Promising start of new human gene editing trials

Gene therapy has a long and sometimes difficult history. Many human genetic disorders can result from problems with a single gene, which makes them attractive targets for modification.But Someone died In a very early gene therapy trial, the entire field retreated considerably. And despite a much more cautious approach, the risk remains significant, as two people died during the trial. Occurred only this year..

But for researchers in the field and those suffering from genetic disorders, this week gives hope that the long-delayed promises in the field may eventually be fulfilled. At the Virtual Science Conference, a group presented the results of a large safety trial that allowed 50 of 52 patients to discontinue treatment for hemophilia. Another treatise describes the use of CRISPR gene editing and blood stem cell transplantation to successfully treat patients with sickle cell anemia or related disorders.

Recovery of coagulation

The hemophilia trial was typical of the earliest efforts in gene therapy. In this case, the disease is caused by a defect in a single gene, so providing a new copy to the cell corrects the problem. Also, the protein encoded by that gene circulates in the blood, so you don’t have to target a small cell population that may be difficult to access to fix the problem. Target a new copy of the gene to any cell. Anything that can export proteins into the bloodstream works.

In this case, the defective gene produces a protein called factor IX that is part of the cascade that allows coagulation. People with defective factor IX genes can receive infusions of purified versions, but they have to be repeated every other week and are quite expensive. Gene therapy promises to eliminate their need. The method used in the new study involves using a virus that carries the Factor IX gene to infect hepatocytes. The virus integrates into the genome of infected cells and provides a permanent copy of the functional factor IX gene.

by Conference presentation summary, A phase III trial enrolled 52 patients, 50 of whom completed treatment. By 6 months after virus injection, factor IX levels in these patients had risen to levels with a risk of bleeding similar to the lower limit of the general population. Its level is classified as very mild hemophilia and does not require treatment. Side effects were primarily related to the immune response to the virus treated with steroids.

Both patients who did not respond to treatment also had immune problems due to previous exposure to the virus used to insert the replacement gene. First, the virus caused a strong immune response during the transfusion of the virus and had to be stopped. The second had very high levels of antibodies that neutralized the virus. However, 40% of other patients were previously exposed to the virus, which did not cause any problems during the trial.

Anemia editing

The second trial of gene therapy was significantly smaller and the treatment was much more complex. It focused on two types of anemia in which the underlying mutations provide some protection from malaria: sickle cell anemia and β-thalassemia morphology. These alter the function of red blood cells and cause serious health problems. Beta thalassemia damages one of the hemoglobin genes, causing a serious underproduction. People with sickle cell disease produce polymer-forming hemoglobin, which causes the red blood cells to lose their shape.

Although it is possible to treat β-thalassemia with gene therapy similar to that for hemophilia, it does not necessarily work for sickle cell anemia, where hemoglobin morphology changes, simply by providing an alternative copy of the defective gene. not. One of the ideas being considered to treat these anemias is to reactivate the fetal hemoglobin gene. It has a high affinity for oxygen and is able to absorb oxygen from the adult placenta. But it stops within a few years of birth.

This shutdown is partially mediated by a protein called BCL11A. Therefore, in theory, elimination of BCL11A can reactivate the fetal hemoglobin gene. Unfortunately, it is not easy to get rid of it because it performs important functions in other cells.

To avoid this problem, the researchers behind the new study obtained blood stem cells from patients with β-thalassemia and sickle cell disease. They were then subjected to CRISPR gene editing to remove some of the DNA essential for activating BCL11A in red blood cells. As you might expect, it wasn’t completely efficient, but it reached a level where editing was performed on about 80% of the copies of BCL11A. And when these edited stem cells formed erythrocytes in culture, they produced high levels of fetal hemoglobin.

The actual clinical trials involved dangerous procedures. Blood stem cells were eliminated in two patients, one with β-thalassemia and the other with sickle cell disease. Gene-edited stem cells were then injected, allowing patients to use them to develop new blood supplies. This is a very aggressive procedure and requires extensive medical assistance. Both had serious events that required treatment during recovery. A Profile of one of the test participants It may help explain why someone puts it at risk.

Not perfect, but good

For both patients, it worked. In patients with β-thalassemia, fetal hemoglobin levels were approximately 30 milligrams per liter before treatment, but gradually increased to reach 1,300 grams per liter one year later. In patients with sickle cell disease, fetal hemoglobin accounted for half of her total one year after surgery. Correspondingly, the amount of sickle cell hemoglobin decreased. The latter patients experienced an average of 7 severe vascular events annually, 3 or more of which usually required hospitalization. She had only three since the surgery, all of which were definitely convalescent.

Only two patients have passed the year since the study began, while another eight patients have been on the same treatment for more than three months. Although not discussed in detail in this article, the results show that they are “almost in line with the findings of the two patients described here.”

“Missing”

There are several reasons why this procedure does not completely switch the patient to fetal hemoglobin. To get started, gene editing wasn’t 100 percent efficient. However, beyond that, its activity is regulated by factors other than the protein targeted here, and activation of it does not shut down other forms of hemoglobin. This is not a problem with β-thalassemia, as mutations only produce normal hemoglobin. However, that means that altered morphology continues to be produced in patients with sickle cell disease. An important factor is that there is a normal shape around it that is sufficient to prevent the formation of the hemoglobin polymer.

One of CRISPR’s major concerns is that its edits are not always directed to the correct sequence. There may be “off target” edits. In this case, researchers performed tests designed to identify off-target edits using cultured cells, but found nothing. That doesn’t mean that rare cases don’t end up being returned to the patient, but it makes it much less.

This does not mean that this particular approach to gene therapy is widely used (although the two companies behind it hope to do so). For one, many other approaches have already been approved for testing and, in some cases, have already been used in patients. And all of them require a lot of additional safety and effectiveness testing. And both procedures require adequate medical treatment so that they are never routine.

However, the results suggest that gene therapy has made some progress again after extensive and necessary pauses to help define and address safety issues. And in the meantime, biologists have developed many additional tools that could make it much more effective.

NEJM, 2020. DOI: 10.1056 / NEJMoa2031054 (((About DOI).