Human Disease Gene Identification and Correction 
manner. Deficiency of the protein dystrophin 
leads to mukiple abnormalities of muscle dys- 
function and death. The gene is large and its 
mRNA complex. Three mouse models of DMD are 
available for study. We have recently developed 
methods for the isolation of a full-length mouse 
dystrophin minigene that functions normally in 
cells and mice. Using special muscle-specific 
promoters controlling the dystrophin minigene, 
correction of the deficiency in the mdx mouse 
model has been achieved. 
Current research efforts on DMD feature mouse 
and human studies. Our direction in the former 
studies is to develop efficient methods for gene 
delivery to muscle cells of the adolescent mdx 
mouse. In the human studies we have embarked 
on a collaborative effort (with Helen Blau, Stan- 
ford University) to isolate DMD muscle satellite 
cells from very young patients. These cells will 
be the target for gene delivery and will be re- 
turned to the patient. 
Gene Isolation and Molecular Diagnostics: 
Fragile X Locus 
The end of the long arm of the X chromosome 
contains some 40 disease genes, including those 
for two mental retardation disorders (fragile X 
syndrome and the oculocerebrorenal syndrome 
of Lowe) and a severe muscular dystrophy 
(Emery- Dreifuss) . Yeast artificial chromosomes 
(YACs) containing 80 percent of this 40-mega- 
base region have been isolated and analyzed for 
possible involvement in these disorders. Clones 
containing the region involved in Lowe's syn- 
drome and fragile X have been obtained, and the 
identification of the genes involved is under way. 
In the case of fragile X, the YAC identified has 
allowed the development of a new marker, based 
on the polymerase chain reaction (PCR) , that im- 
proves fragile X diagnosis; narrows the region re- 
sponsible for the syndrome to about 200 kb; and 
provides the material for deciphering the nature 
of this most common form of inherited mental 
retardation. Molecular diagnosis of fragile X now 
exceeds the accuracy of cytogenetic studies and 
has been used already at Baylor in three prenatal 
cases. 
Gene Isolation and Molecular Diagnostics: 
Duchenne Muscular Dystrophy 
Simultaneous (multiplex) PCR amplification 
of multiple regions of the DMD gene is now possi- 
ble with 81 percent accuracy for deletions and 
duplications, as determined by a cooperative 
study conducted at 14 international laboratories. 
Genetic linkage analysis is developed and tested 
for a new multiplex amplification method, which 
employs short tandem repeat polymorphisms 
(STRs) at six positions in the gene. These devel- 
opments make scanning for new mutations and 
genetic linkage efficient, cost-effective, and accu- 
rate. The techniques apply to both affected males 
and female carriers. 
Gene Isolation and Molecular Diagnostics: 
Lesch-Nyhan Syndrome 
All Lesch-Nyhan syndrome diagnoses for both 
affected males and female carriers are performed 
at the molecular level by direct automated se- 
quencing of genetic defects. The mechanisms of 
mental retardation and choreoathetosis (involun- 
tary movement), associated with Lesch-Nyhan 
syndrome, are being studied with transgenic tech- 
nology. These studies will produce mice defi- 
cient in both HPRT (hypoxanthine guanine phos- 
phoribosyltransferase) and uricase, as is the case 
in the human syndrome. Humans and other homi- 
nids appear to have lost uricase through peptide 
chain termination and splice junction muta- 
tions that occurred approximately 20 million 
years ago. 
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