Molecular Genetics and Gene Therapy for 
Metabolic Disorders 
Savio L. C. Woo, Ph.D. — Investigator 
Dr. Woo is also Professor in the Department of Cell Biology and Institute for Molecular Genetics at Baylor 
College of Medicine. He obtained his undergraduate degree in chemistry from Loyola College, Montreal, 
and his Ph.D. degree in biochemistry from the University of Washington, where he worked with Earl Davie. 
His postdoctoral research was done at the University of British Columbia, Vancouver, in the Division of 
Neurological Sciences of the Department of Psychiatry. 
A major focus in my laboratory has been the 
analysis of human metabolic disorders at the 
molecular level. Phenylketonuria (PKU), the dis- 
ease under investigation, is an inborn error in 
amino acid metabolism that causes severe and 
permanent mental retardation in untreated chil- 
dren. The condition is caused by defects in the 
liver enzyme phenylalanine hydroxylase (PAH) 
and is transmitted from the parents to both boys 
and girls. In the United States it affects 300-400 
new^borns a year, and 1 of every 50 individuals is 
an asymptomatic carrier of the disease trait. 
Prenatal Diagnosis 
Our laboratory has isolated the human PAH 
gene by molecular cloning and used the cloned 
gene to analyze cellular DNA of normal and af- 
fected individuals. Extensive benign variations in 
this gene can be used to trace the transmission of 
individual PAH genes from the parents to the 
children in PKU families. A fetus ^Nho has in- 
herited the same PAH genes as an affected sibling 
will have PKU. This has led to a prenatal diagnosis 
procedure for PKU in families w^ith previously 
affected children, and the procedure has been 
adopted in the United States and many western 
European countries. 
Carrier Detection 
The above procedure is applicable only to fami- 
lies with affected children. To prevent PKU from 
occurring in the first place, it is necessary to 
identify the asymptomatic carriers, which re- 
quires extensive knowledge of the gene defects. 
To determine the molecular basis of PKU among 
Caucasians, patient samples were obtained from 
various European countries and molecularly ana- 
lyzed. The defects in the 10 most common mu- 
tant PAH genes and a number of less prevalent 
ones have been characterized. 
The 1 0 prevalent mutations together represent 
about two-thirds of all PKU genes in Caucasians. 
If carrier screening is carried out today and fol- 
lowed by prenatal diagnosis in couples who are 
both carriers, it is theoretically possible to re- 
duce the incidence of PKU by ninefold. Thus the 
number of PKU patients in this country could po- 
tentially be reduced from 300-400 a year to 27- 
36 a year. It is important to continue with the 
molecular analysis: if 90 percent of PKU genes 
could be detected throughout the Caucasian pop- 
ulation, it would be possible to reduce the num- 
ber of PKU patients to only 3-4 a year. 
Population Dynamics 
A number of these PKU genes have distinctive 
patterns of distribution on the European conti- 
nent. One is very prevalent in eastern Europe, and 
the frequency decreases in a gradual fashion from 
east to west. These results suggested that the mu- 
tation occurred in eastern Europe some time ago 
and was then spread throughout the European 
continent by migration of the people in prehis- 
toric times. Similarly, another PKU gene is very 
frequent in northern Europe but less frequent in 
the neighboring countries. These results sug- 
gested that multiple PKU mutations occurred in- 
dependently in various parts of Europe and then 
spread throughout the Caucasian race. 
Somatic Gene Therapy 
The other major goal of our laboratory is to ex- 
plore the potential for somatic gene therapy of 
the genetic disorder. The PAH gene has been in- 
serted into the genome of an incapacitated virus. 
The recombinant viruses are able to infect mam- 
malian cells and incorporate themselves into the 
genome of the infected cells, but they are no 
longer able to produce new virus to continue the 
infection process. The recombinant viruses were 
used to infect cultured rodent hepatoma cells, 
thereby conferring on them an ability to synthe- 
size the corresponding human enzyme. More re- 
cently we have demonstrated that normal hepatic 
cells from mice can be cultured and efficiently 
infected by the recombinant virus. 
These results have led to experimentation with 
hepatocyte transplantation in laboratory animals. 
A variety of inert substances were used as support 
for the mouse hepatic cells prior to transplanta- 
tion. Unfortunately, the transplanted hepatocytes 
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