Heart Repair with Human Tissue Engineered Myocardium
Heart disease is the number one cause of morbidity and mortality in the US. With an estimated 1.5 million new or recurrent myocardial infarctions, the total economic burden on our health care system is enormous. Although conventional pharmacotherapy and surgical interventions often improve cardiac function and quality of life, many patients continue to develop refractory symptoms. Thus, the development of new therapies is urgently needed. “Tissue engineering” can be broadly defined as the application of novel bioengineering methods to understand complex structure-function relationships in normal or pathological conditions and the development of biological substitutes to restore, maintain, or improve function. It is different from “cell therapy”, which is designed to improve the function of an injured tissue by simply injecting suspensions of isolated cells into the injury site. To date, two main limitations of cell therapy are (1) acute donor cell death due to unfavorable seeding environment and (2) the lack of suitable cell type that genuinely resembles human cardiac cells. Our proposal seeks to use engineered tissue patches seeded with human embryonic stem cell-derived cardiomyocytes for treatment of ischemic heart disease in small and large animal models. It represents a significant development of novel techniques to address both of the main limitations of cell therapy, and will provide a new catalyst for the entire field of stem cell-based tissue engineering.
Patients with end-stage heart failure have a 2-year survival rate of 25% by conventional medical therapy. Not commonly known to the public is that this dismal survival rate is actually worse when compared to patients with AIDS, liver cirrhosis, or stroke. Following a heart failure, the endogenous repair process is not sufficient to compensate for cardiomyocyte death. Thus, novel therapies with stem cells in combination with supportive scaffolds to form engineered cardiac tissue grafts is emerging as a promising therapeutic avenue. Engineered tissues have now been used to make new bladders for patients needing cystoplasty, bioarticial heart patches seeded with bone marrow cells, and more recently new trachea for patient with late stage tracheal cancer. Our multi-disciplinary team intends to push the therapeutic envelop by developing human tissue engineered myocardium for treatment of post-myocardial infarction heart failure. We will first test our engineered cardiac tissue in small and large animal models. We will perform extensive quality control measures to define morphological, molecular, and functional properties. At the end of 3 years, we are confident we will be able to derive a lead candidate that can move into IND-enabling preclinical development. These discoveries will benefit the millions of patients with heart failure in California and globally.
Despite advances in medical and device therapies, patients with end-stage heart failure have a survival rate of only 25% during the first 2 years following their diagnosis. Heart failure typically follows from damage induced by severe myocardial infarction (MI; heart attack). After a severe MI, the human heart may lose up to 1 billion heart muscle cells (cardiomyocytes). For most of these patients, heart transplantation is the only useful therapy, but there is a severe shortage of donor hearts. Recently, left ventricular assist devices (LVADs) have become available to take over the pumping function of the crucial left ventricle chamber of the heart. These devices were originally used as “bridge to transplant” (a temporary measure to keep patients alive until a new heart became available); recently some patients have received LVADs as “destination therapies” (permanent substitutes for transplanted hearts). The problems associated with these mechanical implants, however, include increased risk of stroke (blood clots that form due to the devices) and infection (the LVADs are powered from batteries that are carried outside the body and require wires to pierce the skin).
We are working to develop cardiac regenerative medicine using Engineered Heart Muscle (EHM). We are using human embryonic stem cells (hESCs) because they can be grown in very large quantities and, with the appropriate methods, can be triggered to differentiate into the cardiomyocytes, fibroblasts and smooth muscle that are lost after MI. Because these cells can be produced in essentially unlimited quantities, we could theoretically treat a very large number of patients who currently have no options.
During the first year of this project, we have a) established methods for producing the multi-billion quantities of hESC-derived cells needed to address this problem; b) developed methods to freeze and ship these cells to our collaborator in Germany for EHM assembly, and c) used these cells to generate 2 different forms of EHMs to compare their survival and function both in vitro (composition, force generated) and in vivo (after transplantation into rats that have been given MIs). We are now refining the EHM design with the goal of moving forward to testing them in animals with more human-like hearts (based on size and heart rate); this step will be essential to evaluate their safety and function before any clinical trial.
The project “Heart Repair with Human Tissue Engineered Myocardium” is designed to find a new option for the treatment of heart failure. Because of the shortage of donor hearts, many patients in need never receive this life-saving therapy. We are generating engineered heart muscles (EHMs) that are made from cardiomyocytes (heart muscle cells) derived from human embryonic stem cells. The ultimate goal of this work is to produce a beating human heart “patch” that can be transplanted onto damaged hearts, and help restore function.
Through the joint efforts of researchers in Dr. Joseph Wu’s laboratory at Stanford and Dr. Larry Couture’s team at City of Hope, we have developed a process that allows essentially unlimited generation of cardiomyocytes using a process that is fully compatible with eventual clinical use. Our collaborator at Gottingen University, Dr. Wolfram Zimmerman, uses these cells to produce EHMs, which are then shipped to Stanford. At Stanford, the EHMs are evaluated for their structure, overall health, and ability to generate force as measured in vitro. These EHMs are also transplanted into rodents that have been given heart attacks, to see if the EHMs can survive and improve heart function.
In the first year of this project, we compared different methods of making EHMs and the results that could be measured both in vitro and in vivo. We established a model of heart disease in rats with defective immune systems (necessary for the survival of human cells/tissues in this extremely foreign setting). We found that a specific grid-like patch design was both easier to construct than other options and was able to survive in the rat model of heart disease.
In the 2nd year, we focused on this patch design and performed a larger number of transplants. Using EHMs made from genetically engineered cells that give off a fluorescent signal, we were able to track the long-term survival of the EHMs (at least 7 months) without having to sacrifice many of the animals. Our analysis of the transplanted EHMs showed that they had survived transplantation and had taken on characteristics that made them closer to normal heart tissue. In addition, EHM transplantation resulted in improved heart function, as compared to rats that either received no transplants or received a control EHM transplant that contained dead cells.
The next phase of our project will be to evaluate the function of larger EHMs in swine model of heart disease, since these animals have hearts that are similar in size and heart rate to humans. This is a crucial step before considering translating this work into human patients.