Types I and II Keratin Intermediate Filaments.

Keratins-types I and II-are the intermediate-filament-forming proteins expressed in epithelial cells. They are encoded by 54 evolutionarily conserved genes (28 type I, 26 type II) and regulated in a pairwise and tissue type-, differentiation-, and context-dependent manner. Here, we review how keratins serve multiple homeostatic and stress-triggered mechanical and nonmechanical functions, including maintenance of cellular integrity, regulation of cell growth and migration, and protection from apoptosis. These functions are tightly regulated by posttranslational modifications and keratin-associated proteins. Genetically determined alterations in keratin-coding sequences underlie highly penetrant and rare disorders whose pathophysiology reflects cell fragility or altered tissue homeostasis. Furthermore, keratin mutation or misregulation represents risk factors or genetic modifiers for several additional acute and chronic diseases.

Mice expressing a mutant Krt75 (K6hf) allele develop hair and nail defects resembling pachyonychia congenita.

KRT75 (formerly known as K6hf) is one of the isoforms of the keratin 6 (KRT6) family located within the type II cytokeratin gene cluster on chromosome 12 of humans and chromosome 15 of mice. KRT75 is expressed in the companion layer and upper germinative matrix region of the hair follicle, the medulla of the hair shaft, and in epithelia of the nail bed. Dominant mutations in members of the KRT6 family, such as in KRT6A and KRT6B cause pachyonychia congenita (PC) -1 and -2, respectively. To determine the function of KRT75 in skin appendages, we introduced a dominant mutation into a highly conserved residue in the helix initiation peptide of Krt75. Mice expressing this mutant form of Krt75 developed hair and nail defects resembling PC. This mouse model provides in vivo evidence for the critical roles played by Krt75 in maintaining hair shaft and nail integrity. Furthermore, the phenotypes observed in our mutant Krt75 mice suggest that KRT75 may be a candidate gene for screening PC patients who do not exhibit obvious mutations in KRT6A, KRT6B, KRT16, or KRT17, especially those with extensive hair involvement.