Exon Utilization Improves Risk Stratification for Advanced Heart Failure in Titin Cardiomyopathy

Truncating variants in the titin (TTN) gene (TTN-TV) are the most common genetic cause of dilated cardiomyopathy (DCM) and confer a significant risk of progression to advanced heart failure (AHF). Penetrance of TTN-TV has been linked to the level of expression of the exon containing the TTN-TV, quantified using the percent spliced in (PSI). We performed long-read RNA sequencing on cardiac tissue from 8 unused organ donors and 14 DCM patients and identified expression of 16 TTN isoforms. We used these data to recalculate PSI (PSI-LR). PSI-LR reclassified 5% of exons compared to the original PSI calculated using short-read RNA sequencing (PSI-SR). We then analyzed a cohort of 98 patients with cardiomyopathy due to TTN-TV, 34 (35%) of whom developed AHF, as defined as the need for left ventricular assist device implantation or heart transplant. PSI-LR, but not the original PSI-SR, predicted AHF risk (odds ratio 1.35 per 0.1 increase, p=0.038). A PSI-LR threshold of <0.75 best identified patients who did not progress to AHF. Our PSI metric, PSI-LR, which accounts for the expression of the multiple TTN isoforms, predicts the expressivity of TTN-TV with regards to the progression to AHF, which has important implications for prognosis and management.