A genetic variation might be responsible for a rare but serious side effect of cholesterol-lowering drugs or statins, British researchers have found.

Statins are prescribed for lowering risk of heart attack and strokes by reducing levels of harmful cholesterol in the blood. Nearly 3.4 million people in England alone are prescribed statins, preventing almost 10,000 deaths annually.

However, statins can sometimes cause myopathy, popularly known as muscle pain and weakness. Myopathy occurs in roughly one out of every 1,000 person on statins. Generally, myopathy is not severe and it disappears a day after the treatment is stopped. On very rare occasions, muscle tissue begins to break down leading to a potentially fatal condition called rhabdomyolisis.

Study leader Professor Rory Collins, a British Heart Foundation scientist at Oxford University and colleagues analyzed 300,000 genetic markers in 85 patients who developed myopathy and 90 patients who had no problems after taking a high dose of Merck & Co.’s Zocor, a cholesterol-lowering drug.

Other statins include Pfizer Inc.’s Lipitor, AstraZeneca Plc’s Crestor, (both generated $33.7 billion in worldwide sales in 2007), Mevacor (lovastatin), Pravachol (pravastatin), Lescol (fluvastatin) and Baycol (cerivastatin).

The researchers discovered that a variation in the DNA code of a gene called SLC01B1 was responsible for 60 percent of the myopathy cases in people taking high dose statin therapy.

This gene regulates the uptake of statins into the liver, and the genetic variant seems to affect its function, causing higher levels of the statin to be present in the blood.

The researchers found that carriers of one copy of the genetic variation were four times more likely to develop myopathy that those without a copy. The risk increased 16 times for carriers of two copies of the rogue gene.

Based on these findings, researchers could develop a genetic test specially designed to discover who is at risk of myopathy, Dr. Collins said.

“A DNA test based on these findings could guide doctors as to whether a patient at high risk of heart disease will cope with a high dose of a statin, which might be more effective than a standard dose at preventing a heart attack or stroke. If the patient doesn’t have the gene variation, they are likely to be at low risk of myopathy even on high statin dose.”

The study, funded by Merck & Co., was published in the July 24 issue of the New England Journal of Medicine.