New therapeutic strategy could target toxic protein in most patients with Huntington's disease

Apr 09, 2009

Howard Hughes Medical Institute researchers have designed tiny RNA molecules that shut off the gene that causes Huntington's disease without damaging that gene's healthy counterpart, which maintains the health and vitality of neurons. Laboratory studies suggest that a single small interfering RNA could reduce production of the damaging Huntingtin protein in nearly half of people with the disease. Another 25 percent of patients might benefit from one of a set of four additional small interfering RNAs.

Phillip D. Zamore, an HHMI investigator at the University of Massachusetts Medical School in Worcester, and his colleagues reported their findings in an article published April 9, 2009, in the journal .

There is no treatment for Huntington's disease, which is caused by a mutant form of the Huntingtin gene. Huntingtin is required for healthy nerve cells, but the mutant gene makes a toxic protein that contains excess amounts of the amino acid glutamine.

The key to whether the Huntingtin gene is normal or defective lies in a kind of genetic stutter: a repetitive sequence of the DNA triplet CAG, which codes for the amino acid glutamine. Stretches of CAG "repeats" appear in every human being's Huntingtin gene, but the length varies. Whereas the normal gene has a sequence of between six and 34 CAG repeats, the abnormal gene contains many more. In fact, any stretch of DNA containing more than 40 of these repeats ensures that its bearer will develop Huntington's—the greater the number of repeats, the earlier the disease strikes and the greater its ferocity. The abnormal causes movement disorders, cognitive failure, and ultimately, death. Children who have a parent with Huntington's disease have a 50 percent chance of inheriting the disease themselves.

Zamore studies how can be used to silence selectively. In the 1990s, he and other scientists learned they could shut down the production of specific proteins by introducing double-stranded RNA into the cell that is identical to the RNA they wanted to turn off. These strands of RNA, known as short interfering RNA (siRNA), slice apart the original RNA, which the cell then destroys.

But nine years ago, when researcher Neil Aronin, who is also at UMass Medical School, proposed using the technique to attack Huntington's, Zamore couldn't see a way.

"I explained to him that you can't," Zamore said. The problem was that the disease gene and its healthy allele are almost identical, and Zamore told Aronin that he wouldn't be able to distinguish between the two forms of Huntingtin. "Then, as he was leaving my office, it occurred to me that you could," he recalled. The key was something called a single nucleotide polymorphism or SNP.

A SNP is any place on the genetic code that varies by a single unit. The genetic code is written with four letters, A, C, T, and G, which stand for the four nucleotides, adenosine, cytidine, thymidine, and guanosine. The pattern of these nucleotides dictates which protein is encoded by a given gene. DNA in the nucleus is transcribed as messenger RNA, which leaves the nucleus and begins making proteins based on the order of these four bases. A person's two copies of any gene may vary at these locations "simply because the two parents have different ancestries," Zamore said.

Zamore, Aronin, and their collaborators decided to look for such variation in the Huntingtin gene. It was a bit of a long shot. Even if the lab found relevant SNPs, it was likely few people would share the same polymorphisms, making drug development and testing nearly -- if not completely -- impossible.

Then they got lucky. The search for SNPs in the genetic material of 109 Huntington patients uncovered a single SNP carried by 48 percent of people with Huntington's. "This SNP is actually associated with the disease. We don't know why," Zamore said. That meant a single siRNA could shut off expression in the mutant Huntingtin gene - while leaving cells' healthy Huntingtin genes intact -- in almost half of all U.S. and European Huntington's patients.

"The most exciting part of the study was finding one siRNA that clearly is the top candidate for a clinical trial, where the patient population is predicted to be sufficiently large that it merits the development of a drug you could take into trial," Zamore said.

"That takes away the biggest worry we had, which was that the number of siRNAs we would have to test in order to have impact on the disease would be too large, and as a consequence the FDA would never approve any trial," he continued.

By adding an siRNA against one of two other common SNPs, Zamore says gene silencing could be effective in 75 percent of patients with Huntington's disease in the U.S. and Europe. Although the group found other SNPs, targeting more of them failed to increase the number of patients who could be helped, he said.

The next problem became developing siRNAs that could discriminate between target mRNAs and non-target. "That turned out to be frustratingly difficult," Zamore said. In tests of human cells, the siRNA sometimes sliced up the disease RNA, as it should. But sometimes it destroyed the normal Huntingtin RNA as well. To prevent this error, Zamore and his colleagues changed one more nucleotide base on the siRNA. Now, the silencing RNA was different from the healthy mRNA by two nucleotides, making it less likely to grab the good RNA.

Further research in mice will examine the efficacy of the siRNA tool. "The siRNA has to be sufficiently stable, and has to get into the right cells, and has to discriminate between the two (genes). It's incredibly expensive work," he said.

Zamore acknowledges that even with this progress, they're a long way from a treatment for Huntington's. "The Huntington's community is very savvy about understanding that scientific progress is always plodding. It's the sum of lots of little steps. From our perspective, the most important thing is to keep taking those steps."

Source: Howard Hughes Medical Institute (news : web)

Explore further: Refining the language for chromosomes

add to favorites email to friend print save as pdf

Related Stories

Team demos safety of RNA therapy

Sep 26, 2007

Researchers from MIT, Alnylam Pharmaceuticals and other institutions have demonstrated the safety of a promising type of genetic therapy that could lead to treatments for a wide range of diseases such as cancer.

Recommended for you

Refining the language for chromosomes

15 hours ago

When talking about genetic abnormalities at the DNA level that occur when chromosomes swap, delete or add parts, there is an evolving communication gap both in the science and medical worlds, leading to inconsistencies in ...

Down's chromosome cause genome-wide disruption

Apr 16, 2014

The extra copy of Chromosome 21 that causes Down's syndrome throws a spanner into the workings of all the other chromosomes as well, said a study published Wednesday that surprised its authors.

User comments : 0

More news stories

Leeches help save woman's ear after pit bull mauling

(HealthDay)—A pit bull attack in July 2013 left a 19-year-old woman with her left ear ripped from her head, leaving an open wound. After preserving the ear, the surgical team started with a reconnection ...

Scientists tether lionfish to Cayman reefs

Research done by U.S. scientists in the Cayman Islands suggests that native predators can be trained to gobble up invasive lionfish that colonize regional reefs and voraciously prey on juvenile marine creatures.

Venture investments jump to $9.5B in 1Q

Funding for U.S. startup companies soared 57 percent in the first quarter to a level not seen since 2001, as venture capitalists piled more money into an increasing number of deals, according to a report due out Friday.

White House updating online privacy policy

A new Obama administration privacy policy out Friday explains how the government will gather the user data of online visitors to WhiteHouse.gov, mobile apps and social media sites. It also clarifies that ...