Molecular Biology of Obesity and Diabetes 
complement. These data suggest that mutations 
in the same gene can cause obesity in two differ- 
ent rodent species and raise the possibility that 
similar mutations can cause obesity in humans. 
We are endeavoring to test this possibility, in col- 
laboration with Rudolph Leibel, by using the in- 
terferon-a and complement genes to characterize 
human families with a high incidence of obesity. 
To identify other RFLPs that are more tightly 
linked than the probes mentioned above, we have 
used the technique of chromosomal microdissec- 
tion, in which small slices of individual chromo- 
somes are dissected and cloned. Separate libraries 
have been made from proximal chromosome 6, 
where ob maps, and from mid chromosome 4, 
where db maps. Two probes have been isolated 
from the chromosome 4 library that flank db and 
are about 1 cM apart. Similarly, two probes that 
flank ob and are about 0.2 cM apart were isolated 
from the chromosome 6 library. A genetic dis- 
tance of 0.2 cM corresponds to about 400,000 
base pairs of DNA. 
We are currently attempting to clone the DNA 
near these probes by using techniques such as 
pulsed-field gel electrophoresis and cloning of 
large fragments in yeast artificial chromosomes 
(YACs). YACs of 300,000 to 1 million base pairs 
have already been isolated for each of the probes 
that flank ob, and the search for the gene in these 
artificial chromosomes should begin shortly. The 
cloning of these genes should further our under- 
standing of the mechanisms that control food in- 
take and body weight. 
Polygenic Inheritance of Type II Diabetes 
The mice carrying the ob and db obesity muta- 
tions develop diabetes of adult onset that is quite 
similar to type II diabetes in humans. However, 
differences in the type II phenotype are seen in 
mutant mice, depending on the strain carrying 
the mutation. C57BL/6J ob/ob mice develop a 
mild insulin-resistant diabetes with high levels of 
plasma concentration of glucose and insulin. In 
contrast, DBA/2J mice develop a severe diabetes 
characterized by very high plasma levels of glu- 
cose and relatively low plasma insulin concentra- 
tion. These data suggest that genetic differences 
in insulin sensitivity and output from pancreatic 
/3-ceIls can influence the severity of the diabetes 
in genetically obese mice of different inbred 
strains. 
Further analyses of these data have suggested 
that the genetic differences are a result of poly- 
genic inheritance — genetic variation in several 
genes. Advances in molecular genetics now make 
it possible to dissect polygenic traits (such as dia- 
betes in mice) into single gene components. This 
requires that a collection of genetic markers, 
such as RFLPs, be available for genotyping the 
diabetic animals. To facilitate the analysis of poly- 
genic traits, we have helped Eric Lander and Wil- 
liam Dietrich (Massachusetts Institute of Technol- 
ogy) generate a linkage map of the mouse 
genome, using a new type of genetic marker 
known as the SSR (simple sequence repeat). 
These markers greatly facilitate genetic mapping 
experiments. We are now analyzing obese prog- 
eny of various genetic crosses with these SSR 
markers to identify novel genes that predispose 
ob animals to diabetes. 
Regulation, Function, and Expression 
of Cholecystokinin in Human Tumors 
The hormone cholecystokinin (CCK) was origi- 
nally found in the small intestine by virtue of its 
ability to stimulate gallbladder contraction and 
pancreatic secretion in response to feeding. High 
levels of CCK have also been found in neurons of 
the mammalian brain, where it functions as a neu- 
rotransmitter. The first demonstration that CCK 
could affect behavior was reported by Gerry Smith 
and Dick Gibbs, who showed that peripherally 
administered CCK had an appetite-suppressing 
effect on rats. It has also been demonstrated that 
CCK antagonists increase feeding behavior in ro- 
dents. These observations suggest that the regula- 
tion and function of this gene is important in the 
control of appetite. 
At present, we are using a variety of techniques 
to explore CCK's function(s). One of our first 
objectives was to determine whether any human 
tumors associated with weight loss overexpress 
this peptide, and we therefore began screening 
tumor cell lines for CCK production. Several pe- 
diatric tumors were found to synthesize the appe- 
tite depressant, including peripheral neuroepi- 
thelioma (a rare nerve tumor that usually 
develops in the chest wall) , Ewing's sarcoma of 
bone, and rhabdomyosarcoma (a malignant mus- 
cle tumor) . These data suggest that measurement 
of the CCK levels in the blood may be of diagnos- 
tic and prognostic value in the management of 
these tumors. Studies are being initiated to ascer- 
tain whether overproduction of CCK in patients 
with these tumors is associated with excessive 
weight loss. 
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