Editing our DNA with Molecular Scissors
Scientists Are Building a Toolbox of Molecules that Could Cure Many Different Genetic Diseases
Scientists are making tiny scissors called TALENs that can cut and fix a broken gene in a cell. The technology isn’t ready for use in people yet but when it is, it could help us cure many different genetic diseases. As long as those diseases are caused by problems with a single gene.
So this new tool has the potential to help with diseases like sickle cell anemia and cystic fibrosis. These are diseases where a single gene is broken.
What this new tool probably can’t do is cure more complicated diseases like cancer where lots of genes are affected. That’s because scientists would have to make a different pair of scissors for each broken gene. We’d simply need too many different scissors swarming around in our cells.
This all sounds fantastic but we are at least 5 to 10 years away from seeing this technology being used in patients. Scientists first need to make sure that it works properly in cells in the lab, and that it is safe. Then they need to get the things into a patient’s cells. After that, doctors can finally begin clinical trials with a small number of patients. If those go well, we may see TALENs being used in many hospitals and clinics around the world.
If all goes according to plan, millions of people might be cured of their genetic disease. Not bad for a bit of cutting and pasting.
Editing Broken Genes
Sometimes when a gene is broken, it can cause a disease. So certain differences in the CFTR gene can lead to cystic fibrosis. And certain changes in the HBB gene can lead to sickle cell anemia.
An obvious way to cure diseases like this is to fix the broken gene. But it isn’t as easy as it sounds!
We can’t just search and replace mistakes in our DNA like with a Word document. But what we can do is make a sort of biological editing tool called a TALEN. This strange hybrid of bacterial genes can work with the cell’s own machinery (and a copy of the right DNA provided by the scientists) to fix a broken gene.
The first thing a TALEN needs to do is to find the broken gene in the patient’s DNA. This is an important first step because if it can’t find that one broken gene, then it could land on other genes in the cell. If it does, the patient will have lots of cuts in his or her DNA. This would be very bad.
Targeting only one spot in one single gene was by far the hardest part of making a TALEN. Why? Because humans have over 25,000 genes spread out over their six billion letters of DNA. Without the help of a plant bacterial gene called TAL, scientists could have struggled for years to discover a way to find this needle in a haystack.
A certain family of plant bacteria infects its host by injecting a protein, called a TAL, into the plant cell first. The instructions for making this TAL protein are found in the TAL gene.
These TAL proteins can recognize a unique DNA sequence of A’s ,C’s, G’s, and T’s. The TAL proteins use these DNA sequences to find and turn on genes that help the bacteria get inside the cell. Usually, this is bad news for the plant, but it’s good news for scientists (and maybe for people with genetic diseases).
Scientists have cracked the code that TAL proteins use to find the right gene. It turns out that TAL proteins are made up of different building blocks. Each building block recognizes one DNA letter – either an A, C, G, or T.
Using some molecular tricks, we can now mix and match these building blocks in the lab. The result is a TAL protein that can find and go to almost any gene that a scientist wants.
Now that scientists have a way to find the broken gene, they need to get their editing tool to cut the DNA where there is a mistake. For this they turned to a second bacterial gene.
Bacteria have genes called endonucleases designed to chop up any foreign, invading DNA. This primitive immune system helps protect bacteria from viruses. (Bacteria get sick too!)
To make a TALEN (as the editing tool is called), scientists combine the TAL that recognizes the broken gene with the endonuclease that will cut it. TAL + endonuclease = TALEN.
But they need to do more than cut the DNA. They need to repair it too.
So the next step is to paste in a working copy of the broken gene. A cell doesn’t “know” what a working gene looks like, so it can’t just add the right DNA letters to correct the mistake by itself. Luckily scientists can help it out by adding the right DNA sequence.
Once the TALEN makes a cut and the right DNA sequence is added, a cell does the rest of the work. Cells already can and do fix mistakes in their DNA through a process called homologous recombination. Scientists are simply hijacking a system that is already in place and making it do what they want.
The system has worked pretty well so far in the lab. The big question is whether or not it will work in people.
A video about TALENs
One Day TALENs Could Be Used To Cure Disease
So far, scientists have been able to use TALENs to edit genes in cells in the lab. They do this by designing TALENs to a gene that they want to edit. Then they add TALENs to the cells along with the new DNA that they want to paste in to the broken gene.
Editing genes in cells in a petri dish is one thing, but the next challenge is to use TALENs to cure human disease. Scientists are still working on solving this problem. And they have their work cut out for them.
The main issue is fixing the gene in a high percentage of cells. Getting TALENs into a cell is not easy. If the procedure is gentle, too few cells pick up the TALENs. But if the procedure is harsh, too many cells end up dying.
Other issues include making sure the cell with the fixed gene (or its descendants) stays around for the lifetime of the patient. And making sure that the gene editing is done at a beneficial time in the patient’s life.
Sickle cell disease (SCD) is a great candidate for TALEN treatment. Scientists have already created the necessary TALENs that can target and cut the HBB gene. Now they just need to get it into enough cells!
SCD happens when the one of the genes for making hemoglobin has one tiny mistake in its instructions. The mistake causes red blood cells (where hemoglobin lives) to sometimes go from a flat donut shape to a sickle shape. The round flat shape of normal red blood cells helps them fit into all of the tiny blood vessels all over your body.
The “sickled” cells in a SCD patient, though, have a much harder time squeezing through tiny spaces. This causes them to get clogged in small blood vessels leading to extreme pain and deadly problems like strokes.
Right now, there is no good cure for SCD. A small number of patients may get a bone marrow transplant from someone without the disease, but this is a risky procedure that doesn’t always work. Some people can’t find a compatible donor. And even when they do, a patient’s immune system sometimes will still reject the transplant.
Scientists hope that gene editing could be a cure for SCD some day. Since SCD affects the blood, if scientists can fix the gene in most of a patient’s bone marrow cells, then the patient will have a permanent source of fixed cells. This is because bone marrow cells are constantly making new blood cells to replace old dead ones.
The downside is that these cells are pretty fragile. Any current techniques used to get TALENs into cells would cause high levels of cell death. Scientists probably need to come up with a better way to get TALENs in before effectively treating SCD this way.
Here’s how it might work in the future. First doctors would remove some of the patient’s own bone marrow cells. Then, these cells would be handed to scientists so they could be treated with TALENs in the lab. Scientists would make sure that the HBB gene is fixed and then hand the fixed cells back to the doctor so they could be put back into the patient. The great thing about this strategy is that the chances of a patient’s immune system rejecting its own cells is pretty low.
Of course, SCD is also ideal for any other gene repair therapies out there. But none of these has proven to be widely successful yet.
Many thought gene therapy would be a cure, but this hasn’t worked well yet either. In gene therapy, a virus delivers a working gene to the cell. Unfortunately one (among many) problems with gene therapy is that the cell sees viral DNA and shuts it down. Another problem is that sometimes the virus triggers the cell to become cancerous. Gene editing with TALENs gets rid of viruses completely, so presumably a cell will be much healthier.
So maybe TALENs are the answer. Of course as we use them more, we may find other issues that need to be resolved too.
For example, scientists design TALENs to cut only once in a specific gene, but in reality, sometimes they cut more often than that and in more than one gene. Scientists still don’t know why this happens, but they are working on an answer.
These issues will be important to figure out before we can use TALENs in patients. But scientists are getting closer and closer to understanding how to use them safely and effectively in human cells.
By Dr. Stacey Wirt, Stanford University