Mild recessive epidermolytic hyperkeratosis associated with a novel keratin 10 donor splice-site mutation in a family of Norfolk terrier dogs

A de novo variant in the keratin 1 gene (KRT1) in a Chinese shar-pei dog with severe congenital cornification disorder and non-epidermolytic ichthyosis

A 3-months old Chinese shar-pei puppy with ichthyosis was investigated. The dog showed generalized scaling, alopecia and footpad lesions. Histopathological examinations demonstrated a non-epidermolytic hyperkeratosis. The parents of the affected puppy did not show any skin lesions. A trio whole genome sequencing analysis identified a heterozygous de novo 3 bp deletion in the KRT1 gene in the affected dog. This variant, NM_001003392.1:c.567_569del, is predicted to delete a single asparagine from the conserved coil 1A motif within the rod domain of KRT1, NP_001003392.1:p.(Asn190del). Immunohistochemistry demonstrated normal levels of KRT1 expression in the epidermis and follicular epithelia. This might indicate that the variant possibly interferes with keratin dimerization or another function of KRT1. Missense variants affecting the homologous asparagine residue of the human KRT1 cause epidermolytic hyperkeratosis. Histologically, the investigated Chinese shar-pei showed a non-epidermolytic ichthyosis. The finding of a de novo variant in an excellent functional candidate gene strongly suggests that KRT1:p.Asn190del caused the ichthyosis phenotype in the affected Chinese shar-pei. To the best of our knowledge, this is the first description of a KRT1-related non-epidermolytic ichthyosis in domestic animals.

Inheritance of Monogenic Hereditary Skin Disease and Related Canine Breeds

The plasticity of the genome is an evolutionary factor in all animal species, including canines, but it can also be the origin of diseases caused by hereditary genetic mutation. Genetic changes, or mutations, that give rise to a pathology in most cases result from recessive alleles that are normally found with minority allelic frequency. The use of genetic improvement increases the consanguinity within canine breeds and, on many occasions, also increases the frequency of these recessive alleles, increasing the prevalence of these pathologies. This prevalence has been known for a long time, but mutations differ according to the canine breed. These genetic diseases, including skin diseases, or genodermatosis, which is narrowly defined as monogenic hereditary dermatosis. In this review, we focus on genodermatosis sensu estricto, i.e., monogenic, and hereditary dermatosis, in addition to the clinical features, diagnosis, pathogeny, and treatment. Specifically, this review analyzes epidermolytic and non-epidermolytic ichthyosis, junctional epidermolysis bullosa, nasal parakeratosis, mucinosis, dermoid sinus, among others, in canine breeds, such as Golden Retriever, German Pointer, Australian Shepherd, American Bulldog, Great Dane, Jack Russell Terrier, Labrador Retriever, Shar-Pei, and Rhodesian Ridgeback.

ABHD5 frameshift deletion in Golden Retrievers with ichthyosis

Ichthyoses are hereditary skin disorders characterized by the formation of scales and defects in the outermost layer of the epidermis. In dogs, at least six different breed-specific ichthyoses including a relatively common PNPLA1-related autosomal recessive ichthyosis in Golden Retrievers are known. In this study, we investigated 14 Golden Retrievers with scales that were not homozygous for the mutant PNPLA1 allele suggesting a genetically distinct new form of ichthyosis. Histopathological examinations showed lamellar, orthokeratotic hyperkeratosis, and mildly hyperplastic epidermis that led to the diagnosis of a nonepidermolytic ichthyosis. Combined linkage and homozygosity mapping in 14 cases and 30 nonaffected family members delimited a critical interval of ∼12.7 Mb on chromosome 23. Whole-genome sequencing of an affected dog revealed a single protein-changing variant within this region that was not present in 795 control genomes. The identified variant is a 14 bp deletion in the ABHD5 gene (c.1006_1019del), leading to a frameshift and altering the last 14 codons p.(Asp336Serfs*6). The genotypes at this variant showed perfect cosegregation with the ichthyosis phenotype in a large family comprising 14 cases and 72 controls. ABHD5 encodes an acyltransferase required for lipid metabolism. In humans, variants in ABHD5 cause Chanarin-Dorfman syndrome, a neutral lipid storage disease with ichthyosis. Our data in dogs together with the knowledge on the effects of ABHD5 variants in humans strongly suggest ABHD5:c.1006_1019del as candidate causative genetic variant for a new canine form of ichthyosis, which we propose to designate as Golden Retriever ichthyosis type 2 (ICH2).

Genes of Congenital Dermatologic Disorders in Dogs—A Review

This article presents an overview of up-to-date identified genes responsible for congenital canine skin diseases of dogs and the characteristics of these diseases. Congenital skin diseases constitute a specific group of dermatologic disorders that plays an important role in breeding of purebred dogs. They include primary seborrhoea, ichthyosis, hereditary nasal parakeratosis, dermatomyositis, colour dilution alopecia, skin mucinosis, dermoid sinus, lethal acrodermatitis, acral mutilation syndrome, keratoconjunctivitis sicca, ichthyosiform dermatosis, bullous epidermolysis, exfoliative dermal lupus erythematosus, congenital footpad hyperkeratosis and sebaceous adenitis. In the majority of cases, their occurrence is linked to particular breeds. In more than half of these diseases a specific defective gene variant responsible for the disease has been identified. Genetic tests for identification of the relevant defective genes serve as an important tool in the diagnostics of diseases in veterinary practice and in breeding of purebred dogs.

Genetics of inherited skin disorders in dogs

Canine genodermatoses represent a broad spectrum of diseases with diverse phenotypes. Modern genetic technology including whole genome sequencing has expedited the identification of novel genes and greatly simplified the establishment of genetic diagnoses in such heterogeneous disorders. The precise genetic diagnosis of a heritable skin disorder is essential for the appropriate counselling of owners regarding the course of the disease, prognosis and implications for breeding. Understanding the underlying pathophysiology is a prerequisite to developing specific, targeted or individualized therapeutic approaches. This review aims to create a comprehensive overview of canine genodermatoses and their respective genetic aetiology known to date. Raising awareness of genodermatoses in dogs is important and this review may help clinicians to apply modern genetics in daily clinical practice, so that a precise diagnoses can be established in suspected genodermatoses.

Ichthyosis and hereditary cornification disorders in dogs

The stratum corneum (SC), the outermost layer of the epidermis, serves a crucial role in maintaining body hydration and protection from environmental insults. When the stratum corneum is injured or when the genetic blueprints are flawed, the body is at risk of dehydration, secondary infections and allergen sensitization. Advancements in veterinary dermatology have revealed a wide gamut of disease from relatively benign to lethal that specifically arise from flawed structural proteins, enzymes or lipids needed to create the corneocytes and lipid bilayers of the SC. Some conditions closely mimic their human counterparts while others are unique to the dog. This review will focus on forms of ichthyosis in the dog.

Ichthyosis in Dogs—Congenital Dermatologic Disorder

The skin provides protective functions, such as thermoregulation, resorption, provision of immune responses, storage and sensory functions, which all play an important role in the internal stability of the organism. The skin has 3 major layers: the epidermis, the dermis and subcutis. The outermost protective layer of the epidermis, the stratum corneum, consists of 20 to 30 overlapping layers of anucleate cells, the corneocytes. Ichthyosis is an autosomal recessive congenital skin disease, in which the corneocytes form defects that appear like individual steps of the stratum corneum. Ichthyosis is characterized by excessive scaling over the entire body surface and is not curable; the symptoms can only be alleviated. Several genetic variants have been identified in specific dog breeds: PNPLA1 in the Golden Retrievers, SLC27A4 in the Great Danes, NIPAL4 in the American Bulldogs, TGM1 in the Jack Russel Terriers, ASPRV1 in the German Shepherds, which cause different forms of nonepidermolytic ichthyosis and KRT10 in the Norfolk Terriers, which causes epidermolytic ichthyosis. When classifying breeds of dogs predisposed to ichthyosis, it is necessary to determine the presence of defective genes in the genome of the individual animals involved in mating.

Grainyhead-like transcription factors: guardians of the skin barrier

There has been selective pressure to maintain a skin barrier since terrestrial animals evolved 360 million years ago. These animals acquired an unique integumentary system with a keratinized, stratified, squamous epithelium surface barrier. The barrier protects against dehydration and entry of microbes and toxins. The skin barrier centres on the stratum corneum layer of the epidermis and consists of cornified envelopes cemented by the intercorneocyte lipid matrix. Multiple components of the barrier undergo cross-linking by transglutaminase (TGM) enzymes, while keratins provide additional mechanical strength. Cellular tight junctions also are crucial for barrier integrity. The grainyhead-like (GRHL) transcription factors regulate the formation and maintenance of the integument in diverse species. GRHL3 is essential for formation of the skin barrier during embryonic development, whereas GRHL1 maintains the skin barrier postnatally. This is achieved by transactivation of Tgm1 and Tgm5, respectively. In addition to its barrier function, GRHL3 plays key roles in wound repair and as an epidermal tumour suppressor. In its former role, GRHL3 activates the planar cell polarity signalling pathway to mediate wound healing by providing directional migration cues. In squamous epithelium, GRHL3 regulates the balance between proliferation and differentiation, and its loss induces squamous cell carcinoma (SCC). In the skin, this is mediated through increased expression of MIR21, which reduces the expression levels of GRHL3 and its direct target, PTEN, leading to activation of the PI3K-AKT signalling pathway. These data position the GRHL family as master regulators of epidermal homeostasis across a vast gulf of evolutionary history.

LAMB3 Missense Variant in Australian Shepherd Dogs with Junctional Epidermolysis Bullosa

In a highly inbred Australian Shepherd litter, three of the five puppies developed widespread ulcers of the skin, footpads, and oral mucosa within the first weeks of life. Histopathological examinations demonstrated clefting of the epidermis from the underlying dermis within or just below the basement membrane, which led to a tentative diagnosis of junctional epidermolysis bullosa (JEB) with autosomal recessive inheritance. Endoscopy in one affected dog also demonstrated separation between the epithelium and underlying tissue in the gastrointestinal tract. As a result of the severity of the clinical signs, all three dogs had to be euthanized. We sequenced the genome of one affected puppy and compared the data to 73 control genomes. A search for private variants in 37 known candidate genes for skin fragility phenotypes revealed a single protein-changing variant, LAMB3:c.1174T>C, or p.Cys392Arg. The variant was predicted to change a conserved cysteine in the laminin β3 subunit of the heterotrimeric laminin-322, which mediates the binding of the epidermal basement membrane to the underlying dermis. Loss-of-function variants in the human LAMB3 gene lead to recessive forms of JEB. We confirmed the expected co-segregation of the genotypes in the Australian Shepherd family. The mutant allele was homozygous in two genotyped cases and heterozygous in three non-affected close relatives. It was not found in 242 other controls from the Australian Shepherd breed, nor in more than 600 other controls. These data suggest that LAMB3:c.1174T>C represents the causative variant. To the best of our knowledge, this study represents the first report of a LAMB3-related JEB in domestic animals.

Transcriptome Profiling and Differential Gene Expression in Canine Microdissected Anagen and Telogen Hair Follicles and Interfollicular Epidermis

The transcriptome profile and differential gene expression in telogen and late anagen microdissected hair follicles and the interfollicular epidermis of healthy dogs was investigated by using RNAseq. The genes with the highest expression levels in each group were identified and genes known from studies in other species to be associated with structure and function of hair follicles and epidermis were evaluated. Transcriptome profiling revealed that late anagen follicles expressed mainly keratins and telogen follicles expressed GSN and KRT15. The interfollicular epidermis expressed predominately genes encoding for proteins associated with differentiation. All sample groups express genes encoding for proteins involved in cellular growth and signal transduction. The expression pattern of skin-associated genes in dogs is similar to humans. Differences in expression compared to mice and humans include BMP2 expression mainly in telogen and high KRT17 expression in the interfollicular epidermis of dogs. Our data provide the basis for the investigation of the structure and function of canine skin or skin disease and support the use of dogs as a model for human cutaneous disease by assigning gene expression to specific tissue states.

Equine Hoof Canker: Bovine Papillomavirus Infection Is Not Associated With Impaired Keratinocyte Differentiation

Impaired keratinocyte differentiation has recently been suggested as a key event in equine hoof canker development. Koilocytotic appearance of keratinocytes, one of the most characteristic morphological alterations in hoof canker tissue, is also a common marker for papillomavirus (PV) infection, and bovine PV-1 and/or -2 (BPV-1/2) has previously been detected in equine canker patients. Therefore, the present study aimed to correlate the frequency and severity of koilocytotic keratinocytes with BPV detection in hoof canker samples. Hoof tissue of 5/18 canker-affected horses and 2/6 control horses tested positive for BPV-1/2 DNA using polymerase chain reaction. Thus, no association between the presence of BPV-1/2 papillomaviral DNA and koilocytotic appearance was found. Proteins associated with but not specific for PV infection were also investigated. Using immunohistochemistry, specific adhesion molecules (E-cadherin and β-catenin) and intermediate filaments (keratins 6 and 14) important for intact epidermal barrier function and keratinocyte differentiation were documented in control samples (n = 6) and in hoof canker tissue samples (n = 19). Altered expression patterns of intermediate filaments and adhesion molecules were demonstrated in canker tissue, confirming the importance of incomplete keratinocyte differentiation, as well as the crucial role of keratinocyte differentiation in hoof canker.

Prevalence of PNPLA1 Gene Mutation in 48 Breeding Golden Retriever Dogs

A non-epidermolytic ichthyosis has been identified in Golden Retrievers due to a variant in the PNPLA1 gene, and a genetic test is available to detect wild-type, heterozygous and homozygous dogs. The aims of this study were to investigate the prevalence of the PNPLA1 gene variant in Golden Retrievers used for breeding and to provide more information to breeders in order to restrict the spread of this disease. Clinical examination and assessment of the PNPLA1 genotype using PCR testing of oral swabs were performed in 48 breeding Golden Retrievers. Wild-type, heterozygous or homozygous variants of the PNPLA1 gene were demonstrated in 10 (21%), 23 (48%), and 15 (31%) of the 48 dogs, respectively. In only 3 of the 48 dogs were clinical signs suggestive of ichthyosis identified. Data collected agreed with data reported in the literature. The high prevalence of homozygous and heterozygous variants makes the exclusion of mutated dogs from breeding impractical. Furthermore, the reliability of the PNPLA1 mutation in prediction of clinical signs of ichthyosis is unclear. Additional studies are needed to investigate if PNPLA1 is the only gene involved or if other genes and environmental factors have a role in the development of ichthyosis in Golden Retrievers.

A de novo variant in the ASPRV1 gene in a dog with ichthyosis

Ichthyoses are a heterogeneous group of inherited cornification disorders characterized by generalized dry skin, scaling and/or hyperkeratosis. Ichthyosis vulgaris is the most common form of ichthyosis in humans and caused by genetic variants in the FLG gene encoding filaggrin. Filaggrin is a key player in the formation of the stratum corneum, the uppermost layer of the epidermis and therefore crucial for barrier function. During terminal differentiation of keratinocytes, the precursor profilaggrin is cleaved by several proteases into filaggrin monomers and eventually processed into free amino acids contributing to the hydration of the cornified layer. We studied a German Shepherd dog with a novel form of ichthyosis. Comparing the genome sequence of the affected dog with 288 genomes from genetically diverse non-affected dogs we identified a private heterozygous variant in the ASPRV1 gene encoding "aspartic peptidase, retroviral-like 1", which is also known as skin aspartic protease (SASPase). The variant was absent in both parents and therefore due to a de novo mutation event. It was a missense variant, c.1052T>C, affecting a conserved residue close to an autoprocessing cleavage site, p.(Leu351Pro). ASPRV1 encodes a retroviral-like protease involved in profilaggrin-to-filaggrin processing. By immunofluorescence staining we showed that the filaggrin expression pattern was altered in the affected dog. Thus, our findings provide strong evidence that the identified de novo variant is causative for the ichthyosis in the affected dog and that ASPRV1 plays an essential role in skin barrier formation. ASPRV1 is thus a novel candidate gene for unexplained human forms of ichthyoses.

A Defect in NIPAL4 Is Associated with Autosomal Recessive Congenital Ichthyosis in American Bulldogs

Autosomal recessive congenital ichthyosis in the American bulldog is characterized by generalized scaling and erythema with adherent scale on the glabrous skin. We had previously linked this disorder to NIPAL4, which encodes the protein ichthyin. Sequencing of NIPAL4 revealed a homozygous single base deletion (CanFam3.1 canine reference genome sequence NC_06586.3 g.52737379del), the 157th base (cytosine) in exon 6 of NIPAL4 as the most likely causative variant in affected dogs. This frameshift deletion results in a premature stop codon producing a truncated and defective NIPAL4 (ichthyin) protein of 248 amino acids instead of the wild-type length of 404. Obligate carriers were confirmed to be heterozygous for this variant, and 150 clinically non-affected dogs of other breeds were homozygous for the wild-type gene. Among 800 American bulldogs tested, 34% of clinically healthy dogs were discovered to be heterozygous for the defective allele. More importantly, the development of this canine model of autosomal recessive congenital ichthyosis will provide insight into the development of new treatments across species.

Genetic testing in veterinary dermatology

BACKGROUND

Molecular genetics has made significant advances in the analysis of hereditary dermatoses during the last several years.

OBJECTIVES

To provide an update on currently available genetic tests for skin diseases of dogs, cats and horses, and to aid the veterinary clinician in the appropriate selection and applications of genetic tests.

METHODS

The scientific literature on the topic was critically reviewed. The list of known causative variants for genodermatoses and hair morphology traits was compiled by searching the Online Mendelian Inheritance in Animals (OMIA) database.

RESULTS

Genetic testing has become an important diagnostic method in veterinary medicine. Genetic tests can help to establish the correct diagnosis in some diseases with relatively nonspecific signs. Genetic tests are also essential for sustainable breeding programmes and to help minimize the frequency of animals with hereditary diseases. Advances in genetic methodology and bioinformatics already allow genome-wide screening for potential disease causing mutations for research purposes. It is anticipated that this will become a routine process in clinical practice in the future.

CONCLUSION AND CLINICAL IMPORTANCE

As specific DNA tests and broad genome-wide analyses come into more common use, it is critical that clinicians understand the proper application and interpretation of these test results.

A Novel SLC27A4 Splice Acceptor Site Mutation in Great Danes with Ichthyosis

Ichthyoses are a group of various different types of hereditary disorders affecting skin cornification. They are characterized by hyperkeratoses of different severity levels and are associated with a dry and scaling skin. Genome-wide association analysis of nine affected and 13 unaffected Great Danes revealed a genome-wide significant peak on chromosome 9 at 57-58 Mb in the region of SLC27A4. Sequence analysis of genomic DNA of SLC27A4 revealed the non-synonymous SNV SLC27A4:g.8684G>A in perfect association with ichthyosis-affection in Great Danes. The mutant transcript of SLC27A4 showed an in-frame loss of 54 base pairs in exon 8 probably induced by a new splice acceptor site motif created by the mutated A- allele of the SNV. Genotyping 413 controls from 35 different breeds of dogs and seven wolves revealed that this mutation could not be found in other populations except in Great Danes. Affected dogs revealed high amounts of mutant transcript but only low levels of the wild type transcript. Targeted analyses of SLC27A4 protein from skin tissues of three affected and two unaffected Great Danes indicated a markedly reduced or not detectable wild type and truncated protein levels in affected dogs but a high expression of wild type SLC27A4 protein in unaffected controls. Our data provide evidence of a new splice acceptor site creating SNV that results in a reduction or loss of intact SLC27A4 protein and probably explains the severe skin phenotype in Great Danes. Genetic testing will allow selective breeding to prevent ichthyosis-affected puppies in the future.

Congenital Ichthyosis in 14 Great Dane Puppies With a New Presentation

The present study describes a generalized congenital skin condition in 14 Great Dane puppies. Macroscopically, all dogs showed generalized gray to yellow scaling and skin wrinkles on the head and all 4 extremities. Skin sections were histologically examined using hematoxylin and eosin, Heidenhain's Azan, and Sudan red III staining methods and by conducting the alcian blue/periodic acid Schiff (AB/PAS) reaction technique on sections. Furthermore, incubation with hyaluronidase was performed. Skin samples were ultrastructurally analyzed using transmission electron microscopy. All affected Great Dane puppies had epidermal and follicular orthokeratotic hyperkeratosis, enlarged keratohyaline granules, vacuolated keratinocytes, and accumulations of an eosinophilic and alcianophilic, lipid-rich material within dilated hair follicular lumina and the cytoplasm of sebocytes. The macroscopic, histopathologic, and ultrastructural skin changes in all 14 Great Dane puppies indicate a new variant of a primary disorder of cornification with congenital, non-epidermolytic, lamellar ichthyosiform appearance.

Cutaneous mosaicism, in KRT1 pI479T patient, caused by the somatic loss of the wild-type allele, leads to the increase in local severity of the disease

BACKGROUND

Epidermolytic ichthyosis (BCIE, OMIM 113800), is an autosomal dominant disorder of the skin caused by mutations in keratin genes KRT1 and KRT10. We present two sporadic patients showing a mild diffuse ichthyosis with palmoplantar keratoderma. Interestingly, one of them shows a significant hyperkeratosis of palms and soles similar to those present in the Meleda disease (OMIM 248300).

OBJECTIVE

In this paper we would clarify the genetic difference between the two patients, giving rise to the different phenotype.

METHODS

Clinical evaluation, followed by histological and molecular analysis has been established for these patients.

RESULTS

We demonstrated the presence of a genetic cutaneous mosaicism. Both patients carry the KRT1 pI479T substitution, but in the palmoplantar areas of one of them, only the mutated allele is expressed (hemizygous). This leads to highlight a new type of cutaneous mosaic, the palmoplantar mosaicism.

Autosomal Recessive Congenital Ichthyosis in American Bulldogs Is Associated With NIPAL4 (ICHTHYIN) Deficiency

A minority of patients with nonsyndromic autosomal recessive congenital ichthyosis (ARCI) display mutations in NIPAL4 (ICHTHYIN). This protein plays a role in epidermal lipid metabolism, although the mechanism is unknown. The study describes a moderate form of ARCI in an extended pedigree of American Bulldogs that is linked to the gene encoding ichthyin. The gross phenotype was manifest as a disheveled pelage shortly after birth, generalized scaling, and adherent brown scale with erythema of the abdominal skin. Pedigree analysis indicated an autosomal recessive mode of inheritance. Ultrastructurally, the epidermis showed discontinuous lipid bilayers, unprocessed lipid within corneocytes, and abnormal lamellar bodies. Linkage analysis, performed by choosing simple sequence repeat markers and single-nucleotide polymorphisms near genes known to cause ACRI, revealed an association with NIPAL4. NIPAL4 was identified and sequenced using standard methods. No mutation was identified within the gene, but affected dogs had a SINE element 5' upstream of exon 1 in a highly conserved region. Of 545 DNA samples from American Bulldogs, 32 dogs (17 females, 15 males) were homozygous for the polymerase chain reaction fragment. All affected dogs were homozygous, with parents heterozygous for the insertion. Immunolabeling revealed an absence of ichthyin in the epidermis. This is the first description of ARCI associated with decreased expression of NIPAL4 in nonhuman species.

The stratum corneum: the rampart of the mammalian body

BACKGROUND

The stratum corneum (SC) is the outermost region of the epidermis and plays key roles in cutaneous barrier function in mammals. The SC is composed of 'bricks', represented by flattened, protein-enriched corneocytes, and 'mortar', represented by intercellular lipid-enriched layers. As a result of this 'bricks and mortar' structure, the SC can be considered as a 'rampart' that encloses water and solutes essential for physiological homeostasis and that protects mammals from physical, chemical and biological assaults.

STRUCTURES AND FUNCTIONS

The corneocyte cytoskeleton contains tight bundles of keratin intermediate filaments aggregated with filaggrin monomers, which are subsequently degraded into natural moisturizing compounds by various proteases, including caspase 14. A cornified cell envelope is formed on the inner surface of the corneocyte plasma membrane by transglutaminase-catalysed cross-linking of involucrin and loricrin. Ceramides form a lipid envelope by covalently binding to the cornified cell envelope, and extracellular lamellar lipids play an important role in permeability barrier function. Corneodesmosomes are the main adhesive structures in the SC and are degraded by certain serine proteases, such as kallikreins, during desquamation.

CLINICAL RELEVANCE

The roles of the different SC components, including the structural proteins in corneocytes, extracellular lipids and some proteins associated with lipid metabolism, have been investigated in genetically engineered mice and in naturally occurring hereditary skin diseases, such as ichthyosis, ichthyosis syndrome and atopic dermatitis in humans, cattle and dogs.

Canine ichthyosis and related disorders of cornification

The stratum corneum acts as a permability barrier to keep the body hydrated while preventing environmental damage and exposure to pathogens and noxious substances. Disorders of cornification (DOC) arise from an inability to form a normal stratum corneum. Most DOC arise secondary to skin allergies, ectoparasitism, endocrine and metabolic diseases. Primary DOC typically arise from single gene mutations. As there is no cure for primary DOC (ichthyoses), a stepwise diagnostic approach is fundamental to establishing a correct diagnosis. Treatment involves a regimen of topical therapy as well as medical scrutiny to address secondary bacterial and yeast infections.

Histologic morphology and involucrin, filaggrin, and keratin expression in normal canine skin from dogs of different breeds and coat types

The purpose of this study was to measure the thickness of canine epidermis at various anatomical sites according to localization of cornified envelopes (involucrin and filaggrin), keratins (keratin 10, 5), and their mRNA expression. This was done in the skin of five breeds of dogs including seven poodles, six golden retrievers, six Shih Tzus, four pugs, and four Labrador retrievers. Epidermal thickness of the stratum corneum and nucleated epidermal layer was significantly different. The greatest thickness was observed in the digital web area and the thinnest epidermis was in the axilla. Epidermal thickness was also significantly different between the breeds (p < 0.05). Immunohistochemical staining scores revealed significant decreases of involucrin, filaggrin, and keratin 10 in the ventral and weight-bearing sites, and a relative increase of keratin 5 (p < 0.05). q-PCR analysis showed that their the levels of mRNA were positively correlated with expression of the corresponding proteins in skin samples (p < 0.05). The present study is the first to report the relationship between epidermal gene expression and histologic morphology of the skin in normal dogs. Further studies will be essential to fully understand the pathogenesis of skin barrier dysfunctions in canines.

PNPLA1 mutations cause autosomal recessive congenital ichthyosis in golden retriever dogs and humans

Ichthyoses comprise a heterogeneous group of genodermatoses characterized by abnormal desquamation over the whole body, for which the genetic causes of several human forms remain unknown. We used a spontaneous dog model in the golden retriever breed, which is affected by a lamellar ichthyosis resembling human autosomal recessive congenital ichthyoses (ARCI), to carry out a genome-wide association study. We identified a homozygous insertion-deletion (indel) mutation in PNPLA1 that leads to a premature stop codon in all affected golden retriever dogs. We subsequently found one missense and one nonsense mutation in the catalytic domain of human PNPLA1 in six individuals with ARCI from two families. Further experiments highlighted the importance of PNPLA1 in the formation of the epidermal lipid barrier. This study identifies a new gene involved in human ichthyoses and provides insights into the localization and function of this yet uncharacterized member of the PNPLA protein family.

Parallel mapping and simultaneous sequencing reveals deletions in BCAN and FAM83H associated with discrete inherited disorders in a domestic dog breed

The domestic dog (Canis familiaris) segregates more naturally-occurring diseases and phenotypic variation than any other species and has become established as an unparalled model with which to study the genetics of inherited traits. We used a genome-wide association study (GWAS) and targeted resequencing of DNA from just five dogs to simultaneously map and identify mutations for two distinct inherited disorders that both affect a single breed, the Cavalier King Charles Spaniel. We investigated episodic falling (EF), a paroxysmal exertion-induced dyskinesia, alongside the phenotypically distinct condition congenital keratoconjunctivitis sicca and ichthyosiform dermatosis (CKCSID), commonly known as dry eye curly coat syndrome. EF is characterised by episodes of exercise-induced muscular hypertonicity and abnormal posturing, usually occurring after exercise or periods of excitement. CKCSID is a congenital disorder that manifests as a rough coat present at birth, with keratoconjunctivitis sicca apparent on eyelid opening at 10–14 days, followed by hyperkeratinisation of footpads and distortion of nails that develops over the next few months. We undertook a GWAS with 31 EF cases, 23 CKCSID cases, and a common set of 38 controls and identified statistically associated signals for EF and CKCSID on chromosome 7 (P_raw 1.9×10⁻¹⁴; P_genome = 1.0×10⁻⁵) and chromosome 13 (P_raw 1.2×10⁻¹⁷; P_genome = 1.0×10⁻⁵), respectively. We resequenced both the EF and CKCSID disease-associated regions in just five dogs and identified a 15,724 bp deletion spanning three exons of BCAN associated with EF and a single base-pair exonic deletion in FAM83H associated with CKCSID. Neither BCAN or FAM83H have been associated with equivalent disease phenotypes in any other species, thus demonstrating the ability to use the domestic dog to study the genetic basis of more than one disease simultaneously in a single breed and to identify multiple novel candidate genes in parallel.

Congenital keratoconjunctivitis sicca and ichthyosiform dermatosis in 25 Cavalier King Charles spaniel dogs. Part I: clinical signs, histopathology, and inheritance

The clinical presentation and progression (over 9 months to 13 years) of congenital keratoconjunctivitis sicca and ichthyosiform dermatosis (CKCSID) in the Cavalier King Charles spaniel dog are described for six new cases and six previously described cases. Cases presented with a congenitally abnormal (rough/curly) coat and signs of KCS from eyelid opening. Persistent scale along the dorsal spine and flanks with a harsh frizzy and alopecic coat was evident in the first few months of life. Ventral abdominal skin was hyperpigmented and hyperkeratinized in adulthood. Footpads were hyperkeratinized from young adulthood with nail growth abnormalities and intermittent sloughing. Long-term follow-up of cases (13/25) is described. Immunomodulatory/lacrimostimulant treatment had no statistically significant effect on Schirmer tear test results, although subjectively, this treatment reduced progression of the keratitis. Histopathological analysis of samples (skin/footpads/lacrimal glands/salivary glands) for three new cases was consistent with an ichthyosiform dermatosis, with no pathology of the salivary or lacrimal glands identified histologically. Pedigree analysis suggests the syndrome is inherited by an autosomal recessive mode.

Identification and characterization of hierarchical structures in dog breeding schemes, a novel method applied to the Norfolk Terrier

Popular sires, a limited population size, and the founder event are widely considered the main reasons for the low genetic diversity observed in many dog breeds. However, these factors have had only a small role in the historic decrease in diversity observed in the Norfolk Terrier breed. We show that the decrease in this breed has been mainly due to large, popular kennels. Dogs from these kennels have, on average, larger genetic contributions to subsequent generations than others. A test for the presence of a popular kennel effect is proposed and applied (P < 0.001). These kennels were found to be the same as nuclei of selection existing in other livestock species. This result revealed a hierarchical structure of dog breeding schemes, with an asymmetric gene flow predominantly from the nuclei toward the main population. Possible reasons for this structure and implications for future population management are discussed. The main reason is probably that the breed type was established by large, popular kennels and that small kennel breeders used their stud dogs to benefit from the achievements of the popular kennels. Many kennels, however, were unable to make their own substantial genetic contributions to the breed.

Clinical, histopathological and genetic data of ichthyosis in the golden retriever: a prospective study

OBJECTIVES

We described epidemiological, clinical, histopathological and ultrastructural features of ichthyosis in the golden retriever breed in a prospective study. We also investigated the mode of transmission of this disease.

MATERIALS AND METHODS

We examined 150 golden retrievers, 73 of which were affected by ichthyosis (35 males and 38 females). We carried out detailed clinical and histopathological examinations for 40 affected dogs. Transmission electron microscopy was performed for two of them. We used pedigree analysis with the Cyrillic software to determine the mode of transmission.

RESULTS

Dermatological signs included a mild to moderate or severe generalised scaling with initially small to large whitish scales and progressively blackish scales. The ventral glabrous skin was hyperpigmented and rough, similar to sandpaper. Histopathological features were characterised by moderate to severe laminated or compact orthokeratotic epidermal hyperkeratosis without significant involvement of the stratum granulosum. Ultrastructural findings revealed laminated or compact keratin layers and numerous persistent corneodesmosomes within the stratum corneum. Analysis of the pedigree suggested an autosomal recessive inheritance.

CONCLUSION

The histopathological and ultrastructural characteristics strongly suggest that golden retriever ichthyosis is a retention ichthyosis, caused by absence of corneodesmosomal degradation, transmitted through an autosomal recessive mode.

Cornification defect in the golden retriever: clinical, histopathological, ultrastructural and genetic characterisation

Veterinarians have recognised a nonpruritic skin disease in the Golden retriever breed characterised by excessive scaling of large, variably pigmented flakes of skin in otherwise healthy dogs. This prospective case series describes clinical, histopathological, ultrastructural and genetic features of this cornification defect in 17 affected dogs. The condition affects young dogs of either sex and is characterised by symmetrical, predominantly ventro-lateral scaling and hyperpigmentation of the trunk. Histopathological and ultrastructural changes of the stratum corneum are suggestive of delayed degradation of corneodesmosomes. A genetic aetiology is proposed and a single-trait autosomal recessive mode of inheritance discussed.

The clinical and morphologic features of nonepidermolytic ichthyosis in the golden retriever

A scaling disorder specific to Golden Retriever dogs has been recognized by both dermatologists and pathologists, but to date has not been well characterized. At the University of Pennsylvania's Laboratory of Toxicology and Pathology, 46 cases of ichthyosis were diagnosed histologically in Golden Retriever dogs from January 2004 to January 2007. A total of 22 dogs had skin lesions documented at younger than 1 year of age; 3 dogs between 1 and 2 years of age; 13 dogs developed lesions at older than 2 years; and the time of onset was unknown for 8 dogs. A total of 25 dogs were female, and 21 were male. All dogs had strikingly similar histopathologic changes that consisted of mild to moderate laminar orthokeratotic hyperkeratosis with an absence of epidermal hyperplasia and dermal inflammation. Ultrastructural analysis using a ruthenium tetroxide fixation method was performed on punch biopsy samples from 5 dogs and compared with 2 control dogs (1 clinically and histologically normal sibling of an affected dog and 1 Cairn Terrier). All affected dogs had retained and convoluted membranes with crystalline structures in the stratum corneum. Scattered keratinocytes in the granular cell layer had prominent, clear, membrane-bound, cytoplasmic vacuoles. Pedigree analysis of 14 dogs was compatible with autosomal recessive inheritance, but incomplete dominance could not be ruled out. This unique hyperkeratotic/scaling disorder in Golden Retrievers has distinctive clinical, histologic, and ultrastructural features, which are consistent with a primary cornification defect.

Mild recessive bullous congenital ichthyosiform erythroderma due to a previously unidentified homozygous keratin 10 nonsense mutation

We have identified a previously unreported homozygous nonsense mutation p.Cys427X in the keratin 10 (K10) gene (KRT10) in a Turkish girl with recessive bullous congenital ichthyosiform erythroderma (BCIE) showing superficial blistering. p.Cys427X is located upstream of the previously reported homozygous truncation mutation within the same exon 6 causing mRNA decay. Immunohistochemical examination showed a complete absence of K10 protein in the patient's epidermis. The findings of this study suggest that K10 knockout patients show unique clinicopathological features of clinically mild BCIE with blisters occurring within the granular layer. In addition, the unaffected, heterozygous carriers of the mutation indicate that the K10 peptide from one normal allele alone is sufficient for keratin network formation.

Topics in pediatric dermatology

Skin conditions of puppies and kittens are often infectious, such as ectoparasites or dermatophytosis. Hereditary and congenital skin problems are often detected at an early age. Young animals may be more prone to toxicity from medications, and labels should be read carefully for age limits. Husbandry factors, including nutrition, ectoparasites,temperature and humidity, cleaning products, and bedding, should be considered. Fleas are still a common problem despite recent improvements in flea control and can be debilitating in young animals because of blood loss.

A human keratin 10 knockout causes recessive epidermolytic hyperkeratosis

Epidermolytic hyperkeratosis (EHK) is a blistering skin disease inherited as an autosomal-dominant trait. The disease is caused by genetic defects of the epidermal keratin K1 or K10, leading to an impaired tonofilament network of differentiating epidermal cells. Here, we describe for the first time a kindred with recessive inheritance of EHK. Sequence analysis revealed a homozygous nonsense mutation of the KRT10 gene in the affected family members, leading to a premature termination codon (p.Q434X), whereas the clinically unaffected consanguineous parents were both heterozygous carriers of the mutation. Semi-quantitative RT-PCR and western blot analysis demonstrated degradation of the KRT10 transcript, resulting in complete absence of keratin K10 protein in the epidermis and cultured keratinocytes of homozygous patients. This K10 null mutation leads to a severe phenotype, clinically resembling autosomal-dominant EHK, but differing in form and distribution of keratin aggregates on ultrastructural analysis. Strong induction of the wound-healing keratins K6, K16 and K17 was found in the suprabasal epidermis, which are not able to compensate for the lack of keratin 10. We demonstrate that a recessive mutation in KRT10 leading to a complete human K10 knockout can cause EHK. Identification of the heterogeneity of this disorder has a major impact for the accurate genetic counseling of patients and their families and also has implications for gene-therapy approaches.