Inside out: regenerative medicine for recessive dystrophic epidermolysis bullosa

Review nature publishing group Inside out: regenerative medicine for recessive dystrophic epidermolysis bullosa Michael Vanden Oever1, Kirk Twaroski2, Mark J. Osborn1, John E. Wagner1 and Jakub Tolar1 Epidermolysis bullosa is classified as a genodermatosis, an inherited genetic skin disorder that results in severe, chronic skin blistering with painful and life-threatening complications. Although there is currently no cure for epidermolysis bullosa, concurrent advances in gene and stem cell therapies are converging toward combinatorial therapies that hold the promise of clinically meaningful and lifelong improvement. Recent studies using hematopoietic stem cells and mesenchymal stromal/stem cells to treat epidermolysis bullosa have demonstrated the potential for sustained, effective management of the most severe cases. Furthermore, advances in the use of gene therapy and gene-editing techniques, coupled with the development of induced pluripotent stem cells from patients with epidermolysis bullosa, allow for autologous therapies derived from a renewable population of cells that are patient-specific. Here we describe emerging treatments for epidermolysis bullosa and other genodermatoses, along with a discussion of their benefits and limitations as effective therapies. E pidermolysis bullosa (EB) belongs to a group of rare genetic skin disorders primarily caused by mutations in genes that encode for extracellular matrix proteins. EB is characterized by persistent skin blistering and painful lesions. Recessive dystrophic epidermolysis bullosa (RDEB), one of the most severe forms of EB, is due to loss-of-function mutations in the essential extracellular matrix protein type VII collagen (C7) (1). Loss or diminished function of C7 leads to weakness in the structural architecture of the dermal–epidermal junction (DEJ) and mucosal membranes where C7 is deposited (Figure 1). This loss of structural support leads to skin blistering and complications, including esophageal strictures, mitten deformities, itching, and painful blistering (2,3). Children with RDEB are affected from birth, with little to no respite despite constant care and extensive wound dressing (4). In normal skin, fibroblasts and keratinocytes produce C7 near the DEJ and in response to injury or tissue damage (5–7). Through local coordination between systemic immune cells that regulate inflammatory and wound-healing responses, human skin is capable of regenerating injured tissue and maintaining a homeostatic state. However, in the case of RDEB, the supporting cells near the DEJ are unable to produce functional C7 and generate a normal healing response. Furthermore, chronic wounds such as those found in patients with RDEB have shown a limited ability to remodel the extracellular matrix in a productive manner, further limiting the ability for RDEB skin to regenerate itself. Although many of the signs and symptoms of RDEB are apparent at birth, certain aspects of the disease pathology are progressive in nature. Loss of the physical barrier and disruption of the immunologic function of the skin leads to persistent chronic infections, acquired resistance to antibiotics, and can become refractory to interventional therapies. Sustained TGF-β activity and resulting contractile fibrosis from chronic wound healing leads to pseudosyndactyly (8). In addition, RDEB patients struggle with eating solid food, which leads to malnutrition, and develop corneal abrasions that worsen with time (9). Furthermore, many RDEB patients develop an aggressive form of squamous cell carcinoma later in life (10–13). Due to the life-altering, severe nature of this systemic disease, stem cell therapies—with the potential to address the underlying cause of the disorder by providing a lifelong source of normal C7—should be considered. STEM CELL THERAPY FOR RDEB Stem cells have the capacity to address the diverse nature of RDEB symptoms. Initial research in animal models showed that hematopoietic cell transplantation (HCT) of donor-derived cells contributed to wound healing in the skin (14). In this study, green fluorescent protein-labeled bone marrow was transplanted into non-green fluorescent proteinlabeled mice following cutaneous wounding. Interestingly, wounding stimulated bone marrow cell engraftment and induced production of non-hematopoietic skin structures from bone marrow-derived cells. HCT in an RDEB mouse model demonstrated an improvement in animal survival and skin strength (15), and transplanted CD150+/48 − cells homed to injured skin. Deposits of C7 protein and rudimentary anchoring fibrils—structures required for skin integrity that are absent in RDEB mice—were found in treated 1 Division of Blood and Marrow Transplantation, Department of Pediatrics, University of Minnesota, Minneapolis, Minnesota.; 2Stem Cell Institute, University of Minnesota, Minneapolis, Minnesota. Correspondence: Jakub Tolar () Received 21 July 2017; accepted 14 September 2017; advance online publication 1 November 2017. doi:10.1038/pr.2017.244 318 Pediatric RESEARCH Volume 83 | Number 1 | January 2018 Copyright © 2018 International Pediatric Research Foundation, Inc. Regenerative medicine for RDEB a | Review b Figure 1. An illustration of the differences between healthy skin (a) and RDEB skin (b) at the dermal–epidermal junction (80). Copyright © 2015 Massachusetts Medical Society. Reprinted with permission. RDEB, recessive dystrophic epidermolysis bullosa. mice, providing evidence that a subset of cells of hematopoietic origin are capable of correcting the basement membrane zone defect in a murine RDEB model. This work was the foundational platform upon which the first-in-human studies were based (16). Six children with RDEB received allogeneic HCT, all of whom exhibited improved wound healing and a reduction in blister formation. Between 30 and 130 days after transplantation, an increase in C7 deposition was found at the DEJ in five of the six recipients; however, there was no observed normalization of the anchoring fibrils at the times measured. In all six recipients, substantial numbers of donor cells were found in the skin. This initial experience not only supported the potential effectiveness of allogeneic HCT but also the future use of gene-corrected autologous hematopoietic stem cells (17). Although HCT appears to be a viable option for RDEB patients, the inherent risks and safety concerns of HCT are still present, and other approaches with stem cells of non-hematopoietic origin could be used as an alternative or auxiliary approach to HCT. Because of the role of mesenchymal stromal/stem cells (MSCs) in wound healing and their beneficial effect in animal models of EB, we and others hypothesized that MSCs may play a supportive role in the transplant setting. Initial work studied the role of MSCs in wound healing (18). The differentiation potential of MSCs allowed them to be recruited to sites of skin injuries and to transdifferentiate int (...truncated)