Severe combined immunodeficiency (SCID), a genetic disorder characterised by a reduced number of immune cells in the body. Some children with severe combined immunodeficiency (SCID), a genetic disorder characterised by a reduced number of immune cells, have been treated using gene therapy.
The first ever gene therapy trial
Gene therapy is a medical technique, first developed in 1972, that uses genes to treat or prevent disease.
The first ever gene therapy trial was initiated in 1990 by Dr William French Anderson. The patient was a four year old girl called Ashanthi who was suffering from a very rare disease known as severe combined immunodeficiency (SCID).
In Ashanthi’s case, the disease was caused by the absence of the enzyme adenosine deaminase (ADA). This deficiency prevented her body from producing the white blood cells that are required to fight infections. It left her vulnerable to even the mildest of infections, resulting in symptoms such as recurrent ear or chest infections, and persistent thrush in the mouth or throat. Antimicrobial drugs can be used to treat these infections in individuals with SCID but they only provide short-term benefits.
Before the advent of gene therapy, there were only two ways to treat Ashanthi’s SCID. The first was regular injections with the adenosine deaminase enzyme (ADA-PEG), which Ashanthi started receiving from the age of two. Initially, she responded well and developed some resistance to infections. By the age of four however, her health began to deteriorate and another option was needed.
“At that time the only way SCID-affected children could survive was by total isolation in an artificial, germ-free environment.
The second treatment option was a bone marrow transplant from a compatible donor. Bone marrow is a spongy tissue found in the centre of our bones. It produces red blood cells, platelets and white blood cells. In the case of Ashanthi, a transplanted bone marrow would increase white blood cell production and give her an effective immune system with the ability to fight infection. Unfortunately however, this option was ruled out due to the lack of compatible bone marrow donors.
At that time, if neither of these treatments were possible, the only way affected children could survive was by total isolation in an artificial, germ-free environment. For this reason, children with SCID were often known as ‘bubble babies’.
David Vetter from Texas also had SCID and had to live in a sterile environment for most of his life during the 1970/80s. He was known to the media as ‘the boy in the plastic bubble’ and wore a special ‘spacesuit’ to protect him from infections.
The ideal target for gene therapy
In the early 1990s, while Ashanthi’s parents were desperately searching for another option for their child, permission to perform the first gene therapy trial on humans was being obtained. Scientists had already shown it was possible to insert new genes into plants and animals, but it had never been done in humans.
ADA deficiency was an ideal target for the first set of gene therapy trials for a number of reasons:
- the effects of the disease are reversible and do not cause irreversible, long-term damage in the individual
- the disease results from the loss of function of a single gene
- adenosine deaminase levels vary widely in the normal population so tight control of the introduced gene is not important
- the adenosine deaminase gene is very small and easy to manipulate in the laboratory
- the target cells for the therapy are lymphocytes? (white blood cells), which are accessible, easy to grow and easy to put back into the body of a patient
- the alternative treatments are expensive and/or hazardous.
KEY FACTVectors are ‘vehicles’ used by scientists to insert new genes into DNA.
The process of gene therapy to treat Ashanthi’s condition involved getting some of Ashanthi’s white blood cells from her blood. Once outside of the body, new, working copies of the adenosine deaminase gene were inserted in to the cells via a vector?. Vectors are ‘vehicles’ used by scientists to insert new genes into DNA. In this case, the vector was a virus? that had been modified so it no longer caused disease. Once the functioning adenosine deaminase gene had been inserted into white blood cells by the virus, these white blood cells were injected back into Ashanthi’s blood. Because the white blood cells originally came from her body, there was no risk of her immune system attacking the cells.
The initial impact of the gene therapy on Ashanthi was amazing. Within six months of the procedure, her white blood cell count had risen to normal levels, and over the next two years she continued to improve.
Unfortunately the effects of Ashanthi’s gene therapy were short lived. Her adenosine deaminase enzyme therapy was continued during the trial so that if the gene therapy was unsuccessful, Ashanthi’s condition would not deteriorate too quickly. However, this made it more difficult to establish the efficacy of the gene therapy alone. To see if it had worked they had to briefly stop Ashanti’s enzyme therapy. When they did this the symptoms of her disease returned. This meant that the gene therapy didn’t completely cure Ashanthi’s condition and she had to continue using the enzyme therapy.
KEY FACTHaematopoietic stem cells are a unique type of stem cell that are found in the body and have the ability to develop into all types of blood cells.
Further trials investigated the use of haematopoietic stem cells for treating SCID. Haematopoietic stem cells are a unique type of stem cell that are found in the body and have the ability to develop into all types of blood cells, including white blood cells. These stem cells can be reprogrammed to become white blood cells containing the adenosine deaminase gene to replace the white blood cells lacking the gene in people with Ashanthi’s condition.
Transplanting these reprogrammed stem cells into an ADA-SCID patient showed moderate success. The resulting white blood cells did produce adenosine deaminase but only at very low levels.
In 2002, there was a major breakthrough in adenosine deaminase gene therapy. This came following the development of non-myeloblative conditioning. Haematopoietic stem cells are isolated from the patient and reprogrammed to contain the adenosine deaminase gene. The bone marrow of the patient is then partially destroyed to reduce the number of ADA-deficient white blood cells in the patient. The reprogrammed haematopoietic stem cells are then transplanted back into the patient. The transplanted cells can re-establish themselves as the dominant population of white blood cells, but this time containing functioning adenosine deaminase genes.
The first successful use of non-myeloblative conditioning was seen in a two-year-old Palestinian child called Salsabil. Her white blood cell count, which had originally been very low, increased considerably and for some specific cells returned to normal. Importantly, the therapy was found to completely restore adenosine deaminase activity in her white blood cells, curing Salsabil of ADA-SCID.
Now, Salsabil is able to live a normal life. Her body is producing antibodies, and she even managed to recover from chicken pox, which would almost certainly have killed her before.
As trials of this technique continued, all of the success stories were in children that had never received adenosine deaminase enzyme therapy (ADA-PEG). Unlike the gene therapy trial with Ashanthi, who continued to receive enzyme therapy during her stem cell therapy, this meant researchers were able to assess the exact effectiveness of the gene therapy on its own. This observation also suggested that enzyme therapy may have contributed to the lack of success in gene therapy trials.
Other applications of gene therapy
Currently, gene therapy is being used to treat a whole range of conditions. Some research is only in the early stages with techniques still being tested in animal models, however the findings are promising.
The SUMO-1 gene is a gene that is missing or decreasing in heart failure patients. Researchers have found that using gene therapy to replace the SUMO-1 gene in patients can significantly improve the function of a damaged heart.
Heart failure continues to be a leading cause of hospitalisation in the elderly. It occurs when a person’s heart is too weak to pump properly and circulate blood around the body.
Gene therapy with the SUMO-1 gene was found to result in stronger heart contractions and better blood flow when tested in mice. It is hoped that the same will be true when the therapy is translated to clinical trials in humans.
Acute lymphoblastic leukaemia
Progress has also been made in people with acute lymphoblastic leukaemia.
Leukaemia is a cancer of the white blood cells. The term lymphoblastic means that the cancer affects a type of white blood cell called a lymphocyte, which are mostly used to fight viral infections. ‘Acute’ refers to the rapid nature at which this particular cancer progresses.
In acute lymphoblastic leukaemia, a subset of lymphocytes called B-cells become cancerous. Scientists therefore decided to modify another type of immune cell, the T-cell, to attack only B-cells. They did this by reprogramming T-cells to attack all cells with a protein called CD19 on their surface. CD19 is a molecule that is only found on the surface of B-cells. When these reprogrammed T-cells were reintroduced back into acute lymphoblastic leukaemia patients they attacked and destroyed all cancerous and normal B-cells in the body. With all of the body’s B-cells destroyed the immune system could make new, normal, non-cancerous B cells over the next few months.
“In some patients it took just a few weeks for all of their cancer cells to be removed.
Although it is early days, the results from this trial are very promising, with 10 out of 13 individuals treated ending up in remission from the cancer. In some patients it took just a few weeks for all of their cancer cells to be removed.
Currently there are few effective treatments for Leber’s congenital amaurosis (LCA), a rare inherited eye disease caused by an abnormality in a gene called RPE65. However, a trial involving young patients with the condition, found that inserting healthy copies of the RPE65 gene into the cells of the retina helped them to function normally.
An additional positive result from this research was that no side effects to the therapy were recorded.
It is thought that the younger the patients are when they are treated with this technique, the more successful the results will be. This is because the condition is treated early before it has progressed too far. Trials are continuing.
Bone cancers, such as osteosarcoma, often have to be treated by removing the diseased section of bone and replacing it with a donated section of bone from a dead body donated to medical research. The donated bone is then screwed into place and the patient can carry on as normal. The only problem is that the bone wears out quickly because it is not alive and generating new cells. This means that the implant sometimes only lasts about 10 years before more extensive surgery is required.
To find a way to make the transplanted bone more robust, researchers looked into the genes and proteins involved in bone growth and health in mice. They found that the transplanted “dead” bone did not express two key genes normally expressed in and around living bone, RANKL and VEGF. This suggested that they needed to find a way to restart expression of these two genes to extend the life of the bone transplant.
To do this, they inserted the RANKL and VEGF genes into a modified, harmless virus. This virus was then applied in a paste directly onto the mouse bone graft during surgery.
The viral vector was absorbed into the tissue surrounding the dead bone graft, activating the RANKL and VEGF genes. As a result the mouse’s body began to treat the bone graft as if it was its own tissue, instead of a foreign object that needs to be attacked. The researchers found that the dead bone graft was even incorporated into the mouse’s own, living bone. New blood vessels began to grow around the graft and the dead bone was converted to new, healthy bone, within a year. It was predicted that a human bone could do the same in as little as five years.