Gene Therapy Creates a New Heart Pacemaker

When a patient’s heart beats too rapidly, too slowly or erratically, and if the usual heart medicines fail to properly regulate the heart rhythm, then the patient’s cardiologist may prescribe the implantation of an electronic pacemaker to regulate the heart rhythm. Even though implanted pacemakers are widely used, their installation requires an invasive surgery, they carry some risk of infection, and they also set off metal detectors during airport security checks. However, gene therapy might soon join the electronic pacemaker as a treatment for a poorly-regulated heart. It runs out that inserting a specific gene into heart-muscle cells can allow researchers to restore a normal heart rhythm in pigs, albeit temporarily.

Electronic pacemakers restore regular function to hearts by sending small electrical currents to the heart muscle in order to stimulate a heartbeat. This function is usually donned by the sinoatrial node, which is a cluster of a few thousand cardiac cells in the upper part of the right atrium that signals the heart to initiate a heartbeat and, therefore, sets the heart rate.

Heart Conducting System.  1) is the sinoatrial node or pacemaker and 2 is the atrioventricular node that receives the beat signal from the sinoatrial node and sends it to the ventricles.
Heart Conducting System. 1) is the sinoatrial node or pacemaker and 2 is the atrioventricular node that receives the beat signal from the sinoatrial node and sends it to the ventricles.

A research team led by Eduardo Marbán, who is a cardiologist at Cedars-Sinai Medical Center in Los Angeles, California, attempted to engineer heart cells outside the sinoatrial node to act as the pacemaker of the heart. The findings from Marbán’s laboratory were reported in the journal Science Translational Medicine.

Marbán and his colleagues used 12 laboratory pigs for their laboratory experiments. In these animals, Marbán and others induced a fatal heart condition in which electrical activity that originates from the sinoatrial node cannot spread through the heart. This forces other, less capable parts of the heart to take over and act as a pacemaker. Then, Marbán’s group used high-frequency radiowaves to destroy the sinoatrial nodes in the pigs’ hearts. This caused the animals’ average heart rate to slow to about 50 beats per minute (compared to the normal rate of 100 or more beats per minute). Such animals, if they were a human, would require an electronic pacemaker.

Next, Marbán and other injected the pigs’ hearts with a genetically modified virus that carried a pig gene called Tbx18, which is involved in heart development. Within one day, infected heart cells infected with the virus began to express those genes usually found in sinoatrial node cells. These cells acted as the pacemaker and began to direct the pumping the heart at a normal rate. The animals maintained this steady beating for the two-week study period, whether resting, moving or sleeping.

In an interview, Marbán said that his method is simpler than other biological approaches to restore a normal heart rhythm to hearts. These other approaches include inducing cardiac muscle cells to a pluripotent state, then coaxing them to differentiate into pacemaker cells. However, Marbán cautioned that the effects of gene therapy might be temporary. Over time, the body’s immune system would probably recognize the virus used to deliver Tbx18 to the heart and attack and destroy the infected cells. Marbán’s team is presently monitoring pigs that have received the gene-therapy treatment for several months to measure the persistence of this pacemaker effect.

However, even if the treatment’s effects are limited, it could still prove useful, according to Marbán. For example, if a pacemaker patient suffers from an infection as a result of the pacemaker, that pacemaker must be temporarily removed. This patient could then receive a biological pacemaker that could keep the heart pumping steadily until the infection clears and a new device is implanted. The gene-therapy approach could also help unborn children with heart defects, or even children who quickly outgrow implanted pacemakers or people for whom surgery is simply too risky.

“I think it’s a truly creative idea,” says Ira Cohen, a cardiac electrophysiologist at Stony Brook University Medical Center in New York. He would like to see the therapy tested in dogs, whose average heart rate is 60-100 beats per minute, which is more similar to that of a human.

Marbán is presently in talks with the US Food and Drug Administration about developing a human trial, which he says could be just two to three years away.

Induced Pluripotent Stem Cell Treatments for Heart Attacks

Several recent papers have used induced pluripotent stem cells (iPSCs) to treat heart attacks in laboratory animals. These papers followed similar strategies that included culturing iPSCs, differentiating those iPSCs into heart muscle cells, surgically inducing a heart attack in laboratory rodents, and then transplanting the iPSC-induced heart muscle cells into the hearts of the animals that suffered a heart attack. The results are beyond encouraging; they are remarkable.

The first paper is from James Thomson’s laboratory at the University of Wisconsin, Madison (Zhang J, et al., Circ Res. 2009 Feb 27;104(4):e30-41). In this paper, human iPSCs were differentiated into heart muscle cells. These heart muscle cells expressed many heart muscle-specific genes and proteins, and also had mixed characteristics. Some of the cells resembled heart muscle from the upper part of the heat (atrial), some looked like heart muscle from the lower part of the heart (ventricular), and still others had similarities to heart pacemaker cells. These iPSC-derived heart muscle cells also showed the same response to heart medicines that normal, native heart muscle would show. Thus human iPSCs can form functional heart muscle cells.

While experiment paralleled experiments with mouse iPSCs (see Mauritz C, et al., Circulation. 2008 Jul 29;118(5):507-17; & So KH, et al., Int J Cardiol. 2011 Dec 15;153(3):277-85), it begged the question: “Could these iPSC-derived heart muscle cells integrate into a working heart and act like normal heart muscle cells?” These answer to this question in a certifiable “Yes!”

The first paper – Mauritz C., et al., Eur Heart J. 2011 Nov;32(21):2634-41; examined mouse iPSCs and their ability to differentiate into heart muscle cells that could be used to treat laboratory animals with heart attacks.  These workers from Kutschka’s lab found that iPSC-derived heart muscle came in two forms; those that expressed an enzyme called “fetal liver kinase-1” and those that did not.  Fetal liver kinase-1 or Flk-1 is a receptor for a growth hormone called vascular endothelial growth factor (VEGF).  VEGF is a major stimulator of blood vessel formation, and Flk-1 confers upon cells the ability to respond to VEGF and form blood vessels.

Kutschka’s lab workers isolated the Flk-1-positive cells from the Flk-1-negative cells by means of a cell sorter, but they did not other extremely important experiment.  They used cells that expressed a fluorescent protein if and only if they had not completely differentiated.  This way, all incompletely differentiated cells were removed by the cell sorter.  Since incompletely differentiated iPSCs can cause tumors, this is an important safety consideration if this technology is ever to see the light of clinical trials.  The two heart muscle cell populations were then implanted into the hearts of laboratory mice that had suffered heart attacks.  Control mice were injected with saline.

The results showed that both populations of iPSC-derived heart muscle cells integrated into the injured hearts and increased heart function and structure.  However the Flk-1-positive cells conferred even more benefits onto the hearts.  Furthermore, because these iPSC-derived heart muscle cells were produced from adult cells that came from the animals that had suffered heart attacks, there was no need to use mice that had defective immune systems, or were given immunosuppressive drugs.  This definitely shows that iPSC-based treatments are ready for human clinical trials at some time in the near future.

The second paper – Singla DK, et al., Mol Pharm. 2011 Oct 3;8(5):1573-81; takes a slightly different approach.  This research group from the University of Central Florida made iPSCs from a cultured heart muscle cell line called H9c2.  Singla and colleagues transfected these cells with four genes (Oct3/4, Sox2, Klf4, and c-Myc) and this successfully transformed the cultured heart muscle cells into iPSCs.  Then they differentiated those iPSCs into heart muscle cells that beat in culture and also expressed essential heart muscle proteins.  When transplanted into the hearts of laboratory animals that had recently suffered a heart attack, these iPSC-derived muscle cells made proper contacts with other heart muscle cells, properly communicated with them, and improved heart function much better than transplantation of H9c2 cells, or those injected with no cells.  Once again, we have the same strain of mouse from which H9c2 was made.  These mice did not require any suppression of the immune system to receive this treatment because they received cells made from their own genetic stock, and the immune system recognized them as part of themselves.

Finally, a paper by Nelson TJ, et al., Circulation. 2009 Aug 4;120(5):408-16 – from the Mayo Clinic in Rochester, Minnesota also shows the feasibility of iPSCs for regenerative therapy in laboratory animals.  In this paper, undifferentiated iPSCs were transplanted into the hearts of laboratory rodents that had recently suffered heart attacks.  The iPSCs were placed into differentiation media and transplanted into the hearts of mice with poorly operating immune systems and those with normal immune systems.  In the mice with poorly functional immune systems, the implanted iPSCs formed aggressive tumors that overtook the heart and eventually killed the animal.  However, in those animals with normally-operating immune systems, no tumors formed, and the iPSCs form heart-specific cell types and properly engrafted into the heart without disrupting te structure of the heart.  iPSC treatment also regenerated cardiac, smooth muscle, and endothelial tissue, and restored post-heart attack function when it came to contractile performance, the thickness of the ventricular wall, and the electrophysiology of the heart.

These experiments show that iPSCs can fix injured hearts.  There are even protocols to safely differentiate them into heart muscle cells.  Clearly the safety of these must be better investigated and established before they can transition to clinical trials.  However, these papers are definitely a good start to what will hopefully become, some day, personalized stem cells to treat an ailing heart.