Arrhythmogenic right ventricular dysplasia/cardiomyopathy (ARVD/C) is a dominantly inherited cardiomyopathy that histologically is mainly localized in the right ventricle. The hallmark of the condition, seen in endomyocardial biopsies, is fibrofatty replacement of cardiomyocytes.1 ARVD/C is intriguing, not only because it is associated with a high frequency of sudden death caused by ventricular arrhythmias at a relatively young age, but also because of its clinical and genetic characteristics: first, although there is evidence for an underlying genetic basis, genetic penetrance is low and the progressive clinical picture is highly variable; second, mild manifestations of the disease do not exclude the risk of sudden death. The low penetrance and clinical variability present a challenge for accurate clinical diagnosis, which so far has relied on criteria proposed by an International Task Force.2 These criteria are specific but lack sensitivity, particularly in the early stages of disease, and some diseases, such as myocarditis, sarcoidosis and idiopathic right ventricular outflow tract tachycardias, can mimic ARVD/C. Asimaki et al. have now described a highly sensitive and specific immunohistochemical analysis of intercalated disk proteins in myocardial samples.3
The clinical criteria currently used2 are based on family history, the presence of structural alterations and right ventricular dysfunction, tissue characterization, arrhythmias and abnormalities in repolarization, depolarization and conduction. Routine assessment should include two-dimensional echocardiography, electrocardiography, signal-averaged electrocardiography, 24-hour Holter monitoring, stress test and family history. If the results are inconclusive, additional imaging techniques (cine-angiography, MRI or radionuclide scintigraphy) and an endomyocardial biopsy should be considered. Appropriate interpretation requires experience and a complete evaluation but this is not always feasible. Endomyocardial biopsy, for example, is not always performed because of the risk of complications (perforation) during the procedure—the typical histological features (fibrofatty replacement of cardiomyocytes) occur predominantly in the thin and fragile right ventricular free wall.
The discovery of genetic mutations as the underlying cause of ARVD/C was an important breakthrough
The discovery of genetic mutations as the underlying cause of ARVD/C was an important breakthrough. Mutations were identified in genes encoding proteins of the cardiac desmosome (Figure 1),4, 5 which is important for mechanical cell-to-cell adhesion. Pathogenic mutations in the gene encoding the desmosomal protein plakophilin 2 (PKP2) are particularly prevalent and have been identified in up to 43% of cases.5, 6, 7 Mutations in other genes encoding desmosomal proteins, including desmocollin 2, desmoglein 2, desmoplakin and plakoglobin, have also been identified.8 The expanding genetic possibilities could facilitate diagnosis, especially in equivocal cases. In addition, the presence of a mutation in an individual with an affected, mutation-carrying relative identifies those at risk and thus requiring regular monitoring, whereas those without the disease-causing mutation would not need monitoring. However, the genetic revolution in ARVD/C is not complete as mutations have been identified in only approximately 50% of cases. In addition, more and more patients are being identified who have more than one mutation in at least one ARVD/C-related gene (compound-heterozygous, bigenic involvement).9 This suggests that disease severity might be attributed to multiple genetic variants; however, their pathogenicity is not always clear because functional characterization is often lacking.
Figure 1 | Schematic representation of the cardiac desmosome showing the major proteins involved.
Abbreviations: DES, desmin; DM, dense midline; DSC2, desmocollin 2; DSG2, desmoglein 2; DSP, desmoplakin; IDP, inner dense plaque; ODP, outer dense plaque; PG, plakoglobin; PKP2, plakophilin 2; PM, plasma membrane. Permission obtained from Wolters Kluwer Health © van Tintelen, J. P. et al. Curr. Opin. Cardiol. 22, 185–192 (2007).
To address the problems in diagnosing ARVD/C, Asimaki et al. focused on the levels of desmosomal proteins at myocardial cell–cell junctions. They used immunohistochemistry to assess samples from three groups of patients. The first group consisted of 11 patients with documented ARVD/C. Myocardial samples were obtained either from autopsy (n = 9) or endomyocardial biopsy (n = 2). Eight of these patients carried a desmosomal protein gene mutation. Interestingly, all patients showed a clear reduction in the immunoreactive signal for plakoglobin at the intercalated disks, whereas this was not observed in control samples. In most of the patients, levels of desmoplakin and plakophilin 2 were also reduced.
To determine whether the decrease in plakoglobin levels is specific for ARVD/C, myocardium from a second group, consisting of 15 patients with end-stage heart disease (five individuals each with hypertrophic, dilated or ischemic heart disease), was assessed. Decreased levels of desmoplakin were found in two patients and an increased level in one patient, all with dilated cardiomyopathy. Levels of all other proteins, including plakoglobin, were normal.
Finally, to confirm the usefulness of immunohistochemistry in identifying ARVD/C, immunostaining was performed on tissue samples from a third group consisting of ARVD/C (n = 11), non-ARVD/C (n = 14) and possible ARVD/C (n = 5) patients. In five patients the tissue quality precluded immunohistochemical diagnosis. In 22 patients, in whom immunohistochemical interpretation was possible, a definite diagnosis was made on the basis of clinical criteria. Immunohistochemistry led to a correct diagnosis in 20 of 22 patients: 10 of 11 with documented ARVD/C and 10 of 11 without ARVD/C. The one non-ARVD/C patient with reduced plakoglobin levels had been diagnosed with sarcoidosis. This disorder can mimic ARVD/C, but no information on the clinical presentation was given. A diagnosis of ARVD/C was made in one of three patients classified as having possible ARVD/C. A reduced level of plakoglobin showed a sensitivity of 91% and a specificity of 82%, with positive and negative predictive values of 83% and 90%, respectively, indicating that a reduced immunoreactive signal of plakoglobin at the intercalated disk is a consistent feature in patients with ARVD/C.
An important observation by Asimaki et al. was that plakoglobin was also reduced at the intercalated disks in seemingly normal regions of the myocardium, such as the left ventricle and the interventricular septum. This has important clinical consequences as routine endomyocardial sampling from the right side of the septum is a relatively safe procedure, compared with sampling from the affected right ventricular free wall, which is associated with the risk of perforation. The discovery that the immunohistochemical alterations are not related specifically to the right ventricle, but can be found at all right and left ventricular sites, is particularly intriguing from the pathogenic point of view. Would it be justified to delete the word 'right' from arrhythmogenic right ventricular dysplasia/cardiomyopathy?
The presence of immunohistochemical abnormalities in the absence of histological abnormalities suggests that the immunohistochemical changes precede the histological ones. Further studies are required to validate these results in larger groups of patients with ARVD/C and phenocopies. However, confirmation of this hypothesis will represent a major step towards early diagnosis of ARVD/C, which is pivotal in preventing sudden, unexpected death at a young age.
