Uncovering cardiac molecular remodelling in racing Thoroughbreds with paroxysmal atrial fibrillation

BACKGROUND: Atrial fibrillation (AF) is the most common arrhythmia in both humans and horses, with a comparable prevalence of ~2%. In horses, AF is a major cause of poor athletic performance and linked to sudden cardiac death, yet its underlying molecular mechanisms remain poorly understood. OBJECTIVE: To investigate transcriptomic remodelling associated with spontaneous paroxysmal AF (PAF). STUDY DESIGN: Case-control study. METHODS: We performed RNA sequencing on left atrial (LA) tissue from three Thoroughbred racehorses with PAF and four matched controls. All horses were euthanised due to musculoskeletal reasons. Cell type deconvolution was performed with BayesPrism. The differential expression analysis was performed with EdgeR. Alternative splicing analysis was performed with SUPPA2. RESULTS: Cell-type deconvolution revealed a modest, non-significant shift towards increased fibroblast and immune cell proportions in PAF, but not enough to account for widespread transcriptional changes. 1400 of the 12,012 included genes were considered significantly differentially expressed (Benjamini-Hochberg adjusted p-value <0.05). These genes included ion channels, calcium-handling proteins, and conduction-related genes. In addition, several genes directly involved in cardiac muscle contraction and mitochondrial energy metabolism were differentially expressed. Alternative splicing analysis revealed over 20 significantly altered events, including skipped exons in genes with potential relevance to cardiac function and remodelling. MAIN LIMITATIONS: The number of biological replicates was modest. CONCLUSIONS: Overall, despite no major changes in cell composition, the findings highlight intrinsic structural and molecular changes in the LA of PAF-affected horses. This study provides the first transcriptomic and splicing analysis of atrial tissue in racehorses with PAF, revealing molecular remodelling consistent with mechanisms of arrhythmia maintenance and suggesting potential targets for further functional investigation.