Gene therapy encounters another setback, can it still be "qualified" for rare disease nemesis
Our reporter Xie Kaifei
Gene therapy can "correct" the mutated gene that causes the disease, thereby bringing hope of cure to patients suffering from congenital genetic diseases. But recently, in a gene therapy clinical trial 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 defect diseases. However, gene therapy has recently encountered setbacks.
Recently, a US biotechnology company (Audentes Therapeutics) issued a letter stating that out of 17 children with neuromuscular rare diseases who participated in the company's "AT132" gene therapy, 2 children died after receiving high-dose gene therapy.
The key research of this gene therapy started in 2017 and aims to treat a fatal disease caused by a single gene mutation-X chromosome linked myotubular myopathy. The treatment was originally intended to submit an application this year and expect to be approved by the US Food and Drug Administration by the end of the year, but the plan is now shelved.
There were also children who died when receiving gene therapy 20 years ago. How was the gene therapy at that time different from this time? After decades of development, what improvements have been made in gene therapy and what problems are still facing? In this regard, a reporter from Science and Technology Daily interviewed relevant experts.
"Make up for what is missing", "correct" the mutated gene
In the human body, there is a gene called "MTM1", which expresses a protein called "myosin" for the development and maintenance of muscle cells. X chromosome-linked myotubular myopathy is caused by mutations in the "MTM1" gene.
X chromosome-linked myotubular myopathy mainly affects skeletal muscle, and the symptoms are muscle weakness and hypotonia. Muscle lesions can impair the patient's ability to sit, stand, and walk, and even cause difficulty in eating and breathing. It may also cause ophthalmoplegia, facial muscle weakness, and loss of muscle reflexes.
Currently, the main treatments for this 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 trials have been launched yet. In general, there is currently no effective treatment for this disease.
To this end, researchers began to explore the use of gene therapy to "correct" the mutated gene. "Based on the treatment logic of'replenishing what is missing', this gene therapy uses adeno-associated virus (AAV) as a vector to introduce the normal'MTM1' gene into muscle cells to produce normal functional myotubes, thereby repairing damaged muscles The function of cells." According to researcher Qiu Zilong from the Center for Excellence in Brain Intelligence, Chinese Academy of Sciences, this is a "permanent cure" method, and ideally, it can even achieve "one-time treatment and lifetime benefits."
How to achieve this "correction" process? "The whole process is very similar to'dotting'." Qiu Zilong pointed out that the researchers injected the adeno-associated virus loaded with the normal "MTM1" gene into the patient's blood, and delivered the adeno-associated virus vector to the muscle cells through the blood circulation, and in it Produces the "myosin protein" with normal function.
Before these children participated in clinical trials, the gene therapy had undergone preclinical studies, including verification of the safety and effectiveness of the therapy in cells cultured in vitro and mouse disease models. This therapy was generally safe.
But in order to improve the treatment effect, the researchers tested a higher dose of gene therapy. "High doses are particularly important for neuromuscular diseases, because more gene copies are needed to reach the targeted tissues through the blood circulation." said Dr. Cheng Cheng from the School of Bioengineering, East China University of Science and Technology. The dose used in the genetic research is the highest dose in gene therapy. : 300 trillion genome copies per kilogram of body weight.
The "upgraded" transmission carrier is safer
In recent years, gene therapy has developed rapidly. The same incident of pressing the "brake button" occurred in 1999. An 18-year-old American patient named Jesse Kissinger died unfortunately in a clinical trial of gene therapy using adenovirus as a carrier.
At that time, researchers used adenovirus as a vector to insert genes into patients’ cells to replace parts that were missing or dysfunctional that caused the disease. A few days later, Jesse died of multiple organ failure due to a 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 is the simplest type of single-stranded DNA defect virus discovered so far. Adeno-associated virus gene therapy uses adeno-associated virus as a carrier to deliver therapeutic genes to specific tissues and organs. These therapeutic genes are stably present and expressed in the form of episomes in these non-dividing cells, thereby effectively treating single genes Genetic disease.
At present, adeno-associated virus is the more mainstream gene therapy vector platform, and gene therapy of adenovirus and lentivirus is also available. Qiu Zilong pointed out that compared with adenoviruses, adeno-associated virus vectors have the advantages of high safety, low immunogenicity, longer curative effect and easy modification. The newly discovered adeno-associated viruses such as AAV8 and AAV9 have the ability to transfer genes into cells. 10-100 times stronger than adenovirus. The lentivirus has the characteristic of randomly integrating the genome and has the potential risk of disrupting normal gene function.
In addition to the rare diseases caused by single-gene mutations reported this time, gene therapy at home and abroad is also used to treat Parkinson's disease, Alzheimer's disease, cancer and other complex diseases. Currently, about 42 companies and nearly 100 drug development projects use adeno-associated virus vectors for gene therapy. In this fatal accident, the vector used by the researchers for gene therapy was AAV8.
Too high dose is prone to toxic side effects
Regarding the insecurity of gene therapy, Cheng Cheng believes that in the case of “AT132” gene therapy, the cause of death of children may be that they received too high doses of adeno-associated virus intravenous injection, resulting in a large number of Antibodies against adeno-associated virus. The combination of virus and antibody can further activate the complement pathway or innate immune response, thereby triggering strong side effects.
"Another insecurity is manifested by the misplaced expression of normal genes." Cheng Cheng took the gene therapy "Zolgensma" for spinal muscular atrophy as an example, and said that intravenous injection would cause the normal genes carried by "Zolgensma" to accumulate excessively in the liver. , But this gene does not function in the liver, which leads to a more common hepatotoxicity after treatment, which is manifested as an increase in transaminases.
As early as a few years ago, American scientist James Wilson, a pioneer in the field of gene therapy and one of the developers of adeno-associated viruses, expressed concern about the high-dose vectors used in gene therapy. In February 2018, Wilson's team published a paper stating that high-dose adeno-associated virus injection caused severe toxic reactions in rhesus monkeys and piglets. One of the rhesus monkeys had to be euthanized due to liver failure. For this reason, Wilson called on researchers to carefully monitor similar toxic effects when performing gene therapy.
Wilson’s collaborator, Professor Gao Guangping of the Massachusetts Institute of Technology School of Medicine, once expressed similar views. He expressed concern in an interview in 2019: the rapid development of this field will make people less cautious. Will make Jesse Kissinger happen again.
"Science and technology progress will experience setbacks." Qiu Zilong said that although the high-dose gene therapy has caused the death of patients, the effectiveness of gene therapy cannot be denied. Faced with the lack of effective treatments for single-gene rare diseases, gene therapy has indeed brought hope to many patients and their families. At present, gene therapy still has many shortcomings, which requires continuous improvement and improvement by experts and scholars in the field to make it safer and more effective, and should not give up food due to choking.
In terms of promoting the development of gene therapy, Qiu Zilong suggested that continuous transformation is needed to obtain new types of adeno-associated viruses with special tissue affinity, reduce the body's immune response after injection, and achieve more accurate delivery of normal genes, and further reduce Toxic and side effects; at the same time, develop better gene regulatory elements so that the normal genes delivered to patients can produce functional proteins at the right time and in the right amount.
Cheng Cheng also suggested that the production and purification process of adeno-associated virus can be further optimized, the impurities in the virus can be reduced, and the purity can be improved; a better delivery mode can be selected. For example, for diseases of the nervous system, intrathecal injection or intraventricular injection can be used. Vector delivery mode.