Early and Late Myocardial Effects of Proton Beam Cardiac Radioablation: Insights From Gene Expression Analysis

Cardiac radioablation (CRA) is an emerging treatment modality for refractory ventricular tachycardia. The mechanisms underlying the antiarrhythmic effect of CRA remain incompletely understood.

The goal of this study was to investigate the nature and time course of key gene expression of apoptosis, conduction, and fibrosis after 40 Gy proton beam CRA.

In 18 swine, left ventricular (LV) targets were treated with a single-fraction 40 Gy proton beam. Myocardium was harvested from an irradiated LV target (IR-40Gy LV), the border area (IR-Border LV), and non-irradiated remote location (Remote LV) 1 to 32 weeks later. Apoptosis was assessed with the terminal deoxynucleotidyl transferase dUTP nick end labeling method that detects DNA breaks in the late phase of apoptosis. Markers of cellular survival, electrical conduction, and fibrosis were also assessed with real-time polymerase chain reaction.

Terminal deoxynucleotidyl transferase dUTP nick end labeling staining suggested ongoing apoptosis in IR-40Gy LV through 20 to 32 weeks, with a peak at 4 to 8 weeks. Statistically significant expression reduction of TNNT2 (encoding cardiac troponin T) was noted in the IR-40Gy LV group compared with the Remote LV group at 1 to 2 weeks and through 20 to 32 weeks. IR-40Gy LV and IR-Border LV showed patterns of expression reduction of genes of myocardial conduction, including SCN5A, GJA1, and GJA5 (encoding Na_V1.5, connexins 43 and 40, respectively) beginning at 4 to 8 weeks. Fibrosis genes COL1A1 and COL3A1 (encoding collagen types I and III) exhibited statistically significant expression up-regulation late after irradiation (20-32 weeks) in IR-40Gy LV relative to Remote LV.

Apoptosis peaked at 4 to 8 weeks and continued during several months of follow-up after proton beam irradiation. There was no evidence of up-regulation of conduction markers early after irradiation. Activation of fibrosis pathways occurred several weeks after irradiation.