Having high levels of high density lipoprotein (HDL), also called "good" cholesterol, is believed to reduce our heart disease risk, but a new study found an exception to the generally accepted medical maxim.

The study, led by researchers at the University of Pennsylvania and published Thursday in the U.S. journal Science, showed that a rare genetic cause of increased HDL may actually be a "bad" thing.

"Our results indicate that some causes of raised HDL actually increase risk for heart disease," said senior author Daniel Rader, chair of the department of genetics at the University of Pennsylvania.

"This is the first demonstration of a genetic mutation that raises HDL but increases risk of heart disease."

Rader and his colleagues compared lipid-modifying genes of 328 people with markedly elevated HDL to a control group with lower HDL and identified one noteworthy individual who carried two copies of a gene variant.

Normally, HDL is involved in moving cholesterol from peripheral tissues to the liver, where the cholesterol is metabolized and excreted.

A rare mutation in a gene called SCARB1, however, prevents HDL from doing its job, therefore resulting in a buildup of HDL in the blood, the researchers said.

This increase in HDL levels "does not reflect an increase in clearing of cholesterol from the peripheral tissue," explained Michael Holmes, who was not involved in the study, at the University of Oxford.