Expanding the Clinical and Genetic Spectrum of KRT1, KRT2 and KRT10 Mutations in Keratinopathic Ichthyosis

Treating epidermolytic ichthyosis and ichthyosis with confetti with epidermal autografts cultured from revertant skin

BACKGROUND

No efficient treatment has been established yet for epidermolytic ichthyosis (EI) caused by pathogenic variants in KRT1 or KRT10. Patients with ichthyosis with confetti (IWC) show multiple normal-appearing spots, caused by the revertant somatic recombination of pathogenic variants that occurs at each spot independently. Additionally, some patients with EI have large areas of normal skin due to revertant postzygotic mosaicism.

OBJECTIVE

To assess the feasibility transplanting cultured epidermal autografts (CEAs) produced from revertant epidermal keratinocytes in patients with EI and IWC.

METHODS

We performed a clinical trial of treatment with CEAs produced from each patient's own revertant epidermal keratinocytes as a proof-of-concept study. This is a single-arm, open (masking not used), uncontrolled, single-assignment, treatment purpose study. The primary outcome was the rate of areas without the recurrence of ichthyosis lesions 4 weeks after the final transplant (%). The secondary outcome was the rate of areas without the recurrence of ichthyosis lesions 24 weeks after initial transplantation (%).

RESULTS

We successfully produced CEAs from the genetically confirmed revertant skin of the two mosaic EI patients and one IWC patient and genetically confirmed that CEAs mainly consist of revertant wild-type cells by amplicon sequencing and droplet digital PCR analysis. Single-cell RNA sequencing analysis confirmed the normal proliferation and safety profiling of CEAs. CEAs were transplanted to desquamated lesional sites of the patients. Four weeks after this transplantation, the rate of areas without the recurrence of ichthyosis lesions in the three cases was 39.52%, 100.0%, and 100.0% respectively, although the recurrence of ichthyosis lesions was seen at the site of CEA transplantation in all three patients at 24 weeks after transplantation.

CONCLUSION

CEAs from normal skin have the potential to be a safe and local treatment option for EI and IWC.

TRIAL REGISTRATION

jRCTb041190097.

Autosomal Dominant Lamellar Ichthyosis Due to a Missense Variant in the Gene NKPD1

The identification of monogenic causes for cornification disorders has enhanced our understanding of epidermal differentiation and skin barrier function. Autosomal dominant lamellar ichthyosis is a rare condition, and ASPRV1 was the only gene linked to autosomal dominant lamellar ichthyosis to date. We identified a heterozygous variant (ENST00000686631.1:c.1372G>T, p.[Val458Phe]) in the NKPD1 gene in 7 individuals from a 4-generation German pedigree with generalized lamellar ichthyosis by whole-exome sequencing. Segregation analysis confirmed its presence in affected individuals, resulting in a logarithm of the odds score of 3.31. NKPD1 encodes the NKPD1 protein, implicated in the plasma membrane; its role in human disease is as yet unknown. Skin histology showed moderate acanthosis and compact orthohyperkeratosis, and the ultrastructure differed clearly from that in ASPRV1-autosomal dominant lamellar ichthyosis. Although NKPD1 mRNA expression increased during keratinocyte differentiation, stratum corneum ceramides exhibited no significant changes. However, affected individuals showed an elevated ratio of protein-bound ceramides to omega-esterified ceramides. This highlights NKPD1's role in autosomal dominant lamellar ichthyosis, impacting ceramide metabolism and skin lipid barrier formation, as demonstrated through functional characterization.

Inherited ichthyosis as a paradigm of rare skin disorders: Genomic medicine, pathogenesis, and management

Great advances have been made in the field of heritable skin disorders using next-generation sequencing (NGS) technologies (ie, whole-genome sequencing, whole-exome sequencing, whole-transcriptome sequencing, and disease-targeted multigene panels). When NGS first became available, the cost and lack of access to these technologies were limiting factors; however, with decreasing sequencing costs and the expanding knowledge base of genetic skin diseases, fundamental awareness of NGS has become prudent. The heritable ichthyoses comprise a genotypically and phenotypically heterogeneous group of monogenic keratinization disorders characterized by persistent scaling, with at least 55 distinct genes currently implicated in causing nonsyndromic and syndromic forms of the disease. By providing a simplified overview of available NGS techniques and applying them in the context of ichthyosis, one of the most common genodermatoses, we hope to encourage dermatologists to offer, when appropriate, genetic testing earlier in patients with unsolved presentations. With the aid of NGS, dermatologists can provide diagnostic certainty in cases of suspected genodermatoses and offer potentially life-changing genome-guided and targeted therapies as they become available.

Identification of Critical Genes for Ovine Horn Development Based on Transcriptome during the Embryonic Period

Horns, also known as headgear, are a unique structure of ruminants. As ruminants are globally distributed, the study of horn formation is critical not only for increasing our understanding of natural and sexual selection but also for the breeding of polled sheep breeds to facilitate modern sheep farming. Despite this, a significant number of the underlying genetic pathways in sheep horn remain unclear. In this study, to clarify the gene expression profile of horn buds and investigate the key genes in horn bud formation, RNA-sequencing (RNA-seq) technology was utilized to investigate differential gene expression in the horn buds and adjacent forehead skin of Altay sheep fetuses. There were only 68 differentially expressed genes (DEGs) identified, consisting of 58 up-regulated genes and 10 down-regulated genes. RXFP2 was differentially up-regulated in the horn buds and had the highest significance (p-value = 7.42 × 10^-14). In addition, 32 DEGs were horn-related genes identified in previous studies, such as RXFP2, FOXL2, SFRP4, SFRP2, KRT1, KRT10, WNT7B, and WNT3. Further, Gene Ontology (GO) analysis showed that the DEGs were mainly enriched with regard to growth, development, and cell differentiation. Pathway analysis revealed that the Wnt signaling pathway may be responsible for horn development. Further, through combining the protein-protein interaction networks of the DEGs, it was found that the top five hub genes, namely, ACAN, SFRP2, SFRP4, WNT3, and WNT7B, were also associated with horn development. Our results suggest that only a few key genes, including RXFP2, are involved in bud formation. This study not only validates the expression of candidate genes identified at the transcriptome level in previous studies but also provides new possible marker genes for horn development, which may promote our understanding of the genetic mechanisms of horn formation.

Congenital Ichthyosis: Clinical and Genetic Characteristics of the Disease

Congenital ichthyosis is a group (almost 100 clinical variants) of rare genetic skin diseases caused by pathogenic changes in more than 50 genes. Clinical features of ichthyosis, regardless of its genotype, are dry skin, peeling, hyperkeratosis frequently accompanied with erythroderma. These patients have extremely low quality of life due to changes in appearance, discomfort due to itching and functional limitations (pain during walking, impaired hands motor skills and functions due to hyperkeratosis foci in functionally relevant areas), as well as impaired functions of various organs and systems in syndromic forms of disease. Patients need daily skin care and systemic medications. By now, there is no definitive treatment for ichthyosis. Diagnostic difficulties in determining the clinical forms of congenital ichthyosis are associated with their clinical heterogeneity and with similarity in external manifestations. Difficulties in differential diagnosis with other dermatoses are particularly crucial in case of syndromic forms of disease. This review presents the modern classification of ichthyoses, provides data on disease clinical and genetic variants, diagnostic algorithms, treatment methods for patients with this severe disease.

A de novo variant in the keratin 1 gene (KRT1) in a Chinese shar-pei dog with severe congenital cornification disorder and non-epidermolytic ichthyosis

A 3-months old Chinese shar-pei puppy with ichthyosis was investigated. The dog showed generalized scaling, alopecia and footpad lesions. Histopathological examinations demonstrated a non-epidermolytic hyperkeratosis. The parents of the affected puppy did not show any skin lesions. A trio whole genome sequencing analysis identified a heterozygous de novo 3 bp deletion in the KRT1 gene in the affected dog. This variant, NM_001003392.1:c.567_569del, is predicted to delete a single asparagine from the conserved coil 1A motif within the rod domain of KRT1, NP_001003392.1:p.(Asn190del). Immunohistochemistry demonstrated normal levels of KRT1 expression in the epidermis and follicular epithelia. This might indicate that the variant possibly interferes with keratin dimerization or another function of KRT1. Missense variants affecting the homologous asparagine residue of the human KRT1 cause epidermolytic hyperkeratosis. Histologically, the investigated Chinese shar-pei showed a non-epidermolytic ichthyosis. The finding of a de novo variant in an excellent functional candidate gene strongly suggests that KRT1:p.Asn190del caused the ichthyosis phenotype in the affected Chinese shar-pei. To the best of our knowledge, this is the first description of a KRT1-related non-epidermolytic ichthyosis in domestic animals.

Novel mutations of the ABCA12, KRT1 and ST14 genes in three unrelated newborns showing congenital ichthyosis

Background

Congenital ichthyosis (CI) is a heterogeneous group of genetic disorders characterized by generalized dry skin, scaling and hyperkeratosis, often associated to erythroderma. They are rare diseases, with overall incidence of 6.7 in 100,000. Clinical manifestations are due to mutations in genes mostly involved in skin barrier formation. Based on clinical presentation, CI is distinguished in non-syndromic and syndromic forms. To date, mutations of more than 50 genes have been associated to different types of CI.

Cases presentation

We report on three Italian unrelated newborns showing clinical signs compatible with different forms of CI of variable severity, namely Harlequin ichtyosis (HI), epidermolytic ichtyosis (EI) and autosomal recessive ichtyosis with hypotrichosis (ARIH). Target next generation sequencing (NGS) analysis identified three novel mutations of the ABCA12, KRT1 and ST14 genes, respectively associated to such congenital ichtyoses, not reported in literature. Genomic investigation allowed to provide the more appropriate management to each patient, based on an individualized approach.

Conclusions

Our report highlights the wide genetic heterogeneity and phenotypic variability of CI. It expands the current knowledge on such diseases, widening their genomic database, and providing a better clinical characterization. Furthermore, it underlines the clinical relevance of NGS, which is essential to address the management of patients. Indeed, it may guide towards the most adequate approach, preventing clinical obstinacy for subjects with more severe forms and unfavorable outcomes (together with the support, in such situations, of bioethicists included within the multidisciplinary care team), as well as reassuring families in those with milder course and favorable evolution.

The Genomic and Phenotypic Landscape of Ichthyosis: An Analysis of 1000 Kindreds

Importance

Ichthyoses are clinically and genetically heterogeneous disorders characterized by scaly skin. Despite decades of investigation identifying pathogenic variants in more than 50 genes, clear genotype-phenotype associations have been difficult to establish.

Objective

To expand the genotypic and phenotypic spectra of ichthyosis and delineate genotype-phenotype associations.

Design, Setting, and Participants

This cohort study recruited an international group of individuals with ichthyosis and describes characteristic and distinguishing features of common genotypes, including genotype-phenotype associations, during a 10-year period from June 2011 to July 2021. Participants of all ages, races, and ethnicities were included and were enrolled worldwide from referral centers and patient advocacy groups. A questionnaire to assess clinical manifestations was completed by those with a genetic diagnosis.

Main Outcomes and Measures

Genetic analysis of saliva or blood DNA, a phenotyping questionnaire, and standardized clinical photographs. Descriptive statistics, such as frequency counts, were used to describe the cases in the cohort. Fisher exact tests identified significant genotype-phenotype associations.

Results

Results were reported for 1000 unrelated individuals enrolled from around the world (mean [SD] age, 50.0 [34.0] years; 524 [52.4%] were female, 427 [42.7%] were male, and 49 [4.9%] were not classified); 75% were from the US, 12% from Latin America, 4% from Canada, 3% from Europe, 3% from Asia, 2% from Africa, 1% from the Middle East, and 1% from Australia and New Zealand. A total of 266 novel disease-associated variants in 32 genes were identified among 869 kindreds. Of these, 241 (91%) pathogenic variants were found through multiplex amplicon sequencing and 25 (9%) through exome sequencing. Among the 869 participants with a genetic diagnosis, 304 participants (35%) completed the phenotyping questionnaire. Analysis of clinical manifestations in these 304 individuals revealed that pruritus, hypohydrosis, skin pain, eye problems, skin odor, and skin infections were the most prevalent self-reported features. Genotype-phenotype association analysis revealed that the presence of a collodion membrane at birth (odds ratio [OR], 6.7; 95% CI, 3.0-16.7; P < .001), skin odor (OR, 2.8; 95% CI, 1.1-6.8; P = .02), hearing problems (OR, 2.9; 95% CI, 1.6-5.5; P < .001), eye problems (OR, 3.0; 95% CI, 1.5-6.0; P < .001), and alopecia (OR, 4.6; 95% CI, 2.4-9.0; P < .001) were significantly associated with TGM1 variants compared with other ichthyosis genotypes studied. Skin pain (OR, 6.8; 95% CI, 1.6-61.2; P = .002), odor (OR, 5.7; 95% CI, 2.0-19.7; P < .001), and infections (OR, 3.1; 95% CI, 1.4-7.7; P = .03) were significantly associated with KRT10 pathogenic variants compared with disease-associated variants in other genes that cause ichthyosis. Pathogenic variants were identified in 869 (86.9%) participants. Most of the remaining individuals had unique phenotypes, enabling further genetic discovery.

Conclusions and Relevance

This cohort study expands the genotypic and phenotypic spectrum of ichthyosis, establishing associations between clinical manifestations and genotypes. Collectively, the findings may help improve clinical assessment, assist with developing customized management plans, and improve clinical course prognostication.

Epidermolytic epidermal nevus caused by a somatic mutation in KRT2

Superficial epidermolytic ichthyosis (formerly Ichthyosis bullosa of Siemens) is an uncommon condition caused by dominant mutations in KRT2 encoding keratin 2. Epidermolytic epidermal nevus due to somatic mutations in KRT2 is even rarer. Here, we report the third case of KRT2-associated epidermal nevus and review the literature.

Visualization of Keratin with Diffuse Reflectance and Autofluorescence Imaging and Nonlinear Optical Microscopy in a Rare Keratinopathic Ichthyosis

Keratins are one of the main fluorophores of the skin. Keratinization disorders can lead to alterations in the optical properties of the skin. We set out to investigate a rare form of keratinopathic ichthyosis caused by KRT1 mutation with two different optical imaging methods. We used a newly developed light emitting diode (LED) based device to analyze autofluorescence signal at 405 nm excitation and diffuse reflectance at 526 nm in vivo. Mean autofluorescence intensity of the hyperkeratotic palmar skin was markedly higher in comparison to the healthy control (162.35 vs. 51.14). To further assess the skin status, we examined samples from affected skin areas ex vivo by nonlinear optical microscopy. Two-photon excited fluorescence and second-harmonic generation can visualize epidermal keratin and dermal collagen, respectively. We were able to visualize the structure of the epidermis and other skin changes caused by abnormal keratin formation. Taken together, we were able to show that such imaging modalities are useful for the diagnosis and follow-up of keratinopathic diseases.

Types of congenital nonsyndromic ichthyoses

Congenital ichthyoses are a very heterogeneous group of diseases manifested by dry, rough and scaling skin. In all forms of ichthyoses, the skin barrier is damaged to a certain degree. Congenital ichthyoses are caused by various gene mutations. Clinical manifestations of the individual types vary as the patient ages. Currently, the diagnosis of congenital ichthyoses is based on molecular analysis, which also allows a complete genetic counseling and genetic prevention. It is appropriate to refer the patients to specialized medical centers, where the cooperation of a neonatologist, a pediatric dermatologist, a geneticist and other specialists is ensured.

First Case of KRT2 Epidermolytic Nevus and Novel Clinical and Genetic Findings in 26 Italian Patients with Keratinopathic Ichthyoses

Keratinopathic ichthyoses (KI) are a clinically heterogeneous group of keratinization disorders due to mutations in KRT1, KTR10, or KRT2 genes encoding keratins of suprabasal epidermis. Characteristic clinical features include superficial blisters and erosions in infancy and progressive development of hyperkeratosis. Histopathology shows epidermolytic hyperkeratosis. We describe the clinical, histopathological, and molecular findings of a series of 26 Italian patients from 19 unrelated families affected with (i) epidermolytic ichthyosis due to KRT1 or KRT10 mutations (7 and 9 cases, respectively); (ii) KTR10-mutated ichthyosis with confetti (2 cases); (iii) KRT2-mutated superficial epidermolytic ichthyosis (5 cases); and (iv) KRT10-mutated epidermolytic nevus (2 cases). Of note, molecular genetic testing in a third case of extensive epidermolytic nevus revealed a somatic missense mutation (p.Asn186Asp) in the KRT2 gene, detected in DNA from lesional skin at an allelic frequency of 25% and, at very low frequency (1.5%), also in blood. Finally, we report three novel dominant mutations, including a frameshift mutation altering the C-terminal V2 domain of keratin 1 in three familiar cases presenting a mild phenotype. Overall, our findings expand the phenotypic and molecular spectrum of KI and show for the first time that epidermolytic nevus can be due to somatic KRT2 mutation.

Molecular Modeling of Pathogenic Mutations in the Keratin 1B Domain

Keratin intermediate filaments constitute the primary cytoskeletal component of epithelial cells. Numerous human disease phenotypes related to keratin mutation remain mechanistically elusive. Our recent crystal structures of the helix 1B heterotetramer from keratin 1/10 enabled further investigation of the effect of pathologic 1B domain mutations on keratin structure. We used our highest resolution keratin 1B structure as a template for homology-modeling the 1B heterotetramers of keratin 5/14 (associated with blistering skin disorders), keratin 8/18 (associated with liver disease), and keratin 74/28 (associated with hair disorder). Each structure was examined for the molecular alterations caused by incorporating pathogenic 1B keratin mutations. Structural modeling indicated keratin 1B mutations can harm the heterodimer interface (R265P^K5, L311R^K5, R211P^K14, I150V^K18), the tetramer interface (F231L^K1, F274S^K74), or higher-order interactions needed for mature filament formation (S233L^K1, L311R^K5, Q169E^K8, H128L^K18). The biochemical changes included altered hydrophobic and electrostatic interactions, and altered surface charge, hydrophobicity or contour. Together, these findings advance the genotype-structurotype-phenotype correlation for keratin-based human diseases.

The first case of a mosaic superficial epidermolytic ichthyosis diagnosed by Ultra-Deep Sequence

Background

Superficial epidermolytic ichthyosis (SEI), known as ichthyosis bullosa of Siemens (IBS; OMIM No. 146800) before, is a type of keratinopathic ichthyosis due to the KRT2 mutations (NM_000423.3; OMIM No. 600194). Here, we report the first case of SEI caused by a KRT2 mosaic mutation.

Methods

We presented the clinical data of a 5‐year‐old Chinese boy who suffered from SEI. The histopathological examination and immunofluorescence were performed to rule out immunobullous skin diseases and diseases with subepidermal blisters. Genomic DNA samples were extracted from the lesion tissue and next‐generation sequencing was performed. We also confirmed the variant allele frequency (VAF) in different tissues by an Ultra‐Deep Sequencing technology.

Results

The patient presented with blisters on the lower extremities and linear, superficially hyperkeratotic lesions. Immunofluorescence of IgG, IgA, IgM, C3, C4, and C1q were negative, and the histopathological results showed intraepidermal blisters containing lymphocytes and eosinophils. A heterozygous missense mutation, c.G1459A (p. Glu487Lys), in exon 7 of the KRT2 gene was detected at a 31.17% allele frequency. The same mutation p. Glu487Lys has been described several times in the literature.

Conclusion

Thus, in our patient, the mosaic mutation explains the blaschkoid ichthyosiform phenotype. To our knowledge, this is the first case of SEI with a KRT2 mosaic mutation.

A novel frameshift truncation mutation in the V2 tail domain of KRT1 causes mild ichthyosis hystrix of Curth-Macklin

Ichthyosis hystrix, Curth-Macklin type (IHCM) is an extremely rare autosomal dominant dermatosis caused by mutations in the keratin genes, KRT1 or KRT10, which often manifests as extensive, dark, spiky or verrucous plaques and severe palmoplantar keratoderma. We report a novel frameshift truncation mutation, c.1596_1597insAT (p.Gly533Metfs*82) in exon 7 (V2 tail domain) of KRT1, which, by replacing the glycine-serine-rich tail of KRT1 with a series of 75 alanine-rich amino acids, produces a mild IHCM phenotype. The patient with the mutation presented with localized ichthyosis and progressive hyperkeratosis of the palms and soles with no history of blistering.

Ichthyosis with confetti caused by new and recurrent mutations in KRT10 associated with varying degrees of keratin 10 mis-localization

BACKGROUND

Ichthyosis with confetti (IWC) is an extremely rare autosomal-dominant genodermatosis characterized by erythroderma with numerous confetti-like pale spots. IWC is caused by mutations in KRT10 (IWC-I) or KRT1 (IWC-II) which affect their tail domains. In IWC-I, the mutations lead to replacement of glycine/serine-rich keratin 10 (K10) tail with arginine- or alanine-rich frameshift motifs, causing K10 mis-localization which might trigger loss of the mutant KRT10 allele via mitotic recombination, leading to genetic reversion.

OBJECTIVE

To investigate mutations in five IWC-I patients and their functional consequences.

METHODS

We performed Sanger sequencing of KRT1 and KRT10 in peripheral blood samples of five patients, with highly polymorphic KRT10 SNPs genotyped to confirm loss-of-heterozygosity in the epidermis of pale spots. K10 expression pattern was examined in both patient skin biopsies and HaCaT cells overexpressing mutant KRT10-enhanced green fluorescence protein fusion.

RESULTS

Four novel and one recurrent KRT10 mutations were identified in patient peripheral blood samples but not in the corresponding pale spot epidermis. Two of the mutations, c.1696_1699dupCACA and c.1676dupG, affected residues close to K10 carboxyl terminus and encoded only 3 and 6 arginine residues, which were far fewer than reported previously. Interestingly, imaging analyses for K10 in HaCaT cells overexpressing either of these two mutations and in the corresponding patients' affected skin, showed a remarkably lower level of K10 mis-localization compared to that of other mutations reported in this study.

CONCLUSIONS

Our findings suggest that the number of arginine residues in the mutant tail may correlate with the level of K10 mis-localization in IWC-I keratinocytes. These results expand the genotypic and phenotypic spectrum of IWC-I.

Next-generation sequencing through multi-gene panel testing for diagnosis of hereditary ichthyosis in Chinese

Inherited ichthyoses are a heterogeneous group of rare disorders related to over 40 genes. To identify underlying molecular causes in inherited ichthyosis among Chinese and to correlate genotype and phenotype, 35 probands clinically diagnosed inherited ichthyosis, except ichthyosis vulgaris and X-linked ichthyosis, were included in our study. Molecular analysis was performed using next-generation sequencing (NGS) through multi-gene panel testing targeting all ichthyosis-related genes. Genetic variants causative for the ichthyosis were identified in 32 of 35 investigated patients. In all, 43 causative mutations across 12 genes were disclosed, including 16 novel variants. Thirteen keratinopathic ichthyosis, fourteen autosomal recessive congenital ichthyosis (ARCI) including one caused by mutations in SDR9C7, and five syndromic ichthyoses were confirmed. Four probands, with presumptive ARCI, turned out to be keratinopathic ichthyosis (2), neutral lipid storage disease (1), and Sjogren-Larsson syndrome (1), respectively. Next-generation technology has been demonstrated to be an effective tool in diagnosing inherited ichthyosis constituting a diverse group of cornification disorders. Our study further expands mutation spectrum and clinical phenotype associated with inherited ichthyosis in Chinese.

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.

Arginine- but not alanine-rich carboxy-termini trigger nuclear translocation of mutant keratin 10 in ichthyosis with confetti

Ichthyosis with confetti (IWC) is a genodermatosis associated with dominant-negative variants in keratin 10 (KRT10) or keratin 1 (KRT1). These frameshift variants result in extended aberrant proteins, localized to the nucleus rather than the cytoplasm. This mislocalization is thought to occur as a result of the altered carboxy (C)-terminus, from poly-glycine to either a poly-arginine or -alanine tail. Previous studies on the type of C-terminus and subcellular localization of the respective mutant protein are divergent. In order to fully elucidate the pathomechanism of IWC, a greater understanding is critical. This study aimed to establish the consequences for localization and intermediate filament formation of altered keratin 10 (K10) C-termini. To achieve this, plasmids expressing distinct KRT10 variants were generated. Sequences encoded all possible reading frames of the K10 C-terminus as well as a nonsense variant. A keratinocyte line was transfected with these plasmids. Additionally, gene editing was utilized to introduce frameshift variants in exon 6 and exon 7 at the endogenous KRT10 locus. Cellular localization of aberrant K10 was observed via immunofluorescence using various antibodies. In each setting, immunofluorescence analysis demonstrated aberrant nuclear localization of K10 featuring an arginine-rich C-terminus. However, this was not observed with K10 featuring an alanine-rich C-terminus. Instead, the protein displayed cytoplasmic localization, consistent with wild-type and truncated forms of K10. This study demonstrates that, of the various 3' frameshift variants of KRT10, exclusively arginine-rich C-termini lead to nuclear localization of K10.

Novel and recurrent mutations in keratin 1 cause epidermolytic ichthyosis and palmoplantar keratoderma

Mutations in keratin genes underlie a variety of epidermal and nonepidermal cell-fragility disorders, and are the genetic basis of many inherited palmoplantar keratodermas (PPKs). Epidermolytic PPK (EPPK) is an autosomal dominant disorder that can be due to mutations in the keratin 1 gene, KRT1. Epidermolytic ichthyosis (EI), the major keratinopathic ichthyosis, is characterized by congenital erythroderma, blistering and erosions of the skin. Causative mutations in KRT1 and KRT10 have been described, with PPK being present primarily in association with the former. We report four unrelated cases (one with sporadic EI and three with autosomal dominant PPK), due to two novel and two recurrent KRT1 mutations. Mutations in KRT1 are not only scattered throughout the keratin 1 protein, as opposed to being clustered, but can result in a range of phenotypes as further confirmed by these mutations, giving a complex genotype/phenotype pattern.

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.

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.

Inherited Nonsyndromic Ichthyoses: An Update on Pathophysiology, Diagnosis and Treatment

Hereditary ichthyoses are due to mutations on one or both alleles of more than 30 different genes, mainly expressed in the upper epidermis. Syndromic as well as nonsyndromic forms of ichthyosis exist. Irrespective of etiology, virtually all types of ichthyosis exhibit a defective epidermal barrier that constitutes the driving force for hyperkeratosis, skin scaling, and inflammation. In nonsyndromic forms, these features are most evident in severe autosomal recessive congenital ichthyosis (ARCI) and epidermolytic ichthyosis, but to some extent also occur in the common type of non-congenital ichthyosis. A correct diagnosis of ichthyosis-essential not only for genetic counseling but also for adequate patient information about prognosis and therapeutic options-is becoming increasingly feasible thanks to recent progress in genetic knowledge and DNA sequencing methods. This paper reviews the most important aspects of nonsyndromic ichthyoses, focusing on new knowledge about the pathophysiology of the disorders, which will hopefully lead to novel ideas about therapy.

Epidermolytic Ichthyosis without Keratin 1 or 10 Mutations: A Case Report

In this paper, the authors report a case of an 11-year-old boy with epidermolytic ichthyosis who presented with multiple scattered erosions and typical hyperkeratotic plaques over the face, upper and lower extremities, the trunk, palms and soles. Family history revealed an affected older male sibling and an affected first-degree female relative. In addition, there was a positive history of generations of consanguinity in the patient's family pedigree, increasing the probability of an autosomal recessive inheritance. The clinical diagnosis was confirmed by histopathology; however, mutations in the keratin 1 and 10 genes were absent. This case report addresses the importance of establishing correct diagnosis and mode of inheritance, with literature review of genetic mutations, possible differential diagnosis and the most common and successful treatment modalities for epidermolytic ichthyosis.

Expanding the Mutation Spectrum of Ichthyosis with Confetti

Ichthyosis with confetti is a rare, autosomal dominant disorder caused by frameshift mutations in KRT10 or KRT1 and characterized by the development of white, genetically revertant macules in red, diseased skin. All cases result from mutations affecting the tail domains of keratin-10 or keratin-1, and Suzuki et al. expand the mutation spectrum for ichthyosis with confetti caused by mutations in KRT1, showing that a polyarginine frameshift in the keratin-1 tail can also cause this disorder.

Coexistence of mutations in keratin 10 (KRT10) and the mitochondrial genome in a patient with ichthyosis with confetti and Leber's hereditary optic neuropathy

Ichthyosis with confetti (IWC) is a severe congenital genodermatosis characterized by ichthyosiform erythroderma since birth and confetti-like spots of normal skin appearing in childhood as a results of revertant mosaicism. This disorder is caused by mutations in KRT10 or KRT1 genes. We report a 16-year-old boy who presented ichthyosiform erythroderma with severe desquamation since birth and gradually worsening psycho-neurological symptoms (mental retardation, ataxia, dystonia, hypoacusis). The patient conspicuously lacked typical confetti-like spots at the age of 16. The molecular diagnostics by the whole exome sequencing showed a novel de novo (c.1374-2A>C) mutation in the KRT10 gene responsible for the development of IWC (KRT10 defect was confirmed by immunofluorescent study). Concurrently, the m.14484T>C mutation in mitochondrial MTND6 gene (characteristic for Leber's hereditary optic neuropathy or LHON) was detected in patient, his mother and brother. LHON causes frequent inherited blindness typically appearing during young adult life whose expression can be triggered by additional factors such as smoking or alcohol exposure. We speculate the effects of KRT10 and LHON mutations influence each other-skin inflammatory reaction due to severe ichthyosis might trigger the development of psychoneurological abnormalities whereas the mitochondrial mutation may reduce revertant mosaicism phenomenon resulting in the lack of confetti-like spots characteristic for IWC. However, based on a single case we should be cautious about attributing phenotypes to digenic mechanisms without functional data.

Mutations in SULT2B1 Cause Autosomal-Recessive Congenital Ichthyosis in Humans

Ichthyoses are a clinically and genetically heterogeneous group of genodermatoses associated with abnormal scaling of the skin over the whole body. Mutations in nine genes are known to cause non-syndromic forms of autosomal-recessive congenital ichthyosis (ARCI). However, not all genetic causes for ARCI have been discovered to date. Using whole-exome sequencing (WES) and multigene panel screening, we identified 6 ARCI-affected individuals from three unrelated families with mutations in Sulfotransferase family 2B member 1 (SULT2B1), showing their causative association with ARCI. Cytosolic sulfotransferases form a large family of enzymes that are involved in the synthesis and metabolism of several steroids in humans. We identified four distinct mutations including missense, nonsense, and splice site mutations. We demonstrated the loss of SULT2B1 expression at RNA and protein levels in keratinocytes from individuals with ARCI by functional analyses. Furthermore, we succeeded in reconstructing the morphologic skin alterations in a 3D organotypic tissue culture model with SULT2B1-deficient keratinocytes and fibroblasts. By thin layer chromatography (TLC) of extracts from these organotypic cultures, we could show the absence of cholesterol sulfate, the metabolite of SULT2B1, and an increased level of cholesterol, indicating a disturbed cholesterol metabolism of the skin upon loss-of-function mutation in SULT2B1. In conclusion, our study reveals an essential role for SULT2B1 in the proper development of healthy human skin. Mutation in SULT2B1 leads to an ARCI phenotype via increased proliferation of human keratinocytes, thickening of epithelial layers, and altered epidermal cholesterol metabolism.

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".