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Stem cells

Afflicted by a bone disease he now treats, Oliver Semler hopes a new therapy can arrest it before birth.

David Klammer

Ailing fetuses to be treated with stem cells

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At the time of his birth, 41 years ago in Germany, Oliver Semler had already fractured one bone, a pencil-thin rib. That, his bowed femurs, and a second fracture at 2 days old told doctors he had osteogenesis imperfecta (OI), a debilitating genetic disease in which brittle bones break easily and often—from being jostled at school, for example, or slipping on wet leaves outdoors. Or even, as in Semler’s case, from acrobatics in the womb. “I had a cast two or three times a year,” says Semler, who’s lost count of his fractures but endured 27 surgeries to repair the worst of them. “For my colleagues at school, it was normal. Oliver is not there 2 or 3 days, and he comes back to school with a cast.”

Improbably, given the severity of his disease, Semler avoided life in a wheelchair—though his growth was stunted and he stands 4 feet 8 inches tall. He attended medical school and became a pediatrician at the University of Cologne in Germany, where he cares for about 250 children with his condition. Semler is now preparing for a first in OI and well beyond: a clinical trial of a stem cell treatment for afflicted fetuses, which aims to give them a healthier start to life than he had himself.

The trial, still being planned, is one of a handful being designed in Europe and the United States for arguably the trickiest patient population there is: pregnant women and their fetuses. After decades of hopes raised and dashed, pediatricians, immunologists, and others are cautiously hopeful that new biological insights and a push for treatment from parents-to-be could turn the tide for prenatal stem cell therapy. “I think [there’s] a lot more of an understanding about what’s feasible, what’s safe, what’s ethical,” says Anna David, who studies fetal therapy at University College London and is collaborating with Semler on the OI trial. 

Even trials with just 30 or so mothers, the target for the OI study, require herculean effort: a dozen academic and industry partners, tens of doctors, millions of dollars. A high-profile failure—or worse, injury to the mother or her fetus—could set the field back decades. “That’s what keeps me up at night,” David says. “Safety, trying to tick all the boxes, thinking as much as possible” about everything that could go wrong. Some fetuses have already been treated on an ad hoc basis, with encouraging results, and that’s one factor spurring the trials. But so is the chance that a vial of the right cells, administered at the right moment, could arrest a devastating disease before birth.

Back in the 1980s, when Semler was in elementary school, hopes ran high for treating fetuses with stem cells. Because the fetal immune system is still developing and some of its sentries, such as the T cells, are not fully functional, doctors believed that fetuses would easily accept foreign cells—unlike even a baby, who might need chemotherapy or other toxic treatment to suppress the immune system. The transplanted cells would flourish into healthy blood cells, immune cells, or other elements the fetus lacked, the thinking went, thereby halting damage that a disease was already inflicting in the womb. Maria Grazia Roncarolo, then a young pediatric immunologist, remembers the heady days in Lyon, France, where this theory was put to the test. 

In 1988 and 1989 at Edouard Herriot Hospital, two fetuses whose blood cell markers showed they had severe immune disorders became the first ever to receive a cell transplant. Doctors led by Jean-Louis Touraine infused them through the umbilical vein with blood stem cells from 7- to 10-week-old aborted fetuses. Roncarolo helped monitor the first baby after he was born. With a little boy wriggling in front of her, “I was really totally excited,” she says. “I thought that I found a way to cure every genetic disease. In my complete young naïveté, I thought I’d found a solution.”

A third fetus brought everyone back to Earth. Instead of an immune deficiency, this one had thalassemia, a life-threatening blood disorder. Hoping to treat the fetus when its immune system was especially primitive and the disease in a nascent stage, doctors infused stem cells into the fetus’s abdominal cavity at just 12 weeks’ gestation. To their shock, the cells were rejected and the transplant failed.

We’ve got to do a clinical trial before people do this willy-nilly.

Anna David, University College London

“The assumption we had that the fetus was tolerant [of foreign cells] was wrong,” Roncarolo says. Brutally disappointed, she overhauled her career, returned to basic research, and now works at Stanford University in Palo Alto, California, where she studies stem cell transplants and immune tolerance. Meanwhile, the rest of the Lyon team persevered, but outcomes were dismal. Two fetuses died before birth from the treatment; in two others the cells—all from aborted fetuses—didn’t engraft. At that point, the doctors halted their efforts.

Outwardly, interest in stem cell therapy for fetuses faded. But behind the scenes, a small cohort of researchers embarked on a years-long journey to understand why it hadn’t worked when immunology suggested it should. Pregnancy is a unique setting, with two genetically distinct beings that intertwine without rejecting each other—a setting that, in theory at least, should offer an opening for cell therapy.

In the late 2000s came a string of discoveries that helped explain both the successes and the failures. For one thing, researchers learned that fetal T cells are, in fact, able to reject foreign invaders—whether a microbe or a cell transplant—more readily than thought. Later, other researchers found that it probably wasn’t just the fetus that had doomed those earlier cell transplants. Maternal immune cells are inevitably circulating through the fetus, and in mouse studies, these cells rebelled against the donor cells.

A group led by Mike McCune at the University of California, San Francisco (UCSF), however, discovered an encouraging phenomenon: Some of the fetus’s T cells are primed to convert to so-called T-regulatory cells, which can tolerate outsiders, particularly cells from the mother. In mice, “if you match the transplant to the mom, [nearly] all of the fetuses you transplant can engraft,” says Tippi Mackenzie, a pediatric surgeon at UCSF. She began wondering whether donor cells from the mother might be the most likely to get accepted. “It was this eureka moment,” she says.

Mackenzie and others also found that in animals, higher doses of donor maternal cells were more effective, and that injecting them into the fetal bloodstream further enhanced success. In humans, this becomes possible after 18 weeks’ gestation, by funneling cells through the umbilical vein.

As science plugged along, desperate parents sought drastic solutions. In early 2002, Stefan and Madeleine Karlsson, living in Uppsala, Sweden, and pregnant with their first child, were riding a roller coaster of uncertainty. A 20-week ultrasound had raised red flags, but additional testing turned up nothing. At 25 weeks, doctors “discovered that something was wrong,” Stefan Karlsson recalls. “After this we did ultrasounds every week, and the next week all was fine, and the week after that something was wrong again, but no one knew what.” At 30 weeks’ gestation—a little more than 2 months before normal delivery—doctors shipped off fetal cells for analysis. The results were devastating. The Karlssons learned that their daughter had OI, later diagnosed as the most severe form, type III. Only one other child in the world, in Canada, has been identified with the same mutation. That baby died at 5 months old.

As a child, Oliver Semler had repeated fractures.

As a child, Oliver Semler had repeated fractures.

Courtesy J. Oliver Semler

The Karlssons were referred to the Karolinska University Hospital in Stockholm, where physicians proposed a radical strategy: Harvest a specific type of stem cell from the liver of an aborted fetus and infuse it through the umbilical vein into the fetus that Madeleine was carrying. The family agreed. 

“We transplanted about 6 million cells,” says Cecilia Götherström of Karolinska, who produced the cells. The stem cells used, mesenchymal stem cells (MSCs), are thought to elicit a less vigorous immune reaction than blood stem cells, and MSCs can develop into bone, along with other connective tissues. The hope was that MSCs would take hold and
produce new, healthy bone.

The transplanted cells were not a genetic match for the mother or the fetus, whom the Karlssons would name Olivia. Nonetheless, at least some of them engrafted: When Olivia was 9 months old a bone biopsy revealed donor cells mixed in with her own.

At first Olivia did better than the doctors anticipated. But around her sixth birthday she began to deteriorate and suffered a spate of fractures. Eventually, Götherström and her colleagues retransplanted Olivia with more MSCs from the same donor tissue. Olivia’s fractures abated, and she has been retransplanted every 4 years since. Now 14 years old, she has not had a fracture in 18 months, surprising her doctors. Olivia’s father says she swims for physical therapy, enjoys sewing, arts and crafts, and, like most teenagers, shopping for clothes and spending time with her friends.

Götherström knows that, without a control group, there’s no way to be sure that MSCs helped Olivia. Still, heartened by Olivia’s experience, she and her colleagues later shipped fetal MSCs to Singapore for another treatment of a fetus with OI, and infused a third whose parents traveled to Sweden from their home in Ireland; that child is now a toddler. And, with others, Götherström began laying the groundwork to test whether in utero therapy really was making a difference. “We’ve got to do a clinical trial before people do this willy-nilly,” says David in the United Kingdom—especially because interest in such treatments could surge as it becomes easier to diagnose serious disorders in utero.

Two years ago, Semler and Götherström met at a conference on OI in Wilmington, Delaware. Until then, Semler hadn’t considered stem cell transplants a viable treatment for the disorder. But he knew better than almost anyone that severe OI takes hold before birth, when the skeleton is still forming. On ultrasounds, doctors detect shortened leg bones and even fractures. Supplying MSCs as early as possible, Semler thought, might give a fetus a better chance to build healthy bone, without the side effects of a bone marrow transplant after birth. And Götherström believed that infusing the cells before birth had a better chance of working in part because of differences in fetal circulation: Cells infused into the bloodstream are less likely to get stuck in the lungs and never make it to their destination than they are in a baby.

V. Altounian/Science

An OI trial, called BOOSTB4, is now edging toward the starting line. Götherström, David, Semler, and others have secured more than $9 million from the European Union and Swedish Research Council, and are crafting their protocol. Starting later this year, they hope to begin recruiting 15 families across Europe with fetuses shown by ultrasound and DNA testing to have severe OI and in the second or third trimester infuse them with MSCs from fetal liver cells, just like those Olivia received. The babies will receive additional infusions after birth. They’ll be compared with another cohort of 15, who will receive MSC transplants only after birth, and with historical controls. All the children will be followed for 10 years, to see whether those treated in utero and afterward have fewer fractures, better bone mineral density, better growth, and better quality of life.

“We are not aiming for a cure,” Götherström cautions. “It’s not that they will get up from a wheelchair and start running.” Rather, she hopes that halving the number of fractures, which, along with demonstrating safety, is the trial’s goal, will make a major difference to patients and families. And simply pulling off a trial like this one “could open up a wider field of other disorders being treated” before birth, she says. 

In the United States, Mackenzie’s group and another have embraced a different approach: treating fetuses with blood stem cells culled from their mothers, the strategy she began exploring in animals almost a decade ago. Last month she filed a request with the U.S. Food and Drug Administration to offer hematopoietic stem cells to fetuses with α thalassemia, a severe form of the disease that is fatal before or soon after birth. Prenatal cell therapy failed for thalassemia nearly 30 years ago, but Mackenzie hopes for a different outcome this time.

Because the cells come from the mother, she expects them to be better tolerated and will infuse a much higher dose into the umbilical vein after 18 weeks’ gestation. “If that trial doesn’t work, then we would like to go earlier,” Mackenzie says, potentially by injecting cells into the fetus’s heart.

Alan Flake at The Children’s Hospital of Philadelphia (CHOP) in Pennsylvania is planning a similar clinical trial of maternal stem cells in fetuses with sickle cell anemia, a painful, debilitating disease that often shortens life. (Flake led the only known fetal stem cell transplant in the United States. Done in the mid-1990s on a fetus with a severe immune deficiency, it used stem cells donated by the father, and was considered largely successful.) Flake declined to speak with Science, citing a hectic schedule and concerns about hyping a treatment not yet in trials, but in a video on CHOP’s website he was upbeat. “The research results are absolutely convincing that we can achieve a cure of sickle cell disease,” Flake says. The hospital is raising several million dollars to support a trial.

The medical risks of in utero cell transplants are generally considered modest, though they include pregnancy loss. Similar procedures, such as fetal blood transfusions for anemia, are already offered. But some worry about risk that may come after birth. “We are relying on the idea that the stem cells will proliferate and differentiate into what we want,” says David Chitayat, a medical geneticist at The Hospital for Sick Children in Toronto, Canada. Chitayat wonders whether proliferation could proceed in unexpected ways, for example leading to tumors—a worry that cuts across many stem cell therapies. (Mackenzie notes that it shouldn’t be a problem for her strategy, because she plans to use the same kind of cells as bone marrow transplants, which haven’t caused tumors.)

Some also worry that the mother’s interests will be subsumed by her fetus’s, especially as fetal treatment programs are usually based in children’s hospitals, and pediatricians consider the fetus their patient. “There’s a person who has that uterus in her body” that needs to be accessed, says Anne Drapkin Lyerly, a bioethicist at the University of North Carolina, Chapel Hill, who has studied fetal surgeries for spina bifida and other conditions. Any treatment will affect not just the fetus, but “the people who are going to be caring for this baby,” she says. Lyerly admits she doesn’t know exactly how these concerns should be addressed, beyond careful attention to the mother’s experience, including how she may react if treatment causes pregnancy loss.

The doctors driving these trials are acutely aware of the risks, but emphasize that the conditions they aim to treat cause lifelong suffering and often early death. If prenatal stem cell therapy proves successful and safe, ethicists and other physicians agree, it could be life-changing. And as Chitayat notes, “if you don’t start, you don’t know.”

Sharing why he embraced cell therapy 14 years ago, when his wife was 7 months pregnant and the field far murkier than it is today, Stefan Karlsson reaches for an answer any parent can appreciate. “For us it was that we did not see any drawbacks in trying stem cells, there were only advantages,” he explains. “And that it could help Olivia.”