Human heart disease research should benefit from the development of two new strains of genetically altered mice by collaborations that included two scientists from LBL's Life Sciences Division (LSD).
LSD's Edward M. Rubin and Judy Verstuyft joined forces with groups at other institutions on separate projects to create new strains of mice for use as models in heart disease research and for testing potential therapies. One project involved the addition of a human gene into mice that increases the risk of atherosclerosis, and one involved the removal of a murine gene that helps protect mice from high cholesterol.
"In both cases, the mice became severely atherosclerotic," says Rubin. "This supports other studies indicating that genetics as well as diet plays a significant role in heart disease."
Heart disease remains the leading cause of death in the United States and the majority of these deaths are the result of atherosclerosis -- the hardening of arteries through the buildup of plaque deposited primarily by low density lipoprotein (LDL) or "bad cholesterol."
In the first project, the introduction of a human gene that codes for a protein called apolipoprotein(a), or apo(a), made mice far more susceptible to the development of fatty lesions that lead to hardening of the arteries.
Says Rubin, "Apo(a) transgenic mice developed lesions at a rate of about 20 times greater than non-transgenic litter mates when both were fed an atherogenic diet. The mechanism by which mice with high levels of apo(a) became atherosclerotic appeared to mimic the development of atherosclerosis in humans."
Apo(a) is a glycoprotein whose addition to low density lipoprotein (LDL) increases the risk of atherosclerosis in humans. It is also associated with other related health problems such as myocardial infarction, stroke, and restenosis. Because apo(a) is found only in primates, there have not been any animal models available for its study until now.
The apo(a) transgenic mice were developed in collaboration with Richard Lawn and David Wade, of the Stanford University School of Medicine, and Robert Hammer, of the University of Texas Southwestern Medical School, in Dallas. Already these mice are providing new insight into the relationship between apo(a) and atherosclerosis.
Says Rubin, "The highly significant differences in lesion areas between the transgenic and the control groups and the co- localization of apo(a) to the lesions in the transgenic animals strongly indicate that the difference in atherogenesis between these two groups is due to apo(a). However, that the apo(a) did not bind to mouse LDL suggests that it acted independently in increasing atherosclerotic susceptibility. This is a complete surprise."
Rubin and his colleagues say that further studies with transgenic animals expressing human apo(a) as well as other apolipoprotein genes will be needed. A paper on the research they have already done appeared in the December 17 issue of Nature.
In the second project, a new research technique called "gene targeting," or "knockout," was used to create mice that were missing an apolipoprotein called apoE. This protein, which is normally found in humans as well as mice, is primarily responsible for the removal of lipoproteins from the liver. In tests conducted at LBL by Rubin and Verstuyft, apoE-deficient mice developed high levels of cholesterol even while on a low-fat diet. By the time they were three months of age, these mice all had extensive atherosclerosis.
"On a low fat, low cholesterol mouse chow diet, apoE- deficient mice had plasma cholesterol levels of 494 milligrams per deciliter compared with 60 milligrams per deciliter levels in control animals," says Rubin. "When challenged with a high fat Western-type diet, the apoE-deficient mice had levels of 1821 mg/dl, compared with control levels of 132 mg/dl."
The high cholesterol levels in the apoE-deficient mice were the result of an increase in the presence of LDLs, which, according to Rubin, shows that apoE is essential for clearing lipoproteins from the blood. He sees the mice as a valuable model for the testing of possible atherosclerosis treatments. He also believes that future studies involving these and other transgenic mice that have been developed should help clarify the respective roles of genetics and diet in the development of atherosclerosis.
The apoE-deficient mice were created by a team from the Rockefeller University in New York that included Andrew Plump, Jonathon Smith, Tony Hayek, Katrina Aalto-Setala, Annmarie Walsh, and Jan Breslow. This research was reported in a recent issue of Cell.