ORCID
https://orcid.org/0000-0002-4054-3312
Date of Award
2-24-2024
Degree Name
Doctor of Philosophy (PhD)
Degree Type
Dissertation
Abstract
Ventricular tachycardia (VT) is frequently associated with morbidity in patients with heart disease and can lead to sudden cardiac death, which accounts for over 350,000 deaths per year and nearly half of all cardiovascular-related deaths in the United States. Despite its status as a major public health concern, arrhythmia management strategies are severely limited in safety and efficacy. Standards-of-care include defibrillators, anti-arrhythmic drugs, and/or ablations. Implantable cardioverter defibrillators improve survival in at-risk populations but severely reduce quality-of-life and increase risks of further-declining ventricular function. Pharmacologic interventions are limited in efficacy and have dozens of off-target effects which are major sources of morbidity and mortality. Invasive radiofrequency catheter ablations frequently fail due to anatomical factors or limitations in the physics of heat transfer to create full-thickness, gap-free ablations. Radiotherapy is a modern oncologic intervention that uses photon beams to deliver high-dose radiation, precisely and noninvasively, to any target tissue in the body. Hypothetically, ablative doses of radiation to the heart could noninvasively replicate the effect of CA, with a fibrotic response expected over months to years. Early preclinical studies that explored radiobiology for arrhythmia treatment attempted to create ablative scar by inducing mitotic catastrophe, apoptosis, and fibrosis. In these models, extremely high doses ranging from 40-160 Gy to the myocardium were sufficient to produce moderate fibrosis over a period of several months, while lower doses and earlier timeframes doses did not achieve these effects. Recent clinical evidence has demonstrated the safety and efficacy of cardiac radiotherapy for treatment of drug-refractory VT in humans. In 2017, the first case series of cardiac RT reported a 99.9% reduction in patient VT burden after only 25 Gy radiation. A subsequent 2020 prospective Phase I/II clinical trial of 19 reported a 94% reduction in episodes of VT or premature ventricular contractions after 25 Gy in patients who had either previously failed or were not eligible for CA. Several studies have now replicated these clinical results at multiple independent medical centers. In nearly all cases, VT reduction occurred within days to weeks; thus, the onset of VT reduction is inconsistent with expected and reported timeframes of radiation-induced fibrosis. As such, the mechanisms by which 25 Gy radiation sub-acutely reduces VT are unknown. In the setting of heart disease, structural and electrical heterogeneity promote areas of slow conduction in surviving myocardium, which subsequently lead to electrical reentry from delays in impulse prorogation longer than the effective refractory period. Although radiation was presumed to prevent VT by creating radiation-induced fibrosis to homogenize scar, the anti-arrhythmic effects preventing reentry could potentially be mediated through effects on enhancement and restoration of electrical conduction. Herein, I first asked whether radiation-induced fibrosis is an important radiobiologic effect in patients. Using post-mortem or explanted patient specimens previously treated with 25 Gy, I observed that radiation does not replicate the fibro-ablative effects of thermal catheter ablation. Irrespective of specimen fibrosis, all patients exhibited suppression of VT within 1 month of treatment. As a follow-up, I tested a small animal model of 25 Gy cardiac radiation and detected no evidence gross fibrosis on histology or collagen deposition at 6 weeks post-treatment. To test fibrosis-independent effects of radiation to the post-mitotic heart, I evaluated murine cardiac electrophysiology and molecular biology after radiotherapy. Within 6 weeks post-treatment, irradiated hearts exhibited enhanced electrophysiologic properties on electrocardiogram and voltage optical mapping, attributed to observed upregulations in the cardiac sodium channel NaV1.5 and the gap junction subunit connexin 43. These effects were observed to occur at doses as low as 15-25 Gy and persisted for a minimum of 42 weeks, akin to an electrical reprogramming of the conducting substrate. I further demonstrated that reprogramming occurs primarily in surviving border zone myocardium and not scar myofibroblasts using a surgical model of myocardial infarction. To understand cell signaling mechanisms that may contribute to these effects, I utilized unbiased RNA sequencing of the irradiated murine ventricle and observed reactivation of the Notch signaling pathway in adult cardiomyocytes as a potential mechanistic contributor. Using an adult-inducible, cardiomyocyte-specific murine model of Notch activation, I demonstrated that transient reactivation of Notch signaling in adult left ventricles alone is sufficient to upregulate NaV1.5 for at least 1 year, and these effects correlated with persistently increased conduction velocities. To test whether Notch signaling is also necessary for conduction reprogramming, I utilized an adult-inducible Notch loss-of-function transgenic mouse model that reduced the overall effect size of conduction velocity reprogramming and NaV1.5 upregulation by 30%. Assessment of cardiomyocyte nuclei and genome accessibility revealed larger nuclear sizes in the absence of changes to DNA content, as well as chromatin compaction of cardiomyocyte DNA at 48 hours post-treatment. Within this dissertation, I demonstrate that cardiac radiotherapy regulates and reprograms cardiomyocyte electrophysiology without ablative fibrosis in human and mouse models. Herein, I further discuss therapeutic strategies for electrically-reprogramming ventricular cardiomyocytes to prevent reentry and tachycardia. Indeed, insights in cardiomyocyte radiobiology such as those presented within this dissertation are expected to lead to improvements and refinement in the cardiac radiation protocol, wider adoption of this emerging treatment technique, and potential expansion of therapy into greater arrhythmia populations.
Language
English (en)
Chair and Committee
Stacey Rentschler
Committee Members
Charles Kaufman, Jeanne Nerbonne, David Ornitz, Julie Schwarz
Recommended Citation
Zhang, David Meng, "Mechanisms of Electrical Substrate Reprogramming after Cardiac Radiotherapy" (2024). Arts & Sciences Electronic Theses and Dissertations. 3238.
https://openscholarship.wustl.edu/art_sci_etds/3238