HDL, or ‘good’ cholesterol, can remove cholesterol from arteries and shuttle it to the liver where it is eliminated, but this process can be disrupted in certain circumstances (such as deficiency of SCARB1). Credit: The lab of Daniel Rader, MD, Perelman School of Medicine, University of Pennsylvania
The medical maxim that elevated HDL cholesterol (HDL-C) is “good” has been overturned by a multi-center, international study, led by Perelman School of Medicine researchers at the University of Pennsylvania. They show that a certain genetic cause of increased HDL-C may actually be “bad,” noting that a specific mutation in a gene which encodes a cell receptor protein that binds to HDL prevents the receptor from functioning. The mutation causes an increased risk of coronary heart disease even in the presence of elevated levels of HDL-C or “good” cholesterol.
Previous research raised the possibility that HDL might not be quite as protective against heart disease as generally believed by cardiologists, especially after several clinical trials of HDL-raising drugs showed little or no effect. “The thinking about HDL has evolved recently to the concept that it may not directly protect against all heart disease,” said Daniel J. Rader, MD, chair of the department of Genetics. “Our results indicate that some causes of raised HDL actually increase risk for heart disease. This is the first demonstration of a genetic mutation that raises HDL but increases risk of heart disease.”
Rader et al sequenced the lipid-modifying regions of the genomes of 328 people with markedly elevated HDL (along with a control group with lower HDL) to identify genetic causes of high HDL. One of the genes they focused on was SCARB1, which encodes for Scavenger Receptor B1 (SR-B1), major receptor for HDL on cell surfaces. They identified, for the first time, a person without any SCARB1 function, typified by an extremely high HDL-C level of about 150 mg/dL, whereas the normal level is about 50 mg/dL. The subject had 2 copies of a SCARB1 mutation called P376L, which the team showed caused a breakdown in HDL receptor function.
The researchers generated induced pluripotent stem cells (iPSCs) from the SCARB1-deficient person, used them to create liver cells, and showed these new cells had profound reduction in their ability to take up HDL. “This mutation prevents the receptor from getting to the cell surface where it needs to be situated in order to bind and take up HDL,” Rader explained. “This disruption in the receptor’s job is due to mistakes in its folding and processing during protein synthesis.”
Going back to the other sequenced genomes, persons with only 1 copy of SCARB1 P376L mutation have significantly higher HDL-C levels. The Penn team and colleagues plan to characterize and test other SCARB1 mutations for their relationship to HDL levels and heart disease. Other genes may also have similar effects. “Eventually we may want to perform genetic testing in persons with high HDL to make sure they don’t have mutations -like this one -that raise HDL but don’t protect against, or may even increase, risk for heart disease,” Rader said. Since P376L mutation in SCARB1 appears to be specific to people of Ashkenazi Jewish descent, testing in this ethnic group might be particularly important.
A therapeutic approach to increase the expression or activity of SCARB1 could be a new way to reduce the risk of heart disease even though it would reduce HDL blood levels. “The work demonstrates that the protective effects of HDL are more dependent upon how it functions than merely how much of it is present,” Rader concluded. “We still have a lot to learn about the relationship between HDL function and heart disease risk.” http://www.uphs.upenn.edu/news/News_Releases/2016/03/rader/
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