Several lines of evidence strongly implicate genes involved in desmosomal architecture in the pathogenesis of the keratodermas and related phenotypes. These include the targeting of desmosomal proteins by pemphigus autoantibodies, the presence of mutations in desmosomal genes underlying human skin and hair diseases, and the generation of epidermal phenotypes in mice when these genes are targeted or manipulated. Collectively, these lines of investigation support the hypothesis that desmosomal proteins are critical in epidermal integrity. Human disease phenotypes provide a powerful starting point for classical genetic studies to find new genes by linkage analysis, positional cloning and functional studies of the new proteins. In fact, this approach represents one of the only methods by which to identify truly novel proteins with essential roles in skin and hair follicle integrity. In recent years, our laboratory has made considerable progress taking this classical genetic approach to the study of human and mouse genetic diseases of the skin. To date, this approach has yielded insights into the hairless phenotype in mouse and humans, and the nude phenotype with alopecia and immunodeficiency. In the final year of our initial funding period, using a positional cloning and in silico comparative genomic approach, we unexpectedly discovered three novel mouse desmoglein genes (dsg1b, dsg1c and dsg4), and one additional human desmoglein (Dsg4). Moreover, we have recently demonstrated a critical role human and mouse dsg4 in epidermal and hair follicle differentiation, as well as in pemphigus. We demonstrated the Dsg4 is the gene responsible for both localized autosomal recessive hypotrichosis (LAH) in humans as well as the lanceolate hair (lah) phenotype in the mouse. To continue advancing along this fruitful avenue of research in this Competing Continuation, we have identified three new genetic diseases whose genes are currently unidentified, and which will serve as the basis of this proposal going forward. They consist of Punctate Keratoderma, Naxos Disease and Generalized Basaloid Follicular Hamartoma Syndrome. We propose a comprehensive strategy toward characterization of the molecular basis of these novel disorders, beginning with genetic linkage studies in Specific Aim 1, positional cloning of the underlying genes using comparative animal approaches in Specific Aim 2, and finally in Specific Aim 3, functional analysis of new proteins using spontaneous or engineered mouse mutations. As an example of functional analysis of new proteins, desmoglein 4 will be studied in detail using a combined molecular, cellular and biochemical approaches using four animal models, the lanceolate hair mouse and rat, in Aim 3 of this renewal application. We have found dsg4 to be expressed in many tissues outside the skin, such as the breast, heart, and microvasculature, suggesting it may have a more widespread role in cell adhesion and disease. We anticipate that the approach of classical genetics will continue to identify new components of the epidermis in the next award period, and yield new insights into proteins with critical roles in skin biology [unreadable] [unreadable]