Novel molecular therapies for heritable skin disorders

Abstract

Tremendous progress has been made in the past two decades in molecular genetics of heritable skin diseases, and pathogenic mutations have been identified in as many as 500 distinct human genes. This progress has resulted in improved diagnosis with prognostic implications, has refined genetic counseling, and has formed the basis for prenatal and presymptomatic testing and preimplantation genetic diagnosis. However, there has been relatively little progress in developing effective and specific treatments for these often devastating diseases. However, very recently, a number of novel molecular strategies, including gene therapy, cell-based approaches, and protein replacement therapy, have been explored for the treatment of these conditions. This overview will focus on the prototypic heritable blistering disorders, epidermolysis bullosa, and related keratinopathies, in which significant progress has been made recently toward treatment, and it will illustrate how some of the translational research therapies have already entered the clinical arena.

Figure 1

Postulated mechanism by which fibroblast therapy may ameliorate the blistering tendency in RDEB. (a) In normal skin, keratinocytes synthesize type VII collagen molecules (red), which assemble into anchoring fibrils. These fibrils entrap the interstitial collagen fibers in the dermis, securing the stable association at the dermal-epidermal junction. (b) In some patients with RDEB, there are only a few rudimentary anchoring fibrils, allowing formation of blisters below the lamina densa as a result of minor trauma. (c) Allogeneic fibroblasts injected directly into dermis elicit a subclinical immune reaction that leads to synthesis of heparin binding-EGF-like growth factor (HB-EGF), which upregulates the synthesis and assembly of patient’s own mutated type VII collagen. The increase in the rudimentary anchoring fibrils, which are partially functional, stabilizes the association of epidermis to the underlying dermis and ameliorates the blistering tendency. (Adapted from Uitto, 2011a).

Figure 2

Schematic steps of reprogramming somatic cells, such as fibroblasts, to induced pluripotent stem (iPS) cells, and their differentiation into epidermal keratinocytes capable of forming skin-like structures. The reprogramming process is initiated by introduction of transcription factors (cMYC, SOX2, OCT4 and KLF4) into the somatic cells by transduction of expression vectors, synthetic mRNA or recombinant protein. The iPS cells have characteristic features that allow their identification and enrichment. The iPS cells can then be differentiated into keratinocytes under specific culture conditions, e.g., medium supplemented with retinoic acid (RA) and bone morphogenic protein-4 (BMP-4). BMZ, basement membrane zone. (Adapted from Uitto, 2011b).

Figure 3

siRNA strategies for autosomal dominant keratin 6a disorders by targeting either mutant or both mutant/wild-type alleles. (a) In normal keratinocytes, synthesis of K6a (blue), K6b (red) and K6c (green) occurs; (b) in PC keratinocytes with a heterozygous missense mutation in KRT6A there is dominant-negative interference between the wild-type and mutant K6a protein that perturbs the keratin network and compromises cell integrity, leading to skin blistering as a result of minor trauma; (c) one siRNA approach is to target the mutant KRT6A allele to leave only residual wild-type KRT6A allele expression; (d) an alternative siRNA strategy is to silence all KRT6A, both mutant and wild-type – blistering does not occur in the absence of K6a because of functional redundancy with K6b and K6c, allowing normal intermediate filament network integrity.