Second-Hit, Postzygotic PMVK and MVD Mutations in Linear Porokeratosis

MVD Variants Identified in a Rare Clinical Variant of Porokeratosis: A Case Report of Disseminated Superficial Porokeratosis (DSP) in a Chinese Patient

Porokeratosis comprises a diverse range of both hereditary and acquired disorders characterized by clonal hyperproliferation of keratinocytes. These disorders manifest with a variety of clinical presentations but are histologically unified by the presence of the cornoid lamella. In this study, we report an unusual presentation of a rare clinical variant of porokeratosis, namely disseminated superficial porokeratosis, in which mutations in the Mevalonate decarboxylase (MVD) gene have been identified. This finding contributes to the growing understanding of the genetic underpinnings of this complex dermatological condition and may have implications for diagnosis and treatment.

Conventional and Novel Treatment Strategies for Porokeratoses: A Narrative Review

Porokeratoses are a heterogenous group of autoinflammatory keratinization disorders all characterized by the presence of a cornoid lamella. In addition to gene mutations affecting the mevalonate pathway, environmental factors such as UV radiation, immunosuppression, trauma, and infection are also thought to contribute to porokeratoses. To date, there are no management guidelines or levels of evidence for commonly used pharmacologic and non-pharmacologic treatment options for porokeratoses. Conventional treatment strategies encompass topical and systemic drugs (e.g., salicylic acid, topical glucocorticoids, and retinoids), phototherapy, laser, and surgical interventions. Better insights into the pathogenesis of porokeratoses have paved the way for the development of novel therapeutic approaches, such as topical statins or the use of monoclonal antibodies. This narrative review aims to summarize both conventional and novel treatment options, including their level of evidence, advantages, and disadvantages.

Konventionelle und neue Behandlungsstrategien bei Porokeratosen: Eine narrative Übersicht

Porokeratosen sind eine heterogene Gruppe autoinflammatorischer Keratinisierungsstörungen, die durch kornoide Lamellen gekennzeichnet sind. Neben Genmutationen, die sich auf den Mevalonat‐Stoffwechselweg auswirken, werden auch Umweltfaktoren wie UV‐Strahlung, Immunsuppression, Traumata und Infektionen für die Entstehung von Porokeratosen verantwortlich gemacht. Bislang gibt es keine Behandlungsrichtlinien oder Evidenzgrade für die gängigen pharmakologischen und nicht‐pharmakologischen Behandlungsoptionen bei Porokeratosen. Zu den konventionellen Behandlungen zählen topische und systemische Medikamente wie Salicylsäure, topische Glucocorticoide und Retinoide, Phototherapie, Laser und chirurgische Verfahren. Bessere Erkenntnisse über die Pathogenese von Porokeratosen haben die Entwicklung neuartiger therapeutischer Ansätze ermöglicht, etwa topische Statine oder monoklonale Antikörper. In dieser narrativen Übersichtsarbeit werden sowohl die herkömmlichen als auch neuen Behandlungsmöglichkeiten einschließlich ihres Evidenzgrads sowie ihrer Vor‐ und Nachteile zusammengefasst.

Gene-specific somatic epigenetic mosaicism of FDFT1 underlies a non-hereditary localized form of porokeratosis

Porokeratosis is a clonal keratinization disorder characterized by solitary, linearly arranged, or generally distributed multiple skin lesions. Previous studies showed that genetic alterations in MVK, PMVK, MVD, or FDPS-genes in the mevalonate pathway-cause hereditary porokeratosis, with skin lesions harboring germline and lesion-specific somatic variants on opposite alleles. Here, we identified non-hereditary porokeratosis associated with epigenetic silencing of FDFT1, another gene in the mevalonate pathway. Skin lesions of the generalized form had germline and lesion-specific somatic variants on opposite alleles in FDFT1, representing FDFT1-associated hereditary porokeratosis identified in this study. Conversely, lesions of the solitary or linearly arranged localized form had somatic bi-allelic promoter hypermethylation or mono-allelic promoter hypermethylation with somatic genetic alterations on opposite alleles in FDFT1, indicating non-hereditary porokeratosis. FDFT1 localization was uniformly diminished within the lesions, and lesion-derived keratinocytes showed cholesterol dependence for cell growth and altered expression of genes related to cell-cycle and epidermal development, confirming that lesions form by clonal expansion of FDFT1-deficient keratinocytes. In some individuals with the localized form, gene-specific promoter hypermethylation of FDFT1 was detected in morphologically normal epidermis adjacent to methylation-related lesions but not distal to these lesions, suggesting that asymptomatic somatic epigenetic mosaicism of FDFT1 predisposes certain skin areas to the disease. Finally, consistent with its genetic etiology, topical statin treatment ameliorated lesions in FDFT1-deficient porokeratosis. In conclusion, we identified bi-allelic genetic and/or epigenetic alterations of FDFT1 as a cause of porokeratosis and shed light on the pathogenesis of skin mosaicism involving clonal expansion of epigenetically altered cells.

Presentations of Cutaneous Disease in Various Skin Pigmentations: Porokeratosis

Description Porokeratosis was first described in 1893. It is a relatively rare disorder with over 9 subtypes. Lesions are clinically characterized as well-demarcated, erythematous papules (raised, <1 cm) or plaques (raised, >1 cm), with an atrophic center, and raised scaly border. Porokeratosis is an important diagnosis to identify because it may undergo malignant transformation and mimics many commonly encountered diagnoses. These commonly mimicked diagnoses include squamous cell carcinoma, tinea corporis, nummular dermatitis, and psoriasis vulgaris, to name a few. The clinical images in this review focus on identifying porokeratosis along the full spectrum of skin tones.

Pathogenesis-directed treatment of linear porokeratosis with topical cholesterol-lovastatin

A 2-year-old boy presented with an extensive, asymptomatic, photosensitive eruption refractory to topical steroids and tretinoin; examination and biopsies were consistent with generalized linear porokeratosis involving the face, limbs, and trunk. Treatment with topical cholesterol-lovastatin was initiated, and it successfully improved early erythematous lesions. Whole exome sequencing that targeted mevalonate pathway genes crucial in cholesterol synthesis later revealed a pathogenic, paternally inherited, porokeratosis-associated MVD, c.70+5 G>A, mutation. Topical cholesterol-lovastatin is a safe and effective empiric treatment for porokeratosis when used in the early, erythematous phase, and its success is likely mediated through its role in targeting mevalonate pathway mutations.

Porokeratoses-A Comprehensive Review on the Genetics and Metabolomics, Imaging Methods and Management of Common Clinical Variants

Porokeratosis is a heterogeneous group of keratinising disorders characterised by the presence of particular microscopic structural changes, namely the presence of the cornoid lamella. This structure develops as a consequence of a defective isoprenoid pathway, critical for cholesterol synthesis. Commonly recognised variants include disseminated superficial actinic porokeratosis, disseminated superficial porokeratosis, porokeratosis of Mibelli, palmoplantar porokeratosis (including porokeratosis palmaris et plantaris disseminata and punctate porokeratosis), linear porokeratosis, verrucous porokeratosis (also known as genitogluteal porokeratosis), follicular porokeratosis and porokeratoma. Apart from the clinical presentation and epidemiology of each variant listed, this review aims at providing up-to-date information on the precise genetic background, introduces imaging methods facilitating the diagnosis (conventional and ultraviolet-induced fluorescence dermatoscopy, reflectance confocal microscopy and pathology), discusses their oncogenic potential and reviews the literature data on the efficacy of the treatment used, including the drugs directly targeting the isoprenoid-mevalonate pathway.

Targeted anti-interleukin-17 therapy for linear porokeratosis

We report a case involving a 12-year-old female patient with linear porokeratosis (LP) associated with the PMVK pathogenic variant c.329C&gt;A, p.R110Q (monoallelic in DNA from blood and biallelic in DNA from LP). Therapeutic measures, which included a statin/cholesterol cream, did not ameliorate the patient’s skin lesions. Our findings demonstrate a strong interleukin (IL)-17A-positive inflammatory infiltrate, upregulation of IL-17-responsive genes and a significant clinical response to anti-IL-17A therapy in LP.

Bilateral Linear Porokeratosis Treated With Topical Lovastatin 2% Monotherapy

Linear porokeratosis is a rare skin disorder that presents along dermatomal or Blashko lines. While the mechanism of linear porokeratosis formation is unknown, both disrupted cholesterol synthesis and mevalonate accumulation have been proposed as possible theories. There is a small chance of transforming into cutaneous malignancies, most commonly squamous cell carcinomas. The patient is a 61-year-old male with an unusual presentation of bilateral linear porokeratosis. His condition provided a unique opportunity to compare the efficacy of topical treatments in a single individual. A previous trial had successfully cleared the porokeratosis plaques with topical cholesterol 2%/lovastatin 2% on the patient's right arm. After a 12-week trial of topical lovastatin 2% monotherapy on the left arm, our current study demonstrated a comparable reduction of porokeratosis lesions. In our PubMed search, there has been a single reported case of disseminated superficial actinic porokeratosis successfully treated with topical lovastatin 2% monotherapy, but there have not been any reported cases of linear porokeratosis treated with this therapy. While topical lovastatin monotherapy for porokeratosis subvariants requires further studies, this case demonstrates similar efficacy of treating linear porokeratosis with topical lovastatin compared to cholesterol/lovastatin dual therapy. These findings support the theory of mevalonate accumulation as a more likely cause of linear porokeratosis compared to disruption of cholesterol synthesis.

Decreased Imiquimod-Induced Psoriasis-Like Skin Inflammation in a Novel MvdF250S/+ Knock-In Mouse Model

The mevalonate-diphosphate decarboxylase (MVD) gene, a member of the mevalonate pathway, plays a critical role in regulating the biosynthesis of cholesterol, steroid hormones, and non-steroid isoprenoids. Previous studies have suggested that the MVD c.746 T > C mutation is a major pathogenic gene of porokeratosis (PK), an autoinflammatory keratinization disease (AIKD) with unclear pathogenesis, few effective treatments, and no suitable animal model. To investigate the function of Mvd^F250S/+ mutation, we developed a novel Mvd^F250S/+ mouse model carrying an equivalent point mutation to the most common genetic variation among Chinese PK patients (MVD^F249S/+) using CRISPR/Cas9 technology, which exhibited reduced cutaneous expression of Mvd protein. In the absence of external stimulation, Mvd^F250S/+ mice did not display specific phenotypes. However, upon induction with imiquimod (IMQ), Mvd^F250S/+ mice exhibited decreased susceptibility to skin acute inflammation compared to wild-type (WT) mice, as evidenced by reduced cutaneous proliferation and lower protein levels of IL-17a and IL-1β. Additionally, after IMQ induction, the Mvd^F250S/+mice exhibited downregulated collagen generation and upregulated expression of Fabp3 compared to WT mice, whereas no significant changes in the key genes related to cholesterol regulation were found. Furthermore, the Mvd^F250S/+ mutation activated autophagy. Our findings provided insights into the biological function of MVD in the skin.

Case Report: A Novel MVK Missense Mutation in the Sporadic Porokeratosis Ptychotropica in China

Porokeratosis ptychotropica (PPt) is a rare type of porokeratosis (PK) characterized by pruritic, reddish-brownish verrucous papules, and plaques usually around genital area or buttocks. Here, a case of a 70-year-old woman who was diagnosed as PPt was reported. The patient suffered from severe pruritic papules and plaques in the buttock region and pubis for 4 years. The skin lesions were giant, well-defined brown plaques with many satellite papules scattered around. Both clinical manifestations and histopathological features supported the diagnosis of PPt. In review of the identified mutation was found in patients with disseminated superficial actinic porokeratosis (DSAP) combined with PPt, while its unclear in PPt. To investigate the hypothesis that the variant reported in the present case report may played as an independent "likely pathogenic factor" of PPt. Consequently, a de novo missense pathogenic mutation in the MVK gene was identified in this case. Unexpectedly, it is a first report of a novel MVK mutation in sporadic PPt. This rare case suggested an isogenetic background between PPt and DSAP, which may help to explore the underlying pathogenesis of PPt.

Combining Isotretinoin and Topical Cholesterol/Atorvastatin in the Treatment of Linear Porokeratosis: A Case Report

Linear porokeratosis (LP) is an epidermal keratinization disorder manifesting in the form of annular plaques with an atrophic center and hyperkeratotic margins. Although rare, LP carries a significant risk of skin cancer. Histological examination usually reveals the cornoid lamella, a parakeratosis column visualized in the outer layer of the epidermis. First-line treatment of LP is retinoids. However, the effects of combination therapy of isotretinoin and topical statins on LP are not well-understood. Herein, we attempted treatment with both isotretinoin and 2% cholesterol/atorvastatin ointment, with considerable improvement observed using the former but not the latter. These findings suggest that 2% topical cholesterol/atorvastatin treatment may not carry any additional benefits, even if used alongside retinoids. Further studies are needed to assess the potential effects of statins on LP.

A possibly new autoinflammatory disease due to compound heterozygous phosphomevalonate kinase gene mutation

BACKGROUND

Mevalonate kinase (MVK) plays a role in cholesterol and non-sterol isoprenoid biosynthesis and its deficiency-related diseases are caused by bi-allelic pathogenic mutations in the MVK gene, (MVK), which leads to rare hereditary autoinflammatory diseases. The disease may manifest different clinical phenotypes depending on the degree of the deficiency in the enzyme activity. The complete deficiency of the enzyme activity results in the severe metabolic disease called mevalonic aciduria, while a partial deficiency results in a broad spectrum of clinical presentations called hyper-immunoglobulin D syndrome (HIDS). Serum immunoglobulin (Ig) D and urine mevalonic acid levels may be increased during inflammatory attacks of HIDS.

CASE PRESENTATION

Herein, for the first time in the literature, we present a 6-year-old male patient who suffered from recurrent episodes of fever, polyarthritis, skin rash, diarrhea, abdominal pain, and inflammatory bowel disease-like manifestations with elevated levels of serum IgD, and urine mevalonic acid. Eventually we detected compound heterozygous mutations in the phosphomevalonate kinase (PMVK) gene coding the second enzyme after mevalonate kinase in the mevalonate pathway.

CONCLUSION

For patients presenting with HIDS-like findings, disease exacerbations and persistent chronic inflammation, and having high urinary mevalonic acid and serum IgD levels, raising suspicion in terms of MVK deficiency (MVKD), it is recommended to study all mevalonate pathway enzymes, even if there is no mutation in the MVK gene. It should be kept in mind that novel mutations might be seen such as PMVK gene.

Segmental basaloid follicular hamartomas derive from a post-zygotic SMO p.L412F pathogenic variant and express hair follicle development-related proteins in a pattern that distinguish them from basal cell carcinomas

Basaloid follicular hamartomas (BFH) are benign small basaloid skin tumors that can present as solitary or multiple lesions. Congenital BFH lesions arranged in a segmental distribution have been described, suggesting they derive from a somatic post-zygotic mutational event. Previously, BFH were described in Happle-Tinschert syndrome, which results from a post-zygotic SMO variant and is characterized by segmental BFH with variable involvement of the teeth, skeleton, and central nervous system. Here, we describe two patients with isolated segmental BFH and no systemic involvement. Paired whole exome sequencing of BFH and normal tissue revealed a pathogenic SMO c.1234 C>T, p.L412F variant restricted to BFH tissue. We characterized the proliferation index and expression of Hedgehog and Wnt/beta-catenin pathway related proteins in segmental BFH compared to sporadic basal cell carcinomas (BCCs) and found that segmental BFH had a lower proliferation index. Although segmental BFH expressed a similar level of Gli-1 compared to BCCs, levels of LEF-1 and SOX-9 expression in BFH were weaker for both and patchier for LEF-1. Our results show that a somatic SMO activating variant causes segmental BFH. Since these patients are prone to developing BCCs, differences in SOX9, LEF1, and Ki-67 expression can help distinguish between these two basaloid lesions.

Inflammatory linear verrucous epidermal nevus (ILVEN) encompasses a spectrum of inflammatory mosaic disorders

BACKGROUND

Inflammatory linear verrucous epidermal nevus (ILVEN) is a rare skin disease characterized by pruritic erythematous scaly plaques distributed along the lines of Blaschko. Two cases of ILVEN with CARD14 mutations and one case with a GJA1 mutation have been previously reported.

OBJECTIVE

To elucidate the genetic cause of a cohort of patients diagnosed based on clinical and histopathological evaluation with ILVEN.

METHODS

We recruited patients diagnosed with ILVEN based on clinical and histopathological criteria. Exome sequencing of affected skin with or without blood/saliva was performed and germline and somatic pathogenic variants were identified.

RESULTS

Five patients were enrolled. All had skin lesions from birth or early childhood. Two patients developed psoriasis vulgaris after the diagnosis of ILVEN. The first had a germline heterozygous CARD14 mutation and a post-zygotic hotspot mutation in KRT10. The histopathologic evaluation did not show epidermolytic hyperkeratosis. The second had a post-zygotic hotspot mutation in HRAS. Her ILVEN became itchy once psoriasis developed. One patient was re-diagnosed with linear porokeratosis based on a germline mutation in PMVK and a post-zygotic second-hit mutation. Two patients were re-diagnosed with congenital hemidysplasia with ichthyosiform nevus and limb defect nevus based on germline NSDHL mutations.

CONCLUSION

ILVEN is a clinical descriptor for a heterogenous group of mosaic inflammatory disorders. Genetic analysis has the potential to more precisely categorize ILVEN and permits pathogenesis-directed therapies in some cases.

Assessing Cutaneous Mosaicism at the Molecular Level

Mosaicism results from postzygotic alterations during embryogenesis leading to genetically distinct populations of cells within individuals and has been historically recognized by phenotypes with visible, often patterned manifestations. Before the advent of molecular profiling assays and high-throughput sequencing, it was challenging to study mosaicism in human disease; however, the study of mosaic disorders has recently revealed unexpected and novel pathways for disease pathogenesis. In this paper, we will review the techniques for discovery of disease-causing alleles using Proteus syndrome; phakomatosis pigmentokeratotica; linear porokeratosis; and vacuoles, E1 enzyme, X-linked, autoinflammatory somatic syndrome as models. These tools represent powerful approaches for dissecting the genetic basis for human disorders.

Pathogenesis of annular lesions: Why a ring presentation?

Annular lesions represent a distinct morphology which characterizes many well-known dermatologic conditions. Little is definitively known regarding the pathogenesis of annular lesions, however there a few well-regarded hypotheses. Lesions that clear centrally while enlarging peripherally may result from a local central tissue anergy, or tolerance. The central area in lesions due to dermatophyte infections or subacute cutaneous lupus erythematous may have a central immunity to the antigen that trigged the lesion. The peripheral spread of inflammatory mediators may also contribute to lesions that expand centrifugally. In a highly active immune response, some of the inflammatory mediators may spread to adjacent tissue, which can propagate the inflammatory reaction. The additional hypotheses regarding pathogenesis are disease specific with individual mechanisms having been proposed. This chapter will describe both general and disease specific mechanisms that may contribute to the formation of annular lesions.

Bilateral Linear Porokeratosis Treated With Topical Cholesterol 2%/Lovastatin 2%

Linear porokeratosis is a cutaneous disorder that typically presents in a unilateral linear formation. While the exact cause of linear porokeratosis is unknown, it is thought to be a downstream effect of disrupted cholesterol synthesis and mevalonate accumulation. Our patient is a 61-year-old male with an unusual case presentation of bilateral linear porokeratosis. He had failed numerous standard therapies. Pathologic examination of a skin biopsy was consistent with bilateral linear porokeratosis. Through a PubMed search, there have been limited reported cases of unilateral linear porokeratosis, but there have not been any reported cases of bilateral linear porokeratosis. There are currently limited therapies with satisfactory outcomes for variants of porokeratosis. While there are some studies on the topical application of cholesterol/lovastatin, limited studies have been performed on the linear form. Our study evaluates the efficacy of compounded topical cholesterol 2%/lovastatin 2% ointment on bilateral linear porokeratosis. The patient demonstrated a significant reduction of porokeratotic lesions on the treated arm compared to the untreated arm. Cholesterol/lovastatin is alternative therapy that can be considered in the treatment of linear porokeratosis and other porokeratosis variants.

Twists and turns of the genetic story of mevalonate kinase-associated diseases: A review

Mevalonate kinase (MK)-associated diseases encompass a broad spectrum of rare auto-inflammatory conditions, all resulting from pathogenic variants in the mevalonate kinase gene (MVK). Their clinical manifestations are highly variable, ranging from more or less serious systemic disorders, such as hereditary recurrent fevers, to purely localized pathologies such as porokeratosis. The oldest condition identified as linked to this gene is a metabolic disease called mevalonic aciduria, and the most recent is disseminated superficial actinic porokeratosis, a disease limited to the skin. The modes of inheritance of MK-associated diseases also diverge among the different subtypes: recessive for the systemic subtypes and dominant with a post-zygotic somatic genetic alteration for MVK-associated porokeratosis. This review quickly retraces the historical steps that led to the description of the various MK-associated disease phenotypes and to a better understanding of their pathophysiology, then summarizes and compares the different genetic mechanisms involved in this group of disorders, and finally discusses the diverse causes that could underlie this phenotypic heterogeneity.

Life-long Skin Eruptions along Blaschko's Lines in a 27-year-old Woman: A Quiz

Abstract is missing (Quiz)

Generalized type 2 segmental disseminated superficial actinic porokeratosis coexisted with multiple cutaneous squamous cell carcinomas: Analysis of two cases

Porokeratosis (PK) is defined as hyperpigmented macules or patches with a distinctive, ridge-like hyperkeratotic border which is histologically characterized by a cornoid lamella. Here, we report two cases of linear porokeratosis which converted to multiple cutaneous squamous cell carcinoma after long history progression. In addition, patient 2 was accompanied by secondary dermal amyloid deposits, which was rare reported.

Progressive Osseous Heteroplasia is not an Autosomal Dominant Trait but Reflects Superimposed Mosaicism in Different GNAS Inactivation Disorders

Progressive osseous heteroplasia (POH) is a rarely occurring genetic condition characterized by severe segmental ossification involving the skin and deep connective tissues including the muscles. So far, the disorder is generally described as an autosomal dominant trait. By contrast, the following arguments are in favor of the alternative concept that POH should rather be taken as a non-specific segmental manifestation of different GNAS inactivation disorders such as Albright hereditary osteodystrophy (AHO) with hormone resistance, AHO without hormone resistance, and osteomatosis cutis. Presently, POH has got its own OMIM number 166350 but this is obviously wrong because the disorder does not reflect heterozygosity for a GNAS mutation. Conversely, the disorder is most likely due to an early event of postzygotic loss of heterozygosity with loss of the corresponding wild-type allele. This alternative concept, as proposed in 2016, offers a plausible explanation for the following features of POH. Familial occurrence is usually absent. POH is usually observed in families with one of the three GNAS inactivation disorders as mentioned above. Mosaicism is suggested by the pronounced segmental manifestation of POH and by its lateralization. Some patients have, in addition to POH, bilaterally disseminated features of osteomatosis cutis or AHO, and other patients have family members with one of these nonsegmental disorders. Remarkably, POH tends to appear much earlier than the nonsegmental GNAS inactivation disorders. - Molecular support of the concept was documented in a superficial variant of POH called 'plate-like osteoma cutis'. In several other autosomal dominant skin disorders, molecular corroboration of the theory of superimposed mosaicism has been provided. - For all of these reasons, it is unlikely that POH can further be taken as a distinct autosomal dominant trait. Generation of more molecular data in multiple cases of POH occurring in GNAS inactivation disorders will be crucial to corroborate the proposed concept.

Linear porokeratosis with bone abnormalities treated with compounded topical 2% cholesterol/2% lovastatin ointment

We describe a case of linear porokeratosis with associated bone resorption in a 17-year-old female with marked improvement after 2% cholesterol/2% lovastatin ointment application. Porokeratosis is a heterogenous group of keratinization disorders characterized by a cornoid lamella, consisting of focal dyskeratotic cells in the granular layer and columns of parakeratosis. The pathogenesis of porokeratosis is not fully elucidated; however, germline mutations have recently been identified in the mevalonate pathway which can lead to a buildup of metabolites that could play a role in dysmaturation. There has only been one prior report of an affected distal digit with underlying bone resorption in association with linear porokeratosis.

Second-Hit Somatic Mutations in Mevalonate Pathway Genes Underlie Porokeratosis

Familial and sporadic porokeratosis are associated with germline heterozygous mutations in mevalonate pathway genes. Kubo et al. show that each skin lesion of disseminated superficial actinic porokeratosis originates from a postnatal keratinocyte clone with a different second-hit genetic event in the wild-type allele of the corresponding gene. They also confirm that linear porokeratosis derives from a single prenatal clone of keratinocytes with a second-hit genetic event.

Mosaicism in genodermatoses

Genodermatoses are inherited disorders presenting with cutaneous manifestations with or without the involvement of other systems. The majority of these disorders, particularly in cases that present with a cutaneous patterning, may be explained in the context of genetic mosaicism. Despite the barriers to the genetic analysis of mosaic disorders, next-generation sequencing has led to a substantial progress in understanding their pathogenesis, which has significant implications for the clinical management and genetic counseling. Advances in paired and deep sequencing technologies in particular have made the study of mosaic disorders more feasible. In this review, we provide an overview of genetic mosaicism as well as mosaic cutaneous disorders and the techniques required to study them.

Mutations in KRT10 in epidermolytic acanthoma

BACKGROUND

Epidermolytic acanthoma (EA) is a rare acquired lesion demonstrating a characteristic histopathological pattern of epidermal degeneration referred to as epidermolytic hyperkeratosis (EHK). On histopathological analysis, EA appears nearly identical to inherited EHK-associated dermatoses such as epidermolytic ichthyosis and ichthyosis bullosa of Siemens. While it has been speculated that EA is caused by mutations in KRT10, KRT1, or KRT2 found in these inherited dermatoses, none have yet been identified. Herein, we aim to identify the contributions of keratin mutations to EA.

METHODS

Using genomic DNA extracted from paraffin-embedded samples from departmental archives, we evaluated a discovery cohort using whole-exome sequencing (WES) and assessed remaining samples using Sanger sequencing screening and restriction fragment length polymorphism (RFLP) analysis.

RESULTS

DNA from 16/20 cases in our sample was of sufficient quality for polymerase chain reaction amplification. WES of genomic DNA from lesional tissue revealed KRT10 c.466C > T, p.Arg156Cys mutations in 2/3 samples submitted for examination. RFLP analysis of these samples as well as eight additional samples confirmed the mutations identified via WES and identified four additional cases with Arg156 mutations. In sum, 6/11 screened cases showed hotspot mutation in KRT10.

CONCLUSIONS

Hotspot mutations in the Arg156 position of KRT10, known to cause epidermolytic ichthyosis, also underlie EA.

Clinical course of porokeratosis ptychotropica over 7 years in an otherwise healthy child

Porokeratosis ptychotropica is an unusual variant of porokeratosis characterized by papules and plaques located on the buttocks and gluteal cleft and showing multiple coronoid lamellae on histology. In this case report, we present the longitudinal clinical course of porokeratosis ptychotropica in a pediatric patient with individual red-brown hyperkeratotic lesions that enlarged and became confluent prior to surgical intervention. We also discuss the etiology of porokeratosis ptychotropica and review current as well as future treatment options for the disease.

Topical cholesterol/lovastatin for the treatment of porokeratosis: A pathogenesis-directed therapy

BACKGROUND

Porokeratosis is associated with mevalonate pathway gene mutations. Therapeutic options are few and often limited in efficacy. We hypothesized that topical therapy that aims to replenish cholesterol, an essential mevalonate pathway end-product, and block the accumulation of mevalonate pathway toxic metabolites could alleviate porokeratosis.

OBJECTIVE

To study the efficacy of topical cholesterol/lovastatin in different variants of porokeratosis.

METHODS

We enrolled a series of 5 porokeratosis patients,1 with disseminated superficial actinic porokeratosis, 2 with porokeratosis palmaris et plantaris disseminata, and 2 with linear porokeratosis. Patients were genotyped before initiation of therapy. Patients then applied topical cholesterol/lovastatin twice daily to a unilaterally defined treatment area for up to 3 months. The response was evaluated and patients photographed at every visit.

RESULTS

Three patients had MVD mutations, and 2 patients had PMVK mutations. Treatment with topical cholesterol/lovastatin (but not cholesterol alone) resulted in near complete clearance of disseminated superficial actinic porokeratosis lesions after 4 weeks of therapy and moderate improvement of porokeratosis palmaris et plantaris disseminata lesions and linear porokeratosis lesions. There were no adverse events.

LIMITATIONS

Case series design with a small number of patients.

CONCLUSION

Topical cholesterol/lovastatin is an effective and well-tolerated therapy for porokeratosis that underscores the utility of a pathogenesis-based therapy that replaces deficient end products and prevents accumulation of potentially toxic precursors.

Phenotypic expansion of POFUT1 loss of function mutations in a disorder featuring segmental dyspigmentation with eczematous and folliculo-centric lesions

Appearance of mosaic disorders in thin Blaschko lines suggests that somatic mutations in keratinocyte precursors underlie their pathogenesis. Germline heterozygous mutations in POFUT1 gene cause Dowling-Degos disease (DDD), a skin disease that features flexural reticulated hyperpigmentation and follicular-based lesions. POFUT1 mosaicism has not been described to date. Here, we describe a 9-year-old female with segmental hyper- and hypopigmented patches with overlying eczematous plaques and follicular papules. Employing paired whole exome sequencing of saliva and keratinocytes isolated from affected skin, we found a novel germline heterozygous POFUT1 deletion causing frameshift and premature codon termination and somatic copy-neutral loss of heterozygosity on chromosome 20 encompassing POFUT1. Expression levels of POFUT1 as well as other key regulators of the notch signaling pathway-NOTCH1, NOTCH2, and HES1-were reduced in affected keratinocytes compared with normal keratinocytes. Our findings provide the first evidence of POFUT1 postzygotic mutation and a phenotypic expansion of POFUT1 loss of function mutations. We show that a recessive loss of function mutation in POFUT1 produces a distinct clinical presentation with features (e.g., dermatitis) that are absent in the generalized form of DDD. This study demonstrates how analysis of mosaic disorders can reveal unexpected phenotypes for known genes.

Clonal Expansion of Second-Hit Cells with Somatic Recombinations or C>T Transitions Form Porokeratosis in MVD or MVK Mutant Heterozygotes

Patients with disseminated superficial actinic porokeratosis (DSAP) and linear porokeratosis (LP) exhibit monoallelic germline mutations in genes encoding mevalonate pathway enzymes, such as MVD or MVK. Here, we showed that each skin lesion of DSAP exhibited an individual second hit genetic change in the wild-type allele of the corresponding gene specifically in the epidermis, indicating that a postnatal second hit triggering biallelic deficiency of the gene is required for porokeratosis to develop. Most skin lesions exhibited one of two principal second hits, either somatic homologous recombinations rendering the monoallelic mutation biallelic or C>T transition mutations in the wild-type allele. The second hits differed among DSAP lesions but were identical in those of congenital LP, suggesting that DSAP is attributable to sporadic postnatal second hits and congenital LP to a single second hit in the embryonic period. In the characteristic annular skin lesions of DSAP, the central epidermis featured mostly second hit keratinocytes, and that of the annular ring featured a mixture of such cells and naïve keratinocytes, implying that each lesion reflects the clonal expansion of single second hit keratinocytes. DSAP is therefore a benign intraepidermal neoplasia, which can be included in the genetic tumor disorders explicable by Knudson's two-hit hypothesis.