Genetics of inherited skin disorders in dogs

Intragenic PNPLA1 duplication in Labrador retrievers with nonepidermolytic ichthyosis

BACKGROUND

Ichthyoses represent a heterogeneous group of cornification disorders characterised by epidermal scaling.

OBJECTIVES

To describe the clinical, histopathological and genetic analysis of a Labrador retriever with nonepidermolytic ichthyosis, and the results of a population screening for a newly detected PNPLA1 genomic duplication.

ANIMALS

Two 7-year-old male littermates, 531 population samples.

MATERIALS AND METHODS

Clinical and histopathological analysis, whole genome sequencing and digital PCR-based genotyping were performed.

RESULTS

Generalised scaling and histological laminar orthokeratotic hyperkeratosis confirmed the ichthyosis diagnosis on Dog 1. Dog 2 showed mild clinical signs possibly associated with allergies and not ichthyosis. The genome of Dog 1 was sequenced and compared to 1469 genetically diverse control genomes. The analysis identified a 6099-bp duplication spanning three internal exons of the PNPLA1 gene, which is predicted to result in an altered C-terminal tail of the protein, NP_001277038.2:p.(E558Lfs*17). Dog 2 had a heterozygous genotype and carried one copy of the duplicated PNPLA1 allele. Of the screened 531 additional Labrador retrievers, 491 were homozygous wild-type, 36 were heterozygous carriers and four carried the duplication in a homozygous state.

CONCLUSIONS AND CLINICAL RELEVANCE

Previously identified PNPLA1 variants cause autosomal recessive ichthyosis in golden retrievers and humans. Given the well-established function of PNPLA1, the identified genomic duplication represents a likely candidate causal variant for the observed ichthyosis in the examined Labrador retriever. This is the first report of a new form of autosomal recessive ichthyosis in Labrador retrievers, which provides the basis for genetic testing.

A homozygous LAMB3 frameshift variant in junctional epidermolysis bullosa-affected Bleu du Maine sheep

Epidermolysis bullosa (EB) is a group of inherited skin disorders characterized by skin fragility and blistering. Here, four Bleu du Maine lambs, sired by one ram, were diagnosed with EB very early in life. Due to the severity of the clinical signs, the lambs had to be euthanized. The affected lambs exhibited hoof sloughing and multiple ulcerations on the head, oral cavity, skin over the joints, and the ruminal pillars. Histopathology showed abrupt subepidermal clefts, epidermal detachment directly above the basal membrane, and ulcerations consistent with junctional EB (JEB). Two cases underwent whole-genome sequencing (WGS) to identify the genetic cause. Genomic analyses with the hypothesis of autosomal recessive inheritance identified the most likely pathogenic homozygous 1-bp deletion in the LAMB3 gene (NC_056065.1:g.73166198delG). Recessive forms of JEB in humans and dogs are caused by variants in LAMB3 gene, which encodes β3 subunit of laminin 332, a critical component of the epidermal basal membrane. The ovine frameshift variant putatively introduces a premature stop codon and disrupts the donor splice site of exon 20. The variant allele was homozygous in both sequenced cases and heterozygous in three unaffected close relatives and was absent in 1075 unrelated control sheep of various other breeds. This study highlights the importance of genetic investigation in veterinary diagnostics of and represents the first report of a LAMB3-related recessive EB in sheep. The findings enable genetic testing to inform breeding strategies and provide a second spontaneous large animal model for LAMB3-related JEB in humans.

Skin barrier, phenotypic and genotypic characterisation of autosomal recessive ichthyosis in TGM1-deficient Jack Russell Terriers and response to topical ceramide

BACKGROUND

Autosomal recessive ichthyosis leads to structural or biochemical changes that impair skin barrier function.

HYPOTHESIS/OBJECTIVES

To assess (1) the phenotype and genotype in a litter of Jack Russell Terriers with autosomal recessive congenital ichthyosis (ARCI), and (2) the defective skin barrier and determine if a topical ceramide can modulate the barrier.

ANIMALS

A healthy dam and litter of Jack Russell Terrier puppies (healthy male, affected male and female), one affected adult Jack Russell Terrier and one unrelated healthy Jack Russell Terrier.

MATERIALS AND METHODS

A severe cornification defect was identified via examination of affected puppies. As the phenotype worsened, the affected puppies received a topical application of ω-0-acylceramide for 10 days. Before humane euthanasia, the skin barrier was evaluated via transepidermal water loss (TEWL), corneometry and pH in affected dogs. Genomic testing was performed, and skin samples were analysed by light and electron microscopy.

RESULTS

Affected puppies were homozygous for the 1980 bp LINE-1 insertion in the TGM1 (transglutaminase 1) gene; the unaffected littermate and the dam were heterozygous carriers. ARCI puppies were underweight and had a severe hyperkeratotic phenotype that impaired mobility. TEWL was markedly higher in affected dogs. The cutaneous pH of affected puppies was higher than the normal littermate. Treatment of the skin with ω-0-acylceramide normalised the pH to match the littermate and decreased TEWL. Electron microscopy revealed marked attenuation of the cornified envelope.

CONCLUSIONS AND CLINICAL RELEVANCE

Dogs with TGM1-deficient ARCI have an impaired skin barrier. Topical therapy can partially repair the barrier defect.

KRT5 missense variant in a Cardigan Welsh Corgi with epidermolysis bullosa simplex

Epidermolysis bullosa (EB) is a group of blistering disorders that includes several subtypes, classified according to their level of cleavage. Typical clinical signs are blisters and erosions resulting from minimal trauma. The disease has been described in many mammalian species and pathogenic variants in at least 18 different genes have been identified. In the present study, we investigated a Cardigan Welsh Corgi with congenital clinical signs consistent with epidermolysis bullosa. The puppy had blisters and erosions on the paw pads, and the oral mucosa. Histologic examination demonstrated the typical clefting between the dermis and epidermis and confirmed the clinical suspicion. We obtained whole genome sequencing data from the affected puppy and searched for variants in candidate genes known to cause EB. This revealed a heterozygous missense variant, KRT5:p.(E476K), affecting the highly conserved KLLEGE motif of keratin 5. The mutant allele in the affected puppy arose owing to a de novo mutation event as it was absent from both unaffected parents. Knowledge of the functional impact of KRT5 variants in other species together with the demonstration of the de novo mutation event establishes KRT5:p.(E476K) as causative variant for the observed EBS.

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.