Out of Sync: How genetic variation can disrupt the heart's rhythm
In the August 31 issue of Science Translational Medicine, new research from the University of Chicago shows how deficits in a specific pathway of genes can lead to the development of atrial fibrillation, a common irregular heartbeat, which poses a significant health risk.
Researchers describe a complex system of checks and balances, including the intersection of two opposing regulatory methods that work to maintain normal cardiac rhythm, and offer insights that could lead to individualized treatment in humans.
"We hope that this and similar studies contribute to a mechanistic understanding underlying the genetic basis of heart arrhythmias" said study author Ivan Moskowitz, MD, PhD, associate professor in the Department of Pediatrics, Pathology, and Human Genetics at the University of Chicago. "Such studies will allow clinicians to stratify patients based on their likely natural history of disease and potentially their response to specific therapeutics."
Atrial fibrillation (AF) is the most common cardiac arrhythmia in the world. It affects more than 2.7 million Americans, according to the American Heart Association. AF occurs when the normal rhythm of the heart goes awry, causing a rapid, irregular heartbeat. When blood is not properly ejected from the heart, blood clots can form, leading to high risk of stroke.
The U.S. spends billions of dollars annually to treat AF. Current treatments focus not on the source of the arrhythmia, but on the secondary health effects it causes.
"Now we prescribe the equivalent of rat poison (Coumadin® or other blood-thinning medications) to prevent blood clots and decrease the risk of stroke," said Moskowitz. "Or, doctors will use a laser to burn part of your heart to try to stop the arrhythmia. Of course neither of these options is where we want to be."
Patients with other forms of heart disease, such as congestive heart failure or hypertension, have an increased risk of AF. For decades this observation caused doctors to believe that AF was just a side effect of other heart-related issues. However, some patients with AF have no other cardiac issues and not all patients with congestive heart failure have AF. Having a family member with AF is associated with a greatly increased risk for the arrhythmia, suggesting a genetic component.
"This led the field to ask what is it about one person who has predisposing cardiac disease and gets AF from someone who doesn't? It turns out there is a profound genetic component associated with the risk," said Moskowitz.
Genetic factors lead to atrial fibrillation in humans
Genome wide association studies, which use the genomes of many people to find differences in the DNA associated with people who have a particular disease, have identified specific genes in humans that are involved in AF. While these studies are incredibly powerful in identifying the 'who' (the important genes) they do little in the way of offering a 'how'.
One of the regions in the genome implicated in AF is near a gene named Tbx5. Although its role in AF was not understood, Tbx5 is known to control other genes and to be important in both the structure and the rhythm of the heart. People missing one of their two copies of Tbx5 have a condition called Holt-Oram syndrome, which leads to impaired limb and heart development.
"An important starting point was a genome wide association study that implicated Tbx5 in patients with a structurally normal heart," said Rangarajan Nadadur, first author on the study. "That indicated to us that Tbx5 has a role in cardiac rhythm outside of its role in cardiac structure."
It was long thought that a mouse heart could not develop primary AF, but when Nadadur and others in Moskowitz's team knocked out the Tbx5 gene from adult mice, they found that the mice developed spontaneous AF.
Using this tool - mice with structurally healthy hearts and AF - the researchers investigated what role Tbx5 played by looking for the genes it controlled. About 30 genes have been linked to AF in humans. The researchers found that half of those genes were decreased in the absence of Tbx5 and that Tbx5 directly targeted some of those genes.
A balancing act for the heartRegular electrical activity propagating through a normal heart.
Pitx2, a gene controlled by Tbx5, is the most commonly identified gene in genome wide association studies for AF. This finding prompted the researchers to reach out to James Martin's research group at Baylor College of Medicine, collaborators on a Leducq Foundation grant to study AF, who were studying Pitx2. They found a common set of genes, important to cardiac rhythm, which were not properly controlled when either Tbx5 or Pitx2 was removed from a mouse. These genes were also increased after Pitx2 was removed, the opposite of what was seen when Tbx5 was removed.
"Both Tbx5 or Pitx2 directly control important rhythm genes in the heart, but in opposite directions" said Moskowitz. "Removing either causes a susceptibility to AF." Surprisingly, Moskowitz's group found that when Tbx5 and Pitx2 were decreased together in the same mouse, the predisposition to arrhythmia went away. These observations describe a system known as an 'incoherent feed forward loop', controlled by Tbx5 and Pitx2. Although described in simpler systems, this is the first time this model has been described in the heart. An incoherent feed forward loop enables the system to maintain a delicate balance, almost like a shock absorber. If the system is disturbed, and one side shifts up or down, the incoherent feed forward loop intervenes to keep it from moving too far. This network allows for tight control of genes that control the rhythm of the heartbeat.
Irregular electrical activity propagating through a Tbx5-deleted heart (undergoing atrial fibrillation).
"The clinical application of this model is that we may be able to provide more precisely targeted treatments to AF patients depending on whether their cardiac rhythm network is up- or down-regulated," said Moskowitz. For example, if an important calcium channel is too active and causing AF, blocking it with medication would be helpful. However, if that calcium channel is not active enough and contributing to AF, prescribing a calcium channel blocker may be ineffective or even harmful.
"We believe that a better understanding of the mechanisms underlying the genetic risk of the disease will ultimately have a significant impact on treatment," said Moskowitz.
The study was funded by the National Institutes of Health, the Leducq Foundation, and the American Heart Association. Additional authors include Xinan Yang, Jenna Bekeny, Margaret Gadek, Michael Broman and Stefan Mazurek from the University of Chicago; Yun Qiao, Igor Efimov and Bastiaan Boukens from George Washington University; Malou van den Boogaard and Vincent Christoffels from Amsterdam, the Netherlands; Tarsha Ward, Christine Seidman and Jon Seidman from Harvard Medical School; Min Zhang and James Martin from Baylor; and Elizabeth McNally from Northwestern University.