Pathobiology of arrhythmogenic right ventricular cardiomyopathy Paediatric

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1 Annals of Diagnostic Paediatric Pathology 2006, 10 (3 4): Copyright by Polish Paediatric Pathology Society Annals of Diagnostic Pathobiology of arrhythmogenic right ventricular cardiomyopathy Paediatric Pathology El bieta Czarnowska, Bo ena Cukrowska Department of Pathology The Children's Memorial Health Institute Warsaw, Poland Abstract Arrhythmogenic right ventricular cardiomyopathy is a disease characterised by replacement of mainly right ventricular myocardium by fatty or fibro-fatty tissue but its pathomechanizm still not completely known. In the review clinical features of the disease, histopathology of endomyocardial biopsy specimens, disease pathobiology are presented, and pathomechanism in the context of genetic mutations is discussed. Key words: arrhythmogenic right ventricular cardiomyopathy, anatomopathology, histopathology, gene mutation, pathomechanism Arrhythmogenic right ventricle cardiomyopathy (ARVC) is a myocardial disease characterized by replacement of myocardium by fatty or fibro-fatty tissue, primarily in the right ventricle (RV) and involvement of the left ventricle in nearly half cases. [29]. The disease processes originate from subepicardium and extend to the subendocardium, usually (in 80% cases) sparing ventricular septum [8]. ARVC familial occurrence with autosomal dominant inheritance, variable expression and incomplete penetrance is estimated from 1/2000 to 1/5000 [10]. The disease affects men more frequently than women in ratio 3:1. Heredofamilial background is found in about 50% of cases. Usually the disease inheritance is autosomal dominant with incomplete penetrance and variable expression. Till now, six of disease genes (plakoglobin, desmoplakin, plakophilin-2, desmoglein- 2, RyR2, TGß3) have been identified [4, 13, 22, 27, 29, 31]. The disease is at high risk of sudden death, particularly in young people and athletes [9]. Pathogenesis of the disease despite of genetic findings is still poorly known. Clinical features Characteristic clinical abnormalities include delayed depolarization, epsilon waves and inverted T waves in the right precordial leads, late potentials, ventricular dilatation, systolic and wall motion injury. Palpitation and syncope due to occurrence of ventricular tachycardia of right ventricular origin, with left bundle branch block are frequent. Electrical instability may be triggered by exercise or stress. Standarized diagnostic criteria of ARVC diagnosis are based on the presence of 2 major or 1 major plus 2 minor or 4 minor criteria established by an international task force [21], and shown in Table 1. It could be pointed that clinical presentation of the disease is related to range of myocardial pathology and can subdivided into three phases:! Concealed phase without symptoms and minor electrical instability; patients are frequently asymptomatic but at risk of sudden death, particularly during intense physical exertion;! Overt phase with symptomatic ventricular arrhythmia, patients present palpitation, and/or syncope, morphological abnormalities can be detect by imaging techniques (Echo, Magnetic Resonance);! Advanced phase with sever myocardial loss causing impairment contractility of the right ventricle [7]. In the advanced phase the disease may clinically mimic dilated cardiomyopathy and in the end stage the left ventricle abnormalities may be involved. It should be emphasized that patients often present broader spectrum of disease Address for correspondence El bieta Czarnowska tel Department of Pathology fax The Children s Memorial Health Institute e.czarnowska@czd.pl Warsaw Al. Dzieci Polskich 20

2 66 Table 1 Major and minor criteria for clinical diagnosis of ARVC. [adopted from Corrado et al. 2006] MAJOR MINOR amily history disease confirmed at necroscopy sudden death at <35 year age due or surgery to suspected ARVC ECG depolarization/ epsilon waves or QRS complex late potentials on signal averaged ECG conduction abn in right precordial leads (V 1 V 3 ) Arrhythmias inverted T waves in right precordial leads (V 2 &V 3 ) in patients >12year and absence RBBB sustained or nonsustained LBBB, ventricular tachycardia Documented in ECG or Holter, extrasystoles (>1000/24h on Holter) global or regional severe dilation and reductionof RV ejection mild global RV dilation or ejection fraction dysfunction and fraction withwith no or mild LV involvement; reduction with normal LV; segmental structural alterations localized RV aneurysms severe segmental dilation of RV, regional hypokinesia dilatation of RV EMBs fibrofatty replacement of myocardium Abbreviations: RBBB right bundle branch block, LBBB left bundle branch block, abn abnormal, EMBs endomyocardial biopsies, ECG electrocardiogram and the phases mentioned above can be hardly recognized. The disease expression is hardly diagnosed particularly at early stages and in children, who did not yet develop full clinical disease features. Anatomopathological features Two variants of ARVC hearts according to the nature of myocardium replacement by fatty and fibrofatty tissue can be distinguished (ig. 1) [11]. This replacement occurs in a segmental, patchy fashion. The consequences of ventricle wall structural injury are diffuse hypokinesis or regional wall motion abnormalities. The presence of fat and/or fibrosis in the RV coexisted with the degenerative changes of the myocytes trapped within area and their loss are adequate in diagnosis of ARVC [8]. In the fatty variant diffuse (in 20% cases) or segmental (80% cases) pattern of fatty localization in the RV is observed. Moreover, septum is involved in 20% cases [2]. A marked decrease of myocardial cells associated with replacement by fatty cells starts in the area called triangle of dysplasia that is the right ventricle outflow tract, the apex and the inferior wall. In this type of heart infiltration of inflammatory cells is rarely observed. The fibro-fatty variant hearts exhibit thickening of the right ventricle wall < 3mm and inflammatory cell infiltration in 2/3 of cases [2]. This infiltration may contribute to electrical instability and sudden death and the disease progress to heart failure. What is an etiologic agent triggering inflammatory process remains not clear since the presence of cardiotropic viruses is controversial [7]. Some investigators did not find cardiotropic viruses [6] while other reported the presence of retroviruses and adenoviruses in 59% cases of 12 sporadic ARVC [5,14]. indings in endomyocardial biopsy samples ig. 1 Progressive fatty replacement of the myocardium in the right ventricle wall (RV) with thinning of the RV wall The histological findings of fatty or fibrofatty tissue in endomyocardial biopsy (EMB) samples are not diagnostic per

3 67 se and they can be observed in myocarditis and other cardiomyopathies. In ARVC fatty or fibrofatty tissue in EMBs coexist with cardiomyocytes loss. Analysis of significant number of biopsy samples made by Angelini et al. suggests that the amount of fibro-fatty tissue exceeding 43% of the samples together with loss of myocytes is diagnostic [1] (ig. 2). It has also been shown that EMBs analysis improves diagnostic accuracy in ARVC, being more sensitive tool than nuclear magnetic resonance (89% vs. 56%) [23]. It must be however taken into consideration that analysis of biopsy specimens can be negative at early stages of the disease and in children due to segmental pattern of myocardial replacement. ig. 3 Cardiomyocyte couplings at intercalated disc. A) Pale contrast of junctions and long desmosome in this area (arrow). Microscopic magnification B) Region of the junctions with series of short desmosomes (arrows). Microscopic magnification Genetic ig. 2 Section of the right ventricle endomyocardial biopsy samples stained with Trichrom. Microscopic magnification 20. Adipose tissue, myocytes red staining, fibrous tissue blue staining A high cell death by apoptosis was reported in ARVC, although there is discrepancy among investigators about intensity of this process [20, 25, 32]. The apoptosis has been discussed as potential mechanism of myocardial atrophy and consequence of desmosomes injury. Recent electron microscopical studies revealed abnormalities of cellular junction located at intercalated disc, i.e. desmosomes, fascia adherens and nexuses. In the past has been noticed that couplings at intercalated disc are pale [15]. The recent morphometric analysis have shown the increase of desmosome mean length and wideness of gap between cell membrane in the area of desmosmes and fascia adherens (ig. 3A), and presence of abnormally short junctions composed of series reaping couplings (found in 11 out of 21 cases) (ig. 3B), and decreased number of nexuses per 10 m cell membrane unit [3]. No statistically significant differences among the ultrastructural features have been found between cases with diagnosed gene mutation and genotype negative patients [3]. ARVC is mostly inherited in autosomal dominant fashion but recessive variant called Naxos disease is also known. Ten types of disease are known (Table 2). Naxos disease is characterized by common heart abnormalities coexisted with palmoplantar keratoderma and wool hair. Two base pair deletion in the plakoglobine gene has been identified [21]. Plakoglobin is one of major constituent of desmosmes. Table 2 ARVC types and genetics. D autosomal dominant, R autosomal recessive, TG-ß3 transforming growth factor beta-3, RyR2 ryanodine receptor 2, DSP desmoplakine, PKP2 plakophilin 2, DSG2 desmoglein 2, PKG plakoglobin Disease Chromosome Gene Mode of type locus defect transmission AKPK1 14q24.3 TG- 3 D AKPK2 1q42-q43 RyR2 D AKPK3 14q12-q22? D AKPK4 2q32.1-q32.3? D AKPK5 3p23? D AKPK6 10p12-p14? D AKPK7 10q22.3? D AKPK8 6p24 DSP D AKPK9 PKP2 D AKPK10 18q12,1 DSG2 D Naxos disease 17q21 PKG R

4 68 Among gene linked to autosomal dominant ARVC were found genes coding other than plakoglobin proteins related to mechanical junctions, i.e. desmoplakin [28], plakophilin-2 [13], desmoglein-2 [27]. Desmoplakin together with plakoglobin anchores to cadherins, desmoglein or desmocolin, which comprise the transmembrane component of desmosome. Plakophilin is located in the outer dense plaque of desmosomes and links cadherins to desmoplakine. Because these four proteins are involved in ARVC the disease is considered as a disease of the desmosomes. Desmosomes proteins are linked to actin network while fascia adherens proteins to intermediate filaments e.g. desmin network. Therefore, abnormalities in proteins of both desmosomes and fascia adherens might lead to impairment of cell-cell contact [17]. Impaired cell mechanical coupling may predispose cardiomyocytes to detachment and death under conditions of mechanical stress. Destabilization of cell-cell adhesion complex also disturb normal turnover of conexines, the element of gap junction (nexuses), which play a role in electrical cell coupling. Diminished expression of connexin 43 at the intercalated disc has been also observed in patients with Naxos disease [16]. This feature in the context of the role of nexuses in ion transport and transfer of electrical stimulation may result in heterogeneous electrical conduction and arrhythmia. Other found genes linked to autosomal dominant ARVC are cardiac ryanodine receptor 2 (RyR2) and transforming growth factor beta-3 (TG -3). Mutations in gene RyR2 results from substitution of amino acids in domains which are critical for regulation of the calcium channel. Up to present mutations in RyR2 association with cardiac diseases beside of ARVC 2 has been also found in catecholaminergic polymorphic ventricular tachycardia [28] and familial polymorphic ventricular tachycardia diseases [18]. These diseases and ARVC are characterized by effort-induced polymorphic ventricular arrhythmias and a risk of sudden death. These might be due to fact that mutation in RyR2 alter the ability of calcium channel to remains closed and thus stress or physical effort initiate intercellular calcium overload leading to severe arrhythmias. In affected probants of ARVC type 1 a nucleotide substitution in 5 UTR and in one subject substitution in 3 UTR in the TG-ß3 gene were detected [4]. It is known that transforming growth factor of TG-ß family inhibits proliferation of many type of cells and stimulate mesenchymal cells to proliferate and promote fibrosis [19]. Pathomechanism In the context of defects in intercellular junction of cardiomyocytes it suggested that volume overload in the right ventricle would produce myocardial over-stretch. It is known that stretched cardiomyocytes exhibit abnormal release of calcium from ryanodine receptor channel [26] what affects intracellular calcium concentration. Volume overload in subjects, carriers of mutation in RYR2 can also lead to calcium overload. Both disconnection of cellular junctions and calcium overload can be also related to increased cardiomyocyte apoptosis (ig. 5). It has been suggested that fibrofatty replacement is repair process. However recently Garcia- -Gras et al. (2006) exploring the effects desmoplakine deficiency on Wnt/ß catenin signaling found that translocation of plakoglobin into cardiomyocyte nuclei led to up-regulation of genes implicated in adipogenesis [12]. This signaling pathway could clear increased quantity of fat in ARVC hearts. ig. 4 Molecular model of adherens junction and desmosome organization ig. 5 Proposed patomechanism of ARVC

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