GENETIC CONTROL OF NEMATODE DEVELOPMENT 
H. Robert Horvitz, Ph.D., Investigator 
How do genes control animal development? Tak- 
ing a primarily genetic approach to answer this ques- 
tion, members of Dr. Horvitz's laboratory have iso- 
lated developmental mutants of the nematode 
Caenorhabditis elegans and characterized them, 
using both genetic and molecular techniques. Be- 
cause the complete ultrastructure, including the 
complete wiring diagram of the nervous system, and 
the complete cell lineage of C. elegans are known, 
mutant animals can be studied at the level of single 
cells and even single synapses. Genes involved in 
cell lineage, cell signaling, cell death, cell migra- 
tion, and cell differentiation have been identified 
and analyzed. 
Cell Lineage 
The problem of cell lineage — how a single ferti- 
lized egg cell undergoes a complex pattern of cell 
divisions to generate a multiplicity of distinct cell 
types — is a major focus of this laboratory's research. 
Hundreds of cell lineage mutations have been iden- 
tified that perturb the normal pattern of cell divi- 
sions and cell fates. Molecular studies of the genes 
defined by these cell lineage mutations suggest that 
much of the cell diversity that is generated in the 
course of the C. elegans development involves cas- 
cades of transcription factors. 
For example, the genes lin-26, lin-39, and lin-1 1 
act sequentially during the cell lineages that gener- 
ate the hermaphrodite vulva. The lin-26 gene, 
which determines whether the Pn.p cells become 
neuronal or hypodermal, encodes a protein with 
two zinc fingers; lin-39, which determines whether 
hypodermal Pn.p cells become potential vulval 
cells or other hypodermal cell types, encodes a pro- 
tein with a homeodomain; and lin-1 U which deter- 
mines which vulval cell fate a potential vulval 
cell will express, encodes another homeodomain- 
containing protein. 
Cell Signaling 
Much of C. e/e^««s development involves cell sig- 
naling. The cell interactions that control the devel- 
opment of the hermaphrodite vulva have been ana- 
lyzed in some detail. Studies in this laboratory have 
defined a genetic pathway for vulval development, 
which now consists of almost 50 genes. One of these 
genes, let- 60, functions as a genetic switch during 
vulval induction. Dr. Paul Sternberg (HHMI, Califor- 
nia Institute of Technology) and his colleagues 
showed that let- 60 encodes a Ras protein. 
Based on this observation, Dr. Horvitz's laboratory 
discovered that mutations that activate let- 60 Ras in 
C. elegans are like those that cause oncogenic acti- 
vation of Ras proteins in humans. The laboratory 
also identified a regulator of the let-60 Ras gene, 
sem-5, and found that the gene encodes a novel sig- 
nal transduction protein consisting entirely of SH2 
and SH3 domains, protein regions originally ob- 
served in the mammalian src oncogene. The com- 
bined studies of Drs. Horvitz and Sternberg have re- 
vealed that the pathway of signal transduction in C. 
elegans vulval development is strikingly similar 
to that seen in mammalian cells and that many of 
the genes that function in cell signaling during 
vulval development correspond to mammalian 
oncogenes. 
Cell Death 
Naturally occurring or "programmed" cell death 
is common during the development of many ani- 
mals, including C. elegans. Dr. Horvitz and his col- 
leagues have identified and characterized geneti- 
cally and molecularly two genes, ced-3 and ced-4, 
that are necessary for the initiation of programmed 
cell death. Both of these genes act within the cells 
that die to cause their deaths. The gene ced-9 nega- 
tively regulates the activities of ced-3 and ced-4 and 
protects cells against programmed cell death. In- 
triguingly, ced-9 seems to be necessary for many, 
and perhaps all, cells that normally survive to avoid 
undergoing programmed cell death. The genes 
ced-3, -4, and -9 have been cloned and are being 
analyzed molecularly. Three other genes, ces-1, 
ces-2, and egl-1, regulate ced-9 in a cell-specific 
manner and thus determine whether specific cells 
live or undergo programmed cell death. 
Seven additional genes, ced-1, -2, -5, -6, -7, -8, 
and -10, are necessary for the process of phagocyto- 
sis that causes the corpse of a cell undergoing pro- 
grammed cell death to be engulfed by a neighboring 
cell. A number of other mutations have been identi- 
fied that cause cells to die that normally survive, 
including some that result in a spongiform morphol- 
ogy reminiscent of certain human neurodegenera- 
tive diseases. Because cell death is the cause of the 
major clinical features of human neurodegenerative 
diseases, the genes responsible for familial amyotro- 
phic lateral sclerosis (ALS; Lou Gehrig's disease) are 
being sought. Current evidence indicates that there 
are at least three such genes, one of which is located 
on chromosome 2 1 . 
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