A Step Towards Curing Type 1 Diabetes

Scientists have used gene therapy to make insulin-producing cells in a mouse's pancreas

by Dr. Barry Starr, Stanford University

September 5, 2008

A new study out in Nature shows how to turn one kind of pancreas cell into an insulin-producing beta cell. This research is an important step in finding a cure for Type 1 diabetes.

In Type 1 diabetes, a patient's body destroys his or her own beta cells in the pancreas. Now the patient's body can't make insulin anymore. Which is why people with Type 1 diabetes treat their illness by injecting insulin.

In this study the researchers turned one kind of pancreas cell, an exocrine cell, into a beta cell in the pancreas of a live mouse. They did this by using gene therapy to reprogram the cell.

The hope is that scientists might eventually be able to do this in people too. Around 15,000 new cases of Type 1 diabetes happen every year in the U.S. Right now there are only treatments. If this works in people, then it would be a cure.

Unfortunately the procedure is nowhere near ready for people yet. One problem, of course, is that mice aren't people. What worked in a mouse might not work in a person.

An even bigger problem is that the created beta cells are scattered here and there throughout the pancreas. Beta cells work best in clumps (as they exist naturally). So scientists will need to figure out how to get them to clump together.

But it is a great first step towards a cure. To understand in more detail what the researchers did, we'll first go over how cells are programmed in the first place. Then we'll talk about how the researchers reprogrammed some of the mouse's pancreas cells.

Programming a Cell

Cell type is
programmed with
transcription
factors.

Every cell has the same DNA. And in that DNA is the same set of genes. What makes one cell different from another is what genes are on and what genes are off. (And what genes were on and off in the past.)

This cell type programming can happen in a number of ways. But the most relevant way for this research is programming by transcription factors.

Transcription factors are proteins that work to turn genes on and off. Usually, they can turn many different genes on and off at once. A cell is programmed to be a certain cell type by the mix of transcription factors it has (or has had in the past).

In nature, many transcription factors program stem cells into final cell types. For example, certain transcription factors might work together to turn a blood stem cell into a red or white blood cell.

The authors of this study worked on changing one cell type into another by adding new transcription factors. They couldn't simply add transcription factors though. Transcription factors are proteins and proteins are hard to add to a cell and they disappear pretty quickly after they are added.

To get around these problems, the authors added transcription factor genes instead of the transcription factors themselves. Just like any other protein, the instructions for a transcription factor are found in its gene. So if a scientist puts a transcription factor gene into a cell in the right way, it will make the transcription factor for long enough to have an effect. Put the right ones in and a scientist can reprogram a cell.

Reprogramming a Cell with Gene Therapy

The first step in reprogramming an exocrine cell into an insulin producing cell is to find the right transcription factors for the job. This is not easy. There are over 2,000 transcription factor genes scattered throughout mouse (and human) DNA.

The researchers set out to find the right transcription factors through a process of elimination. First they eliminated any transcription factors that weren't in a pancreas at some point during development. Unfortunately, that left 1,100 transcription factors.

Next they focused on transcription factors present in beta cells and the cells that beta cells come from. (Remember, beta cells are the cells in the pancreas that make insulin.) They found 20 different ones.

The next step was to mutate each of these 20 transcription factor genes and see what happens. Nine of them had an effect on beta cells when they were mutated. So the researchers focused on these nine.

Next the researchers needed to figure out which cells of the pancreas to add these transcription factor genes to and how to get them there. They decided to add these genes to exocrine cells using a virus.

Exocrine and beta cells come from a common cell type. And exocrine cells can be made to make insulin in a lab dish. So exocrine cells were a natural candidate for this experiment.

Scientists often use viruses to
add genes to a cell.

The researchers wanted to find a way to add these genes to the mouse's exocrine cells and not its beta cells. They settled on using a type of cold virus called adenovirus. Adenovirus tends to infect exocrine cells but not beta cells.

A common way that scientists add genes to cells is with viruses. The idea is to take the dangerous genes out of the virus and add back the ones scientists are interested in. Then the virus infects a cell, adds its DNA including the gene of interest, and the cell makes the protein the scientist is interested in.

Because there isn't a lot of room in a virus' DNA, researchers usually put just one gene in. Which means these researchers made nine different viruses each with one of the transcription factor genes they were interested in.

Now they started injecting the virus into the mouse pancreas. First they injected all nine viruses. After a month, there were a few insulin producing cells in two of the three animals they used.

Next they wanted to narrow down which of the nine transcription factors were absolutely required. First they tried only eight at a time. This eliminated three of them.

They continued this process until they found the three key transcription factors—Ngn3, Pdx1, and Mafa. All three were required and they were potent. Around 20% of the infected cells went on to make insulin.

The researchers then did a number of experiments to show that the infected cells were originally exocrine cells. And that the new cells were very similar to beta cells and could make insulin.

Finally they were able to show that the transcription factor genes didn't have to stay on forever to have their effect. One of the big problems with gene therapy is that eventually the body recognizes the viral DNA as other and shuts it down. So the best gene therapies are the ones like this—the hit and run kinds.

These results are definitely promising (at least in mice). After three months, the cells are still making insulin. Hopefully they'll keep making insulin for the rest of their lives.

Content provided by the Department of Genetics, Stanford University.