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 and the American Academy 
of Arts and 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 and grown in bacte- 
rial plasmids, using standard recombinant DNA 
techniques. Then the regions of interest are ex- 
cised 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 in 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 we make transgenic mice to deter- 
mine 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 expression of the gene in 
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 
identifying sequences responsible for directing 
appropriate expression of globin genes in red 
blood cells, albumin in hepatocytes, and prota- 
mine I in male germ cells. 
Because regulatory elements from one gene 
can often be used to control another; the expres- 
sion of many interesting genes can be directed to 
a specific cell type and the consequences on the 
development and function of those cells can be 
assessed. For example, using the elastase en- 
hancer element, we have been able to make 
strains of mice that reproducibly develop pancre- 
atic cancer as a consequence of expressing the 
transforming gene from simian virus 40, the 
mouse myc gene, or the human Yi-ras oncogene. 
Similarly, we have developed models of liver 
cancer by directing the expression of these genes 
to hepatocytes 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 these genes mediate. By 
means of simple genetic crosses, mice carrying 
any pair of these transforming genes can be cre- 
ated. They develop tumors that appear more rap- 
idly and grow more aggressively than those in 
mice carrying a single gene, suggesting that these 
genes act cooperatively. 
Some genes that are not generally considered 
oncogenes may also predispose cells to malignant 
transformation and cancerous growth. In one ex- 
ample, we expressed the surface antigens of hepa- 
titis B virus (HBV) in the liver, using the albumin 
enhancer. In transgenic mice, expression of this 
gene resulted in the synthesis of the viral surface 
antigen and envelope protein, which aggregated 
within the secretory apparatus of the liver cells, 
causing cellular injury and death. This is accom- 
panied by liver cell regeneration. However, 
when the mice were more than a year old, they 
developed liver cancer. Because HBV infects mil- 
lions of people worldwide, and the incidence of 
liver cancer among them is very high, this result 
may indicate that chronic expression of HBV sur- 
face antigens may be a contributing factor. Simi- 
larly, expression of plasminogen activator in the 
liver using the albumin enhancer results in defec- 
tive blood clotting and liver injury. In this case, 
315 
