Function of Proto-oncogenes in Early 
Emhryogenesis 
Roeland Nusse, Ph.D. — Associate Investigator 
Dr. Nusse is also Associate Professor of Developmental Biology at the Stanford University School of Medi- 
cine. He obtained his Ph.D. degree from the University of Amsterdam and was a postdoctoral fellow with 
Harold Varmus at the University of California, San Francisco, before returning to Amsterdam, where he 
became head of the Department of Molecular Biology at the Netherlands Cancer Institute. last year he 
moved to the Beckman Center of Stanford University and the Howard Hughes Medical Institute. He is a 
member of the European Molecular Biology Organization (EMBO). 
THE major goal of our work is to elucidate how 
intercellular signals control the proper ar- 
rangement of cells and tissues during early em- 
hryogenesis. These signals can be in the form of 
polypeptides that are secreted from one cell and 
received by others through binding to specific 
receptors. A systematic approach to the identifi- 
cation of communication molecules involved in 
embryogenesis is only possible in organisms 
where saturation mutagenesis can be applied to 
screen for genes with a relevant phenotype. Both 
in Drosophila and in Caenorhabditis elegans, 
such approaches have led to the discovery of a 
series of developmental genes, whose functions 
can be analyzed by molecular cloning. In combi- 
nation with cell transplantation and cell ablation 
experiments, this method is extremely powerful 
and has led to illuminating insights. 
From another line of research, it has become 
clear that some proteins originally identified as 
growth factors in adult organisms also act as im- 
portant regulators early in embryogenesis. And 
other molecules implicated in embryonic signal- 
ing have turned out to be the products of proto- 
oncogenes: genes that normally have essential 
functions in the regulation of cell proliferation, 
but whose altered expression can lead to cancer. 
The Wnt/wingless gene family is one of the 
best examples of the link between cancerous 
growth and the control of normal development. 
The prototypic member of this group is Wnt- 1 , an 
oncogene frequently activated in mouse mam- 
mary cancer. The Wnt-l gene is normally not ex- 
pressed in mammary gland or in most other adult 
tissues, but its transcription can be induced by 
nearby insertion of proviral DNA of a retrovirus, 
the mouse mammary tumor virus. Wnt-l encodes 
a secreted protein rich in cysteine residues, and 
the gene is normally expressed only during early 
mouse embryogenesis, in particular in the clos- 
ing neural tube and the developing brain. Proof 
that Wnt- 1 is an oncogene came from transfection 
experiments and from the finding that, placed as 
a transgene in the germline of mice, it can lead to 
tumor induction. 
Recently we and others have found that Wnt- 1 
is part of a gene family comprising, in the mouse, 
at least nine members. Most of these Wnt-l -re- 
lated genes have a very restricted pattern of ex- 
pression during early developmental stages, and 
at least several members of this group can behave 
as oncogenes when activated by insertion of pro- 
viral DNA in mouse mammary tumors. The Wnt-5 
gene, for example, is more than 50 percent iden- 
tical to Wnt- 1 and is activated in a low percentage 
of mammary tumors. 
We have performed a detailed in situ RNA hy- 
bridization analysis of the expression of Wnt-5 
and a highly related gene, Wnt-^k. Both genes are 
expressed in the developing neural tube, in some 
areas that overlap the expression domain of 
Wnt-l but also in unique domains. In particular, 
the anterior boundary of expression of Wnt-i and 
Wnt-5^^ is interesting; the genes are expressed in 
the diencephalon and in the cerebral hemi- 
spheres, suggesting that they play important roles 
in establishing these compartments in the devel- 
oping brain. This could now be tested by generat- 
ing mice with mutations in these genes, using ho- 
mologous recombination in embryonic stem 
cells. 
Our present aim is to understand the mecha- 
nism of action of the Wnt- 1 gene family during 
embryogenesis and to extrapolate these findings 
to cancerous growth. We wish, for example, to 
identify the receptors for these secreted mole- 
cules. One of our model systems is the fruit fly 
Drosophila. Some years ago we made the surpris- 
ing observation that the homologue of the Dro- 
sophila Wnt- 1 gene was identical to the segment 
polarity gene wingless. Because of the extensive 
genetic analysis of Drosophila embryogenesis, in 
particular the mechanism of segmentation, this 
observation has allowed us to study the interac- 
tions of Wnt-l /ivingless with other genes. 
The basic body plan of the fruit fly is set up by 
several classes of genes that progressively divide 
the embryo into smaller compartments: the gap 
genes, the pair-rule genes, and the segment polar- 
ity genes. The gap genes and the pair-rule genes 
encode nuclear proteins and are active before the 
Drosophila embryo becomes cellularized, and 
the segment polarity genes are the first ones that 
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