Gene Targeting 
Mario R. Capecchi, Ph.D. — Investigator 
Dr. Capecchi is also Professor of Human Genetics at the University of Utah School of Medicine and 
Professor of Biology at the University of Utah. He received his B.S. degree in chemistry and physics from 
Antioch College and his Ph.D. degree in biophysics from Harvard University, where he worked with James 
Watson. Dr. Capecchi remained at Harvard as a Junior Fellow of the Society of Fellows and then joined 
the Harvard faculty. Before moving to the University of Utah, he was Associate Professor of Biochemistry 
at Harvard School of Medicine. Dr. Capecchi is a member of the National Academy of Sciences. 
GENE targeting — homologous recombination 
between DNA sequences in the chromo- 
somes of mouse embyro-derived stem (ES) cells 
and newly introduced, exogenous DNA se- 
quences — has been used to create mice with null 
(ablating) mutations in members of two families 
of genes. The first set of genes are members of the 
int proto-oncogene family (int l, int-2, etc.). 
This set of genes is believed to be involved in 
localized developmental decisions mediated 
through cell-cell signaling. Their protein prod- 
ucts resemble growth factors, and int-2 is a 
member of the fibroblast growth factor family. 
These genes were initially identified through 
their pathological role in the genesis of mouse 
mammary carcinomas. We are addressing the ques- 
tion of their normal role during embryogenesis. 
Targeted disruption of int-1 (wnt-l) resulted 
in mice with a range of phenotypes from death at 
birth to survival to adulthood. Those mice surviv- 
ing exhibited severe ataxia (loss of balance and 
coordinated movement). Those int- 1~ / int- 1~ 
mice that died at or near birth showed severe ab- 
normalities in the formation of the entire cerebel- 
lum and the major portion of the midbrain, 
whereas the defect in the survivors was restricted 
to the formation of the anterior region of the cere- 
bellum. We have shown that a pre-existing mu- 
tant mouse, identified by its ataxic behavior, con- 
tains a frameshift mutation in the int-1 {wnt-1) 
gene. This allele also appears to be a null muta- 
tion and exhibits the same range of variation in 
expressivity as our targeted null allele. 
We have created mice with null mutations in 
int-2. These mice also exhibit a variable pheno- 
type from death at birth to survival to adulthood. 
The survivors are even fertile. The int-2'' /int- 2~ 
mice have defects in the formation of the vesti- 
bule and cochlea, resulting in loss of balance and 
deafness. However, the degree of malformation 
of these inner ear compartments varies. 
The second set of genes we are analyzing con- 
stitute part of the developmental program that 
specifies positional information along the antero- 
posterior axis of the early embryo. These genes, 
collectively known as the Hox genes, code for 
transcription factors in both the human and 
mouse. The 38 Hox members are present on four 
linkage groups of four separate chromosomes. 
The four linkage groups are believed to have 
arisen early in chordate evolution, as a result of 
quadruplication of a single ancestral group com- 
mon to both vertebrates and invertebrates. Ex- 
pansion of this gene complex may have had a 
critical role in the progression from invertebrates 
to vertebrates, by supplying the necessary com- 
plexity to this network of genes to accommodate 
the development of our complex body plan. 
In Drosophila these genes act as master 
switches directing the course of morphogenic de- 
velopment of each parasegment. A mutation in 
one gene can result in dramatic homeotic trans- 
formations of one body part into another, such as 
the conversion of antennae to legs. Determining 
the function of the corresponding genes in mam- 
mals is just beginning. 
We have initiated a systematic genetic analysis 
of the Hox genes in mice. First, we are creating 
mice with null mutations in each of these genes 
to define their individual functions. From such an 
analysis, patterns should emerge that define the 
zones governed by these genes. Second, through 
epistasis and molecular analysis of combinations 
of hox mutations, we hope to define how this set 
of genes functions as a network to specify posi- 
tional information along the body axis of the em- 
bryo. To date we have analyzed mice containing 
targeted disruptions in the two closely linked 
Hox genes, hox- 1.5 and hox- 1.6. 
Disruption of hox- 1.5 or -7.6 resulted in mice 
with complex but regionally restricted develop- 
mental defects. Interestingly, the sets of defects 
associated with disrupting these two genes are 
distinct and non-overlapping. The hox-1.5~/ 
hox- 1.5'^ mice are athymic, aparathyroid, and 
have reduced thyroid and submaxillary tissue. 
They also exhibit a wide spectrum of throat abnor- 
malities (including shortened necks, abnormal 
larynx, truncated soft palate, and poor organiza- 
tion of throat musculature) and defects of the 
heart and major arteries. This collection of defi- 
ciencies is remarkably similar to those afflicting 
humans with the congenital DiGeorge syndrome. 
On the other hand, hox- 1 .6~ /hox- 1 .6" mice 
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