In the spring of this year, researchers from the University of Iowa have excited the scientific community by demonstrating that it is possible to silence a mutant gene (like the one that causes Huntington disease) using RNA interference, or RNAi. The findings suggest that this gene-silencing technique might one day be useful in treating many human diseases, including HD.

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DNA holds all of our genetic information, and contains the thousands of genes that make the proteins that in turn make up who we are. We are made up of millions of individual cells, each one having a copy of the same DNA. Only a small amount of the total DNA is expressed in individual cells. The part of the DNA that is expressed in individual cells is copied into an RNA copy of the DNA. The RNA or more specifically messenger RNA provide the information for the production of a particular protein.

As an analogy, think of the human body as being like a house. If DNA is a home builder's impression of a what a finished house looks like, RNA consists of the essential detailed, three-dimensional drawings for the construction workers who make the house to follow. This means that RNA interference is not about changing the original design of the house, but focuses instead on specific features of the detailed plans such as where to put the bathroom or a set of stairs.

How does RNAi work?

Genes come in pairs, one copy from your mother and one copy from your father. In Huntington disease, inheriting one bad, or mutant, gene copy from either parent is enough to cause the disease. What the University of Iowa researchers have shown is that it is possible to silence the mutant gene copy without affecting the normal copy. Specifically, RNAi is used to destroy the messenger RNA that is made from the mutant huntingtin gene while leaving the normal huntingtin messenger RNA to be expressed as normal huntingtin protein. Since previous research has shown that the protein that is made from the normal gene copy plays a role in normal cell function, being able to silence just the mutant copy was a critical part of the research.

"If you have a bad gene, you simply switch it off and leave the good copy alone to perform its normal function," said Victor Miller, a UI graduate student and the lead author of the paper. "It is an intellectually simple but technically difficult thing to do … This work is an important proof of principle but it is still a long way from clinical application."

How did they do it?

Working in cell culture (which basically means they worked with some special cells in a dish), the researchers used the relatively new technology known as RNA interference (RNAi) to silence or destroy the messenger RNA made from the mutant gene that causes the neurodegenerative condition called Machado-Joseph disease (or Spinocerebellar Ataxia Type 3), while leaving the normal gene product alone.

Machado-Joseph disease (MJD), Huntington's disease and at least seven other neurodegenerative disorders all are caused by the same type of genetic mutation. The genetic defect in these diseases produces a mutant protein that is produced in brain cells and then clumps together, forming protein aggregates, ultimately damaging and killing brain cells. The research team studies Machado-Joseph disease because it is a good model for investigating these types of neurodegenerative diseases.

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Remember, individual genes are made up of important codes or sequences of amino acids. These sequences are incredibly long and complex. These sequences are also not always perfect. Sometimes, the sequence in a gene has little errors or differences in them that aren't serious - these are sometimes called single nucleotide polymorphisms (SNPs). Other times, the errors in the sequences are quite serious - these kinds of errors are what can cause the mutant gene in Huntington disease.

When the researchers first focused the RNA interference, they targeted the mutant copy of the gene, specifically the part of the copy that is faulty. But when they did this, the RNAi technique suppressed not only the mutant copy of the gene, but the normal copy as well.

Then the researchers noticed that on the mutant copy of the gene there was a single sequence difference in the coding of the gene right next to the mutated sequence of the gene. This little flaw is called a single nucleotide polymorphism or SNP. When the researchers targeted the SNP with RNA interference, the technique was able to tell the difference between the mutant copy and the normal copy of the messenger RNA, and only suppress the mutant copy.

The discovery that RNA interference could distinguish between genes on the basis of a single nucleotide polymorphism (SNP) was very exciting because every person's DNA differs mostly on the basis of these unique single letter variations in the genetic code. Thus it might be possible to use RNA interference to target unique single nucleotide polymorphisms associated with specific genes in order to manipulate those genes.

Just how important is this?

RNA interference has been hailed as the greatest breakthrough of the year by the American journal Science. And, when first commenting on RNA interference, Nancy Wexler, president of the Hereditary Disease Foundation in New York said, "When I first heard of this work, it just took my breath away."

While the work done in tissue culture is extremely exciting and has demonstrated that it will be possible, in future, to use RNA interference-based gene therapy techniques to reduce the expression of mutant huntingtin, there is still a huge amount of basic research that needs to be done before this could be used in HD patients. For example, the molecules that carry out the job of destroying the mutant copy of the huntingtin messenger RNA need to be delivered to the human brain so they can do their job, and scientists must ensure that these molecules are effective and safe in cells and animals before any testing in humans can be done. Dr. Eileen Denovan-Wright, who is a member of the Huntington Society of Canada's Research Council and does research in RNA interference, is cautiously optimistic. So far, there is every reason to be hopeful that a real treatment for HD may be available in the foreseeable future.

Can you translate this for me?

Everyone's genes are made up of two copies, one copy you inherit from your mother, the other from your father. Huntington disease occurs when an individual inherits the mutant gene that causes HD from one of their parents, and a copy of the normal huntingtin gene from the other.

In experiments using living cells, scientists have figured out a way to stop the mutant copy of a gene (they used a genetic disease just like Huntington's) from producing the toxic protein that leads to brain cell death - and they did it without damaging the normal copy of the gene that would be needed for normal function. This process is called RNA interference because the scientists used RNA (an important chemical that works with DNA inside cells) to interfere with or silence the mutant copy of the gene.

RNA interference is a type of gene therapy, and is extremely experimental. So, while the result of the research is important news, it will take a number of years before scientists could even begin to think about using this kind of technique in human beings.

Developed using material from:

Bob Holmes. Gene Therapy may switch off Huntington's. March 13, 2003. As posted at <http://www.newscientist.com>.

John Cornwell. The Elixir of Life. May 25, 2003. As posted at <http://www.hdfoundation.org>.

University of Iowa News Release. UI Researchers Selectively Silence Disease-Causing Gene. May 27, 2003.