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 Pro- 
fessor of Biology at the University of Utah. He received his B.S. degree in chemistry and physics from An- 
tioch 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 was recently elected to the National Academy of Sciences. 
HOMOLOGOUS recombination between DNA 
sequences residing in the chromosome and 
newly introduced DNA sequences, termed gene 
targeting, provides the means for specifically 
modifying any gene in any desired manner in cul- 
tured mammalian cells. The desired alteration is 
first introduced into a chosen cloned DNA se- 
quence, using standard recombinant DNA tech- 
nology. Gene targeting is then used to transfer the 
gene modification to the cognate gene in the ge- 
nome of cultured cells. If the recipient cell for 
the gene modification is a pluripotent, mouse 
embryo-derived stem (ES) cell, then the means 
are available for creating chimeric mice from 
these cells that will transmit the altered gene to 
their progeny. With this technology the biologi- 
cal function of any cloned gene can be 
determined. 
Our laboratory is using this technology to de- 
termine the function of two sets of genes in the 
early development of the mouse. The first set of 
genes includes members of the int proto-onco- 
gene family (int-1, int-2 . . . ), which are be- 
lieved to be involved in localized developmental 
decisions through cell-cell signaling. This family 
of genes contains at least 20 members. Targeted 
disruption of the murine int-1 proto-oncogene 
resulted in mice with severe abnormalities in 
midbrain and cerebellar development, indicating 
a prominent role for the int- 1 protein in the in- 
duction, from the neural tube, of the mesenceph- 
alon and metencephalon. Some of these mice 
survive birth and exhibit severe ataxia (i.e., 
loss of balance and coordinated movement) . Sur- 
prisingly, despite missing a large portion of 
their midbrain, these mice respond fairly nor- 
mally to stimuli of light, sound, smell, and touch. 
We have also created mice with null mutations in 
int-2. Mice heterozygous for this mutation ap- 
pear normal and are fertile. We are in the process 
of analyzing the phenotype of int-2 homo- 
zygotes. 
The second set of genes that we are analyzing 
are transcription factors participating in a devel- 
opmental program for specifying positional in- 
formation along the anteroposterior axis of the 
early mouse embryo. These mammalian genes are 
homologues of the Drosophila Ultrabithorax 
and Antennapedia genes. In invertebrates these 
genes specify the identity of cells within each 
parasegment. The function of the corresponding 
genes in the human and mouse is not known. 
However, their expression pattern argues for an 
equally important role in mammalian develop- 
ment. In the human and mouse this set of genes, 
known collectively as the Hox genes, contains at 
least 30 members. We have used gene targeting to 
disrupt specifically a dozen of these genes in 
mouse ES cells. These cells are in turn being used 
to generate chimeric mice that transmit the mu- 
tant gene to their progeny. Disruption of the box- 
1.5 gene resulted in mice with multiple, but 
regionally restricted, developmental defects: 
box- 1 .5~/hox- 1 .5~ mice are athymic, aparathy- 
roid, and have reduced thyroid and submaxillary 
tissue. They also exhibit a wide spectrum of 
throat abnormalities, including shortened necks, 
abnormal larynx, truncated soft palate, and poor 
organization of muscle tissue. In addition, these 
mice often feature defects of the heart and arter- 
ies, as well as craniofacial abnormalities. This 
collection of deficiencies is remarkably similar to 
the pathology afflicting humans with DiGeorge 
syndrome, a congenital disorder. 
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