Gene Therapy Cures Phenylketonuria in Mice

ST. LOUIS, MISSOURI--A new kind of gene therapy can cure the metabolic disorder phenylketonuria in mice, researchers reported here this week at the annual meeting of the American Society of Gene Therapy. The method uses a viral protein rather than a virus itself to insert a corrective gene into a specific location in the mouse's DNA, making it a potentially safer way of delivering new genes.

Traditional gene therapy involves using a virus to deliver normal copies of genes into a subject's body. But because such genes integrate into a host's genome at random locations, the approach can disrupt normal genes, like those that guard against cancer. Such risks recently became clear when three patients in a gene therapy trial in France that had successfully treated 10 children with X-linked severe combined immunodeficiency disease later developed leukemia (ScienceNOW, 7 March).

Ideally, researchers would like to place a corrective gene only in a specific, safe location away from other genes. Geneticist Michele Calos's group at Stanford University School of Medicine in California has invented such a method using a type of enzyme called an integrase, which is used by viruses that infect bacteria and which latches onto a specific sequence of the host cell's DNA. Li Chen in Savio Woo's lab at Mount Sinai School of Medicine in New York City wondered if he could use a similar approach to treat mice with phenylketonuria--a disease that prevents people from making enough of the liver enzyme phenylalanine hydroxylase (PAH). As a result, the body cannot convert the amino acid phenylalanine to tyrosine, and the buildup of phenylalanine leads to severe mental retardation. Currently, the main treatment for phenylketonuria is a strict diet low in phenylalanine.

Chen made loops of bacterial DNA containing an integrase and a good copy of the PAH gene, and then injected these plasmids into a mouse's tail vein. After three injections, the mice made more PAH enzyme and their blood levels of phenylalanine fell to within the normal range for more than 6 weeks, Chen said. Another sign that it worked: the mice's fur turned from gray to black because they now had the tyrosine they needed to make melatonin.

The Stanford group has shown that bacterial integrases can be used to cure several diseases in animals, such as hemophilia, but Woo's team used a different integrase, which may work better in certain tissues, notes Barrie Carter of Targeted Genetics in Seattle, Washington. However, researchers first need to find more efficient ways of getting the plasmids into cells. "This one is not yet ready for prime time," says Woo.

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