Overcoming functional redundancy to elicit pachyonychia congenita-like nail lesions in transgenic mice

Shifts in keratin isoform expression activate motility signals during wound healing

Keratin intermediate filaments confer structural stability to epithelial tissues, but the reason this simple mechanical function requires a protein family with 54 isoforms is not understood. During skin wound healing, a shift in keratin isoform expression alters the composition of keratin filaments. If and how this change modulates cellular functions that support epidermal remodeling remains unclear. We report an unexpected effect of keratin isoform variation on kinase signal transduction. Increased expression of wound-associated keratin 6A, but not of steady-state keratin 5, potentiated keratinocyte migration and wound closure without compromising mechanical stability by activating myosin motors to increase contractile force generation. These results substantially expand the functional repertoire of intermediate filaments from their canonical role as mechanical scaffolds to include roles as isoform-tuned signaling scaffolds that organize signal transduction cascades in space and time to influence epithelial cell state.

Altered keratinocyte differentiation is an early driver of keratin mutation-based palmoplantar keratoderma

The type I intermediate filament keratin 16 (KRT16 gene; K16 protein) is constitutively expressed in ectoderm-derived appendages and in palmar/plantar epidermis and is robustly induced when the epidermis experiences chemical, mechanical or environmental stress. Missense mutations at the KRT16 locus can cause pachyonychia congenita (PC, OMIM:167200) or focal non-epidermolytic palmoplantar keratoderma (FNEPPK, OMIM:613000), which each entail painful calluses on palmar and plantar skin. Krt16-null mice develop footpad lesions that mimic PC-associated PPK, providing an opportunity to decipher its pathophysiology, and develop therapies. We report on insight gained from a genome-wide analysis of gene expression in PPK-like lesions of Krt16-null mice. Comparison of this data set with publicly available microarray data of PPK lesions from individuals with PC revealed significant synergies in gene expression profiles. Keratin 9 (Krt9/K9), the most robustly expressed gene in differentiating volar keratinocytes, is markedly downregulated in Krt16-null paw skin, well-ahead of lesion onset, and is paralleled by pleiotropic defects in terminal differentiation. Effective prevention of PPK-like lesions in Krt16-null paw skin (via topical delivery of the Nrf2 inducer sulforaphane) involves the stimulation of Krt9 expression. These findings highlight a role for defective terminal differentiation and loss of Krt9/K9 expression as additional drivers of PC-associated PPK and highlight restoration of KRT9 expression as a worthy target for therapy. Further, we report on the novel observation that keratin 16 can localize to the nucleus of epithelial cells, implying a potential nuclear function that may be relevant to PC and FNEPPK.

Pathophysiology of pachyonychia congenita-associated palmoplantar keratoderma: new insights into skin epithelial homeostasis and avenues for treatment

BACKGROUND

Pachyonychia congenita (PC), a rare genodermatosis, primarily affects ectoderm-derived epithelial appendages and typically includes oral leukokeratosis, nail dystrophy and very painful palmoplantar keratoderma (PPK). PC dramatically impacts quality of life although it does not affect lifespan. PC can arise from mutations in any of the wound-repair-associated keratin genes KRT6A, KRT6B, KRT6C, KRT16 or KRT17. There is no cure for this condition, and current treatment options for PC symptoms are limited and palliative in nature.

OBJECTIVES

This review focuses on recent progress made towards understanding the pathophysiology of PPK lesions, the most prevalent and debilitating of all PC symptoms.

METHODS

We reviewed the relevant literature with a particular focus on the Krt16 null mouse, which spontaneously develops footpad lesions that mimic several aspects of PC-associated PPK.

RESULTS

There are three main stages of progression of PPK-like lesions in Krt16 null mice. Ahead of lesion onset, keratinocytes in the palmoplantar (footpad) skin exhibit specific defects in terminal differentiation, including loss of Krt9 expression. At the time of PPK onset, there is elevated oxidative stress and hypoactive Keap1-Nrf2 signalling. During active PPK, there is a profound defect in the ability of the epidermis to maintain or return to normal homeostasis.

CONCLUSIONS

The progress made suggests new avenues to explore for the treatment of PC-based PPK and deepens our understanding of the mechanisms controlling skin tissue homeostasis. What's already known about this topic? Pachyonychia congenita (PC) is a rare genodermatosis caused by mutations in KRT6A, KRT6B, KRT6C, KRT16 and KRT17, which are normally expressed in skin appendages and induced following injury. Individuals with PC present with multiple clinical symptoms that usually include thickened and dystrophic nails, palmoplantar keratoderma (PPK), glandular cysts and oral leukokeratosis. The study of PC pathophysiology is made challenging because of its low incidence and high complexity. There is no cure or effective treatment for PC. What does this study add? This text reviews recent progress made when studying the pathophysiology of PPK associated with PC. This recent progress points to new possibilities for devising effective therapeutics that may complement current palliative strategies.

Imaging Functional Nucleic Acid Delivery to Skin

Monogenic skin diseases arise from well-defined single gene mutations, and in some cases a single point mutation. As the target cells are superficial, these diseases are ideally suited for treatment by nucleic acid-based therapies as well as monitoring through a variety of noninvasive imaging technologies. Despite the accessibility of the skin, there remain formidable barriers for functional delivery of nucleic acids to the target cells within the dermis and epidermis. These barriers include the stratum corneum and the layered structure of the skin, as well as more locally, the cellular, endosomal and nuclear membranes. A wide range of technologies for traversing these barriers has been described and moderate success has been reported for several approaches. The lessons learned from these studies include the need for combinations of approaches to facilitate nucleic acid delivery across these skin barriers and then functional delivery across the cellular and nuclear membranes for expression (e.g., reporter genes, DNA oligonucleotides or shRNA) or into the cytoplasm for regulation (e.g., siRNA, miRNA, antisense oligos). The tools for topical delivery that have been evaluated include chemical, physical and electrical methods, and the development and testing of each of these approaches has been greatly enabled by imaging tools. These techniques allow delivery and real time monitoring of reporter genes, therapeutic nucleic acids and also triplex nucleic acids for gene editing. Optical imaging is comprised of a number of modalities based on properties of light-tissue interaction (e.g., scattering, autofluorescence, and reflectance), the interaction of light with specific molecules (e.g., absorbtion, fluorescence), or enzymatic reactions that produce light (bioluminescence). Optical imaging technologies operate over a range of scales from macroscopic to microscopic and if necessary, nanoscopic, and thus can be used to assess nucleic acid delivery to organs, regions, cells and even subcellular structures. Here we describe the animal models, reporter genes, imaging approaches and general strategies for delivery of nucleic acids to cells in the skin for local expression (e.g., plasmid DNA) or gene silencing (e.g., siRNA) with the intent of developing nucleic acid-based therapies to treat diseases of the skin.

Gene expression profiling in pachyonychia congenita skin

BACKGROUND

Pachyonychia congenita (PC) is a skin disorder resulting from mutations in keratin (K) proteins including K6a, K6b, K16, and K17. One of the major symptoms is painful plantar keratoderma. The pathogenic sequelae resulting from the keratin mutations remain unclear.

OBJECTIVE

To better understand PC pathogenesis.

METHODS

RNA profiling was performed on biopsies taken from PC-involved and uninvolved plantar skin of seven genotyped PC patients (two K6a, one K6b, three K16, and one K17) as well as from control volunteers. Protein profiling was generated from tape-stripping samples.

RESULTS

A comparison of PC-involved skin biopsies to adjacent uninvolved plantar skin identified 112 differentially-expressed mRNAs common to patient groups harboring K6 (i.e., both K6a and K6b) and K16 mutations. Among these mRNAs, 25 encode structural proteins including keratins, small proline-rich and late cornified envelope proteins, 20 are related to metabolism and 16 encode proteases, peptidases, and their inhibitors including kallikrein-related peptidases (KLKs), and serine protease inhibitors (SERPINs). mRNAs were also identified to be differentially expressed only in K6 (81) or K16 (141) patient samples. Furthermore, 13 mRNAs were identified that may be involved in pain including nociception and neuropathy. Protein profiling, comparing three K6a plantar tape-stripping samples to non-PC controls, showed changes in the PC corneocytes similar, but not identical, to the mRNA analysis.

CONCLUSION

Many differentially-expressed genes identified in PC-involved skin encode components critical for skin barrier homeostasis including keratinocyte proliferation, differentiation, cornification, and desquamation. The profiling data provide a foundation for unraveling the pathogenesis of PC and identifying targets for developing effective PC therapeutics.

Gene Silencing in Skin After Deposition of Self-Delivery siRNA With a Motorized Microneedle Array Device

Despite the development of potent siRNAs that effectively target genes responsible for skin disorders, translation to the clinic has been hampered by inefficient delivery through the stratum corneum barrier and into the live cells of the epidermis. Although hypodermic needles can be used to transport siRNA through the stratum corneum, this approach is limited by pain caused by the injection and the small volume of tissue that can be accessed by each injection. The use of microneedle arrays is a less painful method for siRNA delivery, but restricted payload capacity limits this approach to highly potent molecules. To address these challenges, a commercially available motorized microneedle array skin delivery device was evaluated. This device combines the positive elements of both hypodermic needles and microneedle array technologies with little or no pain to the patient. Application of fluorescently tagged self-delivery (sd)-siRNA to both human and murine skin resulted in distribution throughout the treated skin. In addition, efficient silencing (78% average reduction) of reporter gene expression was achieved in a transgenic fluorescent reporter mouse skin model. These results indicate that this device effectively delivers functional sd-siRNA with an efficiency that predicts successful clinical translation.

Building models for keratin disorders

Palmoplantar keratoderma is a hallmark of pachyonychia congenita (PC) and focal non-epidermolytic palmoplantar keratoderma (FNEPPK). By generating keratin 16 (Krt16)-deficient mice, Lessard and Coulombe, as described in this issue, have generated a mouse model to replicate these palmoplantar lesions. Studies using this model may provide novel insights into the molecular mechanisms responsible for the formation of palmoplantar lesions in PC and FNEPPK patients.

Keratin 16-null mice develop palmoplantar keratoderma, a hallmark feature of pachyonychia congenita and related disorders

Keratin 16 (KRT16 in human, Krt16 in mouse), a type I intermediate filament protein, is constitutively expressed in epithelial appendages and is induced in the epidermis upon wounding and other stressors. Mutations altering the coding sequence of KRT16 cause Pachyonychia Congenita (PC), a rare autosomal dominant disorder characterized by hypertrophic nail dystrophy, oral leukokeratosis, and palmoplantar keratoderma (PPK). PPK associated with PC are extremely painful and compromise patient mobility, making them the most debilitating PC symptom. In this study, we show that, although inherited in a recessive fashion, the inactivation of Krt16 in mice consistently causes oral lesions as well as PPK-like hyperkeratotic calluses on Krt16^−/− front and hind paws, which severely compromise the animals’ ability to walk. Our findings call into question the view that PC-related PPK arise exclusively as a gain-of-function on the account of dominantly acting mutated keratins, and highlight the key role of modifiers in the clinical heterogeneity of PC symptoms.

First-in-human mutation-targeted siRNA phase Ib trial of an inherited skin disorder

The rare skin disorder pachyonychia congenita (PC) is an autosomal dominant syndrome that includes a disabling plantar keratoderma for which no satisfactory treatment is currently available. We have completed a phase Ib clinical trial for treatment of PC utilizing the first short-interfering RNA (siRNA)-based therapeutic for skin. This siRNA, called TD101, specifically and potently targets the keratin 6a (K6a) N171K mutant mRNA without affecting wild-type K6a mRNA. The safety and efficacy of TD101 was tested in a single-patient 17-week, prospective, double-blind, split-body, vehicle-controlled, dose-escalation trial. Randomly assigned solutions of TD101 or vehicle control were injected in symmetric plantar calluses on opposite feet. No adverse events occurred during the trial or in the 3-month washout period. Subjective patient assessment and physician clinical efficacy measures revealed regression of callus on the siRNA-treated, but not on the vehicle-treated foot. This trial represents the first time that siRNA has been used in a clinical setting to target a mutant gene or a genetic disorder, and the first use of siRNA in human skin. The callus regression seen on the patient's siRNA-treated foot appears sufficiently promising to warrant additional studies of siRNA in this and other dominant-negative skin diseases.

Hedgehog signaling, keratin 6 induction, and sebaceous gland morphogenesis: implications for pachyonychia congenita and related conditions

Keratins 6a and b (K6a, K6b) belong to a subset of keratin genes with constitutive expression in epithelial appendages, and inducible expression in additional epithelia, when subjected to environmental challenges or disease. Mutations in K6a or K6b cause a broad spectrum of epithelial lesions that differentially affect nail, hair, and glands in humans. Some lesions reflect a loss of the structural support function shared by K6, other keratins, and intermediate filament proteins. The formation of sebaceous gland-derived epithelial cysts does not fit this paradigm, raising the question of the unique functions of different K6 isoforms in this setting. Here, we exploit a mouse model of constitutively expressed Gli2, a Hedgehog (Hh) signal effector, to show that K6a expression correlates with duct fate in sebaceous glands (SGs). Whether in the setting of Gli2 transgenic mice skin, which develops a prominent SG duct and additional pairs of highly branched SGs, or in wild-type mouse skin, K6a expression consistently coincides with Hh signaling in ductal tissue. Gli2 expression modestly transactivates a K6a promoter-driven reporter in heterologous systems. Our findings thus identify K6 as a marker of duct fate in SGs, partly in response to Hh signaling, with implications for the pathological expansion of SGs that arises in the context of certain keratin-based diseases and related disorders.

Genetically engineered mouse models for skin research: taking the next step

Genetically engineered mouse models are invaluable to investigators in nearly all areas of biomedical research. The use of genetically engineered mice has allowed researchers to explore fundamental functions of genes in a mammal that shares substantial similarities with human physiology and pathology. Genetically engineered mice are often used as animal models of human diseases that are vital tools in investigating disease development and in developing and testing novel therapies. Gene targeting in embryonic stem cells allows endogenous genes to be specifically altered. As knowledge regarding precise genetic abnormalities underlying a variety of dermatological conditions continues to emerge, the ability to introduce corresponding alterations in endogenous gene loci in mice, often at a single base pair level, has become essential for detailed studies of these genetic diseases. In this review, we provide examples of mouse models harboring modified endogenous gene(s), generated using the technique commonly referred to as the "knock-in" approach, to exemplify the important and sometimes superior role of this methodology in dermatological research.

Single-Nucleotide-Specific siRNA Targeting in a Dominant-Negative Skin Model

RNA interference offers a novel approach for developing therapeutics for dominant-negative genetic disorders. The ability to inhibit expression of the mutant allele without affecting wild-type gene expression could be a powerful new treatment option. Targeting the single-nucleotide keratin 6a (K6a) N171K mutation responsible for the rare monogenic skin disorder pachyonychia congenita (PC), we demonstrate that small interfering RNAs (siRNAs) can potently and selectively block expression of mutant K6a. To test whether lead siRNAs could discriminate mutant mRNA in the presence of both wild-type and mutant forms, a dominant-negative PC cell culture model was developed. As predicted for a dominant-negative disease, simultaneous expression of both wild-type and mutant K6a resulted in defective keratin filament formation. Addition of mutant-specific siRNAs allowed normal filament formation, suggesting selective inhibition of mutant K6a. The effectiveness of our siRNA in skin was tested by co-delivering a firefly luciferase/mutant K6a bicistronic reporter construct and mutant-specific siRNAs to mouse footpads. Potent inhibition of the fluorescent reporter was demonstrated using the Xenogen IVIS200 in vivo imaging system. Additionally, wild type-specific siRNAs knocked down the expression of pre-existing endogenous K6a in human keratinocytes. These results suggest that efficient delivery of these "designer siRNAs" may allow effective treatment of numerous genetic disorders including PC.

Inhibition of estrogen-mediated uterine gene expression responses by dioxin

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) exhibits antiestrogenic properties, including the inhibition of estrogen-induced uterine growth and proliferation. The inhibition of estrogen-mediated gene expression through ER/AhR cross-talk has been proposed as a plausible mechanism; however, only a limited number of inhibited responses have been investigated that are unlikely to fully account for the antiuterotrophic effects of TCDD. Therefore, the effects of TCDD on ethynyl estradiol (EE)-mediated uterine gene expression were investigated using cDNA microarrays with complementary physiological and histological phenotypic anchoring. Mice were gavaged with vehicle, 3 daily doses of 10 mug/kg EE, a single dose of 30 mug/kg TCDD, or a combination of EE plus TCDD and sacrificed after 4, 12, 24, and 72 h. TCDD cotreatment inhibited EE-induced uterine wet weight by 37, 23, and 45% at 12, 24, and 72 h, respectively. TCDD cotreatment also reduced EE-mediated stromal edema, hypertrophy, and hyperplasia and induced marked luminal epithelial cell apoptosis. A 2 x 2 factorial microarray design was used to identify EE- and TCDD-specific differential gene expression responses as well as their interactive effects. Only 133 of the 2753 EE-mediated differentially expressed genes were significantly modulated by TCDD cotreatment, indicating a gene-specific inhibitory response. The EE-mediated induction of many genes, including trefoil factor 1 and keratin 14, were inhibited by greater than 90% by TCDD. Functional annotation of inhibited responses was associated with cell proliferation, water and ion transport, and maintenance of cellular structure and integrity. These inhibited responses correlate with the observed histological alterations and may contribute to the antiuterotrophic effects of TCDD.

Delivery and Inhibition of Reporter Genes by Small Interfering RNAs in a Mouse Skin Model

RNA interference offers the potential of a novel therapeutic approach for treating skin disorders. To this end, we investigated delivery of nucleic acids, including a plasmid expressing the reporter gene luciferase, to mouse skin by intradermal injection into footpads using in vivo bioluminescence imaging over multiple time points. In order to evaluate the ability of RNA interference to inhibit skin gene expression, reporter gene constructs were co-injected with specific or non-specific siRNAs and the in vivo effects measured. Our results revealed that specific unmodified and modified siRNAs (but not nonspecific matched controls) strongly inhibit reporter gene expression in mice. These results indicate that small interfering RNA, delivered locally as RNA directly or expressed from viral or non-viral vectors, may be effective agents for treating skin disorders.

Keratin expression provides novel insight into the morphogenesis and function of the companion layer in hair follicles

Hair follicles cycle between stages of growth (anagen) and metabolic quiescence (telogen) throughout life. In mature follicles, transition from telogen back into anagen involves the activation, proliferation, and differentiation of epithelial stem cells located in the bulge, a specialization of the outer root sheath. Recent studies identified keratin 6a (K6a) transcripts as enriched in bulge epithelial stem cells in mouse skin. We used messenger RNA probes, antibodies, a LacZ reporter mouse model, and whole-mount staining assays to investigate the regulation of mK6a during mouse postnatal hair cycling, and compare it to mK75, a companion layer (Cl) marker. We find that mK75 regulation parallels that of inner root sheath (IRS) markers, with expression onset at anagen IIIa above the new hair bulb and subsequent spreading towards the bulge. Although also occurring in the Cl, mK6a expression begins at anagen IIIb in differentiating cells located proximal to the bulge, and subsequently spreads towards the hair bulb. mK6a and mK75 thus exhibit temporally distinct, and spatially opposed, expression patterns in the Cl during postnatal anagen. These findings provide novel insight into the morphogenesis and properties of the Cl, and raise the distinct possibility that it is an integral part of the IRS compartment.

Great promises yet to be fulfilled: defining keratin intermediate filament function in vivo

Keratins are abundant proteins in epithelial cells, in which they occur as a cytoplasmic network of 10 - 12 nm wide intermediate filaments (IFs). They are encoded by a large family of conserved genes in mammals, with more than 50 individual members partitioned into two sequence types. A strict requirement for the heteropolymerization of type I and type II keratin proteins during filament formation underlies the pairwise transcriptional regulation of keratin genes. In addition, individual pairs are regulated in a tissue-type and differentiation-specific manner. Elucidating the rationale behind the diversity and differential distribution of keratin proteins offers the promise of novel insight into epithelial biology. At present, we know that keratin IFs act as resilient yet pliable scaffolds that endow epithelial cells with the ability to sustain mechanical and non-mechanical stresses. Accordingly, inherited mutations altering the coding sequence of keratins underlie several epithelial fragility disorders. In addition, keratin IFs influence the cellular response to pro-apoptotic signals in specific settings, and the routing of membrane proteins in polarized epithelia. Here we review studies focused on a subset of keratin genes, K6, K16 and K17, showing a complex regulation in vivo, including a widely known upregulation during wound repair and in diseased skin. Progress in defining the function of these and other keratins through gene manipulation in mice has been hampered by functional redundancy within the family. Still, detailed studies of the phenotype exhibited by K6 and K17 null mice yielded novel insight into the properties and function of keratin IFs in vivo.

Defining the properties of the nonhelical tail domain in type II keratin 5: insight from a bullous disease-causing mutation

Inherited mutations in the intermediate filament (IF) proteins keratin 5 (K5) or keratin 14 (K14) cause epidermolysis bullosa simplex (EBS), in which basal layer keratinocytes rupture upon trauma to the epidermis. Most mutations are missense alleles affecting amino acids located in the central alpha-helical rod domain of K5 and K14. Here, we study the properties of an unusual EBS-causing mutation in which a nucleotide deletion (1649delG) alters the last 41 amino acids and adds 35 residues to the C terminus of K5. Relative to wild type, filaments coassembled in vitro from purified K5-1649delG and K14 proteins are shorter and exhibit weak viscoelastic properties when placed under strain. Loss of the C-terminal 41 residues contributes to these alterations. When transfected in cultured epithelial cells, K5-1649delG incorporates into preexisting keratin IFs and also forms multiple small aggregates that often colocalize with hsp70 in the cytoplasm. Aggregation is purely a function of the K5-1649delG tail domain; in contrast, the cloned 109 residue-long tail domain from wild type K5 is distributed throughout the cytoplasm and colocalizes partly with keratin IFs. These data provide a mechanistic basis for the cell fragility seen in individuals bearing the K5-1649delG allele, and point to the role of the C-terminal 41 residues in determining K5's assembly properties.