Personalized Stem Cells Treat Sickle Cell Anemia in Mice

Scientists Convert a Mouse's Skin Cell to a Stem Cell and Treat its Sickle Cell Anemia

by Dr. Barry Starr, Stanford University

December 14, 2007

Stem cell research moves fast! A few weeks ago the news was that scientists had created human embryonic stem (ES) cells without destroying an embryo. Now a group of scientists has used this type of cell to treat and possibly cure sickle cell anemia in mice.

But don't look for a cure in humans this year. The procedure works pretty well in mice but some of the steps haven't been optimized for people yet.

Some of the steps also come with the unacceptable risk of cancer. This will need to be worked out before this research can be translated to people. But it is definitely a huge first step towards curing sickle cell anemia.

The Promise of Personalized Embryonic Stem Cells

Stem cells can divide
forever and turn into
any other kind of cell.

Embryonic stem (ES) cells have the potential for treating and even curing many diseases that are currently incurable. This potential comes from the fact that ES cells can make new copies of themselves forever. And because they can turn into any other kind of cell.

This makes ES cells of obvious use in diseases where cells die off or are injured and the body can't replace them. Scientists would grow up ES cells in the lab and turn them into whatever cell the patient needs. The new cells would then be put into the patient to replace the missing or damaged cells.

Of course any treatment will be much trickier than this in practice. But this will be the general idea in replacing dead neurons in Parkinson's patients. Or pancreatic cells in diabetes patients. Or nerve cells in patients with spinal cord injuries.

ES cells might also be used to cure genetic blood diseases. In fact, these may be one of the first applications as we already know how to get new stem cells into a patient to make new blood. It is called a bone marrow transplant.

The idea would be to replace the patient's blood stem cells with new blood stem cells created from ES cells. Now the patient would make blood free of the genetic disease. This approach could work to cure diseases like hemophilia or sickle cell anemia.

A huge problem with any treatments that uses ES cells is rejection. ES cells are usually taken from embryos and then transplanted into the patient. Like any tissue or organ transplant, the body will try to reject the tissue.

But if the ES cells could come from the patient, then there would be little or no risk of rejection. These kinds of ES cells are called personalized ES cells.

The tricky part here is getting embryonic cells from a patient who is not an embryo. One option is to clone the patient. Few people are thrilled with this option for many obvious reasons.

The other option is to turn one of the patient's cells into an ES cell. Now the ES cell would come from the patient and not be rejected. Scientists recently worked out the technology for doing this.

But no one really knew if cells made this way could be used to treat illnesses. Until now. A group of researchers showed that these cells can be used to successfully treat sickle cell anemia in a mouse.

More Information

The future of stem cell research

Using a Skin Cell to Cure Sickle Cell Anemia

Researchers focused on curing sickle cell anemia because the procedures for replacing blood stem cells are well worked out. They simply need to do a bone marrow transplant.

The researchers used a mouse's own skin cell to cure it of sickle cell anemia. Here's a brief version of how they did it:

1) Turn a skin cell into an ES cell
2) Fix the gene that causes sickle cell anemia
3) Turn the ES cell into a blood stem cell
4) Perform a bone marrow transplant

This is a very difficult, time consuming process but it has worked so far in curing the mouse's sickle cell anemia. Scientists are continuing to watch the mouse for any signs that the disease might return. Or that cancer might develop. After 12 weeks, the mouse still looks disease-free.

So the researchers have apparently cured a sickle cell mouse using ES cells. A key point here is that the researchers used ethically pure cells—no one was cloned and no embryos were destroyed to get the ES cells. Hopefully new findings with these kinds of cells will keep coming at this furious pace.

Additional Information

For anyone interested, here are the four steps in more detail (along with the researchers' results):

1) Turning a skin cell into an embryonic stem (ES) cell

The first step in all of this was to get an ES cell from the mouse. They used a mouse that had the human form of sickle cell anemia.

They did this the way other scientists have done it—by adding four genes to a skin cell. After adding these genes to mouse skin cells, they isolated 24 that acted like ES cells. They chose one at random to use for the rest of the procedure.

A problem with ES cells made this way is that they can sometimes grow into a cancer. To get around this problem, the researchers removed one of the genes that they added back—the myc gene. Previous studies had shown that myc was the most likely culprit in causing cancer.

It is not easy to remove a gene but they set up the system so that they could remove myc with a Cre recombinase. Basically, they added a gene that makes a protein (Cre recombinase) that chopped the gene out. This worked in one of the ten cells they tried it on.

So now they have an ES cell that is less likely to grow into a cancer and are ready to move onto the next step.

Scientists fixed the mistake
in the mouse's Hb gene.

2) Fixing the hemoglobin (Hb) gene

Sickle cell anemia is caused by a single change in the Hb gene. To cure the disease, this "mistake" needed to be fixed.

Basically they added DNA that contained a corrected version of the Hb gene to the cell. The cell then used homologous recombination to replace the sickle cell version of the Hb gene with a version that does not cause the disease. Click here to learn more about how this works.

This procedure is very inefficient. Out of 72 possible cells, the gene was fixed in only one. And only one of the two copies of Hb was corrected. (Remember mice and people have two copies of most of their genes.)

What this means is that both the sickle cell and the normal versions of the Hb gene were present in the cell. In essence, any "cured" mouse would go from one that had sickle cell anemia to one that was a carrier of sickle cell anemia.

Before accomplishing this, no one was sure how well homologous recombination would work in this type of converted ES cell. The researchers showed that it worked as well as any ES cell.

So now they have a personalized ES cell that has a corrected Hb gene.

3) Turning the ES cell into a blood stem cell

To do a bone marrow transplant, doctors need blood (or hematopoietic) stem cells not ES cells. So the next step was to turn the ES cell into a hematopoietic stem cell.

The researchers did this by adding a gene called HoxB4 to the cell using a virus. Previous work had shown that adding this gene could turn an ES cell into a blood stem cell. And it worked here too.

Using a virus like this introduces the risk that the cell could become cancerous. This is because scientists can't yet control where the virus lands in the cell's DNA. If it lands in the wrong place, it can turn on an oncogene and cause cancer.

If this sort of research is to progress into humans, this step will have to be changed. The risk of cancer is too high.

Now the researchers have a hematopoietic stem cell with a corrected Hb gene.

4) Bone marrow transplant

The final step is to destroy the bone marrow of the original sickle cell mouse and to add back the new cells. The researchers did this and the stem cells successfully took.

When the researchers studied the mice, they found that they no longer had sickle cell symptoms. Their blood type was more similar to a carrier. A carrier does not normally have the symptoms.

Content provided by the Department of Genetics, Stanford University.