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. 
EXPRESSION of immunoglobulin genes is lim- 
ited to one cell type, namely B lymphocytes. 
Only B cells express the functions required for 
gene rearrangement, the process whereby mature 
immunoglobulin genes are formed from discrete 
gene segments; and only B lymphocytes possess 
the components necessary for immunoglobulin 
gene transcription, the process that creates an 
RNA copy of the rearranged genes. 
Within these genes, there are at least two major 
transcriptional regulatory elements: the pro- 
moter, a DNA domain located close to the site 
where transcription begins, and the enhancer, 
another DNA domain that stimulates initiation 
from the promoter. The activities of each of these 
elements are restricted to the B cells and are con- 
trolled by proteins that bind them. We have fo- 
cused our studies on the proteins binding the im- 
munoglobulin heavy-chain (IgH) gene enhancer, 
in an attempt to understand how they elicit the 
enhancer's B cell-specific activity. 
Contributions from a number of laboratories, 
including our own, led to a detailed map of the 
IgH enhancer. The map indicates the protein- 
binding locations and gives some information 
about the regulatory mechanism. The enhancer, 
while relatively small (200 base pairs), proves to 
be exceedingly complex. Interestingly, many of 
the perhaps nine or more distinct enhancer- 
binding proteins are found in multiple cell types, 
even those in which the enhancer is normally in- 
active. To explain this, it has been argued that 
some of the proteins may serve to stimulate the 
enhancer's activity, while others may repress it. 
During the past few years, our efforts have been 
directed toward the isolation of cDNAs that en- 
code these enhancer binders. Thus far, we have 
identified six. We are presently using segments of 
these genes (cDNAs) to manipulate and character- 
ize their encoded proteins both structurally and 
functionally. 
We have shown that two of the IgH enhancer- 
binding proteins, E2-5 and TFE3, are involved in 
a fascinating transcriptional regulatory scheme. 
In B cells the situation is relatively straightfor- 
ward, as both proteins bind the enhancer and act 
in concert to stimulate its activity. In non-B cells 
the situation is more complicated. In vivo exper- 
iments suggest the presence of an additional pro- 
tein, a repressor, that binds the enhancer and pos- 
sibly precludes E2-5 action. 
Binding of this putative repressor has two ef- 
fects. First, the enhancer is less active as a result 
of the absence of bound E2-5 protein. Second, the 
repressor can attenuate at a distance the function 
of TEE 3. Hence the presence of the repressor in 
non-B cells results in the shutdown of both 
E2-5- and TFE3-mediated activation. The effects 
of the repressor can be overcome in these cells by 
artificially overproducing the E2-5 protein. Pre- 
sumably such overexpression is sufficient to dis- 
place the bound repressor. 
A third gene segment isolated in the laboratory 
likely encodes the repressor. Although we have 
yet to prove this directly, the encoded protein, 
denoted Zeb, is structurally distinct from the 
aforementioned proteins that activate transcrip- 
tion and, moreover, falls into the family of so- 
called zinc finger proteins, many of which have 
been shown to repress transcription of other 
genes. The DNA sites to which Zeb binds in the 
test tube overlap, but are distinct from, those that 
the activator proteins bind, yet are identical to 
those that confer the repressing activity within 
cells. Ongoing experiments should further char- 
acterize Zeb's mode of action and possible roles 
in other gene systems. 
E2-5 can be specifically repressed by yet an- 
other protein, a structural relative named Id 
(identified and initially characterized by Harold 
Weintraub [HHMI, Ered Hutchinson Cancer Re- 
search Center]). Unlike Zeb, Id binds directly to 
E2-5 (and related proteins), prevents its binding 
to DNA, and thus keeps it from stimulating tran- 
scription. Id expression was observed to fall off 
in several cell lines when they were induced to 
differentiate, suggesting that it may serve as a gen- 
eral antagonist of cellular differentiation by in- 
hibiting DNA-binding proteins specifically re- 
quired for differentiation. 
The importance of E2-5 for IgH enhancer activ- 
ity led to our asking whether Id plays a role dur- 
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