SUMMARY Orphan diseases are individually rare but collectively affect a significant fraction of the population. Such conditions are often misdiagnosed and lacking in effective therapies, leading to suffering and reduced quality of life. Pachyonychia congenita (PC) is a rare skin disease that entails severe nail dystrophy, oral leukoplakia, sebaceous gland defects and, most significantly, highly painful palmoplantar keratoderma (PPK). PC is caused by mutations in any one of five keratin genes (KRT6A-C, KRT16, and KRT17) that encode intermediate filament-forming proteins. These keratin genes are expressed in all ectoderm-derived epithelial appendages and in palmoplantar skin, accounting for the tissues mainly affected in PC. In addition, the “PC genes” (including the mutated alleles) are robustly induced upon stress of various etiologies (e.g., wounding, infection, pressure stress), altering the balance between keratin proteins in diseased keratinocytes and likely contributing to the complex phenotypes of PC. A better understanding of the pathophysiology of PC will not only lead to better therapies for this condition but also deepen our knowledge of basic skin homeostasis, in ways that may impact the clinical management of addtional skin disorders with features related to PC. The lack of genetically-relevant transgenic mouse models represents a crippling limitation in the study of PC and the development and testing of novel therapies for this disorder. The major goal of this two-year project is to create and validate mouse models that carry PC-causing mutations in relevant keratin genes and exhibit key attributes of this disorder, in particular, PPK-like lesions in footpad skin. In Aim 1 we will complete the characterization of a novel mouse strain carrying a R123C allele at the Krt16 locus, which we recently generated in the laboratory. The corresponding allele in the human accounts for 21% of known cases of PC involving KRT16 mutations (33 out of 157 cases). Aim 2 entails the creation and characterization of a new strain carrying a N161del allele at the Krt6a locus. The latter occurs in >30% of KRT6A pedigrees (76 out of 249 cases) and also causes a classic PC presentation in the human. Strategies have been devised to use chemical and genetic stressors to accentuate skin lesions relevant to PC in Krt16 R123C and Krt6a N171del mouse strains, should they not arise spontaneously. The production and vetting of mouse models involving mutations in two of the PC genes will yield an much needed in vivo platform to develop and test therapies for this disorder.