Treating Genetic Disorders and Diseases using Gene Therapy
Our body is made up of trillions of cells that each contain DNA (Deoxyribonucleic Acid). This DNA is the genetic instruction our body needs to create how we look and how we react to certain situations. However, sometimes the genes mutate causing a change in the DNA sequence.
This can be caused by DNA copying mistakes made during cell division, exposure to radiation or mutagens and infection by viruses. The mutations would eventually lead to genetic disorders if left untreated. Previously, we weren’t able to treat or put a stop to these mutations. Nevertheless, in the past few decades, we’ve come across a variety of solutions to treat them using Gene Therapy.
So what is Gene Therapy? Gene Therapy is a medical approach to cure diseases by altering the underlying genetic makeup. The earliest method of Gene Therapy called Gene Transfer or Gene Addition involved adding a new gene into the cells to help fight disease or to replace a faulty gene with a functioning one. However, newer methods such as Gene Editing use a different approach to correct genetic problems. Gene Editing introduced a variety of ways to alter genes such as:
- Fixing a genetic alteration underlying a disease
- Turning on a gene to fight a disease
- Turning off a gene that isn’t functioning properly
- Removing DNA that is causing disease or impairing gene function
Gene Therapy was first used on September 14, 1990, on a 4-year-old girl born with severe combined immunodeficiency (SCID). This initial trial was a large success and paved the path for future trials. However, a trial performed on 18-year-old Jesse Gelsinger resulted in his death on September 17, 1999. This trial was supposed to give him a modified virus to help cure his OTCD, but due to his death, Gene Therapy became a black label. Slowly, it was able to come back to life due to its promising success after a few years.
A few years ago, Brian Madeux who had Hunter Syndrome received a treatment aimed at the genes in his liver cells. Hunter Syndrome results from a mutation in a gene for an enzyme that a cell needs to break down certain sugars. When the enzyme is defective or missing, the sugars build up causing developmental delays, brain damage, organ problems and early death. Brian had a mild version of the syndrome but that didn’t stop him from needing 2 dozen operations.
The goal of this trial, which was sponsored by Sangamo Therapeutics, was to insert a replacement copy of the gene. They would first snip the DNA strand using CRISPR near the promoter of the cell for the protein called Albumin. After they insert the replacement copy, the cell starts fixing the area and uses the new DNA as part of the strand, and the gene’s activity is then controlled by the Albumin promoter. The plan is to turn these liver cells into factories that manufacture the missing enzyme in Hunter Syndrome.
Sangamo’s approach was considered to be very safe since it avoided the risks of traditional Gene Therapy in which the new gene would be inserted at random and could potentially turn into a cancer cell. The body also doesn’t need that much of the enzyme, so only treating part of the liver should be more than enough.
Hunter Syndrome patients normally receive weekly infusions of the missing enzyme, however, their blood levels drop within a day. The goal is that this treatment would allow the body to steadily produce the enzyme for years to come. There is a problem though, the enzyme Hunter patients now receive does not cross the blood-brain barrier, a tight network of cells that protects the brain from pathogens, and the liver-made enzyme produced by the gene edit may not either. To put it simply, this treatment may not stop the brain damage that could occur in Hunter Syndrome.
However, this isn’t quite the first gene-edited human. This trial used a variation of genetic scissors called Zinc Finger Nucleases (ZFNs). Like CRISPR, ZFNs can cut both strands of the DNA helix. In trials several years ago, Sangamo used ZFNs to protect patients from HIV by getting their blood cells, altering the gene, and then transfusing the blood cells back into the patient.
Nevertheless, this is the first time ZFNs have been used to directly modify DNA in a living patient which is called in vivo gene-editing. This is much more complicated than normal editing a cell’s DNA in a lab. Researchers have to use a viral vector to carry the DNA encoding ZFNs and the new gene to the liver cells. Although this adeno-associated virus is widely used in Gene Therapy and considered safe, it can trigger a potentially dangerous response in some patients, however, it can be managed with steroids.
In vivo gene-editing also poses some additional risks because it faces the same problem as traditional Gene Therapy. ZFNs could potentially make cuts in the wrong place and turn on a cancer gene. Compounding this risk, the liver cells that contain the DNA for ZFNs can now make nucleases for years, even though they no longer need them. Sangamo also says that off-target cutting would be rare and only happen in non-functioning stretches of DNA. They are also going to investigate the off-target cuts by taking liver biopsies from patients.
Another example of evolving Gene Therapy is how a wife and husband team swap genes to prevent vision loss. Some babies are born with severe vision loss due to retinal diseases that lead to total blindness. Jean Bennett and Albert Maguire, the wife and husband team, are the ones who created a treatment for this situation.
When they first started their research in 1991, none of the genes known to cause vision loss were discovered. In 1993, researchers found one potential target, RPE65. Several years later, Jean and Albert tested a therapy that targeted vision loss genes on 3 dogs with severe vision loss and it was successful.
A similar gene in humans is called Leber congenital amaurosis (LCA). LCA doesn’t allow the retina to perform properly or send signals to the brain. This situation could even cause severe shaking of the eye called Nystagmus and usually causes the host to go completely blind by the age of 40. Researchers have connected these diseases and mutations to a total of 27 genes related to retina development and function. The gene causing these issues could be any one of the 27 and until Gene Therapy came along, it was untreatable.
The mutations in RPE65 are just one of the multiple inherited retinal dystrophies. However, it was the main cause that allowed Jean and Albert to act on this situation. They inserted an adeno-associated virus that was programmed to find retina cells with unhealthy genes. It was then supposed to insert a healthy copy of that gene. This whole procedure was directly inserted into the patient’s eyes a bit under the retina. In 2017, their treatment was approved by the FDA for use on heritable retinal dystrophies caused by the mutated RPE65 gene including LCA type 2 and retinitis pigmentosa which were similar diseases. It was called Voretigene Neparvovecrzyl but it was marketed as Luxturna. Luxturna was the first FDA-approved in vivo gene therapy that was directed to target the cells in the body.
One of the biggest leading causes of death worldwide is cancer. So you’ll be happy to know that gene therapy is also being used to train our immune system to fight this unsolvable disease. A way known as chimeric antigen receptor (CAR) T cell therapy is to essentially program the patient’s immune cells to recognize and target cells with cancerous mutations. Steve Rosenberg who is the chief of surgery helped develop a therapy and published the results for the treatment of Lymphoma in 2010.
“That patient had massive amounts of disease in his chest and his belly, and he underwent a complete regression,” Rosenberg says.
CAR T cell therapy essentially uses the patient’s own white blood cells called T cells which are the first line of defence against pathogens. These T cells are taken out of the patient and altered with specific receptors to cancer cells. Once they are infused back into the patient, they recognize and kill any cancer cells that they might encounter. They also have the ability to reproduce and remain on alert for future encounters.
The University of Pennsylvania reported results from a CAR T cell treatment trial, called Tisagenlecleucel, for acute lymphoblastic leukemia (ALL), one of the most common childhood cancers in 2016. Patients with ALL have the ability that causes mutations in the DNA of bone marrow cells. This mutation causes an unnatural amount of lymphoblasts, or undeveloped white blood cells to be dumped into their bloodstream. This disease is so deadly that less than half the adults die 5 years after being treated for it.
CAR T cells are extremely efficient in dealing with ALL. A single CAR T cell has the power to eliminate as many as 100,000 lymphoblasts. In the study conducted by UPENN, 29 out of 52 ALL patients went into sustained remission. The FDA approved this therapy and the following year the agency approved it for use against diffusing large B cell lymphoma.
Recently, the results from a clinical trial for treating sickle cell disease were published. Sickle cell disease affects millions of people worldwide and it causes crescent-shaped blood cells to be produced. These cells are stickier and more rigid than red blood cells and can cause Anemia and life-threatening diseases.
Beta thalassemia which affects millions more, causes the body to produce less Hemoglobin which is the iron-rich protein that allows red blood cells to carry oxygen. Bone marrow transfers are the go-to treatment method for people with matching donors, otherwise, the treatments usually consist of blood transfusions and medications to treat likewise complications.
Both sickle cell disease and Beta thalassemia are caused by inherited single-cell mutations making them really good candidates for gene therapy. CRISPR Cas9 is being used to edit the patient’s genome and eliminate the mutation. The targeted sequence is transcribed onto the guide RNA which leads the protein to the specific DNA strand so that it can make the cut to repair or delete the gene entirely.
Vertex Pharmaceuticals and CRISPR Therapeutics are using CRISPR to boost the production of healthy fetal hemoglobin which is normally turned off after birth. The patients that received this treatment showed astounding results when their hemoglobin levels spiked a few short months later.
SMA or spinal muscular atrophy is a neurodegenerative disease that causes motor neurons that control muscle movements and connect the spinal cord to the muscles and organs to die, degrade or malfunction. Toddlers and infants are the ones that are typically diagnosed with it. The underlying cause is a genetic mutation that hinders the production of a protein that is required to build and maintain motor neurons.
There are 4 types of SMA ranked by severity of production of each motor neuron’s proteins. The worst cases can make it difficult for the patient to breathe, sit or even swallow. Infants that have been diagnosed had a 90% mortality rate by one year.
Adrian Krainer, who was a biochemist at Cold Spring Harbor Laboratory, was researching how RNA mutations can cause cancer and genetic diseases when they disrupt a process called splicing when he suspected that the same thing was causing SMA. When RNA is transcribed from the DNA template, it needs to be spliced into the mRNA before it can watch over protein production. In that process, some sequences are cut out called introns and the ones that remain are put together called exons. Adrian figured out that one of these important exons was missing called SMN1 in SMA patients.
Their solution was to make SMN2, another exon, produce a lot more of the protein so that it could essentially fill in for SMN1. Since the therapy doesn’t build itself into the genome, it has to be administered every 4 months. Although they can’t completely restore motor functions, they can help the patients live longer and more active lives.
All in all, Gene Therapy could be a global phenomenon that changes the world drastically given some time. It’s already being widely used in the medical field as proven by my examples above. Its possibilities are endless and there is so much that could be accomplished through it.
Its already treated so many of the world’s deadliest diseases and for the ones that weren’t, treatments are making steady progress. This could impact the world in a massive way and completely change the way we look at healthcare.
