Ryan Turner

Research Has Implications for Treatment of Strokes and Heart Attacks

Ryan Turner, an HMS student who has conducted research in the laboratory of HSPH associate professor Guy Reed, has engineered a form of the enzyme called plasmin that holds promise for improving treatment of heart attacks and strokes.

Turner is entering his fourth year of a five-year medical program. During his first two years, he conducted research part-time in the Reed lab under the auspices of a National Institutes of Health grant for minority students. He then won a grant from the Howard Hughes Medical Institute (HHMI) to conduct research full-time in the Reed lab last year.

Turner’s results were significant enough that he has earned a second grant from HHMI that will pay for his last two years of medical school. Turner was one of a small number of medical students from across the country to receive the second grant.

Heart attacks and strokes are caused by blood clots called thrombi that block blood flow in the arteries of the heart and of the brain. Body tissues become deprived of needed oxygen and nutrients, eventually dying.

Cardiovascular disease is the number one killer of Americans each year—and has been since 1900 with the exception of one year, according to a recent report by the American Heart Association.

Turner has spent the last year doggedly researching plasmin, a naturally occurring enzyme in the body with the primary responsibility of dissolving blood clots. Specifically, plasmin attacks fibrin, a substance that makes up clots.

A protein inhibitor called alpha 2-antiplasmin in blood stops the clot-busting effects of plasmin. Unfortunately, the quick action of alpha 2-antiplasmin is sometimes too fast, leaving clots in the body to do their damage.

"What I endeavored to do was to take plasmin and allow it to stay around longer in the body, increase its half-life so to speak, make it resistant to its natural inhibitor," said Turner.

To achieve his goal, Turner engineered a new form of plasmin. He took elements from another protein found in the body called Factor D. The protein looks almost identical to plasmin in two characteristics called primary sequence and three-dimensional fold, but is different in at least one very important way: Factor D is resistant to inhibition.

"I wanted to know why Factor D was so special and could not be inhibited," said Turner.

He used three-dimensional molecular viewing techniques to look at the crystal structures of proteins and decided that there were "loop" regions on the surface described as exosites.

"Essentially what I did was swap loops from the protein that was completely resistant to inhibition and insert them into plasmin, creating what we call a chimera, or a protein that is something in between the two original proteins," said Turner.

Turner’s findings have implications for developing new ways to treat thrombosis. When people come to an emergency room with coronary thrombosis or a heart attack, they may be given a common clot-busting enzyme called TPA. The enzyme cleaves plasminogen, producing plasmin. Turner envisions skipping the need to cleave plasminogen and instead directly infusing the body with the engineered form of plasmin, providing faster treatment to patients. A second infusion containing a different agent would then stop the anti-clotting effects of the engineered plasmin.

"People have tried things like this before with TPA, trying to engineer it, making it resistant to inhibitors, and they were successful in increasing its resistance," said Turner, "but no one has tried this before with plasmin. It’s completely new."

Added Reed, "This enzyme system has already been exploited by novel biological agents to save lives of people who have had heart attacks, but it’s clear we can do better. About one-half of people treated do not see the desired therapeutic results with these agents. If we could improve the therapeutic results, we could perhaps reduce mortality by 50 percent. That is why this enzyme system is such a good target."

The plasmin research is the latest in a string of studies that 25-year-old Turner has taken part in as a scientist. As an undergraduate at the University of Maryland Baltimore County, he worked on five published scientific papers, studying protein structures and nucleic acid structures related to HIV.

When he came to Harvard, Turner decided to try something new and joined Reed’s lab to do cardiovascular work. He credits Reed with providing him with the resources and guidance to learn about a new field and with giving him the freedom to plan his own research project.

"I was very fortunate to find Guy Reed as a mentor," said Turner. "He’s perfect because he practices cardiology and does basic science research, and I hope to do something similar when I graduate from medical school."

Turner has traveled to Paris, Washington, DC, San Diego and Florida to present his findings. He has been an author on three published papers since coming to Harvard.

This fall, Turner returns full time to medical school.

"Ryan exemplifies the ability to think about a human disease process, reduce it to a scientific question, and then try to answer that question in a way that would have implications for improving disease treatment," said Reed. "He is an extremely talented young man with a bright future in both biomedical research and medicine."