Genetic Studies in Cardiovascular Disease 
Jean-Marc Lalouel, M.D., D.Sc. — Investigator 
Dr. Lalouel is also Professor of Human Genetics at the University of Utah School of Medicine. He obtained 
a medical doctorate, a master's degree in microbiology and genetics, and a doctorate of sciences in 
genetics at the University of Paris, France. He furthered his training as a postdoctoral fellow and a research 
associate with Newton Morton at the University of Hawaii and was Professor of Human Biology at the 
University of Paris before joining the faculty of the University of Utah. 
COMMON cardiovascular disorders such as 
coronary artery disease and essential hyper- 
tension exhibit a familial tendency. Such broad 
clinical classes represent complex etiological en- 
tities, where many genes and environmental de- 
terminants are likely to be involved. Multiplicity 
and heterogeneity in causation stretch the ability 
of genetic methods to the limit of their investiga- 
tive power. The consideration of biochemical pa- 
rameters can reduce this degree of complexity. 
Before the global picture can be comprehended, 
however, details of the landscape will need to be 
scrutinized first. This perspective is highlighted 
in two examples from our laboratory. Work on 
lipoprotein lipase was led by Mitsuru Emi and 
Akira Hata, while work on hypertension was led 
by Richard Lifton. 
Abnormal lipoprotein concentrations in 
plasma are commonly observed in the relatives of 
patients with early coronary disease, yielding 
various patterns of hypercholesterolemia and/or 
hypertriglyceridemia within families. Such com- 
plex phenotypes are thought to result either from 
the variable expression of a single-gene defect or 
from the independent contribution of two or 
more genes. The contribution of such genes to 
the clinical expression of hyperlipidemia is fur- 
ther blurred by the fact that hormonal influences, 
diet, and habitus exert major influences on the 
regulation of lipid metabolism. A severe form of 
familial hypercholesterolemia, which accounts 
for about 5 percent of myocardial infarction, has 
been linked to molecular defects of a lipoprotein 
cellular receptor. More than 90 percent of di- 
etary fat is hydrolyzed by lipoprotein lipase (LPL) 
in an initial step controlling its delivery to periph- 
eral tissues. The enzyme, secreted by adipocytes 
and muscle cells, acts at a distance from its site of 
synthesis, becoming anchored to the luminal sur- 
face of capillaries by an ionic interaction with 
heparan sulfate. In the presence of a specific co- 
factor, apolipoprotein C-II, the enzyme hydro- 
lyzes triglycerides of intestinal or hepatic origin 
by binding to the surface of circulating lipopro- 
teins, thereby releasing fatty acids for uptake in 
the tissues where they can be used as fuel or rees- 
terified for storage. Defective functional enzyme 
is the diagnostic feature of a rare recessive chylo- 
micronemia syndrome, familial LPL deficiency. 
This condition is characterized by massive chy- 
lomicronemia in the fasting state, episodes of ab- 
dominal pain, life-threatening acute pancreatitis, 
and eruptive xanthomas. By investigating the rela- 
tives of a homozygous subject, we showed pre- 
viously that heterozygotes for such molecular 
defects tend to express a common form of hyper- 
triglyceridemia. However, when we screened 
unrelated subjects for molecular defects of the 
LPL gene, we found that mutations of this gene 
probably account for 3-5 percent of such hyper- 
lipidemias. This situation, similar to receptor de- 
fects in familial hypercholesterolemia, again 
stress that most forms of hyperlipidemia are of 
heterogeneous origin. 
We and others have now identified a host of 
mutations in the LPL gene of subjects with LPL 
deficiency, including a large number of simple 
atnino acid substitutions. Such mutations may 
provide natural, specific probes of functional do- 
mains of the enzyme. When they are superim- 
posed to the known three-dimensional structure 
of the homologous enzyme, pancreatic lipase, it 
becomes clear that they are spread over the pro- 
tein's first folding domain, which includes the 
triad of residues directly involved in catalysis 
(see figure) . 
We reproduced four such mutations in vitro, 
expressed them in cultured cells, and analyzed 
the corresponding mutant proteins. A common 
feature of these mutations is that they aff^ected the 
assembly and stability of the two identical sub- 
units characterizing the active enzyme. Hence 
the majority of these mutations lead to loss of 
catalytic activity through rather nonspecific 
mechanisms. 
To probe other domains of the enzyme, it be- 
came clear that we could not rely solely on natu- 
rally occurring mutations. LPL binds to heparan 
sulfate, a member of a family of ubiquitous and 
abundant complex polysaccharides. Interactions 
of this nature play a role in many biological pro- 
cesses, such as lipolysis, hemostasis, cell adhe- 
sion, cell proliferation, or angiogenesis. The pro- 
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