Saved from the light

Scientists use gene therapy to cure day blindness in mice

by Dr. Barry Starr

Scientists have used gene therapy to treat a severe form of vision problem in mice. Basically the mice can now see because scientists added a gene to their eyes. Maybe those three mice would've escaped the farmer's wife with this.

The mice had something called congenital achromatopsia. This is just a fancy way of saying their cones don't work.

As you probably know, a big part of our eyesight comes from rods and cones. The rods help us see at night. The cones are useful in the day.

The cones also let us see more clearly. And we need them to see color too.

Some people are born with cones that don't work. If some of them don't work (or aren't there), you get common forms of color blindness like the red-green kind. But the problems are much more severe for people who are born with no working cones.

These people (and mice) are nearly blind during the day. And everything is in shades of gray.

This kind of vision problem is not common in most populations. Something like 1 in 20,000 or 1 in 40,000 people are born with it. But congenital achromatopsia is much more common in at least one isolated community.

Around 4-10% of the Pingelapese people of the eastern Caroline Islands in the Pacific suffer from this condition. Imagine what a boon this would be to their society. Assuming, of course, that the cure works for their slightly different form of achromatopsia.

Day blindness

Scientists created a virus
that cured mice of
day blindness.

As I said, people with congenital achromatopsia have cones that don't work. The light hits the cones and the cones don't send the information to the brain.

There are a number of genes involved in sending this information. If any of them don't work right, then the whole system breaks down.

Scientists have so far found three genes that when broken are known to cause congenital achromatopsia. The mice had a broken copy of one of these genes—GNAT2. The Pingelapese have a broken version of a different gene—CNGB3.

To cure the mice, the scientists put a working copy of the GNAT2 gene into their eyes. The working copy rescued their vision so that they could now see.

There were a number of things the scientists needed to do to get this to work. First, they needed to know what exactly was wrong with the mice. Since they engineered the mice to have a broken GNAT2 gene, that was easy.

Next they needed to have a working copy of the GNAT2 gene. This is easy too. There are lots of ways to get a gene copy that works.

Now it gets tricky. They need for the gene to work in the eye. This means they need to get the gene into the eye's DNA. And have it work and keep working once it is there.

The way they got the gene into the eye's DNA was with a virus. Many viruses work by sticking themselves into a cell's DNA. What the scientists did was take out some of the nonessential virus DNA and put in the GNAT2 gene.

Unfortunately, this isn't enough. A gene by itself will be ignored by the cell. A gene gets noticed because of the DNA around it.

So the scientists needed some of this DNA too. But it can be hard to figure out what DNA around the gene is important.

Scientists used a piece of human DNA that works in people's eyes. They showed that this DNA worked in mouse eyes too.

So the final virus had the GNAT2 gene and the fetching piece of human DNA. The scientists injected billions of these viruses into the mouse's eyes. The virus then inserted itself into the DNA. Then the gene turned on and 19 out of 21 mice could see after 1-2 months.

And 18 of them could still see 6 months later. This is important because a big problem with gene therapy is that the effects tend to wear off over time. Cells don't like viral DNA and have ways of shutting it off.

This is a wonderful defense against bad viruses. But a real pain when it comes to gene therapy. We'll have to wait and see if the gene stays on in these mice.

Down the road we'll also have to see if the virus works with the CNGB3 gene in mice with broken CNGB3 genes. And then see if both viruses work in people. Just think, if all of this works out, many of the Pingelapese people may be able to finally see how beautiful their island is.

More Information

Why not just fix the gene?

Fixing a gene
in a chromosome
is hard.

With all of the problems with gene therapy, why not just go in and fix the broken gene the mice (and people) already have? Because it is an incredibly difficult thing to do.

Let's focus on the Pingelapese with congenital achromatopsia. Many of them have a single letter change on chromosome 8—a C to a T. So what we need to do is change that T back into a C. Sounds simple enough.

In fact, this isn't too hard to do in the lab with the just the gene. But it gets much trickier in a cell. Right now it is essentially impossible.

The reason for the difference between the lab and the cell has to do with how much DNA we're dealing with. And how the DNA is packaged.

In the cell, we want to change one spot on chromosome 8. Chromosome 8 is 155,000,000 letters long. The CNGB3 gene is pretty close to the middle of the chromosome.

It is important to realize that this isn't like a very long book or Word document where you can just Control F and then edit. This is an inches long invisibly thin molecule with 155 million letters that we can't see.

Not only that, but this thread is wrapped around millions of little spools all trapped inside the nucleus of the cell. When we try to fix it, we can't unravel it all and splice in the new letters, like type setting or old-fashioned film editing.

We need to break chromosome 8 at just the right place and make the right fix without messing up anything else. Inside the nucleus, without being able to really see the letters. And we have to do it with all of the other chromosomes all packed into the nucleus too.

This is a truly daunting task. We can't do it alone—we need the help of the cell's own machinery. We haven't yet been able to pull this off reliably.

The most promising work so far is to send in a guided pair of scissors for the cell's machinery to use to fix the gene (see "How to fix a gene" below). Even though there have been some success stories, this procedure is not yet ready for primetime. And who knows if it ever will be.

No, we'll need to stick with gene therapy for the time being. We can't fix broken genes but we can add back a working copy.

If we can get the gene to stay on in people, think of all of those people that will be helped. I have focused on the Pingelapese Islanders but there are many other people who can be helped too.

Even though the condition is rare, a lot of people in the world suffer from it. If we take the 1 in 40,000 number, we're looking at 175,000 people who will be able to see during the day. These lucky folks may one day be able to see because of a virus.

More Information

Content provided by the Department of Genetics, Stanford University.