U study finds key RNA link to disease

Liz Kohman

Two University researchers have found the missing piece of a puzzle they’ve been putting together for more than a decade.

Laura Ranum and John Day, associate professors and researchers in the Institute of Human Genetics, discovered an irregularity in the RNA on human chromosome 3. The alteration is the cause of type 2 myotonic dystrophy – a form of muscular dystrophy.

They said they hope their findings will lead to better diagnoses and eventually, a cure. Their findings will appear in today’s issue of the journal Science.

Myotonic dystrophy affects more than 30,000 Americans and is characterized by muscle stiffness and deterioration, a higher propensity to develop diabetes, heart trouble and hormonal problems. This unusual grouping of clinical features are symptoms of two types of myotonic dystrophy.

The discovery is unusual because the genetic alteration was found in the RNA, said Ranum.

“We think all the features common of these two diseases are caused by the expansion in the RNA,” she said.

Ranum and Day identified 100 people from more than 20 families for the study and collaborated with researchers studying families in Germany.

The disease is most common in people of Germanic descent. Ranum and Day said the source of the alteration could have been a mutation in one person’s genetic code.

If one parent has myotonic dystrophy, a child has a 50 percent chance of getting the disease.

Day said more people in Minnesota probably have myotonic dystrophy and haven’t been diagnosed. The study’s findings will allow doctors to test for myotonic dystrophy.

“Because the disorder can cause so many different kinds of effects, and the people with them end up seeing so many different kinds of physicians, they might just focus on one or another aspect and not really see the whole picture,” Day said. “Now to have a genetic test identify that they actually have this is going to help a lot in terms of making sure people get the right diagnosis.”

RNA was not thought to be a likely source of disease because it acts as the messenger between DNA and protein synthesis. Previously, scientists believed diseases came from genetic alterations expressed in proteins.

The irregularity found in chromosome 3 is similar to an alteration in chromosome 19 – the source of type 1 myotonic dystrophy.

In 1998, Ranum and Day discovered the location of the faulty genetic material on chromosome 3 after searching all 26 human chromosomes.

This finding followed a 1992
discovery of faulty genetic material on chromosome 19 made by other scientists.

The similar alterations in the RNA on both chromosomes
led scientists to believe RNA causes the disease.

Scientists had a different model explaining how the genetic alteration on chromosome 19 caused the disease but couldn’t explain how the alteration on chromosome 3 caused myotonic dystrophy type 2.

Ranum said the disease mechanism is unusual because the focus is on mutant RNA and not mutant proteins. This discovery has caused controversy in the scientific community.

“Some people believe that we have very strong evidence that it’s directly involved in the disease process. We believe we have strong evidence that it is involved in the disease process,” Ranum said. “But it’s hard (for some scientists) to really believe that this mutation in the RNA can really be responsible for the disease.”

The study was funded by the National Institutes of Health and the Muscular Dystrophy Association.

Sharon Hesterlee, director of research and development for the Muscular Dystrophy Association, said Ranum and Day’s findings are exciting.

“The most immediate effect will be better diagnostics, but at the same time it does help us focus our efforts to find a cure for the disease,” Hesterlee said.

She said researchers will now focus their efforts on removing the mutation from the RNA. “It changes our whole strategy for how we develop therapy,” she said.

Liz Kohman covers the Academic Health Center. She welcomes comments at [email protected]