Gene therapy advances shine the spotlight on epidermolysis bullosa, bringing hope to patients

Recent breakthroughs in gene therapy continue to transform the landscape of therapies for epidermolysis bullosa (EB), offering encouraging outcomes for patients suffering from this severe blistering skin disease. Two different therapeutic strategies that have already reached the clinic share the common goal of restoring the missing or dysfunctional protein via gene replacement. Notably, while ex vivo approaches have shown success in the past, a ground-breaking in vivo gene therapy product has recently gained approval from the US Food and Drug Administration, marking a significant milestone for EB therapy.

To date, two forms of gene therapy have shown promise for application in EB: the clinically advanced gene replacement strategy and gene editing based on designer nucleases.¹ EB belongs to a group of genodermatoses caused by mutations in genes expressed by epidermal keratinocytes and, to some extent, dermal fibroblasts.² Currently, mutations in more than 16 genes have been described that lead to functional impairment, reduction or absence of the respective protein within the skin’s basement membrane zone, the junction between the epidermis and the underlying dermis. As a consequence, the connectivity between both skin layers is reduced, leading to skin fragility characterized by the formation of extended blisters and lesions upon mechanical stress. The affected gene determines the plane of cleavage within the tissue and consequently the EB type. Mutations within keratin 5 and 14, as well as within plectin, lead to intraepidermal blistering, a hallmark of EB simplex. Junctional EB (JEB) is caused by mutations in genes encoding laminin-332, type XVII collagen, and integrin-α6β4. In the very severe and debilitating dystrophic EB (DEB) form, causal mutations are localized within the type VII collagen-encoding gene.² In general, the treatment of EB is limited to wound management, highlighting the need for long-lasting and curative gene therapies that would reduce the daily burden of affected families.

Up until recently, the most successful gene replacement therapies in EB had been achieved in JEB patients carrying mutations in the LAMB3 gene.^3,4,5 In an ex vivo approach, epidermal stem cells, residing both in the epidermal basal layer and in the bulge of the hair follicle,^6,7 were isolated from patient skin biopsies and treated with a retroviral vector expressing the full-length cDNA of LAMB3. Corrected cells were then expanded into autologous epidermal sheets, which were transplanted back onto the patient. The transgenic epidermis is sustained by a defined number of regenerative epidermal stem cells, resulting in long-term clinical outcomes with no severe adverse effects reported to date.^8,9,10 The most recent clinical application in a 7-year-old JEB patient, who had lost almost his entire skin due to concurrent sepsis, was lifesaving. The fast execution of the ex vivo treatment enabled the transplantation of 0.85 m² transgenic epidermal grafts—maintained by somatic stem cells with high proliferative potential^5,9—to cover ∼80% of the patient’s body surface area over the course of three surgeries. At 8 years post treatment, the outcome shows the effectiveness and life-changing potential of the ex vivo approach.

Curiously, applying a similar ex vivo strategy in recessive DEB did not result in an equivalent success. Although the treatment was well tolerated, long-term graft maintenance at 12 months was limited, and the initial improvement in type VII collagen expression and anchoring fibril formation declined over time, resulting in re-blistering at the graft site at 6 months. A possible explanation may lie in an insufficient number of corrected epidermal stem cells ex vivo.^11,12 An alternative ex vivo strategy involving intradermal injection of genetically modified fibroblasts showed variable increases in type VII collagen expression in treated versus non-treated skin areas in two out of four treated patients at 12 months, but no new mature anchoring fibril formation was detected.¹³ Only one patient reported improved wound healing at the treated site. Overall, compared with the successes achieved in JEB, the clinical experience with ex vivo gene and cell therapy in recessive DEB has been unsatisfactory, especially considering the substantial burden of the procedure. The variance in clinical outcomes across and within EB types underscores the impact of the complexity and heterogeneity of EB on therapeutic success.

In vivo gene therapy is largely viewed as the ultimate goal in the development of EB therapeutics, especially given the accessibility of skin. Thus, the recent US Food and Drug Administration approval for the gene therapy product beremagene geperpavec (B-VEC) has ignited excitement and expectations in both patients and the scientific community alike. Developed by Krystal Biotech, this ground-breaking treatment, marketed under the name Vyjuvek, marks the first topical and redosable in vivo gene therapy for DEB. Vyjuvek utilizes a modified non-replicating herpes simplex virus to deliver two copies of COL7A1 cDNA into skin cells within wounds.^14,15 The phase III, double-blind, intrapatient randomized, placebo-controlled study conducted at Stanford University included 31 patients aged 1–44 years.¹⁴ Skin wounds were treated weekly with B-VEC or placebo for 26 weeks. The most clinically relevant outcome: 67% of wounds treated with B-VEC had healed by 3 months and remained healed at 6 months. By contrast, only 22% of wounds treated with placebo achieved complete healing at 6 months, and these wounds showed a highly dynamic course of healing and recurrence over the 6-month observation period. Thus, considering the mild systemic side effects and pruritus observed with treatment, this in vivo approach represents a momentous achievement in the field of gene therapy for genodermatoses. Future trials will reveal the durability and potential long-term side effects of B-VEC.

As remarkable as B-VEC is, it does not mediate permanent correction of the skin cells and will need to be repeatedly applied throughout the patient’s lifetime. Currently, gene replacement using an integrating viral vector (as for JEB) or gene editing via designer nucleases such as CRISPR-Cas represent the only means to achieve a local permanent cure. The good news is that we are inching closer toward achieving this goal. Enabled by continuous innovations in both designer nucleases and editing strategies, gene editing has become an efficient, versatile, and increasingly safe option to restore gene function in genodermatoses. In an ex vivo setting,^16,17 safety issues concerning nuclease-mediated genomic toxicity, including genetic modifications and chromosomal aberrations at on- and off-target sites, can be analyzed in depth, facilitating selection of corrected clones with the best safety profile before expansion and transplantation back to the patient. However, as the in vivo application onto the skin is a less-invasive procedure, the development and establishment of transcutaneous in vivo delivery platforms for designer nucleases will facilitate the bench-to-bedside translation of promising gene editing candidates and is considered a primary aim of current research. In this context, B-VEC gives hope to patients and their families and has set a benchmark upon which new therapies can be built upon. This will undoubtedly inspire researchers in this field to set and achieve new milestones in the therapy of EB.