Complex Control of the Immunoglobulin 
Heavy-Chain Gene 
Thomas R. Kadesch, Ph.D. — Associate Investigator 
Dr. Kadesch is also Associate Professor of Human Genetics at the University of Pennsylvania School of 
Medicine. He received his Ph.D. degree in biochemistry from the University of California, Berkeley, where 
he studied with Michael Chamberlin. His postdoctoral research was done with Paul Berg at the Stanford 
University School of Medicine. 
THE expression of immunoglobulin genes is 
limited to only one cell type in the body, 
namely B lymphocytes. In addition to exhibiting 
this cell-type specificity, the expression of these 
genes is temporally regulated during lymphocyte 
development, initiating in the pre-B and B cell 
stages and later increasing as those cells differen- 
tiate to become mature plasma cells. Results from 
many laboratories have led to the conclusion that 
a major component of this regulation occurs at 
the level of transcription, the process of creating 
an RNA copy of the gene. Obtaining a detailed 
understanding of the transcriptional control of 
immunoglobulin genes will aid in elucidating 
the mechanisms of transcription regulation of 
other genes. Thus the study of immunoglobulin 
gene transcription should shed light on a variety 
of transcriptional control processes, including 
those that go awry and result in deleterious con- 
sequences (e.g., the transcriptional activation of 
oncogenes, leading to cancer) . 
Studies of the structure of immunoglobulin 
genes and of the DNA sequences that influence 
their rate of transcription in lymphocytes indi- 
cate that there are at least two major transcrip- 
tional regulatory signals (elements) within the 
genes. The first is the promoter, a conventional 
DNA element located close to the site of tran- 
scription initiation. The second is an enhancer, 
which stimulates activity of the promoter. Al- 
though many enhancers are found close to pro- 
moters, the enhancers within the immunoglobu- 
lin genes are located in the central portion of the 
genes, a few thousand bases from the promoter 
elements. 
Since the activities of the immunoglobulin 
promoters and enhancers are restricted to B lym- 
phocyte cells, it has been suggested that those 
cells may uniquely express transcription factors 
(proteins) that activate those control elements. 
One of our prime interests is to understand the 
mechanisms underlying the B cell-specific ex- 
pression of the enhancer located in the immuno- 
globulin heavy-chain gene. 
Our previous work and that of others has led to 
the drawing of a detailed enhancer map. This map 
provides the precise locations for the binding of 
these regulatory proteins and additional informa- 
tion as to how those proteins act. The enhancer is 
a relatively small (200 base pairs), but exceed- 
ingly complex, segment of DNA. Many of the per- 
haps nine or more distinct proteins that bind the 
enhancer are found in multiple cell types, even 
in cells where the enhancer is normally inactive. 
It is assumed that some of these proteins act to 
stimulate enhancer activity, while others may 
function to repress it. 
Recently our efforts have been directed toward 
the cloning and functional characterization of the 
genes that encode these enhancer-binding pro- 
teins. Thus far we have cloned segments (i.e., 
cDNAs) that correspond to at least six (possibly 
seven) distinct genes that encode enhancer-bind- 
ing proteins. We are using these gene segments to 
manipulate and characterize the proteins, both 
structurally and functionally. 
Two of the encoded proteins (E2-5 and TFE3) 
are involved in a fascinating transcriptional regu- 
latory scheme. In B cells the situation is relatively 
straightforward: both E2-5 and TFE3 bind the en- 
hancer and act in concert to activate transcrip- 
tion. In non-B cells, the situation is more com- 
plex. In vivo experiments suggest the presence 
of an additional repressor protein that binds the 
enhancer and precludes binding of E2-5. 
Binding of this putative repressor has two ef- 
fects. First, the enhancer is less active due to the 
absence of bound E2-5 protein. Second, the re- 
pressor has the ability to attenuate, at a distance, 
the ability of the TFE3 protein to function. Thus 
the presence of the repressor in non-B cells re- 
sults in the shutdown of both E2-5-mediated and 
TFE3-mediated activation. These effects can be 
overcome by artificially overproducing the E2-5 
protein in non-B cells. Presumably this overex- 
pression is sufficient to displace the bound 
repressor. 
Because of its key role in transcriptional regula- 
tion, we are interested in identifying and charac- 
terizing the repressor. Currently, however, we 
have only genetic evidence that it exists. Our re- 
cent isolation of a cDNA that encodes yet another 
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