Revertant mosaicism in skin: natural gene therapy

Tissue mosaicism following stem cell aging: blood as an exemplar

Loss of stem cell regenerative potential underlies aging of all tissues. Somatic mosaicism, the emergence of cellular patchworks within tissues, increases with age and has been observed in every organ yet examined. In the hematopoietic system, as in most tissues, stem cell aging through a variety of mechanisms occurs in lockstep with the emergence of somatic mosaicism. Here, we draw on insights from aging hematopoiesis to illustrate fundamental principles of stem cell aging and somatic mosaicism. We describe the generalizable changes intrinsic to aged stem cells and their milieu that provide the backdrop for somatic mosaicism to emerge. We discuss genetic and nongenetic mechanisms that can result in tissue somatic mosaicism and existing methodologies to detect such clonal outgrowths. Finally, we propose potential avenues to modify mosaicism during aging, with the ultimate aim of increasing tissue resiliency.

Clinical and molecular diagnosis of genodermatoses: Review and perspectives

Genodermatoses are a complex and heterogeneous group of genetic skin disorders characterized by variable expression and clinical and genetic heterogeneity, rendering their diagnosis challenging. DNA-based techniques, like whole-exome sequencing, can establish a diagnosis in 50% of cases. RNA-sequencing is emerging as an attractive tool that can obtain information regarding gene expression while integrating functional genomic data with regard to the interpretation of variants. This increases the diagnostic rate by an additional 10-15%. In the present review, we detail the clinical steps involved in the diagnosis of genodermatoses, as well as the current DNA-based technologies available to clinicians. Herein, the intention is to facilitate a better understanding of the possibilities and limitations of these diagnostic technologies. In addition, this review could guide dermatologists through new emerging techniques, such as RNA-sequencing and its applications to familiarizing them with future techniques. Currently, this multi-omics approach is likely the best strategy designed to promote the diagnosis of patients with genodermatoses and discover new skin disease genes that could result in novel targeted therapies.

Recurring germline mosaicism in a family due to reversion of an inherited derivative chromosome 8 from an 8;21 translocation with interstitial telomeric sequences

BACKGROUND

Mosaicism for chromosomal structural abnormalities, other than marker or ring chromosomes, is rarely inherited.

METHODS

We performed cytogenetics studies and breakpoint analyses on a family with transmission of mosaicism for a derivative chromosome 8 (der(8)), resulting from an unbalanced translocation between the long arms of chromosomes 8 and 21 over three generations.

RESULTS

The proband and his maternal half-sister had mosaicism for a der(8) cell line leading to trisomy of the distal 21q, and both had Down syndrome phenotypic features. Mosaicism for a cell line with the der(8) and a normal cell line was also detected in a maternal half-cousin. The der(8) was inherited from the maternal grandmother who had four abnormal cell lines containing the der(8), in addition to a normal cell line. One maternal half-aunt had the der(8) and an isodicentric chromosome 21 (idic(21)). Sequencing studies revealed microhomologies at the junctures of the der(8) and idic(21) in the half-aunt, suggesting a replicative mechanism in the rearrangement formation. Furthermore, interstitial telomeric sequences (ITS) were identified in the juncture between chromosomes 8 and 21 in the der(8).

CONCLUSION

Mosaicism in the proband, his half-sister and half-cousin resulting from loss of chromosome 21 material from the der(8) appears to be a postzygotic event due to the genomic instability of ITS and associated with selective growth advantage of normal cells. The reversion of the inherited der(8) to a normal chromosome 8 in this family resembles revertant mosaicism of point mutations. We propose that ITS could mediate recurring revertant mosaicism for some constitutional chromosomal structural abnormalities.

Postzygotic mutations and where to find them - Recent advances and future implications in the field of non-neoplastic somatic mosaicism

The technological progress of massively parallel sequencing (MPS) has triggered a remarkable development in the research on postzygotic mutations. Although the overwhelming majority of studies in the field focus on oncogenesis, non-neoplastic diseases are attracting more and more attention. The aim of this review was to summarize some of the most recent findings in the field of somatic mosaicism in diseases other than neoplastic events. We discuss the abundance and role of postzygotic mutations, with a special emphasis on disorders which occur only in a mosaic form (obligatory mosaic diseases; OMDs). Based on the list of OMDs compiled from the published literature and three databases (OMIM, Orphanet and MosaicBase), we demonstrate the prevalence of cancer-related genes across OMDs and suggest other sources to further explore OMDs and OMD-related genes. Additionally, we comment on some practical aspects related to mosaic diseases, such as approaches to tissue sampling, the MPS coverage required to detect variants at a very low frequency, as well as on bioinformatic and molecular tools dedicated to detect somatic mutations in MPS data.

Challenges in Treating Genodermatoses: New Therapies at the Horizon

Genodermatoses are rare inherited skin diseases that frequently affect other organs. They often have marked effects on wellbeing and may cause early death. Progress in molecular genetics and translational research has unravelled many underlying pathological mechanisms, and in several disorders with high unmet need, has opened the way for the introduction of innovative treatments. One approach is to intervene where cell-signaling pathways are dysregulated, in the case of overactive pathways by the use of selective inhibitors, or when the activity of an essential factor is decreased by augmenting a molecular component to correct disequilibrium in the pathway. Where inflammatory reactions have been induced by a genetically altered protein, another possible approach is to suppress the inflammation directly. Depending on the nature of the genodermatosis, the implicated protein or even on the particular mutation, to correct the consequences or the genetic defect, may require a highly personalised stratagem. Repurposed drugs, can be used to bring about a "read through" strategy especially where the genetic defect induces premature termination codons. Sometimes the defective protein can be replaced by a normal functioning one. Cell therapies with allogeneic normal keratinocytes or fibroblasts may restore the integrity of diseased skin and allogeneic bone marrow or mesenchymal cells may additionally rescue other affected organs. Genetic engineering is expanding rapidly. The insertion of a normal functioning gene into cells of the recipient is since long explored. More recently, genome editing, allows reframing, insertion or deletion of exons or disruption of aberrantly functioning genes. There are now several examples where these stratagems are being explored in the (pre)clinical phase of therapeutic trial programmes. Another stratagem, designed to reduce the severity of a given disease involves the use of RNAi to attenuate expression of a harmful protein by decreasing abundance of the cognate transcript. Most of these strategies are short-lasting and will thus require intermittent life-long administration. In contrast, insertion of healthy copies of the relevant gene or editing the disease locus in the genome to correct harmful mutations in stem cells is more likely to induce a permanent cure. Here we discuss the potential advantages and drawbacks of applying these technologies in patients with these genetic conditions. Given the severity of many genodermatoses, prevention of transmission to future generations remains an important goal including offering reproductive choices, such as preimplantation genetic testing, which can allow selection of an unaffected embryo for transfer to the uterus.

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.

Current topics in Epidermolysis bullosa: Pathophysiology and therapeutic challenges

Epidermolysis bullosa (EB) is a group of inherited skin and mucosal fragility disorders resulting from mutations in genes encoding basement membrane zone (BMZ) components or proteins that maintain the integrity of BMZ and adjacent keratinocytes. More than 30 years have passed since the first causative gene for EB was identified, and over 40 genes are now known to be responsible for the protean collection of mechanobullous diseases included under the umbrella term of EB. Through the elucidation of disease mechanisms using human skin samples, animal models, and cultured cells, we have now reached the stage of developing more effective therapeutics for EB. This review will initially focus on what is known about blister wound healing in EB, since recent and emerging basic science data are very relevant to clinical translation and therapeutic strategies for patients. We then place these studies in the context of the latest information on gene therapy, read-through therapy, and cell therapy that provide optimism for improved clinical management of people living with EB.

Somatic Mutations in Rheumatologic Diseases: VEXAS Syndrome and Beyond

Discovery of the VEXAS (vacuoles, E1 enzyme, X-linked, autoinflammatory, somatic) syndrome demonstrates that somatic mutations in hematologic precursor cells can cause adult-onset, complex inflammatory disease. Unlike germline mutations, somatic mutations occur throughout the lifespan, are restricted to specific tissue types, and may play a causal role in non-heritable rheumatologic diseases, especially conditions that start in later life. Improvements in sequencing technology have enabled researchers and clinicians to detect somatic mutations in various tissue types, especially blood. Understanding the relationships between cell-specific acquired mutations and inflammation is likely to yield key insights into causal factors that underlie many rheumatologic diseases. The objective of this review is to detail how somatic mutations are likely to be relevant to clinicians who care for patients with rheumatologic diseases, with particular focus on the pathogenetic mechanisms of the VEXAS syndrome.

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.

Somatic mutations provide important and unique insights into the biology of complex diseases

Somatic evolution of cells within the body is well known to lead to cancers. However, spread of somatic mutations within a tissue over time may also contribute to the pathogenesis of non-neoplastic diseases. Recent years have seen the publication of many studies aiming to characterize somatic evolution in healthy tissues. A logical next step is to extend such work to diseased conditions. As our understanding of the interplay between somatic mutations and non-neoplastic disease grows, opportunities for the joint study of germline and somatic variants will present themselves. Here, we present our thoughts on the utility of somatic mutations for understanding both the causes and consequences of common complex disease and the challenges that remain for the joint study of the soma and germline.

Phenotypic heterogeneity and mosaicism in Xia-Gibbs syndrome: Five Danish patients with novel variants in AHDC1

Xia-Gibbs syndrome (XGS) is a neurodevelopmental disorder characterized by intellectual disability, developmental delay, seizures, hypotonia, obstructive sleep apnoea and mild facial dysmorphism. Heterozygosity for loss-of-function variants in AHDC1, encoding the AT-hook DNA binding motif containing protein 1, were discovered in 2014 as the likely genetic cause of Xia-Gibbs syndrome. We present five patients with Xia-Gibbs syndrome caused by previously unreported variants in AHDC1. Two of the patients share a frameshift variant: c.2849del (p.(Pro950Argfs*192)) in AHDC1. Despite sharing this variant, the two patients show remarkable phenotypic differences underscoring the clinical heterogeneity of Xia-Gibbs syndrome. In addition, we present a case of Xia-Gibbs syndrome caused by mosaicism for an AHDC1 variant.

Pathogenetic Therapy of Epidermolysis Bullosa: Current State and Prospects

Epidermolysis bullosa is a severe hereditary disease caused by mutations in genes encoding cutaneous basement membrane proteins. These mutations lead to dermal-epidermal junction failure and, as a result, to disturbances in the morphological integrity of the skin. Clinically, it manifests in the formation of blisters on the skin or mucosa that in some cases can turn into non-healing chronic wounds, which not only impairs patient's quality of life, but also is a live-threatening condition. Now, the main approaches in the treatment of epidermolysis bullosa are symptomatic therapy and palliative care, though they are little effective and are aimed at reducing the pain, but not to complete recovery. In light of this, the development of new treatment approaches aimed at correction of genetic defects is in progress. Various methods based on genetic engineering technologies, transplantation of autologous skin cells, progenitor skin cells, as well as hematopoietic and mesenchymal stem cells are studied. This review analyzes the pathogenetic methods developed for epidermolysis bullosa treatment based on the latest achievements of molecular genetics and cellular technologies, and discusses the prospects for the use of these technologies for the therapy of epidermolysis bullosa.

The Combination of Single-Cell and Next-Generation Sequencing Can Reveal Mosaicism for BRCA2 Mutations and the Fine Molecular Details of Tumorigenesis

Germline mutations in the BRCA1 and BRCA2 genes are responsible for hereditary breast and ovarian cancer syndrome. Germline and somatic BRCA1/2 mutations may define therapeutic targets and refine cancer treatment options. However, routine BRCA diagnostic approaches cannot reveal the exact time and origin of BRCA1/2 mutation formation, and thus, the fine details of their contribution to tumor progression remain less clear. Here, we establish a diagnostic pipeline using high-resolution microscopy and laser microcapture microscopy to test for BRCA1/2 mutations in the tumor at the single-cell level, followed by deep next-generation sequencing of various tissues from the patient. To demonstrate the power of our approach, here, we describe a detailed single-cell-level analysis of an ovarian cancer patient we found to exhibit constitutional somatic mosaicism of a pathogenic BRCA2 mutation. Employing next-generation sequencing, BRCA2 c.7795G>T, p.(Glu2599Ter) was detected in 78% of reads in DNA extracted from ovarian cancer tissue and 25% of reads in DNA derived from peripheral blood, which differs significantly from the expected 50% of a hereditary mutation. The BRCA2 mutation was subsequently observed at 17-20% levels in the normal ovarian and buccal tissue of the patient. Together, our findings suggest that this mutation occurred early in embryonic development. Characterization of the mosaic mutation at the single-cell level contributes to a better understanding of BRCA mutation formation and supports the concept that the combination of single-cell and next-generation sequencing methods is advantageous over traditional mutational analysis methods. This study is the first to characterize constitutional mosaicism down to the single-cell level, and it demonstrates that BRCA2 mosaicism occurring early during embryogenesis can drive tumorigenesis in ovarian cancer.

Human pluripotent stem cells: An alternative for 3D in vitro modelling of skin disease

To effectively study the skin and its pathology, various platforms have been used to date, with in vitro 3D skin models being considered the future gold standard. These models have generally been engineered from primary cell lines. However, their short life span leading to the use of various donors, imposes issues with genetic variation. Human pluripotent stem cell (hPSC)-technology holds great prospects as an alternative to the use of primary cell lines to study the pathophysiology of human skin diseases. This is due to their potential to generate an unlimited number of genetically identical skin models that closely mimic the complexity of in vivo human skin. During the past decade, researchers have therefore started to use human embryonic and induced pluripotent stem cells (hESC/iPSC) to derive skin resident-like cells and components. These have subsequently been used to engineer hPSC-derived 3D skin models. In this review, we focus on the advantages, recent developments, and future perspectives in using hPSCs as an alternative cell source for modelling human skin diseases in vitro.

Next-generation sequencing for inborn errors of immunity

Inborn errors of immunity (IEIs) include several hundred gene defects affecting various components of the immune system. As with other constitutional disorders, next-generation sequencing (NGS) is a powerful tool for the diagnosis of these diseases. While NGS can provide molecular confirmation of disease in a patient with a suspected or classic phenotype, it can also identify new molecular defects of the immune system, expand gene-disease phenotypes, clarify mechanism of disease, pattern of inheritance or identify new gene-disease associations. Multiple clinical specialties are involved in the diagnosis and management of patients with IEI, and most have no formal genetic training or expertise. To effectively utilize NGS tools and data in clinical practice, it is relevant and pragmatic to obtain a modicum of knowledge about genetic terminology, the variety of platforms and tools available for high-throughput genomic analysis, the interpretation and implementation of such data in clinical practice. There is considerable variability not only in the technologies and analytical tools used for NGS but in the bioinformatics approach to variant identification and interpretation. The ability to provide a molecular basis for disease has the potential to alter therapeutic management and longer-term treatment of the disease, including developing personalized approaches with molecularly targeted therapies. This review is intended for the clinical specialist or diagnostic immunologist who works in the area of inborn errors of immunity, and provides an overview of the need for genetic testing in these patients (the "why" aspect), the various technologies and analytical approaches, bioinformatics tools, resources, and challenges (the "how" aspect), and the clinical evidence for identifying which patients might be best served by such testing (the "when" aspect).

Revertant somatic mosaicism as a cause of cancer

Revertant (somatic) mosaicism is a spontaneous correction of a causative mutation in patients with congenital diseases. A relatively frequent event, revertant mosaicism may bring favorable outcomes that ameliorate disorders, and is therefore called “natural gene therapy.” However, it has been revealed recently that “overcorrection” of inherited bone marrow failure in patients with sterile alpha motif domain containing 9 (SAMD9)/9L syndromes by revertant mosaicism induces myelodysplastic syndrome (MDS) with monosomy 7 that occasionally proceeds to acute myelogenous leukemia (AML). In this review, we interpret very complex mechanisms underlying MDS/AML in patients with SAMD9/9L syndromes. This includes multiple myeloid tumor suppressors on the long arm of chromosome 7, all of which act in a haploinsufficient fashion, and a difference in sensitivity to interferon between cells carrying a mutation and revertants. Overcorrection of mutants by somatic mosaicism is likely a novel mechanism in carcinogenesis.

Mosaicism in Human Health and Disease

Mosaicism refers to the occurrence of two or more genomes in an individual derived from a single zygote. Germline mosaicism is a mutation that is limited to the gonads and can be transmitted to offspring. Somatic mosaicism is a postzygotic mutation that occurs in the soma, and it may occur at any developmental stage or in adult tissues. Mosaic variation may be classified in six ways: (a) germline or somatic origin, (b) class of DNA mutation (ranging in scale from single base pairs to multiple chromosomes), (c) developmental context, (d) body location(s), (e) functional consequence (including deleterious, neutral, or advantageous), and (f) additional sources of mosaicism, including mitochondrial heteroplasmy, exogenous DNA sources such as vectors, and epigenetic changes such as imprinting and X-chromosome inactivation. Technological advances, including single-cell and other next-generation sequencing, have facilitated improved sensitivity and specificity to detect mosaicism in a variety of biological contexts.

Evidence for immortality and autonomy in animal cancer models is often not provided, which causes confusion on key issues of cancer biology

Modern research into carcinogenesis has undergone three phases. Surgeons and pathologists started the first phase roughly 250 years ago, establishing morphological traits of tumors for pathologic diagnosis, and setting immortality and autonomy as indispensable criteria for neoplasms. A century ago, medical doctors, biologists and chemists started to enhance "experimental cancer research" by establishing many animal models of chemical-induced carcinogenesis for studies of cellular mechanisms. In this second phase, the two-hit theory and stepwise carcinogenesis of "initiation-promotion" or "initiation-promotion-progression" were established, with an illustrious finding that outgrowths induced in animals depend on the inducers, and thus are not authentically neoplastic, until late stages. The last 40 years are the third incarnation, molecular biologists have gradually dominated the carcinogenesis research fraternity and have established numerous genetically-modified animal models of carcinogenesis. However, evidence has not been provided for immortality and autonomy of the lesions from most of these models. Probably, many lesions had already been collected from animals for analyses of molecular mechanisms of "cancer" before the lesions became autonomous. We herein review the monumental work of many predecessors to reinforce that evidence for immortality and autonomy is essential for confirming a neoplastic nature. We extrapolate that immortality and autonomy are established early during sporadic human carcinogenesis, unlike the late establishment in most animal models. It is imperative to resume many forerunners' work by determining the genetic bases for initiation, promotion and progression, the genetic bases for immortality and autonomy, and which animal models are, in fact, good for identifying such genetic bases.

Recombination-induced revertant mosaicism in ichthyosis with confetti and loricrin keratoderma

Revertant mosaicism refers to a condition in which a pathogenic germline mutation is spontaneously corrected in somatic cells, resulting in the presence of two or more cell populations with different genotypes in an organism arising from a single fertilized egg. If the revertant cells are clonally expanded due to a survival advantage over the surrounding mutant cells, patients benefit from this self-healing phenomenon which leads to the development of milder-than-expected clinical phenotypes; in genetic skin diseases, patients with revertant mosaicism present with small islands of healthy skin. To date, revertant mosaicism has been reported in ∼50 genetic diseases involving the skin, blood, liver, muscle, and brain. In this review, I briefly summarize current knowledge on revertant mosaicism in two particular skin diseases, ichthyosis with confetti (IWC) and loricrin keratoderma (LK), both of which develop numerous revertant skin patches. Notably, homologous recombination (HR) is the only mechanism underlying the reversion of pathogenic mutations in IWC and LK, and this was identified following the analysis of ∼50 revertant epidermis samples. All the samples showed long-tract loss of heterozygosity (LOH) that originated at regions centromeric to pathogenic mutations and extended to the telomere of the mutation-harboring chromosomes. Elucidating the molecular mechanisms underlying revertant mosaicism in IWC and LK-especially how mutant proteins induce long-tract LOH-would potentially expand the possibility of manipulating HR to induce the reversion of disease-causing mutations and help devising novel therapies not only for IWC and LK but also for other intractable genetic diseases.

Parental somatogonadal COL2A1 mosaicism contributes to intrafamilial recurrence in a family with type 2 collagenopathy

The COL2A1 gene encodes the alpha-1 chain of procollagen type 2. Pathogenic variants in the COL2A1 gene are associated with several different types of skeletal dysplasia collectively known as type 2 collagenopathies. Type 2 collagenopathies have an autosomal dominant inheritance. Some germline or somatogonadal mosaicism cases have been reported. We investigated whether somatogonadal mosaicism occurred in a family with two children suspected of type 2 collagenopathies or related diseases. First, we detected a pathogenic variant in the COL2A1 gene in the two affected children by whole exome sequencing (WES). Next, we performed targeted deep sequencing to their parents without the variant by WES. A low level of COL2A1 mosaicism was revealed in the mother's tissues. We concluded that the mother had somatogonadal mosaicism with the COL2A1 mutation arose in the epiblast, and that the intrafamilial recurrence rate of the disease by the somatogonadal mosaicism was higher than by the germline mosaicism. This report suggests that parental low-level mosaicism should be evaluated in those parents with children carrying de novo germline mutations and the targeted deep sequencing is useful to detect them.

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.

Cultured Epidermal Autografts from Clinically Revertant Skin as a Potential Wound Treatment for Recessive Dystrophic Epidermolysis Bullosa

Inherited skin disorders have been reported recently to have sporadic normal-looking areas, where a portion of the keratinocytes have recovered from causative gene mutations (revertant mosaicism). We observed a case of recessive dystrophic epidermolysis bullosa treated with cultured epidermal autografts (CEAs), whose CEA-grafted site remained epithelized for 16 years. We proved that the CEA product and the grafted area included cells with revertant mosaicism. Based on these findings, we conducted an investigator-initiated clinical trial of CEAs from clinically revertant skin for recessive dystrophic epidermolysis bullosa. The donor sites were analyzed by genetic analysis, immunofluorescence, electron microscopy, and quantification of the reverted mRNA with deep sequencing. The primary endpoint was the ulcer epithelization rate per patient at 4 weeks after the last CEA application. Three patients with recessive dystrophic epidermolysis bullosa with 8 ulcers were enrolled, and the epithelization rate for each patient at the primary endpoint was 87.7%, 100%, and 57.0%, respectively. The clinical effects were found to persist for at least 76 weeks after CEA transplantation. One of the three patients had apparent revertant mosaicism in the donor skin and in the post-transplanted area. CEAs from clinically normal skin are a potentially well-tolerated treatment for recessive dystrophic epidermolysis bullosa.

Mosaicism in Cutaneous Disorders

Genetic mosaicism arises when a zygote harbors two or more distinct genotypes, typically due to de novo, somatic mutation during embryogenesis. The clinical manifestations largely depend on the differentiation status of the mutated cell; earlier mutations target pluripotent cells and generate more widespread disease affecting multiple organ systems. If gonadal tissue is spared-as in somatic genomic mosaicism-the mutation and its effects are limited to the proband, whereas mosaicism also affecting the gametes, such as germline or gonosomal mosaicism, is transmissible. Mosaicism is easily appreciated in cutaneous disorders, as phenotypically distinct mutant cells often give rise to lesions in patterns determined by the affected cell type. Genetic investigation of cutaneous mosaic disorders has identified pathways central to disease pathogenesis, revealing novel therapeutic targets. In this review, we discuss examples of cutaneous mosaicism, approaches to gene discovery in these disorders, and insights into molecular pathobiology that have potential for clinical translation.

Functional therapies for cutaneous wound repair in epidermolysis bullosa

Chronic wounding as a result of recurrent skin blistering in the painful genetic skin disease epidermolysis bullosa, may lead to life-threatening infections, increased risk of tumor formation, and other serious medical complications. Therefore, epidermolysis bullosa patients have an urgent need for optimal wound care and tissue regeneration. Therapeutic strategies using gene-, protein-, and cell-therapies are being developed to improve clinical symptoms, and some of them have already been investigated in early clinical trials. The most favorable options of functional therapies include gene replacement, gene editing, RNA targeting, and harnessing natural gene therapy. This review describes the current progress of the different approaches targeting autologous skin cells, and will discuss the benefits and challenges of their application.

Chromosomal inversions as a hidden disease-modifying factor for somatic recombination phenotypes

Heterozygous chromosomal inversions suppress recombination. Therefore, they may potentially influence recombination-associated phenotypes of human diseases, but no studies have verified this hypothesis. Here, we describe a 35-year-old man with severe congenital ichthyosis. Mutation analysis revealed a heterozygous splice-site mutation, c.1374-2A>G (p.Ser458Argfs*120), in KRT10 on 17q21.2. This mutation was previously reported in patients with ichthyosis with confetti type I (IWC-I), a prominent skin disease characterized by the frequent occurrence of recombination-induced reversion of pathogenic mutations. Intriguingly, the number of revertant skin areas in this patient is considerably reduced compared with typical IWC-I cases. G-banded karyotyping revealed that the patient harbors a heterozygous nonpathogenic inversion, inv(17)(p13q12), whose long-arm breakpoint was subsequently refined to chromosomal positions (chr17: 36,544,407-36,639,830) via FISH. Collectively, the only chance of revertant mosaicism through somatic recombination appears to involve recombination between the KRT10 mutation and the inversion breakpoint. Indeed, in the examined revertant spot, the KRT10 mutation was diminished by somatic recombination starting from chromosomal positions (chr17: 36,915,505-37,060,285) on 17q12. This study provides the first evidence to our knowledge implicating chromosomal inversions as a potential modifier of clinical phenotypes. Furthermore, the reduced occurrence of revertant spots in the recombination-suppressed patient suggests that somatic recombination is the main mechanism of revertant mosaicism in IWC-I.

Current and Future Perspectives of Stem Cell Therapy in Dermatology

Stem cells are undifferentiated cells capable of generating, sustaining, and replacing terminally differentiated cells and tissues. They can be isolated from embryonic as well as almost all adult tissues including skin, but are also generated through genetic reprogramming of differentiated cells. Preclinical and clinical research has recently tremendously improved stem cell therapy, being a promising treatment option for various diseases in which current medical therapies fail to cure, prevent progression or relieve symptoms. With the main goal of regeneration or sustained genetic correction of damaged tissue, advanced tissue-engineering techniques are especially applicable for many dermatological diseases including wound healing, genodermatoses (like the severe blistering disorder epidermolysis bullosa) and chronic (auto-)inflammatory diseases. This review summarizes general aspects as well as current and future perspectives of stem cell therapy in dermatology.

Revertant mosaicism repairs skin lesions in a patient with keratitis-ichthyosis-deafness syndrome by second-site mutations in connexin 26

Revertant mosaicism (RM) is a naturally occurring phenomenon where the pathogenic effect of a germline mutation is corrected by a second somatic event. Development of healthy-looking skin due to RM has been observed in patients with various inherited skin disorders, but not in connexin-related disease. We aimed to clarify the underlying molecular mechanisms of suspected RM in the skin of a patient with keratitis-ichthyosis-deafness (KID) syndrome. The patient was diagnosed with KID syndrome due to characteristic skin lesions, hearing deficiency and keratitis. Investigation of GJB2 encoding connexin (Cx) 26 revealed heterozygosity for the recurrent de novo germline mutation, c.148G > A, p.Asp50Asn. At age 20, the patient developed spots of healthy-looking skin that grew in size and number within widespread erythrokeratodermic lesions. Ultra-deep sequencing of two healthy-looking skin biopsies identified five somatic nonsynonymous mutations, independently present in cis with the p.Asp50Asn mutation. Functional studies of Cx26 in HeLa cells revealed co-expression of Cx26-Asp50Asn and wild-type Cx26 in gap junction channel plaques. However, Cx26-Asp50Asn with the second-site mutations identified in the patient displayed no formation of gap junction channel plaques. We argue that the second-site mutations independently inhibit Cx26-Asp50Asn expression in gap junction channels, reverting the dominant negative effect of the p.Asp50Asn mutation. To our knowledge, this is the first time RM has been reported to result in the development of healthy-looking skin in a patient with KID syndrome.

Revertant mosaicism for family mutations is not observed in BRCA1/2 phenocopies

In BRCA1/2 families, early-onset breast cancer (BrCa) cases may be also observed among non-carrier relatives. These women are considered phenocopies and raise difficult counselling issues concerning the selection of the index case and the residual risks estimate in negative family members. Few studies investigated the presence of potential genetic susceptibility factors in phenocopies, mainly focussing on BrCa-associated single-nucleotide polymorphisms. We hypothesized that, as for other Mendelian diseases, a revertant somatic mosaicism, resulting from spontaneous correction of a pathogenic mutation, might occur also in BRCA pedigrees. A putative low-level mosaicism in phenocopies, which has never been investigated, might be the causal factor undetected by standard diagnostic testing. We selected 16 non-carriers BrCa-affected from 15 BRCA1/2 families, and investigated the presence of mosaicism through MALDI-TOF mass spectrometry. The analyses were performed on available tumour samples (7 cases), blood leukocytes, buccal mucosa and urine samples (2 cases) or on blood only (7 cases). In one family (n.8), real-time PCR was also performed to analyse the phenocopy and her healthy parents. On the 16 phenocopies we did not detect the family mutations neither in the tumour, expected to display the highest mutation frequency, nor in the other analysed tissues. In family 8, all the genotyping assays did not detect mosaicism in the phenocopy or her healthy parents, supporting the hypothesis of a de novo occurrence of the BRCA2 mutation identified in the proband. These results suggest that somatic mosaicism is not likely to be a common phenomenon in BRCA1/2 families. As our families fulfilled high-risk selection criteria, other genetic factors might be responsible for most of these cases and have a significant impact on risk assessment in BRCA1/2 families. Finally, we found a de novo BRCA2 mutation, suggesting that, although rare, this event should be taken into account in the evaluation of high-risk families.

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".

Different X-linked KDM5C mutations in affected male siblings: is maternal reversion error involved?

Genetic reversion is the phenomenon of spontaneous gene correction by which gene function is partially or completely rescued. However, it is unknown whether this mechanism always correctly repairs mutations, or is prone to error. We investigated a family of three boys with intellectual disability, and among them we identified two different mutations in KDM5C, located at Xp11.22, using whole-exome sequencing. Two affected boys have c.633delG and the other has c.631delC. We also confirmed de novo germline (c.631delC) and low-prevalence somatic (c.633delG) mutations in their mother. The two mutations are present on the same maternal haplotype, suggesting that a postzygotic somatic mutation or a reversion error occurred at an early embryonic stage in the mother, leading to switched KDM5C mutations in the affected siblings. This event is extremely unlikely to arise spontaneously (with an estimated probability of 0.39-7.5 × 10(-28) ), thus a possible reversion error is proposed here to explain this event. This study provides evidence for reversion error as a novel mechanism for the generation of somatic mutations in human diseases.

Unrevealed mosaicism in the next-generation sequencing era

Mosaicism refers to the presence in an individual of normal and abnormal cells that are genotypically distinct and are derived from a single zygote. The incidence of mosaicism events in the human body is underestimated as the genotypes in the mosaic ratio, especially in the low-grade mosaicism, stay unrevealed. This review summarizes various research outcomes and diagnostic questions in relation to different types of mosaicism. The impact of both tested biological material and applied method on the mosaicism detection rate is especially highlighted. As next-generation sequencing technologies constitute a promising methodological solution in mosaicism detection in the coming years, revisions in current diagnostic protocols are necessary to increase the detection rate of the unrevealed mosaicism events. Since mosaicism identification is a complex process, numerous examples of multistep mosaicism investigations are presented and discussed.

Ichthyosis with confetti: clinics, molecular genetics and management

Ichthyosis with confetti (IWC) is an autosomal dominant congenital ichthyosis also known as ichthyosis variegata or congenital reticular ichthyosiform erythroderma. It manifests at birth with generalized ichthyosiform erythroderma or with a collodion baby picture. The erythrodermic and ichthyotic phenotype persists during life and its severity may modify. However, the hallmark of the disease is the appearance, in childhood or later in life, of healthy skin confetti-like spots, which increase in number and size with time. IWC is a very rare genodermatosis, with a prevalence <1/1,000,000 and only 40 cases reported worldwide. The most important associated clinical features include ear deformities, mammillae hypoplasia, palmoplantar keratoderma, hypertrichosis and ectropion. IWC is due to dominant negative mutations in the KRT10 and KRT1 genes, encoding for keratins 10 and keratin 1, respectively. In this context, healthy skin confetti-like spots represent “repaired” skin due to independent events of reversion of keratin gene mutations via mitotic recombination. In most cases, IWC clinical suspicion is delayed until the detection of white skin spots. Clinical features, which may represent hint to the diagnosis of IWC even before appearance of confetti-like spots, include ear and mammillae hypoplasia, the progressive development of hypertrichosis and, in some patients, of adherent verrucous plaques of hyperkeratosis. Altogether the histopathological finding of keratinocyte vacuolization and the nuclear staining for keratin 10 and keratin 1 by immunofluorescence are pathognomonic. Nevertheless, mutational analysis of KRT10 or KRT1 genes is at present the gold standard to confirm the diagnosis. IWC has to be differentiated mainly from congenital ichthyosiform erythroderma. Differential diagnosis also includes syndromic ichthyoses, in particular Netherton syndrome, and the keratinopathic ichthyoses. Most of reported IWC cases are sporadic, but familial cases with autosomal dominant mode of inheritance have been also described. Therefore, knowledge of the mutation is the only way to properly counsel the couples. No specific and satisfactory therapy is currently available for IWC. Like for other congenital ichthyoses, topical treatments (mainly emollients and keratolytics) are symptomatic and offer only temporary relief. Among systemic treatments, retinoids, in particular acitretin, improve disease symptoms in most patients. Although at present there is no curative therapy for ichthyoses, treatments have improved considerably over the years and the best therapy for each patient is always the result of both physician and patient efforts.

Reduced Toxicity Conditioning and Allogeneic Hematopoietic Progenitor Cell Transplantation for Recessive Dystrophic Epidermolysis Bullosa

Recessive dystrophic epidermolysis bullosa is a severe, incurable, inherited blistering disease caused by COL7A1 mutations. Emerging evidence suggests hematopoietic progenitor cells (HPCs) can be reprogrammed into skin; HPC-derived cells can restore COL7 expression in COL7-deficient mice. We report two children with recessive dystrophic epidermolysis bullosa treated with reduced-toxicity conditioning and HLA-matched HPC transplantation.

New approaches to the treatment of orphan genetic disorders: Mitigating molecular pathologies using chemicals

With the advance and popularization of molecular techniques, the identification of genetic mutations that cause diseases has increased dramatically. Thus, the number of laboratories available to investigate a given disorder and the number of subsequent diagnosis have increased over time. Although it is necessary to identify mutations and provide diagnosis, it is also critical to develop specific therapeutic approaches based on this information. This review aims to highlight recent advances in mutation-targeted therapies with chemicals that mitigate mutational pathology at the molecular level, for disorders that, for the most part, have no effective treatment. Currently, there are several strategies being used to correct different types of mutations, including the following: the identification and characterization of translational readthrough compounds; antisense oligonucleotide-mediated splicing redirection; mismatch repair; and exon skipping. These therapies and other approaches are reviewed in this paper.

Cell Competition Modifies Adult Stem Cell and Tissue Population Dynamics in a JAK-STAT-Dependent Manner

Throughout their lifetime, cells may suffer insults that reduce their fitness and disrupt their function, and it is unclear how these potentially harmful cells are managed in adult tissues. We address this question using the adult Drosophila posterior midgut as a model of homeostatic tissue and ribosomal Minute mutations to reduce fitness in groups of cells. We take a quantitative approach combining lineage tracing and biophysical modeling and address how cell competition affects stem cell and tissue population dynamics. We show that healthy cells induce clonal extinction in weak tissues, targeting both stem and differentiated cells for elimination. We also find that competition induces stem cell proliferation and self-renewal in healthy tissue, promoting selective advantage and tissue colonization. Finally, we show that winner cell proliferation is fueled by the JAK-STAT ligand Unpaired-3, produced by Minute^−/+ cells in response to chronic JNK stress signaling.

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In the adult fly gut, wild-type cells outcompete subfit Minute^−/+ cells

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Both stem and differentiated Minute^−/+ cells are eliminated by cell competition

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Cell competition promotes proliferation and self-renewal of normal stem cells

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The growth of healthy cells is boosted by JAK-STAT signaling

Somatic mosaicism in the human genome

Somatic mosaicism refers to the occurrence of two genetically distinct populations of cells within an individual, derived from a postzygotic mutation. In contrast to inherited mutations, somatic mosaic mutations may affect only a portion of the body and are not transmitted to progeny. These mutations affect varying genomic sizes ranging from single nucleotides to entire chromosomes and have been implicated in disease, most prominently cancer. The phenotypic consequences of somatic mosaicism are dependent upon many factors including the developmental time at which the mutation occurs, the areas of the body that are affected, and the pathophysiological effect(s) of the mutation. The advent of second-generation sequencing technologies has augmented existing array-based and cytogenetic approaches for the identification of somatic mutations. We outline the strengths and weaknesses of these techniques and highlight recent insights into the role of somatic mosaicism in causing cancer, neurodegenerative, monogenic, and complex disease.

Induced pluripotent stem cells in dermatology: potentials, advances, and limitations

The discovery of methods for reprogramming adult somatic cells into induced pluripotent stem cells (iPSCs) has raised the possibility of producing truly personalized treatment options for numerous diseases. Similar to embryonic stem cells (ESCs), iPSCs can give rise to any cell type in the body and are amenable to genetic correction by homologous recombination. These ESC properties of iPSCs allow for the development of permanent corrective therapies for many currently incurable disorders, including inherited skin diseases, without using embryonic tissues or oocytes. Here, we review recent progress and limitations of iPSC research with a focus on clinical applications of iPSCs and using iPSCs to model human diseases for drug discovery in the field of dermatology.

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.

Just like the rest of evolution in Mother Nature, the evolution of cancers may be driven by natural selection, and not by haphazard mutations

Sporadic carcinogenesis starts from immortalization of a differentiated somatic cell or an organ-specific stem cell. The immortalized cell incepts a new or quasinew organism that lives like a parasite in the patient and usually proceeds to progressive simplification, constantly engendering intermediate organisms that are simpler than normal cells. Like organismal evolution in Mother Nature, this cellular simplification is a process of Darwinian selection of those mutations with growth- or survival-advantages, from numerous ones that occur randomly and stochastically. Therefore, functional gain of growth- or survival-sustaining oncogenes and functional loss of differentiation-sustaining tumor suppressor genes, which are hallmarks of cancer cells and contribute to phenotypes of greater malignancy, are not drivers of carcinogenesis but are results from natural selection of advantageous mutations. Besides this mutation-load dependent survival mechanism that is evolutionarily low and of an asexual nature, cancer cells may also use cell fusion for survival, which is an evolutionarily-higher mechanism and is of a sexual nature. Assigning oncogenes or tumor suppressor genes or their mutants as drivers to induce cancer in animals may somewhat coerce them to create man-made oncogenic pathways that may not really be a course of sporadic cancer formations in the human.

Modulating the stem cell niche for tissue regeneration

The field of regenerative medicine holds considerable promise for treating diseases that are currently intractable. Although many researchers are adopting the strategy of cell transplantation for tissue repair, an alternative approach to therapy is to manipulate the stem cell microenvironment, or niche, to facilitate repair by endogenous stem cells. The niche is highly dynamic, with multiple opportunities for intervention. These include administration of small molecules, biologics or biomaterials that target specific aspects of the niche, such as cell-cell and cell-extracellular matrix interactions, to stimulate expansion or differentiation of stem cells, or to cause reversion of differentiated cells to stem cells. Nevertheless, there are several challenges in targeting the niche therapeutically, not least that of achieving specificity of delivery and responses. We envisage that successful treatments in regenerative medicine will involve different combinations of factors to target stem cells and niche cells, applied at different times to effect recovery according to the dynamics of stem cell-niche interactions.

Cell therapy in dermatology

Harnessing the regenerative capacity of keratinocytes and fibroblasts from human skin has created new opportunities to develop cell-based therapies for patients. Cultured cells and bioengineered skin products are being used to treat patients with inherited and acquired skin disorders associated with defective skin, and further clinical trials of new products are in progress. The capacity of extracutaneous sources of cells such as bone marrow is also being investigated for its plasticity in regenerating skin, and new strategies, such as the derivation of inducible pluripotent stem cells, also hold great promise for future cell therapies in dermatology. This article reviews some of the preclinical and clinical studies and future directions relating to cell therapy in dermatology, particularly for inherited skin diseases associated with fragile skin and poor wound healing.

Advances in understanding and treating dystrophic epidermolysis bullosa

Epidermolysis bullosa is a group of inherited disorders that can be both systemic and life-threatening. Standard treatments for the most severe forms of this disorder, typically limited to palliative care, are ineffective in reducing the morbidity and mortality due to complications of the disease. Emerging therapies—such as the use of allogeneic cellular therapy, gene therapy, and protein therapy—have all shown promise, but it is likely that several approaches will need to be combined to realize a cure. For recessive dystrophic epidermolysis bullosa, each particular therapeutic approach has added to our understanding of type VII collagen (C7) function and the basic biology surrounding the disease. The efficacy of these therapies and the mechanisms by which they function also give us insight into developing future strategies for treating this and other extracellular matrix disorders.