Novel Mechanism of Revertant Mosaicism in Dowling–Meara Epidermolysis Bullosa Simplex

Inherited Reticulate Pigmentary Disorders

Reticulate pigmentary disorders (RPDs) are a group of inherited and acquired skin conditions characterized by hyperpigmented and/or hypopigmented macules. Inherited RPDs include dyschromatosis symmetrica hereditaria (DSH), dyschromatosis universalis hereditaria (DUH), reticulate acropigmentation of Kitamura (RAK), Dowling-Degos disease (DDD), dyskeratosis congenita (DKC), Naegeli-Franceschetti-Jadassohn syndrome (NFJS), dermatopathia pigmentosa reticularis (DPR), and X-linked reticulate pigmentary disorder. Although reticulate pattern of pigmentation is a common characteristic of this spectrum of disorders, the distribution of pigmentation varies among these disorders, and there may be clinical manifestations beyond pigmentation. DSH, DUH, and RAK are mostly reported in East Asian ethnicities. DDD is more common in Caucasians, although it is also reported in Asian countries. Other RPDs show no racial predilection. This article reviews the clinical, histological, and genetic variations of inherited RPDs.

Revertant Mosaicism in Genodermatoses: Natural Gene Therapy Right before Your Eyes

Revertant mosaicism (RM) is the intriguing phenomenon in which nature itself has successfully done what medical science is so eagerly trying to achieve: correcting the effect of disease-causing germline variants and thereby reversing the disease phenotype back to normal. RM was molecularly confirmed for the first time in a genodermatosis in 1997, the genetic skin condition junctional epidermolysis bullosa (EB). At that time, RM was considered an extraordinary phenomenon. However, several important discoveries have changed this conception in the past few decades. First, RM has now been identified in all major subtypes of EB. Second, RM has also been identified in many other genodermatoses. Third, a theoretical mathematical exercise concluded that reverse mutations should be expected in all patients with a recessive subtype of EB or any other genodermatosis. This has shifted the paradigm from RM being an extraordinary phenomenon to it being something that every physician working in the field of genodermatoses should be looking for in every patient. It has also raised hope for new treatment options in patients with genodermatoses. In this review, we summarize the current knowledge on RM and discuss the perspectives of RM for the future treatment of patients with genodermatoses.

Revertant Mosaicism in Epidermolysis Bullosa

Epidermolysis bullosa (EB) is a group of genetic blistering diseases characterized by mechanically fragile skin and mucocutaneous involvement. Historically, disease management has focused on supportive care. The development of new genetic, cellular, and recombinant protein therapies has shown promise, and this review summarizes a unique gene and cell therapy phenomenon termed revertant mosaicism (RM). RM is the spontaneous correction of a disease-causing mutation. It has been reported in most EB subtypes, some with relatively high frequency, and has been observed in both keratinocytes and fibroblasts. RM manifests as identifiable patches of unaffected, blister-resistant skin and can occur through a variety of molecular mechanisms, including true back mutation, intragenic crossover, mitotic gene conversion, and second-site mutation. RM cells represent a powerful autologous platform for therapy, and leveraging RM cells as a therapeutic substrate may avoid the inherent mutational risks of gene therapy/editing. However, further examination of the genomic integrity and long-term functionality of RM-derived cells, as well in vivo testing of systemic therapies with RM cells, is required to realize the full therapeutic promise of naturally occurring RM in EB.

Investigational Treatments for Epidermolysis Bullosa

Epidermolysis bullosa (EB) is a heterogeneous group of rare inherited blistering skin disorders characterized by skin fragility following minor trauma, usually present since birth. EB can be categorized into four classical subtypes, EB simplex, junctional EB, dystrophic EB and Kindler EB, distinguished on clinical features, plane of blister formation in the skin, and molecular pathology. Treatment for EB is mostly supportive, focusing on wound care and patient symptoms such as itch or pain. However, therapeutic advances have also been made in targeting the primary genetic abnormalities as well as the secondary inflammatory footprint of EB. Pre-clinical or clinical testing of gene therapies (gene replacement, gene editing, RNA-based therapy, natural gene therapy), cell-based therapies (fibroblasts, bone marrow transplantation, mesenchymal stromal cells, induced pluripotential stem cells), recombinant protein therapies, and small molecule and drug repurposing approaches, have generated new hope for better patient care. In this article, we review advances in translational research that are impacting on the quality of life for people living with different forms of EB and which offer hope for improved clinical management.

A six-attribute classification of genetic mosaicism

Mosaicism denotes an individual who has at least two populations of cells with distinct genotypes that are derived from a single fertilized egg. Genetic variation among the cell lines can involve whole chromosomes, structural or copy number variants, small or single nucleotide variants, or epigenetic variants. The mutational events that underlie mosaic variants occur during mitotic cell divisions after fertilization and zygote formation. The initiating mutational event can occur in any types of cell at any time in development, leading to enormous variation in the distribution and phenotypic effect of mosaicism. A number of classification proposals have been put forward to classify genetic mosaicism into categories based on the location, pattern, and mechanisms of the disease. We here propose a new classification of genetic mosaicism that considers the affected tissue, the pattern and distribution of the mosaicism, the pathogenicity of the variant, the direction of the change (benign to pathogenic vs. pathogenic to benign), and the postzygotic mutational mechanism. The accurate and comprehensive categorization and subtyping of mosaicisms is important and has potential clinical utility to define the natural history of these disorders, tailor follow-up frequency and interventions, estimate recurrence risks, and guide therapeutic decisions.

Consensus reclassification of inherited epidermolysis bullosa and other disorders with skin fragility

BACKGROUND

Several new genes and clinical subtypes have been identified since the publication in 2014 of the report of the last International Consensus Meeting on Epidermolysis Bullosa (EB).

OBJECTIVES

We sought to reclassify disorders with skin fragility, with a focus on EB, based on new clinical and molecular data.

METHODS

This was a consensus expert review.

RESULTS

In this latest consensus report, we introduce the concept of genetic disorders with skin fragility, of which classical EB represents the prototype. Other disorders with skin fragility, where blisters are a minor part of the clinical picture or are not seen because skin cleavage is very superficial, are classified as separate categories. These include peeling skin disorders, erosive disorders, hyperkeratotic disorders, and connective tissue disorders with skin fragility. Because of the common manifestation of skin fragility, these 'EB-related' disorders should be considered under the EB umbrella in terms of medical and socioeconomic provision of care.

CONCLUSIONS

The proposed classification scheme should be of value both to clinicians and researchers, emphasizing both clinical and genetic features of EB. What is already known about this topic? Epidermolysis bullosa (EB) is a group of genetic disorders with skin blistering. The last updated recommendations on diagnosis and classification were published in 2014. What does this study add? We introduce the concept of genetic disorders with skin fragility, of which classical EB represents the prototype. Clinical and genetic aspects, genotype-phenotype correlations, disease-modifying factors and natural history of EB are reviewed. Other disorders with skin fragility, e.g. peeling skin disorders, erosive disorders, hyperkeratotic disorders, and connective tissue disorders with skin fragility are classified as separate categories; these 'EB-related' disorders should be considered under the EB umbrella in terms of medical and socioeconomic provision of care. Linked Comment: Pope. Br J Dermatol 2020; 183:603.

Revertant mosaic fibroblasts in recessive dystrophic epidermolysis bullosa

BACKGROUND

Revertant mosaicism has been described previously in recessive dystrophic epidermolysis bullosa (RDEB), manifesting as regions of skin with normal mechanical and biological characteristics. Here we report the discovery of revertant dermal fibroblasts, unique in that all other documented cases of revertant mosaicism occur in epidermal keratinocytes.

OBJECTIVES

To determine the cause of revertant mosaicism found in a patient with RDEB from isolated epidermal keratinocytes and dermal fibroblasts in blister and mosaic skin regions.

METHODS

Skin biopsies were taken from blister and mosaic skin regions of a patient with RDEB. Allele identification was confirmed and the type VII collagen (C7) content and COL7A1 expression profile of isolated keratinocytes and fibroblasts was determined.

RESULTS

Keratinocytes isolated from the mosaic area had a slight increase in C7, although overall expression of COL7A1 was unchanged between blister and mosaic fibroblasts. Differential allele expression was identified in blister and mosaic fibroblasts using targeted RNA sequencing (TREx), where the allele harbouring a point mutation was preferentially expressed over that containing a frameshift mutation. A crossing over event was identified in mosaic fibroblasts that was not present in blister fibroblasts, yielding a functional COL7A1 allele in a subset of cells.

CONCLUSIONS

In documenting a novel case of revertant mosaicism in RDEB, we have identified dermal fibroblasts as having the capacity to correct blistering functionally. We have also pioneered the use of TREx in quantifying allele-specific expression. Using fibroblasts instead of keratinocytes for RDEB therapies offers advantages in the local and systemic therapy of RDEB. What's already known about this topic? Revertant mosaicism has been previously documented in patients with recessive dystrophic epidermolysis bullosa (RDEB), however, it has only been found in epidermal keratinocytes. What does this study add? We have demonstrated that COL7A1 gene reversion in dermal fibroblasts occurs and is able to form functional skin in a patient with RDEB. Additionally, we have pioneered a new application for targeted RNA sequencing in quantifying allele-specific expression in fibroblasts and keratinocytes. What is the translational message? This opens up possibilities for using fibroblasts as local and systemic therapy for patients with RDEB.

Keratins and epidermolysis bullosa simplex

Keratin intermediate filaments play an important role in maintaining the integrity of the skin structure. Understanding the importance of this subject is possible with the investigation of keratin defects in epidermolysis bullosa simplex (EBS). Nowadays, in addition to clinical criteria, new molecular diagnostic methods, such as next generation sequencing, can help to distinguish the subgroups of EBS more precisely. Because the most important and most commonly occurring molecular defects in these patients are the defects of keratins 5 and14 (KRT5 and KRT14), comprehending the nature structure of these proteins and their involved processes can be very effective in understanding the pathophysiology of this disease and providing new and effective therapeutic platforms to treat it. Here, we summarized the various aspects of the presence of KRT5 and KRT14 in the epidermis, their relation to the incidence and severity of EBS phenotypes, and the processes with which these proteins can affect them.

A "late-but-fitter revertant cell" explains the high frequency of revertant mosaicism in epidermolysis bullosa

Revertant mosaicism, or "natural gene therapy", is the phenomenon in which germline mutations are corrected by somatic events. In recent years, revertant mosaicism has been identified in all major types of epidermolysis bullosa, the group of heritable blistering disorders caused by mutations in the genes encoding epidermal adhesion proteins. Moreover, revertant mosaicism appears to be present in all patients with a specific subtype of recessive epidermolysis bullosa. We therefore hypothesized that revertant mosaicism should be expected at least in all patients with recessive forms of epidermolysis bullosa. Naturally corrected, patient-own cells are of extreme interest for their promising therapeutic potential, and their presence in all patients would open exciting, new treatment perspectives to those patients. To test our hypothesis, we determined the probability that single nucleotide reversions occur in patients' skin using a mathematical developmental model. According to our model, reverse mutations are expected to occur frequently (estimated 216x) in each patient's skin. Reverse mutations should, however, occur early in embryogenesis to be able to drive the emergence of recognizable revertant patches, which is expected to occur in only one per ~10,000 patients. This underestimate, compared to our clinical observations, can be explained by the "late-but-fitter revertant cell" hypothesis: reverse mutations arise at later stages of development, but provide revertant cells with a selective growth advantage in vivo that drives the development of recognizable healthy skin patches. Our results can be extrapolated to any other organ with stem cell division numbers comparable to skin, which may offer novel future therapeutic options for other genetic conditions if these revertant cells can be identified and isolated.

Revertant mosaicism in genodermatoses

Inherited monogenic skin disorders include blistering disorders, inflammatory disorders, and disorders of differentiation or development. In most cases, the skin is broadly involved throughout the affected individual's lifetime, but rarely, appearance of normal skin clones has been described. In these cases of revertant mosaicism, cells undergo spontaneous correction to ameliorate the effects of genetic mutation. While targeted reversion of genetic mutation would have tremendous therapeutic value, the mechanisms of reversion in the skin are poorly understood. In this review, we provide an overview of genodermatoses that demonstrate widespread reversion and their corrective mechanisms, as well as the current research aimed to understand this "natural gene therapy".

The categories of cutaneous mosaicism: A proposed classification

Mosaic disorders can most easily be studied in the skin. This article presents a comprehensive overview of the different forms of cutaneous mosaicism. Major categories are genomic versus epigenetic mosaicism and nonsegmental versus segmental mosaicism. The class of nonsegmental mosaics includes single point mosaicism as exemplified by solitary benign or malignant skin tumors; disseminated mosaicism as noted in autosomal dominant tumor syndromes such as neurofibromatosis 1; and patchy mosaicism without midline separation as found in giant melanocytic nevus. The class of segmental mosaics includes segmental manifestation of lethal genes surviving by mosaicism as noted in Proteus syndrome; type 1 segmental mosaicism of autosomal dominant skin disorders reflecting heterozygosity for a postzygotic new mutation; type 2 segmental mosaicism of autosomal dominant skin disorders reflecting loss of heterozygosity that occurred at an early developmental stage in a heterozygous embryo; and isolated or superimposed segmental mosaicism of common polygenic skin disorders such as psoriasis or atopic dermatitis. A particular form of genomic mosaicism is didymosis (twin spotting). Revertant mosaicism is recognizable as one or more areas of healthy skin in patients with epidermolysis bullosa or other serious genodermatoses. The category of epigenetic mosaicism includes several X-linked, male lethal disorders such as incontinentia pigmenti, and the patterns of lyonization as noted in X-linked non-lethal disorders such as hypohidrotic ectodermal dysplasia of the Christ-Siemens-Touraine type. An interesting field of future research will be the concept of epigenetic autosomal mosaicism that may explain some unusual cases of autosomal transmission of linear hypo- or hypermelanosis.

Epidermolysis bullosa in animals: a review

Epidermolysis bullosa (EB) is a hereditary mechanobullous disease of animals and humans, characterized by an extreme fragility of the skin and mucous membranes. The main feature of EB in humans and animals is the formation of blisters and erosions in response to minor mechanical trauma. Epidermolysis bullosa is caused by mutations in the genes that code for structural proteins of the cytoskeleton of the basal keratinocytes or of the basement membrane zone. Based on the ultrastructural levels of tissue separation, EB is divided into the following three broad categories: epidermolysis bullosa simplex, junctional epidermolysis bullosa and dystrophic epidermolysis bullosa. Human types of EB are divided into several subtypes based on their ultrastructural changes and the mode of inheritance; subtypes are not fully established in animals. In humans, it is estimated that EB affects one in 17,000 live births; the frequency of EB in different animals species is not known. In all animal species, except in buffalo with epidermolysis bullosa simplex, multifocal ulcers are observed on the gums, hard and soft palates, mucosa of the lips, cheek mucosa and dorsum of the tongue. Dystrophic or absent nails, a frequent sign seen in human patients with EB, corresponds to the deformities and sloughing of the hooves in ungulates and to dystrophy or atrophy of the claws in dogs and cats. This review covers aspects of the molecular biology, diagnosis, classification, clinical signs and pathology of EB reported in animals.

The genetics of human skin disease

The skin is composed of a variety of cell types expressing specific molecules and possessing different properties that facilitate the complex interactions and intercellular communication essential for maintaining the structural integrity of the skin. Importantly, a single mutation in one of these molecules can disrupt the entire organization and function of these essential networks, leading to cell separation, blistering, and other striking phenotypes observed in inherited skin diseases. Over the past several decades, the genetic basis of many monogenic skin diseases has been elucidated using classical genetic techniques. Importantly, the findings from these studies has shed light onto the many classes of molecules and essential genetic as well as molecular interactions that lend the skin its rigid, yet flexible properties. With the advent of the human genome project, next-generation sequencing techniques, as well as several other recently developed methods, tremendous progress has been made in dissecting the genetic architecture of complex, non-Mendelian skin diseases.

The genetics of skin fragility

Genetic skin fragility manifests with diminished resistance of the skin and mucous membranes to external mechanical forces and with skin blistering, erosions, and painful wounds as clinical features. Skin fragility disorders, collectively called epidermolysis bullosa, are caused by mutations in 18 distinct genes that encode proteins involved in epidermal integrity and dermal-epidermal adhesion. The genetic spectrum, along with environmental and genetic modifiers, creates a large number of clinical phenotypes, spanning from minor localized lesions to severe generalized blistering, secondary skin cancer, or early demise resulting from extensive loss of the epidermis. Laboratory investigations of skin fragility have greatly augmented our understanding of genotype-phenotype correlations in epidermolysis bullosa and have also advanced skin biology in general. Current translational research concentrates on the development of biologically valid treatments with therapeutic genes, cells, proteins, or small-molecule compounds in preclinical settings or human pilot trials.

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.

Local delivery of gene-modifying triplex-forming molecules to the epidermis

Epidermal keratinocytes are particularly suitable candidates for in situ gene correction. Intraperitoneal administration of a triplex-forming oligonucleotide (TFO) was shown previously to introduce DNA base changes in a reporter gene in skin, without identifying which cells had been targeted. We extend those previous experiments using two triplex-forming molecules (TFMs), a peptide nucleic acid (PNA-Antp) and a TFO (AG30), and two lines of transgenic mice that have the chromosomally integrated λsupFG1 shuttle-reporter transgene. Successful in vivo genomic modification occurs in epidermis and dermis in CD1 transgenic mice following either intraperitoneal or intradermal delivery of the PNA-Antennapedia conjugate. FITC-PNA-Antp accumulates in nuclei of keratinocytes and, after intradermal delivery of the PNA-Antp, chromosomally modified, keratin 5 positive basal keratinocytes persist for at least 10 days. In hairless (SKH1) mice with the λsupFG1 transgene, intradermal delivery of the TFO, AG30, introduces gene modifications in both tail and back skin and those chromosomal modifications persist in basal keratinocytes for 10 days. Hairless mice should facilitate comparison of various targeting agents and methods of delivery. Gene targeting by repeated local administration of oligonucleotides may prove clinically useful for judiciously selected disease-causing genes in the epidermis.

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

Hereditary keratin disorders of the skin and its appendages comprise a large group of clinically heterogeneous disfiguring blistering and ichthyotic diseases, primarily characterized by the loss of tissue integrity, blistering and hyperkeratosis in severely affected tissues. Pathogenic mutations in keratins cause these afflictions. Typically, these mutations in concert with characteristic features have formed the basis for improved disease diagnosis, prognosis and most recently therapy development. Examples include epidermolysis bullosa simplex, keratinopathic ichthyosis, pachyonychia congenita and several other tissue-specific hereditary keratinopathies. Understanding the molecular and genetic events underlying skin dysfunction has initiated alternative treatment approaches that may provide novel therapeutic opportunities for affected patients. Animal and in vitro disease modelling studies have shed more light on molecular pathogenesis, further defining the role of keratins in disease processes and promoting the translational development of new gene and pharmacological therapeutic strategies. Given that the molecular basis for these monogenic disorders is well established, gene therapy and drug discovery targeting pharmacological compounds with the ability to reinforce the compromised cytoskeleton may lead to promising new therapeutic strategies for treating hereditary keratinopathies. In this review, we will summarize and discuss recent advances in the preclinical and clinical modelling and development of gene, natural product, pharmacological and protein-based therapies for these disorders, highlighting the feasibility of new approaches for translational clinical therapy.

Natural gene therapy may occur in all patients with generalized non-Herlitz junctional epidermolysis bullosa with COL17A1 mutations

Mutations in the type XVII collagen gene (COL17A1) result in the blistering disorder non-Herlitz junctional epidermolysis bullosa (JEB-nH). The incidence of revertant mosaicism, also called "natural gene therapy", was identified in a cohort of 14 patients with JEB-nH caused by COL17A1 mutations in the Netherlands. Five different in vivo reversions, all correcting the germ-line COL17A1 mutation c.2237delG in exon 30, were found in four mosaic JEB-nH patients. The correcting DNA changes involved a wide variety of somatic mutations, from which an indel mutation (c.2228-101_2263+70delins15) and a large deletion of 2,165 base pairs (c.2227+153_2336-318del) have not been previously observed in patients with revertant mosaicism. Our results show that there is no preference for a repair mechanism. Moreover, revertant mosaicism was confirmed on a DNA level in 6 out of 10 generalized JEB-nH patients. Further, photo-material and clinical history of the other four generalized JEB-nH patients demonstrated that each patient has revertant skin patches. In contrast, revertant mosaicism was not detected in the four localized JEB-nH patients. The fact that so many, if not all, generalized JEB-nH COL17A1 patients have revertant patches offers opportunities for cell therapies in which the patient's own naturally corrected cells are used as a source.

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.

Two novel recessive mutations in KRT14 identified in a cohort of 21 Spanish families with epidermolysis bullosa simplex

BACKGROUND

Basal epidermolysis bullosa simplex (EBS) is a group of blistering genodermatoses mostly caused by mutations in the keratin genes, KRT5 and KRT14. Recessive mutations represent about 5% of all EBS mutations, being common and specific in populations with high consanguinity, where affected patients show severe phenotypes.

OBJECTIVES

To accomplish the first mutational analysis in patients of Spanish origin with EBS and to delineate a comprehensive genotype-phenotype correlation.

METHODS

Twenty-one EBS families were analysed. Immunofluorescence mapping at the dermoepidermal junction level was performed on skin biopsies from patients. Mutation screening of the entire coding sequences of KRT5 and KRT14 in genomic DNA was assessed by polymerase chain reaction and direct sequencing.

RESULTS

KRT5 or KRT14 causative mutations were identified in 18 of the 21 EBS families. A total of 14 different mutations were disclosed, of which 12 were dominant missense mutations and two truncating recessive mutations. Five of the 14 mutations were novel including three dominant in KRT5 (p.V186E, p.T321P and p.A428T) and two recessive in KRT14 (p.K116X and p.K250RfsX8). The two patients with EBS carrying homozygous recessive mutations were affected by severe phenotypes and belonged to consanguineous families. All five families with the EBS Dowling-Meara subtype carried recurrent mutations affecting the highly conserved ends of the α-helical rod domain of K5 and K14. The seven mutations associated with the localized EBS subtype were widely distributed along the KRT5 and KRT14 genes. Two families with mottled pigmentation carried the P25L mutation in KRT5, commonly associated with this subtype.

CONCLUSIONS

This study further confirms the genotype-phenotype correlation established for EBS in other ethnic groups, and is the first in a Mediterranean country (excluding Israel). This study adds two novel recessive mutations to the worldwide record to date, which includes a total of 14 mutations. As in previous reports, the recessive mutations resulted in a lack of keratin K14, giving rise to a generalized and severe presentation.

Rethinking the prosaicism of mosaicism; is it in us?

Exponential advances in the quantitation of DNA variation and epigenetic states seem poised to convert much of biological research into a statistical exercise. But these developments also invite us to reimagine well-worn biological concepts on a grander scale. Somatic mosaicism refers to postzygotic mutations persisting in the individual, occasionally conspicuous to dermatologists as Blaschkoid, checkerboard, phylloid and patchy morphologies. A thoughtful examination of cutaneous mosaicism suggests, however, that virtually all of us may be somatic mosaics. Such genetic variability within individuals might explain localized presentations of disease and implies that some tissues literally evolve throughout life. We discuss here (i) the likely ubiquity of somatic mosaicism, (ii) the broad range of possible biological consequences and (iii) how experimentalists and clinicians may begin establishing genotype-to-phenotype correlates.

Revertant mosaicism in skin: natural gene therapy

Revertant mosaicism is a naturally occurring phenomenon involving spontaneous correction of a pathogenic mutation in a somatic cell. Recent studies suggest that it is not a rare event and that it could be clinically relevant to phenotypic expression and patient treatment. Indeed, revertant cell therapy represents a potential 'natural gene therapy' because in vivo reversion obviates the need for further genetic correction. Revertant mosaicism has been observed in several inherited conditions, including epidermolysis bullosa, a heterogeneous group of blistering skin disorders. These diseases provide a useful model for studying revertant mosaicism because of the visual and accessible nature of skin. This overview highlights the latest developments in revertant mosaicism and the translational implications germane to heritable skin disorders.

Mitotic recombination in patients with ichthyosis causes reversion of dominant mutations in KRT10

Somatic loss of wild-type alleles can produce disease traits such as neoplasia. Conversely, somatic loss of disease-causing mutations can revert phenotypes; however, these events are infrequently observed. Here we show that ichthyosis with confetti, a severe, sporadic skin disease in humans, is associated with thousands of revertant clones of normal skin that arise from loss of heterozygosity on chromosome 17q via mitotic recombination. This allowed us to map and identify disease-causing mutations in the gene encoding keratin 10 (KRT10); all result in frameshifts into the same alternative reading frame, producing an arginine-rich C-terminal peptide that redirects keratin 10 from the cytokeratin filament network to the nucleolus. The high frequency of somatic reversion in ichthyosis with confetti suggests that revertant stem cell clones are under strong positive selection and/or that the rate of mitotic recombination is elevated in individuals with this disorder.

Epidermolysis bullosa simplex: a paradigm for disorders of tissue fragility

Epidermolysis bullosa (EB) simplex is a rare genetic condition typified by superficial bullous lesions that result from frictional trauma to the skin. Most cases are due to dominantly acting mutations in either keratin 14 (K14) or K5, the type I and II intermediate filament (IF) proteins tasked with forming a pancytoplasmic network of 10-nm filaments in basal keratinocytes of the epidermis and in other stratified epithelia. Defects in K5/K14 filament network architecture cause basal keratinocytes to become fragile and account for their trauma-induced rupture. Here we review how laboratory investigations centered on keratin biology have deepened our understanding of the etiology and pathophysiology of EB simplex and revealed novel avenues for its therapy.

Cutaneous mosaicism: a molecular and clinical review

Physicians have long been intrigued by the distinct patterns created by epidermal nevi and other mosaic cutaneous disorders. Although many of the molecular mechanisms underlying these disorders remain unrevealed, with the release of the results of the Human Genome Project our knowledge is rapidly increasing. The underlying genetic defects for many of the X-linked and mosaic disorders have recently been identified. Advances in technology, such as the array comparative genomic hybridization, will provide the tools for continued gene discovery and expanded understanding of the pathogenic mechanisms underlying mosaic skin conditions.

Revertant mosaicism in junctional epidermolysis bullosa due to multiple correcting second-site mutations in LAMB3

Revertant mosaicism due to in vivo reversion of an inherited mutation has been described in the genetic skin disease epidermolysis bullosa (EB) for the genes KRT14 and COL17A1. Here we demonstrate the presence of multiple second-site mutations, all correcting the germline mutation LAMB3:c.628G-->A;p.E210K, in 2 unrelated non-Herlitz junctional EB patients with revertant mosaicism. Both probands had a severe reduction in laminin-332 expression in their affected skin. Remarkably, the skin on the lower leg of patient 078-01 (c.628G-->A/c.1903C-->T) became progressively clinically healthy, with normal expression of laminin-332 on previously affected skin. In the other proband, 029-01 (c.628G-->A/c.628G-->A), the revertant patches were located at his arms, shoulder, and chest. DNA analysis showed different second-site mutations in revertant keratinocytes of distinct biopsy specimens (c.565-3T-->C, c.596G-->C;p.G199A, c.619A-->C;p.K207Q, c.628+42G-->A, and c.629-1G-->A), implying that there is not a single preferred mechanism for the correction of a specific mutation. Our data offer prospects for EB treatment in particular cases, since revertant mosaicism seems to occur at a higher frequency than expected. This opens the possibility of applying revertant cell therapy in mosaic EB of the LAMB3 gene by using autologous naturally corrected keratinocytes, thereby bypassing the recombinant gene correction phase.

Multiple correcting COL17A1 mutations in patients with revertant mosaicism of epidermolysis bullosa

Revertant mosaicism by somatic reversion of inherited mutations has been described for a number of genetic diseases. Several mechanisms can underlie this reversion process, such as gene conversion, crossing-over, true back mutation, and second-site mutation. Here, we report the occurrence of multiple corrections in two unrelated probands with revertant mosaicism of non-Herlitz junctional epidermolysis bullosa, an autosomal recessive genodermatosis due to mutations in the COL17A1 gene. Immunofluorescence microscopy and laser dissection microscopy, followed by DNA and RNA analysis, were performed on skin biopsy specimens. In patient 1, a true back mutation, 3781T-->C, was identified in the specimen from the arm, and a second-site mutation, 4463-1G-->A, which compensated for the frameshift caused by the inherited 4424-5insC mutation, was identified in the 3' splice site of exon 55 in a specimen from the middle finger. Patient 2 showed--besides two distinct gene conversion events in specimens from the arm and hand sites, both of which corrected the 1706delA mutation--a second-site mutation (3782G-->C) in an ankle specimen, which prevented the premature ending of the protein by the 3781C-->T nonsense mutation (R1226X). Thus, both inherited mutations, paternal as well as maternal, reverted at least once by different reversion events in distinct cell clusters in the described patients. The occurrence of multiple correcting mutations within the same patient indicates that in vivo reversion is less unusual than was generally thought. Furthermore, in the male patient, mosaic patterns of type XVII collagen-positive keratinocytes were present in clinically unaffected and affected skin. This latter observation makes it likely that reversion may be overlooked and may happen more often than expected.

Mosaicism in cutaneous pigmentation

PURPOSE OF REVIEW

This article reviews the disorders of patterned dyspigmentation and discusses the pathogenesis of the pigmentary changes.

RECENT FINDINGS

A range of cytogenetic abnormalities has been detected in patterned pigmentary disease. This molecular heterogeneity correlates with the wide spectrum of clinical phenotypes observed. Many of the molecular defects overlap with genes known to play a role in pigmentation. Our understanding of the underlying genetic mechanisms for these mosaic conditions is evolving with advances in technology and dissection of the molecular pathways involved in melanocyte biology.

SUMMARY

The causal heterogeneity of patterned dyspigmentation promises to reveal clues about the differentiation, function, and control of melanocytes in embryonic and postnatal development.

Piecing together the puzzle of cutaneous mosaicism

Autosomal dominant disorders of the skin may present in a pattern following the lines of embryologic development of the ectoderm. In these cases, the surrounding skin is normal, and molecular studies have shown that the causative mutation is confined to the affected ectodermal tissue (type 1 mosaicism). Rarely, an individual shows skin lesions that follow the pattern of type 1 mosaicism, but the rest of the skin shows a milder form of the disorder (type 2 mosaicism). A new study provides the molecular basis for type 2 mosaicism.

Genodermatoses 2003-2004

PURPOSE OF REVIEW

Many genodermatoses have been linked in recent years to their respective genes. The underlying biology and integrative nature of these genes with other genes and organ systems is beginning to be understood. This paper reviews recent advances in neurocutaneous disorders, ectodermal dysplasias, and the phenomenon of revertant gene mosaicism.

RECENT FINDINGS

In neurofibromatosis type 1, molecular assays are being developed to distinguish malignant from benign and premalignant lesions. Clinical mutation analysis for the NF1 gene has been problematic; a sensitive new assay using automated comparative sequence analysis may be helpful. Revision of clinical care guidelines is ongoing. New data for the prospective management of optic pathway gliomas is reviewed. The two genes that underlie tuberous sclerosis complex, tuberin and hamartin, lie at the center of an important signal transduction pathway with significant implications for pharmacologic treatment. Issues in genetic counseling for this highly variable disease are updated. Extensive progress has been made in understanding the basis of several forms of ectodermal dysplasia. Disorders caused by mutations in p63 and the connexin and NF-kappaB gene families will be reviewed. Finally, phenotypic in vivo amelioration of genodermatoses via revertant gene mosaicism will be discussed as a possible mechanism to be exploited in directed therapeutic approaches.

SUMMARY

This paper reviews recent developments in the molecular and biologic bases of neurofibromatosis type 1, tuberous sclerosis, and ectodermal disorders related to p63 and the connexin and NF-kappaB gene families. The concept of revertant gene mosaicism is also discussed as a potential model for gene therapy.

Gene therapy of epidermolysis bullosa

Easy access to the organ and identification of underlying mutations in epidermolysis bullosa (EB) facilitated the first cutaneous gene therapy experiments in vitro in the mid-1990s. The leading technology was transduction of the respective cDNA carried by a retroviral vector. Using this approach, the genotypic and phenotypic hallmark features of the recessive forms of junctional EB, which are caused by loss of function of the structural proteins laminin-5 or bullous pemphigoid antigen 2/type XVII collagen of the dermo-epidermal basement membrane zone, have been corrected in vitro and in vivo using xenograft mouse models. Recently, this approach has also been shown to be feasible for the large COL7A1 gene (mutated in dystrophic EB), applying PhiC31 integrase or lentiviral vectors. Neither of these approaches has made it into a successful Phase I study on EB patients. Therefore, alternative approaches to gene correction, including modulation of splicing, are being investigated for gene therapy in EB.

Natural repair mechanisms in correcting pathogenic mutations in inherited skin disorders

This review assesses molecular aspects of the rescue of disease-causing mutations in genodermatoses by means of naturally occurring secondary genetic phenomena. Such data have important implications for the design of gene therapy approaches for inherited skin diseases. Reversal of the phenotype depends on three elements: the number of cells involved; the degree of gene reversal; and the specific timing of the reversion. If reversion occurs in somatic cells, revertant mosaicism may occur. This is the situation in which a patient's skin is generally affected by the genodermatosis, but islands of normal skin stand out. These reflect the presence of revertant cells that are sufficient to restore a normal local skin phenotype. Reversion of the original mutation may also be partial, in which case the phenotype may display no, or only limited, improvement. Nevertheless, the phenotype may ameliorate with age if the reverted cells preferentially expand in time or if the time of onset of reversion is after birth. In essence, the complexities of naturally occurring rescue processes are important to understand because the inherent mechanisms may provide clues and insight into optimal therapeutic gene manipulation, and the possibility of mimicking nature in the management of patients with diverse genodermatoses.