GENETIC LINKAGE MAPPING IN THE SEARCH FOR DISEASE GENES 
Raymond L. White, Ph.D., Investigator 
I. Human Genetic Linkage Map. 
A. Primary maps. Efforts over the past several years 
have culminated in a nearly complete primary ge- 
netic linkage map of the human genome. In a pri- 
mary map, genetic markers are spaced in such a way 
that any gene that in mutant form causes an inher- 
ited disease can be localized to a specific segment of 
a chromosome by the cosegregation of the mutant 
allele with a polymorphism of a nearby mapped 
marker, if sufficient DNA samples are available from 
families segregating the defective allele. Primary 
maps of chromosomes are constructed from 77 ge- 
netic linkage data obtained from a panel of 60 three- 
generation reference families, most of whom have 
been ascertained in Utah. During the past year, Dr. 
White's laboratory has published primary maps of 
chromosomes 1, 9, 10, 15, 18, and 19; when other 
maps appear that are currently in press or in prepa- 
ration, this laboratory will have contributed primary 
mapping data for all but the two smallest chromo- 
somes, 21 and Y 
B. High-resolution maps. Once primary maps are 
established, the next step in linkage analysis (as 
mandated by the Genome Project) is to develop 
high -resolution maps of markers ~1 centimorgan 
apart, the limit of resolution of linkage studies. 
High-resolution maps make it possible to pinpoint 
with greater accuracy an unknown gene that has 
initially been localized to a chromosomal region by 
primary mapping and to narrow the target suffi- 
ciently that other techniques can be used to isolate 
and characterize the gene. High-resolution maps 
will also serve as the basis for ordering sets of over- 
lapping cosmids into physical maps of the chromo- 
somes. This laboratory has been concentrating on 
chromosomes 16 and 17 for these high-resolution 
mapping studies. 
This year the laboratory has automated enzyme 
digestion and gel loading. Libraries of clones from 
flow-sorted chromosome 16 (provided by the Los 
Alamos National Laboratory) are being searched, 
with the goal of identifying 100 new polymorphic 
marker systems for this chromosome. 
Thirty new DNA markers based on loci contain- 
ing a variable number of tandem repeats (VNTRs) 
have been developed for chromosome 17, by hy- 
bridization of a synthetic oligonucleotide sequence 
(GGNNGTGGG), under conditions of low strin- 
gency to cloned chromosome 17 sequences de- 
rived from somatic cell hybrids. These and an addi- 
tional 35 marker loci that show site polymorphism 
with two or more enzymes and have an average 
heterozygosity >70% are being genotyped in the 60 
reference families for eventual ordering into a high- 
resolution map of this chromosome. 
XL Disease Linkages. 
The large number of polymorphic markers devel- 
oped in this laboratory, in particular the several 
hundred highly polymorphic VNTR markers now 
available for the genomic map, make detection of 
new disease gene locations, by this and other labo- 
ratories worldwide, an ongoing and accelerating 
process. This year, genetic studies under the direc- 
tion of Dr. Mark Leppert detected linkage between 
markers on chromosome 20 and a gene responsible 
for a syndrome of benign familial neonatal convul- 
sions; Dr. Leppert's group also showed that the ge- 
netic lesion in a large kindred that shows a com- 
plex phenotype, including colon cancer in some 
individuals, is at the same locus as the gene on 
chromosome 5 that is responsible for adenomatous 
polyposis coli (APC) in other families. In addition, 
DNA markers developed here have been used for 
high-resolution mapping in the vicinity of genes re- 
sponsible for multiple endocrine neoplasia types 1 
and 2A. VNTRs have also contributed to studies in 
collaborating laboratories that have detected loss of 
heterozygosity in tumor cells as a way to pinpoint 
the molecular changes leading to cancer. 
III. Adenomatous Polyposis Coli. 
The first of two genes actively being sought 
in this laboratory is the locus on chromosome 5 
that harbors a mutation leading to familial APC. 
Dr. Yusuke Nakamura's group, after constructing 
a high-resolution linkage map of markers in the 
region of the APC gene, isolated 50 cosmid 
clones within a 5 Mb region containing the gene 
and ordered them into a physical map. This group 
is attempting to identify mutations within the re- 
gion in 40 sporadic cases of adenomatous poly- 
posis, by looking for new sizes of fragments on 
Southern blots and pulsed-field gels. They have 
also isolated cDNA clones from the region that ex- 
press normal activity in colon tissue, to identify 
genes that might be candidates for harboring the 
APC mutation. 
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