The Ichthyoses—Pathogenesis and Prenatal Diagnosis: A Review of Recent Advances

Abstract: Disturbances in the process of normal cornification leading to pathologic scaling provide the pathophysiologic basis for the ichthyoses. These disturbances may result from either abnormalities in protein metabolism (keratinization) (i.e., the "bricks") or in lipid metabolism (i.e., the "mortar") (Fig. 1). The evidence linking the various ichthyoses to defects in protein or lipid metabolism have been reviewed. It is likely that future advances will lead not only to a more complete understanding of the pathogenesis of these disorders, but also will shed significant light on the normal stratum corneum functions of barrier formation and desquamation, as well as lead the way to more rational and effective therapies. In recent years, prenatal diagnosis has been successfully performed in several of the ichthyoses. It is likely that improvements in our ability to prenatally diagnose those disorders will advance hand-in-hand with further progress in unraveling their underlying causes.     

Pathogenesis of the permeability barrier abnormality in epidermolytic hyperkeratosis

Epidermolytic hyperkeratosis is a dominantly inherited ichthyosis, frequently associated with mutations in keratin 1 or 10 that result in disruption of the keratin filament cytoskeleton leading to keratinocyte fragility. In addition to blistering and a severe disorder of cornification, patients typically display an abnormality in permeability barrier function. The nature and pathogenesis of the barrier abnormality in epidermolytic hyperkeratosis are unknown, however. We assessed here, first, baseline transepidermal water loss and barrier recovery kinetics in patients with epidermolytic hyperkeratosis. Whereas baseline transepidermal water loss rates were elevated by approximately 3-fold, recovery rates were faster in epidermolytic hyperkeratosis than in age-matched controls. Electron microscopy showed no defect in either the cornified envelope or the adjacent cornified-bound lipid envelope, i.e., a corneocyte scaffold abnormality does not explain the barrier abnormality. Using the water-soluble tracer, colloidal lanthanum, there was no evidence of tracer accumulation in corneocytes, despite the fragility of nucleated keratinocytes. Instead, tracer, which was excluded in normal skin, moved through the extracellular stratum corneum domains. Increasing intercellular permeability correlated with decreased quantities and defective organization of extracellular lamellar bilayers. The decreased lamellar material, in turn, could be attributed to incompletely secreted lamellar bodies within granular cells, demonstrable not only by several morphologic findings, but also by decreased delivery of a lamellar body content marker, acid lipase, to the stratum corneum interstices. Yet, after acute barrier disruption a rapid release of preformed lamellar body contents was observed together with increased organelle contents in the extracellular spaces, accounting for the accelerated recovery kinetics in epidermolytic hyperkeratosis. Accelerated recovery, in turn, correlated with a restoration in calcium in outer stratum granulosum cells in epidermolytic hyperkeratosis after barrier disruption. Thus, the baseline permeability barrier abnormality in epidermolytic hyperkeratosis can be attributed to abnormal lamellar body secretion, rather than to corneocyte fragility or an abnormal cornified envelope/cornified-bound lipid envelope scaffold, a defect that can be overcome by external applications of stimuli for barrier repair.     

Ichthyosis induced by cholesterol-lowering drugs. Implications for epidermal cholesterol homeostasis

Ichthyosis and other disorders of cornification may occur as side effects of treatment with several hypocholesterolemic agents. Recent progress in understanding of the functional role of lipids in stratum corneum provides a new pathophysiologic basis for these earlier clinical observations. In stratum corneum, lipids are segregated within intercellular membranes, where they appear to regulate permeability barrier function and desquamation. Cholesterol is an important constituent of these membranes and may be essential to both of these functions. Perturbation of barrier function induces cholesterologenesis locally within the epidermis. Polar sterol metabolites, such as cholesterol sulfate, may also regulate epidermal sterologenesis under normal or pathologic circumstances. Cholesterol homeostasis may also modulate desquamation. For example, hairless mice fed azacosterol hydrochloride (20,25-diazacholesterol) develop a generalized scaling disorder without loss of barrier function. In these mice, total stratum corneum sterol content is markedly decreased, and topical or systemic repletion with cholesterol can correct the scaling abnormalities.     

Impaired cutaneous permeability barrier function, skin hydration, and sphingomyelinase activity in keratin 10 deficient mice

Point mutations in the suprabasal cytokeratins 1 (K1) or 10 (K10) in humans have been shown to be the cause of the congenital ichthyosis epidermolytic hyperkeratosis. Recently, a K10 deficient mouse model was established serving as a model for epidermolytic hyperkeratosis. Homozygotes suffered from severe skin fragility and died shortly after birth. Heterozygotes developed hyperkeratosis with age. To see whether phenotypic abnormalities in the mouse model were associated with changes in skin barrier function and skin water content we studied basal transepidermal water loss and capacity for barrier repair after experimental barrier disruption as well as stratum corneum hydration. Also, we determined the activities of acid and neutral sphingomyelinase key enzymes of the tumor necrosis factor and interleukin-1 signal transduction pathways generating the ceramides most important for epidermal permeability barrier homeostasis. Neonatal homozygotes showed an 8-fold increase in basal transepidermal water loss compared with wild type controls. Adult heterozygotes exhibited delayed barrier repair after experimental barrier disruption. Stratum corneum hydration was reduced in homozygous and heterozygous mice. Acid sphingomyelinase activity, which is localized in the epidermal lamellar bodies and generates ceramides for extracellular lipid lamellae in the stratum corneum permeability barrier, was reduced in homozygous as well as heterozygous animals. Neutral sphingomyelinase activity, which has a different location and generates ceramides involved in cell signaling, was increased. The reduction in acid sphingomyelinase activity may explain the recently described decreased ratio of ceramides to total lipids in K10 deficient mice. In summary, our results demonstrate the crucial role of the keratin filament for permeability barrier function and stratum corneum hydration.     

Harlequin ichthyosis and other autosomal recessive congenital ichthyoses: the underlying genetic defects and pathomechanisms

Autosomal recessive congenital ichthyoses (ARCI) include several severe subtypes including harlequin ichthyosis (HI), lamellar ichthyosis and non-bullous congenital ichthyosiform erythroderma. Patients with these severe types of ichthyoses frequently show severe hyperkeratosis and scales over a large part of the body surface form birth and their quality of life is often severely affected. Recently, research into the pathomechanisms of these severe congenital ichthyoses have advanced dramatically and led to the identification of several causative genes and molecules underlying the genetic defects. To date, seven loci have been identified that are associated with ARCI and, among them, five causative genes and molecules have been detected. The five genes are transglutaminase 1 gene (TGM1), ABCA12, two lipoxygenase genes, ALOXE3 and ALOX12B and ichthyin. One of these components, ABCA12, has recently been shown to be a keratinocyte lipid transporter associated with lipid transport in lamellar granules and loss of ABCA12 function leads to a defective lipid barrier in the stratum corneum, resulting in the HI phenotype. Transglutaminse 1 deficiency was reported to cause a malformed cornified cell envelope leading to a defect in the intercellular lipid layers in the stratum corneum and defective stratum corneum barrier function resulting in an ichthyosis phenotype. Thus, defective intercellular lipid layers are major findings in autosomal recessive congenital ichthyoses. Information concerning ARCI genetic defects and disease pathomechanisms are beneficial for providing better treatments and genetic counseling including prenatal diagnosis for families affect by ichthyoses.     

Normal ultrastructure, but altered stratum corneum lipid and protein composition in a mouse model for epidermolytic hyperkeratosis.

Recently, we established keratin 10-deficient mice, serving as a model for the hyperkeratotic skin disorder epidermolytic hyperkeratosis. The considerable ichthyosis in these mice suggested alterations in terminal differentiation and in the formation of a functional epidermal barrier. Here, we report on the ultrastructural organization and composition of the stratum corneum lipids and on the expression of two major cornified envelope proteins. Electron microscopy of ruthenium tetroxide postfixed skin samples demonstrated a normal extrusion and morphology of lamellar bodies as well as the formation of bona fide lamellar layers in neonatal keratin 10-deficient mice. When we studied the composition of the major stratum corneum lipids, however, we found significant changes. Most importantly, the analysis of ceramide subpopulations revealed that the total amount of ceramide 2 was elevated in keratin 10-deficient mice, whereas ceramides 1, 3, 4, and 5 were decreased among total stratum corneum lipids. The amount of the ceramide precursors sphingomyelin and glucosylceramide was reduced in the stratum corneum without accompanying changes in the mRNA coding for acid sphingomyelinase. Notably, we found an increased mRNA and protein content for involucrin in neonatal keratin 10-deficient mice, whereas the expression of loricrin was not changed. Our data demonstrate that, although the formation of lipid layers in the stratum corneum appeared to be normal, its lipid composition is significantly altered in keratin 10-deficient mice.     

Exogenous origin of n-alkanes in pathologic scale.

BACKGROUND--Although n-alkanes accumulate in some disorders of cornification, recent studies using radioactive carbon 14 content by accelerator mass spectrometry point to an exogenous origin for alkanes in normal stratum corneum, and their derivation in congenital ichthyosiform erythroderma remains controversial. DESIGN AND RESULTS--Using 14C content to measure sample age, the n-alkane fractions from two patients with congenital ichthyosiform erythroderma contained no detectable contemporary materials. By electron microscopy, alkane-enriched emollients (petrolatum [Vaseline]) permeated to all levels of stratum corneum of hairless mice, expanding the intercellular domains and distorting membrane bilayers. Similar ultrastructural changes were also observed in the stratum corneum of patients with congenital ichthyosiform erythroderma. When alkanes were excluded, no differences in lipid content were evident between two forms of autosomal recessive ichthyosis. CONCLUSIONS--These data demonstrate that scale n-alkanes in disorders of cornification derive from environmental sources and indicate the pervasiveness of petroleum-based emollients in skin. Therefore, epidermal lipid analyses must be interpreted with caution. However, these studies do not rule out an important therapeutic and/or pathogenic role for exogenous n-alkanes in skin.     

Ichthyoses: differential diagnosis and molecular genetics

Ichthyoses are a heterogeneous group of cornification disorders characterized by a generalized scaling of the skin. Common types such as ichthyosis vulgaris and X-linked recessive ichthyosis manifest after birth. In contrast, rare congenital ichthyoses are inherited diseases, which at birth typically present collodion membranes or ichthyosiform erythroderma. Syndromic ichthyoses display a variety of outstanding associated non-cutaneous symptoms. Because of their rarity these disorders often pose a diagnostic challenge for the clinician. This review discusses a broad spectrum of 13 isolated types of ichthyoses, 11 different syndromes with associated ichthyosis and four related cornification disorders. The clinical, ultrastructural and biochemical characteristics are described along with the different molecular causes of ichthyosis. Special attention is given to lamellar ichthyosis and non-bullous congenital ichthyosiform erythroderma. The different pathomechanisms causing ichthyosis provide a fascinating insight into the role of various proteins, enzymes, lipids and metabolic pathways involved in terminal epidermal differentiation/keratinisation.     

An update on molecular aspects of the non-syndromic ichthyoses

Abstract:  Ichthyosis includes a number of subtypes from congenital severe forms, such as harlequin ichthyosis (HI), to mild non-congenital forms, such as ichthyosis vulgaris. Recently, research into the pathomechanisms of ichthyoses has dramatically advanced and led to the identification of several causative genes and molecules underlying the genetic defects. In most types of ichthyosis, pathogenic mechanisms are associated with defects in skin barrier function. Three major components of the stratum corneum barrier are (i) intercellular lipid layers, (ii) cornified cell envelope and (iii) keratin-filaggrin degradation products. The causative molecules underlying ichthyosis subtypes include ABCA12, lipoxygenase-3, 12 R -lipoxygenase, CYP4F2 homolog, ichthyin and steroid sulphatase and all these are thought to be related to the intercellular lipid layers. Transglutaminase 1 has a function in cornified cell envelope formation. Keratins 1, 10 and 2 are involved in the keratin network of suprabasal keratinocytes and filaggrin are essential for formation of keratohyalin granules. In fact, loss of ABCA12 function leads to a defective lipid barrier in the stratum corneum, resulting in the HI phenotype and ABCA12 is a known keratinocyte lipid transporter associated with lipid transport in lamellar granules. Filaggrin gene mutations in ichthyosis vulgaris cause keratohyalin granule deficiency. Information concerning genetic defects and ichthyotic disease pathomechanisms are beneficial to develop effective therapy and provide information for genetic counselling including prenatal diagnosis for families affected by ichthyotic disease.     

Hypergranulotic dyscornification: a distinctive histologic pattern of maturation of epidermal epithelium present in solitary keratoses

Hypergranulotic dyscornification is an appellation proposed to designate a newly recognized distinctive pattern of epidermal maturation that is analogous to other epithelial reaction patterns such as epidermolytic hyperkeratosis, focal acantholytic dyskeratosis, cornoid lamellation, pale-cell acanthosis, and follicular mucinosis. The name "benign hypergranulotic keratosis with dyscornification" is proposed to specify solitary keratoses with digitated epidermal hyperplasia that exhibit this exceptional pattern of cornification. This abnormal type of cornification is characterized by hypergranulosis. A pale-staining basophilic substance is present intercellularly within the upper spinous layer and the hyperplastic granular layer. Overlying the thickened granular layer in foci at tips of epidermal papillations are orthokeratotic mounds of large, dull, eosinophilic staining corneocytes that are sharply demarcated from the thickened granular layer. Basophilic keratohyalin granules are focally retained within these corneocytes. There is overlying compact orthokeratosis that extends across the entire lesion. The compact orthokeratosis is slightly basophilic, and lies below a laminated and basket-weave orthokeratotic stratum corneum. There is a predominantly lymphocytic infiltrate at the base of these neoplasms with some spongiosis. There is parakeratosis focally present in the stratum corneum overlying these individual areas of abnormal cornification. The histopathologic and clinical findings in eight lesions that exhibit hypergranulotic dyscornification, a heretofore undescribed unique pattern of epidermal cornification, are presented.     

Structural and functional consequences of loricrin mutations in human loricrin keratoderma (Vohwinkel syndrome with ichthyosis)

Although loricrin is the predominant protein of the cornified envelope (CE) in keratinocytes, loss or gain of loricrin function in mouse models produces only modest skin phenotypes. In contrast, insertional mutations resulting in a frameshift in the C-terminal domain of loricrin produce the characteristic ichthyosis of loricrin keratoderma in mouse and man. To ascertain the basis for the loricrin keratoderma phenotype, we assessed epidermal structure and stratum corneum (SC) function in a previously genotyped human loricrin keratoderma kindred. Our studies revealed abnormal corneocyte fragility and basal permeability barrier function, but accelerated repair kinetics. Despite fragility, increased water loss occurred predominantly via extracellular domains, which correlated with disorganized lamellar bilayers that were linked spatially to discontinuities of the CE. Accelerated barrier recovery was explicable by amplified lamellar body secretion, while partial normalization of the CE in the outer SC correlated with persistence of abundant calcium in the extracellular spaces (positioned to activate transglutaminase-1). These results show that the barrier abnormality in loricrin keratoderma is linked to a defective CE scaffold, resulting in increased extracellular permeability, as shown previously for another "scaffold disorder", lamellar ichthyosis. But in contrast to lamellar ichthyosis, the CE scaffold partially normalizes in the outer SC in loricrin keratoderma.     

Barrier function of the skin: "la raison d'être" of the epidermis

The primary function of the epidermis is to produce the protective, semi-permeable stratum corneum that permits terrestrial life. The barrier function of the stratum corneum is provided by patterned lipid lamellae localized to the extracellular spaces between corneocytes. Anucleate corneocytes contain keratin filaments bound to a peripheral cornified envelope composed of cross-linked proteins. The many layers of these specialized cells in the stratum corneum provide a tough and resilient framework for the intercellular lipid lamellae. The lamellae are derived from disk-like lipid membranes extruded from lamellar granules into the intercellular spaces of the upper granular layer. Lysosomal and other enzymes present in the extracellular compartment are responsible for the lipid remodeling required to generate the barrier lamellae as well as for the reactions that result in desquamation. Lamellar granules likely originate from the Golgi apparatus and are currently thought to be elements of the tubulo-vesicular trans-Golgi network. The regulation of barrier lipid synthesis has been studied in a variety of models, with induction of several enzymes demonstrated during fetal development and keratinocyte differentiation, but an understanding of this process at the molecular genetic level awaits further study. Certain genetic defects in lipid metabolism or in the protein components of the stratum corneum produce scaly or ichthyotic skin with abnormal barrier lipid structure and function. The inflammatory skin diseases psoriasis and atopic dermatitis also show decreased barrier function, but the underlying mechanisms remain under investigation. Topically applied "moisturizers" work by acting as humectants or by providing an artificial barrier to trans-epidermal water loss; current work has focused on developing a more physiologic mix of lipids for topical application to skin. Recent studies in genetically engineered mice have suggested an unexpected role for tight junctions in epidermal barrier function and further developments in this area are expected. Ultimately, more sophisticated understanding of epidermal barrier function will lead to more rational therapy of a host of skin conditions in which the barrier is impaired.     

The skin: an indispensable barrier

Abstract: The skin forms an effective barrier between the organism and the environment preventing invasion of pathogens and fending off chemical and physical assaults, as well as the unregulated loss of water and solutes. In this review we provide an overview of several components of the physical barrier, explaining how barrier function is regulated and altered in dermatoses. The physical barrier is mainly localized in the stratum corneum (SC) and consists of protein‐enriched cells (corneocytes with cornified envelope and cytoskeletal elements, as well as corneodesmosomes) and lipid‐enriched intercellular domains. The nucleated epidermis also contributes to the barrier through tight, gap and adherens junctions, as well as through desmosomes and cytoskeletal elements. During epidermal differentiation lipids are synthesized in the keratinocytes and extruded into the extracellular domains, where they form extracellular lipid‐enriched layers. The cornified cell envelope, a tough protein/lipid polymer structure, resides below the cytoplasmic membrane on the exterior of the corneocytes. Ceramides A and B are covalently bound to cornified envelope proteins and form the backbone for the subsequent addition of free ceramides, free fatty acids and cholesterol in the SC. Filaggrin is cross‐linked to the cornified envelope and aggregates keratin filaments into macrofibrils. Formation and maintenance of barrier function is influenced by cytokines, 3′,5′‐cyclic adenosine monophosphate and calcium. Changes in epidermal differentiation and lipid composition lead to a disturbed skin barrier, which allows the entry of environmental allergens, immunological reaction and inflammation in atopic dermatitis. A disturbed skin barrier is important for the pathogenesis of contact dermatitis, ichthyosis, psoriasis and atopic dermatitis.     

Harlequin ichthyosis with epidermal lipid abnormality.

An infant with phenotypic harlequin ichthyosis survived for nine months, then died a crib death. At autopsy, an enlarged, but structurally normal, thymus was found. Light microscopically, the epidermis showed massive hyperkeratosis and variable parakeratosis, and a stain for neutral fat was positive in the upper epidermis and stratum corneum. Electron microscopic study disclosed crystals resembling cholesterol and masses of autophagic vacuoles, many of them glutted with lipid, deposited within cells of the stratum corneum. Biochemically, cholesterol and triglyceride levels in the stratum corneum were sharply elevated (19.8 and 32.0 mg/g of dry weight, respectively). A defect in epidermal lipid metabolism is postulated.     

Alpha hydroxy acids: procedures for use in clinical practice

Alpha hydroxy acids and alpha keto acids applied topically in lower concentrations reduce the thickness of hyperkeratotic stratum corneum by reducing corneocyte cohesion at lower levels of the stratum corneum. This property permits efficient clinical control of dry skin, ichthyosis, follicular hyperkeratosis, and other conditions characterized by retention of stratum corneum. Applied topically in higher concentrations, these acids cause epidermolysis. This property provides a new alternative for treating seborrheic keratoses, keratoses commonly known as "age spots," actinic keratoses, and verrucae vulgares; all of which lesions involve distinct epidermal hyperplasia as well as retention of stratum corneum. Facial wrinkles can be modified with topical alpha hydroxy acids, applied in higher concentrations as office procedures, and concomitant daily home application of lower concentrations.     

The fibrous proteins in various types of ichthyosis.

The stratum corneum of individuals with ichthyosis vulgaris, sex-linked ichthyosis, lamellar ichthyosis, and epidermolytic hyperkeratosis has been studied. An alpha x-ray diffraction pattern has been observed in all specimens and the solubility of the alpha fibrous proteins appears to be the same as in normal stratum corneum. Sodium dodecyl sulfate (SDS)-polyacrylamide electrophoresis of the fibrous proteins showed variable patterns within the different types of ichthyosis, while amino acid analyses of the proteins were quite similar to those from normal stratum corneum. These data suggest that the fibrous proteins in the ichthyosis are not abnormal, but further studies on the individual polypeptide chains are necessary to rule out more subtle differences.     

Abnormal epidermal barrier in the pathogenesis of psoriasis

Almost 2 decades ago, Williams and Elias suggested a unifying concept for the pathogenesis of disorders of cornification, according to which the integrity of the epidermal barrier and its effective function is an important factor in the regulation of epidermal DNA synthesis. Interference with the barrier integrity or function will result in epidermal hyperplasia and may be the primary event leading to hyperproliferative skin diseases, such as psoriasis. We have analyzed alterations to several structures of the epidermal barrier that might be responsible for barrier dysfunction and thus lead to hyperproliferation of the epidermis in an attempt to repair the barrier and, as a result, might be inducers of psoriasis. There are several convincing reports indicating that inhibiting of epidermal transglutaminase may lead to epidermal hyperproliferation and that this stimulus might trigger psoriasis among genetically predisposed patients. Disturbance of epidermal barrier function caused by derangement of lipid or cholesterol or ceramide synthesis leads to increased DNA synthesis and epidermal hyperplasia and as a result might be an inducer of psoriasis. We could find little evidence to show that defective defense of the epidermis or an abnormal response of it to bacteria plays a role in the pathogenesis of psoriasis. Accumulating data indicate that there is an association of psoriasis and mutations of genes within the epidermal differentiation complex, which are crucial for the development, maturation, cornification, cross-linking, and terminal differentiation of the epidermis, called psoriasis susceptibility locus 4.     

Disaggregation of Corneocytes from Surfactant-treated Sheets of Stratum Corneum in Hyperkeratosis on Psoriasis,Ichthyosis Vulgaris and Atopic Dermatitis

To elucidate the pathogenesis of impaired barrier function and the influence of surfactant on the stratum corneum in hyperkeratosis, we investigated morphological alterations of the corneocytes with soap solution. Groups of five patients each with psoriasis vulgaris (PV), ichthyosis vulgaris (IV), atopic dermatitis (AD), and normal controls were examined. Four samples of the horny layer were obtained from the same site by cyanoacrylate adhesive biopsy. The first sample was used for the superficial layer, and the fourth, for the basal horny layers. Each sample was agitated in 1% stirred soap solution at 60°C. The number and size of isolated corneocytes and the morphologic changes were investigated. The release of corneocytes was greater and the swelling and morphological changes of corneocytes exposed to soap solutions were less in PV and AD than in IV or in healthy subjects. In IV, the release was markedly less than in controls. The release and swelling were greater in the superficial than in the basal horny layers. It was concluded that the cohesiveness of corneocytes was probably less in PV and AD and greater in IV than in normals. It was also suggested that the cohesion of corneocytes from the superficial horny layer was less than that from the deep layer. The permeability of the cornified envelope in PV and AD patients was less than in IV or healthy subjects. It was confirmed that highly potent soaps induce loss of many corneocytes and reduce the barrier function of the stratum corneum.     

A topical heparinoid‐containing product improves epidermal permeability barrier homeostasis in mice

Because of the importance of epidermal functions, including stratum corneum hydration and maintenance of permeability barrier homeostasis, in the pathogenesis of a variety of cutaneous and systemic disorders, a wide range of products has been developed to improve epidermal functions. However, the underlying mechanisms whereby certain products, including heparinoid‐containing product, are far little understood. In the present study, we assessed the impact of a heparinoid‐containing product, Hirudoid^® cream, on epidermal permeability barrier function and expression levels of a panel of epidermal mRNA related to the formation/maintenance of the permeability barrier in mouse skin. Our results showed that while the baseline levels of transepidermal water rates remained unchanged, treatment with Hirudoid^® cream twice daily for 7 days significantly accelerated permeability barrier recovery and increased stratum corneum hydration. In parallel, expression levels of epidermal mRNA for certain differentiation marker‐related proteins, lipid synthetic enzymes, keratinocyte proliferation and antimicrobial peptides also increased significantly. Together, these results provide the underlying mechanisms by which topical Hirudoid^® cream improves epidermal permeability barrier and antimicrobial function. Because of its benefits for epidermal functions, heparinoid‐containing product could be more useful in the management of skin conditions, characterized by abnormal permeability barrier and antimicrobial function.