Different approaches have been studied preclinically including: a) stem-cell based approaches12; b) hybrid (cell- and gene-based approaches);12c) gene therapy over-expressing a single or a combination of ion channels12 (known as functional reengineering), and more recently, d) gene therapy to achieve somatic reprograming.13,14

Different stem-cell based approaches have been used such as embryonic stem cells,15,16 or induced pluripotent cells capable of differentiating into spontaneously-beating heart cells, and generate short-term biological pacemaker activity. While generally effective, they are not free of problems: they can have limited engraftment and rejection (requiring immunosuppressive therapy) and be proarrhythmic, given the heterogeneity of the cell population obtained with present isolation and purification techniques. As a result of this heterogeneity, different action potential durations (affecting myocardial repolarization) can create a substrate for functional reentry-induced arrhythmias.

Hybrid approaches, in which stem cells are “loaded” with ion channel genes, have also been used to deliver the gene of interest without a viral vector.12 The greatest experience with this approach has been obtained using human mesenchymal stem cells that have been “loaded” with one of the HCN (hyperpolarization activated cyclic nucleotide-gated) family of genes (responsible for generating the pacemaker funny current or If). The major advantage of this approach is that human mesenchymal stem cells are somewhat nonimmunogenic. On the other hand, they tend to migrate from the injection site with the potential to create multiple ectopic foci of automaticity.17

The first de novo biological pacemaker by gene therapy was created by Miake et al18 in 2002. Over-expressing a dominant negative mutant (Kir2.1AAA) of the inward rectifier current (IK1) converted a normally-quiescent myocyte into an oscillating one, capable of generating spontaneous action potentials and biological pacemaker activity in vivo. While this approach was effective, the use of a mutant gene could represent a problem for human translation due to safety and regulatory concerns. Other approaches focused on overexpressing wild-type or mutant forms of the HCN family of genes (responsible for the pacemaker current or If) have been developed in different small- and large-animal models. While this approach offered the advantage of expressing an endogenous gene, the heart rates achieved were not optimal and the delivery methods were highly invasive (open-chest or left-sided/arterial approaches).19

In contrast to functional reengineering approaches, somatic reprogramming seeks to create genuine sinus node tissue from ordinary heart muscle. The approach involves reexpressing a gene, TBX18, that figures prominently in the development of the sino-atrial node (SAN) during embryonic life; this suffices to create a biological pacemaker in situ.13,14 Initially, in vitro experiments demonstrated the ability of the human TBX18 gene to reprogram normal working myocytes into pacemaker cells or induced SAN cells. Moreover, when injected into a rodent model of heart block, TBX18-transduced animals exhibited biological pacemaker activity that originated from the injection site, as evidenced by QRS morphology and axis.13 When induced SAN were analyzed by morphometry and molecular techniques, they resembled native SAN cells. Induced SAN cells have a characteristic long lean shape, increased levels of HCN2 (the pacemaker channel gene) and connexin 45 (the predominant gap junction in the SAN), and decreased levels of connexin 43 (the predominant gap junction in the normal working myocardium) and IK1.13 All these features of TBX18-induced SAN cells mimic the signature characteristics of the native SAN.

To further characterize the effects of TBX18-induced biological pacemaker when delivered in a clinically-realistic fashion, we implemented a modification of a protocol we had previously described.14,20 In brief, TBX18 (or control) was injected through a venous catheter into the high posterior septum. The entire procedure was performed by minimally-invasive methods limited to the venous side. The catheter used for TBX18 delivery was advanced through the femoral vein, and has been used extensively in human stem cell trials.21 Interestingly, those animals that received TBX18 exhibited physiologically relevant biopaced rhythms that allowed them to achieve higher levels of physical activity compared with age-matched controls. Additionally, those animals that received the biopacer required minimal backup electronic pacemaker utilization, unlike controls, which were largely pacemaker-dependent.

A fundamental consideration when delivering a biological therapy (or any therapy) to patients is the invasiveness of the method. No physician will consider offering patients a treatment that requires open-chest delivery (unless there is an additional indication to perform surgery), or even a left-sided approach with the consequent risks of hematoma and stroke. As described above, we have recently developed a minimally-invasive (venous catheter-based system) to deliver our biopacer gene without the need for open-heart surgery or access to the left-sided circulation.14,20 This technique is a modification of a cardiac catheterization technique routinely used by clinicians in the human electrophysiology laboratory. The genes are delivered by a commercially-available catheter (NOGA Myostar™; Biological Delivery Systems, Diamond Bar, California, United States) that can be advanced to the heart, construct a 3-dimensional electroanatomic map and inject the biopacer with a high level of precision by a retractable needle.

Other groups have proposed injection into the left bundle (via the arterial circulation) in the setting of device-related infections.22 Accessing the left-sided heart can have catastrophic consequences, such as stroke and vascular complications. A recent editorial by Rosen11 speculates that there may be more risks when injecting in the right side in the setting of a device infection, but this is difficult to rationalize given the systemic nature of blood-borne infections. In our opinion, a right-sided minimally-invasive delivery technique would be optimal when translating this technology to the clinic.

As clinician-scientists, when developing a new class of therapeutic agent to better treat our patients, we must target first those patients at risk that have no good prognosis, or no good alternatives with currently available therapies. Three groups of patients satisfy these criteria: a) patients with pacemaker-related infections who are pacemaker-dependent and need their hardware to be removed to more effectively treat the infection3; b) pediatric congenital heart disease patients who have unfavorable anatomy for endovascular devices and often need multiple system changes when they grow into adult life, and c) fetuses with intrauterine congenital heart block, who often develop hydrops fetalis and stillbirth due to the lack of reliable therapy.7

The thought of recreating a sinus node by a single human gene injection, avoiding the need for implanting electronic devices, may sound like one of those stories that we read in science fiction novels.23 With the advent of minimally-invasive delivery systems, novel biological agents, and accumulating efficacy and safety data in preclinical models of human disease, we may soon see this dream come true.

Supported by NIH (National Institutes of Health)/NCATS (National Center for Advancing Translational Science), UCLA (University of California, Los Angeles) CTSI (Clinical and Translational Science Institute) grant number UL1TR000124, Cedars-Sinai Board of Governors Heart Stem Cell Center, United States.

None declared.