Prof. Dr Marcel F. Jonkman, MD, PhD, Center for Blistering Skin Diseases, Department of Dermatology, University Medical Center Groningen, PO Box 30.001, 9700 RB Groningen, The Netherlands, Tel.: +31 50 3612520, Fax: +31 50 3612624, e-mail: email@example.com
Abstract: Both skin and heart are subject to shear mechanical stress and need to be stress-resistant in a flexible way. The intercellular connecting structures in skin and heart, the desmosomes, that have to resist these forces show remarkable resemblance in epidermis and myocardium. Mutations in desmosomal proteins lead to inherited desmosomal cardiocutaneous syndromes (DCCS): une liaison dangereuse. This article will critically review the cutaneous and cardiac features as well as the molecular background of DCCS, such as Naxos disease and Carvajal syndrome caused by deficiencies of plakoglobin and desmoplakin respectively. In addition, potential other desmosomal gene candidates for an involvement in cardiocutaneous syndromes are considered. The skin features in these syndromes may be the hallmark for the presence of progressive and ultimately lethal cardiac disease. Knowledge of these skin features and early recognition of such a syndrome may provide opportunities to halt or slow down cardiac disease progression, treat arrhythmias and even prevent sudden death.
Skin and heart have to restrain considerable mechanical forces and need to be flexible at the same time. The intercellular connecting structures, the adhering junctions (desmosomes in the skin and desmosome-like structures in the area composita in the heart), which mediate this dynamic resistance, show remarkable ultrastructural similarities between both organs (Fig. 1). The plaque proteins plakoglobin (PG) and desmoplakin (Dsp) are present in both, and mutations in their genes give rise to DCCS such as Naxos disease (MIM #601214) and Carvajal syndrome (MIM #605676), respectively (1,2). Clinically, DCCS are characterized by the quartet woolly hair (WH), palmoplantar keratoderma (PPK), skin fragility and cardiac abnormalities. Each of these features may also be observed separate from each other as non-syndromic disorders. The combination of WH and PPK should be considered a warning sign for the presence of cardiac abnormalities. In this review, the clinical features and molecular background of the DCCS will be discussed.
Desmosomes: similarities and differences between skin and heart
Desmosomes (composition of the Greek words ‘desmos’ meaning bond and ‘soma’ meaning body) are intercellular structures linking the intermediate filament (IF) cytoskeletons from neighbouring cells and providing intercellular bonding in many stress-bearing tissues, in particular skin and heart. Desmosomes also function in differentiation and tissue morphogenesis (reviewed in 3–6). Desmosomal proteins are derived from three gene families: cadherins, armadillo proteins and plakins (reviewed in 4,7). Cadherins [desmocollins (Dsc) 1–3 and desmogleins (Dsg) 1–4] form the extracellular connections by homophilic and heterophilic bonding. The cytoplasmic tails of cadherins bind to the armadillo proteins PG (encoded by the JUP gene) and the plakophilins (PKP) 1–3, which form the outer dense plaque of the desmosome, visible by electron microscopy (EM; Fig. 1). These armadillo proteins in turn bind to the N-terminus of the plakin protein Dsp, which by its C-terminal plakin-repeat domain links the IFs to this plaque and forms the inner dense plaque. Lateral interactions and other proteins strengthen these connections (Fig. 2). Some of the desmosomal proteins in skin and heart differ. The extracellular linkage in epidermis is formed by the desmosomal cadherins Dsg1, Dsg3, Dsg4, Dsc1, Dsc3 and the glycoprotein corneodesmosin (CDSN) while in myocardium, Dsg2 and Dsc2 function as the extracellular linkers. PKP1 and PKP3 are the major epidermal PKPs, whereas PKP2 is the sole PKP present in cardiac tissue. Desmosomal proteins associated with human genetic diseases showing skin and/or ectodermal abnormalities without cardiac features are Dsg1, Dsg4, PKP1, CDSN and DSP, and those related to cardiac disease without skin features are Dsg2, Dsc2, PKP2, Dsp and PG. Desmosomal proteins shared by epidermis and myocardium are Dsc2, Dsg2 (although only in very low level in epidermis), PKP2, Dsp, PG and plectin. These proteins have all been associated with one or more of the DCCS features quartet, but only PG, Dsp, and recently Dsc2, have been associated with the full cardiocutaneous syndrome.
While desmosomes anchor IFs, adherens junctions (fascia adhaerens in myocardium) anchor actin filaments (Fig. 2) and gap junctions allow small molecular exchange between neighbouring cells. Desmosomes, adherens junctions and gap junctions seem to be closely interacting and dependent on each other. For example, Dsp gene knockout mice show adherens junction abnormalities as well (8). In addition, in keratinocytes of PG knockout mice, β-catenin, which normally is only present in adherens junctions, seems to take over part of PG function (9,10). Furthermore, inhibition of PKP2 synthesis in rat cardiac cells caused redistribution of the gap junction protein connexin 43 (Cx43) to the intracellular space and a decrease in coupling of the cells (11,12). Mutations in PG, Dsp and PKP2 in humans also affected Cx43 synthesis and localization in the cell (13–16). That this is not the other way around is indicated by cardiac-specific Cx43 knockout mice, which do not show any abnormalities in desmosomes or adherens junctions (17). Interestingly, of all desmosomal proteins, PG is present in both desmosomes and adherens junctions in skin and heart (18). Moreover, Franke et al. observed that in myocardial intercalated discs (IDs), other desmosomal proteins besides PG are not only present in desmosome-resembling structures, but also in ultrastructurally fascia adhaerens-resembling structures as well, and suggested the term ‘area composita’ (19,20). Goossens et al. (21) made similar observations. They also showed that PKPs associate with αT-catenin, forming another hybrid link between the cadherin–catenin complex and desmosomal proteins. These findings point to a mixed-type junctional structure in the myocardial ID. In skin, the desmosomes and adherens junctions appear as more distinct structures with only PG found in both junctions.
PG (or γ-catenin) links Dsp in desmosomes and actin in adherens junctions to the intercellular cadherins (for review see 6 and 22). Evidence has been found that PG also performs nuclear signalling. In the pathogenesis of pemphigus vulgaris (PV), an autoimmune blistering disease with auto-antibodies against desmosomal cadherins, PG signalling seems to play a crucial role in the pathogenesis, as PG null mice did not show blistering upon exposure to pathogenic auto-antibodies (23,24). PV desmosomes showed intracytoplasmic rupture with pinched off desmosomes similar as observed in PG−/− mice (25). Furthermore, PG translocation initiated nuclear signalling in epidermis (26). That this seems to be the case in myocardium as well is illustrated by studies of Garcia-Gras et al. (27) on Dsp haplo-insufficient mice and siRNA Dsp inhibited HL1 cells. The authors observed changes in the canonical and evolutionary conserved Wnt/β-catenin signalling pathway involved in the regulation of cell fate, proliferation and apoptosis. The Dsp haplo-insufficient mice showed cardiac abnormalities with PG nuclear translocation and upregulation of genes involved in adipogenesis suggesting a shift from a myocytes fate into an adipocyte fate. The macroscopic and microscopic cardiac changes in these mice mimicked cardiac structural changes observed in the hereditary heart disease arrhythmogenic right ventricular cardiomyopathy (ARVC, MIM #107970) in humans [for review see, i.e. (28,29)]. Of note, heterozygous PG deficiency provokes ARVC. Manifestation of the phenotype is accelerated by endurance training. This suggests a functional role for PG and training in the development of ARVC (30).
Dsp is located in the desmosomal plague where it anchors IF proteins (keratins in skin and desmin in heart) through its C-terminal plakin-repeat domain (Fig. 2). The N-terminal head domain binds PG, PKPs and cadherins in the outer dense plaque. Dsp therefore is the major linker in the desmosome. Alternative splicing of the DSP gene transcript generates two isoforms, which differ in the central rod domain (31). DspI is present in both skin and heart, whereas the smaller DspII is mainly present in skin and at very low levels in the heart (32). Dsp knockout studies have shown the essential role of Dsp in adequate tissue differentiation and cell–cell contact. Dsp knockout mice show embryonic lethality after implantation, but before gastrulation at around E6.5. The Dsp null embryos were smaller and had a fragile endoderm with weakened cell–cell junctions. Hardly any desmosomal structures could be detected, and these showed markedly impaired IF insertion (33). When Dsp was rescued in extraembryonic tissue, the mice survived somewhat longer and revealed myocardial and epidermal defects, as well as abnormal microvasculature and neuroepithelial defects (34). Epidermal specific Dsp knockout mice suffered from severe skin fragility showing sheetwise peeling of epidermis upon minor trauma leaving large areas of denuded skin (8). Dsp−/− epidermis displayed marked acantholysis of basal and spinous layers with desmosomes lacking their inner dense plaque. The split took place on the cytoplasmic side of desmosomes similar as observed in PG−/− keratinocytes (10). Keratin filaments formed perinuclear aggregates and lacked desmosomal insertion. Noteworthy, adherens junctions were markedly reduced and abnormal, and also defects in the actin cytoskeleton were observed indicating that normal desmosomes are required for proper adherens junction stabilization and actin cytoskeleton organization as well (8). DSP mutations in humans are associated with the quartet of clinical features that comprise DCCS (Fig. 3, Table S1).
Desmosomal cardiocutaneous syndromes
In 1986, Protonotarios et al. (35) reported about the triad of clinical features of WH, diffuse non-epidermolytic PPK and ARVC in four families from the Greek island Naxos. This DCCS was therefore named ‘Naxos disease’, and later also reported from other parts of the world (36–41) (for review see 42). WH, which may be sparse, brittle and hypopigmented as well, is present from birth and affects scalp, eyebrows, as well as axillary and pubic hair (Fig. 4b) (43,44). PPK develops during the first year of life and is of the diffuse type and may be surrounded by an erythematous border. Both WH and PPK precede clinically overt cardiac disease. No severe blistering or skin erosions have been reported in Naxos disease. Other cutaneous features observed are hyperhidrosis (45) and nail abnormalities (39), although it is not clear whether these are related to Naxos disease itself or comprise comorbidity. Cardiac disease becomes symptomatic during adolescence (youngest patient 13-year old) and syncope is usually the first sign (42). In adults almost 100% of affected people have clear electrocardiogram abnormalities. In the initial stages, arrhythmias can be present without macroscopic myocardial abnormalities. Sudden death due to arrhythmia is a major cause of death (one-third of patients prematurely die in a 10-year follow-up with a mean age of 32) (43). In the course of the disease, affected hearts show right ventricle dilation and fibrofatty displacement of myocardial tissue typical of ARVC (46,47). With further disease progression, left ventricular involvement and heart failure develop. WH was present in 14/40 heterozygous carriers. The cutaneous phenotype made it possible to identify children at risk for developing ARVC in 12 families with Naxos disease (43).
Molecular background Naxos disease
In 2000, McKoy et al. showed that a homozygous 2 bp deletion (c.2157delTG) in the JUP gene encoding PG underlies Naxos disease. The deletion caused a frameshift with subsequent truncation of the last 56 aminoacids of PG (1). Truncated PG was present but failed to localize at IDs in myocardium of patients (13). Interestingly, gap junction remodelling with reduced Cx43 was found early in Naxos disease, possibly explaining the heart rhythm disturbances. Reduction of Cx43 at gap junctions was observed in epidermis of PG−/− mice, DSP−/− mice and Carvajal patients with truncated Dsp (8,10,15,48). In addition, inhibition of PKP2 synthesis in cardiomyocytes results in a notable decrease in Cx43 at cell–cell contact sites (11,12). These findings indicate an intimate cross-talk between desmosomal proteins and gap junctions and dependence of gap junctions on normal amounts and normal functioning of desmosomal proteins. It is tempting to speculate that JUP mutations and changes in PKP2 expression could affect gap junctions indirectly by causing altered PG binding and/or positioning in desmosomes and nuclei with downstream effects on the Wnt signalling pathway which is thought to be involved in regulating GJA1, the gene coding for Cx43, expression as well (49). Considering the non-epidermolytic PPK in Naxos disease, it is interesting to note that mutations in GJA1 have been associated with PPK (50,51). It could be hypothesized that Cx43 alterations caused by mutated PG are involved in the development of PPK in Naxos disease.
Plakoglobin knockout mice show a much more severe phenotype with embryonic lethality caused by cardiac rupture (52,53). The few mice that survive until birth additionally showed severe skin fragility, not observed in humans. Epidermis revealed intercellular widening, and rupture on the cytoplasmic site of the inner dense plaque, so that the complete desmosome is pinched off from the cell membrane (Fig. 5), similarly as in conditional Dsp knockout mice (8). In addition, epidermal desmosomes were reduced in number and larger than normal. Indeed studies indicate that PG plays a role in determining desmosomes size (54). Unfortunately, no EM of epidermal desmosomes in Naxos patients has been described. It would be interesting to investigate whether these are altered in size as well. In myocardium of PG−/− mice, desmosomes were markedly reduced in number (53). Interestingly, β-catenin appeared in desmosomes, indicating that this adherens junction protein can partially, but not sufficiently take over desmosomal PG function (9). This might account for the slightly longer survival of PG null mice compared with Dsp null mice and also for the less severe cardiac phenotype in Naxos patients compared with Carvajal syndrome patients. The milder phenotype of PG deficiency in human versus mice is probably due to the rest function of C-terminally truncated PG preventing embryological death, skin fragility and limiting PPK to non-epidermolytic. An alternative hypothesis is that acantholysis cannot take place because of the altered PG protein. Auto-antibodies against desmosomal cadherins were unable to induce acantholysis in PG−/− epidermis, whereas in the presence of PG, acantholysis developed, indicating that the acantholysis in the acquired autoimmune blistering disease PV is dependant on PG (23,24). Perhaps overt acantholysis in Naxos disease is lacking, because the truncated PG has lost its ability to ‘signal’ loss of cadherin binding in keratinocytes.
Following the description of the JUP:c.2157delTG mutation, only one other PG mutation has been associated with human disease. Asimaki et al. (14) described an autosomal dominant one aminoacid insertion in the PG N-terminus (p.S39_K40insS) in a family with non-syndromic ARVC. Normal PG has been shown to suppress epidermal proliferation and hair growth in vivo (55) and as a result, PG−/− keratinocytes are hyperproliferative (56) with PPK. Apparently, the N-terminal insertion mutation does not have this effect in heterozygous state in vivo as patients show normal hair and skin growth (14).
The Carvajal syndrome was named after Carvajal-Huerta who reported the quartet of WH, striate epidermolytic keratoderma (also referred to as Brunauer-Fohs-Siemens type PPK), skin fragility and a mainly left-sided dilated cardiomyopathy (DCM) in Ecuadorian families (57) (Fig. 4a). The differentiation from ARVC is not so clear cut, as in Naxos patients, ARVC in later stages progressed to a biventricular dilatation, hardly discernable from DCM. The clinical skin features also include linear keratoses in flexural areas, follicular keratosis on elbows and knees or scattered across abdomen and lower limbs, and clubbing of fingernails. Around half of patients experienced transient pruritic blistering on trunk, and extremities and psoriasiform keratoses on knees, extensor legs and dorsal aspect of the feet. Histopathology of affected skin showed acantholysis in spinous layers, a feature not observed in Naxos disease. The age at which the first cardiac abnormalities were observed ranged between 7 and 34 years old and without treatment had high mortality due to sudden death or heart failure within 10 years. Cardiac tissue of patients with Carvajal syndrome revealed ventricular hypertrophy and dilatation of particularly the left side, although the right ventricle and atrium were clearly affected as well. On immunofluorescence antigen mapping, reduced amounts of PG and Cx43 were observed. Desmin showed normal distribution but failed to insert at IDs. In the hearts of Carvajal patients, no fat depositions (pathognomonic for ARVC) were observed (15).
The clinical variations with other mutations in DSP are summoned (Fig. 3, Table S1). In one family with clinically affected members being homozygous for a missense mutation in Dsp C-terminus (DSP:p.G2375R), the skin features consisted of an extremely dry skin and skin blistering from childhood, mainly affecting palmoplantar skin and the knees (58). The index patient fulfilled the criteria for ARVC and therefore the definition ‘Naxos-like’ was used. Other patients with clinically ‘Naxos-like’ disease and a DSP mutation were described (32,59,60).
Molecular background Carvajal syndrome
In 2000, Norgett et al. (2) found that Carvajal syndrome was caused by a homozygous single nucleotide deletion in the last exon of DSP (c.7901delG), leading to truncation of a part of the C-domain in the tail of both Dsp isoforms (Fig. 3, Table S1). The resulting phenotype is much less severe than the phenotype in Dsp null mice and in a human patient with lethal acantholytic epidermolysis bullosa (LAEB). This particular patient was compound heterozygous for mutations, which both lead to truncations of the complete Dsp C-terminus in both DspI and DspII (Fig. 3, Table S1) (61). The patient clinically mimicked the conditional epidermal Dsp knockout mice by displaying severe neonatal shedding of large skin areas and early postnatal death. In addition, complete nail loss, neonatal teeth and universal alopecia were observed. Skin biopsies showed marked acantholysis with rupture of desmosomes at the cytoplasmic side between the inner dense plaque and the keratin filaments (Fig. 5). Desmosomes were normal in size and number, but lacked IF insertion. The child died ten days after birth because of heart failure, most likely caused by the combination of enormous amounts of fluid replacements to prevent dehydration and cardiac disease due to truncated Dsp.
Other DSP mutations associated with cardiocutaneous clinical features have been described. The homozygous non-sense mutation p.R1267X was found in a patient showing PPK, WH and severe biventricular cardiomyopathy with lethal ending at the age of 3 years old due to progressive heart failure (32). The compound heterozygous mutations c.2516del4 and c.3917del4 were found in another patient showing PPK, WH, skin fragility and a similar early-onset, severe biventricular DCM (60). Non-sense mutation p.Q673X in compound heterozygous state with another non-sense mutation, p.Q1446X, was associated with complete alopecia, PPK, skin fragility and mainly LV DCM leading to sudden death in a 9 year old (48). Mutations R1267X, 3917del4 and Q1446X are located in the DspI specific region and result in loss of DspI due to non-sense mutation mediated RNA decay. DspII could still be detected in the skin and probably also in the heart in the patient with the homozygous R1267X mutation (32). The phenotypes of these patients teach us that DspII is sufficient for embryonic development, formation of desmosomes and epidermal integrity. It however is not sufficient to fully compensate for the loss of DspI in the heart. The patients with the compound heterozygous DSP mutations additionally lacked DSPII from one allele and showed skin fragility in contrast to the patient with the homozygous R1267X mutation lacking only DspI, suggesting a dose effect (48,60). In addition, carriers of the truncating mutations in LAEB and Carvajal syndrome do not show skin, hair or heart pathology, implying a dose effect as well (57).
Woolly hair is one of the three cardinal features in DCCS. But what exactly do we consider as WH? WH represents a hair shaft abnormality clinically characterized by curly, fine hair with a soft woolly texture. The curls have an average diameter of around 0.5 cm and the hair shafts are ovoid, flattened or irregular (62). WH also exists as a non-syndromic ‘disease’, with both an autosomal dominant (MIM #194300) and autosomal recessive (MIM #278150) pattern of inheritance. In recessive WH, mutations have been found in the genes P2RY5 and LIPH (63,64). No gene has been found to be involved in the autosomal dominant WH families yet. WH may be part of a syndrome as well. Keratosis pilaris atrophicans faciei (also called ulerythema ophryogenes), Noonan syndrome, cardiofaciocutaneous syndrome and Costello syndrome are syndromes which have WH as one of the clinical features. In addition, WH has been associated with skin fragility in skin fragility-WH syndrome caused by mutations in DSP (65) (Fig. 3, Table S1).
Chien et al. (66) provided a practical evaluation scheme for patients with WH. An early differentiating feature in their algorithm of syndromic WH is keratosis pilaris. The authors consider it not present in the syndromes caused by desmosomal gene mutations. We however do not agree for two reasons: first, mutations in PG and Dsp have been associated with follicular hyperkeratosis [Naxos disease and Carvajal syndrome (57)], and second, keratosis pilaris is a very common feature in atopic constitution present in 10% of population. Therefore, excluding desmosomal protein mutations on the base of presence of keratosis pilaris does not seem appropriate.
WH in combination with PPK provides a valuable ‘warning signal’ for the development of cardiac disease (43). Considering the association of WH with the DCCS, many questions can be posed. By which mechanism do some mutations in DSP and JUP cause WH? First, animal studies showed that abrogation of desmosomal proteins causes hair deformities (67–70), and human mutations in genes encoding desmosomal proteins, like PKP1 and DSG4, have been associated with hair abnormalities (69,71). Secondly, mutations in PG and Dsp could cause morphological changes in hair formation by interfering with cell signalling pathways (27). A third possible mechanism is that mutations in desmosomal proteins exert their effect on hair morphology indirectly through affecting adherens junction proteins, like E-cadherin, P-cadherin, β-catenin and α-catenin, which have been proven to play an important role in hair follicle development (8,72). The presence of β-catenin in desmosomes of PG−/− keratinocytes might alter its cell signalling properties and have its effects on adherens junction composition as well. Some heterozygous carriers of the PG truncating mutation had WH while others had not, indicating that additional unknown genetic and environmental factors determine the outcome (43).
In patients with Dsp mutations, there seems to be no clear phenotype–genotype correlation (Fig. 3, Table S1). WH is only present in combination with PPK, either with or without cardiac disease. Heterozygous carriers are not affected (2,32,48,58,60,61,65).
Palmoplantar keratoderma is one of the hallmarks of DCCS caused by Dsp and PG mutations. Mutations in the proteins Dsg1 (73), keratin 1 (74) and Dsp (75,76) have been found in non-syndromic striate PPK (Fig. 3, Table S1). The mechanisms behind the development of PPK are still unclear. One hypothesis involves impaired keratinocyte integrity in the high-levels-of-stress-bearing palmoplantar skin. This could lead to a compensatory differentiation change and hyperkeratosis to protect from further loss of tissue integrity, or alternatively, cytokines released from ruptured cells could trigger epidermal proliferation and differentiation. Secondly, the mutation itself could affect a signalling site of the protein and thereby exert a change in differentiation. The alteration in differentiation by desmosome signalling may be mediated by secondary downregulation of connexins resulting in PPK. Another hypothesis is that altered desmosome composition in general and/or decreased number of desmosomes cause proliferational and differential changes.
It is also not clear why certain Dsp mutations cause striate PPK and others do not (Fig. 3, Table S1). A particular complicated observation is that loss of protein synthesis from one allele has been found in both striate PPK (75,76) and non-syndromic ARVC (77,78) and arrhythmogenic left ventricular cardiomyopathy (ALVC) (79–81). How can mutations, which are predicted to have exactly the same effect on the protein, result in such different phenotypes? It gets even more confusing when realizing that the heterozygous carriers of the p.R1267X mutation who only produce 50% of the normal DspI are completely healthy. They differ from the striate PPK families in having normal dose DspII, which might protect them from hair, skin and cardiac disease.
Cardiomyopathy: two sides of the same coin/heart?
The major differences between the cardiac disease in Carvajal syndrome and in Naxos disease are early and predominant left ventricular involvement and absence of adipose depositions in the former. The onset and progression of cardiac disease in Carvajal syndrome seems slightly earlier and more severe (42). The additional reports of DSP mutations associated with biventricular involvement or ARVC in patients with WH and PPK, and the involvement of DSP mutations in non-syndromic ARVC (77,78,80,82–85), as well as ALVC (79,81), indicate that Naxos disease and Carvajal syndrome comprise different outcomes of a similar initial desmosomal defect and reflect two sides of a spectrum (77,86).
In general, it is not well understood why some patients reveal a cardiomyopathy with predilection for the left side and others have a classical right-sided ARVC. Originally, ARVC was considered a right-sided matter in the heart, and left ventricle involvement was thought to be an end-stage phenomenon, occurring after development of right ventricle dilatation and dysfunction. This is expressed in the Task Force Criteria for diagnosing ARVC patients, in which left-sided involvement is even an exclusion criterion (47). However, more and more evidence indicates that left-sided involvement early in disease is more common than initially thought (81,87–92). In the large majority of ARVC patients, the end-stage of disease is biventricular DCM. Suggestions are made to use less strict criteria for diagnosing ARVC (28,88,89).
Regarding DSP mutations there seems to be a slight tendency for Dsp N-terminal missense mutations to cause predominant right-sided cardiac disease, whereas the C-terminal truncating mutations cause left-side involvement (Fig. 3, Table S1). Interestingly, the N-terminal mutations are predicted to interfere with PG binding. Considering PG mutations are involved in autosomal dominant and recessive (Naxos) ARVC, it could be hypothesized that mainly right-sided ARVC with fibrofatty cardiomyocyte displacement develops whenever PG functioning and/or signalling is affected. Of note, recently Asimaki et al. (80) showed that in myocardial samples of 11 ARVC affected people (of which eight carried a mutation in a desmosomal protein) immunoreactive signal levels for PG at IDs were markedly reduced compared with that of the normal control myocardial samples and samples from people with other cardiomyopathies who all showed normal PG levels. Predominantly left-sided cardiomyopathy seems to develop when Dsp–IF interaction is interrupted. In general, ARVC has been recognized as a desmosomal disease with mutations reported in all myocardially synthesized desmosomal proteins: Dsc2, Dsg2, PKP2, Dsp and PG (see also the ARVC database, http://www.arvcdatabase.info) (28,29). Initially, it was thought that the disrupted intercellular binding of myocytes leads to cell death inducing a general repair process with fibrosis and fat depositions in affected myocardium. As the right ventricle has a thinner wall, it would be more vulnerable to stress. Supportive for this idea is the notion that myocardial infarction is often associated with fibrosis and fat depositions as well. More recently, Garcia-Gras et al. (27) showed that Dsp suppression in mouse hearts and cultured cardiomyocytes caused PG nuclear translocation, inhibition of the canonical Wnt/β-catenin cell signalling pathway which is involved in the regulation of cell fate, proliferation and apoptosis and a transdifferentiation of myocyte to adipocyte fate of cardiac cells. The mice displayed an ARVC phenotype with fibrofatty displacement of cardiomyocytes. These findings indicate that changes in cell signalling processes and gene expression alterations are induced by mutations in desmosomal proteins (27). Furthermore, as mentioned above, markedly decreased PG was observed in myocardial samples of ARVC patients with mutations in different desmosomal proteins suggesting a common pathway involving PG (80). Of course, the two concepts are not mutually exclusive and perhaps loss of cell–cell contact in itself can induce changes in the Wnt/β-catenin signalling pathway and/or other pathways involved in adipogenesis, fibrosis and apoptosis. Alterations in the Wnt/β-catenin signalling pathway might turn out to be a final common pathway in ARVC. Additional functional protein studies and studies investigating these cell signalling pathway changes in desmosomal protein mutations, specifically the interactions between PG and β-catenin and how they influence the Wnt signalling, can give further insight in pathogenesis and lead to better understanding and eventually treatment of dominant and recessive desmosomal cardiomyopathies, left-sided, right-sided or both.
DCCS without mutations in JUP or DSP
Several cases have been described in which the patients revealed sparse and/or WH, PPK and cardiomyopathy without mutations in DSP or JUP being found (93,94). Recently, Simpson et al. (95) reported a homozygous DSC2 mutation (c.1841delG, p.S614fsX625) in two related patients with the clinical triad WH, PPK and ARVC with left ventricle involvement. Heterozygous DSC2 mutations have been found in a small proportion of non-syndromic ARVC patients (91,92). The heterozygous mutation DSC2:c.631-2A>G detected in an ARVC patient is particularly noteworthy as additional RNA and protein analysis have been performed (92). The results indicated loss of protein production from the mutated allele in cardiac tissue. The mutation was mimicked in zebrafish. Mutant embryos showed reduced DSC2 mRNA expression and developed profound cardiac abnormalities, which could be rescued for by dose-dependant co-injection with wildtype human DSC2 mRNA, but not with mutant mRNA. These findings indicate that the dose of Dsc2 is critical for normal cardiac function. Apparently, this dose effect does not apply to skin as the homozygous patient, nor the carriers described by Simpson et al. (95) showed skin abnormalities.
As they are present in both skin and heart, three other desmosomal candidates for involvement in DCCS consist of Dsg2 (18q12), PKP2 (12p11) and plectin (8q24). Dominant mutations in the former two proteins have already been found in non-syndromic ARVC/D (90,96,97). In addition, a homozygous mutation in Dsc2, the desmosomal heterophilic interaction partner of Dsg2 with similar tissue distribution, causes a DCCS similar to Naxos and Carvajal syndrome (described above) (95). As Dsg2 is closely located to, and interacting with Dsc2, it is a likely candidate for causing a cardiocutaneous syndrome as well. However, DSG2−/− mice showed embryonic lethality around blastocyst implantation, indicating an important function of Dsg2 in early development (98). Thus, mutations affecting both DSC2 alleles might be too detrimental and therefore not found in human disease. Conditional epidermal DSC2 knockout studies could provide further insight in the function of Dsg2 in epidermis.
The second desmosomal protein shared by epidermis and myocardium but without association with a cardiocutaneous syndrome is PKP2. PKP2 exists in two splice variants (a and b) and is present in desmosomes and/or in the nuclei in a wide variety of tissues, including keratinocytes and cardiomyocytes (99,100). PKP2 knockout mice showed embryonic lethality at mid-gestation at similar timing as PG knockout mice, caused by lethal defects in cardiac morphogenesis (101). These findings led Gerull et al. (97) to hypothesize and then prove, that PKP2 mutations can cause ARVC in humans, thus supporting the idea that ARVC is a ‘desmosomal disease’. The PKP2 null mice embryos did not show desmosomal or adherens junction abnormalities in the forming epidermis, however, the early lethality prohibited observation of effects of PKP2 absence in differentiated epidermis. It could be hypothesized that loss of PKP2 in epidermis can be compensated for by PKP1 and PKP3, while in myocardium PKP2 is essential as it is the only PKP present and consequently mutations in PKP2 will only affect myocardium. On the contrary, recessive null mutations in the epidermal and ectodermal specific PKP1 are associated with ectodermal dysplasia/skin fragility syndrome (MIM #604536) and indicate that in skin, PKP2 is not able to compensate for loss of PKP1 (71). PKP2 mutations in humans reported until now were all associated with non-syndromic ARVC without skin features. Downregulation of PKP2 in cultured cardiomyocytes causes loss of appropriate cell–cell coupling and loss of desmoplakin and Cx43 from cell–cell junctions, pointing to an important regulatory role of PKP2 in myocardial architecture and cell–cell coupling (11,12). Additional conditional epidermal PKP2 knockout animal models could shine light on the function of PKP2 in skin and whether lack of PKP2 causes skin abnormalities. It remains to be seen whether mutations in PKP2 in humans can cause a cardiocutaneous syndrome.
Plectin is a rather obscure desmosomal protein which is not considered in the majority of articles reviewing desmosomes as its position and function in desmosomes is not clear and is thought to be accessory. Plectin is a large and versatile cytolinker protein which belongs to the plakin family of proteins (102). Plectin is encoded by the PLEC1 gene. By alternative splicing, multiple alternative plectin isoforms are generated which are expressed in a cell-type and differentiation specific way (103–106). Plectin is present in a multitude of tissues where it is mainly localized at connection structures, like IDs and Z-lines in myocardium; hemidesmosomes, desmosomes and focal contacts in skin; Z-lines and costameres in skeletal muscle; desmosomes in intestinal epithelium (107–112). Plectin functions as a cytolinker and has been connected to the three major cytoskeletons: microfilaments, IFs and microtubules. In polarized cells, plectin was observed at desmosomal structures and associated with desmoplakin and IFs (112). However, the function of plectin in the desmosome seems to be ‘accessory’ as neither of the reported plectin mutations in humans caused desmosomal disintegration and/or acantholysis. Furthermore, desmosomes in plectin−/− mice had normal appearance indicating that plectin is not necessary for desmosome formation (108). Whatever its function in the desmosome may be, plectin knockout mice survive until birth, but die in the first postnatal days revealing considerable skin fragility, skeletal muscle pathology and cardiac abnormalities (108). In addition, specific skeletal and cardiac muscle plectin knockout mice showed cardiac pathology as well (113). Recently, we found plectin to be associated with cardiomyopathy in epidermolysis bullosa simplex with late-onset cardiomyopathy with conduction disturbances (114). It is tempting to speculate that plectin mutations might be involved in ARVC as well.
The DCCS are rare diseases. However, after the initial reports and the discovery of the genes involved additional reports of patients and families with similar clinical features, and in some cases confirmed by mutation detection have occurred in literature, indicating that it is not as rare as was initially thought (2). The WH is present form birth and the PPK from first years of life, anticipating the major cardiac problems. Therefore, knowledge of these syndromes by clinicians and especially dermatologists, general practitioners and cardiologists is of uttermost importance, as with early recognition and current treatment possibilities (no extreme exercise, medications, intra-cardiac devices, heart transplantation) morbidity and mortality caused by the cardiocutaneous syndromes may be delayed or even prevented (115). An illustrative example is provided by Kolar et al. (116) who present a case of an 18-year old girl with PPK initially diagnosed with Papillon-Lefevre syndrome (MIM #245000) who suddenly died of cardiac arrest. On forensic obduction, she turned out to have a DCM, as well as WH (and the already observed PPK), the combination of which suggests Carvajal syndrome. The authors emphasize the importance of early recognition of such a syndrome and early referral. The similarity between desmosomes in skin and heart is a dangerous one when considering DCCS, and the basis of pathogenesis is an altered binding structure: une liaison dangereuse.
What do desmosomal cardiocutaneous syndromes teach us?
• Mutations in widely expressed desmosomal protein encoding genes affect the tissues most exposed to mechanical stress: skin and heart.
• Woolly hair in combination with palmoplantar keratoderma is a ‘warning signal’ for the development of cardiac disease.
• The clinical features in desmosomal cardiocutaneous syndromes (DCCS) may not be the mere result of loss of cell–cell contact, but rather changes in complex signalling pathways induced by altered desmosomal proteins.
• Different cell–cell contacts in skin and heart, like desmosomes, adherens junctions and gap junctions are not independent entities, but show close interaction and interdependence.
• Arrhythmogenic right ventricular cardiomyopathy with fibrofatty deposition develops whenever plakoglobin functioning and/or signalling is affected, and perhaps even more broadly: when the Wnt/β-catenin pathway of signalling is affected.
• Desmoplakin-II from one allele is sufficient for embryonic development and formation of desmosomes. To protect from skin fragility, full doses of desmoplakin-II are necessary.
• The dose of desmocollin-2 is critical for normal cardiac function, but does not apply to skin.
• Plectin is important for maintenance of cardiac integrity in humans.
• Although present in epidermal and myocardial desmosomes, plakophilin-2 and desmoglein-2 have not been linked to a cardiocutaneous syndrome in humans (yet).