Monthly Archives: September 2010

BRCA2 purified, exposed

By my lights, the image below is a kind of scientific marvel, a visual representation of a recent research success that has taken more than15 years to accomplish: isolation of a pure extract of the tumor suppressor protein BRCA2 from human cells.

The photo shows two side-by-side molecules of BRCA2 protein, appropriately shaded pink and bound to a circular DNA constructed to contain a binding site for the protein.

This visualization showing that the protein acts as a pair when bound to DNA was produced  by  Drs. Sarah Compton and Jack Griffith at the  UNC Lineberger Comprehensive Cancer Center.

In a report published in Nature Structural  & Molecular Biology, August 22, 2010,  Stephen West of the London Research Institute, Cancer Research UK, and co-authors, including Compton and Griffith,  describe the first purification of the BRCA2 protein which is produced by a gene whose loss greatly increases the occurrence of breast cancer.

The feat, achieved independently by three labs, were published online August 22 in the journals Nature and Nature Structural & Molecular Biology.

The findings could lead to a better understanding of how the protein works and how BRCA2 sequence mutations cause cancer. They may also open a door to the development of new cancer therapies that could block the disease causing process.

The protein has been notoriously difficult to isolate until now. As one of the largest proteins in a cell, it can’t be expressed in bacteria in order to be isolated like other proteins – it is three to four times too big,  Stephen West told New Scientist magazine. As a result, researchers have until now been using fragments of the protein to understand its function.

As summarized in Nature, the three studies examined the interaction of the full-length BRCA2 protein with other proteins, primarily one called RAD51, which repairs DNA by assembling around breaks in the strands, and forming filaments through which nucleotides (components of DNA) are pulled in to fix the DNA gaps.

By studying the interaction between BRCA2 and RAD51, all three teams confirmed that BRCA2 helps RAD51 to initiate filament growth.

I’m  not surprised that Jack Griffith was involved in this important research. The Kenan distinguished professor’s electron microscopy (EM) work includes a number of breakthroughs beginning in his grad school years.

For his Ph.D. work at Cal Tech, Griffith developed the EM technology needed to directly visualize bare DNA and DNA-protein complexes. His methods involved carefully controlled rotary shadow casting with tungsten and mounting the DNA on very thin carbon films.

Using the methods he developed, Griffith, with Jack Kornberg and Joel A. Huberman published a paper showing an EM image of Escherichia coli DNA polymerase I bound to DNA. This was not only the first EM image of DNA bound to a known protein, but it also showed that electron microscopy had the potential to provide quantitative information about macromolecular assemblies involving DNA.

And in 2002,  Griffith and colleagues used quantitative techniques to map the DNA involved in Fragile X syndrome. It has been known that in people with Fragile X, a particular DNA sequence is repeated too often — as many as two thousand times, compared to only seventeen to thirty times in normal DNA. But it wasn’t known how that repetition, called expansion, contributed to Fragile X syndrome.

“We showed that in Fragile X, that expansion creates a segment of the chromosome that is very unorganized and unprotected relative to the rest of the chromosome,” Griffith told UNC’s Endeavors magazine. The work provides a clue to the molecular causes of the disorder.

Les Lang

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