BioWorld International Correspondent
A new generation of cancer drugs could flow from the discovery that two loops of a cell-signaling protein must be kept firmly bound together in order to prevent the protein from triggering cell division in cancer. Researchers have shown that mutations in the loops allow them to pop apart, causing the protein - a kinase called B-RAF - to activate a cell-signaling pathway leading to cell division.
Richard Marais, team leader at the Cancer Research UK Centre for Cell and Molecular Biology at the Institute of Cancer Research in London, told BioWorld International: "Many people die each year from cancers which have this protein mutated in them, and our work now provides much greater insight into why the mutants activate the cell division pathway. From the drug development point of view, this is quite significant because it allows people to start thinking about what the shape of the drugs they are designing might look like, and how they might improve both the selectivity and potency of these compounds."
B-RAF is a protein kinase that plays a role in relaying signals from outside the cell to the nucleus, thus regulating cell growth, differentiation and death. When activated, it kick-starts a pathway leading to cell division.
An estimated 5,000 people die in the UK every year from cancers that have mutations in B-RAF. Such mutations are present in 70 percent of melanomas, up to 70 percent of thyroid cancers, 30 percent of ovarian cancers and 15 percent of colorectal cancers.
A team of researchers in the UK now has scrutinized how the mutations present in B-RAF bring about their harmful effect and solved the structure of the protein. An account of their investigations appears in the March 19, 2004, issue of Cell, in a paper titled "Mechanism of Activation of the RAF-ERK Signaling Pathway by Oncogenic Mutations of B-RAF."
Co-author David Barford, from the Section of Structural Biology at the Institute of Cancer Research, said: "Most of the mutations we looked at occur in the two regions of the gene that are crucial to holding B-RAF in its inactive form. When either of these regions is damaged, B-RAF is able to rearrange its shape. This change activates the protein and plays a key role in driving the uncontrolled cell proliferation and survival that leads to cancer."
Marais said that about 30 mutations in B-RAF had been described. The team set out to understand what effect those mutations had on the function of the protein.
Marais said: "We found that the majority of the mutations stimulated B-RAF's activity, which is probably what drives the proliferation of cancer cells. To our surprise, we found that some mutants did not activate B-RAF, although they were still able to activate a different member of this family of protein kinases, called C-RAF, with a similar effect on cell division."
Using X-ray crystallography, the team went on to solve the structure of B-RAF. They had noted that the majority of cancer-causing mutations in B-RAF were clustered in two hotspots - one in an area of the protein known as the P-loop and the other in a part known as the activation segment.
In inactive B-RAF, the P-loop and the activation segment are bound to each other.
"In the mutants," Marais said, "it appears that the activation segment folds back on itself in a different way, thus activating the kinase. This provides us with a molecular explanation of how B-RAF might be regulated, because we think that the processes that normally stimulate B-RAF activity effectively do the same thing."
Marais, Barford and their colleagues also showed that a compound called BAY43-9006, which is being developed as a cancer drug by Bayer AG, of Leverkusen, Germany, and Onyx Pharmaceuticals Inc., of Richmond, Calif., can bind to the active site of B-RAF. BAY43-9006 is in clinical trials, but results are not yet available.
"We hope to use the information reported in this study to improve the efficacy and selectivity of the compounds we are developing as part of our own drug discovery program," Marais said. "We also want to gain a better understanding of the biology of the different mutants, and how these are able to activate C-RAF on one hand and B-RAF on the other. We want to know whether the consequences of activating the cell division pathway through these two different routes are different or the same."