Researchers have now determined the molecular mechanism underlying a cardiac arrhythmia syndrome they discovered that can lead to sudden death in young, seemingly healthy people.
The researchers -- from Duke University Medical Center, Vanderbilt University Medical Center and the Howard Hughes Medical Institute (HHMI) -- found that the disorder is caused by a mutation in an integral component of a four-protein complex that coordinates the inflow and outgo of key chemicals - especially calcium - in heart muscle cells. The passage of such ions in and out of heart muscle cells controls the beating of the heart.
The researchers found that patients with a mutation in the gene that codes for the protein known as ankyrin-B are at high risk of sudden death from cardiac arrhythmia. Ankyrin-B serves as a molecular scaffold and "cellular chaperone" for the complex.
These findings are important, the researchers said, because the new knowledge of the structure and location of the ankyrin-B complex -- as well as its importance in maintaining calcium equilibrium within heart muscle cells -- gives them targets for potential new drugs. Furthermore, the researchers said, their findings provide new insights into other disorders involving the pancreas and the eyes.
The results of the researchers were published Nov. 16, 2005, in the Public Library of Science - Biology. The research was supported by the National Institutes of Health and Johnson & Johnson, New Brunswick, N.J.
"We found that ankyrin-B forms a complex with three different proteins that allow the orderly passage of different ions in and out of the cell," said HHMI investigator and Duke cell biologist Vann Bennett, M.D., Ph.D., senior member of the research team. "The most important of these ions is calcium, since its entry into a cell causes the contraction of the cell. However, before the next heartbeat, the calcium needs to be pumped out of the cell. We found that the ankyrin-B complex acts as a safety valve that maintains the appropriate calcium balance."
However, for patients with the mutation in the gene that produces ankyrin-B, the protein complex is much less effective in maintaining this balance, making them vulnerable to the arrhythmia.
The Duke team first reported this specific mutation in ankyrin-B in 2003, based on an international study of a large French family whose members had the mutation. They later identified a specific arrhythmia in humans in 2004 that has since been termed "sick sinus syndrome with bradycardia."
While calcium is important for increasing the heart muscle cell's ability to contract, too much can cause damage to the cell, the researchers said. Once calcium enters the cell, it must quickly be pumped back out, a function that is performed by the sodium/calcium exchanger protein (NCX1), which trades incoming sodium for outgoing calcium. The sodium is then pumped out by the protein enzyme Na/K ATPase. Both of these protein "molecular pumps," NCX1 and Na/K ATPase, are located on the membranes of heart cells.
The researchers knew that ankyrin-B binds individually to NCX1 and Na/K ATPase, as well as a third receptor known as inositol 1,4,5-triphosphate receptor (InsP3R). The exact role of InsP3R is not known and is still under investigation.
"We found for the first time that ankyrin-B binds to all three of the proteins at once, forming a single multi-protein complex," said Peter Mohler, Ph.D., first author of the paper. Mohler, who is now on the faculty of Vanderbilt, performed the research at Duke while he was a HHMI post-doctoral fellow.
"Our analysis showed that the mutant ankyrin-B lost about 60 percent of its ability to bind with the other three proteins," Mohler explained. "Since the physiological effect of the mutation is loss of calcium regulation in heart cells, these results strongly suggest that binding to ankyrin-B is critical for efficiently coordinating the function of the sodium/calcium exchanger with that of Na/K ATPase to remove calcium from the cell."
Interestingly, the researchers pointed out that there are breeds of mice that have the gene for each of the three protein pumps "knocked out," and none of them exhibited heart problems. Nor do humans lacking the gene show heart problems.
"It is exciting to find that the three proteins and ankyrin-B formed one giant protein complex that clearly has a physiological function, and further that it explains why the humans with the mutation of ankyrin-B suffer from these arrhythmias," Mohler said. "Now we know why these people really die - the mutant form of ankyrin-B causes this large protein complex to fall apart."
The researchers pointed out the ankyrin-B complex is not found in skeletal muscle or smooth muscle, and its presence in heart muscle cells probably evolved as a mechanism for controlling the rapid influx, and subsequent rapid pumping out, of calcium, as would be needed during the "fight or flight" response.
Ankyrin-B is also found in nerve cells within the heart, leading researchers to believe that the gene could play important role in other aspects of heart function. It is expressed by cells in different organ systems, including certain nervous system cells, the linings of the lungs and kidneys, the retina and the insulin-producing cells of the pancreas.
"There are excitable cells within the pancreas that use calcium channels to regulate the insulin production," said Bennett, who first discovered ankyrin in 1978. "It may be possible that what we have learned about ankyrin-B's role in maintaining calcium balance in the heart may also hold true in the pancreas."