Curiosity as to why smokers are at an increased risk of developing heart disease has led Cleveland Clinic (Cleveland) researchers, joined by other researchers, to a discovery of a marker that has the potential, they say, to be as important as cholesterol is a predictor of heart disease.
The researchers say the study may help explain why one person suffers a heart attack, and another does not. The Cleveland Clinic researchers’ findings were published in the Sept. 9 issue of the journal Nature Medicine and titled “Protein carbamylation links inflammation, smoking, uremia and atherogenesis.” In addition to smoking, the study also looks at why those with chronic renal disease are more likely to have cardiovascular disease.
“Of all the markers we’ve tested so far, this one outperforms everything,” Stanley Hazen, MD, PhD, section head of Preventive Cardiology at Cleveland Clinic and a researcher in the Lerner Research Institute told Diagnostics & Imaging Week. “And it’s independent of cholesterol, HDL, LDL; it’s independent and additive to MPO and additive to C-reactive protein and other inflammation markers.”
Hazen said the researchers began their effort because they were interested in determining why patients who smoke are at an increased risk to develop heart disease, so they began studying the “chemistry of how smoking affects the inflammation system.”
“And it was through this approach that we discovered that carbamylation, which is a post-translational modification of proteins, is increased due to a product that increases in the blood in smoking,” he said.
While the researchers are not saying carbamylation is the cause of atherosclerosis, they do “think it is a contributor to that,” he said. This mechanism is not only present in high-risk populations, it is also found in those with chronic kidney disease. When proteins in the bloodstream are damaged by carbamylation, it changes the way the cells behave, promoting the accumulation of harmful substances in the arteries and therefore raising the risk of heart disease, the Cleveland Clinic said.
“We think that we have now identified — call it the ‘dark matter’” that is a heretofore “unrecognized underlying pathogenic mechanism that’s involved in the development of atherosclerosis in everyone,” Hazen said, because, he pointed out, doctors already know that cholesterol is not the “be all and end all of how heart disease occurs.” He said that, for example, some people who don’t have high cholesterol develop heart disease, and others with high cholesterol don’t develop atherosclerosis.
The carbamylation process increases the level of a substance called thiocyanate in plasma, which a white blood cell enzyme called myeloperoxidase uses to covalently modify and alter its function to make proteins like “LDL become more atherogenic, and HDL lose some of its protective function.”
The researchers found that a blood test measuring systemic levels of homocitrulline, a molecular marker for the pathway, serves as the “strongest independent predictor of heart disease risk identified thus far,” according to the Cleveland Clinic.
“Then what we found is that this was true not just of smokers but in everyone, and the levels of this in the plasma very much depends upon what your diet is . . .”, Hazen said.
Hazen’s research builds on earlier research conducted at the Cleveland Clinic in the early part of this decade centered around myeloperoxidase (MPO), described then as an “abundant leukocyte enzyme” that is elevated in “culprit lesions that have fissured or ruptured in patients with sudden death from cardiac causes,” according to a study abstract published in The New England Journal of Medicine back in October 2003. The blood test that the Clinic developed was meant to determine whether a person is in danger of experiencing a heart attack within 30 days to six months.
In a prepared statement at the time, Hazen said, “We looked at more than 600 sequential patients who came to the emergency room with chest pain. We found that adding MPO testing to current laboratory-based risk assessments increased our ability to predict future cardiac risks over the next 30 days to six months from 50% to 95% of the time.”
Hazen told D&IW last week that part of the reason he looked at “this particular chemistry” is because he “knew myeloperoxidase had a preference for using this compound [thiocyanate] that is increased in the plasma of smokers, but is present in everybody’s plasma.”
Hazen said that in a normal person’s plasma, thiocyanate is the “very first thing that MPO likes to use as a substrate and chew up before it goes and makes its other products.”
In 2005, the diagnostics division of Abbott Laboratories (Abbott Park, Illinois) entered a licensing agreement with the Cleveland Clinic for the development of an automated in vitro diagnostic test to detect MPO. Abbott obtained nonexclusive rights to the Cleveland Clinic’s existing technologies relating to MPO as a risk indicator for cardiovascular disease. Abbott also maintained worldwide rights to commercialize products using the technology in 2005.
Because of the clinic’s experience with licensing MPO, Hazen says he knows exactly the steps it needs to take. First, the researchers are going to attempt to develop a higher throughput form of the test, which he said is the first step needed “to bring it to clinical use in populations,” because at the moment it is a mass spectrometry test.
In addition to a prognostic test, the researchers are also interested in licensing the technology for development of a drug to combat this process that leads to atherosclerosis.
“People have already described [that when] you introduce MPO into a mouse, it gets accelerated atherosclerosis,” Hazen said. “So, I think this adds more ammunition to trying to accelerate the development of an MPO inhibitor as a novel therapeutic target for cardiovascular disease.”