Epidermolysis bullosa (EB), the paradigm of heritable skin fragility disorders, is associated with mutations in as many as 20 distinct genes. One of the clinical variants, recessive dystrophic EB (RDEB), demonstrates sub-lamina densa blistering accompanied by alterations in anchoring fibrils due to mutations in COL7A1. In this study, we characterized a patient with widespread connective tissue abnormalities, including skin blistering similar to that in RDEB. Whole exome sequencing, combined with genome-wide homozygosity mapping, identified a homozygous missense mutation in PLOD3 encoding lysyl hydroxylase 3 (LH3). No mutations in COL7A1, the gene previously associated with RDEB, were detected. The level of LH3 was dramatically reduced in the skin and fibroblast cultures from the patient. The blistering in the skin occurred below the lamina densa and was associated with variable density and morphology of anchoring fibrils. The level of type VII collagen expression in the skin was markedly reduced. Analysis of hydroxylysine and its glycosylated derivatives (galactosyl-hydroxylysine and glucosyl-galactosyl-hydroxylysine) revealed marked reduction in glycosylated hydroxylysine. Collectively, these findings indicate that PLOD3 mutations can result in a dystrophic EB-like phenotype in the spectrum of connective tissue disorders and add it to the list of candidate genes associated with skin fragility.

Lack of type VII collagen (C7) disrupts cellular proteostasis yet the mechanism remains undescribed. By studying the relationship between C7 and the extracellular matrix (ECM)-associated proteins thrombospondin-1 (TSP1), type XII collagen (C12) and tissue transglutaminase (TGM2) in primary human dermal fibroblasts from multiple donors with or without the genetic disease recessive dystrophic epidermolysis bullosa (RDEB) (n=31), we demonstrate that secretion of each of these proteins is increased in the presence of C7. In dermal fibroblasts isolated from patients with RDEB, where C7 is absent or defective, association with the COPII outer coat protein SEC31 and ultimately secretion of each of these ECM-associated proteins is reduced and intracellular levels are increased. In RDEB fibroblasts, overall collagen secretion (as determined by the levels of hydroxyproline in the media) is unchanged while traffic from the ER to Golgi of TSP1, C12 and TGM2 occurs in a type I collagen (C1) dependent manner. In normal fibroblasts association of TSP1, C12 and TGM2 with the ER exit site transmembrane protein Transport ANd Golgi Organization-1 (TANGO1) as determined by proximity ligation assays, requires C7. In the absence of wild-type C7, or when ECM-associated proteins are overexpressed, C1 proximity and intracellular levels increase resulting in elevated cellular stress responses and elevated TGFβ signaling. Collectively, these data demonstrate a role for C7 in loading COPII vesicle cargo and provides a mechanism for disrupted proteostasis, elevated cellular stress and increased TGFβ signaling in patients with RDEB. Furthermore, our data point to a threshold of cargo loading that can be exceeded with increased protein levels leading to pathological outcomes in otherwise normal cells.

Epidermolysis bullosa (EB) is caused by mutations in as many as 19 distinct genes. We have developed a next-generation sequencing (NGS) panel targeting genes known to be mutated in skin fragility disorders, including tetraspanin CD151 expressed in keratinocytes at the dermal-epidermal junction. The NGS panel was applied to a cohort of 92 consanguineous families of unknown subtype of EB. In one family, a homozygous donor splice site mutation in CD151 (NM_139029; c.351+2T>C) at the exon 5/intron 5 border was identified, and RT-PCR and whole transcriptome analysis by RNA-seq confirmed deletion of the entire exon 5 encoding 25 amino acids. Immunofluorescence of proband's skin and Western blot of skin proteins with a monoclonal antibody revealed complete absence of CD151. Transmission electron microscopy showed intracellular disruption and cell-cell dysadhesion of keratinocytes in the lower epidermis. Clinical examination of the 33-year old proband, initially diagnosed as Kindler syndrome, revealed widespread blistering, particularly on pretibial areas, poikiloderma, nail dystrophy, loss of teeth, early onset alopecia, and esophageal webbing and strictures. The patient also had history of nephropathy with proteinuria. Collectively, the results suggest that biallelic loss-of-function mutations in CD151 underlie an autosomal recessive mechano-bullous disease with systemic features. Thus, CD151 should be considered as the 20th causative, EB-associated gene.

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.

Epidermolysis bullosa simplex (EBS) is usually inherited as an autosomal dominant disease due to monoallelic gain-of-function mutations in KRT5 or KRT14. Although autosomal recessive forms of EBS have been associated with mutations in at least 10 genes, recessive EBS due to homozygous biallelic KRT5 mutations has not been reported previously; it has been hypothesized that it would result in prenatal lethality. We sought the genetic causes of EB in a cohort of 512 distinct EB families by performing whole exome sequencing (WES) and using an EB-targeting next-generation sequencing (NGS) panel of 21 genes. The pathogenicity and consequences of the mutations were determined by expression profiling and at tissue and ultrastructural levels. Two pathogenic, homozygous missense variants of KRT5 in two patients with generalized EBS and a homozygous null mutation in a patient who died as a neonate from complications of EB were found. The two missense mutations disrupted keratin 5 expression on immunofluorescence microscopy, and the human "knock-out" of KRT5 showed no RNA and protein expression. Collectively, these findings identify biallelic KRT5 mutations with a phenotypic spectrum varying from mild, localized and generalized to perinatal lethal, expanding the genotypic profile of autosomal recessive EBS.