Trial and Error - questions and answers
What is gene therapy?
Gene therapy has been called medicine's fourth revolution, after public health, anaesthetics, and antibiotics. Just as those earlier developments helped extend life expectancy, gene therapy seems to offer hope of curing illnesses which current medicine cannot. Gene therapy uses DNA as a drug. There are two main types:
- Gene replacement involves inserting normal genes into cells, and replacing the original, defective genes by means of DNA recombination.
- Gene strengthening introduces new genes to compensate for the deficiency of weak genes, unable to perform their normal function.
What is OTC deficiency and what problems does it cause?
Jesse Gelsinger suffered from the liver disorder, OTC deficiency. OTC is the main enzyme responsible for getting rid of ammonia from the blood. Mutations in the ornithine transcarbamylase (OTC) gene lead to OTC deficiency, which is characterised by raised levels of ammonia in blood. Ammonia is highly toxic and a build up of it leads to varying degrees of mental retardation - and sometimes death.
The gene for OTC is present on the X chromosome. As a result, mutations in it are more severe for men than women because if a female has a defective gene on one of her two X chromosomes, she will be protected from its effects by the normal gene on her second X chromosome. Men have no second X chromosome to compensate if the first is faulty.
What is a gene therapy vector?
A vector is the carrier used to deliver therapeutic genes to cells. The most common method used at present takes advantage of the natural ability of viruses to deliver genetic material to cells, like viral Trojan horses. Viruses are not capable of living as free entities; in fact they are generally inert and inactive outside of their host cells, which are needed for the virus to replicate. Once inside the host cell, the virus hijacks the cell’s genetic machinery and uses it to make more viruses.
To make a viral vector, scientists remove the disease causing genes from the virus, in some cases gutting it entirely, but let it retain the ability to enter the cell and replicate itself. They hijack it.
However, viruses are not the only means of carrying genes into cells. One alternative is to put therapeutic genes into liposomes (microscopic fat capsules). Liposomes can fuse with the outer membrane of a cell and slip DNA into the cell's nucleus. Although liposomes pose fewer problems than virus vectors, they are not very efficient at transferring genes into the nuclei of cells.
What is a retrovirus and why is it so important in gene therapy?
Most viruses and all cellular organisms carry their genetic material as DNA - deoxyribonucleic acid. Retroviruses do not. They carry their genetic blueprint in the form of ribonucleic acid (RNA). Retroviruses are responsible for certain cancers and slow virus infections of animals and cause at least one type of human cancer. A type of retrovirus called a Moloney murine leukaemia virus was the first virus used in human gene therapy.
Retroviruses are useful for gene therapy as they are very good at getting into host cells, and because they combine their genetic material with that of the host permanently, any therapeutic DNA they are carrying can have long term expression - they can offer a permanent cure.
There are some problems with the use of retroviruses. There is no way to have control over exactly where on the cell’s strand of DNA the therapeutic gene inserts itself. This means there is risk of mutation, or even triggering cancer, because insertion may disrupt the host genes. In addition, retroviruses are only capable of infecting dividing cells, meaning they are perfect for gene therapy on things like blood cells, but cannot be used on the brain, heart, liver or lungs, where most diseases occur.
How does the adenovirus differ from a retrovirus?
Adenoviruses are a group of viruses that frequently cause acute upper respiratory tract infections, ie colds. They are considered safer for gene therapy than retroviruses as they replicate in the host cell nucleus without integrating themselves into the host genome. Part of their appeal is that they can affect all the areas where most diseases occur, such as body organs and muscle tissue, and that they can be produced in large quantities.
One obvious problem with adenoviruses is that they only survive in the cell for a few weeks before the immune system manages to fight them off. Many people have antibodies from previous adenovirus infections that fight against the virus when it is put into the body, preventing it from delivering its DNA cargo.
What human testing safeguards are normally in place to prevent tragedies like Jesse's?
A proposed gene therapy experiment ('protocol') must first be approved by a country’s research governing body. Testing on humans can only take place after rigorous testing on animals and tissue culture systems. When a product has been approved for clinical trial (testing on humans), the first step is the Phase 1 trial.
- Phase I trials are designed to assess safety and establish dosing for future trials.
- Phase II studies are undertaken to determine efficacy, whether there is some therapeutic effect.
- Phase III studies are designed to confirm efficacy in larger patient populations and to continue monitoring for rare but significant adverse side effects.
Clinical trials are closely monitored, and any adverse events must be reported. If cases are shown to be unsafe, then trials are modified accordingly, or stopped. If phase III trials are successful, a product can be licensed.
What is the future for gene therapy?
Despite its early promise, there are still only a handful of cases where people have been cured by gene therapy. All of them children, and all of them with the same rare blood disorder. Two of these children have now developed leukaemia as a result of the leukaemia virus used as a vector. As yet, there is little evidence that gene therapy can produce wide therapeutic benefits.
It is clear that gene therapy will fulfil its promise only when vector systems are developed that can safely and efficiently be introduced into patients. Vectors need to be engineered that will target specific cell types, insert their genetic information into a safe site in the genome, be regulated by normal physiological signals, and be coaxed to work throughout the life of the patient.
Why did the adenovirus kill Jesse?
All that can be certain about Jesse’s death is that the adenovirus caused a massive immune reaction in his body, which in turn caused a blood-clotting problem and organ failure. No one is yet certain how this occurred.