Disorders of Intermediate Filaments and Their Associated Proteins

ABSTRACT

• Eukaryotic cytoskeletons are composed of three filament networks. Actin microfilaments (6 nm) and microtubules (30 nm) are composed of proteins that are highly conserved in evolution from yeast to humans. Intermediate filaments (IFs, 10 nm), on the other hand, first appeared in evolution among the brachiopods and seem to have evolved to serve the specialized architectural requirements of higher eukaryotic cells. They constitute a superfamily of more than 40 proteins, which are differentially expressed in most, if not all, cells of higher organisms.

• Despite their diversity in amino acid sequence, all IF proteins have a common secondary structure, consisting of a central, α-helical rod domain and flanking nonhelical head (N-terminal) and tail (C-terminal) segments. IF proteins form parallel, coiled-coil dimers, and more than 10,000 of these are needed to make each 10-nm filament. Dimers associate in a head-to-tail fashion to make linear arrays, which then pack laterally as two antiparallel chains arranged to form apolar protofilaments (2–3 nm). Two protofilaments intertwine to form protofibrils (4.5 nm), and approximately four of these then constitute the overall 10-nm diameter of the IF.

• IF proteins have been subdivided into five distinct subtypes. Type I and II IF proteins are the keratins, which form obligatory heteropolymers in vitro in the absence of other auxiliary proteins or factors. Approximately 30 keratins of two distinct types are coexpressed as pairs in epithelial cells at various stages of differentiation and development. In the epidermis, keratins K5 and K14 are the major structural proteins of basal cells. As keratinocytes commit to terminal differentiation, they switch off expression of this pair and switch on expression of keratins K1 and K10, which constitute approximately 85 percent of total protein in the fully differentiated squame.

• The first disorder of keratin to be genetically defined was epidermolysis bullosa simplex of the Dowling-Meara type (D-M EBS) (MIM Nos. 148066 and 131760), a blistering skin disease where the innermost basal cells of the epidermis rupture and degenerate upon mechanical stress. This form of EBS is typified by clumps or aggregates of keratin material in basal epidermal cells. The elucidation of the genetic basis of EBS as a disorder of keratins K5 (MIM No. 148040) and K14 (MIM No. 148066) represents a novel example of the use of reverse genetics, beginning with the cloning of the basal cell keratin cDNAs and genes, defining key residues important for K5 and K14 filament assembly, engineering dominant negative-acting K14 mutants, discovering that these mutant genes cause EBS in transgenic mice, and finally focusing on human EBS and demonstrating that patients with this disorder have mutations in key residues critical for filament formation. Classic genetics substantiate the reverse genetic findings: Human families with any of four different subtypes of EBS-Dowling-Meara, Koebner (MIM No. 131900), Weber-Cockayne (MIM No. 131800), and mottled pigmentation (MIM No. 131960) have genetic defects that map to either chromosome 12 or chromosome 17, at locations where the respective genes for K5 and K14 reside.

• Many point mutations ...