Status of current therapy approaches
‘Curative’ therapeutic approaches in development over the past decade aim to correct the primary genetic defect at the DNA, mRNA or protein level. These range from stem cells either donor-derived, or patient-derived and gene-corrected, to corrected fibroblast or keratinocyte treatments, injected intradermally or engineered into skin grafts, intradermal or intravenous protein, or antisense oligonucleotides, and premature termination (PTC) readthrough drugs.*
Symptom-relief therapies address pain and other consequences of poor wound healing, including the inﬂammatory and ﬁbrotic processes of some EB subtypes, which predispose to aggressive and fatal squamous cell carcinoma (SCC).
Ex-vivo gene-cell therapy
The headline-making skin graft treatment of 80% of a child’s body surface in 2017 indicated the feasibility of ex-vivo cell-gene transplant therapy, building on an earlier proof-of-concept, limited transplantation in 2005, also of a patient with laminin-deficient JEB. Nevertheless, extensive grafting is surgically traumatic for patients often in poor physical health. While current ex-vivo gene therapy technology is useful for treating recalcitrant non-healing wounds of limited size, it is still an invasive and expensive option. The development of ex-vivo grafting technologies continues to be refined, and multiple clinical trials are in progress.
Alternative approaches to delivering of gene-corrected cells to treat skin and mucosal surfaces are being developed, notably through the use of ‘spray-on’ technologies. All such treatments need to address the problem of the severe microbial colonisation of infected chronic wounds, usually involving some form of debridement to allow cell therapies to take.
EB is a systemic disease, and for some severe subtypes systemic treatments or cures are essential to ensure survival and quality of life. Step-change improvements to tissue integrity for EB patients will be made by addressing the underlying defects in the structural proteins. To remedy these defects, diverse technologies currently being developed include topical and systemic treatments to provide the missing protein through gene/cell therapy or the direct provision of protein.
Bone marrow transplantation
Early bone marrow transplant treatments for both RDEB and JEB showed proof of principle of systemic stem-cell treatment, though the rigour of pre-transplant chemotherapy to ablate the patient’s bone marrow was fatal for many compromised patients. Subsequent protocol refinements to reduce the severity of chemotherapy enhanced survival in RDEB patients greatly, and the chimerism of bone marrow following treatment did not impact on beneficial outcome. Surviving patients showed very variable results however: some sustained improved wound healing and reduced skin blistering over a period of years, while others showed minimal benefit – these benefits correlated with persistence or loss of transplants and consequent levels of wild-type collagen and correctly assembled anchoring fibrils in the skin.
iPSCs (induced pluripotent stem cells)
iPSCs can be developed from small biopsies of a patient’s skin, and iPSCs have been produced from both fibroblasts and keratinocytes. Following correction of the EB mutation, the iPSCs can then be redifferentiated into either keratinocytes or fibroblasts. iPSC-derived therapies are currently being developed for all major forms of EB.
Depending on the EB type and protein, as little as 20-30% of wild-type levels of the missing protein can be effective in strengthening tissues. As early as 2006, intravenous recombinant human collagen VII was shown in an RDEB mouse model to enhance wound healing and strengthen skin; subsequent development by biopharma for translation to clinical treatment has proved problematic but continues. Intradermal injections of recombinant human collagen VII are also progressing to clinical trial. At present, for laminin-deficient JEB, attempts to develop protein therapy are hindered by the tendency of the protein to form misfolded aggregates during manufacture, and is not currently being pursued.
Approaches are also being developed to downregulate production of a faulty protein (appropriate for dominant forms of EB), or to upregulate production of compensatory proteins or prevent premature readthrough termination.
At the level of transcription, antisense oligonucleotides can be used to induce exon-skipping, thus restoring production of slightly shorter, but functional collagen VII; preclinical testing in mouse models shows enhanced skin strength, and clinical trials for several different AON are now planned.
A further transcription-based approach involves the use of drugs to force transcriptional readthrough of premature termination codons (PTCs). With around 10% of genetic diseases estimated to be caused by PTCs, and consequent nonsense mediated RNA decay, this is a promising approach. Clinical trials of aminoglycoside drugs have shown gentamicin as an active PTCs read-through drug; a minor manufacturing component identified as gentamicin B1 shows promise as being potent without the renal and ototoxicity. Amlexanox, also shown to be effective at driving PTCs readthrough is entering clinical trials. Preclinical research on other PTC readthrough drugs continues.
*EB2017— Progress in Epidermolysis Bullosa Research: toward Treatment and Cure, JID 2017) Uitto et al.