Mutations in EXPH5 underlie a rare subtype of autosomal recessive epidermolysis bullosa simplex

Slac2-b Coordinates Extracellular Vesicle Secretion to Regulate Keratinocyte Adhesion and Migration

Slac2-b, also known as exophilin-5, is a Rab27b effector protein with a role in exosome transport and is encoded by the EXPH5 gene. We previously described biallelic loss-of-function mutations in EXPH5 in an autosomal recessive form of epidermolysis bullosa simplex. However, how the loss of Slac2-b expression leads to skin fragility and erosions is unknown. In this study, we demonstrate that keratinocytes (KCs) isolated from two different individuals with mutations in EXPH5 have significant defects in cell‒matrix adhesion. EXPH5-mutant KCs also showed increased perinuclear accumulation and significantly reduced trafficking of CD63⁺ vesicles. These phenotypes were also seen in Slac2-b‒deficient KCs. This was coincident with a reduction in Rab27a protein expression in Slac2-b‒mutant KCs as well as reduced secretion of extracellular vesicles containing extracellular matrix proteins. Live imaging analysis revealed a strong correlation between CD63⁺ vesicle trafficking to the plasma membrane and focal adhesion dynamics. These findings support a role for Slac2-b in regulating local focal adhesion dynamics to support effective KC adhesion and provide insight into the underlying pathophysiology of inherited skin blistering.

Inherited epidermolysis bullosa: New diagnostics and new clinical phenotypes

Inherited epidermolysis bullosa (EB) is a group of heterogeneous genetic disorders characterized by skin fragility. EB comprises a large spectrum of phenotypes, ranging from severe cutaneous and extracutaneous involvement caused by lack of key adhesion proteins, to mild cutaneous fragility caused by subtle molecular defects. Disease-causing variants in 20 different genes account for the genetic and allelic heterogeneity of EB. Here, we discuss the development of laboratory methods that enabled these discoveries and the clinical and molecular features of some new EB entities elucidated during the past 5-6 years.

A novel homozygous deletion in EXPH5 causes a skin fragility phenotype

Epidermolysis bullosa simplex (EBS) is the most common form of EB. Eight different genes have been implicated in the pathogenesis of different types of EBS, but a substantial portion of the cases cannot be attributed to mutations in known genes. Recently, recessive mutations in the gene EXPH5 (encoding exophilin-5, also known as Slac2-b) were identified in patients affected with a mild form of EBS. We used immunohistochemistry, Sanger sequencing and PCR-restriction fragment length polymorphism analysis to identify the cause of mild congenital skin fragility in a 3-year-old girl. No mutations were detected in KRT5 or KRT14, but we identified a novel homozygous deletion in EXPH5, which was found to cosegregate with the disease phenotype in the family. Our results further expand the spectrum of mutations in EXPH5. Appraisal of the present case against previously reported patients indicate that EXPH5 mutations result in a distinctive skin fragility phenotype, with minimal blistering compared with other forms of basal EBS.

Recently Identified Forms of Epidermolysis Bullosa

Epidermolysis bullosa (EB) comprises a collection of clinically diverse inherited blistering diseases that affect the skin and, in some subtypes, mucous membranes and other organs. Currently classified into four main subtypes (EB simplex, junctional EB, dystrophic EB, and Kindler syndrome, mainly based on the level of skin cleavage), the spectrum of EB extends to more than 30 clinical subtypes with pathogenic mutations in at least 18 distinct genes. This review focuses on three recent additions to variants of EB: all are autosomal recessive, and result from mutations in either DST-e (coding for epidermal dystonin, also known as the 230 kDa bullous pemphigoid antigen, BP230), EXPH5 (coding for exophilin-5, also known as Slac2-b), or ITGA3 (coding for the integrin alpha-3 subunit). Each of these new forms of EB is reviewed with respect to the initial gene discovery, clinical features, the current mutation database, and skin pathology. Awareness of these recently described forms of EB is helpful in the clinical evaluation of patients with EB and in defining genotype-phenotype correlation for inherited blistering skin diseases.