Split paw pad disease is a scarcely defined phenotype characterized by skin lesions on the paw pads of dogs. We studied a family of German Shepherd dogs, in which four dogs developed intermittent paw pad lesions and lameness. The paw pads of two of the affected dogs were biopsied and demonstrated cleft formation in the stratum spinosum and stratum corneum, the outermost layers of the epidermis. Whole genome sequencing data from an affected dog revealed a private heterozygous 18 bp in frame deletion in the KRT5 gene. The deletion NM_001346035.1:c.988_1005del or NP_001332964.1:p.(Asn330_Asp335del) is predicted to lead to a loss of six amino acids in the L12 linker domain of the encoded keratin 5. KRT5 variants in human patients lead to various subtypes of epidermolysis bullosa simplex (EBS). Localized EBS is the mildest of the KRT5-related human diseases and may be caused by variants affecting the L12 linker domain of keratin 5. We therefore think that the detected KRT5 deletion in dogs represents a candidate causal variant for the observed skin lesions in dogs. However, while the clinical phenotype of KRT5-mutant dogs of this study closely resembles human patients with localized EBS, there are differences in the histopathology. EBS is defined by cleft formation within the basal layer of the epidermis while the cleft formation in the dogs described herein occurred in the outermost layers, a hallmark of split paw pad disease. Our study provides a basis for further studies into the exact relation of split paw pad disease and EBS.

Epidermolysis bullosa simplex (EBS) is characterized by fragility of the skin (and mucosal epithelia in some instances) that results in non-scarring blisters and erosions caused by minor mechanical trauma. EBS is distinguished from other types of epidermolysis bullosa (EB) or non-EB skin fragility syndromes by the location of the blistering in relation to the dermal-epidermal junction. In EBS, blistering occurs within basal keratinocytes. The severity of blistering ranges from limited to hands and feet to widespread involvement. Additional features can include hyperkeratosis of the palms and soles (keratoderma), nail dystrophy, milia, and hyper- and/or hypopigmentation. Rare EBS subtypes have been associated with additional clinical features including pyloric atresia, muscular dystrophy, cardiomyopathy, and/or nephropathy. The diagnosis of EBS is established in a proband by: the identification of a heterozygous dominant-negative variant or biallelic loss-of-function variants in KRT5, KRT14, or PLEC; a heterozygous pathogenic variant in KLHL24; or biallelic pathogenic variants in CD151, DST, or EXPH5; and/or presence of characteristic findings on examination of a skin biopsy using transmission electron microscopy and/or immunofluorescent mapping. Treatment of manifestations: Supportive care to protect the skin from blistering; use of dressings that will protect the skin and promote healing of wounds. Encourage activities that minimize trauma to the skin; appropriate footwear and physical therapy to preserve ambulation; lance and drain without unroofing new blisters. Dressings consist of three layers: a primary nonadherent contact layer; a secondary layer that provides stability, adds padding, and absorbs drainage; and a tertiary layer with elastic properties. Aluminum chloride (20%) applied to palms and soles can reduce sweating and therefore minimize blister formation in some individuals with EBS. In addition, botulinum toxin, cyproheptadine (Periactin®), tetracycline, erythromycin, diacerein, sirolimus, apremilast, cannabidiol oil, and gentamicin have all been reported to be beneficial. Keratolytic agents for palmar and plantar hyperkeratosis may reduce skin thickening and cracking. Topical and/or systemic antibiotics or silver-impregnated dressings or gels can be used to treat skin infection or reduce bacteria colonization, thereby promoting wound healing. Identification and management of specific causes of pain and itching; management with a pain specialist as needed. Management of fluid and electrolyte imbalance in severely affected infants may be critical during the postnatal period. Nutritional support including vitamin and mineral supplementation, feeding via gastrostomy tube, guided feeding therapy, and Haberman feeder may be necessary for infants and children with oral manifestations of EBS. Iron supplementation for those with anemia as a result of chronic inflammation from blistering and wounding. Weight management and treatment of obesity in older individuals. Standard treatment for basal cell carcinomas in individuals with severe EBS. Psychosocial support when needed. Standard treatments for additional features reported in rare subtypes including pyloric atresia, muscular dystrophy, cardiomyopathy, and nephropathy. Surveillance: Dermatologic assessment for blisters, oral disease, hyperkeratosis, hyperhidrosis, signs and symptoms of wound infection, as well as pruritus and pain. At each visit, assessment of hydration status, growth, nutrition, weight, motor development and mobility, and psychosocial well-being. Consider serum B-type natriuretic peptide (BNP) and creatinine kinase in those with EBS, intermediate with cardiomyopathy; serum renal function studies and urinalysis for those with nephropathy; and neurologic assessment for those with muscular dystrophy as needed. Agents/circumstances to avoid: Excessive heat exposure may exacerbate blistering and infection. Avoid poorly fitting or coarse-textured clothing/footwear and activities that traumatize the skin. Avoid adhesives from regular medical tapes or Band-Aids®. EBS is typically inherited in an autosomal recessive or an autosomal dominant manner. Autosomal recessive EBS is associated with either biallelic loss-of-function variants in KRT5, KRT14, or PLEC or biallelic pathogenic variants in CD151, DST, or EXPH5. Autosomal dominant EBS is associated with either a heterozygous dominant-negative variant in KRT5, KRT14, or PLEC or a heterozygous pathogenic variant in KLHL24. In rare instances, EBS is caused by the presence of heterozygous pathogenic variants in both KRT5 and KRT14 and is inherited in a digenic manner. Autosomal recessive EBS. If both parents are known to be heterozygous for an EBS-related pathogenic variant, each sib of an affected individual has at conception a 25% chance of inheriting biallelic pathogenic variants being affected, a 50% chance of being heterozygous, and a 25% chance of inheriting neither of the familial pathogenic variants. Autosomal dominant EBS. In families with autosomal dominant inheritance, each child of an affected individual has a 50% chance of inheriting the pathogenic variant and (most likely) being affected with EBS (penetrance appears to be <100% for known heterozygous dominant-negative KRT5 and KRT14 variants). Once the EBS-related pathogenic variant(s) have been identified in an affected family member, molecular genetic testing for at-risk family members and prenatal and preimplantation genetic testing are possible.

Epidermolysis bullosa (EB) is a rare genetic disease with widely different clinical manifestations, but the relationship between genotype and phenotype is not fully understood. In the present study, we recruited a Chinese family in which two members had been diagnosed with localized EB simplex (EBS), with clinical manifestation, including blisters and erosions on the soles of the feet since infancy. To identify and confirm the genetic variation in a Chinese family diagnosed as localized EBS. Our study included two patients, other healthy members of the family, and 100 normal controls. Genomic DNA samples were isolated from each participant, and then polymerase chain reaction (PCR) direct sequencing was performed. The results of PCR direct sequencing revealed a novel heterozygous missense mutation in codon 461 of exon 7 of KRT5 (c.1382T>C), which led to an amino acid change (p.L461P) in the patients with EBS but was absent in unaffected family members and 100 unrelated control samples. The present study broadens the mutational spectrum of EBS, and this knowledge could be harnessed for prenatal screening, gene diagnosis, and gene therapy for localized EBS.

Epidermolysis bullosa is a group of inherited blistering skin diseases resulting in most cases from missense mutations in KRT5 and KRT14 genes encoding the basal epidermal keratins 5 and 14. Here, we present a patient diagnosed with a localized subtype of epidermolysis bullosa simplex caused by a heterozygous mutation p.Ala428Asp in the KRT5 gene, that has not been previously identified. Moreover, a bioinformatic analysis of the novel mutation was performed, showing changes in the interaction network between the proteins. Identification of novel mutations and genotype-phenotype correlations allow to better understanding of underlying pathophysiologic bases and is important for genetic counselling, patients' management, and disease course prediction.