Recently, I posted here about an experiment that used induced pluripotent stem cells (iPSCs) from muscular dystrophy (MD) patients to treat mice with MD. A similar but different experiment has successfully treated MD mice using stem cells made from a MD patient.
Francesco Saverio Tedesco from University College London and the San Raffaele Scientific Institute in Milan, Italy, has used a stem cell known to hang out around blood vessels called “mesoangioblasts” to initiate this gene therapy/stem cell treatment strategy. Mesoangioblasts are multipotent progenitors of tissues that form from the middle layer of the embryo (mesoderm). More importantly, mesoangioblasts express genes that are normally found in the circulatory system (blood vessels in particular). Therefore, they seem to be able to form blood vessels rather readily, but there are also indications that they also can differentiate into muscle (Cossu G, Bianco P. (2003). Mesoangioblasts–vascular progenitors for extravascular mesodermal tissues. Curr Opin Genet Dev. 13(5):537-42).
Tedesco and his colleagues examined mesoangioblasts of patients with a moderately mild version of muscular dystrophy called limb-girdle muscular dystrophy. Tedesco and his colleagues found that the numbers of mesoangioblasts in these patients was greatly reduced. This is potentially significant, since mesoangioblasts are being strongly considered as candidate stem cell to treat MD patients.
Next, Tedesco’ laboratory isolated fibroblasts and muscle cells (myoblasts) from the MD patients and converted them into iPSCs, using standard lentiviral-based transfection procedures. They then used the patient-derived iPSCs to to make mesoangioblasts, which they grew in culture. The cultured mesoangioblasts were then subjected to genetic engineering techniques that fixed the mutation that caused MD.
Limb-girdle MD (LGD2D) results from a mutation in a gene that encodes a protein called “alpha-sarcoglycan.” This protein, alpha-sarcoglycan is part of a large complex called the dystrophin-associated protein complex. This complex consists of a host of proteins that includes alpha-sarcoglycan, alpha-dystrobrevin, syncoilin, synemin, sarcoglycan, dystroglycan, and sarcospan. These proteins are embedded in the cell membrane of the muscle cell, and they bind to a long, fibrous protein called dystropin inside the cell and a large protein outside the cell called laminin. Collectively, this structure is called a “costamere.” Costameres bind to to the protein dystrophin and thereby physically link the cell membrane of the muscle cell to the contractile proteins inside the muscle. Likewise, costameres bind to an extracellular protein called laminin and physically link the cell membrane of the muscle to the extracellular matrix. Costameres guarantee that the muscle contracts while firmly anchored to a substratum. Mutations in dystrophin tend to cause Duchenne’s muscular dystrophy (DMD), which is the most severe form of MD. However, mutations in any of the genes that encode costamere proteins can cause a form of MD.
Back to Tedesco and his crew. After genetically fixing the mutation in the mesoangioblasts, Tedesco and his collaborators implanted them into the muscles of mice that completely lacked a functional alpha-sarcoglycan gene and had immune systems that did not work properly. Such mice had muscles that worked very poorly and showed some, though not all the characteristics of MD.
The implanted mesoangioblasts went right to work and made muscle fibers that were normal and contained lots of alpha-sarcoglycan. After growing for a time, Tedesco and co-workers put these mice on a treadmill. The mice that had been implanted with mesoangioblasts performed far better than the control mice. This experiment showed that it is at least possible, in principle to treat MD patients with a combination of stem cells and gene therapy.
Tedesco, who was understandably excited by these results. said: “This is a major proof of concept study. We have shown that we can bypass the limited amount of patients’ muscle stem cells using induced pluripotent stem cells and then produce unlimited numbers of genetically corrected progenitor cells.”
Professor Giulio Cossu, another author of this study from University College London said: “This procedure is very promising, but it will need to be strenuously validated before it can be translated into a clinical setting, also considering that clinical safety for these “reprogrammed” stem cells has not yet been demonstrated for any disease.”