Abstract Body (Do not enter title and authors here): Background: While some chronic pathological substrates for sudden cardiac death (SCD) are well-known (e.g., coronary disease and left ventricular [LV] dysfunction), the vulnerable myocardial state predisposing to fatal arrhythmia remains a critical barrier to near-term SCD prevention.

Hypothesis: Myocardium of autopsy-defined SCDs exhibit distinct expression profiles vs. non-cardiac sudden deaths and trauma deaths that reflect its acute vulnerable state in the hours to days before SCD, as expression arrests upon death.

Goals/Aims: Define the vulnerable myocardial substrate for lethal arrhythmia by targeted RNA profiling.

Methods: We used autopsy to adjudicate arrhythmic from non-arrhythmic causes in 1,265 sudden deaths in San Francisco from 2011-2018. We performed a transcriptomic evaluation of LV sampled at the time of SCD from 245 consented cases using a curated panel of 448 genes with known or hypothesized association with SCD.

Results: Comparison between Arrhythmic (n=129) and Non-Arrhythmic (n=116) cohorts revealed 31 differentially upregulated and 36 downregulated genes (p-adj<0.05), related to collagen-containing extracellular matrix (upregulation of FAP, FMOD, LTBP2), ion transport (upregulation of KCNA5 and KCNN3, and downregulation of KCNJ8, KCNK1, KCNJ5), and contraction (downregulation of MYH6). Fibrosis-related genes showed the highest magnitude increased expression in Arrhythmic vs. Non-arrhythmic deaths and vs. published transcriptomes from end-stage heart failure. After molecular stratification by known markers for mature (COL1A1, COL1A2, COL3A1) and active (POSTN, MEOX1) fibrosis, cases with highest expression of both had the highest proportion of arrhythmic cause of death (27/36 [75%]) vs. cases with low expression of both (87/181 [38%], p=0.006) or vs. mature only (10/14 [71%]) or active only (5/14 [36%]). Activated fibroblast gene expression was enriched in Arrhythmic female vs. Arrhythmic male cases, among other sex-specific differences in ion channel and myosin (upregulation of SCN4B, SCN8A, KCNAB1 in females, KCNJ4 and MYH7B in males) expression.

Conclusions: RNA profiling of myocardium at SCD identifies active fibrosis, undetectable by conventional clinical methods, in the presence of fixed scar and selected ion channel dysregulation (more pronounced among female cases) as an acute vulnerable substrate for fatal arrhythmias, and may identify patients at elevated near-term risk for SCD and novel pathways for intervention.