Mammalian Development and Disease 
Richard D. Palmiter, Ph.D. — Investigator 
Dr. Palmiter is also Professor of Biochemistry at the University of Washington. He received his Ph.D. degree 
from Stanford University and did postdoctoral work at Stanford, Searle Research Laboratories in England, 
and Harvard University. Prior to his current work with transgenic animals, Dr. Palmiter studied the 
mechanism of steroid hormone action in the chick oviduct and the regulation of metallothionein gene 
expression in mice. He is a member of the National Academy of Sciences. 
ABOUT 1 0 years ago we began a fruitful col- 
laboration with Ralph Brinster's laboratory 
at the University of Pennsylvania. Together we 
helped develop methods for introducing func- 
tional genes into all cells of the mouse. The genes 
under study are manipulated in bacterial plas- 
mids, using standard recombinant DNA tech- 
niques. Then the regions of interest are excised 
from the plasmid, and a few hundred copies are 
injected into the pronucleus of a fertilized mouse 
egg (or that of any other mammal) . 
Remarkably, the DNA integrates about 30 per- 
cent of the time into one of the chromosomes 
prior to replication, and the genes are inherited 
by all daughter cells, as any other gene would be. 
Furthermore, many of the genes are functional, 
imparting new genetic characteristics to the ani- 
mal. Mice and other animals carrying foreign DNA 
are referred to as transgenic. Because the new 
genes are also in the germ cells, they are usually 
transmitted to subsequent generations. 
One of our goals has been to discover what 
parts of a gene determine when, where, and how 
efficiently it will be utilized. We often start by 
testing a large piece of DNA that includes the 
gene of interest. In transgenic animals, the gene 
will usually be expressed at the appropriate time 
and place, even though it has integrated at an ab- 
normal chromosomal location and may be de- 
rived from a different mammalian species. Then 
we delete various regions of the genes and, with 
each variant, make transgenic mice to determine 
what regions are essential for appropriate 
expression. 
For example, we have delineated a small re- 
gion (125 base pairs) of the rat elastase I gene 
that is essential for the gene's expression in the 
acinar cells of the pancreas. Furthermore, this se- 
quence (often called an enhancer) can be used to 
direct the expression of another gene (e.g., the 
growth hormone gene) to the acinar cells, and 
the sequence will function when positioned al- 
most anywhere in the vicinity of the growth hor- 
mone gene. 
In similar experiments, we have been identify- 
ing sequences responsible for directing appro- 
priate expression of globin genes in red blood 
cells, albumin to hepatocytes, and protamine I to 
male germ cells. More recently, we have begun to 
locate the elements involved in directing the ex- 
pression of genes in the catecholamine biosynthe- 
sis pathway to specific neurons. 
Because the regulatory elements from one gene 
can often be used to control another, the expres- 
sion of many interesting genes can be directed to 
a particular cell type and the consequences on 
cellular development and function can be as- 
sessed. For example, using the elastase enhancer 
element, we have been able to make strains of 
mice that reproducibly develop pancreatic 
cancer as a consequence of expressing the trans- 
forming gene from SV40 virus, the mouse myc 
gene, or the human H-ras oncogene. Similarly, 
we have developed models of liver cancer by di- 
recting the expression of these genes to hepato- 
cytes with the albumin enhancer. 
Significantly, each of these genes results in a 
characteristic morphological transformation of 
the organ, which probably reflects the particular 
cellular events that the genes mediate. By means 
of simple genetic crosses, mice carrying any pair 
of these transforming genes can be created. They 
develop tumors that appear more rapidly and are 
more aggressive than those in mice carrying a 
single gene, suggesting that these genes act 
cooperatively. 
We have recently shown that the expression of 
genes not generally considered oncogenes may 
also predispose cells to malignant transformation 
and cancerous growth. In one example, we ex- 
pressed the surface antigens of hepatitis B virus 
(HBV) in the liver, using the albumin enhancer. 
HBV infects millions of people worldwide, and 
the incidence of liver cancer among them is high. 
In transgenic mice, expression of this gene re- 
sulted in synthesis of the viral surface antigen and 
envelope protein, which aggregated within the 
secretory apparatus of the liver cells, causing cel- 
lular injury and death. When the mice were more 
than a year old, they developed liver cancer. In a 
similar case, expression in liver of plasminogen 
activator, a protease, results in selection of cells 
345 
