Gene Pattern Expression in Early Embryogenesis 
Alexandra L.Joyner, Ph.D. — International Research Scholar 
Dr. Joyner is a Senior Scientist at the Samuel Lunenfeld Research Institute of Mount Sinai Hospital, 
Toronto, and Associate Professor of Molecular and Medical Genetics at the University of Toronto. She 
received her B.Sc. degree in zoology and her Ph.D. degree in medical biophysics from the University of 
Toronto. She did postdoctoral work in mammalian development with Gail Martin at the University of 
California, San Francisco. 
THE establishment of the basic body plan re- 
quires an intricate coordination of cell-cell 
interactions that appear to be controlled largely 
by the genetic program handed down from gener- 
ation to generation in our DNA. Many of the genes 
that run the program of pattern formation have 
been identified in Drosophila and shown by mu- 
tant analysis to regulate the development of em- 
bryonic regions rather than the differentiation of 
cell types. In keeping with this, many of these 
genes are expressed early in embryogenesis in 
spatially defined patterns. 
The primary focus of research in my laboratory 
has been to identify and study mouse homologues 
of Drosophila pattern-formation genes. This 
work has been based on the premise that a con- 
servation of gene structure through evolution re- 
flects a corresponding conservation of gene func- 
tion. Work over the last few years in many 
laboratories, including mine, has shown this to 
be the case. 
A second research project has involved devel- 
oping and applying a new type of random screen, 
called the gene trap, for genes expressed in a spa- 
tially defined manner during early mouse 
embryogenesis. 
Mouse Homologues of the Drosophila 
Gene engrailed 
The fruit fly body is divided into a number of 
repeated units referred to as segments, and the 
engrailed (en) genes are known to be required 
for proper development of the posterior half of 
each segment. The en gene has characteristics of 
a "switch" that can direct cells down a posterior, 
as opposed to anterior, developmental pathway. 
Molecular characterization of the gene's product 
has shown it to be a transcription factor that binds 
DNA through a motif called a homeodomain. We 
have been studying En 1 and En-2, the mouse 
homologues of the Drosophila en gene, to deter- 
mine whether they also act as transcription fac- 
tors controlling pattern formation. 
One striking difference between these homolo- 
gous mouse and fruit fly genes is that the fly en 
gene is expressed during embryogenesis in 14 
stripes (one for each segment) , whereas the ver- 
tebrate En genes are first expressed in a single 
band across the developing mid- and hindbrain 
junction. En l and En-2 continue to have a spa- 
tially defined expression pattern in the brain un- 
til the cells begin to differentiate into particular 
cell types. En expression then switches and be- 
comes cell-type specific. 
Many other mouse homologues of Drosophila 
pattern-formation genes have also been analyzed. 
A recurring theme is expression in spatially de- 
fined patterns early in development, particularly 
in the nervous system. This suggests that at least 
part of the mechanism for laying down the basic 
body plan, and especially for specifying different 
regions of the nervous system along the anterior- 
posterior axis, involves the coordinate expres- 
sion of different sets of these developmental 
switch genes. 
One of our objectives has been to make trans- 
genic mice that lack the En genes. To date we 
have made mice that are deleted for the En-2 
homeodomain-coding DNA sequences. These 
mice do not show a major disruption of the whole 
mid- and hindbrain region but show a distinct 
disruption in the pattern of folds in the cerebel- 
lum. We are now studying the developmental pro- 
gression of the defect to determine the cellular 
basis of the mutant phenotype. That the cerebel- 
lum, uniquely among adult brain structures, ex- 
presses En-2 in the absence of En-1 indicates a 
functional redundancy between En-1 and En-2. 
Thus both En genes must be deleted before their 
function in development of the mid- and hind- 
brain can be studied. 
A second aspect of our En research is to iden- 
tify other genes in the same genetic pathways as 
En-1 and En-2. One of our approaches has been 
to seek transcription factors that regulate En ex- 
pression. As a first step, we have analyzed the 
DNA sequences around the En-1 and En-2 genes 
and have identified fragments that will direct ex- 
pression of the /acZ reporter gene to the mid- and 
hindbrain junction. These fragments can now be 
further subdivided and used to identify the tran- 
scription factors that bind these sequences and 
regulate En expression. 
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