January 25, 2008

Therapy’s use in healing genetic disorder studied

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James Patton, Ph.D., and colleagues are studying a new gene therapy’s ability to treat stunted growth in animal models. (photo by Daniel Dubois)

Therapy’s use in healing genetic disorder studied

John Phillips, M.D.

John Phillips, M.D.

A team of Vanderbilt researchers has demonstrated for the first time that a new type of gene therapy, called RNA interference, can heal a genetic disorder in a live animal.

The study, in the February issue of Endocrinology, shows that RNA interference can “rescue” growth deficiency in a mouse strain genetically engineered to express a defective human growth hormone.

When the gene that produces the defective human growth hormone is inserted into the mouse's genome, it stunts the mouse's growth.

But when a small snippet of RNA that interferes with the hormone's production is also added, the mouse is restored to normal.

“It has been very satisfying to figure out the underlying cause of this genetic disorder and then identify a way to prevent it,” said John Phillips, M.D., professor of Pediatrics and director of the Division of Pediatric Genetics, who collaborated with James G. Patton, Ph.D., professor of Biological Sciences, and graduate students Nikki Shariat and Robin Ryther.

Growth hormone deficiency has been estimated to occur in between one in 4,000 to 10,000 children. It has a number of different causes, but one that is genetically inherited is called Isolated Growth Hormone Deficiency type II (IGHD-II).

Children with IGHD-II appear fairly normal at birth but do not gain weight or grow as fast as they should, and their bones do not mature properly. The current treatment consists of daily injections of growth hormone until the patients reach adult height.

Not only is this treatment extremely expensive, it also fails to correct the underlying source of the problem: deterioration and death of cells in the pituitary gland that produce growth hormone. As a result, this single hormone deficiency can develop into multi-hormonal deficiency over time.

IGHD-II is a “dominant negative” disorder, caused by a defective form of human growth hormone that not only can't stimulate growth itself, but also blocks the action of normal growth hormone.

“A normal person has a very small amount of this defective hormone — about 1 percent — but people in families with IGHD-II produce 10 percent to 20 percent to 50 percent. And the more they make the slower they grow,” says Patton.

In 2003, co-author Iain Robinson at the National Institute for Medical Research in London created a transgenic mouse expressing the defective human growth hormone gene to mimic growth hormone deficiency.

Although the altered mice still contained the mouse growth hormone genes, he found that high levels of the defective human growth hormone not only stunted their growth, but actually killed the pituitary cells that produce growth hormone.

Progress in RNA interference research gave Patton and Phillips an idea for a way to correct this disorder.

In the last 15 years, scientists have realized that short pieces of double-stranded RNA, called silencing-RNA, regulate gene expression. This has created an opportunity for developing highly targeted therapies for genetic and viral diseases.

“To the best of our knowledge, this is the first time it has been used to correct a dominant negative disorder in a living animal,” says Patton.

Patton and colleagues created a specific silencing-RNA designed to bind uniquely with the messenger-RNA that codes for the defective growth hormone.

“You might call this the 'if you don't like the message, kill the messenger' approach,” Phillips quips.

Having created the special silencing-RNA, the next problem was how to deliver it to the pituitary gland, which, in the case of the mouse, is the size of a grain of uncooked rice and is located at the base of the brain. The researchers created a second strain of mouse which carried the special silencing-RNA and mated them with the growth deficient strain.

Their offspring have both the genetic defect that produces the defective growth hormone and the silencing-RNA that should inhibit its production, allowing the mouse growth hormone to act.

The experiment was successful. The offspring grew normally and showed no defects in their pituitaries.

Now the researchers are investigating ways to deliver their silencing-RNA to the pituitary gland that would be suitable for treating humans. The cells that produce growth hormone have special receptors that signal the cells to release their stocks of growth hormone.

If they can figure out a way to attach the silencing-RNAs to a compound that binds to this receptor, they should be able to deliver them to the cells where they can interfere with the activity of the defective growth hormone.