The major inherited disorders of cornification. New advances in pathogenesis

Experimental Models for the Study of Hereditary Cornification Defects

Ichthyoses comprise a broad spectrum of keratinization disorders due to hereditary defects of cornification. Until now, mutations in more than 50 genes, mostly coding for structural proteins involved in epidermal barrier formation, have been identified as causes for different types of these keratinization disorders. However, due to the high heterogeneity and difficulties in the establishment of valid experimental models, research in this field remains challenging and translation of novel findings to clinical practice is difficult. In this review, we provide an overview of existing models to study hereditary cornification defects with focus on ichthyoses and palmoplantar keratodermas.

Trichohyalin-like 1 protein, a member of fused S100 proteins, is expressed in normal and pathologic human skin

Trichohyalin-like 1 (TCHHL1) protein is a novel member of the fused-type S100 protein gene family. The deduced amino acid sequence of TCHHL1 contains an EF-hand domain in the N-terminus, one trans-membrane domain and a nuclear localization signal. We generated specific antibodies against the C-terminus of the TCHHL1 protein and examined the expression of TCHHL1 proteins in normal and pathological human skin. An immunohistochemical study showed that TCHHL1 proteins were expressed in the basal layer of the normal epidermis. In addition, signals of TCHHL1 proteins were observed around the nuclei of cultured growing keratinocytes. Accordingly, TCHHL1 mRNA has been detected in normal skin and cultured growing keratinocytes. Furthermore, TCHHL1 proteins were strongly expressed in the peripheral areas of tumor nests in basal cell carcinomas and squamous cell carcinomas. A dramatic increase in the number of Ki67 positive cells was observed in TCHHL1-expressing areas. The expression of TCHHL1 proteins also increased in non-cancerous hyperproliferative epidermal tissues such as those of psoriasis vulgaris and lichen planus. These findings highlight the possibility that TCHHL1 proteins are expressed in growing keratinocytes of the epidermis and might be associated with the proliferation of keratinocytes.

A case of bullous congenital ichthyosiform erythroderma (BCIE) caused by a mutation in the 1A helix initiation motif of keratin 1

Bullous congenital ichthyosiform erythroderma (BCIE) is an autosomally dominant inherited disorder characterized by erythematous, erosive, and bullous skin lesions over the entire body at birth and abnormal hyperkeratosis on the palmoplantar sufaces as the patient grows older. BCIE is caused by a mutation in the keratin 1 (K1) and/or keratin 10 (K10) genes, and most pathogenic mutations are found within the helix initiation and termination motifs of the central helical rod domain (K1 and K10) or the upstream H1 homology domain (K10). In addition to inherited cases, sporadic cases due to a new mutation account for approximately half the total cases of BCIE. We report herein a typical sporadic case of BCIE with erythroderma, erosion, and blisters on the entire body surface at birth and palmoplantar and flexuaral areas of hyperkeratosis in the later stage. We found in this case a novel mutation, 559C to T, at amino acid position 187, which resulted in a leucine to phenylalanine substitution within the helix initiation motif of K1.

Epidermolytic hyperkeratosis: a keratin 1 or 10 mutational event

Epidermolytic hyperkeratosis is an unusual type of ichthyosis. This inherited keratinization disorder is characterized clinically by erythema, blistering, and peeling shortly after birth. It may resolve and be replaced with thick scaling. It can lead to life-threatening complications, such as sepsis. Histologically, there is a hyperkeratosis and vacuolar degeneration. Genetically, this is an autosomal dominant disease with complete penetrance; however, 50% are spontaneous mutations. The clinical phenotype is a result of alterations in the gene(s) for keratin 1 and/or 10. We review this disorder and its therapy, which is mainly symptomatic with emollients and retinoids.

Identification of human hornerin and its expression in regenerating and psoriatic skin

We previously isolated a new member of the fused-type S100 protein family (hornerin) from the mouse (Makino, T., Takaishi, M., Morohashi, M., and Huh, N.-h. (2001) J. Biol. Chem. 276, 47445-47452). Mouse hornerin shares structural features, expression profiles, and intracellular localization with profilaggrin, indicating possible involvement of hornerin in cornification. In this study, we identified and partially characterized a human ortholog of mouse hornerin. The human hornerin gene was mapped between trichohyalin and filaggrin on chromosome 1q21.3, the region being completely syntenic with the counterpart of the mouse. The deduced amino acid sequence of 2850 residues shows typical structural features of "fused-type" S100 protein family members. Mature transcripts and protein from human hornerin were not detected in normal stratified epithelium, including the trunk epidermis, tongue, and esophagus. After screening of various normal and pathological human tissues, we found that human hornerin was expressed in psoriatic skin. Hornerin protein was present in the keratinizing region, although at a lower level and in fewer cells compared with filaggrin. Mature transcripts and protein from hornerin were also detected in regenerating human skin after wounding. Hornerin mRNA was induced 5 days after wounding. The mRNA level remained almost constant until 15 days and declined at 30 days after wounding. Hornerin protein was detected in the proximal epidermis (but not in the distal epidermis) at 15 days after wounding. These results indicate that hornerin has a function similar to but distinct from that of filaggrin in cornification.

Epithelial barrier function: assembly and structural features of the cornified cell envelope

Terminally differentiating stratified squamous epithelial cells assemble a specialized protective barrier structure on their periphery termed the cornified cell envelope (CE). It is composed of numerous structural proteins that become cross-linked by several transglutaminase enzymes into an insoluble macromolecular assembly. Several proteins are involved in the initial stages of CE assembly, but only certain proteins from a choice of more than 20 different proteins are used in the final stages of CE reinforcement, apparently to meet tissue-specific requirements. In addition, a variable selection of proteins may be upregulated in response to genetic defects of one of the CE proteins or tissue injury, in an effort to maintain an effective barrier. Additionally, in the epidermis and hair fiber cuticle, a layer of lipids is covalently attached to the proteins, which provides essential water barrier properties. Here we describe our current understanding of CE structure, a possible mechanism of its assembly, and various disorders that cause a defective barrier.

Papulosquamous disorders of the elderly

Several papulosquamous skin conditions occur in the elderly, including inherited and acquired ichthyoses, psoriasis, cutaneous lymphoma, and cutaneous connective tissue diseases. The clinical presentations can be quite helpful in diagnosing these conditions, and confirmatory histology and immunologic testing can often better define the specific entities. Treatment often involves identifying underlying systemic causes, as well as specific approaches based on the diagnosis and severity of clinical presentation.