Loss of Normal Profilaggrin and Filaggrin in Flaky Tail (ft/ft) Mice: an Animal Model for the Filaggrin-Deficient Skin Disease Ichthyosis Vulgaris

Flaky tail (gene symbol ft) is an autosomal recessive mutation in mice that results in a dry, flaky skin, and annular tail and paw constrictions in the neonatal period. Previous studies demonstrated that the ft mutation maps to the central region of mouse chromosome 3, in the vicinity of the epidermal differentiation complex, a gene locus that includes many nonkeratin genes expressed in epidermis. In this study we report a detailed characterization of the flaky tail mouse. Affected homozygous ft/ft mice exhibit large, disorganized scales on tail and paw skin, marked attenuation of the epidermal granular layer, mild acanthosis, and orthokeratotic hyperkeratosis. Biochemical analysis demonstrated that ft/ft mice lacked normal high molecular profilaggrin (≈ 500 kDa), and instead expressed a lower molecular weight form of profilaggrin (220 kDa) that is not proteolytically processed to profilaggrin intermediates or filaggrin. Mutant mice lacked the large, irregular F-type keratohyalin granules that contain profilaggrin, and filaggrin was absent from the cornified layers of ft/ft epidermis. The expression of epidermal keratins was unchanged, whereas the cornified envelope proteins involucrin and loricrin were increased in ft/ft epidermis. Cultured ft/ft keratinocytes also synthesized reduced amounts of profilaggrin mRNA and protein, demonstrating that the defect in profilaggrin expression is intrinsic to epidermal cells. These findings demonstrate that flaky tail mice express an abnormal profilaggrin polypeptide that does not form normal keratohyalin F-granules and is not proteolytically processed to filaggrin. We propose that the absence of filaggrin, and in particular the hygroscopic, filaggrin-derived amino acids that are thought to function in epidermal hydration, underlies the dry, scaly skin characteristic of ft/ft mice. This animal model provides a tool for understanding the role of filaggrin in normal epidermal function and may provide insight into the molecular basis of the filaggrin-deficient human skin disorder ichthyosis vulgaris