Status of current therapy approaches
‘Curative’ therapeutic approaches in development over the past decade aim to correct the primary genetic defect at the level of DNA, mRNA or protein. These range from stem-cell 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
News-headline skin-graft treatment of 80% of a child’s body surface in 2017 [ref 2017 De Luca paper] point to the feasibility of ex-vivo cell-gene transplant therapy, building on earlier proof of concept limited transplantation in 2005, also of a patient with laminin-deficient JEB [ref DeLuca 2005 paper]. 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. Development of ex-vivo grafting technologies continues to be refined, and multiple clinical trials are underway.
Alternative approaches to delivery of gene-corrected cells to treat skin and mucosal surfaces are in development, notably though use of ‘spray-on’ technologies [refs] . All such treatments need to address the problem of the heavy microbial colonization 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 come from addressing the underlying defects in the structural proteins. To remedy these defects, diverse technologies in development include topical and systemic treatments, to provide the missing protein through gene/cell therapy or direct provision of protein.
Early bone marrow transplant treatments [ref Tolar papers] 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.
iPSC (induced pluripotent stem cells)
iPSC can be developed from small biopsies of a patient’s skin, and iPSC have been produced from both fibroblasts and keratinocytes. Following correction of the EB mutation, the iPSC can then be redifferentiated into either keratinocytes or fibroblasts. iPSC-derived therpaies are currently in development for all major forms of EB.
Depending on the EB type and protein, as little as 20-30% of wildtype 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 being currently pursued.
Approaches are also in development to downregulate production of 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 use of drugs to force transcriptional readthrough of premature termination codons (PTC). With around 10% of genetic diseases estimated to be caused by PTC, and consequent nonsense mediated RNA decay this is a promising approach. Clinical trials of aminoglycoside drugs have shown gentamicin as an active PTC read-through drug; a minor manufacturing component identified gentamicin B1 shows promise as potent without the renal and ototoxicity. Amlexanox, also shown to be effective at driving PTC readthrough is entering clinical trials. Prclinical research on other PTC readthrough drugs continues.
*EB2017— Progress in Epidermolysis Bullosa Research: toward Treatment and Cure, JID 2017) Uitto et al.