In 2019, HSCI scientist Amy Wagers, Ph.D. demonstrated that gene-editing machinery can be delivered straight to stem cells where they live, rather than in a lab dish. The findings have major implications for the development of therapeutics for genetic diseases, such as Duchenne muscular dystrophy (DMD).
“If you want to change a genome to correct a disease-causing gene mutation, you have to change it in the relevant stem cells,” said Wagers, an HSCI Executive Committee member. “If you don’t change the stem cells, whatever cells you do fix may eventually be replaced with diseased cells fairly quickly. If you do fix the stem cells, they will create healthy cells that can eventually replace the diseased cells.”
But fixing stem cells is harder than it sounds. Current cell therapies are limited because stem cells have to be extracted, kept alive and healthy, and genetically altered before being returned to the patient’s body. This process is disruptive for the cells, which may ultimately be rejected or fail to engraft back into the patient.
Each type of stem cell is well protected in its own “niche,” often in hard-to-reach places like bone marrow. “When you take stem cells out of the body, you take them out of the very complex environment that nourishes and sustains them, and they kind of go into shock,” Wagers said. “Isolating cells changes them. Transplanting cells changes them. Making genetic changes without having to do that would preserve the regulatory interactions of the cells — that’s what we wanted to do.”
Wagers’ group used an adeno-associated virus (AAV) that infects human (and mouse) cells — but does not cause disease — as a transport vehicle. Building on their earlier work in mice with DMD, Wagers and her colleagues created various AAV packages to deliver gene-editing cargo into several different types of skin, blood, and muscle stem and progenitor cells.
To test whether their AAV complexes managed to deliver, the researchers used mice that act as so-called reporter systems via a “reporter” gene that is normally silenced but can be turned on by gene editing. When the reporter gene is activated, the cell turns bright, fluorescent red.
The researchers observed that in skeletal muscle, up to 60 percent of the stem cells turned fluorescent red. But the utility of the approach extends beyond muscle to other tissues. In cells that give rise to different types of skin cells, up to 27 percent of the cells turned red. Up to 38 percent of the stem cells that make blood in bone marrow were changed. That might seem low, but blood turns over so quickly that in some cases even a single healthy stem cell may be sufficient to rescue a defect.
“We looked at the skin of these AAV-transduced mice from the Wagers lab, and were pleased to see that many dermal cells were successfully edited as well,” said Ya-Chieh Hsu, Ph.D., an HSCI Principal Faculty member. “Those included cells that give rise to dermal adipocytes, and cells that help regulate other stem cells in the skin. We’ve always needed a tool that lets us manipulate dermal cells in vivo rapidly — so for us, this is like a dream come true.”
Delivering a gene therapy directly into a living system has been a barrier for biotech companies trying to develop therapies for diseases like spinal muscular atrophy.
“This is a really important resource for the community,” Wagers said. “It changes the way we can study stem cells in the body — the AAV approach lets researchers investigate different genes for stem cells in their native environment, much more quickly than ever before. The delivery system is robust enough that it can also be used to target genes that affect many different tissues.
“It’s also an important step toward developing effective gene therapies. The approach we developed gets around all the problems you introduce by taking stem cells out of a body and allows you to correct a genome permanently. AAVs are already being used in the clinic for gene therapy, so things might start to move very quickly in this area.”