ASGCT has been full of discussion spanning retrospective analysis of progress in the field of gene and cell therapy through innovative and creative next steps to advance the field. The talks during Wednesday’s Presidential Symposium were no exception.
Charles Murry, PhD, University of Washington Medicine Pathology, presented years of work in a talk titled, “Gene Editing to Enhance Cardiomyocyte-Based Heart Regeneration” to a room full of eager attendees. The general session hall was standing room only, with attendees eager to listen to the speakers. Murry didn’t disappoint as he shared his research story to better inform cardiac regeneration research and therapy options.
“I’d like to start with is just give you a brief glimpse of the pathophysiology of the ischemic heart disease. It should be familiar to most people in the audience, because it’s the number one cause of death in the world, number one cause of death in the United States, number one reason for admission to home from the hospital and patients over 65,” began Murry.
Once an individual develops cardiac damage there is little chance of recovery, let alone survival. Unlike the hearts of some other species, when damaged, human hearts will develop collagen laden scar tissue rather than cardiac muscle cells. This replacement of contractile cardiac tissue with tough and mostly immobile collagen significantly reduces the functionality of the heart.
Transplantation
Regeneration of the cardiac tissues is accomplished by modifying the signaling pathways in hPSCs to specialize the cells into cardiomyocyte progenitor cells. This process has been standardized and scaled up to the point of being able to make large quantities of cells in bioprocessing manufacturing plants. The cells grown in the large vats are multicellular spheroids that can be transplanted into hearts damaged by myocardial infarctions.
Murry quipped, “We started out, like most people do, with very academic starting enterprise, not quite smoking in the tissue culture or pipetting by mouth. But do you know what it’s like and we’ve cleaned up grafting to the point where we have GP protocols in favor of. We also needed to scale up manufacturing.”
Murry and his team tested the functionality of transplanted cardiac tissue in macaques. Over the course of three months, they found improvement in blood ejection, nearly up to the capacity of undamaged hearts, compared to controls.
This was promising data, but these values didn’t fully capture the whole story. Across the various animal species tested, there was variability in quality of heart function. Arrhythmias and tachycardia events were reported to varying degrees. Some animals like mice and ferrets had less detectable negative response, while porcine and non-human primates had more negative responses with some pigs suddenly going into cardiac arrest, while the primates struggled and recovered over time.
“So at this point, I made the decision: You’re not going to take this into the clinic until we solve what is causing this,” Murry declared. This led to the development of Project MEDUSA.
Project MEDUSA
Project MEDUSA (Modifying Electrophysiological DNA to Understand & Suppress Arrhythmias) was established to address and correct the arrhythmia side effect. Murry pointed out that “the root of this arrhythmias seemed to be pacemaking” and he hypothesized that the transplanted graft cells retain contractile automaticity which leads to arrhythmia of the graft with the rest of the heart.
Simply put, the project’s aim was to create cells that lack automaticity but retain the ability to be excitable. Logistically, this is not truly a simple task.
Using gene editing strategies, Murry identified multiple genes that are either knocked out (KO) or over-expressed (OE) to achieve the goal of creating these cells. Using stepwise experimentation, deduction, and deep dedication, the team finally developed a cell that held promise. With a multitiered approach that involved developing the cardiac progenitor cells, and grafting them in pigs, a process that took years to accomplish. Murry pointed out that “this is not going to be just a simple one and done kind of gene editing story.” After many trials and false starts, the cell line to emerge triumphant was a triple KO of excitatory channels combined with a single OE inhibitory channel, which Murry lovingly named “MEDUSA-cells.”
The MEDUSA-cells proved to have about a 95% reduction in arrhythmia in transplantations. However, there were still some arrhythmias and there would be a need to anti-rejection medications. With the KO/OE knowledge, the next step was to create autologous iPSCs to reduce the likelihood of rejection. Using rhesus monkeys that experienced myocardial infarction, Murry found that after 14 weeks, there was no rejection of the tissue, but there was also a maturation of the stem cells to mature, adult cardiomyocytes. There was also no evidence of pro-inflammatory or non-specific immune response.
Murry concluded that while this work holds promise for treatment of cardiomyopathy, the leading cause of death in humans, there are some drawbacks. The basic research has been hard, slow, and expensive, and at this point, individualized therapies would follow suit. He pointed out “there’s remaining challenges that we’ve been making good progress.” He also explained that using iPSCs should work in humans, but the cost and time commitment is likely prohibitive to practical widespread use in humans currently.
Murry will continue his work after he transitions to his new role at the Keck School of Medicine at the University of Southern California.
