ABHD5 frameshift deletion in Golden Retrievers with ichthyosis

Abstract

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).

Keywords: Canis lupus familiaris; animal model; dermatology; dog; genodermatosis; lipid storage disorder; metabolism; precision medicine; veterinary medicine.

Figure 1

Clinical images from an 11-week-old Golden Retriever with ICH2. (A) Adherent scales on the inner pinna. (B) Thick scales on the ventral thorax and (C) abdominal hyperpigmentation.

Figure 2

Histopathology of a Golden Retriever with ICH2. (A) Skin biopsy from an affected dog reveals marked thickening of the epidermis with expansion of the stratum corneum by laminar orthokeratotic hyperkeratosis. Numerous keratinocytes have perinuclear clear spaces (arrows). (B) Normal skin from an unaffected Golden Retriever. H&E 200X.

Figure 3

Pedigree of the investigated Golden Retriever family. Squares represent males and circles represent females. The affected dogs are indicated by filled symbols. Note the multiple inbreeding loops within this pedigree. All affected dogs have shared common ancestors in their maternal and paternal lineages. The 44 dogs that were genotyped on microarrays and used for the linkage analysis are indicated in black and constitute four separate subfamilies. The other dogs are shown in gray. The arrow indicates the dog that was used for whole-genome sequencing.

Figure 4

Mapping of the ICH2 locus and details of the ABHD5:c.1006_1019del variant. (A) Combined linkage and homozygosity mapping revealed a single overlapping region on chromosome 23 indicated by an arrow. (B) Integrative Genomics Viewer (IGV) screenshot showing the position of the deletion in the short-read alignments of the ICH2 affected dog. (C) Sanger sequencing confirmed the detected 14 bp frameshift deletion. The altered reading frame and the premature stop codon are indicated in red. (D) Metabolic pathway for the synthesis of ω-O-acylceramide, an essential lipid required to maintain skin barrier function (Haydar Eskiocak et al. 2019). In the last step of this pathway, ABHD5 acts as a coactivator of PNPLA1 and may help to provide the required triacylglycerides within the endoplasmic reticulum (Ohno et al. 2018). A lack of either ABHD5 or PNPLA1 leads to ichthyosis in humans (Lefevre et al. 2001; Grall et al. 2012).