Release date: 2015-04-28
Gene therapy may finally be able to achieve the original good vision. In the past seven years, experimental gene therapy for implanting healthy genes into cells of congenital blind patients has successfully brought 40 patients back to light. At the same time, clinicians have seen the unprecedented efficacy of gene therapy in more than 120 patients with various types of hematological cancer. Some patients remain cancer-free even after 3 years of treatment. Using gene therapy, the researchers also helped some people with hemophilia (hemophilia is a deadly bleeding disorder) that prolongs their chances of reducing accidents or lowering their need for high-dose coagulants. The survival time.
In contrast to the once-deficient situation of gene therapy, these positive results today are particularly exciting. Sixteen years ago, the accidental death of a teenager named Jesse Gelsinger caused the entire gene therapy study to come to a standstill. Jersinger suffers from a rare digestive tract disease in which his immune system launched an unexpected brutal counterattack and took his life. In the 1990s, it was the time when gene therapy first came to fruition. However, this led to a irrational atmosphere among doctors and researchers that contained too high expectations—and perhaps a sham.
The failure of Gersinger, along with other setbacks in gene therapy, forced scientists to re-examine their methods and consider the feasibility of implementing gene therapy in different populations. They put away their original eager expectations and returned to the most basic research. They examined the potentially lethal side effects of gene therapy (such as the tragedy experienced by Jessinger) and developed ways to circumvent it. At the same time, they place greater emphasis on explaining the possible risks and benefits to subjects and their families involved in gene therapy.
For many people, the turning point in gene therapy took place eight years ago by an 8-year-old boy named Corey Haas. Haas suffers from a degenerative eye disease and his vision is compromised. Doctors used gene therapy to create a protein that could not be synthesized by Haas' defective left eye retina. Less than 4 days after receiving treatment, Haas went to the zoo to play, and to his surprise, he finally saw the sun and the red balloon. Three years later, his right eye received the same treatment. To this day, Haas’ vision is enough for him to join his grandfather in the Thanksgiving turkey hunting.
Although gene therapy has not been carried out in hospitals and clinics to this day, this situation is expected to change in the next 10 years. In 2012, Europe approved the first gene therapy to treat familial lipoproteinase deficiency, a rare but extremely painful genetic disease. At the end of 2013, the National Institutes of Health (NIH) also eliminated some of the non-essential controls on gene therapy.
Some industry observers predict that the United States will approve the first commercial gene therapy in 2016. After more than a decade, gene therapy has finally begun its important mission of revolutionizing medical care.
Heartbreak moment
The failure of early gene therapy highlights the difficulty of importing genes safely and efficiently into target organizations. The safest gene delivery system is often not very effective; and some of the most effective delivery systems can trigger a fatal immune response, which is actually not safe—Jer Singer’s tragedy is an example, and some patients still Therefore, suffering from leukemia.
To understand the mechanisms by which these side effects occur and to find ways to reduce risk, scientists have focused on one of the most common delivery systems in gene therapy: transforming the virus into a "microinjection gun."
First, the researchers were replacing some of the genes in the virus with healthy genes that would be transferred to patients. There is also a benefit to doing this: to prevent the virus from replicating its own genes after entering the human body (the replication of viral genes increases the chance of an immune response after entering the human body).
Next, the researchers injected the modified virus into the patient. Depending on the type of virus, genes can be inserted into different types of human cells.
When Jessinger participated in the clinical trial, the researchers selected a gene delivery system containing adenovirus. Unmodified adenovirus can cause mild upper respiratory tract infections in the human body. Scientists at the University of Pennsylvania believe that the best place to inject the virus is the liver: Jersinger's liver cells are different from normal cells, so they don't produce the digestive enzymes that he lacks. The researchers loaded the gene that made the digestive enzyme into an adenovirus that was "vacated."
Subsequently, the researchers injected about 1 trillion adenoviruses carrying the therapeutic gene directly into the liver of Jer Singh. However, some of these viruses have embarked on a tragic astray. The virus entered the liver as originally planned, and it also infects a large number of macrophages. These huge dendritic cells perform the functions of an immune sentinel and wander around the body. After being infected by the virus, the macrophage sends a signal of foreign body invasion to the body, and the immune system responds and begins to destroy all infected cells. This violent process eventually destroyed Jessinger’s body from the inside.
The fierceness of the immune response was unpredictable, and none of the 17 previously treated patients with the same disease showed such a serious side effect. The researchers did know that adenovirus might trigger an immune response. However, only one monkey in the animal experiment died after injecting another slightly different modified virus; they did not realize that the immune response had such a powerful destructive effect. . “Human individual differences are larger than other animal populations,†said James Wilson, a researcher at the University of Pennsylvania who developed the viral vector used by Jessinger. “We were at that trial. It was observed that one out of 18 subjects developed an extreme host response (the host response refers to the response of the immune system to invading foreign bodies)."
In the circumstances, it would be more sensible to reduce the number of viruses injected into Jessinger's body to one billion instead of directly injecting a trillions of viruses into the patient's body - but this is only a slap in the face. In addition, the researchers did not inform Gersinger and his family about the death of the monkey in the informed consent. On the one hand, they judged that the death of a monkey has nothing to do with clinical trials, which has also criticized researchers.
The death of Jessie Singer is not the only tragedy of gene therapy. Shortly thereafter, 20 children participated in a gene therapy for X-linked severe combined immunodeficiency (SCID-X1), but 5 of them developed leukemia, 1 case So die. The gene delivery system has once again become the source of tragedy. However, the microinjection gun used in this test is a retrovirus that directly inserts the therapeutic gene into the DNA of the host cell.
If entering the genome, the specific integration of the therapeutic gene is somewhat random. However, these genes are sometimes inserted into the oncogene, which in some cases can cause cancer.
Rethink
Given that adenoviruses can cause fatal immune responses, and retroviruses can trigger cancer, researchers are beginning to invest more in other viruses to find better gene carriers. They quickly focused on two more widely used candidate viruses.
The first new gene delivery system was adeno-associated virus (abbreviated AAV). Although most humans have been infected with the virus, it does not cause any disease. Since adeno-associated viruses are so common, it is unlikely to cause an extreme immune response. Another feature of the virus is that it also minimizes the occurrence of side effects: it has several variants (or "serotypes"). These variants can exhibit affinity for specific cells or tissues, for example, AAV2 is best suited for ocular treatment, AAV8 is more suitable for the liver, and AAV9 is particularly suitable for the treatment of heart and brain tissue diseases.
Researchers can select the best AAV serotype based on specific parts of the body, thereby reducing the amount of viral injections to reduce the likelihood of large-scale immune reactions and other adverse reactions. In addition, the therapeutic gene carried by AAV does not enter the chromosome, which avoids the sporadicity of its interaction with the proto-oncogene.
In 1996, adeno-associated virus was first used in clinical trials for the treatment of cystic fibrosis. Since then, the researchers have identified 11 types of AAV serotypes, recombined and engineered them, and constructed hundreds of highly safe gene delivery tools for different tissues. Some recent studies are evaluating a range of therapies using AAV as a carrier for the treatment of brain diseases including Parkinson's disease and Alzheimer's disease, as well as hemophilia, muscle wasting, heart failure and blindness.
Even more surprising is the second viral vector: the human immunodeficiency virus (HIV), which removes the causative gene. HIV is the virus that causes AIDS. However, once you don't care about its "killer" reputation, you will find its advantages in gene therapy. HIV is a member of the Lentivirus family of retroviruses that can circumvent the immune system – a point that is critical for retroviruses and generally does not interfere with proto-oncogenes.
The HIV structure, which removes the disease-causing gene, "has a lot of load capacity," commented Stuart Naylor, a former chief scientist at Oxford Biomedica. Oxford Biomedical is working on the development of "genetic drugs" for the treatment of eye diseases. Compared with the small AAV, "HIV is more suitable for loading multiple genes, or longer gene sequences," Neller said. "This carrier is not toxic and does not cause adverse immune responses." The lentivirus of the causative gene is being used in a number of clinical trials, including the treatment of adrenoleukodystrophy (ALD). The disease was screened in 1992 and appeared in the film Lorenzo's Oil.
Today, the recovery levels of some of the children receiving the above treatment are enough to get them back to school. Although more and more clinical trials have begun to use AAV and HIV, the researchers have modified or modified the original viral vectors so that they can still be used in a few cases. For example, some non-HIV retroviruses can be self-inactivated before they induce leukemia after genetic editing.
Even the adenovirus that once claimed the life of Jessinger is still used as a carrier of gene therapy in clinical trials. Researchers use this vector only in parts of the body where immune responses are less likely to occur. For example, in patients with head and neck cancer who receive radiotherapy, the salivary glands of the lower jaw are damaged, causing symptoms of "dry mouth", and a promising application of adenovirus is to treat this dry mouth.
Currently, the National Institutes of Health is conducting a small-scale clinical trial to insert a gene into salivary gland cells to create a conduit for fluid to enter the gland. Because the salivary glands are small and easy to control, and the number of viruses used in the trial is less than 1/1000 of that used for Jessinger, the chance of an immune response is small. In addition, viral vectors that do not enter the target cells will be swallowed or spit out by the patient as they are saliva, making it difficult to "provoke" the immune system. Since 2006, 6 of the 11 patients who participated in the trial had significantly increased saliva levels. Before the retirement, Bruce Baum was the dentist and biochemist who led the study. His evaluation of the test results was “still cautious but encouragingâ€.
New goal
Encouraged by the above results, medical researchers began to expand the scope of research. They no longer only care about the treatment of genetic diseases, but begin to use genetic technology to reverse the genetic damage that naturally occurs during life.
For example, scientists at the University of Pennsylvania are using gene therapy to treat a common childhood cancer, acute lymphoblastic leukemia (ALL).
Although most children with ALL respond to standard chemotherapy, about 20% of children still do not respond to chemotherapy. Researchers are using gene therapy to activate immune cells in these children to find and destroy stubborn cancer cells.
The method of this test is particularly complex, relying on the so-called "chimeric antigen receptor" technology. In Greek mythology, the term "chimera" originally refers to a monster that is a fusion of different animals, and a "chimeric antigen receptor" that combines two types of immunity that would not have occurred at the same time. The receptor of the molecule.
Once adapted to the chimeric antigen receptor described above, an immune cell called a T cell can "target" certain proteins that are abundantly expressed in leukemia cells, thereby "catching" the leukemia cells and destroying them. The first subjects to receive this type of therapy were adults with chronic leukemia, and they all achieved good results. In the next child patient, the researchers gained more than expected.
In May 2010, 5-year-old Emily Whitehead was diagnosed with leukemia, and the first two rounds of chemotherapy she received did not work. Bruce Levine, one of Whitehead's doctors, said that by the spring of 2012, "Whitehead had received a third dose of chemotherapy, enough to poison an adult. And, Her kidneys, liver and spleen have been damaged." The little girl’s life is at stake.
The doctors took a blood sample from Whitehead and isolated some of her T cells. They then transferred the lentivirus carrying the therapeutic gene into a T cell sample and returned the cells to Whitehead. After a difficult start, gene therapy finally worked in Whitehead, and her condition quickly eased. After 3 weeks of treatment, one-fourth of the white blood cells in Whitehead's bone marrow carry a therapeutic gene, and her T cells begin to declare war on cancer cells, and the latter quickly disappears without a trace. “She also had a small head in April 2012,†Levin recalls. “And on the school day in August, she has already gone to school to report to the second grade.â€
Although this improved T cell may not always exist in Whitehead, doctors can use it repeatedly. The beautiful girl with a fluffy brown hair has survived cancer for almost two years. Whitehead is not the only lucky one. In the second half of 2013, a group of researchers reported that they used chimeric antigen receptor technology in more than 120 patients to treat leukemia of the same type as Whitehead and three other blood cancers. Five adult patients and 22 children were thus free of cancer – meaning their bodies are currently cancer-free.
Entering the clinic
Safe delivery systems are already in the hands of gene therapy experts, and now they are facing the ultimate challenge that all new drugs must face: through the US Food and Drug Administration (FDA) approval.
This long process includes the so-called "phase III clinical trial." Phase III clinical trials are designed to evaluate the effects of drugs or therapies in large-scale subjects and typically take 1-5 years (this time varies widely for different trials). As of the end of 2013, approximately 5% of the nearly 2,000 gene therapy trials reached the phase III clinical phase. The first of these is the trial of Leber congenital amaurosis (LCA). It is this disease that once took away the sight of Haas. At present, the eyes of dozens of patients have seen the light after inserting therapeutic genes.
In 2004, China approved a gene therapy for head and neck cancer, becoming the first country to grant gene therapy. In 2012, Europe approved the gene therapy drug Glybera for the treatment of familial lipoprotein lipase deficiency. The active component of the drug (the mutant gene) was encapsulated in AAV and injected into the patient's leg muscles. . Dutch pharmaceutical company UniQure is negotiating with the FDA to get the drug into the US market. However, Glybera has a potential short board: price. Glybera's single-cure dose is priced at $1.6 million, but if researchers can develop more effective treatments, prices will fall.
Similar to the development of many medical technologies, gene therapy has gone through decades of twists and turns and is far from reaching the end of success. However, with the advent of more and more Corey Haas and Emily Whitehead, gene therapy will gradually become the mainstream of some diseases, and it will also provide new treatment for other diseases. select.
Source: Global Science
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