Our time in the womb is one of the most vulnerable periods of our existence. Pregnant women are warned to steer clear of certain foods and alcohol, and doctors refrain from medical interventions unless absolutely necessary, to avoid the faintest risk of causing birth defects.
Yet it is this very stage that is now being considered for some of the most daring and radical medical procedures yet devised: stem cell and gene therapies. “It’s really the ultimate preventative therapy,” says Alan Flake, a surgeon at the Children’s Hospital of Philadelphia in Pennsylvania. “The idea is to avoid any manifestations of disease.”
The idea may sound alarming, but there is a clear rationale behind it. Use these therapies on an adult, and the body part that you are trying to fix is fully formed. Use them before birth, on the other hand, and you may solve the problem before it even arises. “This will set a new paradigm for treatment of many genetic disorders in future,” says Flake.
Flake has been performing surgery on unborn babies for nearly 30 years, using techniques refined on pregnant animals to ensure they met the challenges of working on tiny bodies and avoided triggering miscarriage. The first operation on a human fetus took place in 1981 to fix a blocked urethra, the tube that carries urine out of the bladder. Since then the field has grown to encompass many types of surgery, such as correction of spinal cord defects to prevent spina bifida.
While fetal surgery may now be mainstream, performing stem cell therapy or gene therapy in the womb would arguably be an order of magnitude more challenging. Yet these techniques seem to represent the future of medicine, offering the chance to vanquish otherwise incurable illnesses by re-engineering the body at the cellular level. Several groups around the world are currently testing them out on animals in the womb.
Of the two, stem cell therapy has the longer history: we have been carrying it out on adults since the 1950s, in the form of bone marrow transplants. Bone marrow contains stem cells that give rise to all the different blood cells, from those that make up the immune system to the oxygen-carrying red blood cells. Bone marrow transplants are mainly carried out to treat cancers of immune cells, such as leukaemia, or the various genetic disorders of red blood cells that give rise to anaemia.
One of Flake’s interests is sickle-cell anaemia, in which red blood cells are distorted into a sickle shape by a mutation in the gene for haemoglobin. People with the condition are usually treated with blood transfusions and drugs to ease the symptoms, but even so they may well die in their 40s or 50s. Some are offered a bone marrow transplant, although perhaps only 1 in 3 can find a donor who is a good match genetically and whose cells are thus unlikely to be rejected by their body. “The biggest issue with treating disease with stem cells is the immune system,” says Flake.
And therein lies the main reason for trying a bone marrow transplant in an unborn baby: its immune system is not fully formed. At around the fourteenth week of pregnancy, the fetus’s immune system learns not to attack its own body by killing off any immune cells that react to the fetus’s own tissues. This raises the prospect of introducing donor stem cells during this learning window and so fooling the immune system into accepting those cells. “You can develop a state of complete tolerance to the donor,” says Flake. “If it works for sickle cell, then there are at least 30 related genetic disorders that could be treated.”
The perfect donor is the mother. For starters, she shares half her genes with the fetus and so is more likely to be a close match. More importantly, if the donated cells cross the placenta into the mother’s bloodstream, they will not trigger a maternal immune response which could harm both mother and baby - as well as kill the donated cells.
As with adult bone marrow transplants, the donor cells can simply be injected into the recipient’s bloodstream and will find their way to the fetal bone marrow. The mother’s bone marrow cells don’t need to replace her child’s faulty cells: animal studies suggest that even if only a minority of the offspring’s bone marrow cells come from the donor, this would be enough to prevent or at least significantly alleviate sickle-cell anaemia. But the real beauty of the treatment is that even if few donor cells survive, they can be topped up after birth by a further bone marrow transplant from the mother, without risk of rejection. It would be like having an identical twin at the ready as a lifelong donor.
So far prenatal bone marrow transplants have only been tried in a handful of cases involving rare genetic disorders. With one exception, results have generally been poor, although those attempts differed from Flake’s current approach in several ways: donor cells did not come from the mother, for instance, or the cells were injected too late in the pregnancy to fool the fetal immune system. “We have spent many years in animal models defining the barriers that caused human attempts to fail,” says Flake. So far he has had promising results in dogs with the canine equivalent of sickle-cell anaemia. “We’ve reached a point that encourages me we can do the same thing in humans,” he says.
Yet there are still dangers. Any kind of injection into a fetus during the first three months of pregnancy carries a risk of miscarriage of between 1 and 5 per cent. The question is whether the potential benefits outweigh this risk. “This is a lifelong treatment,” says Jerry Chan of the National University of Singapore, who is developing a stem cell treatment for thalassaemia, another type of anaemia.
Work on gene therapy has likewise been going on for decades. While there have been safety concerns, in particular after a US teenager died and other children developed leukaemia, a couple of adult treatments have reached the clinic: one in China for cancer and, more recently, one in Europe for a rare metabolic disorder disrupting fat breakdown.
As is the case with adults, gene therapy would only be considered for a fetus if it has a condition that is either fatal or would cause a great deal of suffering. “The diseases most amenable to gene therapy are the ones where there’s absolutely no cure or a poor prognosis after birth,” says Simon Waddington, a gene therapist at University College London (UCL).
Once again, there are advantages to intervening in the womb, besides the principle that prevention is better than cure. The fetus’s small size means that for a given dose of therapeutic DNA, a bigger proportion of its cells will be changed. “There is a clear case for going in early,” says Waddington.
There is another benefit: most gene therapies use a viral vector, a virus modified to deliver the therapeutic DNA into cells. Adults sometimes mount an immune response against the virus; babies in the womb do not as they have not yet encountered it.
The team at UCL is focusing its efforts on treating rare genetic disorders of the liver and its metabolism. Symptoms do not appear in the womb, because the mother’s liver performs the fetus’s metabolic functions. After birth, the baby’s own liver fails to kick in.
One such condition, Gaucher’s disease, is caused by the lack of an enzyme needed to metabolise certain fatty molecules. Its absence leads to the build-up of toxins throughout the body, with one form of the disease damaging nerve cells and causing extensive brain damage. Children with this condition live for only two or three years.
Waddington’s colleague Ahad Rahim has used a viral vector to deliver a copy of the missing gene into mice that have the equivalent of Gaucher’s disease. The therapy is injected into the fetus’s bloodstream and can be given later in the pregnancy, so the risk of miscarriage should, in theory, be minimal.
In mice, the viral vector spreads through the brain, nerves and the rest of the body. “We want it to spread,” says Rahim. Such broad take-up is helpful as Gaucher’s disease affects the brain, spleen, muscles and limbs as well as the liver.
But delivery through the bloodstream raises another issue. Since the idea of gene therapy was first mooted, it has been haunted by the spectre of generating harmful mutations that could be handed down to future generations. The central dogma has thus been to avoid tampering with the “germline”, in other words, people’s sperm and eggs. “There has always been that concern that you might affect the germline,” says Waddington.
The fetal cells that go on to become sperm and eggs develop in a compartment that is relatively impermeable to the bloodstream. Their isolation takes place around the seventh week of pregnancy. In theory, gene therapies given after this stage should not reach them. “The risk is pretty low,” says Waddington.
Choosing the appropriate viral vector also reduces the risk. Some types of gene therapy use a virus that inserts itself into the patient’s DNA, so when cells divide the new gene is passed on to daughter cells indefinitely. The UCL team use a virus called AAV, which sits next to the DNA without integrating into it.
That means that even if the virus reaches the sperm and egg precursor cells, it should not be copied when they divide. “There is more concern over germline transmission with integrating vectors,” says Waddington.
The answer to the germline question should be clarified in the next couple of years, once results are in from a trial of fetal gene therapy using AAV in macaques, an animal model that should more closely resemble what would happen in humans. The monkeys have been treated for haemophilia, a blood clotting disorder for which gene therapy in adult humans is also being investigated. Previous trials using integrating viruses have shown little germline transmission, and use of AAV should cause even less.
The macaques are approaching their fourth birthday and becoming sexually mature. “We will soon see if any eggs or sperm contain viruses,” says Chan, who is running the trial. So far, his tests have not found any germline transmission in the eggs of the macaque mothers, who theoretically could also have been exposed.
Whatever the results, however, interfering with the germline may no longer be the no-no it once was, according to Julian Savulescu, a bioethicist at the University of Oxford and editor of the Journal of Medical Ethics. “If the intervention were safe and [led to a cure], it would be highly desirable if this were passed on to the next generation,” he says.
As Waddington points out, if his approach saves a child’s life without correcting their sperm or eggs, “we’re making people viable that otherwise wouldn’t be viable”. Looked at that way, if the goal is to avoid handing down mutations to future generations, then targeting the germline is positively desirable.
However, most see germline transmission as something to avoid. “As soon as you start getting into germline transmission you are entering new ethical territory that at the moment you’re not allowed to enter and for good reason,” says Andrew George, who heads the UK’s Gene Therapy Advisory Committee. “You are talking about passing on something that is going to have a life beyond the individual you’re treating.”
It is hard to predict when these kinds of techniques could move from animals into people. Chan reckons that babies in the womb could be receiving his stem cell treatment in five years’ time. Others, like Flake, prefer not to be drawn on timescales.
The idea of prenatal therapy has been given new impetus by recent advances in genetic sequencing techniques; it has recently become possible to sequence a fetus’s genes without risk of miscarriage, simply using fetal cells that reach the mother’s blood. “It is very possible that whole-genome sequencing will become standard procedure for prenatal care,” says Chiara Bacchelli of Great Ormond Street Hospital in London. At the moment these sorts of rare genetic disorders may be discovered only when a family’s first child gets ill.
The field has also recently been buoyed by the go-ahead for a human trial in which gene therapy will be given to pregnant women rather than the fetus. The aim is to treat fetal growth restriction, a condition caused by lack of blood flow to the placenta on the mother’s side. In extreme cases, this can cause fetal death or brain damage, and it is responsible for most stillbirths seen today.
Signs are usually picked up at the 24-week scan during pregnancy, but as yet nothing can be done. “This condition is currently untreatable,” says Anna David, an obstetrician at UCL who is running the trial.
The treatment is to deliver a gene coding for a chemical signal called vascular endothelial growth factor to blood vessels on the maternal side of the placenta, which should boost their growth. “Success would show it is possible to do these very difficult gene therapy trials in pregnant women,” says David. If so, that might ease the way for fetal trials.
It’s a big if. But it raises the prospect of a world without such debilitating illnesses as sickle-cell anaemia and Gaucher’s disease. “Why not predict things happening before birth and stop any damage occurring?” asks Waddington. “If you can stop this before it happens, isn’t that the perfect situation?”
-by Meera Senthilingam, New Scientist