Genetics and Biochemistry of Lipoprotein Lipase Deficiency 
As a consequence of these findings, LPL has be- 
come a major focus of research in our laboratory. 
We have initiated a systematic genetic and bio- 
chemical investigation of LPL in classical defi- 
ciency states and in a variety of clinical contexts 
that feature hypertriglyceridemia, including coro- 
nary heart disease, essential hypertension, dia- 
betes, acute pancreatitis, and pregnancy-induced 
hypertension of diabetes. 
The identification of LPL mutations could clar- 
ify the pathophysiologic significance of hyper- 
triglyceridemia in such conditions. Of yet greater 
interest to us, identification of naturally occur- 
ring mutations that impart a deficiency of func- 
tional enzyme will provide the basis for in-depth 
investigations of the functional domains involved 
and the structure-function relationships of LPL. 
Our search has already led to the identification 
of multiple new mutations of the LPL gene. For 
each of those leading to amino acid substitutions, 
in vitro expression experiments have confirmed 
that the mutation produces immunoreactive but 
inactive enzyme. Most amino acid substitutions 
that we or others have so far identified cluster in a 
region that appears to harbor the catalytic domain 
of the enzyme. 
This conclusion can be inferred from our re- 
cent report of the three-dimensional structure of 
a related lipolytic enzyme, pancreatic lipase. We 
predicted the identity of the three key amino 
acids defining the catalytic triad of LPL. System- 
atic amino acid substitutions were generated by 
in vitro mutagenesis at one site and expressed in 
cultured cells, in order to probe what aspect of 
the structure was altered by a naturally occurring 
variant. All substitutions led to the recovery of an 
inactive enzyme, suggesting that none could sub- 
stitute for a glycine strategically located with re- 
spect to the catalytic site. 
In further experiments in progress, we are 
probing other putative functional domains of the 
enzyme, particularly those regions that may be 
involved either with cofactor interaction or with 
heparan sulfate. 
We have extended our investigation of molecu- 
lar variation at the LPL locus by searching system- 
atically for mutations of the gene among more 
than 1 30 unrelated individuals presenting a com- 
mon form of hypertriglyceridemia in association 
with premature coronary artery disease, essential 
hypertension, hyperlipidemia, or pancreatitis in 
pregnancy. A variety of molecular variants have 
been identified on the basis of altered electropho- 
retic mobilities and are currently being charac- 
terized by DNA sequencing. We already know 
that a number of common silent mutations ac- 
count for a significant proportion of the variants 
observed. However, conclusions with respect to 
the relative contribution of LPL defects to the 
pathogenesis of hypertriglyceridemia cannot be 
made at this stage of our ongoing study. 
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