Regulation of Gene Expression During Cell 
Differentiation and Activation 
Jeffrey M. Leiden, M.D., Ph.D. — Associate Investigator 
Dr. Leiden is also Associate Professor of Internal Medicine at the University of Michigan Medical School. He 
received his M.D. and Ph.D. degrees from the University of Chicago. His residency training was at Brigham 
and Women's Hospital, Boston, and his postdoctoral fellowship was in the laboratory of Jack Strominger 
at Harvard University. 
THE processes of cellular differentiation and 
activation are accompanied by complex and 
precisely orchestrated changes in gene expres- 
sion. Abnormalities in the expression patterns of 
these differentiation- and activation-specific 
genes may be involved in the etiology of a num- 
ber of pathologic states, including autoimmune 
disease and malignancy. My laboratory is studying 
gene regulation during T lymphocyte and muscle 
cell differentiation, in order to understand bet- 
ter the molecular mechanisms that regulate 
gene expression during normal and pathologic 
development. 
Regulation of Human T Cell Receptor (TCR) 
Gene Expression During T Cell Development 
Human T cells recognize foreign antigens, such 
as virus-infected cells and tumor cells, via spe- 
cific cell-surface TCR molecules. T cells can be 
divided into two subsets, based on their expres- 
sion of two distinct types of antigen receptor mol- 
ecules. The majority of circulating, peripheral 
blood T cells (including all helper and cytotoxic 
T cells) express the TCR a/^ heterodimer; a small 
but distinct T cell subset of unknown function 
expresses the 7/6 TCR. These a//? and 7/6 T cells 
appear to develop as separate lineages during 
thymic ontogeny. During the past several years, 
my laboratory has been interested in the mole- 
cular mechanisms that regulate the expression 
of these TCR genes during T lymphocyte 
development. 
In an initial set of studies, we identified the 
transcriptional enhancer elements that control 
the expression of the TCR a and |S genes. These 
two enhancers were shown to be required for the 
expression of the TCR a and /3 genes in T cells and 
to function equally well in both mouse and hu- 
man cells. Despite these similarities, they display 
a number of distinct molecular properties. For 
example, the TCR a enhancer functions only in 
a/18 T cells and is equally active on a number of 
different promoters; the TCR ^ enhancer is quite 
active in both a/^ and 7/6 T cells, displays low- 
level activity in B cells, but is inactive in nonlym- 
phoid cells. The identification and localization of 
the human TCR enhancers led us to propose that 
certain T cell tumors previously shown to contain 
chromosomal translocations into the human TCR 
a and jS loci might be caused by the apposition of 
the TCR gene enhancers with translocated 
proto-oncogenes . 
Our more recent studies have focused on iden- 
tifying and characterizing the enhancer DNA se- 
quences and the nuclear proteins bound by these 
sequences that are responsible for controlling 
TCR a and ^ gene expression. These experiments 
have demonstrated that both enhancers contain 
4-5 different binding sites for nuclear proteins. 
At least two of these sites in each are required for 
enhancer activity. Several of the nuclear protein- 
binding sites in each enhancer correspond to pre- 
viously defined enhancer motifs, while others 
represent novel sequence elements. At least one 
site in each enhancer was shown to bind T cell- 
specific nuclear proteins. Both enhancers were 
shown to contain a nuclear protein-binding site 
that was identical to the previously described 
cAMP response element (CRE) . 
To clarify the function of the CRE in regulating 
T cell gene expression, we have cloned two novel 
CRE-binding proteins, CREB-2 and CREB-3, 
which specifically bind to the TCR a CRE and to 
CREs from several other eukaryotic promoters. 
Both of these proteins contain similar basic DNA- 
binding domains and a leucine zipper region that 
allows them to form dimeric complexes both 
with themselves and, potentially, with other 
members of the CREB protein family. 
More recently we have shown that ets- 1 , a pre- 
viously described human proto-oncogene, is actu- 
ally a DNA-binding protein that specifically recog- 
nizes one of the nuclear protein-binding sites in 
the TCR a enhancer. These studies have helped to 
define the Ets proteins as transcriptional regula- 
tors that may play an important role in control- 
ling T cell gene expression. Finally, in collabora- 
tion with Stuart Orkin (HHMI, the Children's 
Hospital, Boston) we have identified a new T 
cell-specific transcription factor called hGATA-3 
that binds to a third nuclear protein-binding site 
within the TCR a enhancer. Ongoing studies are 
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