BioWorld International Correspondent

LONDON - After a search lasting 15 years, a team of scientists in the UK finally identified a protein that plays a key role in DNA repair. The discovery could lead to ways of making radiotherapy or chemotherapy for cancer more efficient through preventing cancer cells from repairing the damage to their DNA.

Since 1988, Stephen West, principal scientist at the London Research Institute of Cancer Research UK, the UK's largest cancer research charity, and his team have been seeking to identify the proteins involved in the later stages of the DNA repair mechanism known as homologous recombination. The repair process uses the same combination of enzymes as the one that brings about the sorting of genes during meiosis.

West described the breakthrough as a "major step forward." He told BioWorld International: "This has been a long-standing puzzle, because although we knew this repair process was going on in cells, the enzymes involved had defied identification, despite considerable effort on the part of scientists. This finding opens the door to the identification of other proteins involved in this step of the repair reaction, which should then allow us to define how repair takes place."

A paper reporting the team's experiments appears in the Jan. 9, 2004, issue of Science. It is titled "RAD51C Is Required for Holliday Junction Processing in Mammalian Cells."

Commenting on the paper, Robert Souhami, Cancer Research UK's director of clinical and external affairs, said: "Many cancer treatments work by causing lethal DNA damage in cancer cells. But if cancer cells are able to repair damaged DNA, they can often survive the effects of treatment. Once we understand the many ways in which DNA repair is brought about, we can look for new ways to inhibit the process in cancer cells, making them more susceptible to treatment."

All cells, including healthy ones, regularly suffer damage to their DNA from radiation or chemicals. As a result, they have evolved mechanisms to repair the damage. One particular repair process uses the same enzymes that are involved in the homologous recombination of chromosomes during meiosis.

Cancer researchers are interested in knowing about how those DNA repair mechanisms work, partly because one of the consequences of inefficient repair is the generation of tumors, and partly because of the possibility of making treatments that induce breaks in the DNA of cancer cells more efficient.

West said: "We had a fairly good idea of the early and middle steps in the recombinational repair process of double-stranded breaks, but no insight into how the later steps were carried out, particularly in mammalian cells."

The team focused on a structure called the Holliday junction, which is the point at which two chromosomes undergoing homologous recombination cross over. When recombination is complete, enzymes cut the two chromosomes apart - and those same enzymes are involved in the later stages of recombinational DNA repair.

West and his team took human cells and filtered out proteins bound to chromatin, passing the proteins through six chromatography columns in succession. He said: "We were able to identify an activity that cuts Holliday junctions by separating it away from the majority of cellular proteins. But the identity of this protein remained a mystery."

The breakthrough came when they tested an antibody that they already had in the lab, which had been raised against a protein known to be involved in recombinational repair, and realized that the protein, called RAD51C, was present in the purified fraction.

"This was the first clue that the fraction contained RAD51C," said West. "We were then able to use the antibody to mop up the protein, which made the activity disappear, and could restore activity by adding back purified RAD51C protein."

They also took cell lines defective in RAD51C and showed that the cells were unable to complete normal homologous recombination.

Further experiments showed that cells lacking the gene encoding a protein called XRCC3, which normally forms complexes with RAD51C, were also unable to separate their chromosomes at the end of homologous recombination.

West said: "This is really only the start of trying to understand how these reactions take place. We have identified one or maybe two factors involved in the reaction, but I am sure that there are many other factors that remain to be identified. We are now trying to use the RAD51C protein as a way to identify these other factors."