Protein Folding and Calcium Binding Defects Account for Errors in Familial Hypercholesterolemia

CAMBRIDGE, Mass. — Familial hypercholesterolemia, a genetic disease characterized by high levels of cholesterol and early mortality, is caused by defects in the receptor for the low-density lipoprotein (LDL)—the bad cholesterol. Now, Boston area scientists have found that this occurs because mutations in the LDL receptor prevent the protein from folding into its normal shape. This in turn impedes the receptor's ability to bind bad cholesterol and remove it from the bloodstream, causing the hypercholesterolemia.

All proteins are strings of amino acids that must fold into a unique shape to perform their function. "Knowing that a protein folding defect is at the root of familial hypercholesterolemia may allow scientists to design better therapies. It could allow one to screen for drugs or identify therapies that enable the defective receptor to fold properly," says Dr. Peter Kim of the Whitehead Institute for Biomedical Research and the Howard Hughes Medical Institute (HHMI). He and colleague Dr. Stephen Blacklow of the Whitehead Institute, Brigham and Women's Hospital and HHMI report their findings in the September issue of Nature Structural Biology.

Protein folding defects have also been implicated in other human diseases, including cystic fibrosis, a-1-antitrypsin deficiency, retinitis pigmentosa, and Marfan's syndrome. "Our studies provide yet another example of how basic science research, such as studies of protein folding, ultimately help us understand human disease," says Dr. Kim.

Normally, cholesterol is removed from the bloodstream and taken into cells by the LDL receptor on the cell surface. Scientists have known that a defective LDL receptor results in high levels of blood cholesterol. In most cases of familial hypercholesterolemia, this happens because the gene for the LDL receptor is slightly mutated.

In the Whitehead-HHMI study, researchers studied a critical fragment of the LDL receptor that frequently contains single amino acid mutations in patients with familial hypercholesterolemia. "We found that the normal copy of this crucial fragment folds into its proper shape, but introducing even single mutations in the fragment interferes with the fragment's ability to fold into this shape," says Dr. Blacklow. Combining nuclear magnetic resonance (NMR) spectroscopy and other biochemical methods, researchers also found that the normal copy of this fragment was unable to fold properly in the absence of calcium, and adding calcium restored its folding ability. Based on these data and other biochemical information, researchers speculate that the mutations in the LDL receptor affect the receptor's ability to bind calcium and therefore its ability to fold into its proper shape.

This work was funded in part by the National Heart, Lung and Blood Institute.

Background

Familial hypercholesterolemia (FH) is an autosomal dominant trait, meaning that a child born to an affected parent has a 50 percent chance of inheriting the gene. About 1 in 500 people are heterozygous for FH&emdash;they have one abnormal and one normal copy of the LDL receptor gene&emdash;making FH one of the most common inborn errors of metabolism. At birth, heterozygotes have a twofold increase in cholesterol levels and develop coronary heart disease in middle age. Homozygotes (people who have two abnormal copies of the LDL receptor gene) number about 1 in a million and have a severe form of the disease in which coronary heart disease begins during childhood and causes death by age 20.

In the 1970s, Drs. Michael Brown and Joseph Goldstein discovered that LDL is removed from the bloodstream by receptors on the cell surface and that these receptors are mutated in familial hypercholesterolemia. The new understanding of the role of these cellular receptors in regulating cholesterol levels in the bloodstream spurred the successful treatment strategy for high cholesterol using a combination of drugs and diet change to lower blood cholesterol levels. Drs. Goldstein and Brown were awarded the 1985 Nobel Prize for Medicine or Physiology for their work.