The CRISPR-Cas9 system can be used to edit out gene mutations and correct diseases. It has also been used to tailor make a number of different experimental animals that express exactly the DNA sequence scientists want. This is done by injecting the cells of developing embryos with the Cas9 enzyme that cuts DNA, along with an RNA guide that tells the enzyme where to cut the DNA, and a corrected DNA sequence which will be copied into the cut DNA. All these ingredients work together to edit the DNA sequence to whatever scientists want. Scientists have even used CRISPR to prevent muscular dystrophy in mice.
It is relatively easy to use CRISPR in embryos because they are transparent and easy to manipulate under a microscope. The big problem with making the jump from editing the DNA of embryos to editing the genes of adult animals is getting the gene-editing ingredients into adult cells. We are not transparent and even if we do surgery or insert a camera-tipped catheter probe into the body to find the diseased tissue, it is not practical to inject every single one of our millions of cells.
The thing is, to cure many diseases, we don’t have to inject every single diseased cell. We just need to cure enough cells to reach a baseline function that we label ‘healthy’.
In a recent study, scientists used the CRISPR-Cas 9 system to cure mice with the liver disease Hereditary Tyrosinema Type 1. This disease is caused by a mutation in the Fah gene. The Fah gene makes the enzyme fumarylacetoacetate hydrolase in liver cells. This enzyme controls the last chemical reaction in the pathway that breaks down tyrosine. Tyrosine is one of 22 amino acids that are the building blocks of proteins. Proteins in our cells are constantly being recycled. Old, damaged ones are removed and shiny new ones replace them. Part of the protein removal process is to break up the proteins into their individual components, amino acids. These amino acids then need to be broken down into something that the cell can use to make energy. The energy is then used to make new amino acids and assemble them into new proteins. The circle of life.
Patients with Hereditary Tyrosinemia Type 1 can’t completely break down tyrosine. This means there is a build up of toxic by products in the cells of these patients. This causes liver and kidney problems and mental retardation. Up until now there has been no good treatment for this disease and it is often fatal. Treatment options are a low protein diet, a liver transplant and the drug nitisone. Nitisone blocks a different enzyme that works earlier in the tyrosine breakdown process. Blocking this enzyme stops the buildup of toxic by products that happens when tyrosine is only partially broken down. Nitisone can alleviate some of the symptoms of Hereditary Tyrosinema but it has some nasty side effects including: abdominal pain and bloating; headache; vomiting; weakness; loss of appetite and weight loss.
Mice with the same Fah mutation that causes Hereditary Tyrosinemia in humans develop significant liver damage, lose weight and die if they are not treated with nitisone. In this study, adult Fah mutant mice were injected with copies of the healthy Fah DNA sequence, guide RNA and Cas9. This resulted in normal Fah protein being expressed in 1/250 liver cells in treated mice. Because these cells were much healthier than the cells that still expressed the mutated Fah, they divided and produced more daughter cells. This rapidly increased the percentage of healthy cells in the livers of the treated mice, up to 33% after 30 days. The treated mice also had significantly less liver damage than untreated mice.
One of the reasons this experiment worked was because the genetic disease investigated was in the liver. The liver filters all blood. Therefore anything injected into the blood will affect the liver. In this study the CRISPR ingredients were injected into the blood via the tail vein. The rate of successful DNA editing in this experiment was very low: 1/250. If they were trying to edit genes in another organ like the heart, or worse the brain which hides behind the blood brain barrier, the uptake would have been much lower and this approach may not have worked. Also the liver is a very regenerative organ with a high rate of cellular division and reproduction. Many other organs like the heart and kidney have an extremely low level of cell turnover. So it would have taken much longer to increase the percentage of healthy cells in these organs.
Still it is a very encouraging result that proves that gene editing can treat disease in adult animals.