The genetic disorder epidermolysis bullosa is the stuff of nightmares. The epidermis contains the cells that form our body's boundary with the outside world; in epidermolysis bullosa, they lose their ability to hold on to the cells underneath them. Small scratches that healthy people wouldn't notice cause the skin to blister off, leaving these patients prone to infection. The constant inflammation makes cancer more likely. More than 40 percent of those afflicted don't even survive to adolescence.

Now, for the first time ever, researchers have restored functioning skin to a young epidermolysis bullosa patient. Their method? His lost skin was entirely replaced using stem cells that had been genetically engineered to replace the inherited defect. The basic outline of the work, published today in Nature, would have sounded like a work of science-fiction less than two decades ago.

The patient

The work was possible because of years of basic research and technology development. To begin with, we have a good handle on the genes involved in epidermolysis bullosa and how the proteins they encode work. The junction between the cells of the epidermis and underlying dermis contains a bed of proteins called the extracellular matrix. The cells on either side have specialized proteins that allow them to latch on to the extracellular matrix. Epidermolysis bullosa is caused by mutations that damage any of a number of genes that encode components of the extracellular matrix or the protein that latches on to them. Because we knew all of this, it's relatively easy to identify the damaged gene when doctors encounter a patient with epidermolysis bullosa.

For the patient at the center of the new work, that encounter took place at the Children’s Hospital of Germany's Ruhr University. Only seven years old, his damaged skin had become infected with bacterial pathogens, leading his condition to deteriorate. Shortly after admission, he lost a horrifying 60 percent of his skin; another 20 percent was lost over the next few weeks. Death was likely, so the parents agreed to a completely experimental treatment; the doctors also received approval from the hospital's ethics review board and the regional government.

What his doctors attempted had only been tried on a small scale with a single patient. It relies on the fact that our skin has a natural healing capacity, with lots of cells that can grow rapidly to cover wounds and other damage. In fact, our epidermis will typically replace itself with fresh cells every month even in the absence of damage. Over the years, we've developed the ability to leverage the skin's natural regenerative capacity by growing epidermal cells in incubators, creating sheets of epidermal tissue that can be used to help burn victims.

Replacement cells

In this case, the doctors started with just a four-centimeter-square bit of undamaged epidermis removed from the patient. The cells were placed into culture and infected with a genetically engineered virus. The virus contained a copy of the specific gene damaged in the patient, along with a DNA sequence that ensured the gene would be active in any cell it was placed into. After infection, the virus' DNA was integrated into the cells' DNA, allowing it to be inherited each time they divided.

And divide they did. The cell population expanded dramatically over the next month, allowing the team to gradually transfer sheets of cultured epidermis to the patient, ultimately replacing more than 80 percent of his original epidermis over the course of two months. By a month after the completion of these procedures, the patient had a nearly complete epidermis again. Two months after that, he had recovered enough to be discharged from the hospital. "His epidermis is currently stable and robust, and does not blister, itch, or require ointment or medications," the team reports.

This is a stunning achievement, but there is one very large concern about the procedure. For example, the virus inserts copies of itself at random in the genome. There's a chance that it could insert somewhere that disables a gene. If the gene helps prevent cells from becoming cancerous and dividing uncontrollably, it might actually help cells with this insertion grow in culture. As a result, there's the chance that the procedure may leave the patient at risk of skin cancers.

To get a handle on this, the researchers looked at where the virus inserted in samples of the cultured cells, both before and after transplantation into the patient. In cultured cells, the authors found there were large numbers of different insertion sites (they identified more than 27,000 of them). Forty percent of these sites, however, were in between genes; another 47 percent were in introns, parts of a gene that aren't used to encode a protein. So, most of the insertion locations aren't anywhere that's likely to be harmful. In addition, there was no sign that any of these insertions caused the cells to grow more quickly in culture.

Finding stem cells

Things looked very different after the cells had been transplanted to the patient, however. By four months afterward, only a few hundred individual insertions showed up in samples taken from biopsies, and the same insertions showed up in many cells. The researchers explain this by proposing that the epidermis is maintained by a small population of stem cells, which produce a much larger population of rapidly dividing epidermal cells. In culture, all these rapidly dividing cells pick up the virus, so you see a lot of insertion sites. Over time, however, they mature and are replaced by newer cells—remember, the epidermis' cells turn over every month. And those newer cells all come from a small population of stem cells.

Based on the numbers they saw, the researchers estimate that about five percent of a typical epidermal cell population is a stem cell.

It's possible that this work could give us a handle on how to identify these stem cells in a culture of epidermal cells. If we can do that, then it's possible we could screen them for virus insertion sites that are unlikely to damage genes, improving the safety of this procedure.

This won't eliminate all the problems with epidermolysis bullosa, since it also affects some internal tissues, like the esophagus and nasal passages. But those don't seem to pick up the same level of damage as the skin. And the authors suggest that, if the procedure is validated in additional patients, it would open the door for gradual treatment of people with epidermolysis bullosa before serious problems arise—in other words, replace their skin through a gradual process while they're infants so that, by the time they're walking and facing injuries, their skin can handle the damage.

Nature, 2017. DOI: 10.1038/nature24487 (About DOIs).