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In this issue of the Journal, Boink et al. (1) reported the study of biological pacemaker function in mongrel dogs that underwent radiofrequency ablation of the atrioventricular node and were then treated with gene transfer using the hyperpolarization-activated cyclic nucleotide–gated current channel 2 (HCN2) construct, the skeletal muscle sodium channel (SkM1) construct, or the dual (HCN2/SkM1) construct injected into the left bundle branch (LBB) or the left ventricular (LV) subepicardium. The researchers coexpressed SkM1 with HCN2 on the basis of the hypothesis that when HCN2 generated the inward current that would drive the membrane toward threshold, SkM1 would create greater availability of sodium channels during diastole because of a more favorable inactivation curve than the cardiac sodium channel, leading to a more negative threshold potential, improved pacemaker stability, and increased beating rates. Indeed, that is what they found. Five to 7 days after injection, upon stable maximal expression of the HCN2/SkM1 construct, LBB-injected dogs demonstrated higher heart rates (80 to 130 beats/min) and better modulation of pacemaker function during circadian rhythm or epinephrine infusion and did not require electronic pacing backup, compared with LV injected dogs and dogs with LBB injections expressing SkM1 or HCN2 alone (1). The researchers concluded that this model is more biologically suitable for pacing than other constructs reported, findings they attribute to the more negative action potential threshold and injection into the LBB. The actual contribution of each compound was not investigated, and likely some cells expressed HCN2 and others SkM1 in varying relationships and quantities, perhaps depending on how they were infected with one gene or the other. Overall, the functional combination capitalizing on their different and complimentary mechanisms of action was successful, maybe from the averaging effects brought about by the cable properties of a functional cardiac syncytium (2,3).
The current report suggested that the expression of HCN2, rather than other isoforms such HCN4, along with SkM1 in the canine LBB, was sufficient to provide an improved automaticity and autonomic responsiveness similar to the highly expressing HCN4 in sinus node pacemaker cells (1). Although cardiac function in LBB-injected dogs appeared to be grossly improved, it remains unclear how exogenous HCN2/SkM1 expression in LBB-injected cells is sufficient to functionally compensate for the difference in anatomic structure and protein expression of LBB cells compared with sinus node pacemaker cardiomyocytes. In addition, further investigation is needed to elucidate whether overexpression of HCN2 in LBB cells leads to the exclusive assembly of functional homotetrameric HCN2 channels or the current observations derived from altered subunit stoichiometry of the existing HCN channels, supporting alternative heterotetrameric HCN channels with intermediate or different activation time constants, steady-state voltage dependence, and cAMP-dependent modulation leading to If channel heterogeneity.
These excellent scientists are to be congratulated on a creative hypothesis established by careful experimentation. Although it appears that the reported construct is better than previous attempts, it is also true that the normally functioning sinus node is a tough act to follow, as is a modern electronic pacemaker, and we need to consider whether the ultimate goal to replace a malfunctioning sinus node with a biologically active pacemaker to avoid the problems associated with electronic pacemakers, particularly leads, is achievable. So, let's put on our clinician hat and evaluate the findings in that light.
First, will the construct work in the atrium? The sinus node contains specialized cardiomyocytes, the pacemaker cells, with a resting potential of approximately −70 mV, that undergo spontaneous depolarization and are characterized by low conductance compared with the adjacent and fast-conducting atrial cardiomyocytes (4). The characteristic conductance of the sinus node allows directional propagation of the depolarization wave and prevents the overhyperpolarization from neighboring atrial cardiomyocytes. This low conductance is granted by the combination between the peculiar anatomic structure and cell composition of the sinus node, as well as the exclusive ion channels, pumps, and gap junction protein milieu of the pacemaker cells, which dramatically diverge from the atrial and ventricular cardiomyocytes (5–8). Granted that the combination construct was not very effective injected into the LV myocardium, the issue now is how well it will work injected into atrial myocardium. Will the unique atrial anatomy and electrophysiology permit the combination to synergize functionally?
A second issue is durability and longevity. The researchers were appropriately cautious and stated that biological pacing is being explored as a possible adjunct or replacement for electronic pacing but that a good deal remains to be done before clinical testing is in order. We would agree with that caveat. Long-term expression of a HCN2/SkM1 construct will require alternative vectors that can integrate into the genome of the transfected cardiomyocytes, thus offering stable and long-lasting expression of the exogenous gene products (9,10). The researchers suggested that because of myocardial cell targeting as well as the DNA size packaging capability of the current available systems, lentiviral vectors may be more suitable to translate their finding into clinical application. However, lentiviral vectors may not be useful for all biological applications because DNA inserts ≥6 kb may dramatically decrease the viral titers due to reduced packaging efficiency, thus hampering the application range of this type of therapy (11). They also offered that overexpressing human mesenchymal stem cells may be a reasonable approach. However, given a projected 8-year survival of an electronic pacemaker, there is a challenge ahead to create a biological substitute capable of initiating roughly 300 million beats without fail and on time.
A third issue is to demonstrate that whichever construct is used, it is not arrhythmogenic and that it responds as a normal pacemaker to autonomic influences (partially shown here), circulating modulators, as well as to ectopic beats, with capture and resetting, to avoid the impact of competing pacemakers and potential arrhythmogenicity. Injection into the LBB in the human ventricle may be quite challenging but may be preferable to right ventricular apical pacing, especially for patients with LBB block.
Finally, if the intent of the researchers is to provide a ventricular pacemaker, rather than a substitute for the sinus node, then the issues of ventricular capture and resetting noted above become even more important. In addition, such a pacing application would function as a VVI pacemaker, generally less favorable than DDD pacing.
In summary, despite the many points that remain to be elucidated and the technical limitations to be addressed, the effort of Boink et al. (1) along with other research groups to establish a biological alternative to electrical pacemakers is extremely significant for the possible clinical applications and the improvement in patient management. None of the clinical problems detract at all from the science but do demonstrate that we will have job security for quite some time to come.
Both authors have reported that they have no relationships relevant to the contents of this paper to disclose.
↵⁎ Editorials published in the Journal of the American College of Cardiology reflect the views of the authors and do not necessarily represent the views of JACC or the American College of Cardiology.
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