252 Plakophilin2b is not expressed in the heart: consequences for the genetic screening of arrhythmogenic right ventricular cardiomyopathy

Background Arrhythmogenic right ventricular cardiomyopathy (ARVC) is an inherited heart disease caused predominantly by heterozygous Plakophilin-2 (PKP2) mutations. However, few molecular studies have been performed to ascertain their pathogenicity. PKP2 encodes two isoforms, the longest (PKP2b) including the alternatively spliced exon 6, which is routinely screened for molecular diagnosis, despite the absence of data on the cardiac expression of PKP2 isoforms. This study addressed the pathogenicity of exon 6 mutations by focusing on a heterozygous missense p.Arg490Trp variant in PKP2. Methods and results The p.Arg490Trp variant of PKP2 was identified in two unrelated ARVC probands and absent from 470 controls. In silico analysis suggests that PKP2 exon 6 is an Alu-derived sequence with very low expression level. Transcriptional and Western blot analysis confirmed that the exon 6 missing PKP2a was the only clearly detectable isoform in all heart samples (four controls and a proband). Moreover, in the proband's heart sample, the variant was not associated with aberrant exon 6 splicing or mutant mRNA down-regulation. Finally, we identified in this proband a heterozygous missense variant (p.Glu2343Lys) in the desmoplakin gene likely to be the disease-causing mutation. Conclusion We demonstrated that only PKP2a is significantly expressed in the heart. Our results strongly suggest that the p.Arg490Trp mutation and other variants located in PKP2 exon 6 can not be considered as disease-causing mutations and therefore that PKP2 exon 6 screening is useless in routine ARVC molecular diagnosis. These results have important consequences for the interpretation of PKP2 mutation screening and subsequent genetic counseling.