Scientists Find Key to Gene Therapy

In mice, silencing RNA cut cholesterol nearly in half

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HealthDay Reporter

WEDNESDAY, Nov. 10, 2004 (HealthDayNews) -- By harnessing a natural gene-silencing mechanism, scientists say they have dramatically reduced the activity of a protein linked to high cholesterol.

The study, conducted in mice, marks the first time that this mechanism, called RNA interference (RNAi), has been successfully manipulated in living tissue as gene therapy to fight disease.

The discovery could pave the way to drugs that fight a range of common illnesses from heart disease to cancer, experts say.

"This discovery has major potential for the discovery of new medicines. It's the type of discovery that really only happens every decade or so," said Phillip A. Sharp, a 1993 Nobel Medicine Prize winner for his work with RNA, and a member of the board of directors at Massachusetts-based Alnylam Pharmaceuticals Inc., the company responsible for the latest findings.

Speaking with reporters at a news teleconference Tuesday, Sharp said the successful application of RNAi gene-silencing "offers a completely new way of treating disease, as it has the potential to target any gene involved in the cause or pathway of human disease."

The Alnylam team, led by Jürgen Soutschek and Hans-Peter Vornlocher, published their findings in the Nov. 11 issue of Nature.

The chief job of all genes is to produce proteins necessary to the function of healthy cells. But sometimes this process goes awry, with genes failing to produce the proper protein, or producing it in inadequate or excessive amounts, resulting in cellular dysfunction and disease. Geneticists have long sought safe, effective ways to switch off the expression of these types of aberrant genes.

Between the activity of the gene, which is comprised of DNA, and the end-product protein, lies an intermediary step, known as transcription. During transcription, a separate molecule, called messenger RNA, copies (transcribes) information from the DNA, which is then used as a blueprint for the manufacture of the protein.

Scientists have been busy looking for a way to silence messenger RNA, as means of interrupting the work of disease-linked genes. And about six years ago, researchers discovered RNA interference -- a naturally occurring mechanism that uses molecules called short-interference RNA (siRNA) to do just that.

However, researchers soon hit a roadblock: Although siRNA works well at silencing genes in laboratory cell cultures, it's proven impossible to replicate that action in living tissue. According to experts, enzymes in living cells quickly degrade siRNA, and the molecules also find it difficult to pass through cell membranes.

But the Alnylam team believes they have solved both those problems.

In studies conducted in mice prone to high cholesterol, "we've found that certain chemical modifications that increase siRNA stability, while providing cellular permeability, significantly improve the [living tissue] pharmacologic properties of siRNAs," Vornlocher told reporters.

Specifically, his team engineered a series of siRNAs that were also chemically bound to a cholesterol molecule. This new molecular partnership boosted the siRNA's ability to pass through the cell membrane, the researchers explained.

As an added bonus, the cholesterol molecule also helped the siRNA link up with specific cellular RNA involved in the production of apoliprotein B (apoB) -- a protein linked to both high cholesterol and cardiovascular disease.

Once the siRNA made that crucial connection within the cell, it snipped off one end of this apoB-generating RNA -- "achieving gene silencing," according to Vornlocher.

The result? Mice injected treated with the new gene-silencing technique got healthier fast.

According to Vornlocher, tests revealed significantly lower blood levels of apoB, and "a significant change in [blood fat] profiles, including a significant reduction in blood cholesterol levels."

In fact, mouse blood levels of the unhealthiest form of cholesterol, LDL, fell by 40 percent soon after the gene-silencing treatment. "This level of cholesterol reduction would be considered highly clinically significant in patients with high blood cholesterol," he said.

Officials at Alnylam stressed that the implications of this research extend far beyond cholesterol control, however.

"The findings enable, for the first time, the potential for RNA interference therapeutics to be developed to address significant diseases, such as cardiovascular disease, diabetes, obesity, hepatitis, cancer and other diseases," said Alnylam president and CEO John Maraganore.

In related study published in the same issue of Nature, researchers led by Dr. Matthew N. Poy of Rockefeller University in New York City, say siRNA gene silencing of a gene responsible for insulin secretion within the pancreas may hold promise as potential diabetes therapy.

In laboratory cell cultures, siRNA-based research identified miR-375, a type of RNA found in pancreatic cells, as a "novel pharmacologic target for the treatment of diabetes," the researchers wrote. Drugs that silence miR-375 might someday help patients improve insulin production within the pancreas and ease diabetes, they say.

More information

For more about the promise of gene therapy, go to the National Human Genome Research Institute.

SOURCES: Nov. 10, 2004, news teleconference with Phillip A. Sharp, Ph.D., member, board of directors, and John Maraganore, Ph.D., president and CEO, Alnylam Pharmaceuticals Inc., Cambridge, Mass; Hans-Peter Vornlocher, researcher, Alnylam Europe AG, Kulmbach, Germany; Nov. 11, 2004, Nature

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