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Spell-Checked Stem Cells Show Promise Against Liver Disease

12 October 2011 1:35 pm
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K. YUSA ET AL., NATURE (ADVANCED ONLINE EDITION) ©2011 MACMILLAN PUBLISHERS LTD.

Gene fix. Red cells in this slice of mouse liver are making a human protein called A1At.

Researchers have taken a step toward showing how stem cells might one day be used to help patients born with a deadly liver disease. The researchers corrected a DNA spelling error in patient skin cells that had been converted into so-called induced pluripotent stem (iPS) cells, then coaxed the cells to form liver cells that seemed to function normally in mice. The approach is still a long way from the clinic, but so far it seems safe.

Alpha 1-antitrypsin deficiency (A1ATD) involves a protein that is released by the liver into the blood and protects the lungs from inflammation. People born with two defective copies of the A1At gene make an improperly folded version of the protein that builds up in the liver. They develop emphysema and cirrhosis, and eventually they need a liver transplant to survive. The disease affects 1 in 2000 northern Europeans and about 100,000 people in the United States.

A group that included researchers at the Wellcome Trust Sanger Institute and the University of Cambridge, both in the United Kingdom, developed a possible treatment for A1ATD by first reprogramming a skin cell from a patient into iPS cells, which are embryonic-like cells that can develop into many tissue types. To these iPS cells they added DNA for proteins called zinc finger nucleases that snipped the cell's DNA at the defect in the A1At gene. They also gave the cells a correct copy of the A1At gene so that the cell's DNA repair machinery would use it as a template to heal the break. The scientists then used another technique to pull out the foreign DNA introduced during the gene repair, "kind of like putting in scaffolding, then taking it down," says mouse geneticist Allan Bradley of the Sanger Institute, who co-authored a paper about the work in Nature.

The point of all this was to create patient-matched iPS cells that had two corrected copies of the A1At gene but no other, unintentional DNA changes that might cause cancer. Using next-generation sequencing, the U.K. team showed that the iPS cells' genome was "clean"—the gene-editing methods had not added any new mutations. The researchers did detect 29 mutations in protein-coding DNA that apparently cropped up while the cells were being reprogrammed or growing in culture. But healthy people carry many mutations like these, and "we don't think it's going to be an issue," Bradley says.

The U.K. team then prompted the repaired iPS cells to differentiate into liver cells and showed that they cranked out the correct version of A1ATD. When injected into the livers of mice with the A1At mutation, the repaired cells appeared to make corrected protein for at least 5 weeks, and the mice did not develop tumors.

The fact that researchers were able to correct errors in iPS cells relatively easily suggests their method has promise for many diseases caused by a single mutation, says Juan Carlos Izpisúa Belmonte of the Salk Institute for Biological Studies in San Diego, California. He and others have recently used gene-editing tools to correct mutations in human iPS cells but haven't used the same combination of methods or done the same set of experiments to determine whether the iPS-derived cells are safe. Although the 29 new mutations seemed harmless in this case, every new iPS cell line will need to be analyzed carefully before it is used to treat a patient, Izpisúa Belmonte says. "The question is whether the mutations have a detrimental effect once the cells are differentiated and transplanted back in vivo, not whether they are present or not," he says.

The U.K. team isn't convinced that all the safety issues are resolved either. "Safety has to be paramount," says co-author David Lomas, a clinical researcher at the University of Cambridge. One idea the team is mulling is to implant in patients a bag of iPS-derived liver cells that are self-contained, so they can't seed tumors in a patient's tissues but can still produce A1ATD. If after several months the cells seemed safe, the researchers would move on to injecting them into a patient's liver, where they should eventually replace A1At-deficient cells.

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