Sci. Aging Knowl. Environ., 30 July 2003
A Keen I
Razor-sharp molecular method mucks up miscreant gene without vandalizing helpful one
Key Words: superoxide dismutase shRNA dominant allele gain of function mutation motoneuronal degeneration
Like a mother who can tell her identical twins apart by the tiniest dimple, a new method can discern two versions of a gene that barely differ. The technique eradicates the bad gene's primary product, messenger RNA (mRNA), which serves as a blueprint to make protein; but it leaves the normal gene's plan intact, at least in mice. The results hint at a treatment for certain genetic disorders--including some forms of Parkinson's, Huntington's, and Alzheimer's diseases--in which a corrupt copy of a gene dominates over a well-behaving one.
Some illnesses can arise from a single spoiler gene. In amyotrophic lateral sclerosis (ALS, or Lou Gehrig's disease), for example, a marred copy of the SOD1 gene differs from the normal sequence at a single letter in the DNA code in about 5% of cases. This faulty gene creates a protein that meddles with the normal protein's ability to do its job. In ALS, that means a breakdown in neurons that control muscle. To fix mutations that cause trouble in the presence of a normal copy of the gene, researchers would have to get rid of the troublemaking protein. In principle, scientists could cut out the sullied gene and replace it with a healthy one. So far, that method has been inefficient even in cultured cells.
As an alternative, researchers are pursuing the technique of RNA interference (RNAi) to shut down misbehaving genes by eradicating their mRNAs. So-called small interfering RNA molecules (siRNAs) that parallel a short portion of the gene's sequence stimulate a cellular process that destroys matching mRNAs. Hongliu Ding and colleagues wanted to determine whether RNAi could destroy a sick gene and leave a nearly identical normal mRNA untouched.
To do so, the team members added three siRNAs to a test tube that contained the contents of cells plus normal and defective SOD1 mRNAs. One siRNA chopped up most of the mutant mRNA while passing over the healthy version. The same siRNA destroyed mutant but not normal mRNA in two human cell cultures--skin and neuronal cells--that contained both flavors of mRNA. To test whether an siRNA that targets a single DNA letter change might work in mammals, the researchers turned to rodents. Because scientists can't efficiently coax siRNAs into nerve cells, the group injected the two versions of the human SOD1 gene into livers of live mice. There, the siRNA eliminated the product of the mutant gene while allowing the normal crop to flourish, indicating that the team can obtain its desired precision.
Neuroscientist Mark Mattson of the National Institute on Aging in Baltimore, Maryland, says that the "nice, basic science study" suggests that "it might be possible to selectively get rid of the RNA encoded by a mutant gene," and the observations in animals hint that the technique could work in humans. But, he adds, delivering gene therapies to nerve cells in people is a "big hurdle to cross," although scientists are toiling away at the problem. If the work eventually succeeds in humans, scientists might be able to put a dimple back in patients' smiles.
July 30, 2003
Science of Aging Knowledge Environment. ISSN 1539-6150