One way to look at medicine is that it attempts to repair malfunctions of the human body, be they internal errors such as genetic diseases or invasions of parasites and pathogens. Synthetic biology aims to re-engineer biological processes, and to do so in such a way as to produce specific, targeted outcomes. It’s no accident, then, that a great deal of hope and effort is being invested in developing synthetic biology for many difficult to treat diseases such as cancers, antibiotic resistant infections or genetic disorders. Even though the discipline is in its infancy, there are already some promising approaches emerging from labs around the world.
In a recent review published in Science, Ruder et al. [Ruder, et al. Science 333, 1248 (2011); DOI: 10.1126/science.1206843], described a number of ways in which synthetic biology may help human health. These approaches are universally elegant and demonstrate an impressive diversity of ideas. This breadth is one of the challenges for the SYBHEL project in that each approach is likely to produce a different set of legal and ethical questions.
The discovery and development of antibiotics has had a profound effect on human health. Many previously deadly diseases are now easily cured. But, inevitably, their impact is starting to wane as bacteria fight back and acquire resistance. Drug development has slowed and new approaches to treating antibiotic resistant infections are needed. Enter the engineered bacteriophage, variations on viruses that infect only specific bacteria.
The first approach described by Ruder et al tackles persistent bacterial infections that hide themselves inside a mucous-like protective coat called a biofilm. A bacteriophage can be engineered to force infected bacteria to do two things. The first is, on infection, to quickly produce large quantities of new virus, burst open and die. This kills the bacterium, helping to reduce the infection and spread the virus rapidly to the remaining bacteria. The virus itself poses no risk to human health. The second is to produce an enzyme that breaks down the biofilm, exposing the bacteria to both antibiotics and the body’s immune system. Lab tests showed this eventually killed 99.997% of biofilm bacteria.
A second study described by Ruder et al produced bacteriophages that, once inside an infected bacterium, turned off the mechanisms that protect it from antibiotics. Treating E. coli infected mice with both antibiotics and the engineered bacteriophage resulted in an 80% survival rate, compared to 20% with antibiotics alone.
A very different approach to infectious disease is seen in a synthetic biology approach that could be used to tackle malaria. The parasite that causes this disease has to spend part of its life cycle inside a mosquito. Genetically modified mosquitoes can be engineered to block this stage, but that will only work if the genetic alterations are spread quickly to the whole mosquito population. A piece of syntheticDNAhas been constructed that should, in principle, encourage another set of genes to spread rapidly throughout a breeding population, almost like an infectious genetic disease. And in laboratory tests it did indeed spread throughout a caged population very quickly. This was a proof of principle, and did not carry any anti-malaria payload, so to speak. The next step would be to add an anti-malaria genetic modification to this system, producing, in theory, a genetic construct that would reduce or even eradicate malaria transmission.
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One of the greatest challenges for cancer treatments is to eliminate cancerous cells in the human body without damaging associated tissue. Ruder et al describe two synthetic biology approaches which have produced promising results in the lab.
The first involved engineering bacteria to produce a protein that makes them stick to human cells, but to produce that protein only in the absence of oxygen. Tumours often grow in low levels of oxygen as their blood supply is poor. The hope is that when the bacteria encounter a low oxygen tumour, they will stick to and invade the cancerous cells. The engineered bacteria were shown to invade human cells in the test tube only when oxygen levels were low. A potential problem with this approach, though, is that it would rely on the blood supply to deliver the bacteria to the cancer. However, tumours often have poor blood supply, making them low in oxygen, which is why this approach works in the first place. A lack of blood makes them susceptible, but also makes delivering the treatment a challenge.
This obstacle has been circumvented in a different approach that tackles one of the genes involved in turning cells cancerous in the first place. The CTNNB1 gene is involved in many colon cancers when it behaves in an abnormal fashion. Reducing the output of this gene when this occurs could potentially stop colon tumours from growing and spreading. The researchers engineered a bacterium that could penetrate colon cancer cells and interfere with the workings of the CTNNB1 gene and demonstrated that it could indeed target colon cancer cells growing in laboratory mice.
Synthetic biology has also been turned onto the challenge of developing new vaccines. A vaccine works by offering the body’s immune system a sample of the disease causing organism, known as an antigen, so that it can remember it and attack it if it invades in the future. An established way of doing this is called attenuation, in which a virus or bacterium is crippled so it cannot cause disease, but retains enough identifying features for the immune system to recognise it. This is, however, a delicate balancing act that doesn’t always work.
One rather elegant piece of synthetic biology has been to produce an artificial cell-like body that can alert an immune system but not cause disease. It works by wrapping some genes and the mechanism to translate them in a membrane similar to our own cell membrane. The genes included can, in theory, be changed for those from any disease causing organism. This will result in little fatty bubbles, called liposomes, that resemble part of the virus or bacterium. However, because they contain nothing else they are not able to produce disease, removing at a stroke the balancing act of attenuation – that tricky process by which an infectious organism is deactivated but not totally destroyed..
We carry inside us 10-100 times more bacteria than we do human cells. These are essential for life, helping us to digest food, produce some vitamins and ward off invading pathogenic bacteria. These bacteria, collectively called the microbiome, live happily inside us and are potentially excellent targets for synthetic biologists.
One example is the development of E. coli, the common gut bacterium, that can help fight off cholera, at least in mice. The E. coli were engineered to produce a chemical signal that cholera bacteria use to coordinate infection. The signal effectively swamped and so jammed the cholera bacteria’s signals. Baby mice given these engineered bacteria followed, 8 hours later, by cholera bacteria had an increase in survival rates of 80% over mice given cholera alone.
A second approach to utilising the microbiome is to engineer bacteria to produce useful drugs inside the body. The trick here would be to make sure that they are produced in the right amounts at the right place in the body, relying on the ability of synthetic biology to build in mechanisms to respond to external stimuli. These would turn on when they detect pathological conditions (rather like the cancer targeting bacteria), and turn off again when the disease has passed.
The final topic Ruder et al discussed was the engineering of human cells to behave in specific, useful, ways. The challenge here is that the majority of synthetic biology to date has been done in bacteria. Mammalian cells like ours are significantly more complex and therefore significantly harder to engineer. However, it has been done.
Mice have been engineered to contain a synthetic biological mechanism that is turned on by the presence of uric acid, the chemical responsible for gout, and produce enzymes that remove the uric acid. Once it is all gone then the mechanism turns off. This mimics the behaviour of many different natural cellular processes, miniature thermostats turning off and on in response to a myriad of chemical signals in the body. The system was tested in mice genetically engineered to over-produce uric acid. This new system succeeded in reducing their uric acid levels and the clinical problems such as gout and crystals in the kidneys associated with excess uric acid.
The review authors conclude by arguing for much more work to be done in mammalian cells, developing systems such as the uric acid removal mechanism. Without them it will be impossible to move synthetic biology from the laboratory into the clinic. The examples here demonstrate that it is possible to build synthetic biological systems that can effectively seek out and treat disease in the body. The potential is enormous, but real clinical treatments are still a long, long way away.