Progress towards genetic and pharmacological therapies for keratin genodermatoses: current perspective and future promise

Keratins as an Inflammation Trigger Point in Epidermolysis Bullosa Simplex

Epidermolysis bullosa simplex (EBS) is a group of inherited keratinopathies that, in most cases, arise due to mutations in keratins and lead to intraepidermal ruptures. The cellular pathology of most EBS subtypes is associated with the fragility of the intermediate filament network, cytolysis of the basal layer of the epidermis, or attenuation of hemidesmosomal/desmosomal components. Mutations in keratins 5/14 or in other genes that encode associated proteins induce structural disarrangements of different strengths depending on their locations in the genes. Keratin aggregates display impaired dynamics of assembly and diminished solubility and appear to be the trigger for endoplasmic reticulum (ER) stress upon being phosphorylated by MAPKs. Global changes in cellular signaling mainly occur in cases of severe dominant EBS mutations. The spectrum of changes initiated by phosphorylation includes the inhibition of proteasome degradation, TNF-α signaling activation, deregulated proliferation, abnormal cell migration, and impaired adherence of keratinocytes. ER stress also leads to the release of proinflammatory danger-associated molecular pattern (DAMP) molecules, which enhance avalanche-like inflammation. Many instances of positive feedback in the course of cellular stress and the development of sterile inflammation led to systemic chronic inflammation in EBS. This highlights the role of keratin in the maintenance of epidermal and immune homeostasis.

Research on genodermatoses using novel genome-editing tools

Genodermatoses comprise a clinically heterogeneous group of mostly devastating disorders affecting the skin. To date, treatment options have in general been limited to symptom relief. However, the recent technical evolution in genome editing has ushered in a new era in the development of causal therapies for rare monogenetic diseases such as genodermatoses. The present review revisits the advantages and drawbacks of engineered nuclease tools currently available: zinc finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs), meganucleases, and - the most innovative - clustered regularly interspaced short palindromic repeats (CRISPR)-associated (Cas) nuclease 9 (CRISPR/Cas9) system. A mechanistic overview of the different modes of action of these programmable nucleases as well as their significance for causal therapy of genodermatoses is presented. Remaining limitations and challenges such as efficient delivery and off-target activity are critically discussed, highlighting both the past and future of gene therapy in dermatology.

RNA-based therapies for genodermatoses

Genetic disorders affecting the skin, genodermatoses, constitute a large and heterogeneous group of diseases, for which treatment is generally limited to management of symptoms. RNA-based therapies are emerging as a powerful tool to treat genodermatoses. In this review, we discuss in detail RNA splicing modulation by antisense oligonucleotides and RNA trans-splicing, transcript replacement and genome editing by in vitro-transcribed mRNAs, and gene knockdown by small interfering RNA and antisense oligonucleotides. We present the current state of these therapeutic approaches and critically discuss their opportunities, limitations and the challenges that remain to be solved. The aim of this review was to set the stage for the development of new and better therapies to improve the lives of patients and families affected by a genodermatosis.

Immune-competent human skin disease models

All skin diseases have an underlying immune component. Owing to differences in animal and human immunology, the majority of drugs fail in the preclinical or clinical testing phases. Therefore animal alternative methods that incorporate human immunology into in vitro skin disease models are required to move the field forward. This review summarizes the progress, using examples from fibrosis, autoimmune diseases, psoriasis, cancer and contact allergy. The emphasis is on co-cultures and 3D organotypic models. Our conclusion is that current models are inadequate and future developments with immune-competent skin-on-chip models based on induced pluripotent stem cells could provide a next generation of skin models for drug discovery and testing.

Keratins K2 and K10 are essential for the epidermal integrity of plantar skin

BACKGROUND

K1 and K2 are the main type II keratins in the suprabasal epidermis where each of them heterodimerizes with the type I keratin K10 to form intermediate filaments. In regions of the ears, tail, and soles of the mouse, only K2 is co-expressed with K10, suggesting that these keratins suffice to form a mechanically resilient cytoskeleton.

OBJECTIVE

To determine the effects of the suppression of both main keratins, K2 and K10, in the suprabasal plantar epidermis of the mouse.

METHODS

Krt2(-/-) Krt10(-/-) mice were generated by crossing Krt2(-/-) and Krt10(-/-) mice. Epidermal morphology of soles of hind-paws was examined macroscopically and histologically. Immunofluorescence analysis and quantitative PCR analysis were performed to analyze the expression of keratins in sole skin of wildtype and Krt2(-/-) Krt10(-/-) mice. Highly abundant proteins of the sole stratum corneum were determined by electrophoretic and chromatographic separation and subsequent mass spectrometry.

RESULTS

K2 and K10 are the most prominent suprabasal keratins in normal mouse soles with the exception of the footpads where K1, K9 and K10 predominate. Mice lacking both K2 and K10 were viable and developed epidermal acanthosis and hyperkeratosis in inter-footpad epidermis of the soles. The expression of keratins K1, K9 and K16 was massively increased at the RNA and protein levels in the soles of Krt2(-/-) Krt10(-/-) mice.

CONCLUSIONS

This study demonstrates that the loss of the main cytoskeletal components of plantar epidermis, i.e. K2 and K10, can be only partly compensated by the upregulation of other keratins. The thickening of the epidermis in the soles of Krt2(-/-) Krt10(-/-) mice may serve as a model for pathomechanistic aspects of palmoplantar keratoderma.

Efficacy of botulinum toxin in pachyonychia congenita type 1: report of two new cases

Pachyonychia congenita (PC) is a rare genodermatosis caused by a mutation in keratin genes, which can lead to hypertrophic nail dystrophy and focal palmoplantar keratoderma (predominantly plantar), amongst other manifestations. Painful blisters and callosities, sometimes exacerbated by hyperhidrosis, are major issues that can have a significant impact on patient quality of life. Many alternative treatments for this condition have been applied with variable and partial clinical response, but a definitive cure for this disease has yet to be discovered. After obtaining informed consent, two patients with genetically confirmed PC type 1 were treated with plantar injections of botulinum toxin type A. Both patients showed a marked improvement in pain and blistering with an average response time of one week, a six-month mean duration of effectiveness, and a lack of any side effects or tachyphylaxis.

Mechanisms of protein-folding diseases at a glance

For a protein to function appropriately, it must first achieve its proper conformation and location within the crowded environment inside the cell. Multiple chaperone systems are required to fold proteins correctly. In addition, degradation pathways participate by destroying improperly folded proteins. The intricacy of this multisystem process provides many opportunities for error. Furthermore, mutations cause misfolded, nonfunctional forms of proteins to accumulate. As a result, many pathological conditions are fundamentally rooted in the protein-folding problem that all cells must solve to maintain their function and integrity. Here, to illustrate the breadth of this phenomenon, we describe five examples of protein-misfolding events that can lead to disease: improper degradation, mislocalization, dominant-negative mutations, structural alterations that establish novel toxic functions, and amyloid accumulation. In each case, we will highlight current therapeutic options for battling such diseases.

KRT9 gene mutation as a reliable indicator in the prenatal molecular diagnosis of epidermolytic palmoplantar keratoderma

Epidermolytic palmoplantar keratoderma (EPPK) is the most frequent form of such keratodermas. It is inherited in an autosomal dominant pattern and is clinically characterized by diffuse yellowish thickening of the skin on the palms and soles with erythematous borders during the first weeks or months after birth. EPPK is generally caused by mutations of the KRT9 gene. More than 26 KRT9 gene mutations responsible for EPPK have been described (Human Intermediate Filament Database, www.interfil.org), and many of these variants are located within the highly-conserved coil 1A region of the α-helical rod domain of keratin 9. Unfortunately, there is no satisfactory treatment for EPPK. Thus, prenatal molecular diagnosis or pre-pregnancy diagnosis is crucial and benefits those affected who seek healthy descendants. In the present study, we performed amniotic fluid-DNA-based prenatal testing for three at-risk pregnant EPPK women from three unrelated southern Chinese families who carried the KRT9 missense mutations p.Arg163Trp and p.Arg163Gln, and successfully helped two families to bear normal daughters. We suggest that before the successful application of preimplantation genetic diagnosis (PGD), and noninvasive prenatal diagnosis of EPPK that analyzes fetal cells or cell-free DNA in maternal blood, prenatal genetic diagnosis by amniocentesis or chorionic villus sampling (CVS) offers a quite acceptable option for EPPK couples-at-risk to avoid the birth of affected offspring, especially in low- and middle-income countries.

Gene therapy for skin diseases

The skin possesses qualities that make it desirable for gene therapy, and studies have focused on gene therapy for multiple cutaneous diseases. Gene therapy uses a vector to introduce genetic material into cells to alter gene expression, negating a pathological process. This can be accomplished with a variety of viral vectors or nonviral administrations. Although results are promising, there are several potential pitfalls that must be addressed to improve the safety profile to make gene therapy widely available clinically.

Modulation of keratin 1, 10 and involucrin expression as part of the complex response of the human keratinocyte cell line HaCaT to ultraviolet radiation

Skin exposure to ultraviolet (UV) light evokes a complex stress response in keratinocytes. Keratin filament organization provides structural stability and mechanical integrity of keratinocytes. Involucrin is a transglutaminase substrate protein contributing to the formation of insoluble cornified envelopes. However, a more complex role for keratins and involucrin has been proposed, including the regulation of cell stress response. The aim was to evaluate modulations of keratin 1, 10 and involucrin expression in HaCaT in the light of the complex response of these cells to UV-B radiation, including effects on c-Jun and matrix metalloproteinase 1 (MMP-1) gene expression and production of interleukin (IL) 6 and 8. A UV-B (300±5 nm) dose of 10 mJ/cm² was selected since this dose resulted in a partial decrease in cell viability in contrast to higher UV-B doses, which induced complete cell death 48 h after treatment. The UV-B radiation induced significant expression of keratin 1 and 10 and decreased expression of involucrin. This was accompanied by increased expression of c-Jun and MMP-1 and IL-6 and IL-8 production. The data suggest that the expression of keratin 1, 10 and involucrin is modulated in HaCaT keratinocytes as a part of the complex stress response to UV radiation.

One goal, different strategies--molecular and cellular approaches for the treatment of inherited skin fragility disorders

Epidermolysis bullosa (EB) is a heterogeneous group of inherited diseases characterized by the formation of blisters in the skin and mucosa. There is no cure or effective treatment for these potentially severe and fatal diseases. Over the past few years, several reports have proposed different molecular strategies as new therapeutic options for the management of EB. From classical vector-based gene therapy to cell-based strategies such as systemic application of bone marrow stem cells or local application of fibroblasts, a broad range of molecular approaches have been explored. This array also includes novel methods, such as protein replacement therapy, gene silencing and the use of induced pluripotent stem cells (iPCs). In this review, we summarize current concepts of how inherited blistering diseases might be treated in the future and discuss the opportunities, promises, concerns and risks of these innovative approaches.

The sound of silence: autosomal recessive congenital ichthyosis caused by a synonymous mutation in ABCA12

Autosomal recessive congenital ichthyosis refers to a heterogeneous group of cornification disorders of major impact on patients' life. The disease has been linked so far to mutations in 8 distinct genes. We report a consanguineous family of Arab Muslim origin with several members displaying a severe form of congenital ichthyosiform erythroderma. Using a panel of polymorphic microsatellite markers, we identified a region of homozygosity shared by all patients on 2q34, in a region harbouring the ABCA12 gene. Direct sequencing of genomic DNA derived from a patient failed to reveal any obviously pathogenic change in the coding sequence of this gene. In contrast, cDNA sequence analysis revealed the existence of a 163-bp-long deletion in exon 24, thus pointing to a splicing defect. Careful reanalysis of the genomic DNA sequence revealed apart from several known single-nucleotide polymorphisms, a hitherto unreported homozygous synonymous mutation in exon 24 (c.3456G>A; p.S1152S), which was found to lead to the formation of a novel splicing acceptor site. Synonymous mutations have been shown to uncommonly cause inherited disorders in humans. Here, we present the first example of a congenital form of ichthyosis resulting from such a genetic defect.

Imbalance of intermediate filament component keratin 14 contributes to increased stress signalling in epidermolysis bullosa simplex

An important characteristic of epidermolysis bullosa simplex Dowling-Meara (EBS-DM) keratinocytes is the increased level of Jun N-terminal kinase (JNK) stress signalling, which is thought to contribute to the disease phenotype. In this work, we report on the dramatic up-regulation of cytokeratin 14 (K14) in the EBS-DM model cell line KEB7 at both the transcriptional and translational levels, which is noteworthy because KEB7 patient cells are heterozygous for a missense mutation (R125P) in K14. By performing functional assays, we show a direct link between overexpressed wild-type K14 and increased JNK signalling in healthy, immortalized keratinocytes. This observation led us to hypothesize a positive feedback model in which mutant (R125P) K14 triggers JNK signalling, leading to increased AP1-dependent expression of K14, which in turn amplifies JNK signalling further. We therefore suggest that an imbalance of cytoplasmic K14 monomers and K14 incorporated into the intermediate filament (IF) network leads to elevated stress signalling, potentially altering IF dynamics by phosphorylation, which as a side effect, weakens EBS-DM keratinocytes.