Keratin disorders: from gene to therapy

Identification of genes important for cutaneous function revealed by a large scale reverse genetic screen in the mouse

The skin is a highly regenerative organ which plays critical roles in protecting the body and sensing its environment. Consequently, morbidity and mortality associated with skin defects represent a significant health issue. To identify genes important in skin development and homeostasis, we have applied a high throughput, multi-parameter phenotype screen to the conditional targeted mutant mice generated by the Wellcome Trust Sanger Institute's Mouse Genetics Project (Sanger-MGP). A total of 562 different mouse lines were subjected to a variety of tests assessing cutaneous expression, macroscopic clinical disease, histological change, hair follicle cycling, and aberrant marker expression. Cutaneous lesions were associated with mutations in 23 different genes. Many of these were not previously associated with skin disease in the organ (Mysm1, Vangl1, Trpc4ap, Nom1, Sparc, Farp2, and Prkab1), while others were ascribed new cutaneous functions on the basis of the screening approach (Krt76, Lrig1, Myo5a, Nsun2, and Nf1). The integration of these skin specific screening protocols into the Sanger-MGP primary phenotyping pipelines marks the largest reported reverse genetic screen undertaken in any organ and defines approaches to maximise the productivity of future projects of this nature, while flagging genes for further characterisation.

Defective channels lead to an impaired skin barrier

Channels are integral membrane proteins that form a pore, allowing the passive movement of ions or molecules across a membrane (along a gradient), either between compartments within a cell, between intracellular and extracellular environments or between adjacent cells. The ability of cells to communicate with one another and with their environment is a crucial part of the normal physiology of a tissue that allows it to carry out its function. Cell communication is particularly important during keratinocyte differentiation and formation of the skin barrier. Keratinocytes in the skin epidermis undergo a programme of apoptosis-driven terminal differentiation, whereby proliferating keratinocytes in the basal (deepest) layer of the epidermis stop proliferating, exit the basal layer and move up through the spinous and granular layers of the epidermis to form the stratum corneum, the external barrier. Genes encoding different families of channel proteins have been found to harbour mutations linked to a variety of rare inherited monogenic skin diseases. In this Commentary, we discuss how human genetic findings in aquaporin (AQP) and transient receptor potential (TRP) channels reveal different mechanisms by which these channel proteins function to ensure the proper formation and maintenance of the skin barrier.

Directed expression of a chimeric type II keratin partially rescues keratin 5-null mice

The crucial role of structural support fulfilled by keratin intermediate filaments (IFs) in surface epithelia likely requires that they be organized into cross-linked networks. For IFs comprised of keratins 5 and 14 (K5 and K14), which occur in basal keratinocytes of the epidermis, formation of cross-linked bundles is, in part, self-driven through cis-acting determinants. Here, we targeted the expression of a bundling-competent KRT5/KRT8 chimeric cDNA (KRT8bc) or bundling-deficient wild type KRT8 as a control to the epidermal basal layer of Krt5-null mice to assess the functional importance of keratin IF self-organization in vivo. Such targeted expression of K8bc rescued Krt5-null mice with a 47% frequency, whereas K8 completely failed to do so. This outcome correlated with lower than expected levels of K8bc and especially K8 mRNA and protein in the epidermis of E18.5 replacement embryos. Ex vivo culture of embryonic skin keratinocytes confirmed the ability of K8bc to form IFs in the absence of K5. Additionally, electron microscopy analysis of E18.5 embryonic skin revealed that the striking defects observed in keratin IF bundling, cytoarchitecture, and mitochondria are partially restored by K8bc expression. As young adults, viable KRT8bc replacement mice develop alopecia and chronic skin lesions, indicating that the skin epithelia are not completely normal. These findings are consistent with a contribution of self-mediated organization of keratin IFs to structural support and cytoarchitecture in basal layer keratinocytes of the epidermis and underscore the importance of context-dependent regulation for keratin genes and proteins in vivo.

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.

A mutation in the FAM83G gene in dogs with hereditary footpad hyperkeratosis (HFH)

Hereditary footpad hyperkeratosis (HFH) represents a palmoplantar hyperkeratosis, which is inherited as a monogenic autosomal recessive trait in several dog breeds, such as e.g. Kromfohrländer and Irish Terriers. We performed genome-wide association studies (GWAS) in both breeds. In Kromfohrländer we obtained a single strong association signal on chromosome 5 (p_raw = 1.0×10⁻¹³) using 13 HFH cases and 29 controls. The association signal replicated in an independent cohort of Irish Terriers with 10 cases and 21 controls (p_raw = 6.9×10⁻¹⁰). The analysis of shared haplotypes among the combined Kromfohrländer and Irish Terrier cases defined a critical interval of 611 kb with 13 predicted genes. We re-sequenced the genome of one affected Kromfohrländer at 23.5× coverage. The comparison of the sequence data with 46 genomes of non-affected dogs from other breeds revealed a single private non-synonymous variant in the critical interval with respect to the reference genome assembly. The variant is a missense variant (c.155G>C) in the FAM83G gene encoding a protein with largely unknown function. It is predicted to change an evolutionary conserved arginine into a proline residue (p.R52P). We genotyped this variant in a larger cohort of dogs and found perfect association with the HFH phenotype. We further studied the clinical and histopathological alterations in the epidermis in vivo. Affected dogs show a moderate to severe orthokeratotic hyperplasia of the palmoplantar epidermis. Thus, our data provide the first evidence that FAM83G has an essential role for maintaining the integrity of the palmoplantar epidermis.

The palms and soles of mammals are covered by the palmoplantar epidermis, which has to bear immense mechanical forces and has therefore a special composition in comparison to the epidermis on regular skin. We studied a Mendelian disease in dogs, termed hereditary footpad hyperkeratosis (HFH). HFH affected dogs develop deep fissures in the paw pads, which are the consequence of a pathological thickening of the outermost layer of the epidermis. We mapped the disease causing genetic variant in the Kromfohrländer and Irish Terrier breeds to a 611 kb interval on chromosome 5. HFH affected Kromfohrländer and Irish Terriers shared the same haplotype indicating descent from a common founder. We re-sequenced the genome of an affected dog and compared it to genome sequences of 46 control dogs. The HFH affected dog had only one private non-synonymous variant in the critical interval, a missense variant of the FAM83G gene. We genotyped this variant in more than 500 dogs and found perfect association with the HFH phenotype. Our data very strongly suggest that the FAM83G variant is causative for HFH. FAM83G is a protein with unknown biochemical function. Our study thus provides the first link between this protein and the palmoplantar epidermis.

Autosomal recessive transmission of a rare KRT74 variant causes hair and nail ectodermal dysplasia: allelism with dominant woolly hair/hypotrichosis

Pure hair and nail ectodermal dysplasia (PHNED) comprises a heterogeneous group of rare heritable disorders characterized by brittle hair, hypotrichosis, onychodystrophy and micronychia. Autosomal recessive (AR) PHNED has previously been associated with mutations in either KRT85 or HOXC13 on chromosome 12p11.1-q14.3. We investigated a consanguineous Pakistani family with AR PHNED linked to the keratin gene cluster on 12p11.1 but without detectable mutations in KRT85 and HOXC13. Whole exome sequencing of affected individuals revealed homozygosity for a rare c.821T>C variant (p.Phe274Ser) in the KRT74 gene that segregates AR PHNED in the family. The transition alters the highly conserved Phe274 residue in the coil 1B domain required for long-range dimerization of keratins, suggesting that the mutation compromises the stability of intermediate filaments. Immunohistochemical (IHC) analyses confirmed a strong keratin-74 expression in the nail matrix, the nail bed and the hyponychium of mouse distal digits, as well as in normal human hair follicles. Furthermore, hair follicles and epidermis of an affected family member stained negative for Keratin-74 suggesting a loss of function mechanism mediated by the Phe274Ser substitution. Our observations show for the first time that homozygosity for a KRT74 missense variant may be associated with AR PHNED. Heterozygous KRT74 mutations have previously been associated with autosomal dominant woolly hair/hypotrichosis simplex (ADWH). Thus, our findings expand the phenotypic spectrum associated with KRT74 mutations and imply that a subtype of AR PHNED is allelic with ADWH.

Inherited blistering skin diseases: underlying molecular mechanisms and emerging therapies

A key function of human skin is the formation of a structural barrier against the external environment. In part, this is achieved through the formation of a cornified cell envelope derived from a stratified squamous epithelium attached to an epithelial basement membrane. Resilient in health, the structural integrity of skin can become impaired or break down in a collection of inherited skin diseases, referred to as the blistering genodermatoses. These disorders arise from inherited gene mutations in a variety of structural and signalling proteins and manifest clinically as blisters or erosions following minor skin trauma. In some patients, blistering can be severe resulting in significant morbidity. Furthermore, a number of these conditions are associated with debilitating extra-cutaneous manifestations including gastro-intestinal, cardiac, and ocular complications. In recent years, an improved understanding of the molecular basis of the blistering genodermatoses has led to better disease classification and genetic counselling. For patients, this has also advanced translational research with the advent of new clinical trials of gene, protein, cell, drug, and small molecule therapies. Although curing inherited blistering skin diseases still remains elusive, significant improvements in patients' quality of life are already being achieved.

Beyond expectations: novel insights into epidermal keratin function and regulation

The epidermis is a stratified epithelium that relies on its cytoskeleton and cell junctions to protect the body against mechanical injury, dehydration, and infections. Keratin intermediate filament proteins are involved in many of these functions by forming cell-specific cytoskeletal scaffolds crucial for the maintenance of cell and tissue integrity. In response to various stresses, the expression and organization of keratins are altered at transcriptional and posttranslational levels to restore tissue homeostasis. Failure to restore tissue homeostasis in the presence of keratin gene mutations results in acute and chronic skin disorders for which currently no rational therapies are available. Here, we review the recent progress on the role of keratins in cytoarchitecture, adhesion, signaling, and inflammation. By focusing on epidermal keratins, we illustrate the contribution of keratin isotypes to differentiated epithelial functions.

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.

Keratin 9 is required for the structural integrity and terminal differentiation of the palmoplantar epidermis

Keratin 9 (K9) is a type I intermediate filament protein whose expression is confined to the suprabasal layers of the palmoplantar epidermis. Although mutations in the K9 gene are known to cause epidermolytic palmoplantar keratoderma, a rare dominant-negative skin disorder, its functional significance is poorly understood. To gain insight into the physical requirement and importance of K9, we generated K9-deficient (Krt9^−/−) mice. Here, we report that adult Krt9^−/−mice develop calluses marked by hyperpigmentation that are exclusively localized to the stress-bearing footpads. Histological, immunohistochemical, and immunoblot analyses of these regions revealed hyperproliferation, impaired terminal differentiation, and abnormal expression of keratins K5, K14, and K2. Furthermore, the absence of K9 induces the stress-activated keratins K6 and K16. Importantly, mice heterozygous for the K9-null allele (Krt9^+/−) show neither an overt nor histological phenotype, demonstrating that one Krt9 allele is sufficient for the developing normal palmoplantar epidermis. Together, our data demonstrate that complete ablation of K9 is not tolerable in vivo and that K9 is required for terminal differentiation and maintaining the mechanical integrity of palmoplantar epidermis.

Highlights of recent developments and trends in cancer nanotechnology research--view from NCI Alliance for Nanotechnology in Cancer

Although the incidence of cancer and cancer related deaths in the United States has decreased over the past two decades due to improvements in early detection and treatment, cancer still is responsible for a quarter of the deaths in this country. There is much room for improvement on the standard treatments currently available and the National Cancer Institute (NCI) has recognized the potential for nanotechnology and nanomaterials in this area. The NCI Alliance for Nanotechnology in Cancer was formed in 2004 to support multidisciplinary researchers in the application of nanotechnology to cancer diagnosis and treatment. The researchers in the Alliance have been productive in generating innovative solutions to some of the central issues of cancer treatment including how to detect tumors earlier, how to target cancer cells specifically, and how to improve the therapeutic index of existing chemotherapies and radiotherapy treatments. Highly creative ideas are being pursued where novelty in nanomaterial development enables new modalities of detection or therapy. This review highlights some of the innovative materials approaches being pursued by researchers funded by the NCI Alliance. Their discoveries to improve the functionality of nanoparticles for medical applications includes the generation of new platforms, improvements in the manufacturing of nanoparticles and determining the underlying reasons for the movement of nanoparticles in the blood.

Comparative study of high-resolution multifrequency ultrasound of the plantar skin in patients with various types of hereditary palmoplantar keratoderma

BACKGROUND

High-variable-frequency ultrasound is used as an imaging tool for various cutaneous disorders. We utilized this tool in pachyonychia congenita (PC) patients, who typically present with plantar hyperkeratosis and often severely debilitating pain, compared to patients with epidermolytic palmoplantar keratoderma (EPPK) and mal de Meleda (MDM).

OBJECTIVE

To ascertain the feasibility of ultrasound technology for the diagnosis of PC.

METHODS

The study included a total of 16 patients, 7 with PC, 5 with EPPK and 4 with MDM, who underwent ultrasound examination of the plantar skin with high-resolution multifrequency ultrasound equipment.

RESULTS

Ultrasound scans performed over the proximal and distal plantar foot calluses in PC patients demonstrated hyperechoic dots and lines within the epidermis compatible with hyperkeratosis, engorged varicose veins in the dermis and an anechoic layer interposed between the epidermis and the dermis, corresponding to blister fluid below the calluses. In contrast to PC patients, patients with MDM and EPPK demonstrated no blisters.

CONCLUSION

PC patients, as opposed to a group of patients with MDM and EPPK, displayed subepidermal blistering beneath their calluses. This finding may help in the diagnosis of PC and in partially explaining plantar pain as part of PC symptomatology.

Superficial epidermolytic ichthyosis: a report of two families

Superficial epidermolytic ichthyosis (SEI), previously known as ichthyosis bullosa of Siemens, is a rare genetic skin condition, characterized by blisters and hyperkeratosis. It can be easily confused with epidermolytic hyperkeratosis, known now as epidermolytic ichthyosis, and genetic testing can be helpful in differentiating between the two conditions. We describe two children with SEI confirmed by genetic testing, including one with a novel mutation. We also describe other affected family members with SEI.

Gene therapy for corneal dystrophies and disease, where are we?

PURPOSE OF REVIEW

We assess the studies on vector systems for delivery of transgenes to the cornea that have been published over the last year and summarize new work on the identification of specific transgenes for corneal diseases.

RECENT FINDINGS

Adeno-associated viral vectors are increasingly being successfully applied to the cornea, although transgene expression requires corneal epithelial debridement or intrastromal injection of the vector. Gene delivery platforms based on nanoparticles of chitosan or gold also show promise. Overexpression of vasoinhibin-1 or decorin, or siRNA-mediated blockade of the cannabinoid receptor CB1, can all reduce corneal neovascularization. Overexpression of decorin or matrix metalloproteinase 14 can reduce corneal fibrosis and haze, whereas overexpression of c-Met accelerates the epithelial wound healing. Induction of corneal endothelial cell replication by overexpression of E2F2, p16 or p21 can maintain or even increase corneal endothelial cell density in eye bank corneas. Overexpression of the antiapoptotic transgenes Bcl-xL or p35 significantly enhances corneal endothelial cell survival and reduces apoptosis in stored human corneas.

SUMMARY

Despite a wealth of information on the methods for the delivery of nucleic acids to the human cornea and ever-increasing information on the transgenes with substantial therapeutic potential, gene therapy for corneal disorders has yet to reach the clinic.

Frizzled6 deficiency disrupts the differentiation process of nail development

Nails protect the soft tissue of the tips of digits. The molecular mechanism of nail (and claw) development is largely unknown, but we have recently identified a Wnt receptor gene, Frizzled6 (Fzd6) that is mutated in a human autosomal-recessive nail dysplasia. To investigate the action of Fzd6 in claw development at the molecular level, we compared gene expression profiles of digit tips of wild-type and Fzd6^−/− mice, and show that Fzd6 regulates the transcription of a striking number of epidermal differentiation-related genes. Sixty-three genes encoding keratins, keratin associated proteins, and transglutaminases and their substrates were significantly down-regulated in the knockout mice. Among them, four hard keratins, Krt86, Krt81, Krt34 and Krt31; two epithelial keratins, Krt6a and Krt6b; and transglutaminase1 were already known to be involved in nail abnormalities when dysregulated. Immunohistochemical studies revealed decreased expression of Krt86, Krt6b and involucrin in the epidermal portion of the claw field in the knockout embryos. We further showed that Dkk4, a Wnt antagonist, was significantly down-regulated in Fzd6^−/− mice along with Wnt, Bmp and Hh family genes; and Dkk4 transgenic mice showed a subtly but appreciably modified claw phenotype. Thus, Fzd6-mediated Wnt signaling likely regulates the overall differentiation process of nail/claw formation.

The expanding significance of keratin intermediate filaments in normal and diseased epithelia

Intermediate filaments are assembled from a diverse group of evolutionary conserved proteins and are specified in a tissue-dependent, cell type-dependent, and context-dependent fashion in the body. Genetic mutations in intermediate filament proteins account for a large number of diseases, ranging from skin fragility conditions to cardiomyopathies and premature aging. Keratins, the epithelial-specific intermediate filaments, are now recognized as multi-faceted effectors in their native context. In this review, we emphasize the recent progress made in defining the role of keratins towards the regulation of cytoarchitecture, cell growth and proliferation, apoptosis, and cell motility during embryonic development, in normal adult tissues, and in select diseases such as cancer.

Best treatment practices for pachyonychia congenita

BACKGROUND

Numerous therapeutic modalities have been proposed to treat the manifestations of pachyonychia congenita (PC). While research hopes lie with molecular therapies, patients are in need of answers regarding the efficacy of conventional treatments.

AIM OF THE STUDY

To determine patients' experience and preferences regarding conventional treatments for PC.

METHODS

The study population included 120 PC patients from 20 countries. The study was based on a patient survey developed by physicians and researchers from the International Pachyonychia Congenita Consortium and conducted via the internet. Using an effectiveness scale of 1 to 5, the patients were asked to grade treatments for different manifestations, including keratoderma, cysts, follicular hyperkeratosis, fingernail and toenail involvement.

RESULTS

Patients reported surgical treatments being most effective for cysts and mechanical treatments the most effective conventional therapeutic approach for all other investigated manifestations. The other conventional medical treatments were found to be non-effective to only slightly effective. Among patients with keratoderma, older people were more likely to report beneficial effect from mechanical treatments (P = 0.04), topical retinoids (P = 0.04) and topical steroids (P = 0.02). Likewise, females were more inclined to report filing and grinding beneficial than males (P = 0.02). Finally, carriers of KRT16 and KRT6a were more likely to benefit from keratolytics than carriers of mutations in KRT17 (P = 0.04).

CONCLUSIONS

None of the currently available therapeutic options for PC are ideal, although they provide some relief, with mechanical/surgical options being preferred over medical therapies. These results emphasize the need for more efficient and targeted therapies.

Allele-specific siRNA silencing for the common keratin 12 founder mutation in Meesmann epithelial corneal dystrophy

PURPOSE

To identify an allele-specific short interfering RNA (siRNA), against the common KRT12 mutation Arg135Thr in Meesmann epithelial corneal dystrophy (MECD) as a personalized approach to treatment.

METHODS

siRNAs against the K12 Arg135Thr mutation were evaluated using a dual luciferase reporter gene assay and the most potent and specific siRNAs were further screened by Western blot. Off-target effects on related keratins were assessed and immunological stimulation of TLR3 was evaluated by RT-PCR. A modified 5' rapid amplification of cDNA ends method was used to confirm siRNA-mediated mutant knockdown. Allele discrimination was confirmed by quantitative infrared immunoblotting.

RESULTS

The lead siRNA, with an IC(50) of thirty picomolar, showed no keratin off-target effects or activation of TLR3 in the concentration ranges tested. We confirmed siRNA-mediated knockdown by the presence of K12 mRNA fragments cleaved at the predicted site. A dual tag infrared immunoblot showed knockdown to be allele-specific, with 70% to 80% silencing of the mutant protein.

CONCLUSIONS

A potent allele-specific siRNA against the K12 Arg135Thr mutation was identified. In combination with efficient eyedrop formulation delivery, this would represent a personalized medicine approach, aimed at preventing the pathology associated with MECD and other ocular surface pathologies with dominant-negative or gain-of-function pathomechanisms.

The chicken frizzle feather is due to an α-keratin (KRT75) mutation that causes a defective rachis

Feathers have complex forms and are an excellent model to study the development and evolution of morphologies. Existing chicken feather mutants are especially useful for identifying genetic determinants of feather formation. This study focused on the gene F, underlying the frizzle feather trait that has a characteristic curled feather rachis and barbs in domestic chickens. Our developmental biology studies identified defects in feather medulla formation, and physical studies revealed that the frizzle feather curls in a stepwise manner. The frizzle gene is transmitted in an autosomal incomplete dominant mode. A whole-genome linkage scan of five pedigrees with 2678 SNPs revealed association of the frizzle locus with a keratin gene-enriched region within the linkage group E22C19W28_E50C23. Sequence analyses of the keratin gene cluster identified a 69 bp in-frame deletion in a conserved region of KRT75, an α-keratin gene. Retroviral-mediated expression of the mutated F cDNA in the wild-type rectrix qualitatively changed the bending of the rachis with some features of frizzle feathers including irregular kinks, severe bending near their distal ends, and substantially higher variations among samples in comparison to normal feathers. These results confirmed KRT75 as the F gene. This study demonstrates the potential of our approach for identifying genetic determinants of feather forms.

With the availability of a sequenced chicken genome, the reservoir of variant plumage genes found in domestic chickens can provide insight into the molecular mechanisms underlying the diversity of feather forms. In this paper, we identify the molecular basis of the distinctive frizzle (F) feather phenotype that is caused by a single autosomal incomplete dominant gene in which heterozygous individuals show less severe phenotypes than homozygous individuals. Feathers in frizzle chickens curve backward. We used computer-assisted analysis to establish that the rachis of the frizzle feather was irregularly kinked and more severely bent than normal. Moreover, microscopic evaluation of regenerating feathers found reduced proliferating cells that give rise to the frizzle rachis. Analysis of a pedigree of frizzle chickens showed that the phenotype is linked to two single-nucleotide polymorphisms in a cluster of keratin genes within the linkage group E22C19W28_E50C23. Sequencing of the gene cluster identified a 69-base pair in-frame deletion of the protein coding sequence of the α-keratin-75 gene. Forced expression of the mutated gene in normal chickens produced a twisted rachis. Although chicken feathers are primarily composed of beta-keratins, our findings indicate that alpha-keratins have an important role in establishing the structure of feathers.

Inhibition of CD44 gene expression in human skin models, using self-delivery short interfering RNA administered by dissolvable microneedle arrays

Treatment of skin disorders with short interfering RNA (siRNA)-based therapeutics requires the development of effective delivery methodologies that reach target cells in affected tissues. Successful delivery of functional siRNA to the epidermis requires (1) crossing the stratum corneum, (2) transfer across the keratinocyte membrane, followed by (3) incorporation into the RNA-induced silencing complex. We have previously demonstrated that treatment with microneedle arrays loaded with self-delivery siRNA (sd-siRNA) can achieve inhibition of reporter gene expression in a transgenic mouse model. Furthermore, treatment of human cultured epidermal equivalents with sd-siRNA resulted in inhibition of target gene expression. Here, we demonstrate inhibition of CD44, a gene that is uniformly expressed throughout the epidermis, by sd-siRNA both in vitro (cultured human epidermal skin equivalents) and in vivo (full-thickness human skin equivalents xenografted on immunocompromised mice). Treatment of human skin equivalents with CD44 sd-siRNA markedly decreased CD44 mRNA levels, which led to a reduction of the target protein as confirmed by immunodetection in epidermal equivalent sections with a CD44-specific antibody. Taken together, these results demonstrate that sd-siRNA, delivered by microneedle arrays, can reduce expression of a targeted endogenous gene in a human skin xenograft model.

Microneedle delivery of plasmid DNA to living human skin: Formulation coating, skin insertion and gene expression

Microneedle delivery of nucleic acids, in particular plasmid DNA (pDNA), to the skin represents a potential new approach for the clinical management of genetic skin diseases and cutaneous cancers, and for intracutaneous genetic immunisation. In this study excised human skin explants were used to investigate and optimise key parameters that will determine stable and effective microneedle-facilitated pDNA delivery. These include (i) high dose-loading of pDNA onto microneedle surfaces, (ii) stability and functionality of the coated pDNA, (iii) skin penetration capability of pDNA-coated microneedles, and (iv) efficient gene expression in human skin. Optimisation of a dip-coating method enabled significant increases in the loading capacity, up to 100μg of pDNA per 5-microneedle array. Coated microneedles were able to reproducibly perforate human skin at low (<1N) insertion forces. The physical stability of the coated pDNA was partially compromised on storage, although this was improved through the addition of saccharide excipients without detriment to the biological functionality of pDNA. The pDNA-coated microneedles facilitated reporter gene expression in viable human skin. The efficiency of gene expression from coated microneedles will depend upon suitable DNA loading, efficient and reproducible skin puncture and rapid in situ dissolution of the plasmid at the site of delivery.

Defining keratin protein function in skin epithelia: epidermolysis bullosa simplex and its aftermath

Epidermolysis bullosa simplex (EBS) is a rare genetic condition typified by superficial bullous lesions following incident frictional trauma to the skin. Most cases of EBS are due to dominantly acting mutations in keratin 14 (K14) or K5, the type I and II intermediate filament (IF) proteins that copolymerize to form a pancytoplasmic network of 10 nm filaments in basal keratinocytes of epidermis and related epithelia. Defects in K5-K14 filament network architecture cause basal keratinocytes to become fragile, and account for their rupture upon exposure to mechanical trauma. The discovery of the etiology and pathophysiology of EBS was intimately linked to the quest for an understanding of the properties and function of keratin filaments in skin epithelia. Since then, continued cross-fertilization between basic science efforts and clinical endeavors has highlighted several additional functional roles for keratin proteins in the skin, suggested new avenues for effective therapies for keratin-based diseases, and expanded our understanding of the remarkable properties of the skin as an organ system.

The most common mutation of KRT9, c.C487T (p.R163W), in epidermolytic palmoplantar keratoderma in two large Chinese pedigrees

Epidermolytic palmoplantar keratoderma (EPPK) is generally associated with dominant-negative mutations of the Keratin 9 gene (KRT9), and rarely with the Keratin 1 gene (KRT1). To date, a myriad of mutations has been reported with a high frequency of codon 163 mutations within the first exon of KRT9 in different populations. Notably, a distinct phenotypic heterogeneity, digital mutilation, was found recently in a 58-year-old female Japanese EPPK patient with p.R163W. Here, we report the most common mutation, c.C487T (p.R163W) of KRT9, in two large EPPK pedigrees from southeast China. The arginine residue in peptide position 163 remains almost constant in at least 47 intermediate filament proteins ranging from snail to human. A substitution in arginine alters both the charge and shape of the 1A rod domain and disrupts the function of the helix initiation motif of keratins, finally compromising the integrity of filaments and weakening their stability in the epidermis of palms and soles. We summarize the clinical symptoms of EPPK in Chinese and show that knuckle pads are associated with KRT9 mutations. We suggest that the frequency of p.R163W in Chinese EPPK patients (31.03%) is consistent with that in the general population (29.33%), and that codon 163 is truly a hotspot mutational site of KRT9.

Development of allele-specific therapeutic siRNA in Meesmann epithelial corneal dystrophy

Background

Meesmann epithelial corneal dystrophy (MECD) is an inherited eye disorder caused by dominant-negative mutations in either keratins K3 or K12, leading to mechanical fragility of the anterior corneal epithelium, the outermost covering of the eye. Typically, patients suffer from lifelong irritation of the eye and/or photophobia but rarely lose visual acuity; however, some individuals are severely affected, with corneal scarring requiring transplant surgery. At present no treatment exists which addresses the underlying pathology of corneal dystrophy. The aim of this study was to design and assess the efficacy and potency of an allele-specific siRNA approach as a future treatment for MECD.

Methods and Findings

We studied a family with a consistently severe phenotype where all affected persons were shown to carry heterozygous missense mutation Leu132Pro in the KRT12 gene. Using a cell-culture assay of keratin filament formation, mutation Leu132Pro was shown to be significantly more disruptive than the most common mutation, Arg135Thr, which is associated with typical, mild MECD. A siRNA sequence walk identified a number of potent inhibitors for the mutant allele, which had no appreciable effect on wild-type K12. The most specific and potent inhibitors were shown to completely block mutant K12 protein expression with negligible effect on wild-type K12 or other closely related keratins. Cells transfected with wild-type K12-EGFP construct show a predominantly normal keratin filament formation with only 5% aggregate formation, while transfection with mutant K12-EGFP construct resulted in a significantly higher percentage of keratin aggregates (41.75%; p<0.001 with 95% confidence limits). The lead siRNA inhibitor significantly rescued the ability to form keratin filaments (74.75% of the cells contained normal keratin filaments; p<0.001 with 95% confidence limits).

Conclusions

This study demonstrates that it is feasible to design highly potent siRNA against mutant alleles with single-nucleotide specificity for future treatment of MECD.