Developmental Genetics in the Mouse and Human 
Gregory S. Barsh, M.D., Ph.D. — Assistant Investigator 
Dr. Barsh is also Assistant Professor of Pediatrics at Stanford University. He received his M.D. and Ph.D. 
degrees from the University of Washington, where he studied inherited diseases of collagen biosynthesis 
in the laboratory of Peter Byers. Dr. Barsh 's postgraduate training was in internal medicine and medical 
genetics at Harbor-UCLA Hospital and the University of California, San Francisco. His research in the 
laboratory of Charles Epstein focused on a molecular and genetic characterization of recessive lethal mu- 
tations at the mouse agouti locus. 
VERY little is known about the genetic control 
of mammalian development. But embryo- 
genesis of all mammals follows a similar plan, and 
the basic rules discovered in one species are 
likely to apply to others. By studying the mouse, a 
species in which the early embryo can be ob- 
served and manipulated, we will better under- 
stand how genes control human development 
and how disruption of these processes may lead 
to abnormalities such as miscarriages and birth 
defects. 
In organisms traditionally subject to experi- 
mental genetic analysis, like fruit flies and nema- 
todes, mutations in a particular developmental 
pathway can be selected in a comprehensive 
screening experiment. In mice, however, this ap- 
proach has been hampered by the inability to 
study and recover conditional mutations and by 
the inefficiency of generating new mutations 
through the insertion of mobile genetic ele- 
ments. As a result, much of our insight into mam- 
malian developmental genetics comes from the 
study of preexisting mutations. My laboratory is 
studying a group of previously identified genes 
that affect development around the time of im- 
plantation. In addition, we are developing a sys- 
tem to allow the conditional disruption of genes 
with recessive phenotypes in cell culture and 
transgenic mice. 
Genomic and Functional Characterization 
of the Mouse Segmentation Gene 
kreisler (fer) 
The mouse kr gene, originally recognized by 
its effects on inner ear development and craniofa- 
cial morphogenesis, interferes with the normal 
formation of metameric units in the developing 
hindbrain, so-called rhombomeres. The ;fer muta- 
tion is x-ray induced and therefore likely to repre- 
sent a structural rearrangement that alters a rela- 
tively large region of the chromosome. Located 
within two map units of ^rare two additional loci 
that affect fundamental aspects of peri-implanta- 
tion development: brachypod {bp), which pro- 
duces limb reduction abnormalities similar to 
those seen in the inherited human disease Ose- 
bold-Remondini syndrome, and agouti (A), 
which represents at least three mutations that, 
when homozygous, are lethal at or around the 
time of implantation. 
Toward the eventual goal of isolation of these 
genes, a physical map of mouse chromosome 2 
surrounding kr, A, and bp has been constructed 
with a cell fusion technique called radiation hy- 
brid mapping. This technique, developed by Da- 
vid Cox, is based on the likelihood that pieces of 
mouse DNA closely linked to each other will stay 
together, or "cosegregate," after x-irradiation 
and fusion to hamster cells. Our results have led 
to the isolation of a mouse/hamster hybrid cell 
line that contains the kr, A, and bp loci along 
with very little "extra" mouse DNA. One of the 
mouse DNA fragments present in this hybrid cell 
line, closely linked to the mouse Src gene, de- 
tects a chromosomal alteration in kr/kr mice. 
The alteration involves a chromosomal break lo- 
cated approximately 100,000 base pairs away 
from the beginning of the Src gene, but does not 
interrupt the Src coding sequences. Small DNA 
fragments recovered and mapped by this tech- 
nique are now being used to reach the fer gene by 
the serial isolation of overlapping cosmid and 
yeast artificial chromosome clones. 
To characterize further the relationship be- 
tween rhombomere formation and kr, we have 
studied the expression of a group of genes, the 
Hox-2 cluster, thought to play a role in determin- 
ing the identity of individual rhombomeres. Dif- 
ferent members of the cluster normally exhibit 
anterior borders of expression in adjacent rhom- 
bomeres. In collaboration with Michael Frohman 
and Gail Martin, we have shown that, in kr/kr 
embryos, several Hox-2 family members are ex- 
pressed in the "wrong" rhombomere (s), which 
suggests that the kr gene product may affect 
neural tube segmentation by controlling the ex- 
pression of Hox-2 genes. By isolating the /fergene 
and further characterizing the kr/kr phenotype, 
we hope to learn more about the molecular path- 
ways of mammalian segmentation and the role of 
Hox-2 genes in rhombomere morphogenesis. 
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