Can gene therapy be ＂competent＂ for rare diseases when it encounters setbacks？
Photo courtesy of vision china
Gene therapy can "correct" the mutant genes that cause diseases, thus bringing hope for the patients suffering from congenital genetic diseases. But recently, in a clinical trial of gene therapy conducted by a biotechnology company in the United States, two children who participated in the trial died unfortunately.
After decades of twists and turns, gene therapy has brought new hope for the treatment of major human diseases, especially many genetic defects. However, gene therapy has recently suffered setbacks.
Recently, an American biotechnology company (Audentes Therapeutics) issued a letter saying that among the 17 children with neuromuscular rare diseases who participated in the company’s "AT132" gene therapy, two children died after receiving high-dose gene therapy.
The key research of this gene therapy began in 2017, aiming at treating a fatal disease caused by a single gene mutation — — X chromosome linked myotubular myopathy. The treatment was originally intended to be submitted this year and expected to be approved by the US Food and Drug Administration by the end of the year, but the plan has now been shelved.
Some children died while receiving gene therapy 20 years ago. What are the differences between the gene therapy at that time and this gene therapy? After decades of development, what improvements have been made in gene therapy and what problems are still facing? In this regard, the Science and Technology Daily reporter interviewed relevant experts.
"What is missing makes up for what is missing" and "corrects" mutated genes.
In human body, there is a gene named "MTM1", which is used for the development and maintenance of muscle cells by expressing a protein called "Myocardin". X chromosome linked myotubular myopathy is caused by the mutation of "MTM1" gene.
X chromosome linked myotubular myopathy mainly affects skeletal muscle, and its symptoms are muscle weakness and hypotonia. Muscle lesions will damage the patient’s ability to sit, stand and walk, even cause difficulty in eating and breathing, and may also lead to ophthalmoplegia, facial muscle weakness and muscle reflex disappearance.
At present, the main treatments for the disease include physical therapy and the use of support equipment such as ventilators. Studies have shown that the chemical drug "pyridostigmine" may be beneficial to improve clinical symptoms, but no formal clinical trial has been carried out. Generally speaking, there is no effective treatment for this disease at present.
To this end, researchers began to explore the use of gene therapy to "correct" mutated genes. "Based on ‘ What is missing is what is needed ’ The gene therapy uses adeno-associated virus (AAV) as a vector to transfer normal ‘ MTM1’ Genes are introduced into muscle cells to produce myotubes with normal function, thus repairing the function of damaged muscle cells. " Qiu Zilong, a researcher at the Center for Excellence and Innovation in Brain Intelligence of China Academy of Sciences, said that this is a "permanent cure" treatment method, and under ideal conditions, it can even achieve "one treatment and lifelong benefit".
How to realize this "correction" process? "The whole process and ‘ Drip ’ Very similar. " Qiu Zilong pointed out that the researchers injected the adeno-associated virus loaded with normal "MTM1" gene into the patient’s blood, delivered the adeno-associated virus vector to muscle cells through blood circulation, and produced "myosin protein" with normal function in it.
Before these children participated in clinical trials, the gene therapy had been pre-clinical studied, including verifying the safety and effectiveness of the therapy in cell and mouse disease models cultured in vitro, and the therapy was generally safe.
But in order to improve the therapeutic effect, the researchers tested a higher dose of gene therapy. "High doses are particularly important for neuromuscular diseases, because more copies of genes are needed to reach the targeted tissues through blood circulation." Dr. Cheng Cheng, from the School of Bioengineering, East China University of Science and Technology, said that the dose used in this gene research is the highest dose in gene therapy: 300 trillion copies of genome per kilogram of body weight.
The upgraded transmission carrier is safer.
In recent years, gene therapy has developed rapidly. The event that the "brake button" was also pressed occurred in 1999. An 18-year-old American patient named Jesse Kissinger died in a clinical trial of gene therapy with adenovirus as the carrier.
At that time, researchers used adenovirus as a vector to insert genes into patients’ cells to replace those parts that were lost or dysfunctional and caused diseases. A few days later, Jesse died of multiple organ failure due to severe immune response, which poured cold water on those who were initially optimistic about gene therapy.
After the tragedy of Jesse Kissinger’s death due to gene therapy, the researchers reduced the size of the laboratory and focused on finding safer virus vectors. Finally, they discovered and promoted the use of adeno-associated viruses.
Adeno-associated virus (AAV) is a kind of single-stranded DNA deficient virus with the simplest structure found at present. Adeno-associated virus gene therapy uses adeno-associated virus as a carrier to deliver therapeutic genes to specific tissues and organs, and these therapeutic genes stably exist and express in the form of free bodies in these non-splinter cell, thus effectively treating monogenic genetic diseases.
At present, adeno-associated virus (AAV) is the mainstream carrier platform for gene therapy, and there are also adenovirus and lentivirus gene therapy. Qiu Zilong pointed out that compared with adenovirus, adeno-associated virus vector has the advantages of high safety, low immunogenicity, longer curative effect and easy modification, and the newly discovered adeno-associated viruses such as AAV8 and AAV9 are 10-mdash stronger than adenovirus in transferring genes into cells. A hundred times. Lentiviruses, on the other hand, have the characteristics of randomly integrating genomes, and there is a potential risk of destroying normal gene functions.
In addition to the rare diseases caused by single gene mutation reported in this report, gene therapy is also used to treat Parkinson’s disease, Alzheimer’s disease, cancer and other complex diseases at home and abroad. At present, about 42 companies and nearly 100 drug research and development projects use adeno-associated virus vectors for gene therapy. In this fatal accident, the carrier used by researchers for gene therapy was AAV8.
Too high dose is easy to produce toxic and side effects.
Regarding the insecurity of gene therapy, Cheng Cheng believes that in the case of "AT132" gene therapy, the cause of children’s death may be that they received an excessive dose of adeno-associated virus intravenous injection, which produced a large number of antibodies against adeno-associated virus. The combination of virus and antibody can further activate complement pathway or innate immune response, thus causing strong toxic and side effects.
"Another kind of insecurity is the dislocation expression of normal genes." Cheng Cheng took the gene therapy of spinal muscular atrophy "Zolgensma" as an example, saying that intravenous injection would lead to excessive accumulation of normal genes carried by "Zolgensma" in the liver, but this gene did not play a role in the liver, which triggered a common hepatotoxicity after treatment, which was manifested by the increase of transaminase.
As early as a few years ago, james wilson, an American scientist who was a pioneer in gene therapy and one of the developers of adeno-associated virus, expressed concern about the high-dose vector used in gene therapy. In February 2018, the Wilson team published a paper saying that after high-dose injection of adeno-associated virus, it caused serious toxic reactions in rhesus monkeys and piglets, and one rhesus monkey had to be euthanized due to liver failure. To this end, Wilson called on researchers to carefully monitor similar toxic effects during gene therapy.
Wilson’s collaborator, Professor Gao Guangping of the Massachusetts Institute of Technology, once expressed similar views. In an interview in 2019, he expressed concern that the rapid development of this field would make people less cautious and might make Jesse Kissinger happen again.
"Scientific and technological progress will experience setbacks." Qiu Zilong said that although this high-dose gene therapy caused the death of the patient, the effectiveness of gene therapy cannot be denied. In the face of the lack of effective treatment for rare diseases with single gene, gene therapy has indeed brought hope to many patients and their families. At present, there are still many shortcomings in gene therapy, which needs continuous improvement and perfection by experts and scholars in the field to make it safer and more effective, and should not be abandoned because of choking.
In promoting the development of gene therapy, Qiu Zilong suggested that it is necessary to constantly transform to obtain new types of adeno-associated viruses with special tissue affinity, reduce the immune response of the human body after injection, achieve more accurate delivery of normal genes, and further reduce toxic and side effects; At the same time, research and develop better gene regulatory elements, so that normal genes delivered to patients can produce functional proteins in a timely and appropriate way.
Cheng Cheng also suggested that the production and purification process of adeno-associated virus can be further optimized to reduce impurities in the virus and improve its purity. Choose a better carrier delivery mode, such as intrathecal injection or intraventricular injection for diseases of nervous system.