Testing Stem Cell Quality

A new paper published in the journal EMBO Molecular Medicine by a team from the Lausanne University Hospital describes a protocol that can ensure the safety of adult epidermal stem cells before they are used as treatments for patients. The approach devised by this team takes cultivated, genetically modified stem cells and isolates single cells that are then used to make clonal cell cultures. These cloned cells are then rigorously tested to ensure that they meet the highest possible safety criteria. This protocol was inspired by approaches designed in the biotechnology industry and honed by regulatory authorities for medicinal proteins produced from genetically engineered mammalian cells.

“Until now there has not been a systematic way to ensure that adult epidermal stem cells meet all the necessary requirements for safety before use as treatments for disease,” says EMBO Member Yann Barrandon, Professor at Lausanne University Hospital, the Swiss Federal Institute of Technology in Lausanne and the lead author of the study. “We have devised a single cell strategy that is sufficiently scalable to assess the viability and safety of adult epidermal stem cells using an array of cell and molecular assays before the cells are used directly for the treatment of patients. We have used this strategy in a proof-of-concept study that involves treatment of a patient suffering from recessive dystrophic epidermolysis bullosa, a hereditary condition defined by the absence of type VII collagen which leads to severe blistering of the skin.”

Barrandon and co-workers have cultivated epidermal cells from patients who suffer from epidermolysis bullosa. These cells were then genetically engineered in order to insert a normal copy of the type VIII collagen gene. Then the genetically fixed cells were grown in culture so that they can be used to regenerate skin. Barrandon and others subjected these cells to an array of tests in order to determine which of the genetically engineered cells meet the requirements for safety and “stemness,” which refers to the stem cell characteristics that distinguish it from regular cells; its developmental immaturity and its ability to grow and self-renew. Clonal analysis revealed that the cultured, genetically engineered stem cells varied in their ability to produce functional type VII collagen. When the most viable, modified stem cells were selected and transplanted into the skin of immunodeficient mice, the cells regenerated skin and produced skin that did not blister in the mouse model system for recessive dystrophic epidermolysis bullosa. Furthermore, the cells produced functional type VII collagen. The safety of the cells was assessed by mapping the sites of integration of the viral vector. Because such viruses and produce gene rearrangements other mutations, the chosen cell lines were subjected to whole genome sequencing. Only the cells with insertions in benign locations were considered for use in their mouse model.

Barrandon concluded: “Our work shows that at least for adult epidermal stem cells it is possible to use a clonal strategy to deliver a level of safety that cannot be obtained by other gene therapy approaches. A clonal strategy should make it possible to integrate some of the more recent technologies for targeted genome editing that offer more precise ways to change genes in ways that may further benefit the treatment of disease. Further work is in progress in this direction.”

This work is certainly fascinating, but I think that using integrating viral vectors is asking for trouble. Certainly it should be possible to fix or replace the abnormal type VII collagen gene. Viruses that randomly insert genes into the genome can cause genetic problems, and even sequencing the genome may not properly address the safety concerns of the use of such viral vectors.

Treating A Genetic Skin Disorder with Induced Pluripotent Stem Cells

Dystrophic epidermolysis bullosa (RDEB) is an inherited skin disease that causes fragile skin. RDEB is caused by mutations in the gene that encoded a protein called type VII collagen. Because collagen is a major structural component of skin, collagen mutations result in fragile skin and mucous membranes that blister easily if they are subjected to even slight mechanical stresses. There are no cures for such diseases, but skin creams and palliative care can decrease the severity of the symptoms.

Induced pluripotent stem cells (iPSCs) have the ability to treat such genetic diseases. In order to provide proof of principle of the applicability of iPSCs for the treatment of RDEB, Daniel Wenzel and his colleagues in the laboratory of Arabella Meixner from the Institute of Molecular Biotechnology of the Austrian Academy of Sciences in Vienna, Austria made iPSCs from mice that harbored mutations in the gene that encodes type VII collagen (Col7a1) and exhibited skin fragility and blistering. The symptoms displayed by these Col7a1-mutant mice resembled human RDEB.

Wenzel and his coworkers then genetically repaired the Col7a1 mutations in these iPSCs, and then differentiated these cells into functional fibroblasts that expressed and secreted normal type VII collagen. When implanted, the genetically-repaired iPSC–derived fibroblasts did not form tumors, and could be successfully traced up to 16 weeks after intradermal injection. Therapy with iPSC-derived fibroblasts also resulted in faithful and long-term restoration of type VII collagen deposition at the epidermal-dermal junction of Col7a1 mutant mice, and restored the resistance of the skin to mechanical stresses.

Thus, intradermal injection of genetically repaired iPSC-derived fibroblasts restored the mechanical resistance of the skin to blistering in RDEB mice. These data demonstrate that, at least in principle, RDEB skin can be effectively and safely repaired using a combination of gene therapy and iPSC-based cell therapy.

A similar study examined another type of epidermolysis bullosa.  Noriko Umegaki-Arao and her colleagues in the laboratory of Angela Christiano from Columbia University used iPSCs to treat mice with a distinct type of epidermolysis bullosa that resulted from mutations in COL17A1 gene, which encodes type XVII collagen (Col17).  In this case, however, the mutation has been observed to revert or fix itself in patients.  Patients tend to have patches of skin that are normal in a sea of abnormal skin.

Therefore, Umegaki-Arao and her coworkers derived iPSCs from Col17-mutant mice, differentiated them into skin cells (keratinocytes) and then cultured them, examining individual clones for reversion to normal Col17, which was fairly easy to do as it turns out.  Once revertant-iPSC keratinocytes were properly secured, and then used them to reconstitute human skin in mutant mice.  Thus, revertant keratinocytes can be a viable source of spontaneously gene-corrected cells for developing iPSC-based therapeutic approaches in the treatment of epidermolysis bullosa.