Creating a Cardiac Pacemaker by Gene Therapy - Biopacemaking - page 51

used to modulate the electrophysiologic substrate in a number of disease settings.
Gene transfer of repolarizing K channel proteins has been used to modulate
repolarization, and the results to date suggest that action potential duration (APD) can
be significantly shortened by such approaches [42-44, 53, 67, 68]. More recently,
Donahue and colleagues [2, 19, 20] performed adenoviral gene transfer of an
inhibitory G protein Į-subunit (G Įi2 ) into the AV node of swine in vivo, and
demonstrated successful control of the ventricular rate response in chronic atrial
fibrillation. The success of these proof-of-principle studies suggest that gene transfer
approaches may be useful for the development of a biological pacemaker.
6 Gene Transfer Approaches for Biopacemakers
Initial gene transfer approaches for a biologic pacemaker were aimed at altering the
adrenergic input to the native cardiac pacemaker(s) by transferring adrenergic
receptors to the surrounding regions [22, 23]. ȕ -ARs regulate cardiac chronotropic (as
well as inotropic) response by G-protein coupled signaling pathways—in part through
modulation of I f [37, 40]. Edelberg et al. [22] first explored the ȕ 2 -AR as a target for
gene transfer, choosing the human ȕ 2 -AR because it is immunologically distinct from,
but functionally and structurally similar to, the murine receptor. They demonstrated
that in vivo injection of the ȕ 2 -AR construct into the right atrium of mouse hearts
increased the endogenous heart rate by ~40% (vs. controlled injected hearts) by 2
days, after which the heart rate returned to baseline (by day 7). In a subsequent study
[23], they showed that fluoroscopically guided injection of the ȕ 2 -AR construct in the
right atrium of pigs resulted in a heart rate increase of 50% (without apparent change
in atrial conduction) and expression of the encoded ȕ 2 -AR gene (by immunostaining).
These short-term studies demonstrated the feasibility of modulating cardiac
pacemaker activity by gene transfer approaches in mouse and large animal models,
critical proof-of-principle showing the potential for gene transfer on modulating heart
rate in vivo. From a practical point of view, limitations to these studies included the
use of recombinant cDNA construct (rather than viral gene transfer), the short-lived
effects, and non-specific yield (as the gene transfer affected not only the activity of
the pacemaker, but that of other neighboring structures as well).
7 Gene Transfer Focusing on Ion Channels
The concept of directly manipulating ion channels to induce pacemaking activity in
otherwise ''quiescent'' cardiomyocytes seems, in this context, more appropriate. The
two main currents whose activity has been modulated to this end are the outward
current of the inward rectifier I K1 and the funny current, I f .
Miake et al. [64] created a dominant negative construct that suppresses the I K1
current when transfected and co-expressed with the wild-type Kir2.1 into normal
cardiomyocytes. Specifically, replacement of amino acids 144-146 in the poreforming
H5 region by alanines (Kir2.1AAA) resulted in a dominant negative construct
encoding Kir2.1 subunits with non-conducting pores. Kir2.1AAA was then packaged