Regulation of Gene Expression During Cellular 
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 patterns of expression 
of these specific genes may be involved in the 
etiology of a number of pathologic states, includ- 
ing autoimmune disease and malignancy. My labo- 
ratory is studying gene regulation during T lym- 
phocyte and muscle cell differentiation in order 
to increase understanding of the molecular mech- 
anisms that regulate gene expression during both 
normal and pathologic development. 
Regulation of the T Cell Receptor Gene 
During T Cell Development 
Human T lymphocytes recognize foreign anti- 
gens, such as virus- infected cells and tumor cells, 
via a specific cell-surface molecule, the T cell 
receptor (TCR) . T cells can be divided into two 
subsets based on their expression of two distinct 
types of antigen receptor. The majority of T cells 
that circulate in peripheral blood, including all 
helper and cytotoxic T cells, express the a//3 
heterodimer receptor, while a small but distinct 
T cell subset of unknown function expresses the 
7/6 receptor. The a//3 and 7/6 T cells appear to 
develop as separate lineages during thymic 
ontogeny. 
During the last several years, my laboratory has 
been interested in identifying and characterizing 
the molecular mechanisms that regulate the ex- 
pression of these different TCR genes during T 
cell development. In an initial set of studies, we 
identified the transcriptional enhancer elements 
that control the expression of the TCR a and /5 
genes. These two enhancers were shown to be 
required for the expression of the a and genes 
and to function equally well in both mouse and 
human cells. The identification and localization 
of the human TCR enhancers led us to propose 
that certain T cell tumors that had previously 
been shown to contain chromosomal trans- 
locations into the human TCR a and loci 
might be caused by the apposition of the 
TCR gene enhancers with translocated human 
proto-oncogenes. 
Our more recent studies have focused on pre- 
cisely identifying and characterizing the en- 
hancer DNA sequences and the nuclear proteins 
they bind that are responsible for controlling 
TCR a and /3 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 enhancer are 
absolutely required for enhancer activity. Several 
of the nuclear protein-binding sites in each en- 
hancer correspond to previously defined en- 
hancer motifs, while others represent novel se- 
quence elements. At least one site in each 
enhancer was shown to bind T cell-specific nu- 
clear proteins. 
Both enhancers were shown to contain a bind- 
ing site that was identical to the previously de- 
scribed cAMP response element (CRE). To learn 
more about the function of this element in regu- 
lating T cell gene expression, we have cloned 
two novel CRE-binding proteins, CREB-2 and 
CREB-3, that specifically bind to the TCR a CRE 
and to CREs from several other eukaryotic pro- 
moters. Both of these new CREB proteins contain 
similar basic DNA-binding domains and a leucine 
zipper region that allows them to form dimeric 
complexes both with themselves and, poten- 
tially, with other members of the CREB protein 
family. 
During the past year, we have shown that ets-\, 
a. previously described human proto-oncogene, is 
actually a DNA-binding protein that specifically 
recognizes one of the nuclear protein-binding 
sites in the TCR a enhancer. These studies have 
helped to define the Ets proteins as transcrip- 
tional regulators that may play an important role 
in controlling T cell gene expression. More re- 
cently, we have cloned several new members of 
the Ets family of transcription factors. One of 
these, which we call Elf- 1 , regulates a set of genes 
during the process of T cell activation. In addi- 
tion, it appears to play an important role in regu- 
lating the expression of HIV-2, one of the AIDS 
viruses, in T cells. Finally, in collaboration 
with Stuart Orkin (HHMI, Children's Hospital, 
Harvard Medical School), we have identified a 
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