Safe, Uncontroversial Embryonic Stem Cells Closer to Reality

Researchers Make Embryonic Stem Cell-Like Cells from Adult Cells Without Using a Virus

by Dr. Barry Starr, Stanford University

March 5, 2009

Embryonic stem (ES) cells hold the promise to cure incurable diseases and to repair irreparable damage. But to be used in these ways, scientists need a steady, uncontroversial supply of ES cells. And the best treatments need the ES cells to come from the patient as well.

Over the past two years, scientists have made huge strides in doing both of these things. They have managed to create ES-like cells (called induced pluripotent stem or iPS cells) from the adult cells of patients. These iPS cells act like ES cells in every test scientists have thrown at them. But they are not perfect.

The methods used so far create cells that are prone to turning cancerous. Two new studies just out in Nature (see below) take a major step forward in eliminating this risk. And in creating an iPS cell that has no change to its DNA.

Turning Adult Cells into Embryonic Stem Cells

Back in June of 2007, a group of Japanese researchers used gene therapy to turn a mouse skin cell into an induced pluripotent (iPS) cell. Then in November of the same year, the same group plus one from Wisconsin managed to do the same thing with human cells. These studies broke the stem cell field wide open.

To convert adult cells into iPS cells, the researchers first put four different genes into a virus. They then infected a cell with this virus. The four genes went on to reprogram the cells’ DNA turning the cell into an iPS cell.

While this was hugely important, the cells and the procedure used were not perfect. First off, using a virus is always a bit risky. The viruses used in gene therapy insert themselves into the cell’s DNA. If they insert into the wrong place, then the cell can turn cancerous. This in fact happened in a gene therapy trial in France where 3/11 children ended up with leukemia.

Embryonic stem cells
can become any
other kind of cell.

A second problem with this viral approach is that the genes stay in the cell after it has turned into an iPS cell. These genes can cause cancer by themselves and one of them, myc, caused 20% of the mouse cells in the first study to turn cancerous.

Since these studies, researchers have been trying different starting cells to see if any of them need fewer genes to be converted into iPS cells. A study published in July of 2008 showed that if an adult stem cell was used instead of a skin cell, then only two genes were needed to change the cell.

This result is important but at the end of the day, the genes are still there. And they still used a virus to get the two genes into the cell.

More recently scientists have tried various non-viral ways to get these genes into the cell. All have turned out to be so inefficient that they are not useful. Until now.

The latest studies published in Nature (see below) show how to efficiently get genes into a mouse cell without using a virus. And how to get the genes back out again.

But the researchers managed only the first step in human cells—getting the genes in. The authors did not report getting the genes back out again. This problem will most likely be overcome as the approach they used to pull the genes out of the mouse cells should eventually work in human cells too.

PiggyBacking on DNA

The non-viral approach the researchers used was one involving a transposon. Transposons are bits of DNA that have a gene that lets them jump around a cell’s DNA. And they are very good at it. Studies have found that more than 40% of human DNA is old transposon DNA!

Researchers also know how to make a transposon jump out of a cell’s DNA permanently. This means that if researchers use a transposon to get the four genes in, then they should be able to remove them after the cell has turned into an iPS cell.

They chose a transposon from moths called piggyBac to do their experiments for two main reasons. First it works very well in many different mammalian cell types. Second, when this transposon jumps, it leaves no trace of itself behind. In other words, if the researchers can make this transposon jump out of the cell’s DNA, the DNA left behind will be unchanged.

This is different than most other transposons that tend to change the DNA where they land. Changed DNA can affect how nearby genes work which can cause problems like cancer. So it is very important for the genes to be removed cleanly from these iPS cells.

More Information

Making iPS Cells Using piggyBac DNA

Scientists could only remove
added genes from
mouse cells.

The researchers put the four genes that can convert a cell into an iPS cell into the piggyBac transposon. They then added this DNA to mouse cells and after anywhere from 12-24 days, some of the cells turned into iPS cells.

One study reported that 2.5% of the cells converted into iPS cells. This is very efficient even when compared to the previous viral approach.

These cells passed all of the usual ES tests. They made certain proteins that only ES cells can make. And they behaved like ES cells in a mouse embryo.

The researchers added these iPS cells to an early mouse embryo and let the embryo develop for 10.5 or 13.5 days. When they looked at these embryos, the researchers found that the iPS cells contributed to many different cell types and, in essence, acted like any other ES cell.

The researchers next removed the transposon and the genes it was carrying from 11 different iPS cells. They found that in 10 of the cases, the DNA where the genes had been was unchanged. In other words, the DNA in that area looked the same as before any genes had been added.

The researchers were also able to do some of this in human cells. They were able to turn human cells into iPS cells by adding the piggyBac transposon that contained the four genes. But they could not remove the genes from the human cells.

Future Work

The next step is to work out the conditions for removing the genes from human cells. If the researchers can pull this off, then they may have a nearly endless supply of safe, noncontroversial ES-like cells.

Of course they still need to be able to add genes to the cells and remove them afterward. Ultimately the best possible way to get iPS cells would be to simply add a chemical cocktail to adult cells to reprogram them into iPS cells. Researchers are currently working on this approach.

Content provided by the Department of Genetics, Stanford University.