Lanceolate hair (lah): a recessive mouse mutation with alopecia and abnormal hair

Phenotyping mice with skin, hair, or nail abnormalities: A systematic approach and methodologies from simple to complex

The skin and adnexa can be difficult to interpret because they change dramatically with the hair cycle throughout life. However, a variety of methods are commonly available to collect skin and perform assays that can be useful for figuring out morphological and molecular changes. This overview provides information on basic approaches to evaluate skin and its molecular phenotype, with references for more detail, and interpretation of results on the skin and adnexa in the mouse. These approaches range from mouse genetic nomenclature, setting up a cutaneous phenotyping study, skin grafts, hair follicle reconstitution, wax stripping, electron microscopy, and Köbner reaction to very specific approaches such as lipid and protein analyses on a large scale.

Independent DSG4 frameshift variants in cats with hair shaft dystrophy

Investigations of hereditary phenotypes in spontaneous mutants may help to better understand the physiological functions of the altered genes. We investigated two unrelated domestic shorthair cats with bulbous swellings of the hair shafts. The clinical, histopathological, and ultrastructural features were similar to those in mice with lanceolate hair phenotype caused by loss-of-function variants in Dsg4 encoding desmoglein 4. We sequenced the genomes from both affected cats and compared the data of each affected cat to 61 control genomes. A search for private homozygous variants in the DSG4 candidate gene revealed independent frameshift variants in each case, c.76del or p.Ile26fsLeu*4 in case no. 1 and c.1777del or p.His593Thrfs*23 in case no. 2. DSG4 is a transmembrane glycoprotein located primarily in the extracellular part of desmosomes, a complex of adhesion molecules responsible for connecting the keratin intermediate filaments of neighbouring epithelial cells. Desmosomes are essential for normal hair shaft formation. Both identified DSG4 variants in the affected cats lead to premature stop codons and truncate major parts of the open-reading frame. We assume that this leads to a complete loss of DSG4 function, resulting in an incorrect formation of the desmosomes and causing the development of defective hair shafts. Together with the knowledge on the effects of DSG4 variants in other species, our data suggest that the identified DSG4 variants cause the hair shaft dystrophy. To the best of our knowledge, this study represents the first report of pathogenic DSG4 variants in domestic animals.

Hair follicle dystrophy in a litter of domestic cats resembling lanceolate hair mutant mice

BACKGROUND

A new congenital hair-shaft abnormality resembling the lanceolate hair phenotype of rodents is described in a litter of four domestic short hair (DSH) cats. Data relating to hair shaft and follicle disorders remain scarce in veterinary medicine.

OBJECTIVES

To describe and compare structural abnormalities in these cats with other hair dystrophies in cats and other mammals.

ANIMALS

A DSH cat litter with progressive noninflammatory alopecia.

METHODS AND MATERIALS

Histopathological evaluation, scanning and transmission electron microscopy, and X-ray based element analysis defined the hair and skin changes in cats born with alopecia. Findings were compared to archival data from normal cats and lanceolate hair (Dsg4lahJ ) and Keratin 75 (Krt75tm1Der ) mutant mice.

RESULTS

Light and scanning electron microscopy of the hairs revealed lance- or spear-head shaped defects of the hair tip. Histological findings were swollen hair shafts, initially above the hair bulb matrix and later found in the distal parts of the telogen hair follicles, similar to those observed in Dsg4lahJ Krt75tm1Der mutant mice. Transmission electron microscopy of the hair shaft and hair follicles showed a loss in the normal structure of the guard hairs in the alopecic cats. There was a statistically significant decrease in sulfur content just below the defects in the hair shafts (trichothiodystrophy).

CONCLUSION AND CLINICAL IMPORTANCE

A rare form of congenital alopecia resulting in follicular dystrophy is described in cats which is similar to hair follicle and hair-shaft changes reported in several mutant mouse strains with single gene mutations in adhesion molecules or keratin genes.

Transgenic Kallikrein 14 Mice Display Major Hair Shaft Defects Associated with Desmoglein 3 and 4 Degradation, Abnormal Epidermal Differentiation, and IL-36 Signature

Netherton syndrome is a rare autosomal recessive skin disease caused by loss-of-function mutations in SPINK5 encoding LEKTI protein that results in unopposed activity of epidermal kallikrein-related peptidases (KLKs), mainly KLK5, KLK7, and KLK14. Although the function of KLK5 and KLK7 has been previously studied, the role of KLK14 in skin homeostasis and its contribution to Netherton syndrome pathogenesis remains unknown. We generated a transgenic murine model overexpressing human KLK14 (TghKLK14) in stratum granulosum. TghKLK14 mice revealed increased proteolytic activity in the granular layers and in hair follicles. Their hair did not grow and displayed major defects with hyperplastic hair follicles when hKLK14 was overexpressed. TghKLK14 mice displayed abnormal epidermal hyperproliferation and differentiation. Ultrastructural analysis revealed cell separation in the hair cortex and increased thickness of Huxley's layer. Desmoglein (Dsg) 2 staining was increased, whereas Dsg3 and Dsg4 were markedly reduced. In vitro studies showed that hKLK14 directly cleaves recombinant human DSG3 and recombinant human DSG4, suggesting that their degradation contributes to hair abnormalities. Their skin showed an inflammatory signature, with enhanced expression of IL-36 family members and their downstream targets involved in innate immunity. This in vivo study identifies KLK14 as an important contributor to hair abnormalities and skin inflammation seen in Netherton syndrome.

KLK5 and KLK7 Ablation Fully Rescues Lethality of Netherton Syndrome-Like Phenotype

Netherton syndrome (NS) is a severe skin disease caused by the loss of protease inhibitor LEKTI, which leads to the dysregulation of epidermal proteases and severe skin-barrier defects. KLK5 was proposed as a major protease in NS pathology, however its inactivation is not sufficient to rescue the lethal phenotype of LEKTI-deficient mice. In this study, we further elucidated the in vivo roles of the epidermal proteases in NS using a set of mouse models individually or simultaneously deficient for KLK5 and KLK7 on the genetic background of a novel NS-mouse model. We show that although the ablation of KLK5 or KLK7 is not sufficient to rescue the lethal effect of LEKTI-deficiency simultaneous deficiency of both KLKs completely rescues the epidermal barrier and the postnatal lethality allowing mice to reach adulthood with fully functional skin and normal hair growth. We report that not only KLK5 but also KLK7 plays an important role in the inflammation and defective differentiation in NS and KLK7 activity is not solely dependent on activation by KLK5. Altogether, these findings show that unregulated activities of KLK5 and KLK7 are responsible for NS development and both proteases should become targets for NS therapy.

Immune status, strain background, and anatomic site of inoculation affect mouse papillomavirus (MmuPV1) induction of exophytic papillomas or endophytic trichoblastomas

Papillomaviruses (PVs) induce papillomas, premalignant lesions, and carcinomas in a wide variety of species. PVs are classified first based on their host and tissue tropism and then their genomic diversities. A laboratory mouse papillomavirus, MmuPV1 (formerly MusPV), was horizontally transmitted within an inbred colony of NMRI-Foxn1(nu)/Foxn1nu (nude; T cell deficient) mice of an unknown period of time. A ground-up, filtered papilloma inoculum was not capable of infecting C57BL/6J wild-type mice; however, immunocompetent, alopecic, S/RV/Cri-ba/ba (bare) mice developed small papillomas at injection sites that regressed. NMRI-Foxn1(nu) and B6.Cg-Foxn1(nu), but not NU/J-Foxn1(nu), mice were susceptible to MmuPV1 infection. B6 congenic strains, but not other congenic strains carrying the same allelic mutations, lacking B- and T-cells, but not B-cells alone, were susceptible to infection, indicating that mouse strain and T-cell deficiency are critical to tumor formation. Lesions initially observed were exophytic papillomas around the muzzle, exophytic papillomas on the tail, and condylomas of the vaginal lining which could be induced by separate scarification or simultaneous scarification of MmuPV1 at all four sites. On the dorsal skin, locally invasive, poorly differentiated tumors developed with features similar to human trichoblastomas. Transcriptome analysis revealed significant differences between the normal skin in these anatomic sites and in papillomas versus trichoblastomas. The primarily dysregulated genes involved molecular pathways associated with cancer, cellular development, cellular growth and proliferation, cell morphology, and connective tissue development and function. Although trichoepitheliomas are benign, aggressive tumors, few of the genes commonly associated with basal cell carcinoma or squamous cells carcinoma were highly dysregulated.

In VivoFunction of Desmosomes

Desmosomes are morphologically and biochemically defined cell-cell junctions that are required for maintaining the mechanical integrity of skin and the heart in adult mammals. Furthermore, since mice with null mutations in desmosomal plaque proteins (plakoglobin and desmoplakin) die in utero, it is also evident that desmosomes are indispensable for normal embryonic development. This review focuses on the role of desmosomes in vivo. We will summarize the effects of mutations in desmosomal genes on pre- and post-embryonic development of mouse and man and discuss recent findings relating to the specific role of desmosomal cadherins in skin differentiation and homeostasis.

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.

Mutations in the desmoglein 4 gene underlie localized autosomal recessive hypotrichosis with monilethrix hairs and congenital scalp erosions

Localized autosomal recessive hypotrichosis (LAH) is a recently defined disorder characterized by fragile, short, sparse hairs on the scalp, trunk, and extremities. Mutations in desmoglein 4 (DSG4), a novel member of the desmosomal cadherin family that is expressed in the hair follicle as well as the suprabasal epidermis, have been found to underlie LAH. Thus far, the allelic series includes a recurrent intragenic deletion identified in affected Pakastani kindreds and a missense mutation detected in an Iraqi family. We report three siblings of Iraqi and Iranian origin with LAH that presented with congenital scalp erosions and monilethrix-like hairs, features that have not been previously described in this disorder. Follicular hyperkeratotic papules and marked pruritus were also prominent clinical findings. Novel compound heterozygous DSG4 mutations, including a splice-site mutation and a missense mutation that disrupts a conserved calcium-binding site in the extracellular (EC)2-EC3 interface, were found to underlie the disease in this family. These observations broaden the phenotypic and genotypic spectrum of LAH, further illustrating the consequences of DSG4 dysfunction on epidermal and hair shaft integrity.

More than one gene involved in monilethrix: intracellular but also extracellular players

Monilethrix, an autosomal dominant human hair disorder, is caused by mutations in three type II hair cortex keratins. Rare cases of the disease with non-vertical transmission have now been found to overlap with localized autosomal recessive hypotrichosis. The underlying gene, desmoglein 4 (DSG4), belongs to the desmosomal cadherin superfamily and is also expressed in the cortex of the hair follicle.

Mutations in the Desmoglein 4 Gene Are Associated with Monilethrix-like Congenital Hypotrichosis

The gene encoding human desmoglein 4 (DSG4) was recently cloned, and a mutation in this gene has been reported in several consanguineous Pakistani families affected with localized autosomal recessive hypotrichosis (LAH). In addition, various mutations in the Dsg4 gene have been identified in animal models of hypotrichosis that share a characteristic phenotype called "lanceolate hair". To date, the features of the hair-shaft anomaly in patients with LAH have not been well described. We report a Japanese patient affected with congenital hypotrichosis that was originally diagnosed as monilethrix because she had a hair-shaft abnormality that resembled moniliform hair. However, no mutations were found in the type II hair keratin genes, hHb1, hHb3, and hHb6, whose mutations cause monilethrix. Instead, we identified novel compound heterozygous mutations in the DSG4 gene of our patient. On the maternal allele is a novel S192P transition within the extracellular cadherin II domain of DSG4; on the paternal allele is a novel 2039insT mutation leading to the generation of unstable transcripts. Here we present the observation that mutations in the DSG4 gene can cause monilethrix-like congenital hypotrichosis. Based on our findings, we propose that LAH and monilethrix could overlap.

Intragenic deletion in the Desmoglein 4 gene underlies the skin phenotype in the Iffa Credo "hairless" rat

The Iffa Credo (IC) "hairless" rat is an autosomal recessive hypotrichotic animal model actively used in pharmacological and dermatological studies. Although the molecular basis of the IC rat phenotype was never defined, the designation "hr/hr" (hairless) has been used for this rat mutation. Despite the observation that IC rats share many phenotypic similarities with Charles River (CR) 'hairless rats', crossbreeding between CR and IC rats indicated that these mutations are not allelic, and moreover, genetic analysis of both CR and IC hairless mutant rats showed no mutations in the hr gene. Here, we present a detailed analysis of the skin phenotype in the IC rat. While the initial stages of hair follicle (HF) morphogenesis reveal no significant abnormalities, the subsequent processes of inner root sheath and hair shaft formation are severely disturbed due to impaired proliferation in the hair matrix and abnormal differentiation in the precortex zone. This results in significant reduction of hair bulb volume, and the formation of dysmorphic "blebbed" hair shafts lacking medullar structure and resembling "lanceolate" hairs. Based on the presence of lance-head hairs typical of rodent lanceolate mutants, we performed molecular analysis of the desmoglein 4 gene and found a large intragenic deletion encompassing nine exons of the gene. This finding, together with specific morphological features of skin and hairs, confirms that the IC rat is allelic with the lanceolate hair (lah) mutations in mice and rats. Our results elucidate the genetic and morphological basis of the IC rat mutation, thus providing a new model to study molecular mechanisms of hair growth control.

A spontaneous mutation in the desmoglein 4 gene underlies hypotrichosis in a new lanceolate hair rat model

A recessive hairless mutation arose spontaneously in a congenic line of spontaneously hypertensive rats SHR.BN-(D1Mit3-Igf2)/Ipcv. The mutant rats develop generalized alopecia except for partial hair growth on their heads. Affected animals of the congenic line were crossed with LEW rats and randomly bred for several generations. A genome scan in 74 affected and 75 unaffected offspring localized the mutant gene on rat chromosome 18p12, near the marker D18Rat107, which is closely linked to the desmosomal cadherin gene cluster, syntenic to mouse chromosome 18 and human chromosome 18q12. Recently, the mouse and rat phenotypes lah/lah (lanceolate hair) and lah(J)/lah(J)(lanceolate hair-J) were found to be caused by mutations in the desmoglein 4 (Dsg4) gene. Direct sequencing of the Dsg4 gene in the SHR revealed a homozygous C-to-T transition generating a premature termination codon within exon 8 in the affected animals. Further studies on the skin histology in affected rats demonstrated features consistent with a lanceolate hair mutation, providing further support for the crucial role of desmoglein 4 in hair shaft differentiation.

The lanceolate hair rat phenotype results from a missense mutation in a calcium coordinating site of the desmoglein 4 gene

Desmosomal cadherins are essential cell adhesion molecules present throughout the epidermis and other organs, whose major function is to provide mechanical integrity and stability to epithelial cells in a wide variety of tissues. We recently identified a novel desmoglein family member, Desmoglein 4 (Dsg4), using a positional cloning approach in two families with localized autosomal recessive hypotrichosis (LAH) and in the lanceolate hair (lah) mouse. In this study, we report cloning and identification of the rat Dsg4 gene, in which we discovered a missense mutation in a naturally occurring lanceolate hair (lah) rat mutant. Phenotypic analysis of lah/lah mutant rats revealed a striking hair shaft defect with the appearance of a lance head within defective hair shafts. The mutation disrupts a critical calcium binding site bridging the second and third extracellular domains of Dsg4, likely disrupting extracellular interactions of the protein.

Desmoglein 4 in hair follicle differentiation and epidermal adhesion: evidence from inherited hypotrichosis and acquired pemphigus vulgaris

Cell adhesion and communication are interdependent aspects of cell behavior that are critical for morphogenesis and tissue architecture. In the skin, epidermal adhesion is mediated in part by specialized cell-cell junctions known as desmosomes, which are characterized by the presence of desmosomal cadherins, known as desmogleins and desmocollins. We identified a cadherin family member, desmoglein 4, which is expressed in the suprabasal epidermis and hair follicle. The essential role of desmoglein 4 in skin was established by identifying mutations in families with inherited hypotrichosis, as well as in the lanceolate hair mouse. We also show that DSG4 is an autoantigen in pemphigus vulgaris. Characterization of the phenotype of naturally occurring mutant mice revealed disruption of desmosomal adhesion and perturbations in keratinocyte behavior. We provide evidence that desmoglein 4 is a key mediator of keratinocyte cell adhesion in the hair follicle, where it coordinates the transition from proliferation to differentiation.

Mutant laboratory mice with abnormalities in hair follicle morphogenesis, cycling, and/or structure: annotated tables

Numerous transgenic, targeted mutagenesis (so-called knockouts), conditional (so-called "gene switch") and spontaneous mutant mice develop abnormal hair phenotypes. The number of mice that exhibit such abnormalities is increasing exponentially as genetic engineering methods become routine. Since defined abnormalities in hair follicle morphogenesis, cycling and/or structure in such mutant mice provide important clues to the as yet poorly understood functional roles of many gene products, it is useful to summarize and classify these mutant mice according to their hair phenotype. This review provides a corresponding, annotated table of mutant mice with hair abnormalities, classifying the latter into 6 categories, 1) abnormally low number of hair follicles, 2) disorders of hair morphogenesis, 3) of hair follicle cycling, 4) of hair follicle structure 5) of sebaceous gland structure, and 6) hair growth disorders as a consequence of immunological abnormalities. This annotated table should serve as a useful source of reference for anyone who is interested in the molecular controls of hair growth, for investigators who are looking for mouse models to explore or compare the functional activities of their gene of interest, and for comparing the hair phenotype of newly generated mouse mutants with existing ones.

The spectrum of pathogenic mutations in SPINK5 in 19 families with Netherton syndrome: implications for mutation detection and first case of prenatal diagnosis

The Comèl-Netherton syndrome is an autosomal recessive multisystemic disorder characterized by localized or generalized congenital ichthyosis, hair shaft abnormalities, immune deficiency, and markedly elevated IgE levels. Life-threatening complications during infancy include temperature and electrolyte imbalance, recurrent infections, and failure to thrive. To study the clinical presentations of the Comèl-Netherton syndrome and its molecular cause, we ascertained 19 unrelated families of various ethnic backgrounds. Results of initial linkage studies mapped the Comèl-Netherton syndrome in 12 multiplex families to a 12 cM interval on 5q32, thus confirming genetic homogeneity of Comèl-Netherton syndrome across families of different origins. The Comèl-Netherton syndrome region harbors the SPINK5 gene, which encodes a multidomain serine protease inhibitor (LEKTI) predominantly expressed in epithelial and lymphoid tissues. Recently, recessive mutations in SPINK5 were identified in several Comèl-Netherton syndrome patients from consanguineous families. We used heteroduplex analysis followed by direct DNA sequencing to screen all 33 exons and flanking intronic sequences of SPINK5 in the affected individuals of our cohort. Mutation analysis revealed 17 distinct mutations, 15 of which were novel, segregating in 14 Comèl-Netherton syndrome families. The nucleotide changes included four non-sense mutations, eight small deletions or insertions leading to frameshift, and five splice site defects, all of which are expected to result in premature terminated or altered translation of SPINK5. Almost half of the mutations clustered between exons 2 and 8, including two recurrent mutations. Genotype-phenotype correlations suggested that homozygous nucleotide changes resulting in early truncation of LEKT1 are associated with a severe phenotype. For the first time, we used molecular data to perform prenatal testing, thus demonstrating the feasibility of molecular diagnosis in the Comèl-Netherton syndrome.

Characterization of the desmosomal cadherin gene family: genomic organization of two desmoglein genes on human chromosome 18q12

The human desmoglein genes, desmogleins 1--3, are members of the desmosomal cadherin superfamily, and encode critical components of the desmosome. These genes are tightly clustered within 150--200 kb of chromosome 18q12.1 and represent excellent candidate genes for genetic disorders of the epidermis linked to this region of the genome. Mutations in desmoglein 1 have already been implicated in the genetic disorder striate palmoplantar keratoderma. Similarly, a mutation in desmoglein 3 underlies the balding mouse phenotype, although no human mutations in desmoglein 3 have been identified to date. In this study, we have characterized the genomic organization of two of the three desmoglein genes mapped to chromosome 18q12. Comparison of their exon-intron structure reveals the high level of evolutionary conservation expected from these related genes. The identification of the genomic structure of the desmoglein genes will facilitate mutation detection in genodermatoses with desmosomal abnormalities resulting from underlying defects in these genes.

Structural analysis reflects the evolutionary relationship between the human desmocollin gene family members

Desmocollins, members of the desmosomal cadherin family, are known to play an important role in desmosomal intercellular adhesion. The human desmosomal cadherin cluster is located on chromosome 18q12, and consists of three desmoglein and three desmocollin genes. The cDNAs of all six of these genes have been cloned and sequenced, however, the exon-intron organization was reported for only one human desmocollin gene, DSC2. We elucidated the exon-intron structures of the DSC1 and DSC3 genes using PCR amplification of genomic DNA and direct sequencing of BAC clones. The results suggest a strong evolutionary conservation between the genomic organization of the desmocollin genes.

Hair on a gene string: recent advances in understanding the molecular genetics of hair loss

The hair follicle is finally, after remaining a mystery for many years, beginning to yield some of its molecular secrets. The past decade has seen unprecedented and ever quickening advances in understanding the molecular genetics of the many single gene disorders, which have alopecia as a major feature. This article reviews recent novel clinical and experimental observations, which have shed new light on the basic molecular mechanisms underlying hair morphogenesis, differentiation, keratinization and cycling. We consider recent progress in understanding structural hair defects and complex traits and consider where future developments are likely to occur.

Construction of a long-range YAC physical map spanning the 10-cM region between the markers D18Mit109 and D18Mit68 on mouse proximal chromosome 18

Four yeast artificial chromosome (YAC) contigs, physically approximately 8 Mb, have been constructed spanning a 10-cM region on mouse proximal chromosome 18 and include the sites of 21 known genes, including those near the twirler (Tw) locus and the recently isolated Niemann-Pick type C1 (npc1) gene, formerly designated as the spm locus. This physical map consists of 49 YAC clones that cover roughly 15% of the chromosome. The physical order of 38 microsatellite sequence-tagged sites (STSs) could be assembled and confirmed based on their presence or absence in individual YACs, from proximal D18Mit109 through distal D18Mit68. These YACs provide an important resource for the further characterization and identification of known and unknown genes. The physical map has been integrated with our previously published genetic linkage map and showed an average genetic to physical distance of cM/Mb > 1.1.

Localization of the Netherton syndrome gene to chromosome 5q32, by linkage analysis and homozygosity mapping

Netherton syndrome (NS [MIM 256500]) is a rare and severe autosomal recessive disorder characterized by congenital ichthyosis, a specific hair-shaft defect (trichorrhexis invaginata), and atopic manifestations. Infants with this syndrome often fail to thrive; life-threatening complications result in high postnatal mortality. We report the assignment of the NS gene to chromosome 5q32, by linkage analysis and homozygosity mapping in 20 families affected with NS. Significant evidence for linkage (maximum multipoint LOD score 10.11) between markers D5S2017 and D5S413 was obtained, with no evidence for locus heterogeneity. Analysis of critical recombinants mapped the NS locus between markers D5S463 and D5S2013, within an <3.5-cM genetic interval. The NS locus is telomeric to the cytokine gene cluster in 5q31. The five known genes encoding casein kinase Ialpha, the alpha subunit of retinal rod cGMP phosphodiesterase, the regulator of mitotic-spindle assembly, adrenergic receptor beta2, and the diastrophic dysplasia sulfate-transporter gene, as well as the 38 expressed-sequence tags mapped within the critical region, are not obvious candidates. Our study is the first step toward the positional cloning of the NS gene. This finding promises a better understanding of the molecular mechanisms that control epidermal differentiation and immunity.

Vesicle formation and follicular root sheath separation in mice homozygous for deleterious alleles at the balding (bal) locus

The balding (bal) mutation of the mouse is an autosomal recessive mutation that causes alopecia and immunologic anomalies. A new allele was identified by allelism testing after using an interspecific backcross to localize the mutation to the centromeric end of mouse chromosome 18. We investigated the skin and hair histologic lesions of two alleles (bal(J) and bal(Pas)) at this locus and analyzed the expression of several keratinocyte markers and the production of autoantibodies by immunofluorescence on frozen skin sections. The lesions observed included separation of the inner and outer root sheath in anagen follicles resulting in the hair fiber being very easily plucked from the follicle. Vesicles on the ventral tongue, mucocutaneous junction of the eyelid, foot pads, and rarely in skin were also evident. Separation occurred between the basal and suprabasilar cells forming an empty cleft, resembling that observed in human pemphigus vulgaris. Immunofluorescence studies did not reveal the presence of tissue-bound or circulating autoantibodies. Expression of keratinocyte markers in hair follicles was normal. Keratin 6-positive cells were found on either side of the follicular separation suggesting a molecular defect in adhesion molecules between the inner layer of the outer root sheath cells to layers on either sides. This hypothesis has been confirmed by another group who demonstrated that the bal(J) mutation is due to the insertion of a thymidine in the desmoglein 3 gene, resulting in a premature stop codon.

A YAC contig joining the desmocollin and desmoglein loci on human chromosome 18 and ordering of the desmocollin genes

The desmocollins and desmogleins are members of the cadherin family of adhesive proteins present in the desmosome type of cell-cell junction. All of the known desmoglein and desmocollin isoforms, which have differing tissue and developmental distributions, are coded by very closely linked genes at 18q12.1. We have previously described YAC clones carrying all three known desmoglein (DSG) genes. We have now isolated YAC clones that carry all three known desmocollin genes (DSC1, 2, and 3) from two libraries and also isolated clones that join the DSC locus to the DSG locus, forming a complete contig for the region. Absence of chimeric ends for some of the YACs was confirmed by isolating Vectorette PCR products for the YAC ends and mapping the derived DNA sequences back to other YACs from CEPH. The whole DSC/DSG gene complex occupies no more than about 700 kb, and the genes are arranged in the order cen-3'-DSC3-DSC2-DSC1-5'-5'-DSG1-DSG3-D SG2-3'-tel, so that the two gene clusters are transcribed outward from the interlocus region. A P1 clone carrying part of DSC2 and DSC3 confirmed the relative orientation of transcription of these two genes. The conservation of close genetic linkage may be of trivial importance related to the recent duplication of these genes or may be because there is a region within the locus that is involved in coordinating the expression of the desmoglein and desmocollin genes.

Mouse models for the study of human hair loss

A comparison has been presented to illustrate many of the similarities in patterns of disease between mouse and human hair follicle diseases and how various mouse mutations can be used as research tools to investigate these observations. The powerful genetic tools available for investigating mouse mutations and human homologues will continue to result in many breakthroughs in the understanding of hair follicle biology and pathology. Many more mouse mutations are available than are described here. Information on these mutations fills books and computer databases, providing an unlimited resource.