Heart base irradiation causes functional changes in the conduction system: a pre-clinical model

Abstract

Purpose or Objective Radiation cardiotoxicity affects one third of patients with intrathoracic cancer and is largely characterised by vascular, electrical and pump dysfunction in the months-years following radical doses. The heart is composed of a diverse range of specialised tissues but recent clinical studies suggest that the dose received by the superior region has the highest correlation with post-treatment morbidity and mortality. This region contains several critical substructures, including the proximal coronary arteries and the sinoatrial and atrioventricular nodes. In this study, we investigated the longitudinal effects of radiation on the cardiac conduction system in a partial heart irradiation mouse model and characterised the transcriptional, histological and functional changes following base irradiation. Materials and Methods 8-week old C57BL/6 mice were irradiated with a single fraction of 16Gy to the cardiac base, middle or apex of the heart using a small animal radiotherapy research platform. The base was defined as the caudal third of the heart volume on cone-beam CT, encompassing both atria and both conduction system nodes. Electrocardiography was performed at baseline and 10-week intervals for one year. Mice aged 9 months similarly underwent 16Gy/1# base irradiation, and had electrocardiography and tissue collection at baseline and 30 weeks. Paraffin-embedded tissues were stained with haematoxylin and eosin and Masson’s trichrome, and immunohistochemistry was performed for cell surface markers for endothelium, leucocytes and conduction system cardiomyocytes. Whole-organ spatial transcriptomics was used to characterise regional variations in gene expression. Results Time-dependent increases in the P wave duration and PR interval were observed in young base-irradiated mice, most apparent at 20 weeks post-irradiation. Changes resolved at 40 and 50 weeks. Similarly, P wave duration and PR interval were increased at 30 weeks in aged mice. QRS duration was not prolonged in any groups. Atrial cardiomyocyte density and myocardial fibrosis were not increased compared with controls at 30 weeks in the aged mice. Microvascular density and white cell infiltration were comparable between irradiated and control aged groups. Spatial transcriptomic analysis revealed distinct clusters of gene expression unique to the control and irradiated animals. Conclusion The cardiac conduction system exhibits subacute and self-limiting dysfunction independent of fibrosis following irradiation of the sinoatrial and atrioventricular nodes. Differential effects of radiation were identified on expression of genes linked to cardiac conduction and other cardiomyocyte functions. Further work is planned to establish the radiobiology underpinning the specialised cardiomyocytes of the conduction system nodes. This research has potential implications for radiation oncologists treating both thoracic malignancies and refractory tachyarrhythmias.