By Susan Young of Nature magazine
A team of researchers has corrected a faulty gene in induced pluripotent stem (iPS) cells derived from skin cells of people with an inherited metabolic liver disease. The researchers then developed the stem cells into something resembling liver cells. Their work is published today in Nature.
Alpha-1-antitrypsin deficiency (A1ATD) is the most common genetic disease of the liver, and is caused by a single nucleotide change in the gene that codes for alpha-1-antitrypsin (A1AT), an enzyme inhibitor that normally protects bodily tissues. The condition can result in cirrhosis of liver, which can be treated only with a liver transplant, and in an increased risk of lung cancer and emphysema. A genetic cure for the disease would require completely replacing the mutated gene because any remaining dysfunctional protein would continue to form damaging aggregates. "You can't just put in a normal copy because that's not sufficient to change the disease," says Allan Bradley, a geneticist at the Wellcome Trust Sanger Institute in Hinxton, UK, who is a co-author of the latest study.
To correct the mutation, Bradley and his colleagues used an engineered molecule called a zinc-finger nuclease to find and cleave the faulty A1AT gene in iPS cells derived from skin cells of people with A1ATD. They used a self-inserting DNA molecule called piggyBac to replace the damaged portion.
The researchers then stimulated the gene-corrected iPS cells to differentiate into cells that exhibited some traits of hepatocytes, the liver cells most affected by A1ATD. They transplanted the hepatocyte-like cells into mice; 14 days later, some of the corrected cells had integrated into the rodent liver and were able to produce human A1AT.
Although the corrected cells possessed several features of hepatocytes, it is unclear whether they would be able to act as fully mature liver cells and eventually repopulate the entire damaged organ. No one has yet been able to reprogram iPS cells into fully mature cells of any type, be they hepatocytes, cardiac cells or neurons, says Markus Grompe, who studies liver stem cells at the Oregon Health and Science University in Portland. "No one can make the real lookalike for any cell type that I'm aware of," he says.
Stem cells cultured in labs are known to build up mutations (see Gene defects plague stem-cell lines), so, to check that their changes weren't making things worse, the researchers sequenced the genome of one of the corrected iPS cell lines and compared it to the genomes of the parental skin cells and the uncorrected iPS cells. Around two dozen mutations had arisen in the uncorrected iPS cell compared with the parental skin cell, but there were only a few extra point mutations in the corrected cell lines.
"The genetic correction doesn't increase the number of genetic anomalies that you can find in iPS cells," says Ludovic Vallier, a stem-cell biologist at the University of Cambridge, UK, and one of the study's co-authors. Nevertheless, Vallier is concerned about the number of changes in the reprogrammed cell lines. Before the cells can be developed into a clinical therapy, researchers must understand the biological consequences of these mutations, he says.
Such sequencing should become standard in the field, says Stephen Duncan, a stem-cell biologist at the Medical College of Wisconsin in Milwaukee. Given discoveries about these cells in culture, "we have to ensure that when one takes on gene-replacement therapy in the context of iPS cells, very considerable effort is made to make sure the cells are normal", he says.
This article is reproduced with permission from the magazine Nature. The article was first published on October 12, 2011.