MOLECULAR BIOLOGY OF DLVBETES MELLITUS 
Graeme L Bell, Ph.D., Associate Investigator 
Dr. Bell's laboratory is studying the molecular bi- 
ology and genetics of human endocrine disorders. 
Its primary interest is non-insulin-dependent diabe- 
tes mellitus (NIDDM). 
L Molecular Biology and Genetics of Non-Insulin- 
Dependent Diabetes Mellitus. 
About 10% of adults in the United States are af- 
fected by NIDDM, a disorder of carbohydrate me- 
tabolism. NIDDM is characterized by fasting hyper- 
glycemia, which if untreated contributes to the 
development of the chronic complications of diabe- 
tes mellitus and results in early mortality. Both ge- 
netic and nongenetic factors contribute to the de- 
velopment of this disorder. In most families the 
mode of inheritance of NIDDM does not conform 
to that of either a dominant or recessive disorder, 
thereby confounding genetic studies. NIDDM is 
also likely to be heterogeneous genetically, with a 
number of different diabetogenic genes segregating 
in human populations; any one of these genes may 
increase or even decrease susceptibility to this dis- 
order. The goal of this laboratory is to identify these 
diabetogenic genes. A strategy that includes molec- 
ular biology and genetics is being used to help un- 
ravel the etiology of this disorder. 
Dr. Stefan S. Fajans (University of Michigan) has 
described several families with a slowly progressing 
form of diabetes that occurs in some children, ado- 
lescents, and young adults and has a strong familial 
association. This form of NIDDM, maturity-onset di- 
abetes of young people (MODY), is characterized by 
an autosomal-dominant mode of inheritance and is 
a good model for investigating the natural history 
of NIDDM. The largest and most thoroughly stud- 
ied of such families is the RW pedigree; this family 
offers a unique opportunity to use reverse genetics 
to identify a diabetogenic gene. The MODY form of 
NIDDM in this family is characterized by in- 
sulinopenia and impaired P-cell function. Forty-five 
restriction fragment length polymorphisms (RFLPs) 
associated with 32 different loci have been tested 
for linkage to the MODY phenotype. A molecular 
marker for this disorder has not been identified. 
Two-point analyses exclude —6% of the human ge- 
nome from consideration. Since MODY is an auto- 
somal-dominant disorder, the X and Y chromo- 
somes can also be excluded from consideration; 
thus the MODY gene can be excluded from 12% of 
the human genome. Linkage studies in this family 
are continuing. 
Genetic variation in the human insulin receptor 
gene is associated with insulin resistance and diabe- 
tes. Six missense mutations, one nonsense muta- 
tion, and one deletion have been described in the 
insulin receptor gene. These mutations provide in- 
sight into the structure and function of this protein. 
For example, identification and characterization of 
a mutation causing an Arg-»Ser substitution at resi- 
due 735 in the proreceptor-processing site indi- 
cated that proteolytic processing of the proreceptor 
is necessary for its normal full insulin-binding sensi- 
tivity and signal-transducing activity. The human in- 
sulin receptor gene and its promoter have been iso- 
lated and characterized. The gene spans > 120,000 
base pairs (bp) and has 22 exons varying in size 
from 36 bp to > 2,500 bp. The sequence of 
—13,000 bp of the human insulin receptor gene 
has now been determined; this represents —10% of 
the total gene. Using the sequences of the introns 
and flanking regions as a guide, Dr. Bell has se- 
lected primer pairs specific for each of the 22 exons 
and used the polymerase chain reaction (PGR) to 
amplify each exon specifically. The PGR and direct 
sequencing of the amplified DNA are being used to 
examine the sequences of both alleles of the insu- 
lin receptor gene of two patients who are extremely 
insulin resistant: one patient has the type A syn- 
drome of severe insulin resistance, and the other 
has lipoatrophic diabetes. 
The isolation and characterization of cDNAs/ 
genes encoding proteins that may contribute di- 
rectly or secondarily to the development of glucose 
intolerance and its metabolic sequelae have contin- 
ued, in the belief that these genes may represent 
primary susceptibility determinants or that their al- 
tered regulation in the diabetic state contributes to 
the metabolic derangements characteristic of this 
disorder. These clones also represent molecular 
probes that can be used for genetic studies as well 
as for physiological studies examining the effects of 
diabetes on cellular processes. 
The isolation and characterization of cDNAs en- 
coding human facilitative glucose transporters have 
revealed that facilitative glucose transport is not the 
property of a single protein but is a feature of a 
family of structurally related proteins that have dis- 
tinct but overlapping tissue distributions. These 
proteins have been designated glucose transporter 
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