems in DNA replication – another hallmark of cancer cells.
The work was done in collaboration with the labs of Genevieve Almouzni at the Centre National de la Recherche Scientifique (CNRS) in France and David Gilbert at Florida State University in the USA, as well as EMBL's Electron Microscopy Core Facility.
Cornacchia, D., Dileep, V., Quivy, J.-P., Foti, R., Tili, F., Santarella-Mellwig, R., Anthony, C., Almouzni, G., Gilbert, D. & Buonomo, S.B.C. Mouse Rif1 is a key regulator of the replication-timing programme in mammalian cells. The EMBO Journal, published online 31 July 2012. DOI: 10.1038/emboj.2012.214.