Caption: An energy shortage in the cells causes the heart to pump poorly.

For biologist Cecilia Lo, every day at work revolves around medical mysteries. But there was one particular diagnosis that was so confounding and devastating, she couldn’t rest until she figured it out. In the United States, an average of one-third of babies born with hypoplastic left heart syndrome (HLHS) die before their first birthday. What made the condition fatal for some, while others grew up to live fairly normal lives?

Lo, Distinguished Professor and F. Sargent Cheever Professor, who chairs developmental biology at the University of Pittsburgh, has spent four years trying to pinpoint the mechanism that leads to cardiac failure in certain babies born without a functional left ventricle, the heart chamber responsible for pumping oxygenated blood to the rest of the body. Some children struggle even after major surgery to reconfigure the organ—in which a surgeon creates a one-chamber heart that allows the right ventricle to pick up the slack—and even when the operation is technically successful.

Although outcomes are better at UPMC Children’s Hospital of Pittsburgh and are improving generally with advances in surgical options and medical approaches, for a subset of children with HLHS—“It’s heartbreaking,” says Lo. “There is interest to see if we can rescue these babies and to develop therapies to treat them if they go into acute heart failure.”

Many speculated that the poor outcomes were because the surgically enhanced right ventricle becomes overworked to the point that it just gives out.

But based on a study of the condition in mice, Lo theorized the problem was at the cellular level. If she could prove her hypothesis, it might allow doctors to identify the highest-risk patients and move them to the top of the long waiting list for a heart transplant. “If we don’t understand what causes the heart failure, there is no way to identify who is at risk.”

Medical researchers expect surprises— behind many medical breakthroughs are hypotheses that don’t exactly pan out and force scientists down a new path. But when Lo peered into the microscope to examine the heart cells, she tingled with excitement. The surprise was that there were no surprises. Her theory held up—the mitochondria of people with severe HLHS were damaged.

“It was very gratifying, very exciting. What we predicted actually happened in a very clear-cut fashion. A lot of the time your hypothesis doesn’t necessarily go exactly the way you think. But this just came out beautifully.”

She likens the heart to a car and mitochondria to the gasoline that fuels it—if the mitochondria are defective, the car won’t move.

To test her hypothesis, Lo and her team started with research on mice, looking for genetic mutations that cause congenital heart disease. “We proved that you could get HLHS in mice.” And by studying the animals, they discovered the problem was the mitochondria in the diseased mouse cells.

To test the theory on humans, they collected fibroblast cells (which build connective tissue) from three healthy children and 10 with HLHS. They divided the HLHS cells into two groups—those from patients with severe HLHS who either died or had a transplant before their first birthday and those from a mildly affected group of patients who had lived past age 5 without a transplant.

They converted the fibroblast cells into pluripotent stem cells, which have the capacity to develop into any kind of cell in the body. By adding a specific mixture of nutrients and growth factors, the researchers were able to turn the stem cells into human heart cells that actually beat in the dish.

Just as a heart pulsates as it pumps blood, so do individual heart cells.

Peering at the heart cells through a microscope at the John G. Rangos Sr. Research Center, Lo noticed differences between the two study groups immediately. The heart cells of the healthy patient pulsated robustly. The heart cells of patients who had mild HLHS were similar to the healthy heart cells, pumping a little slower but still steadily. In contrast, the cells from patients with severe HLHS pumped slowly, their labored movement similar to what doctors saw in the hearts of the sickest patients.

Lo discovered that both groups of HLHS cells had defective mitochondria, but the damage was more significant in the group with severe disease. These cells were also unable to use natural defenses to compensate for the damaged mitochondria. The languid pumping could be fatal for a baby, even after a procedure to reroute the anatomy of the underdeveloped heart. At the root of this energy shortage in HLHS is a process known as oxidative stress.

Typically, when the body works as it’s supposed to, mitochondria produce the energy required for cells to function. This happens through a series of steps in which electrons are passed along by protein molecules and then accepted by oxygen. But when there’s oxidative damage, the electrons get lost before they reach the oxygen molecule, holding up the energy production process. The result is damage to the mitochondria, resulting in damage to the cell’s ability to function and, ultimately, a severely damaged organ. “The heart is not going to work very well if you have cells that die and can’t pump blood,” says Lo. Her team uncovered a metabolic marker that may one day be accessed with a simple blood test.

“There’s a lot of data that shows that these patients also have poor neurodevelopmental outcomes such as autism or cognitive impairment. The genes that regulate heart development also regulate brain development.”
Having figured out what caused the problem, Lo and her team set out to test various drugs in search of potential therapies to remedy the mitochondrial defect. They found two promising candidates—sildenafil, which is commonly known by the brand name Viagra, a drug that increases blood flow. The other one is a supplement called TUDCA, a “molecular chaperone” that restores the folded structure of protein molecules that have fallen apart because of oxidative stress. Xinxiu “Cindy” Xu was a lead author on a May 2022 Cell Stem Cell paper that described the findings.

Both of these drugs are already known to be safe in adult humans and won’t have to go through a lengthy approval process. “We don’t have to do safety studies like you would if the drug had never been used clinically.” TUDCA is sold over the counter in health food stores and is often used by body builders. Whether either would be safe and effective for babies with HLHS, we don’t yet know.

First, clinical studies have to be conducted in this patient population. But that’s difficult with a relatively rare condition. “Any one medical center might not have enough patients,” says Lo. “You might only get 10 or 15 a year.” So clinical trials would require multiple centers to participate, which would require more funding.

Nationally HLHS affects about 1,000 babies born each year.

Mary Sanderson (not her real name), a new mother in Harrisburg, had never heard of HLHS until she was pregnant with her first child and had a prenatal screening.

The first doctor she saw told her that surgery wouldn’t be an option and that her baby wouldn’t survive. That was agonizing.

Then a second doctor, a cardiologist, referred her and her husband to UPMC Children’s Hospital, which has some of the best HLHS patient outcomes in the country. Children’s cardiology and surgery teams are led by Jacqueline Kreutzer, professor of pediatrics who is the Peter and Ada Rossin Professor of Pediatric Cardiology, and Victor Morell, surgeon-in-chief who is the Eugene S. Wiener Professor of Cardiothoracic Surgery.

Sanderson’s baby boy was born on May 7 and had his first major operation on the 12th. “He is recovering well,” she says.

Sanderson says the condition made the entire birth process incredibly stressful: “I couldn’t enjoy any part of the pregnancy. I was always worried. I went into labor in Harrisburg, and I worried I wouldn’t get to Pittsburgh on time.”

She is relieved that doctors at Children’s could do surgery­—“Science is amazing,” she says­—and gladly allowed researchers in Lo’s lab to take cell and blood samples from her son. “It is so important that they learn different ways to treat it.”

“Despite major advances in pediatric cardiology over the last few decades, there continue to be major challenges and unsolved life threatening conditions that affect children with congenital heart disease,” says Kreutzer. “Dr. Lo’s groundbreaking research is critical to improve their quality of life and shift traditional care approaches to a new level.”

Even if all goes smoothly with Lo’s studies, it’s difficult to predict how long it will take to finish them and get approval for treatment. Lo and her team plan to test other drugs, as well, to see if they can reverse the effects of mitochondrial damage safely.

A biomarker might come sooner, perhaps within five years, if funding is available. Her research opens the possibility of a simple blood test that will let doctors know which babies are at the highest risk of going into cardiac failure, bumping them to the top of the long transplant list. “There aren’t enough to go around,” Lo says.

“This will tell us who should have priority to get the transplant.”

Check out this related story: This family carried a rare mutation that should have been lethal.