The S100 proteins comprise a family of 21 low molecular weight (9-13 kDa) proteins that are characterized by the presence of two calcium-binding EF-hand motifs. Fourteen S100 protein genes are located within the epidermal differentiation complex on human chromosome 1q21 and 13 S100 proteins (S100A2, S100A3, S100A4, S100A6, S100A7, S100A8, S100A9, S100A10, S100A11, S100A12, S100A15, S100B, and S100P) are expressed in normal and/or diseased epidermis. S100 proteins exist in cells as anti-parallel hetero- and homodimers and upon calcium binding interact with target proteins to regulate cell function. S100 proteins are of interest as mediators of calcium-associated signal transduction and undergo changes in subcellular distribution in response to extracellular stimuli. They also function as chemotactic agents and may play a role in the pathogenesis of epidermal disease, as selected S100 proteins are markedly overexpressed in psoriasis, wound healing, skin cancer, inflammation, cellular stress, and other epidermal states.

The epidermis is a dynamic, continually renewing structure that provides the organism with a life-sustaining interface with the environment. The major cell type of the epidermis, the epidermal keratinocyte, undergoes a complex and carefully choreographed program of differentiation. Aberrations in this process result in the genesis of a variety of debilitating and life-threatening diseases. In the present paper, we discuss the keratinocyte differentiation program and the exogenous agents that regulate differentiation. We describe the marker genes that have been utilized to study the process of gene regulation in epidermis. We describe the keratin proteins and studies that have identified keratin mutations that cause epidermal disease. We present recent information on regulation of keratinocyte gene expression and attempt to summarize current knowledge on the role of transcription factors in this process. We also discuss the process of cornified envelope assembly and the structure and function of the proteins that are thought to be precursors of this structure.

Hemophilia B is an X-linked coagulopathy caused by absence of functional coagulation factor IX (FIX). Using adeno-associated virus (AAV)-mediated, liver-directed gene therapy, we achieved long-term (> 17 months) substantial correction of canine hemophilia B in 3 of 4 animals, including 2 dogs with an FIX null mutation. This was accomplished with a comparatively low dose of 1 ؋ 10 12 vector genomes/kg. Canine FIX (cFIX) levels rose to 5% to 12% of normal, high enough to result in nearly complete phenotypic correction of the disease. Activated clotting times and whole blood clotting times were normalized, activated partial thromboplastin times were substantially reduced, and anti-cFIX was not detected. The fourth animal, also a null mutation dog, showed transient expression (4 weeks), but subsequently developed neutralizing anti-cFIX (inhibitor IntroductionHemophilia B is a sex-linked bleeding disorder caused by a deficiency of functional coagulation factor IX (FIX). Current replacement therapy consists of intravenous infusion of protein concentrate. However, this treatment is costly and inconvenient and carries with it the risk of blood-borne disease transmission. Furthermore, bleeds are often treated only after they have occurred, rather than prophylactically, so that chronic joint damage occurs and the risk of a fatal bleed is always present. Hemophilia is an ideal model for gene therapy because precise regulation and tissue-specific transgene expression are not required. 1,2 A number of animal models are available including knockout mice and well-described hemophilic dog colonies with phenotypes corresponding to the human disease. [3][4][5] Clinical end points for treatment are well defined. An increase of factor levels to more than 1% will improve the phenotype of the disease from severe to moderate, with reduced frequency of spontaneous bleeds, and a further increase to more than 5% will result in a mild phenotype; that is, patients would likely require factor infusion only after severe injury or during surgery. Currently the most serious complication of treatment is the formation of inhibitory antibodies to the deficient protein, which occurs with a frequency of 3% to 4% in patients with hemophilia B. 6,7 Inhibitor formation is observed mostly in those patients with extensive loss of FIX coding information. 6,8 Sustained expression of canine FIX (cFIX) in dogs with a missense mutation has been observed following administration of an adeno-associated virus (AAV) vector into the portal vein for hepatic gene transfer or into skeletal muscle. 9-11 The latter approach is currently being tested in a phase 1 clinical trial. 12 AAV vectors can be produced in a helper virus-free system, are devoid of any viral gene products, and often fail to activate antigen-specific cytotoxic T lymphocytes. 13 However, inhibitor formation is still a frequent complication following intramuscular administration of AAV vector in hemophilia B mice (with a large F9 gene deletion) and dogs with a FIX null mutation. 14,15 In these anima...