by Nessa Carey
The signs in Europe look negative, given the very gloomy history of GM generally, and the recent ruling about edited plants. In the USA, the signs are both confusing and confused, and this is partly down to a turf war between two powerful agencies. The US Department of Agriculture wants to apply to animals the same logic that it has applied in plants. If the change introduced by gene editing is one that could have arisen through traditional breeding practices, there should be no need for regulation. But at the moment the Food and Drug Administration is taking a different view. It wants prior authorisation to be required before meat or other products from gene-edited animals can enter the human food chain.
It’s really important to remember that the experimental animals, the ones in whom the editing took place, will never themselves enter the human food chain. These are the founder stock of pedigree herds and are far too valuable to turn into meat. Essentially, this means that the Food and Drug Administration wants control over animals that have simply inherited genetic changes through perfectly natural breeding.9
This reaches the very depths of regulatory inconsistency when we think of the muscled sheep and cows that could be produced by editing the myostatin gene. Under the current rules, farmers won’t be able to sell meat from these livestock if the originating animal in the line was created by gene editing.
But there are already loads of cattle and sheep in the human food chain which are highly muscled because of changes to the myostatin gene. The Belgian Blue and Piedmont strains of cattle arose naturally through random mutation of this gene. The same is true of Texel sheep. It’s these ‘natural’ mutations that are commonly reintroduced by gene editing.
Two lamb chops, each with the same characteristics. Both have the change in the myostatin gene that created nice chunky animals. If you have access to a DNA sequencing machine you can analyse the myostatin gene but you won’t be able to tell which chop came from a ‘natural’ line of Texel sheep and which from an edited one. They will be identical. But the Food and Drug Administration wants to regulate one and not the other, not because of the DNA sequence but because of the intention behind the DNA sequence. For any scientist this is horribly close to some kind of bizarre magical thinking.
This has also created an entertaining (at best) dilemma for opponents of gene editing, who want complete traceability all the way back through several generations of animal breeding. If editing results in something indistinguishable from a naturally occurring variant, you can’t monitor the food chain to know how the change was originally produced. The DNA sequence will be the same, whether the forebear was a naturally occurring variant like a Belgian Blue, or a gene-edited bull. So the opponents of gene editing have proposed a solution. Their suggestion is that when gene editing is carried out on livestock, the scientists should be forced to include an additional DNA change that can be detected by laboratory tests. This additional sequence will be passed on to offspring, acting as a tag. Which means that opponents of editing want to add ‘foreign’ DNA to the genome, when the scientists working in the field are trying to minimise that, partly to assuage the concerns of the people who objected to ‘foreign’ DNA in the first place.
The healing power of animals
Humans have used animals for thousands of years. Views may vary on how acceptable this is, but there’s little debate about the truth of that statement. Most commonly we have used them as food sources, particularly focusing on their meat, milk and blood, but we interact with them in so many other ways as well. They are our companions, our guards, our hunting allies, our entertainment.
We have also used them as sources of medicinal products for millennia. Ancient Egyptian manuscripts from nearly four thousand years ago detail the use of animal-derived products for medical applications.10 Today we extract venom from snakes and inject tiny amounts into other domesticated animals in order to create antibodies we can use to treat potentially lethal snakebites. Entire species are being driven to extinction to meet the demand for certain traditional Chinese medicines. But with the advent of gene editing, we can use animals in more sophisticated ways than ever before to create therapeutic drugs for human conditions.
The pharmaceuticals that most of us are familiar with are called small molecules. They are things like aspirin, paracetamol, the anti-histamines in hayfever remedies, the statins that lower cholesterol, and the active ingredient in Viagra. These sorts of drugs are quite easy to synthesise using chemical reactions.
Increasingly, however, modern drugs are of the type called biologicals. These are large molecules that can be found in living organisms. The antibodies to treat snakebite are one example, as is the insulin that is vital for people with type 1 diabetes. The best treatments for rheumatoid arthritis and for certain types of breast cancer are biological drugs. A recent market analysis suggested that by 2024 the global market for biological drugs could reach $400 billion a year.11
These drugs are typically very expensive and part of the reason is because they cost an awful lot of money to produce. You can’t make them in a test tube through chemical synthesis, like aspirin, as they are simply too large and complicated. They have to be made by living cells, as only living systems can carry out the complex sets of sophisticated reactions required.
Let’s imagine the molecule you want to use as a drug is usually produced in the human body. Under these circumstances the obvious thing to do might be to isolate the molecule from humans. The commonest example of this is a blood transfusion. But we can spare blood to share with each other because we create more blood rather quickly, so the donor isn’t compromised. But many of the molecules that humans need are only produced in tiny amounts by specific organs. Under these circumstances, it may be that the only way you can harvest enough of the drug you need is by extracting it from post-mortem tissues.
Some children fail to gain height because they don’t produce an essential molecule called growth hormone. At one time, the only way to obtain growth hormone to treat these children was to extract it from dead people. Specifically, it was extracted from the pituitary gland, a tiny structure in the brain, and then injected into the children. What nobody realised at the time was that occasionally the dead donors had been developing a rare form of dementia. This dementia, called Creutzfeldt-Jakob disease, is caused by abnormal proteins that develop in brain cells. When growth hormone was harvested from the brains of people with this type of dementia, nobody realised that the dangerous abnormal protein was accidentally carried over into the preparation. Tragically, when injected into patients who needed the hormone, this abnormal protein triggered the onset of brain degeneration, dementia and finally death. Just under 200 people in the UK are estimated to have died through this route of transmission.12
Because of this, all human growth hormone has been produced in genetically modified bacteria since the mid-1980s. This is safer, cheaper, and can be scaled more easily than extracting the same molecule from human cadavers.
Sometimes animals quite fortuitously produce a protein that is so similar to the human one that we can use it as a medicine. For about 60 years type 1 diabetics were treated with insulin extracted from the pancreas of pigs. This wasn’t ideal as the insulin was a relatively minor component of all the proteins in the pig pancreas and required a lot of expensive purification to produce a relatively small amount of the drug. The pig insulin wasn’t quite identical to the normal human version and it wasn’t suitable for some patients. It was also very difficult to ramp up supply quickly when demand increased. In the 1980s, the drug firm Eli Lilly produced and sold human insulin that had been created in genetically modified bacteria. Now, virtually all insulin is made in bacteria or yeast.
The vast majority of biological drugs are produced in bacteria, in yeast or in cultures of cells from humans or other animals. These systems have their advantages, but also drawbacks. Bacterial cells are less sophisticated than human ones, and aren’t always able to produce a complex protein that has all the same features and characteristics require
d for effective therapies. When mammalian cells are cultured, it can be hard to get the relevant protein produced at high concentration and this adds substantially to the production costs. As a consequence, there are drug programmes where companies are looking for a different approach, and this is where gene editing holds great promise.
There have been some precedents for this using older genetic modification technologies. Researchers added a gene to rabbits so that they would produce a complex biological product required for people who suffer from a genetic disease called hereditary angioedema. In this condition the small blood vessels become leaky and fluid accumulates in the tissues. Not only is this excruciatingly painful, it can be life-threatening if it happens around the airways.13 Injecting the drug produced in the genetically modified rabbits brings these awful episodes under control.14
Let’s imagine you are a scientist who wants to produce good biological drugs. You’d probably want to use systems with certain key features. The obvious ones would be:
Easy to create animals with the necessary genetic change, and with no other disruption to their genome
A production system that is accessible
A production system geared for high levels of production
A production system that can be used for a long time in each animal, rather than having to kill the individual each time you want to harvest the biological agent
For number 1 – well, hello gene editing. And for numbers 2 to 4 – meet eggs.
It’s entirely logical that gene editing for production of biologicals in chicken eggs is gathering pace. Remarkable strides have already been made, especially when we remind ourselves that gene editing really only became technically feasible in 2012.
One of the most advanced programmes has combined gene editing and the naturally high protein potential of eggs in the production of a biological called beta-interferon. This biological is used in the treatment of the form of multiple sclerosis called ‘relapsing’ and it is usually very expensive to produce. In a collaboration between the Institute of Livestock and Grassland Science in Tsukuba, Japan and a company called Cosmo Bio in Tokyo, gene editing was used to create hens whose eggs are rich in beta-interferon.15 The researchers claim that this could drive down production costs by as much as 90%.
Driving down the costs of drugs is vital for both producers and patients. One of the biggest problems facing the pharmaceutical industry is that the drugs they create are too expensive for healthcare providers’ budgets. A biological called Kanuma has been produced in eggs using an older genetic modification technology. Kanuma was developed to treat an ultra-rare disease which affects only nineteen patients in the UK.16 Kanuma is authorised for use by the European Union’s regulatory agency, meaning it is safe and it improves clinical outcomes. But the UK’s National Institute for Health and Care Excellence ruled that at £500,000 per patient, it didn’t create enough long-term benefit to justify this level of expenditure. This issue of reimbursement – where no one will pay for drugs – is the biggest headache the pharmaceutical industry faces. If gene editing can drive down costs substantially, it may increase the chances that patients will be able to access new therapies that can save or improve their lives. But the likelihood is that the decreased production costs will only really make a difference where there are hundreds or thousands of patients who need the drug. For the ultra-rare disorders all the other costs involved in purifying, formulating and distributing the drug, and particularly of running clinical trials, may still result in an unfavourable economic decision.
From sandwich to organ
There are situations where a patient’s clinical needs are just too extreme to be effectively treated or cured by the use of drugs or other existing technologies. Sometimes, nothing less than a whole new organ will do. Maybe a liver, maybe a kidney, maybe heart and lungs. Without a transplant the patient will inexorably decline and ultimately die.
The technology for transplantation exists and the clinical practice is well established. The healthcare benefits are clear and quantifiable. Yet every year huge numbers of people die before receiving a transplant. In the United States there are just under 115,000 people who need this life-saving intervention, and on average twenty people die each day while on a waiting list.17 This picture is repeated throughout the world, because there just aren’t enough people willing to donate their organs after death, even though each donor saves, on average, eight lives.
Public awareness campaigns have highlighted this issue in an attempt to drive up donation rates. Some countries, such as Belgium and Austria, have moved to an opt-out system where implicit consent to donation is assumed unless the donor has made very clear that they are against the decision. But there is still a huge shortfall in available organs worldwide, especially as road traffic deaths fall, since people who died in car crashes were one of the major sources of organ donation. We urgently need a different supply of compatible organs.
What if instead of relying on human donors, we could use organs from animals? This approach is called xenotransplantation, where ‘xeno’ is the Greek word for ‘of foreign origin’. It has long been a dream of transplant specialists. Pig organs are often the most likely candidates, because they are close in size and structure to human organs, and physiologically very similar when we look at structures such as the heart. In mechanical and electrical terms, a pig heart might do pretty well in a human chest cavity.
Unfortunately, there are a number of barriers we need to overturn before the pig supplies us not just with bacon and pork but also with replacement organs. Once again, however, gene editing may help us pick our way past the obstacles.
The genomes of almost all mammals harbour sleeper agents. These are viruses that long ago changed from rampaging around and getting passed on from one sick host to another. Instead, they inserted their own genetic material into the genomes of their hosts. There they slumber, copied every time the host copies its own DNA when one cell divides to form two. When we reproduce we pass them on to our offspring, a set of stowaways gift-wrapped in our own genetic ribbons. Mammals have evolved various molecular defences that keep these interlopers quiescent. But if these mechanisms break down, the sleeping viruses can awaken, and enter a more active phase, marauders once again.
Pigs are no exception to this. Researchers have identified the viruses that lurk unseen in the pig genome. They have shown that these viruses are indeed just lying in wait. They are not dead, they are not broken, they are just silent. And given the right stimulus they can be woken.
What is particularly worrying for the xenotransplantation field is that these pig viruses can also infect human cells. Imagine a human receives a heart transplant from a pig. It’s extremely likely that the human will be taking drugs to dampen their immune system, to minimise the risk of rejecting the pig organ. If the pig viruses reactivate, the immune system may fail to respond to them with sufficient strength and speed. The virus may get a good grip, causing illness in that recipient. Even worse, the recipient may transmit the virus to other people. As a species we are not great at dealing with infections we haven’t encountered before – when Europeans invaded what is now known as Latin America, they brought with them viruses which wiped out between 75% and 90% of the indigenous population.
That level of mortality probably wouldn’t occur in response to the scenario outlined above for pig transplants, but there is certainly a risk to immunosuppressed people in contact with the infected recipients. This includes the very young, the elderly, and the sick. The sick are rather common in hospitals, where we would expect our transplant recipients to be quite regular visitors.
George Church is a Professor at Harvard Medical School. He’s published about 500 papers in his lifetime and has adopted the new gene editing techniques with all the zeal of the 19th-century explorer/missionary his beard so makes him resemble. He has been instrumental in pushing the limits of what the techniques can achieve and his work on the viruses in pig genomes is a great exemplar of this. There ar
e 62 of these sleeping viruses in the pig genome. Church and his team used gene editing to inactivate every single one of these simultaneously. This would have been virtually impossible and a logistical nightmare with older forms of genetic modification. Transmission of the viruses from pig cells to human cells dropped one thousand-fold.18
Two years later, George Church was one of the leaders of the team that took the next jump forwards. Their original research had been conducted in the laboratory, and only in cell culture. In 2017, they combined gene editing with animal cloning approaches and created edited pigs that could not reactivate the viral hijackers in their genomes.19
Church has been quoted as saying that we could see pig-to-human transplants by the end of 2019.20 This seems extraordinarily optimistic, at least in the west where it’s unlikely you could even gain ethical approval that fast for such a radical procedure. There are also many other barriers that have to be overcome, particularly to prevent rapid rejection of the foreign heart. But if we combine the findings of different groups working on all the separate technical problems, we can be increasingly confident that we will be able to use gene editing to hack the genome of the pig in multiple ways, creating herds of porkers with the exact characteristics required for success. At the very minimum we could expect to achieve this for the heart, lungs and kidneys.
Perhaps one day it won’t be the dog that we think of as (wo)man’s best friend, but the pig.
Notes
1. Burkard, C., Lillico, S.G., Reid, E., Jackson, B., Mileham, A.J., et al. ‘Precision engineering for PRRSV resistance in pigs: Macrophages from genome edited pigs lacking CD163 SRCR5 domain are fully resistant to both PRRSV genotypes while maintaining biological function’. PLOS Pathogens (2017); 13(2): e1006206.