Persistence pays off in search for clue to heart defects

A team of scientists led by Benoit Bruneau (left), including Irfan Kathiriya (center) and Kavitha Rao (right), make inroads into understanding what genes are improperly deployed in some cases of congenital heart disease.  Photo courtesy Gladstone Institute

For more than 20 years Dr. Benoit Bruneau has been trying to identify the causes of congenital heart disease, the most common form of birth defect in the U.S. It turns out that it’s not one cause, but many.

Congenital heart disease covers a broad range of defects, some relatively minor and others life-threatening and even fatal. It’s been known that a mutation in a gene called TBX5 is responsible for some of these defects, so, in a CIRM-funded study ($1.56 million), Bruneau zeroed in on this mutation to see if it could help provide some answers.

In the past Bruneau, the director of the Gladstone Institute of Cardiovascular Disease, had worked with a mouse model of TBX5, but this time he used human induced pluripotent stem cells (iPSCs). These are cells that can be manipulated in the lab to become any kind of cell in the human body. In a news release Bruneau says this was an important step forward.

“This is really the first time we’ve been able to study this genetic mutation in a human context. The mouse heart is a good proxy for the human heart, but it’s not exactly the same, so it’s important to be able to carry out these experiments in human cells.”

The team took some iPSCs, changed them into heart cells, and used a gene editing tool called CRISPR-Cas9 to create the kinds of mutations in TBX5 that are seen in people with congenital heart disease. What they found was some genes were affected a lot, some not so much. Which is what you might expect in a condition that causes so many different forms of problems.

“It makes sense that some are more affected than others, but this is the first experimental data in human cells to show that diversity,” says Bruneau.

But they didn’t stop there. Oh no. Then they did a deep dive analysis to understand how the different ways that different cells were impacted related to each other. They found some cells were directly affected by the TBX5 mutation but others were indirectly affected.

The study doesn’t point to a simple way of treating congenital heart disease but Bruneau says it does give us a much better understanding of what’s going wrong, and perhaps will give us better ideas on how to stop that.

“Our new data reveal that the genes are really all part of one network—complex but singular—which needs to stay balanced during heart development. That means if we can figure out a balancing factor that keeps this network functioning, we might be able to help prevent congenital heart defects.”

The study is published in the journal Developmental Cell.

How stem cells know the right way to make a heart . And what goes wrong when they don’t

Gladstone scientists Deepak Srivastava (left), Yvanka De Soysa (center), and Casey Gifford (right) publish a complete catalog of the cells involved in heart development.

The invention of GPS navigation systems has made finding your way around so much easier, providing simple instructions on how to get from point A to point B. Now, a new study shows that our bodies have their own internal navigation system that helps stem cells know where to go, and when, in order to build a human heart. And the study also shows what can go wrong when even a few cells fail to follow directions.

In this CIRM-supported study, a team of researchers at the Gladstone Institutes in San Francisco, used a new technique called single cell RNA sequencing to study what happens in a developing heart. Single cell RNA sequencing basically takes a snapshot photo of all the gene activity in a single cell at one precise moment. Using this the researchers were able to follow the activity of tens of thousands of cells as a human heart was being formed.

In a story in Science and Research Technology News, Casey Gifford, a senior author on the study, said this approach helps pinpoint genetic variants that might be causing problems.

“This sequencing technique allowed us to see all the different types of cells present at various stages of heart development and helped us identify which genes are activated and suppressed along the way. We were not only able to uncover the existence of unknown cell types, but we also gained a better understanding of the function and behavior of individual cells—information we could never access before.”

Then they partnered with a team at Luxembourg Centre for Systems Biomedicine (LCSB) of the University of Luxembourg which ran a computational analysis to identify which genes were involved in creating different cell types. This highlighted one specific gene, called Hand2, that controls the activity of thousands of other genes. They found that a lack of Hand2 in mice led to an inability to form one of the heart’s chambers, which in turn led to impaired blood flow to the lungs. The embryo was creating the cells needed to form the chamber, but not a critical pathway that would allow those cells to get where they were needed when they were needed.

Gifford says this has given us a deeper insight into how cells are formed, knowledge we didn’t have before.

“Single-cell technologies can inform us about how organs form in ways we couldn’t understand before and can provide the underlying cause of disease associated with genetic variations. We revealed subtle differences in very, very small subsets of cells that actually have catastrophic consequences and could easily have been overlooked in the past. This is the first step toward devising new therapies.”

These therapies are needed to help treat congenital heart defects, which are the most common and deadly birth defects. There are more than 2.5 million Americans with these defects. Deepak Srivastava, President of Gladstone and the leader of the study, said the knowledge gained in this study could help developed strategies to help address that.

“We’re beginning to see the long-term consequences in adults, and right now, we don’t really have any way to treat them. My hope is that if we can understand the genetic causes and the cell types affected, we could potentially intervene soon after birth to prevent the worsening of their state over time.

The study is published in the journal Nature.