Therapies for epidermolysis bullosa: delivery is key.

Variants in the KRT14 gene that encodes keratin 14, are responsible for approximately 37% of all cases of epidermolysis bullosa simplex (EBS).1 EBS is characterized by skin fragility and blistering upon minor friction. Currently, treatment of the disease is merely symptomatic and, therefore, research into therapeutic approaches is of great interest. Over recent years, several therapeutic approaches have been explored, among which are RNA‐based approaches,2 such as trans‐splicing.

The study by Peking and colleagues, in this issue of the BJD,3 aimed to correct a mutation in exon 1 of KRT14 by using a previously reported 5′‐trans‐splicing module that replaces exons 1–7 of the KRT14 mRNA.4 The current study aims to elaborate on the molecular integrity of trans‐splicing corrected keratinocytes. Accordingly, the RNA trans‐splicing module was stably transduced into an EBS patient‐derived keratinocyte cell line. Subsequently, skin equivalents were generated out of these corrected cells and grafted onto the backs of mice. These skin equivalents revealed a stable, well‐differentiated epidermis showing the great potential of trans‐splicing as a therapeutic approach for EBS.

The major advantage of trans‐splicing over many other approaches is that one RNA trans‐splicing module can be employed to correct multiple variants in several exons, eliminating the need for designing patient‐specific or variant‐specific therapies (Fig. 1). However, this comes at a price, as it requires relatively large trans‐splicing modules to be delivered into the target cells.5 And this could pose a problem. Viral delivery into cultured keratinocyte stem cells, followed by fluorescence activated cell sorting, subsequent generation of skin equivalents and, finally, transplantation back into the patient does not seem a viable approach for routine clinical application. No doubt the preclinical results with trans‐splicing have been absolutely encouraging so far; however, the crucial factor to determine whether or not it will eventually earn a place on the list of genetic therapies for genetic skin conditions is whether it will be possible to deliver the trans‐splicing module into the target cells in situ safely and efficiently. Unfortunately, this is true for other approaches as well, and not only RNA‐based ones.6, 7 But there is hope, as methods to deliver nucleic acids such as trans‐splicing modules into cells are rapidly improving and becoming safer.8

Figure 1

Schematic overview of trans‐splicing in the KRT14 gene. The RNA trans‐splicing molecule (RTM) RTM163 comprises three major elements: (i) a green fluorescent protein (GFP) reporter; (ii) the wild‐type (WT) KRT14 cDNA exon 1–7 (purple); and (iii) an intronic binding domain (BD) that binds to intron 7 (blue). Binding of the RTM to the endogenous pre‐mRNA (grey) results into two mRNAs: the cis‐spliced mutant KRT14 mRNA and the trans‐spliced WT mRNA. In this example, exon 1, a hotspot for pathogenic variants, is shown in red. In theory, pathogenic variants located anywhere in exon 1–7 could be corrected by RTM163. In a clinical setting the GFP reporter would be removed resulting in WT KRT14 mRNA only. In contrast to the trans‐splicing approach, which is capable of correcting all variants in exon 1–7, other approaches would most likely need the design and optimization of numerous molecules as more than 60 variants in the KRT14 have been described as causing epidermolysis bullosa.

There are high hopes of a future in which most genetic diseases will be curable by virally delivered gene‐editing tools such as CRISPR (clustered regularly interspaced short palindromic repeats)‐guided endonucleases. CRISPR has already proven its value in the generation of numerous disease models and correction of diseases in vitro.9 However, it is still of great importance that other strategies, such as trans‐splicing and other RNA‐targeting therapies, are not pushed aside for this gene‐editing‐future. The magic bullet of gene‐editing is still far off and complications of viral delivery, viral tropism, off‐target effects and unforeseen carcinogenicity of the CRISPR approach and viral delivery, make RNA‐based and RNA‐targeting therapeutics highly relevant. At least, for a period of time, until gene‐editing can be safely employed as it should. Therefore, it is of great importance that reports like the one by Peking and colleagues are published in journals like the BJD.3 Thus, bringing to the attention of a broader range of researchers and clinicians the numerous other therapeutic approaches in the pipeline for genodermatoses, rather than waiting for the magic bullet. For the heterogeneous group of patients with epidermolysis bullosa, for whom one therapy will not cure all, having options is good.

Conflicts of interest

None to declare.