Genomic characterization of the human type I cuticular hair keratin hHa2 and identification of an adjacent novel type I hair keratin gene hHa5

A human identification system for hair shaft using RNA polymorphism

Hair is one of the common pieces of evidence at crime scenes, with abundant mitochondrial DNA but limited nuclear DNA in its shaft. It also helps to narrow the investigation scope to maternal lineage but fails to provide unique individual information. We assumed that RNA in hair shafts would be an alternative resource used to perform human identification based on the facts that (1) RNA retains the polymorphic information; (2) the multi-copy of RNA in a cell resists degradation as compared to the one-copy of nuclear DNA. In this study, we explored the potential of RNA polymorphism in hair shafts for forensic individual identification. A SNaPshot typing system was constructed using 18 SNPs located on 11 genes (ABCA13, AHNAK, EXPH5, KMT2D, KRT35, PPP1R15A, RBM33, S100A5, TBC1D4, TMC5, TRPV2). The RNA typing system was evaluated for sensitivity, species specificity, and feasibility for aged hair samples. Hair samples from a Shanxi population in China were used for the population study of the system. The detection limit of the assay was 2 ng RNA. The CDP of these 11 genes was 0.999969 in the Shanxi population. We also identified the concordance of the RNA and DNA typing results. In summary, we developed an RNA typing method to perform human identification from hair shafts, which performed as accurately as nuclear DNA typing. Our method provides a potential basis for solving the human identification problem from hair shafts, as well as other biological materials that lack nuclear DNA.

De novo filament formation by human hair keratins K85 and K35 follows a filament development pattern distinct from cytokeratin filament networks

In human hair follicles, the hair‐forming cells express 16 hair keratin genes depending on the differentiation stages. K85 and K35 are the first hair keratins expressed in cortical cells at the early stage of the differentiation. Two types of mutations in the gene encoding K85 are associated with ectodermal dysplasia of hair and nail type. Here, we transfected cultured SW‐13 cells with human K85 and K35 genes and characterized filament formation. The K85–K35 pair formed short filaments in the cytoplasm, which gradually elongated and became thicker and entangled around the nucleus, indicating that K85–K35 promotes lateral association of short intermediate filaments (IFs) into bundles but cannot form IF networks in the cytoplasm. Of the K85 mutations related to ectodermal dysplasia of hair and nail type, a two‐nucleotide (C¹⁴⁴⁸T¹⁴⁴⁹) deletion (delCT) in the protein tail domain of K85 interfered with the K85–K35 filament formation and gave only aggregates, whereas a missense mutation (233A>G) that replaces Arg⁷⁸ with His (R78H) in the head domain of K85 did not interfere with the filament formation. Transfection of cultured MCF‐7 cells with all the hair keratin gene combinations, K85–K35, K85(R78H)–K35 and K85(delCT)–K35, as well as the individual hair keratin genes, formed well‐developed cytoplasmic IF networks, probably by incorporating into the endogenous cytokeratin IF networks. Thus, the unique de novo assembly properties of the K85–K35 pair might play a key role in the early stage of hair formation.

Effect of varied hair protein fractions on the gel properties of keratin/chitosan hydrogels for the use in tissue engineering

Keratin/chitosan composite is a readily available source for a hybrid hydrogel in tissue engineering. While human hair keratins could provide biological functions, chitosan could further enhance the mechanical strength of the hybrid hydrogels. However, hair keratin is a group of natural proteins, and the uncontrolled hair protein contents in a hydrogel may lead to the batch-to-batch inconsistent gel properties. The purpose of this study was to investigate the role of hair protein composition, including the keratin-associated proteins (KAPs, 6-30 kDa) and keratin intermediate filaments (KIFs, 45-60 kDa) on gel characteristics of the keratin/chitosan hydrogel. The various compressive and tensile modulus of the gel was observed based on the selection of different protein fractions as the significant gel components. These results thus suggest a straightforward method of preparing hair keratin/chitosan hydrogel with much more controllable gel properties by merely modulating the KAPs/KIFs ratios in a gel.

The expression of KRT2 and its effect on melanogenesis in alpaca skins

In order to investigate the effects of the keratin 2 (KRT2) on alpaca melanocyte in vivo and vitro, the immunohistochemistry (IHC), quantitative real-time PCR (qPCR), Western blot, and alpaca melanocytes transfection methods were used. The results showed that mRNA and protein expression of KRT2 was highly expressed in brown skin in comparison with that in white skin. Moreover, we found that KRT2 was expressed in alpaca melanocytes in vitro by immunocytochemistry. After transfection with KRT2 in alpaca melanocytes, the relative mRNA and protein expression of KRT2, microphthalmia-associtated transcription factor (MITF), tyrosinase (TYR) and tyrosinase-related protein 1 (TYRP1) in alpaca skin melanocytes was increased with significant differences; a further result was the increase of melanin production. The results suggested that KRT2 functions in alpaca hair color formation, which offered an essential theoretical basis for further exploration of the role of melanogenesis.

The human type I keratin gene family: characterization of new hair follicle specific members and evaluation of the chromosome 17q21.2 gene domain

In general concurrence with recent studies, bioinformatic analysis of the chromosome 17q21.2 DNA sequence found in the EBI/Genebank database shows the presence of 27 type I keratin genes and five keratin pseudogenes present on 8 contiguous Bacterial Artificial Chromosome (BAC) sequences. This constitutes the 970 kb type I keratin gene domain. Inserted into this domain is a 350 kb region harboring 32 previously characterized keratin-associated protein genes. Of the 27 keratin genes found in this region, six have not been characterized in detail. This study reports the isolation of cDNA sequences for these keratin genes, termed K25irs1-K25irs4, Ka35, and Ka36, as well as cDNA sequences for the previously reported hair keratins hHa3-I, hHa7, and hHa8. RT-PCR analysis of 14 epithelial tissues using primers for the six novel keratins, as well as for keratins 23 and 24, shows that the six novel keratins appear to be hair follicle associated. Previous expression data, coupled with evolutionary analysis studies point to K25irs1-K25irs4 probably being inner root sheath specific keratins. Ka35 and Ka36 are, based on their exon-intron structure and expression characteristics, hair keratins. In contrast, K23 and K24 appear to be epithelial keratins associated with simple/glandular or stratified, non-cornified epithelia, respectively. A literature analysis coupled with the data presented here confirms that all of the 27 keratin genes found on this domain have been characterized at the transcriptional level. Together with K18, a type I keratin gene found on the type II keratin domain, this seems to be the entire complement of functional type I keratins in humans.

Expression of ΔNLef1 in mouse epidermis results in differentiation of hair follicles into squamous epidermal cysts and formation of skin tumours

To examine the consequences of repressing β-catenin/Lef1 signalling in mouse epidermis, we expressed a ΔNLef1 transgene, which lacks the β-catenin binding site, under the control of the keratin 14 promoter. No skin abnormalities were detected before the first postnatal hair cycle. However, from 6 weeks of age, mice underwent progressive hair loss which correlated with the development of dermal cysts. The cysts were derived from the base of the hair follicles and expressed morphological and molecular markers of interfollicular epidermis. Adult mice developed spontaneous skin tumours, most of which exhibited sebaceous differentiation, which could be indicative of an origin in the upper part of the hair follicle. The transgene continued to be expressed in the tumours and β-catenin signalling was still inhibited, as evidenced by absence of cyclin D1 expression. However, patched mRNA expression was upregulated, suggesting that the sonic hedgehog pathway might play a role in tumour formation. Based on our results and previous data on the consequences of activating β-catenin/Lef1 signalling in postnatal keratinocytes, we conclude that the level of β-catenin signalling determines whether keratinocytes differentiate into hair or interfollicular epidermis, and that perturbation of the pathway by overexpression of ΔNLef1 can lead to skin tumour formation.

A role for the 1A and L1 rod domain segments in head domain organization and function of intermediate filaments: structural analysis of trichocyte keratin

A dynamic model is proposed to explain how the 1A and linker L1 segments of the rod domain in intermediate filament (IF) proteins affect the head domain organization and vice versa. We have shown in oxidized trichocyte IF that the head domain sequences fold back over and interact with the rod domain. This phenomenon may occur widely in reduced IF as well. Its function may be to stabilize the 1A segments into a parallel two-stranded coiled coil or something closely similar. Under differing reversible conditions, such as altered states of IF assembly, or posttranslational modifications, such as phosphorylation etc., the head domains may no longer associate with the 1A segment. This could destabilize segment 1A and cause the two alpha-helical strands to separate. Linker L1 would thus act as a hinge and allow the heads to function over a wide lateral range. This model has been explored using the amino acid sequences of the head (N-terminal) domains of Type I and Type II trichocyte keratin intermediate filament chains. This has allowed several quasi-repeats to be identified. The secondary structure corresponding to these repeats has been predicted and a model has been produced for key elements of the Type II head domain. Extant disulfide cross-link data have been used as structural constraints. A model for the head domain structure predicts that a twisted beta-sheet region may wrap around the 1A segment and this may reversibly stabilize a coiled-coil conformation for 1A. The evidence in favor of the swinging head model for IF is discussed.

c-Myc activation in transgenic mouse epidermis results in mobilization of stem cells and differentiation of their progeny

BACKGROUND

The epidermis is maintained throughout adult life by pluripotential stem cells that give rise, via daughter cells of restricted self-renewal capacity and high differentiation probability (transit-amplifying cells), to interfollicular epidermis, hair follicles, and sebaceous glands. In vivo, transit-amplifying cells are actively cycling, whereas stem cells divide infrequently. Experiments with cultured human keratinocytes suggest that c-Myc promotes epidermal-stem cell differentiation. However, Myc is a potent oncogene that suppresses differentiation and causes reversible neoplasia when expressed in the differentiating epidermal layers of transgenic mice. To investigate the effects of c-Myc on the stem cell compartment in vivo, we targetted c-MycER to the basal layer of transgenic mouse epidermis.

RESULTS

The activation of c-Myc by the application of 4-hydroxy-tamoxifen caused progressive and irreversible changes in adult epidermis. Proliferation was stimulated, but interfollicular keratinocytes still underwent normal terminal differentiation. Hair follicles were abnormal, and sebaceous differentiation was stimulated at the expense of hair differentiation. The activation of c-Myc by a single application of 4-hydroxy-tamoxifen was as effective as continuous treatment in stimulating proliferation and sebocyte differentiation, and the c-Myc-induced phenotype continued to develop even after the grafting of treated skin to an untreated recipient.

CONCLUSIONS

We propose that transient activation of c-Myc drives keratinocytes from the stem to the transit-amplifying compartment and thereby stimulates proliferation and differentiation along the epidermal and sebaceous lineages. The ability, demonstrated here for the first time, to manipulate exit from the stem cell compartment in vivo will facilitate further investigations of the relationship between stem cells and cancer.

A unique type I keratin intermediate filament gene family is abundantly expressed in the inner root sheaths of sheep and human hair follicles

A unique type I keratin intermediate filament group, comprising three highly related proteins and expressed in the inner root sheath of hair follicles, has been identified in both sheep and human. The first members from these species are named oIRSa1 and hIRSa1 and each encodes a protein of 450 amino acids, with compositional characteristics intermediate between those of previously described hair keratin and epidermal cytokeratin type I intermediate filaments. Detection of abundant mRNA transcripts derived from the sheep and human genes by cRNA in situ hybridization only in the inner root sheath and not in the medulla concurs with the findings of earlier ultrastructural analyses that have reported intermediate filaments only in the inner root sheath. Clustering of the IRSa keratin genes is apparent in the genomes of both species. The three hIRSa genes, known to reside on human chromosome 17, are closely linked to three further type I keratin intermediate filament genes of unknown function. This new gene complex, contained almost entirely within a 156 kb BAC (hRPK.142_H_19), is likely to lie near the type I intermediate filament cytokeratin and hair keratin gene loci at 17q12-q21. A phylogenetic analysis including all known human type I intermediate filament cytokeratins, hHa keratins, hIRSa, and hIRSa-linked keratins suggests that origin of the IRSa keratin intermediate filament linkage group preceded origin of most of the epidermal cytokeratins and all hair keratins during emergence of the keratin intermediate filament genes.

Characterization of a 300 kbp region of human DNA containing the type II hair keratin gene domain

Screening of an arrayed human genomic P1 artificial chromosome DNA library by means of the polymerase chain reaction with a specific primer pair from the human type II hair keratin hHb5 yielded two P1 artificial chromosome clones covering approximately 300 kb of genomic DNA. The contig contained six type II hair keratin genes, hHb1-hHb6, and four keratin pseudogenes psihHbA-psihHbD. This hair keratin gene domain was flanked by type II epithelial keratins K6b/K6hf and K7, respectively. The keratin genes/pseudogene are 5-14 kbp in size with intergenic distances of 5-19 kbp of DNA and do not exhibit a single direction of transcription. With one exception, type II hair keratin genes are organized into nine exons and eight introns, with strictly conserved exon-intron boundaries. The functional hair keratin genes are grouped into two distinct subclusters near the extremities of the hair keratin gene domain. One subcluster encodes the highly related hair keratins hHb1, hHb3, and hHb6; The second cluster encodes the structurally less related hair keratins hHb2, hHb4, and hHb5. Reverse transcription-polymerase chain reaction shows that all hair keratin genes are expressed in the hair follicle. Pseudogene psihHbD is also transcriptionally expressed, albeit with alterations in splicing and frameshift mutations, leading to premature stop codons in the splice forms analyzed. Evolutionary tree analysis revealed a divergence of the type II hair keratin genes from the epithelial keratins, followed by their segregation into the members of the two subclusters over time. We assume that the approximately 200 kbp DNA domain contains the entire complement of human type II hair keratin genes.

Keratin expression in the normal nail unit: markers of regional differentiation

Differentiation within the nail unit was examined using a range of antikeratin monoclonal antibodies including the recently described antibody LHTric-1, specific to the acidic hair-type keratin Ha1. Keratinocytes of the nail matrix, nail bed and the digit pulp were characterized by different patterns of keratin expression. Nail matrix was the sole site of expression of Ha1, which colocalized in suprabasal matrix epidermis with epidermal keratins K1 and K10. Small amounts of K17 were found at the apex of the matrix in some cases. K6 and K16 were found where the epidermal surface folds forwards to become the ventral aspect of the proximal nail fold. The nail bed was distinguished by the absence of hair-type keratin Ha1 and the absence of markers of cornified epidermis and mucosal differentiation K1/K10 and K4/K13, respectively, while K6, K16 and K17 were detected. The basal keratin conformation marker, LH6, was expressed suprabasally throughout the nail bed. This complement of keratins exists in the nail bed in the absence of notable proliferative activity, and suggests a state of minimally developed differentiation which may be afforded by the physical or biological properties of the overlying nail. Keratins, K6, K16 and K17 were all found in the digit pulp in limited amounts, possibly in association with the epidermal component of the eccrine duct. The simple epithelial keratins, K7, K8 and K18, were found in small amounts in the specimens from younger individuals, mainly in epibasal cells of the apex of the matrix and in putative Merkel cells.

Two different mutations in the same codon of a type II hair keratin (hHb6) in patients with monilethrix

Monilethrix is an autosomal dominant hair disorder characterized by a beaded appearance of the hair due to periodic thinning of the shaft. The phenotype shows variable penetrance and results in hair fragility and patchy dystrophic alopecia. Mutations of the helix-encoding region in two hair-specific keratins (hHb1 and hHb6) have been identified. We have now investigated two unrelated monilethrix patients and identified two different novel heterozygous point mutations of the same codon in exon 7 of the hHb6 gene. Dystrophic hair samples obtained from both patients showed the typical beaded appearance by scanning electron microscopy. Both mutations affected the first base of codon 402 (glutamic acid). In patient A, a G to C transition occurred causing a glutamine substitution (GAG to CAG: E402Q) whereas in patient B, the transition was G to A yielding a lysine substitution (GAG to AAG: E402K). The sequence of the 1A helical regions of hHb1 and hHb6 as well as the 2B helical region of hHb1, were normal. Unaffected relatives did not have the hHb6 mutation and this codon was found to be highly conserved showing no alteration in the normal population (100 alleles examined). Both mutations disrupted a Taq I restriction site and restriction fragment length polymorphism analysis showed that a diagnostic 361 bp fragment could confirm the mutation. Thus, two new point mutations of the hair-specific keratin gene hHb6 have been identified in this genetic disease.

The catalog of human hair keratins. I. Expression of the nine type I members in the hair follicle

The human type I hair keratin subfamily comprises nine individual members, which can be subdivided into three groups. Group A (hHa1, hHa3-I, hHa3-II, hHa4) and B (hHa7, hHa8) each contains structurally related hair keratins, whereas group C members hHa2, hHa5, and hHa6 represent structurally rather unrelated hair keratins. Antibodies produced against these individual hair keratins, first analyzed for specificity by one- dimensional Western blots of total hair keratins, were used to establish the two-dimensional catalog of the human type I hair keratin subfamily. The catalog comprises two different series of type I hair keratins: a strongly expressed, Coomassie-stainable series containing hair keratins hHa1, hHa3-I/II, hHa4, and hHa5, and a weakly expressed, immunodetectable series harboring hHa2, hHa6 hHa7, and hHa8. In situ hybridization and immunohistochemical expression studies on scalp follicles show that two hair keratins, hHa2 and hHa5, define the early stage of hair differentiation, i.e. hHa5 expression in hair matrix and hHa5/hHa2 coexpression in the early hair cuticle cells. Whereas cuticular differentiation proceeds without the expression of further type I hair keratins, matrix cells embark on the cortical pathway by sequentially expressing hHa1, hHa3-I/II, and hHa4, which are supplemented by hHa6 at an advanced stage of cortical differentiation, and hHa8, which is expressed heterogeneously in cortex cells. Thus, six type I hair keratins are involved in the terminal differentiation of anagen hairs. The expression of hHa7 is conspicuously different from that of the other hair keratins in that it does not occur in the large anagen follicles of terminal scalp hairs but only in central cortex cells of the rare and small follicle type that gives rise to vellus hairs.

Expression of human hair keratin basic 1 in pilomatrixoma. A study of 128 cases

Hard keratins are expressed in normal hair and nails, and are characterized by a higher cysteine content than cytokeratins. Previous studies have suggested a coexpression of hard keratins and cytokeratins in pilomatrixoma, a benign follicular tumour which could originate from the hair matrix. Human hair keratin basic 1 (hHb1) is a newly characterized hair keratin which is expressed specifically by cortical cells of the normal hair shaft. A preliminary study has suggested that hHb1 could be expressed in pilomatrixoma. In order to confirm this hypothesis, we have studied a series of 128 pilomatrixomas by in situ hybridization, using a 35S-labelled hHb1-specific probe. The anti-sense probe was used as a negative control. Among these pilomatrixomas, six were early cases, 60 were classified into the intermediate stage (either fully developed or early regressive cases) and 62 were late regressive tumours made of shadow cells only. Forty-seven tumours showed hHb1 expression (37%), all being intermediate stage pilomatrixomas. The areas positively stained by the probe were band-like structures made of transitional cells only, which were very close to cells showing tricholemmal keratinization features. Neither the basophilic matrix cells nor the shadow cells expressed hHb1. Our results suggest that pilomatrixomas can differentiate towards cortical cells during their maturation process, as this keratin is specifically expressed in the cortex of the normal hair shaft. These data are consistent with previous studies which showed the expression of a hard keratin group in transitional cells by immunohistochemistry. The histogenesis of basophilic cells of pilomatrixoma is controversial, but it is likely that transitional cells represent an equivalent of the hair cortex.

A novel human type II cytokeratin, K6hf, specifically expressed in the companion layer of the hair follicle

In an attempt to identify new members of the human type II hair keratin family by means of 3'- and 5'-RACE methods and cDNA from anagen hair follicles, we detected a sequence that encoded a hitherto unknown type II cytokeratin. The novel cytokeratin comprises 251 amino acids and exhibits the highest sequence homology with K5. Comparative one- and two-dimensional western blots of keratins from anagen hair bulbs, containing or not containing the outer and inner root sheaths (ORS/IRS), and from footsole epidermis with an antibody against the new cytokeratin, revealed its comigration with K6 and its expression in the ORS/IRS complex. We have therefore named the new cytokeratin K6hf, to distinguish it from the various K6 isoforms and to indicate its expression in the hair follicle. Both in situ hybridization with a K6hf-specific cRNA probe and indirect immunofluorescence with the K6hf antibody showed that K6hf is exclusively expressed in the so-called "companion layer" of the hair follicle, a single layered band of flat and vertically oriented cells between the cuboidal ORS cells and the IRS that stretches from the lowermost bulb region to the isthmus of the follicle. Concomitant K17 and K16 expression studies showed that besides suprabasal ORS cells, these cytokeratins are sequentially expressed subsequent to K6hf in companion cells above the hair bulb. Our study confirms the view of a vertically oriented companion layer differentiation. The clearly delayed K17 and K16 expression relative to that of K6hf in companion cells most probably excludes these keratins as possible type I partners of K6hf and suggests the existence of a still unknown type I partner of its own. Thus, not only morphologically but also biochemically, the companion layer is different from the ORS and can therefore be regarded as an independent histologic compartment of the hair follicle.

Regulation of a hair follicle keratin intermediate filament gene promoter

During hair growth, cortical cells emerging from the proliferative follicle bulb rapidly undergo a differentiation program and synthesise large amounts of hair keratin proteins. To identify some of the controls that specify expression of hair genes we have defined the minimal promoter of the wool keratin intermediate filament gene K2.10. The region of this gene spanning nucleotides −350 to +53 was sufficient to direct expression of the lacZ gene to the follicle cortex of transgenic mice but deletion of nucleotides −350 to −150 led to a complete loss of promoter activity. When a four base substitution mutation was introduced into the minimal functional promoter at the binding site for lymphoid enhancer factor 1 (LEF-1), promoter activity in transgenic mice was decreased but specificity was not affected. To investigate the interaction of trans-acting factors within the minimal K2.10 promoter we performed DNase I footprinting analyses and electrophoretic mobility shift assays. In addition to LEF-1, Sp1, AP2-like and NF1-like proteins bound to the promoter. The Sp1 and AP2-like proteins bound sequences flanking the LEF-1 binding site whereas the NF1-like proteins bound closer to the transcription start site. We conclude that the LEF-1 binding site is an enhancer element of the K2.10 promoter in the hair follicle cortex and that factors other than LEF-1 regulate promoter tissue- and differentiation-specificity.

Specialized keratin expression pattern in human ridged skin as an adaptation to high physical stress

We have analysed the expression of keratins in the epidermis of normal human palm and sole skin (ridged skin) using immunohistochemistry and in situ hybridization. The epidermis of human ridged skin expresses a more complex pattern of keratins than thin skin, which is probably due to the greater stress that ridged skin has to withstand. In addition to keratin K9, we document specific expression patterns of keratins K6, K16 and K17 which are suggestive of regional adaptations of this epidermis to a high cell turnover rate. In particular, the sequestered location of nests of K17-positive cells at the bottom of the deep primary epidermal ridges supports the notion of functional heterogeneity of basal cells and suggests that the K17-positive sites may include stem cells. Expression of K6 and K16 in some basal and most suprabasal keratinocytes is compatible with a constitutively high proliferative activity of normal ridged epidermis, but may also reflect different physical properties of the suprabasal cells, in contrast with regions expressing K9. The distinct labelling patterns observed in primary and secondary epidermal ridges as well as epidermal layers above dermal papillae suggest the existence of local microenvironmental niches leading to differences in keratinocyte differentiation.

The two functional keratin 6 genes of mouse are differentially regulated and evolved independently from their human orthologs

The type II keratin 6 (K6) features a complex expression pattern, with a constitutive component in a subset of stratified epithelia and an inducible component following injury and other types of acute challenges. Multiple genes encoding highly related K6 isoforms have been described for human and bovine, a unique feature among mammalian keratin genes. Here we report on the cloning and characterization of two functional genes and their cDNAs encoding the K6 isoforms in mouse and two related pseudogenes. A systematic comparison of the mouse and human K6 genes suggests that they evolved independently after these species diverged. The mK6alpha and mK6beta genes are organized in tandem with the same transcriptional orientation in the mouse genome. Similar to the human isoforms, the coding sequences for mK6alpha and mK6beta isoforms show approximately 95% identity. The two mouse K6 genes are differentially regulated at the mRNA level in several stratified epithelia. The mK6alpha isoform mRNA clearly predominates in intact trunk skin of adult mice, where it is restricted to the outer root sheath of hair follicles. Both mRNAs are induced in epidermis and proximal hair follicles as early as 1 h following acute injury or topical application of phorbol esters and subsequently increase to a comparable extent but with different kinetics. These novel findings have important implications for the evolution, regulation, and function of K6 genes in mammalian species.

Characterization of a 190-kilobase pair domain of human type I hair keratin genes

Polymerase chain reaction-based screening of an arrayed human P1 artificial chromosome (PAC) library using primer pairs specific for the human type I hair keratins hHa3-II or hHa6, led to the isolation of two PAC clones, which covered 190 kilobase pairs (kbp) of genomic DNA and contained nine human type I hair keratin genes, one transcribed hair keratin pseudogene, as well as one orphan exon. The hair keratin genes are 4-7 kbp in size, exhibit intergenic distances of 5-8 kbp, and display the same direction of transcription. With one exception, all hair keratin genes are organized into 7 exons and 6 positionally conserved introns. On the basis of sequence homologies, the genes can be grouped into three subclusters of tandemly arranged genes. One subcluster harbors the highly related genes hHa1, hHa3-I, hHa3-II, and hHa4. A second subcluster of highly related genes comprises the novel genes hHa7 and hHa8, as well as pseudogene PsihHaA, while the structurally less related genes hHa6, hHa5, and hHa2 are constituents of the third subcluster. As shown by reverse transcription-polymerase chain reaction, all hair keratin genes, including the pseudogene, are expressed in the human hair follicle. The transcribed pseudogene PsihHaA contains a premature stop codon in exon 4 and exhibits aberrant pre-mRNA splicing. Evolutionary tree construction reveals an early divergence of hair keratin genes from cytokeratin genes, followed by the segregation of the genes into the three subclusters. We suspect that the 190-kbp domain contains the entire complement of human type I hair keratin genes.

Basonuclin as a cell marker in the formation and cycling of the murine hair follicle

Basonuclin, a zinc-finger protein, is found in stratified squamous epithelia and hair follicles. In the basal keratinocytes of mouse epidermis, basonuclin is detected mainly in the cytoplasm. During the development of murine hair follicles, this protein concentrates in the nuclei of the basal cells that form the primary hair germs. As follicle morphogenesis proceeds, the epithelial cells possessing nuclear basonuclin invade the dermis and surround the follicular papilla. In mature anagen follicles, nuclear basonuclin is principally restricted to the basal layers of the outer root sheath and bulbar matrix; these regions are known to contain cells capable of proliferation, and to lack the features of terminal differentiation. During catagen, the compartment of cells containing nuclear basonuclin regresses, and in telogen, only a small number of these cells remain to form the secondary hair germ at the follicle base. During the next anagen, this basonuclin-containing population expands and regenerates the hair-producing portion of the follicle. It is concluded that in all hair cycles, the transient segment of the follicle originates from germinative cells possessing nuclear basonuclin.

Characterization and chromosomal localization of human hair-specific keratin genes and comparative expression during the hair growth cycle

During anagen, cell proliferation in the germinative matrix of the hair follicle gives rise to the fiber and inner root sheath. The hair fiber is constructed from structural proteins belonging to four multigene families: keratin intermediate filaments, high-sulfur matrix proteins, ultra high-sulfur matrix proteins, and high glycine-tyrosine proteins. Several hair-specific keratin intermediate filament proteins have been characterized, and all have relatively cysteine-rich N- and C-terminal domains, a specialization that allows extensive disulfide cross-linking to matrix proteins. We have cloned two complete type II hair-specific keratin genes (ghHb1 and ghHb6). Both genes have nine exons and eight introns spanning about 7 kb and lying about 10 kb apart. The structure of both genes is highly conserved in the regions that encode the central rod domain but differs considerably in the C-terminal coding and noncoding sequences, although some conservation of introns does exist. These genes have been localized to the type II keratin cluster on chromosome 12q13 by fluorescence in situ hybridization. They, and their type I partner ghHa1, are expressed in differentiating hair cortical cells during anagen. In cultured follicles, ghHa1 expression declined in cortical cells and was no longer visible after 6 d, whereas the basal epidermal keratin hK14 appeared in the regressing matrix. The transition from anagen to telogen is marked by downregulation of hair cortical specific keratins and the appearance of hK14 in the epithelial sac to which the telogen hair fiber is anchored. Further studies of the regulation of these genes will improve our understanding of the cyclical molecular changes that occur as the hair follicle grows, regresses, and rests.

A splice site mutation in the gene of the human type I hair keratin hHa1 results in the expression of a tailless keratin isoform

In this study, we have elucidated the molecular mechanisms underlying the expression of an acidic 41-kDa protein inherited as an autosomal dominant trait of the hair keratin pattern of about 5% of the human population. We show that this protein is a size variant of the large type I hair cortex keratin hHa1 due to a genetic polymorphism in the hHa1 gene. We detected a G-A substitution in the 5' splice site of intron 6 of the hHa1 gene, which segregates with the 41-kDa protein phenotype in two pedigrees and is responsible for the formation of an abnormally spliced hHa1 mRNA species. The use of an alternative 5' splice site leads to the retention of 41 nucleotides of the initial intron 6 sequences in the mature transcript. The open reading frame of the aberrant mRNA creates a premature stop codon immediately downstream of the mutation site. The resulting hHa1 protein variant, hHa1-t, is about 6-kDa smaller than the 47-kDa hHa1 hair keratin and lacks the complete nonhelical tail domain. We show that the tailless hHa1-t is functional, since both recombinant hHa1 and hHa1-t form identical keratin intermediate filaments when assembled in vitro with a type II hair keratin partner. This finding confirms the view of a noninvolvement of the keratin tail domain in filament assembly and explains the lack of a pathological hair phenotype in hHa1-t positive individuals.

Mutations in the hair cortex keratin hHb6 cause the inherited hair disease monilethrix

Pathogenic mutations in a large number of human epithelial keratins have been well characterized. However, analogous mutations in the hard alpha-keratins of hair and nail have not yet been described. Monilethrix is a rare autosomal dominant hair defect with variable expression. Hairs from affected individuals show a beaded structure of alternating elliptical nodes and constrictions (internodes). These internodes exhibit a high prospensity to weathering and fracture. Strong evidence that trichocyte keratin defects might underlie this hair disorder was provided by genetic linkage analyses that mapped this disease to the type-II keratin gene cluster on 12q13. All affected individuals from a four-generation British family with monilethrix, previously linked to the type-II keratin gene cluster, as well as three unrelated single monilethrix patients, exhibited a heterozygous point mutation in the gene for type-II hair cortex keratin hHb6, leading to lysine substitution of a highly conserved glutamic acid residue in the helix termination motif (Glu 410 Lys). In a three-generation French family with monilethrix of a milder and variable phenotype, we detected another heterozygous point mutation in the same glutamic acid codon of hHb6, which resulted in a conservative aspartic acid substitution (Glu 410 Asp). These mutations provide the first direct evidence for involvement of hair keratins in hair disease.

Sequences and differential expression of three novel human type-II hair keratins

As part of a program designed to characterize human hair keratin genes and their expression, we present the cDNA sequences and deduced amino acid sequences of three type-II hair keratins hHb3, hHb5, and hHb6, which by virtue of their amino acid homologies are the orthologs of the previously described sheep wool keratins, K2.10, K2.12, and K.211 [29]. Amino acid sequences comparisons of these keratins, including the previously characterized human K2.9 ortholog hHb1, show extreme conservation not only in the alpha-helices but also in the aminoterminal and proximal carboxyterminal domains. They also demonstrate higher sequence relationships between hHb1, hHb3, and hHb6 as compared to hHb5, which exhibits chain-specific sequences in both the head and tail domains. In situ hybridization studies using specific 3'-probes for the four type-II hair keratins reveal sequential patterns of gene expression in human anagen follicles. Remarkably the onset of hHb5 mRNA synthesis occurs immediately above a small population of matrix cells at the base of the hair bulb and the trichocytes lining the dermal papilla. hHb5 mRNA synthesis extends upward through the matrix and ends in the lower part of the cortex of the hair shaft. In contrast, both hHb1 and hHb3 mRNA synthesis begins simultaneously in the cortex 10-15 cell layers above the apex of the dermal papilla, thus partially overlapping that of hHb5 but continuing to a point well beyond hHb5 in the upper cortex. Synthesis of hHb6 mRNA starts slightly higher than either hHb1 or hHb3 mRNA and proceeds much farther up into the keratogenous zone of the hair shaft. Our study demonstrates that the differentiation of human hair in terms of hair keratin expression begins much earlier than previously assumed, i.e. in lower matrix cells of the hair bulb. This early phase of hair differentiation is followed by a late cortical phase of terminal differentiation which comprises at least three type-II hair keratins in the zone of elongation and the keratogenous zone of the hair shaft.