By AMY ADAMS
Medical center researchers have successfully shut down genes made by the hepatitis B virus, preventing it from replicating in mice. This work, published last month in the online version of Nature Biotechnology, could lead to the first gene therapy treatment for hepatitis B, according to Mark Kay, MD, PhD, professor of genetics and of pediatrics.
Most gene therapy techniques involve inserting a healthy copy of a gene to compensate for one that’s malfunctioning. But some disorders such as cancers, Huntington’s disease, and viral diseases result from genes producing too much protein or an abnormal protein, making these diseases unlikely targets for routine gene therapy. A recently discovered phenomenon called RNA interference now shows promise for flipping the genetic "off" switch to shut down protein production in these disorders.
RNA interference is a process cells use to prevent genes from producing proteins. It works by destroying the protein-coding message called mRNA made by an active gene. A short RNA molecule combined with proteins in the cell bind to the mRNA, targeting it for destruction. With no mRNA, no protein gets made.
In recent years, researchers have hijacked this process to shut down genes of their own choosing. In studies published in Nature last year, Kay and his colleagues used RNA inhibition to thwart proteins from being made by DNA sequences inserted into mice.
The latest work takes the procedure one step forward, using RNA interference to prevent the hepatitis B virus from making proteins and the viral RNA needed for normal replication. "This establishes that RNA interference might work in a gene therapy setting," said Kay, who led the study.
The hepatitis B virus normally infects human liver cells, putting a person at high risk for liver cancer and other liver diseases. There is no effective treatment for the potentially lethal infection.
The researchers first tested RNA interference in mouse cells that had been infected with a form of the hepatitis B virus. The researchers inserted a piece of DNA that produced an inhibitory RNA targeted to hepatitis B mRNA. After eight days, the cells containing the inhibitory RNA made about 95 percent less hepatitis B protein than untreated cells.
The researchers then tested RNA interference in mice that had been infected with the same version of the hepatitis B virus. Mice receiving DNA coding for the inhibitory RNA in addition to the virus made 92 percent less of the hepatitis B mRNA than mice lacking RNA inhibition. The researchers also found that any viral DNA within the mice came from the original virus injected into the liver rather than from a virus that had replicated — further proof that RNA inhibition prevents the virus from multiplying its numbers.
So far, RNA interference seems effective with no detectable downsides, Kay said. "We’ve worked on a gene therapeutic approach against viral hepatitis for about 10 years and this is the first thing we’ve done that really looks promising," he said.
Kay said the eventual goal is to treat people before they have life-threatening liver disease. "If you treat someone before this point, the liver has a remarkable ability to repair the damage," he said. Kay hopes to use the technique in human trials within the next two to three years.
Stanford researchers on the study include senior research fellows Anton McCaffrey, PhD, who was first author on the paper; and Hiroyuki Nakai, MD, PhD; and research assistants Kusum Pandey, Zan Huang and Hui Xu.
Stanford Report, June 11, 2003