A transgenic mouse model that recapitulates the clinical features of both neonatal and adult forms of the skin disease epidermolytic hyperkeratosis.

Keratins are the major structural proteins of keratinocytes, which are the most abundant cell type in the mammalian epidermis. Mutations in epidermal keratin genes have been shown to cause severe blistering skin abnormalities. One such disease, epidermolytic hyperkeratosis (EHK), also known as bullous congenital ichthyosiform erythroderma, occurs as a result of mutations in highly conserved regions of keratins K1 and K10. Patients with EHK first exhibit erythroderma with severe blistering, which later is replaced by thick patches of scaly skin. To assess the effect of a mutated K1 gene on skin biology and to produce an animal model for EHK, we removed 60 residues from the 2B segment of HK1 and observed the effects of its expression in the epidermis of transgenic mice. Phenotypes of the resultant mice closely resembled those observed in the human disease, first with epidermal blisters, then later with hyperkeratotic lesions. In neonatal mice homozygous for the transgene, the skin was thicker, with an increased labeling index, and the spinous cells showed a collapse of the keratin filament network around the nuclei, suggesting that a critical concentration of the mutant HK1, over the endogenous MK1, was required to disrupt the structural integrity of the spinous cells. Additionally, footpad epithelium, which is devoid of hair follicles, showed blistering in the spinous layer, suggesting that hair follicles can stabilize or protect the epidermis from trauma. Blisters were not evident in adult mice, but instead they showed a thick, scaly hyperkeratotic skin with increased mitosis, resulting in an increased number of corneocytes and granular cells. Irregularly shaped keratohyalin granules were also observed. To date, this is the only transgenic model to show the typical morphology found in the adult form of EHK.

Mutations in the 1A domain of keratin 9 in patients with epidermolytic palmoplantar keratoderma.

Epidermolytic palmoplantar keratoderma is an autosomal dominant skin disorder characterized by hyperkeratosis of the palms and soles. Ultrastructurally the disease exhibits abnormal keratin filament networks and tonofilament clumping like that found in the keratin disorders of epidermolysis bullosa simplex and epidermolytic hyperkeratosis. The disease has been mapped to chromosome 17q11-q23 in the region of the type 1 keratin gene locus and more recently mutations have been found in the palmoplantar specific keratin, keratin 9. We have analyzed six unrelated incidences of epidermolytic palmoplantar keratoderma for mutations in their keratin 9 genes. In two of these, we have identified mutations that alter critical residues within the highly conserved helix initiation motif at the beginning of the rod domain of keratin 9. In a three-generation Middle Eastern kindred we found a C to T transition at codon 162 that results in an arginine to tryptophan substitution at position 10 of the 1A alpha-helical domain, thus confirming this codon as a hot spot for mutation in keratin 9. The other mutation found involves a T to C transition at codon 167 that results in the expression of a serine residue in place of the normal leucine at position 15 of the 1A segment and is the first documentation of this mutation in this gene. The identification of these substitutions extends the current catalog of disease causing mutations in keratin 9.

Targeted overexpression of transforming growth factor alpha in the epidermis of transgenic mice elicits hyperplasia, hyperkeratosis, and spontaneous, squamous papillomas.

To assess the effects of transforming growth factor alpha (TGF-alpha) on mammalian skin in vivo, we have targeted its expression to the epidermis of transgenic mice using a vector based on the human K1 (HK1) gene. Neonatal mice expressing the HK1.TGF-alpha transgene were often smaller than normal littermates and had precocious eyelid opening and wrinkled, scaly skin with diffuse alopecia. Juvenile transgenic mouse epidermis was uniformly hyperkeratotic, but this pattern was generally less pronounced in adult transgenic mice unless they expressed high levels of the HK1.TGF-alpha transgene. Spontaneous, squamous papillomas occurred at sites of wounding in adult mice expressing high levels of HK1.TGF-alpha; however, most were prone to regression. Immunoreactive TGF-alpha was 2-6 times higher in the epidermis of these HK1.TGF-alpha lines. Immunoreactive epidermal growth factor receptor had a normal pattern of expression in nonphenotypic adult epidermis, but a marked reduction in the receptor population was detected in hyperplastic newborn epidermis and phenotypic adult epidermis. Autoradiographic localization of 125I-epidermal growth factor showed a similar pattern of distribution, suggesting that the sites of increased TGF-alpha expression induced epidermal growth factor receptor down-regulation. These data demonstrate the in vivo effect of deregulated TGF-alpha expression on epidermal proliferation and differentiation and suggest a potential role for TGF-alpha in carcinogenesis and other hyperproliferative epidermal disorders.

Transgenic models of skin diseases.

BACKGROUND: Transgenic animals have greatly enhanced our understanding of the contribution of various structural and regulatory components to epidermal biology. The expression of mutant versions of these components in the epidermis of transgenic mice has generated animal models of specific human skin diseases. OBSERVATIONS: The expression of mutant keratin genes has produced animal models of epidermolysis bullosa simplex and epidermolytic hyperkeratosis and, in doing so, has focused attention on the genetics of keratins in these and other skin disorders. Similarly, the generation of mice overexpressing growth factors and/or oncogenes, exclusively in the epidermis, has identified the role of these factors in normal skin and produced models of disease states where the regulation of these factors is perturbed. CONCLUSIONS: These models of keratin disorders and other diseases not only enable the determination of the cause of these disorders, but also allow evaluation of novel therapeutic techniques for the amelioration of these skin diseases.

Mutations in the rod domains of keratins 1 and 10 in epidermolytic hyperkeratosis.

Epidermolytic hyperkeratosis is a hereditary skin disorder characterized by blistering and a marked thickening of the stratum corneum. In one family, affected individuals exhibited a mutation in the highly conserved carboxyl terminal of the rod domain of keratin 1. In two other families, affected individuals had mutations in the highly conserved amino terminal of the rod domain of keratin 10. Structural analysis of these mutations predicts that heterodimer formation would be unaffected, although filament assembly and elongation would be severely compromised. These data imply that an intact keratin intermediate filament network is required for the maintenance of both cellular and tissue integrity.

Targeting gene expression to the epidermis of transgenic mice: potential applications to genetic skin disorders.

The ability to specifically target gene expression to the epidermis of transgenic mice offers the exciting possibility of creating animal models of certain skin disorders that are inherited in man. It may be possible to produce mouse models of dominantly inherited keratinization disorders by targeting the expression of mutant genes encoding the major differentiation products of the epidermis, such as the differentiation specific keratins, filaggrin and cell envelope proteins. Mouse models for other skin disorders associated with abnormal regulation of growth, such as psoriasis, may be generated by targeting the overexpression of cytokines and growth factors, which are thought to play important roles in the pathogenesis of this disease. The development of currently unavailable animal models for certain inherited human skin diseases would not only contribute to our understanding of the pathogenesis of these diseases at the molecular level, but also provide interesting models for therapeutic intervention.

Cutaneous manifestations of dysproteinemias.

A number of cutaneous lesions may represent signs of underlying dysproteinemia. The lesions may result from direct infiltration by plasmacytes and immunoblasts--such as plasmacytomas seen with multiple myeloma--or they may result from indirect effects of the immunoglobulins by a number of different mechanisms. Because the spectrum of cutaneous lesions is so large and at times nonspecific, it is important to keep dysproteinemia in one's differential diagnosis at all times. Careful monitoring of these patients is important since the latency period for the actual development of a plasma cell malignancy may be years.