Gene therapy has a long and sometimes difficult history. Plenty of human genetic disorders can be found in problems with the same gene, and that makes them a tempting target for improvement. But someone died in a very early gene-therapy trial, which set the whole area back significantly. And, despite a very cautious approach, the risks are still significant, as two deaths have occurred during one trial this year alone.
But for field researchers and those suffering from genetic diseases, this week offers some hope that the field’s long-delayed promise may finally be fulfilled. At the Virtual Scientific Conference, a group presented the results of a large safety trial in which 50 out of 52 patients were able to discontinue treatment for hemophilia. And a separate paper describing the use of CRISPR gene-acquisition and blood stem cell transplants for the successful treatment of patients with sickle-cell anemia or related disorders.
Restore the clot
Hemophilia trials were typical of early attempts at gene therapy. In this case, the disease is caused by a defect in the same gene, so providing the cells with a new copy will correct the problem. And, the protein encoded by that gene circulates in the blood, so you don’t have to target a small and potentially-difficult population of cells to modify things – target any cell to a new copy of the gene. Anything that can export protein into the bloodstream will work.
In this case, the defective gene produces a protein called factor IX, which is part of the cascade that enables clotting. People with the defective factor IX gene may be inspired by the pure version, but they have to repeat it every other week and it is very expensive. Gene therapy promises to eliminate their need. The method used in the new trial involved a virus that carries the factor IX gene to infect liver cells. The virus integrates into the genome of infected cells, providing them with a permanent copy of the functional factor IX gene.
According to the summary of the meeting presentation, 52 patients were enrolled in the Phase III trial, and 50 of them completed the treatment. Six months after exposure to the virus, the factor of these patients reached the ninth level where they were at risk of bleeding like the lower end of the general population. That level is classified as very mild hemophilia and does not require treatment. Side effects mostly include an immune response to the virus, which was 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. In one, the virus induced strong immunity during virus transfer, which had to be stopped. The other had very high levels of antibodies that neutralized the virus. But 40 percent of the other patients also had pre-exposure to the virus, and did not have difficulty during this trial.
Acquisition of anemias
The second trial of gene therapy was significantly less, and the treatment was much more complex. It focuses on two types of anemia where the underlying mutation provides little protection against malaria: a form of sickle-cell anemia and tha-thalassemia. This alters the function of red blood cells and causes serious health problems. β- Thalassemia damages a gene for hemoglobin, resulting in severe underproduction; People with sickle-cell disease produce hemoglobin which forms a polymer, resulting in red blood cells being missed.
While those working for hemophilia who are able to treat tha-thalassemia through gene therapy like this – provide a copy of the defective gene replacement – which will not work as required for sickle-cell anemia, producing an altered form of hemoglobin. . One of the considerations that has been considered for the treatment of this anemia is to reactivate the fetal hemoglobin gene. It has an affinity for oxygen, which allows it to pick up oxygen from the adult form in the placenta. But it closes within a few years of birth.
This shutdown is mediated by a protein called BCL11A. So, in theory, if you remove BCL11A, you can reactivate the fetal hemoglobin genes. Unfortunately, it is not easy to get rid of, as it performs essential functions in other cells.
To address this problem, the researchers behind the new work obtained blood stem cells from patients with blood-thalassemia and sickle-cell disease. This was then subjected to CRISPR gene acquisition, which removed a portion of the DNA that was needed to activate BCL11A in red blood cells. It wasn’t as efficient as you’d expect, but it did reach the level where about 80 percent of the BCL 11’s were edited. And when red blood cells were formed in culture by these edited stem cells, they produced elevated levels of fetal hemoglobin.
The actual medical trial involved a risky procedure: the blood stem cells of two patients, one with thalassemia, one with sickle-cell disease, were removed. Gene-edited stem cells were then transplanted, allowing patients to develop a new blood supply with their help. This is a very invasive procedure and requires extensive medical attention; Both had serious events that required treatment during recovery. The profile of one of the trial participants can help explain why someone might risk it.
Not perfect, but good
For both patients, it worked. For a Tha-thalassemia patient, fetal hemoglobin levels were about 30 mg per liter before the procedure and gradually increased to 1,300 mg per liter after one year. For the sickle-cell patient, the fetal hemoglobin was about half of its total in one year after processing. Sickle-cell hemoglobin levels decreased accordingly. The subsequent patient averaged seven serious vascular events a year, with more than three usually requiring hospitalization. Since the process, she has only three, all of them were arguing from her recovery period.
Only two patients passed the year figure after the trial began, while the other eight patients crossed the three-month timeline after undergoing the same treatment. While the paper does not go into details, it suggests that the results are “broadly consistent with the findings of the two patients described here.”
“Off target”
There are many reasons why this procedure does not completely change patients’ fetal hemoglobin. For starters, gene acquisition was not 100 percent efficient. But beyond that, its activity here is controlled by factors other than the target protein, and activating it does not stop other forms of hemoglobin. This is not an issue in ss-thalassemia, as the mutation only produces a normal hemoglobin. But that does not mean that the altered form continues to be produced in sickle-cell patients. The main factor is that there is a sufficient amount of surrounding normal forms to interfere with the formation of hemoglobin polymers.
A major concern of CRISPR is that its acquisition does not always direct the correct sequence; There are sometimes “off target” edits. In this case, the researchers performed tests designed to identify -f-target edits using cultured cells and found none. This does not mean that rare cases are returned to patients, but it does make them less likely.
None of this is to say that this particular approach to gene therapy will see widespread use (although you can be sure that the two companies behind it hope that is the case). For one, a number of other approaches have already been approved for testing and, in some cases, are already being used in patients. And they all need a lot of extra safety and effectiveness testing. And both procedures require adequate medical assistance that will never be routine.
But the results suggest that, after extensive and necessary barriers to determining and addressing safety issues, gene therapy is again seeing little progress. And, in the intervening years, biologists have developed many additional tools that have the potential to make it more effective.
NEJM, 2020. DOI: 10.1056 / NEJMOA 2031054 (about DOI).
