It is 35 years since the first description of arrhythmogenic right ventricular cardiomyopathy (ARVC) and more than 20 years since the first reports establishing desmosomal gene mutations as a major cause of the disease. Early advances in the understanding of the clinical, pathological and genetic architecture of ARVC resulted in consensus diagnostic criteria, which proved to be sensitive but not entirely specific for the disease. In more recent years, clinical and genetic data from families and the recognition of a much broader spectrum of structural disorders affecting both ventricles and associated with a propensity to ventricular arrhythmia have raised many questions about pathogenesis, disease terminology and clinical management. In this paper, we present the conclusions of an expert round table that aimed to summarise the current state of the art in arrhythmogenic cardiomyopathies and to define future research priorities.

Hyperproliferative keratinocytes induced by trauma, hyperkeratosis and/or inflammation display molecular signatures similar to those of palmoplantar epidermis. Inherited gain-of-function mutations in RHBDF2 (encoding iRHOM2) are associated with a hyperproliferative palmoplantar keratoderma and squamous oesophageal cancer syndrome (termed TOC). In contrast, genetic ablation of rhbdf2 in mice leads to a thinning of the mammalian footpad, and reduces keratinocyte hyperproliferation and migration. Here, we report that iRHOM2 is a novel target gene of p63 and that both p63 and iRHOM2 differentially regulate cellular stress-associated signalling pathways in normal and hyperproliferative keratinocytes. We demonstrate that p63-iRHOM2 regulates cell survival and response to oxidative stress via modulation of SURVIVIN and Cytoglobin, respectively. Furthermore, the antioxidant compound Sulforaphane downregulates p63-iRHOM2 expression, leading to reduced proliferation, inflammation, survival and ROS production. These findings elucidate a novel p63-associated pathway that identifies iRHOM2 modulation as a potential therapeutic target to treat hyperproliferative skin disease and neoplasia.

Hyperproliferative keratinocytes induced by hyperkeratosis and or inflammation display molecular signature similar to the palmoplantar epidermis. Recently, we have shown that genetic depletion of the inactive Rhomboid protease iRHOM2 leads to a much thinner mammalian footpad with reduced keratinocyte hyperproliferation. In contrast, we identified inherited gain-of-function mutations in RHBDF2 (encoding iRHOM2) associated with a palmoplantar keratoderma and squamous oesophageal cancer syndrome (termed TOC). iRHOM2 is a key mediator of ADAM17 (TNFα converting enzyme; TACE). Gain of function TOC-associated iRHOM2 mutations cause an increase in activity and maturation of ADAM17 with increased shedding of ADAM17 substrates. Towards understanding the upstream regulation of iRHOM2, the sequence was analysed by bioinformatics software. A p63 binding site was found and binding was confirmed by utilising a ChIP-seq dataset in both human and mouse. We found that iRHOM2 is a novel target of p63 and both p63 and iRHOM2 can differentially regulate cellular stress associated signalling pathways. In normal keratinocytes, p63 positively regulates iRHOM2, while iRHOM2 antagonizes p63 expression. In contrast, in hyperproliferative keratinocytes including palmoplantar skin, there is an auto-regulatory feedback loop occurring between p63 and iRHOM2. We demonstrate that this p63-iRHOM2 axis regulates cell survival and oxidative stress via modulation of survivin and cytoglobin respectively. Furthermore, the antioxidant compound Sulforaphane can downregulate p63-iRHOM2 expression, leading to reduced inflammation, cell survival and ROS production. Together these findings elucidate a novel p63 associated pathway that identifies iRHOM2 modulation as a potential therapeutic target to treat hyperproliferative skin disease and neoplasia.

Keratin 16 (K16) is a cytoskeletal scaffolding protein highly expressed at pressure-bearing sites of the mammalian footpad. It can be induced in hyperproliferative states such as wound healing, inflammation and cancer. Here we show that the inactive rhomboid protease RHBDF2 (iRHOM2) regulates thickening of the footpad epidermis through its interaction with K16. K16 expression is absent in the thinned footpads of irhom2 −/− mice compared with irhom2 +/+ mice, due to reduced keratinocyte proliferation. Gain-of-function mutations in iRHOM2 underlie Tylosis with oesophageal cancer (TOC), characterized by palmoplantar thickening, upregulate K16 with robust downregulation of its type II keratin binding partner, K6. By orchestrating the remodelling and turnover of K16, and uncoupling it from K6, iRHOM2 regulates the epithelial response to physical stress. These findings contribute to our understanding of the molecular mechanisms underlying hyperproliferation of the palmoplantar epidermis in both physiological and disease states, and how this ‘stress’ keratin is regulated.

The vitally important skin barrier is formed by extensive cross-linking activity of transglutaminases (TGs) during terminal epidermal differentiation. We have previously shown that epidermal deficiency of a disintegrin and metalloproteinase 17 (ADAM17), the principal EGFR ligand sheddase, results in postnatal skin barrier defects in mice due to impeded TG activity. However, the mechanism by which ADAM17/EGFR signalling maintains TG activity during epidermal differentiation remains elusive. Here we demonstrate that ADAM17-dependent EGFR signalling promotes TG activity in keratinocytes committed to terminal differentiation by direct induction of TG1 expression. Restored TG1 expression of EGF-stimulated differentiated Adam17 −/− keratinocytes was strongly repressed by inhibitors for PLCγ1 or protein kinase C (PKC) pathways, while treatment with the PKC stimulator 12- O -tetradecanoylphorbol-13-acetate restored TG activity in the epidermis of keratinocyte-specific Adam17 −/− ( AD17 Δ KC ) mice. Further investigations emphasized the expression of PKCη, a mediator of TGM1 transcription, to be sensitive to EGFR activation. In agreement, topical skin application of cholesterol sulfate, an activator of PKCη, significantly improved TG activity in epidermis of AD17 Δ KC mice. Our results suggest ADAM17/EGFR-driven PLCγ1 and PKC pathways as important promoters of TG1 expression during terminal keratinocyte differentiation. These findings may help to identify new therapeutic targets for inflammatory skin diseases related to epidermal barrier defects.

The protease ADAM17 (a disintegrin and metalloproteinase 17) catalyzes the shedding of various transmembrane proteins from the surface of cells, including tumor necrosis factor (TNF) and its receptors. Liberation of TNF receptors (TNFRs) from cell surfaces can dampen the cellular response to TNF, a cytokine that is critical in the innate immune response and promotes programmed cell death but can also promote sepsis. Catalytically inactive members of the rhomboid family of proteases, iRhom1 and iRhom2, mediate the intracellular transport and maturation of ADAM17. Using a genetic screen, we found that the presence of either iRhom1 or iRhom2 lacking part of their extended amino-terminal cytoplasmic domain (herein referred to as ΔN) increases ADAM17 activity, TNFR shedding, and resistance to TNF-induced cell death in fibrosarcoma cells. Inhibitors of ADAM17, but not of other ADAM family members, prevented the effects of iRhom-ΔN expression. iRhom1 and iRhom2 were functionally redundant, suggesting a conserved role for the iRhom amino termini. Cells from patients with a dominantly inherited cancer susceptibility syndrome called tylosis with esophageal cancer (TOC) have amino-terminal mutations in iRhom2. Keratinocytes from TOC patients exhibited increased TNFR1 shedding compared with cells from healthy donors. Our results explain how loss of the amino terminus in iRhom1 and iRhom2 impairs TNF signaling, despite enhancing ADAM17 activity, and may explain how mutations in the amino-terminal region contribute to the cancer predisposition syndrome TOC.

iRHOM2 is a highly conserved, catalytically inactive member of the Rhomboid family, which has recently been shown to regulate the maturation of the multi-substrate ectodomain sheddase enzyme ADAM17 (TACE) in macrophages. Dominant iRHOM2 mutations are the cause of the inherited cutaneous and oesophageal cancer-susceptibility syndrome tylosis with oesophageal cancer (TOC), suggesting a role for this protein in epithelial cells. Here, using tissues derived from TOC patients, we demonstrate that TOC-associated mutations in iRHOM2 cause an increase in the maturation and activity of ADAM17 in epidermal keratinocytes, resulting in significantly upregulated shedding of ADAM17 substrates, including EGF-family growth factors and pro-inflammatory cytokines. This activity is accompanied by increased EGFR activity, increased desmosome processing and the presence of immature epidermal desmosomes, upregulated epidermal transglutaminase activity and heightened resistance to Staphylococcal infection in TOC keratinocytes. Many of these features are consistent with the presence of a constitutive wound-healing-like phenotype in TOC epidermis, which may shed light on a novel pathway in skin repair, regeneration and inflammation.

Dermatologists are frequently asked to see patients with pustular eruptions caused by EGFR mAbs or tyrosine kinase inhibitors. In this issue, Campbell et al. describe an infant with severely inflammed skin and bowel and lung disease caused by a homozygous mutation in the EGFR gene. This commentary discusses the power of exome sequencing in disease gene discovery within the rare genodermatoses and the role of aberrant EGFR signaling in a subset of monogenic skin and epithelial syndromes.

The Rhomboids represent a relatively recently discovered family of proteins, consisting in a variety of intramembrane serine proteases and their inactive homologues, the iRhoms. Rhomboids typically contain six or seven transmembrane domains (TMD) and have been classified into four subgroups: Secretase A and B, Presenilin-Associated-Rhomboid-Like (PARL) and iRhoms. Although the iRhoms, iRhom1 and iRhom2, have lost their protease activity during evolution, they retain key non-protease functions and have been implicated in the regulation of epidermal growth factor (EGF) signalling. EGF is moreover a substrate of RHBDL2, their active Rhomboid relative. Other substrates of RHBDL2 include members of the EphrinB family and thrombomodulin. RHBDL2 has also previously been demonstrated to be important in wound healing in cutaneous keratinocytes through the cleavage of thrombomodulin. Additional roles for these intriguing proteins seem likely to be revealed in the future. This review focuses on our current understanding of Rhomboids and, in particular, on RHBDL2 and iRhom2 and their roles in cellular processes and human disease.

Objective Cryptogenic multifocal ulcerating stenosing enteritis (CMUSE) is an extremely rare, but devastating, disease of unknown aetiology. We investigated the genetic basis of this autosomal recessive condition in a pair of affected siblings who have 40-year histories of catastrophic gastrointestinal and extraintestinal disease. Design Genome-wide single-nucleotide polymorphism homozygosity mapping in the two affected family members combined with whole-exome sequencing of one affected sibling. This was followed by confirmatory Sanger sequencing of the likely disease-causing sequence variant and functional studies in affected and unaffected family members. Results Insertion/deletion variation analysis revealed the presence of a homozygous 4 bp deletion (g.155574_77delGTAA) in the PLA2G4A gene, located in the splice donor site directly after exon 17 (the penultimate exon) of the gene in both affected siblings. This introduces a frameshift of 10 amino acids before a premature stop codon (p.V707fsX10), which is predicted to result in the loss of 43 amino acids (residues 707–749) at the C-terminus of cytosolic phospholipase A2-α (cPLA2α). cPLA2α protein expression was undetectable in the gut of both siblings, with platelet aggregation and thromboxane A2 production, as functional assays for cPLA2α activity, grossly impaired. Conclusions We have identified mutations in PLA2G4A as a cause of CMUSE in two affected siblings. Further studies are needed to determine if mutations in this gene are also responsible for disease of a similar phenotype in other cases.

We performed genetic and immunohistochemical studies in a sister and brother with autosomal recessive neonatal inflammatory skin and bowel lesions. The girl died suddenly at 12 years of age from parvovirus B19-associated myocarditis; her brother had mild cardiomyopathy. We identified a loss-of-function mutation in ADAM17, which encodes a disintegrin and metalloproteinase 17 (also called tumor necrosis factor α [TNF-α]-converting enzyme, or TACE), as the probable cause of this syndrome. Peripheral-blood mononuclear cells (PBMCs) obtained from the brother at 17 years of age showed high levels of lipopolysaccharide-induced production of interleukin-1β and interleukin-6 but impaired release of TNF-α. Despite repeated skin infections, this young man has led a relatively normal life. (Funded by Barts and the London Charity and the European Commission Seventh Framework Programme.).

Cell-cell connectivity is an absolute requirement for the correct functioning of cells, tissues and entire organisms. At the level of the individual cell, direct cell-cell adherence and communication is mediated by the intercellular junction complexes: desmosomes, adherens, tight and gap junctions. A broad spectrum of inherited, infectious and auto-immune diseases can affect the proper function of intercellular junctions and result in either diseases affecting specific individual tissues or widespread syndromic conditions. A particularly diverse group of diseases result from direct or indirect disruption of desmosomes--a consequence of their importance in tissue integrity, their extensive distribution, complex structure, and the wide variety of functions their components accomplish. As a consequence, disruption of desmosomal assembly, structure or integrity disrupts not only their intercellular adhesive function but also their functions in cell communication and regulation, leading to such diverse pathologies as cardiomyopathy, epidermal and mucosal blistering, palmoplantar keratoderma, woolly hair, keratosis, epidermolysis bullosa, ectodermal dysplasia and alopecia. Here, as well as describing the importance of the other intercellular junctions, we focus primarily on the desmosome, its structure and its role in disease. We will examine the various pathologies that result from impairment of desmosome function and thereby demonstrate the importance of desmosomes to tissues and to the organism as a whole.