Following myocardial infarction (MI), a heterogenous transition zone between the scar and healthy myocardium develops. The region of myocardium bordering an infarct (border zone, BZ) undergoes many transcriptional changes, some of which may drive adverse remodelling. RUNX1, a master-regulator transcription factor intensively studied in the cancer and blood research fields, is increased in BZ cardiomyocytes at 1-day post-MI. Previously, we have shown that cardiomyocyte-specific Runx1 deficient (Runx1^Δ/Δ) mice demonstrate remarkably preserved systolic function at 1-day post-MI via increased cardiac sarcoplasmic reticulum (SR) calcium uptake highlighting RUNX1 as a novel modulator of contractile function (McCarroll et al. 2018). We then interrogated regional differences in Ca²⁺-handling at 1-day post-MI by performing Ca²⁺ imaging in BZ and remote zone (RZ) cardiomyocytes from Runx1^Δ/Δ and flox control (Runx1^fl/fl) mice. Overall, we found that BZ cardiomyocytes from Runx1^fl/fl mice displayed impaired calcium handling compared to RZ cardiomyocytes, whereas there were no regional differences in Runx1^Δ/Δ mice (Martin et al. 2023).

Ventricular arrhythmias, which can be driven by regional electrophysiological heterogeneity and impaired calcium handling, are a major complication of MI, with the BZ region being the most vulnerable site for arrhythmogenesis (Qin et al. 1996; Smaill et al. 2013). As such, we hypothesised that RUNX1 could play a crucial mechanistic role in the arrhythmogenicity of the ventricle post-MI.

To study this, we performed surface ECG studies in Runx1^Δ/Δ and Runx1^fl/fl mice in the early days post-MI (specifically days 1, 7, and 14) when the ventricle is most susceptable to arrhythmia to elucidate the role of RUNX1 on electrophysiology and arrhythmogenesis. ECG morphology was consistent between Runx1^Δ/Δ and Runx1^fl/fl mice pre-MI. However, we found that although QT interval was prolonged post-MI in both groups reflecting increased electrical heterogeneity, QT interval was shorter in Runx1^Δ/Δ compared to Runx1^fl/fl mice. We then used an intraperitoneal injection containing isoproterenol and caffeine was used to stress the cardiovascular system to evaluate the propensity for arrhythmia. We found that Runx1^Δ/Δ had significantly fewer premature ventricular excitations compared to Runx1^fl/fl mice.

Further, in ventricular cardiomyocytes isolated from C57BL/6 mice and incubated with Runx1 small molecule inhibitor Ro5-3335 or with vehicle control DMSO, we utilised a burst pacing protocol followed by a two-minute period of rest and measured spontaneous calcium events as a measure of arrhythmogenicity. We found that cells treated with Ro5-335 had significantly fewer spontaneous events in the rest period compared cells incubated in DMSO.

These findings indicate that RUNX1 contributes to arrhythmogenic calcium heterogeneity in the BZ post-MI, and its inhibition may mitigate arrhythmia risk by preserving calcium handling and reducing electrophysiological heterogeneity.