Abstract Body (Do not enter title and authors here): Background information: Arrhythmogenic cardiomyopathies (ACM) account for >10% of sudden cardiac deaths (SCD) in the paediatric population. There is urgent need to drastically improve diagnosis and risk stratification. Immunohistochemical studies have shown the buccal mucosa to mirror changes in junctional and signalling protein distribution observed in the myocardial tissue of ACM patients. Our previous pilot work showed that children bearing ACM variants do not exhibit such abnormalities unless and until clinical onset of disease.

Aim: We aimed to establish the timepoint at which key protein redistribution occurs in a large cohort of paediatric ACM gene carriers alongside gene-elusive children showing minor cardiac abnormalities.

Methods: Paediatric silent desmosomal (n=44) or Titin (TTN) (n=7) variant carriers, patients with a definite/probable ACM diagnosis (n=27), and individuals showing clinical abnormalities from gene elusive families (n=21) were recruited and buccal smears collected every 3-6 months over a maximum period of 7 years. Buccal smears were subjected to immunocytochemistry to investigate the distribution of desmosomal proteins desmoplakin (DSP), plakophilin1 (PKP1) & plakoglobin (JUP), and the major gap junction protein connexin43 (Cx43).

Results: All children with an established/likely diagnosis of ACM showed protein distribution changes. Further protein shifts were observed to correlate with disease progression/worsening symptoms. 70% of ACM gene carriers (n=36) exhibited protein shifts prior to clinical onset of disease. Of 10 gene carriers that developed disease during the study, 80% showed 1 or more protein shifts before disease manifestation. All TTN carriers displayed abnormal protein distribution prior to disease onset. 57% of gene-elusive children showed abnormal protein distribution before disease onset. Of the patients who progressed to ACM diagnosis, 80% showed protein abnormalities before fulfilment of diagnostic criteria. For these patients, buccal cell changes accompanied clinical changes. Only 2 patients (2%) in the cohort showed disease onset in the absence of protein abnormalities.

Conclusion: This longitudinal study shows that protein shifts do not correlate with ACM manifestation as previously thought but instead precede disease onset. This method may allow for identification of at-risk individuals in advance of signs exhibited by clinical modalities, with important implications for early risk stratification.