A new study led US scientist has demonstrated the ability to edit stem cell genes not in a dish, but directly in their natural habitat. The results are important for biotechnology research and the development of therapeutic agents for the treatment of genetic diseases.
“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 Amy Wagers, Professor of Stem Cell and Regenerative Biology. “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.”
However, editing stem cells is more difficult than it seems. To date, this method consists in extracting stem cells, creating favorable conditions for maintaining their viability, their genetic transformation and transplantation back to the patient. This process is disruptive for the cells, with the result that they may be rejected after transplantation.
Each type of stem cells lives in its own well-protected “niche” in hard-to-reach areas such as 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.”
Gene transfer using viruses
Wagers’ group used an adeno-associated virus (AAV) to transport genetic material that infects human and mouse cells, but does not cause disease. Based on the results of their previous work on Duchenne muscular dystrophy on a mouse model, Wagers and her colleagues created various AAV packages to deliver genome-modifying “cargo” to skin, blood, muscle and progenitor stem cells.
“This was a true collaboration between labs specializing in several different organs”, – said Jill Goldstein, a postdoctoral fellow in the Wagers lab and co-first author of the study, published in Cell Reports. “We set up experiments in our organs of interest, analyzed them, compared notes, and made adjustments in a kind of scientific assembly line. None of us could have done it alone — it takes a lot of hands, and the team approach made it really fun.”
To test whether the AAV complexes could be delivered to the required cells, the researchers used a line of transgenic reporter mice.
Reporter systems are used to identify a specific gene with the help of a “reporter” gene, which is normally inactive, but can be turned on by genetic editing. In the present study, reporter gene activation caused the cell to fluoresce in red.
Up to 60% effective
The researchers found that in skeletal muscle up to 60% of stem cells changed color. Up to 27% of stem cells that form various types of skin cells have become fluorescent red. Transformed hematopoietic stem cells of the bone marrow was about 38%. Such a percentage may seem insignificant, however, the blood turns over so quickly that in some cases even one healthy stem cell may be enough to correct the pathology.
“So far, the concept of delivering healthy genes to stem cells using AAV hasn’t been practical because these cells divide so quickly in living systems — so the delivered genes will be diluted from the cells rapidly”, – said Sharif Tabebordbar, an alumnus of Harvard’s Department of Stem Cell and Regenerative biology. “Our study demonstrates that we can permanently modify the genome of stem cells, and therefore their progenies, in their normal anatomical niche. There is a lot of potential to take this approach forward and develop more durable therapies for different forms of genetic diseases. That includes different forms of muscular dystrophy, where tissue regeneration is such an important factor.”
“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, Associate Professor of Stem Cell and Regenerative Biology. “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.”
Breakthrough in gene therapy
Gene therapy applied directly to living systems has hitherto not been available to biotechnology companies developing new methods of treating such diseases as, for example, spinal muscular atrophy.
“This is a really important resource for the community for two reasons”, – Wagers said. “First, it changes the way we can study stem cells in the body. The AAV approach lets researchers investigate the importance of different genes for stem cells in their native environment, much more quickly than ever before. Because the delivery system is so robust, it can also be used to target genes that affect many different tissues.
“Secondly, it’s 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.”