Human Molecular Genetics and Comparative Gene Mapping 
only rarely in LD patients, which indicates molec- 
ular genetic heterogeneity of this disorder, and 
point mutations are yet to be identified. 
Our laboratory is engaged in a collaborative 
study of a population in the mountains of south- 
em Ecuador that is derived from early Spanish 
settlers and suffers from a high incidence of LD. 
E>etailed clinical and biochemical studies of af- 
fected individuals by Arlan L. Rosenbloom (Uni- 
versity of Florida, Gainesville) and his colleagues 
have revealed the classical features of LD, in addi- 
tion to other manifestations not previously re- 
ported. We have established lymphoblastoid cell 
lines from several affected individuals and family 
members from different geographic locations. 
Because no gross rearrangements of the GHR 
gene have been identified, we have designed 
primers to use in the PGR (polymerase chain reac- 
tion) amplification of each of the 1 0 exons of this 
gene based on the published sequence. Single 
nucleotide changes in the amplified fragments 
are detected by denaturing gradient gel electro- 
phoresis and confirmed by sequencing. A poly- 
morphism in a GHR exon that cosegregates with 
the disease allele in this population has been de- 
tected and is currently used to develop a PGR- 
based allele-specific oligonucleotide hybridiza- 
tion test for the eventual detection of gene 
carriers. Our search for the molecular defect that 
renders the GHR gene nonfunctional is continu- 
ing. We are especially interested in learning 
whether a single mutation is responsible or 
whether more than one mutation contributes to 
the high incidence of LD in this population. 
Another form of inherited dwarfism, autosomal 
recessive hypopituitarism, is due to a combined 
defect in three hormones produced by distinct 
cell types in the anterior pituitary gland: growth 
hormone, thyroid-stimulating hormone, and fol- 
licle-stimulating hormone. Besides failing to 
grow, these individuals fail to mature sexually 
and may suffer from hypothyroidism. We have 
begun a collaborative study of an inbred Hispanic 
family with several affected members. Our ap- 
proach to unraveling the molecular defect is 
based on the human-mouse comparative map and 
on candidate genes. Two mutations in the mouse 
are phenotypically similar to this disorder: Ames 
dwarf {df) on mouse chromosome 1 1 and Snell 
dwarf {dw) on chromosome 16. By studying the 
inheritance of genetic markers on the human 
chromosome regions that are homologous to the 
relevant regions of mouse chromosomes 1 1 and 
16, we hope to establish whether df or dw is a 
model for this disorder. 
The candidate gene approach suggested that 
Pit- 1 , a pituitary transcription factor, may be a 
likely candidate. Indeed, recent studies from Mi- 
chael Rosenfeld's laboratory (HHMI, University 
of Galifornia, San Diego) have documented a de- 
fect in the Pit-1 gene in two different mutant al- 
leles at the dw locus. We have ruled out a struc- 
tural rearrangement of the Pit-1 gene in our 
hypopituitarism family and have mapped the Pit- 
1 locus to a region of human chromosome 3. If 
the human disease gene cosegregates with 
markers in this region, we will analyze the Pit-1 
gene for mutations. Alternatively, if the mutation 
is homologous to df, it should be linked to 
markers on either human chromosome 1 7 or 5q, 
and the responsible gene will be identified after 
it has been localized. This project exemplifies 
the goals of this laboratory — the generation of 
gene mapping information that is then used to 
identify and test candidate genes for human in- 
herited disorders or mouse mutation models. 
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