Pathomechanisms of epidermolysis bullosa: Beyond structural proteins.

Epidermolysis bullosa (EB), a phenotypically and genetically heterogeneous disorder, has been linked to mutations in the genes encoding structural proteins that reinforce skin integrity via dermal-epidermal adhesion. Breakdowns in these adhesion mechanisms result in four different subtypes of EB classified on the basis of the level of tissue separation within the cutaneous basement membrane zone (BMZ). Mutations in as many as 17 distinct genes that encode structural proteins in the BMZ have been linked to EB. Despite the clinical and histopathological confirmation of EB, many cases remain genetically unsolved. Technical advancements in next-generation sequencing have paved the way for the identification of genes involved in the pathophysiology of EB. Structural proteins have long been identified as the candidate molecules altered in EB, however, recently non-structural proteins, encoded for example by PLOD3, USB1, EXPH5, and KLHL24, involved in enzymatic modification or migration of structural proteins have been implicated. In this overview, we discuss recent work regarding these proteins vis-à-vis their function, associated clinical manifestations, and involvement in the pathogenesis of EB.

Kindler epidermolysis bullosa-like skin phenotype and downregulated basement membrane zone gene expression in poikiloderma with neutropenia and a homozygous USB1 mutation.

Epidermolysis bullosa (EB) is a genotypically heterogeneous group of disorders characterized by cutaneous blistering and erosions with a tremendous spectrum of severity. One of the distinct forms of EB, Kindler EB (KEB), manifests with blistering and poikiloderma; this subtype of EB is caused by mutations in the FERMT1 gene encoding kindlin-1. In this study, we investigated a patient clinically diagnosed as KEB with reduced FERMT1 gene expression and intensity of immunostaining for kindlin-1. Transmission electron microscopy showed lamina densa reduplication, frequently observed in KEB. However, no mutations were identified in FERMT1 in this patient with consanguineous parents, and this gene resided outside of genomic regions of homozygosity (ROH). Instead, whole-exome sequencing and homozygosity mapping identified a homozygous sequence variant at the +4 position of intron 2 in the USB1 gene, encoding an exoribonuclease required for processing of U6 snRNA, a critical component of spliceosomes. Examination of the patient's RNA by RNA-Seq confirmed the pathogenicity of this variant, causing aberrant splicing predicted to result in loss of function of USB1. Mutations in this gene have been reported in patients with poikiloderma and neutropenia, with a few reported cases in association with skin fragility, a condition distinct from the KEB phenotype. Transcriptome analysis revealed that several genes, expressed in the cutaneous basement membrane zone and previously associated with different subtypes of EB, were differentially downregulated at the mRNA level. EB-associated mRNA downregulation was confirmed at protein levels by skin immunofluorescence. These observations provide a novel mechanism for blistering and erosions in the skin as a result reduced presence of adhesion complexes critical for stable association of epidermis and dermis at the level of cutaneous basement membrane zone.

Stromal microenvironment in type VII collagen-deficient skin: The ground for squamous cell carcinoma development.

Recessive dystrophic epidermolysis bullosa (RDEB) is a skin fragility disease caused by mutations that affect the function and/or the amount of type VII collagen (C7), the major component of anchoring fibrils. Hallmarks of RDEB are unremitting blistering and chronic wounds leading to tissue fibrosis and scarring. Nearly all patients with severe RDEB develop highly metastatic squamous cell carcinomas (SCC) which are the main cause of death. Accumulating evidence from a murine RDEB model and human RDEB cells demonstrates that lack of C7 also directly alters the wound healing process. Non-healing RDEB wounds are characterized by increased inflammation, high transforming growth factor-ß1 (TGF-ß1) levels and activity, and are heavily populated by myofibroblasts responsible for enhanced fibrogenesis and matrix stiffness. These changes make the RDEB stroma a microenvironment prone to cancer initiation, where cells with features of cancer-associated fibroblasts are found. Here, we discuss recent knowledge on microenvironment alterations in RDEB, highlighting possible therapeutic targets to prevent and/or delay fibrosis and SCC development.

Molecular pathology of the basement membrane zone in heritable blistering diseases:: The paradigm of epidermolysis bullosa.

Epidermolysis bullosa (EB), a phenotypically heterogeneous group of skin fragility disorders, is characterized by blistering and erosions with considerable morbidity and mortality. Mutations in as many as 18 distinct genes expressed at the cutaneous basement membrane zone have been shown to be associated with the blistering phenotype, attesting to the role of the corresponding proteins in providing stable association of the epidermis to the dermis through adhesion at the dermo-epidermal basement membrane zone. Thus, different forms of EB have been highly instructive in providing information on the physiological functions of these proteins as integral components of the supramolecular adhesion complexes. In addition, precise information of the underlying genes and distinct mutations in families with EB has been helpful in subclassification of the disease with prognostic implications, as well as for prenatal testing and preimplantation genetic diagnosis. Furthermore, knowledge of the types of mutations is a prerequisite for application of allele-specific treatment approaches that have been recently developed, including read-through of premature termination codon mutations and chaperone-facilitated intracellular transport of conformationally altered proteins to proper physiologic subcellular location. Collectively, EB serves as a paradigm of heritable skin diseases in which significant progress has been made in identifying the underlying genetic bases and associated aberrant pathways leading from mutations to the phenotype, thus allowing application of precision medicine for this, currently intractable group of diseases.