Atherosclerosis and the mouse: a decade of experience

Atherosclerotic disease is the leading killer in Western societies, accounting for close to 50% of deaths. Relatively little is known about the genetics of this disease in the general population despite its high prevalence. Several experimental systems exist for studying the pathology of this disease, but these larger animal models fail to offer insights into the genetics. Over the past decade the mouse has supplanted larger animal models of atherosclerosis to become the primary organism for the study of atherosclerosis genetics and, to some extent, pathophysiology. Lipoprotein biologists in particular have benefited from the ability to modify the mouse genetically to study the relationship of lipoprotein abnormalities to atherosclerosis. Given the complexity of the disease an in vivo system is requisite and since the mid-1980s the mouse has served as that system. Initial studies using classical mouse genetics first defined differing susceptibilities to atherosclerosis among distinct strains of mice. These studies acted at least in part to shed preconceptions among lipoprotein and vascular biologists that the mouse could not serve as an atherosclerosis model. Subsequent studies taking advantage of the ability to overexpress and knock genes out have advanced understanding of the in vivo function of genes involved in lipoprotein transport and the relationship between these genes (and their attendant lipoprotein disorders) and atherosclerotic disease. This review chronicles the advances made over the past 10 years and reviews the contribution that the mouse has provided lipoprotein and atherosclerosis research.

Profound induction of hepatic cholesteryl ester transfer protein transgene expression in apolipoprotein E and low density lipoprotein receptor gene knockout mice. A novel mechanism signals changes in plasma cholesterol levels

The plasma cholesteryl ester transfer protein (CETP) mediates the transfer of cholesteryl esters from HDL to other lipoproteins and is a key regulated component of reverse cholesterol transport. Dietary hypercholesterolemia results in increased hepatic CETP gene transcription and higher plasma CETP levels. To investigate the mechanisms by which the liver senses hypercholesterolemia, mice containing a natural flanking region CETP transgene (NFR-CETP transgene) were bred with apo E or LDL receptor gene knockout mice (E0 or LDLr0 mice). Compared to NFR-CETP transgenic (Tg) mice with intact apo E genes, in NFR-CETP Tg/E0 mice there was an eightfold induction of plasma CETP levels and a parallel increase in hepatic CETP mRNA levels. Other sterol-responsive genes (LDL receptor and hydroxymethyl glutaryl CoA reductase) also showed evidence of altered regulation with decreased abundance of their mRNAs in the E0 background. A similar induction of plasma CETP and hepatic CETP mRNA levels resulted from breeding the NFR-CETP transgene into the LDL receptor gene knockout background. When placed on a high cholesterol diet, there was a further increase in CETP levels in both E0 and LDLr0 backgrounds. In CETP Tg, CETP Tg/E0, and CETP Tg/LDLr0 mice on different diets, plasma CETP and CETP mRNA levels were highly correlated with plasma cholesterol levels. The results indicate that hepatic CETP gene expression is driven by a mechanism which senses changes in plasma cholesterol levels independent of apo E and LDL receptors. Hepatic sterol-sensitive genes have mechanisms to sense hypercholesterolemia that do not require classical receptor-mediated lipoprotein uptake.

A role for DNA mutations in diabetes-associated teratogenesis in transgenic embryos

Congenital malformations are the leading cause of death in infants of insulin-dependent diabetic mothers. Although there are data to suggest that hyperglycemia itself is teratogenic, few mechanisms have been proposed to explain diabetic embryopathy. To address the possibility that DNA mutations play a role in the fetal malformations associated with diabetes, we developed a transgenic mouse model system to measure the mutation frequency of a neutral target gene, lacI, during embryonic development in a maternal hyperglycemic environment. Despite the short 21-day gestational period of the mouse, we observed a twofold increase in the mutant frequency of the lacI transgene in fetuses that developed in a mild diabetic environment (blood glucose > 8.3 mmol/l) compared with those that developed under normoglycemic conditions (blood glucose < 8.3 mmol/l). These data provide the first direct evidence of the genotoxic effect of diabetes in vivo and suggest a mechanism for the teratogenicity of the maternal diabetic environment.