Mammalian Developmental Genetics 
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- University of California Hospital, Los Angeles, and at the University of California, 
San Francisco. His research in the laboratory of Charles Epstein focused on a molecular and genetic 
characterization of recessive lethal mutations 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 such abnormalities as miscarriages and birth 
defects. 
In organisms traditionally subject to experi- 
mental genetic analysis, such as fruit flies and 
nematodes, mutations in a particular develop- 
mental pathway can be selected in a comprehen- 
sive screening experiment. In mice, however, 
this approach 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. We are examining 
a group of previously identified genes that affect 
development around the time of implantation. In 
addition, we are developing a system to allow the 
conditional disruption of genes with recessive 
phenotypes in cell culture and in transgenic 
mice. 
Characterization of the Mouse agouti 
and kreisler Genes 
Located within a small region of mouse chro- 
mosome 2 are a group of genes required for fun- 
damental aspects of peri-implantation develop- 
ment. The mousc^agouti {A) locus, originally 
described as a coat color gene, controls the tim- 
ing and type of pigment deposition in developing 
hair follicles. Several A mutations, including A^, 
a'^", and are lethal when homozygous, and at 
least three genes required for embryonic develop- 
ment have been defined at or near the A locus. 
Closely linked to A is the segmentation gene 
kreisler (kr), which affects formation of the in- 
ner ear by interfering with the number and pat- 
tern of metameric units, the so-called rhombo- 
meres, in the developing hindbrain. 
Toward the eventual goal of isolating these 
genes, we have constructed high-resolution ge- 
netic and physical maps of mouse chromosome 2, 
using classical and somatic cell genetic ap- 
proaches. These maps are based on a variety of 
molecular markers in the area, including the in- 
sertion sites of two endogenous retroviruses, 
Emv-15 and Xmv-10, which have been closely 
associated with the A^ and a mutations, respec- 
tively. Our results have shown that neither Emv- 
15 nor Xmv-10 has caused an A mutation. We 
have also established the order and relative dis- 
tances between many closely linked molecular 
markers in the region and are beginning to lead 
toward candidate cDNAs affected by the A and kr 
mutations. 
We have further characterized the relation- 
ships among rhombomere formation, segmenta- 
tion, and the kr mutation, based on a detailed 
analysis of branchial arch derivatives in kr/kr 
adults. In collaboration with Michael Frohman 
and Gail Martin, we have studied the expression 
of rhombomere-specific genes in fer/^r embryos. 
Two of the rhombomeres affected by the kr muta- 
tion contribute to the second and third branchial 
arches, which ultimately form, among other 
structures, the hyoid bone. In kr/kr adults, the 
lower part of the hyoid, normally derived from 
the third branchial arch, exhibits some morpho- 
logic characteristics of the upper part of the 
bone, normally derived from the second bran- 
chial arch. 
In kr/kr embryos, the expression domains of 
Krox-20, Hox-2.6, Hox-2. 7, and Hox-2.9, which 
probably code for transcription factors, and Int- 
2, which codes for a growth factor and may play a 
role in morphogenesis of the inner ear, are all 
shifted rostrally toward the head. Taken together, 
these results suggest that fer affects the location of 
position-specific gene expression along the ros- 
trocaudal axis and that the kr* gene product plays 
a key role in the acquisition of positional iden- 
tity. By isolating the kr gene and further charac- 
terizing the fer/fer phenotype, we hope to clarify 
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