Molecular Studies of Human Genetic Disease 
Arthur L. Beaudet, M.D. — Investigator 
Dr. Beaudet is also Professor in the Institute for Molecular Genetics and the Departments of Pediatrics and 
Cell Biology at Baylor College of Medicine. He received his B.S. degree in biology from Holy Cross College 
and his M.D. degree from Yale University. After completing his pediatric residency at the Johns Hopkins 
Hospital, he performed postdoctoral research at the NIH. 
RESEARCH in our laboratory is aimed at molec- 
ular analysis of human genetic diseases, with 
the dual goals of learning more about the diseases 
themselves and shedding light on normal human 
biology. Currently we are studying cystic fibrosis, 
deficiencies in urea cycle enzymes, spinocere- 
bellar ataxia, and defects in cell adhesion mole- 
cules. Most of this research is conducted in close 
collaboration with William O'Brien. 
Cystic fibrosis (CF) is a common genetic dis- 
ease affecting approximately 1 in 2,500 Cauca- 
sians. Most CF patients die of progressive lung 
disease during childhood or young adult life. 
About 1 in 25 Caucasians carry an abnormal CF 
gene, and about 1 in 625 couples are at high risk 
of having an affected child. Since 1985 our labo- 
ratory has used DNA analysis for carrier detection 
and prenatal diagnosis for CF. This work was 
changed dramatically in late 1989 when other 
laboratories cloned the gene for CF and identified 
the most common defect (mutation) in the gene. 
We analyzed more than 200 North American fami- 
lies for this mutation; the defect was present on 
76 percent of the CF chromosomes. This defect 
was present on only 30 percent of CF chromo- 
somes in Ashkenazic Jewish families, however, 
indicating that other mutations predominate in 
that population. Direct analysis of mutations 
greatly improves the genetic diagnostic tests that 
can be offered to families. 
One option for prevention of CF would be to 
test all couples prior to reproduction, to identify 
carriers of the abnormal gene. Although a single 
common defect is present in the majority of CF 
chromosomes, dozens of different mutations af- 
fect the remaining fraction of abnormal chromo- 
somes. Thus no one simple DNA test can be used 
to identify all gene carriers. Our laboratory and 
other laboratories participating in a large collabo- 
ration have identified many of these additional 
mutations. The laboratory has developed more 
efficient methods for detection of multiple muta- 
tions. We currently test for the five most common 
mutations causing CF in North Americans, and 
this identifies 85 percent of carriers. The ability 
to detect 90-95 percent of carriers would be 
preferable before offering carrier testing to all re- 
productive-age couples, and the laboratory is 
working to achieve this level of detection. 
To develop a treatment for CF, it is desirable to 
have an animal model for easier analysis and for 
therapeutic trials. Methods are available to de- 
velop an animal model in mice, and we have 
characterized the sequence of the mouse CF gene 
as one step in this process. The mouse gene, 
which is similar to the human gene, has been 
used to prepare DNA clones that are suitable for 
disrupting the normal mouse gene in cultured 
cells. These altered cells can be used to generate 
mice affected with CF. Despite substantial effort, 
it has proven difficult to disrupt the CF gene in 
the mouse. It would also be desirable to study the 
equivalent of the CF gene in Drosophila (fruit 
flies) or yeast, if such a gene exists. Preliminary 
work has identified some related genes in these 
organisms, but it is not clear if these genes are 
functionally equivalent to the human CF gene. 
For many years our laboratory has studied en- 
zymes of the urea cycle, with particular focus on 
argininosuccinate synthetase (AS). Genetic defi- 
ciency of AS causes citrullinemia, a disease char- 
acterized by accumulation of ammonia in the 
body, which leads to mental retardation, coma, 
and death. Patients with citrullinemia have a re- 
cessive disease, with two defective copies of the 
gene. Mutations causing citrullinemia include 
deletions of segments of the gene, numerous dif- 
ferent single-amino acid substitutions, and de- 
fects that cause abnormal splicing of the messen- 
ger RNA. Fourteen different mutations have been 
defined; almost all patients inherit a different 
mutation from each parent. The laboratory has 
also used citrullinemia as a model disease for de- 
veloping somatic gene therapy. The human gene 
for AS was introduced into retrovirus vectors in 
the form of a cDNA. The viruses produced are 
able to infect mouse and human cells, and they 
have been used to infect mouse bone marrow 
cells, which are then transplanted into lethally 
irradiated recipient mice. The human gene for AS 
is expressed for many months in a proportion of 
the transplanted animals, and this long-term ex- 
pression in mice is encouraging. 
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