By Dierk Thomas
Atrial fibrillation (AF) is the most common sustained cardiac arrhythmia and accounts for significant morbidity and mortality. The arrhythmia has a prevalence of ~1% in the general population and is age-dependent with 24% (men) and 16% (women) of patients >85 years being affected. The epidemiological significance of AF is further illustrated by a predicted two-fold increase of AF prevalence in the European Union by the year 2060. Despite its high epidemiological and clinical relevance, effective and safe management of AF still constitutes a major clinical challenge. Medical therapy represents the initial standard treatment for most AF patients. However, pharmacotherapy is limited by reduced efficacy, side effects, and safety risks in a significant number of patients. Non-pharmacological therapy is improving rapidly, but only a fraction of AF patients are currently treated by catheter ablation.
Basic research has revealed insights into fundamental mechanisms contributing to the arrhythmia: AF results from a variety of pathophysiological processes, leading to electrical and structural remodelling. The generation of substrates that support slow conduction, shortening of atrial refractory periods, and electrical reentry is particularly relevant as it provides the basis for maintenance of AF. Atrial alterations are observed as early as 24–48 hours after the onset of AF. Multifactorial aetiology and pathogenesis of the condition requires multimodal treatment, tailored to patient-specific mechanisms. The efficacy of an antiarrhythmic intervention to prevent AF largely depends on its capacity to suppress the underlying mechanisms.
In search for mechanism-based treatment modalities, gene therapy offers greater selectivity than small molecule-based or interventional approaches. The gene of interest is packaged into viral carriers and delivered to the target area via direct injection or using catheter-based interventional techniques, providing the advantage of site-restricted action in contrast to systemic application of drugs. Antiarrhythmic gene therapy for rate and rhythm control was evaluated in a porcine model of burst pacing-induced atrial fibrillation (Bikou et al, 2011; Lugenbiel et al, 2012; Soucek et al, 2012; Trappe et al, 2013).
During AF, normal atrioventricular node conduction leads to rapid ventricular rate response, resulting in impairment of left ventricular function and of exercise capacity. To achieve genetic rate control, an activating component of the β-stimulatory pathway, Gαs, was suppressed with an adenovirus encoding for a respective silencing RNA administered via percutaneous access and cardiac catheterisation. This approach reduced atrioventricular nodal conduction, decelerated ventricular heart rates by 20%, and improved cardiac function compared to control animals exhibiting tachymyopathy (Lugenbiel et al, 2012). Potentially limiting side effects such as increased adenylyl cyclase expression and heart rate acceleration upon catecholamine application were not detected. Thus, targeted biological modification of atrioventricular conduction may represent a viable strategy for heart rate control in atrial fibrillation.
To further explore this emerging field, we sought to suppress atrial fibrillation by specifically preventing AF-associated remodelling via targeted atrial gene therapy. A hybrid method of atrial gene transfer was employed in rhythm control approaches, combining direct virus injection and epicardial in vivo-electroporation to yield high efficiency. Shortening of atrial refractory periods and action potentials is critical to AF perpetuation and may be prevented by suppression of repolarising outward potassium currents in atrial myocytes. Biologic rhythm control was achieved in a pre-clinical study by direct atrial gene transfer of a dominant-negative ether-a-go-go-related gene (ERG) K+ channel mutant. Inactivation of ERG channels resulted in reduced repolarising IKr current, induced action potential prolongation and successfully suppressed pacing-induced AF (Soucek et al, 2012). Furthermore, impairment of left ventricular ejection fraction during AF was prevented by anti-ERG gene therapy. In addition to delayed repolarisation, electrical reentry and AF are facilitated by deceleration of electrical atrial conduction. Gap junctions serve as regulators of conduction velocity in the heart and are formed by connexin proteins. AF is associated with reduced connexin 43 expression. We found that genetic correction of AF-associated connexin remodelling by targeted atrial connexin 43 gene transfer prevented persistent AF and preserved left ventricular ejection fraction in pigs (Bikou et al, 2011). Finally, AF is linked to atrial cardiomyocyte apoptosis, leading to structural remodelling and reduction of electrical conduction velocity. We evaluated a genetic approach to rhythm control using siRNA-mediated inactivation of a key apoptotic enzyme, caspase 3 (Trappe et al, 2013). Apoptosis was successfully suppressed by targeted gene therapy. As a result, deceleration of atrial conduction was prevented and the development of persistent AF was inhibited or delayed. Taken together, these pre-clinical studies revealed that gene therapeutic targeting of structural and electrical remodelling represent novel avenues to optimise and personalise AF management.
It is important to recognise that follow-up was short (two weeks) and sample sizes were small due to the animal model. Additional studies need to be conducted in larger groups of animals with extended observation periods prior to application of antiarrhythmic gene therapy in humans. Remaining obstacles of therapeutic gene transfer include optimised control over local gene distribution, potential tumorigenicity, and prevention of local and systemic inflammatory responses. Adenoviral vectors were previously used owing to their ability to induce peak expression within a short time and to their high efficacy in infecting cardiac myocytes. For long-term applications and to study stability, efficacy, and safety of gene therapy, the use of adeno-associated virus or lentivirus as vector is required. The application technique combining local virus injection and electroporation for anti-remodelling treatment could be readily performed during open-chest cardiac surgery in humans. To further refine the gene transfer method, thoracotomy should be replaced by interventional virus application via specific catheters in future studies.
In summary, proof-of-concept gene therapy studies confirmed fundamental mechanistic hypotheses of AF pathophysiology. Suppression of AF (rhythm control) or reduction of ventricular heart rates during the arrhythmia (rate control) was achieved by targeted biological modification of specific substrates in the atria or in the atrioventricular node. After successful establishment of minimally-invasive techniques and following safety assessment, antiarrhythmic gene therapy could expand the current polymodal treatment strategy to eliminate the most debilitating of arrhythmias.
Dierk Thomas is head of Electrophysiology at the Department of Cardiology, University of Heidelberg, Heidelberg, Germany