Hemophilia Is Target of Therapy on Genome

Researchers using a new technique for editing the genome of living cells have shown that they can cure hemophilia in mice, at least in principle, with a couple of injections that carry out the “cut” and “paste” operations needed to insert a corrective gene. This is the first time this genome-editing technique has succeeded in a live animal. Along with other applications, like two AIDS treatments in preliminary stages, the new technique could be the decisive improvement that gives credibility to the long-struggling field of gene therapy.

“This may well revolutionize the field, but it won’t do so overnight,” said Dr. Katherine A. High of the Children’s Hospital of Philadelphia, the research team leader. “Any novel kind of therapeutic takes time to develop.” The essence of the technique is the molecular scissors custom-designed to cut the genome at a unique site. This allows the corrective gene to be inserted at the right place in a chromosome. In previous forms of gene therapy, corrective genes have been inserted into the genome at random sites, for lack of the ability to control where they go. This approach means they are not under their natural control systems and, worse, may be inserted in the middle of some other gene that they disrupt.

“There’s a huge interest in this,” said Dr. Mark A. Kay, a gene therapist at the Stanford University School of Medicine. The genome editing approach “could be game-changing in some applications,” he said. The technique depends on natural agents called zinc finger proteins whose role is to bind to specific sites on the genome and control adjacent genes. By mixing and matching the DNA of different natural zinc finger proteins, researchers can create artificial zinc fingers for any chosen target site on the genome. In their genome-editing role, the zinc fingers are attached to a DNA-cutting enzyme derived from a bacterium. When a pair of zinc finger proteins line up on opposing strands of DNA, their DNA cutters face each other and scissor the DNA apart.

The zinc finger technique has been developed by Sangamo BioSciences and by academic researchers who belong to the Zinc Finger Consortium. “We are fairly inundated with requests,” said Philip D. Gregory, Sangamo’s chief scientific officer. Sangamo designed the zinc finger pairs for Dr. High’s hemophilia project. But the fingers are designed to cut the human Factor 9 gene, not the mouse version, which has a different sequence of DNA units. So Dr. High genetically engineered a strain of hemophiliac mice that carry a mutated version of the human Factor 9 gene in place of their own.

After the cut-and-paste operation, the mice possessed a good working copy of human Factor 9, producing enough to make their blood clot much faster, and well enough to prevent hemophilia, Dr. High and her colleagues report in the journal Nature. To show that the new gene was stably incorporated into cells, they then cut out part of the mice’s livers. The liver regenerated from existing cells, retaining their ability to produce good copies of Factor 9.

Dr. High said it was too soon to try the technique in people, given that an adequate treatment for hemophilia already exists. She plans to test it next in dogs, which are a standard model for new hemophilia treatments. One of the possible problems with the technique is that the zinc fingers sometimes cut at sites other than the intended target site. Dr. High said that besides hemophilia, the zinc finger technique could address many other liver-based genetic diseases. Zinc fingers are being used in a different way in a treatment for AIDS, at present in early clinical trials. The fingers are used to disrupt a gene called CCR5, which makes the receptor used by the AIDS virus to gain access to cells. People with no CCR5 receptor are naturally immune to AIDS.

The hope is that patients will acquire the same immunity after their T cells are treated with zinc fingers and returned to the body. “We are very hopeful that zinc finger technologies will have a spectacular impact on gene therapy and genetic medicine in general,” Dr. Gregory said.

Source : http://www.nytimes.com