Duchenne muscular dystrophy (DMD) is one of the most common forms of inherited dystrophies today. It touches mainly young boys, and the majority of those with DMD become wheelchair ridden by their teenage years, and most would not expect to live into their 30's. Currently, there is not an effective treatment for the disease, though, right now, the light is visible on the horizon and things are may change in the near future.
Researchers are making every effort to find the treatment or cure for DMD; they are working in three directions. First is the classical approach of finding some chemical drug that could help by stopping the disease process or even reversing it. The second approach is developing the practical methods of gene editing, so that defective genes in muscles could be edited and, thus, curing the disease. The third approach is to regenerate healthy muscle cells with the use of stem cell therapy. Each method has its pros and cons.
Stem cell therapy or regenerative medicine has come into it's own over the last couple of decades. By the mid of 20th century, researchers knew that some of the body tissues continue to grow and regenerate for their whole life, like blood and immune cells, while other tissues have limited capability to regrow. Soon they discovered the so-called pluripotent cells from a human embryo with the ability to multiply and transform into just any body organ or cell under specific conditions. However, research on embryo cells had ethical issues attached to it, and so little progress was made in the direction.
Soon researchers understood that such stem cells are present in almost every tissue or organ, and more so in a dynamic organ system like skeletal muscles. By the early 21st century, researchers were able to isolate stem cells or satellite cells from muscles and grow them in the lab with limited success. Cells derived from a particular organ are multipotent, with the ability to transform into various other tissues, but not just for any tissue. However, researchers were able to solve this problem with the help of genetic engineering, thus successfully converting multipotent cells to pluripotent cells.
This allowed researchers to create pluripotent cells, and the ability to convert one seed cell to another. In theory, it gives them the power to grow just about any organ in lab conditions. Naturally, it's not that simple. Converting a cell or producing a colony of cells is one thing, developing the part of the organ to be functional is entirely different.
Gradually researchers are making progress in the right direction. In the latest study researchers developed the skeletal muscles cells from stem cells in the laboratory condition, and for the first time, researchers were able to create cells in artificial conditions that were functional, raising the hopes that soon muscle replacement therapy would be available for Duchenne muscular dystrophy. This new methodology of creating mature, transplantable, skeletal muscles cells from pluripotent cells in lab conditions was developed by a research group from the University of California, Los Angeles.
Researchers studied the muscle development process in humans and identified many earlier unknown factors. Using the new learning, researchers created healthy muscle fibers that could restore dystrophin, the protein that is defective in those living with Duchenne. Without dystrophin, muscles cannot efficiently repair the damage caused due to daily wear and tear instigated by various physical movements. Inability to restore means that muscles soon degenerate or wear out, leaving the person physically weaker with each passing day. This failure to repair itself due to dystrophin defect makes respiratory and cardiac muscles progressively feebler, ultimately leading to their defeat. At present no medication can stop this process, reverse it, or cure this disease completely.
Optimism as skeletal muscles are developed from stem cells
Researchers have long been trying to create fully functional muscle tissue in lab conditions that could be used in replacement therapy, thus enabling regeneration of normal dystrophin producing skeletal muscle cells. However, a research team at the University of California headed by Hicks noted that most of the earlier efforts have failed to provide cells that were mature enough to be transplanted, thus causing a failure of efforts.
Hicks says that they noted that just because the cells developed in the lab were showing few biomarkers, the characteristics of skeletal muscles do not make these cells functionally viable muscular cells. For the success in transplantation, it is vital to develop cells that are incredibly close in structure to the naturally occurring muscular cells in human body. To solve the puzzle Hicks et al. looked closer at the development processes of human skeletal muscles.
During the research on fetal skeletal muscle cells, they identified the two earlier unknown surface markers called NGFR and ERBB3; this helped the research team to accurately isolate the right type of muscles cells from the tissue developed from human pluripotent cells.
Once they were able to isolate these new kind of cells, they were able to multiply them in laboratory conditions; however, soon they realized that something was still missing in their understanding, as new cells did produce the healthy dystrophin-producing muscle fibers. However, these fibers were extremely smaller than naturally occurring skeletal muscle fibers.
Hicks says that they understood that they were still missing the vital link. Thus, the isolated cells were failing to mature properly. For transplantation therapy, there is a need for much larger and stronger muscular fibers, with the ability to contract and function.
Looking deeper into the development process in fetal muscle cells they found that so-called TGF beta signal pathway has to be turned off to enable the generation of larger and viable muscles fibers.
Finally, the team was ready to put their discovery into practice by transplanting these labs grown large muscle fibers in a mouse model.
Hicks and the team says that the final aim of their research is to create muscles cells that could be transplanted into humans living with Duchenne, thus in the development process, they followed all the steps required in developing cells for human transplant.
Thus, first they acquired the stem cells with a defective gene from the mouse model that was having Duchenne muscular dystrophy, then they used the technology to convert these cells into pluripotent cells, next they applied the gene editing tool called CRISPR-cas9 to edit the defective gene. Finally, they isolated the lab-grown skeletal muscle cells from the stem cells that had NGFR and ERBB3 markers; then they injected these healthy cells into the mice along with TGF beta inhibitor.
The team at the University of California was able to get the expected results. It is the first study that could demonstrate the success of lab-grown muscle cells in transplant therapy.
Therefore, the team is quite close to reaching their final objective, all that is required now is to find the skeletal muscle cells that could also regenerate and respond to continuous injury after the transplantation.
- Hicks MR, Hiserodt J, Paras K, et al. ERBB3 and NGFR mark a distinct skeletal muscle progenitor cell in human development and hPSCs. Nature Cell Biology. 2018;20(1):46-57. doi:10.1038/s41556-017-0010-2