Genetic Analysis of Growth Control 
Donna L. George, Ph.D. — Assistant Investigator 
Dr. George is also Associate Professor of Human Genetics at the University of Pennsylvania School of 
Medicine. She received her Ph.D. degree in zoology and genetics from Rutgers University. Her postdoctoral 
research was done at the Oak Ridge National Laboratory, where she characterized basic mechanisms as- 
sociated with the ultraviolet induction of mutagenesis in Escherichia coli. After completing a postdoctoral 
research program with Uta Francke at the University of California, San Diego, she was Assistant Professor 
in the Department of Medicine at the fohns Hopkins University School of Medicine, where she investigated 
chromosomal aberrations and their role in tumorigenesis. 
CELLULAR growth is an intricately regulated 
process that is tightly controlled by the coor- 
dinated interactions of many genes. Mutations 
that alter the structure or expression of a broadly 
based class of genes can result in the perturbation 
of normal cell proliferation; such uncontrolled 
cellular division is a hallmark of cancer. To un- 
derstand better the molecular events that dictate 
the complex course of tumor development and 
progression, we have focused on the identifica- 
tion of these critical growth control genes and 
characterization of the mechanisms by which 
they are regulated. 
Oncogenic Potential of a Novel 
Amplified Gene 
Studies performed in this and other laborato- 
ries have provided substantial evidence that the 
physical amplification and consequent overex- 
pression of some growth control genes are asso- 
ciated with the initiation and/or progression of a 
variety of mammalian tumor cells. A functional 
analysis of amplified DNA in tumor cells should 
allow the identification of other genes that have a 
central role in the regulation of normal cellular 
proliferation and that may contribute to cellular 
transformation. We have identified a candidate 
for such a gene. Amplified DNA sequences pres- 
ent in the tumorigenic mouse cell line 3T3-DM 
have been analyzed to determine whether the 
presence of cellular transforming activity is 
correlated with the elevated expression of any of 
the amplified genes. Our results have provided 
evidence that the mdm-2 gene, which is ampli- 
fied more than 50-fold in the 3T3-DM cell line, 
induces tumorigenicity when overexpressed in 
established, nontumorigenic cell lines. Analysis 
of the predicted amino acid composition of the 
mdm-2 product reveals features that are similar 
to those that are functionally significant in cer- 
tain DNA-binding proteins. Thus mdm-2 may par- 
ticipate in growth control pathways by affecting 
the expression of other genes. We are raising anti- 
sera directed against the product of this evolu- 
tionarily conserved gene to facilitate analysis of 
its normal cellular function. 
Expression of the c-Ki-ras Proto-oncogene 
Despite its potentially pivotal role in growth 
control pathways, relatively little is known about 
the genetic elements that mediate the expression 
of the c-Ki-ras proto-oncogene. Because this 
gene is normally expressed constitutively at low 
levels in all cell types, it is generally considered a 
member of a class of "housekeeping" or growth 
control genes. Using the c-Ki-ras gene as a proto- 
type, we have carried out experiments to identify 
mediators of expression that might be common to 
these genes. Functionally important domains 
within the transcriptional control (or promoter) 
region have been identified, and we have begun 
to examine specific DNA sequence motifs that 
may represent binding sites for particular nuclear 
regulatory factors. 
One promoter element exhibits a particular 
transcriptional influence. This element has an un- 
usual sequence composition (rich in C + T nu- 
cleotides on one strand, referred to as a pur/pyr 
domain) and binds one or more nuclear factors. 
Several lines of evidence indicate that the same or 
related DNA-binding proteins recognize similar 
pur/pyr domains that are located in the promoter 
regions of other growth control genes. Thus the 
sequence similarities of these regions may reflect 
a functional relatedness. The potential of the 
pur/pyr regions to adopt unusual structural con- 
formations may also have implications for the ex- 
pression of genes in which they are contained; 
such elements may allow for the maintenance of 
these promoters in a structure poised for 
transcription. 
We have isolated recombinant DNA clones rep- 
resenting genes encoding DNA-binding proteins 
that recognize the pur/pyr motif. The availability 
of these clones will allow the in vitro and in vivo 
studies needed to define the role that these un- 
usual sequence elements may play in mediating 
gene expression control. 
The Genesis of Meningiomas 
A new avenue of investigation is aimed at eluci- 
dating the molecular changes involved in the de- 
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