Research Report 2000 - MDC

Growth Control and
Gene Regulation in
the Hematopoietic
System
Achim Leutz
Red and white blood cells originate
from hematopoietic stem cells located
in the bone marrow. Stem cells give
rise to progenitors that may
differentiate into one of at least eight
hematopoietic cell types, such as
erythrocytes, neutrophils, or
macrophages. Hematopoietic
proliferation, differentiation, and
leukemogenesis are intimately linked
to a number of key transcription
factors that regulate expression of
lineage-specific genes or entire
developmental programs.
Dysregulation of the pathways that
control the expression or activity of
critical transcription factors, e.g., by
mutation or viral interference, may
cause various diseases such as
immune defects, anemia, or leukemia.
Thus, hematopoiesis provides striking
opportunities to address both
fundamental biological questions and
clinically relevant issues such as: How
are cell growth and differentiation
regulated? How are proliferation and
differentiation connected? How is cell
identity achieved during lineage
commitment? Obtaining answers to
these questions will improve our
understanding and treatment of many
diseases. We have set out to determine
how gene regulatory proteins control
cellular growth and differentiation
programs and how their dysregulation
may cause disease.
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A bipartide gene switch
Proteins of the CCAAT/Enhancer
Binding Protein family (C/EBP)
induce expression of genes which
account for myelomonocytic
commitment, differentiation, and
proliferation arrest. This became
evident when a conditional nuclear
receptor-C/EBP chimera was
expressed and activated in progenitor
cells that subsequently induced their
differentiation into eosinophils. In
collaboration with the cellular Myb
proto-oncoprotein (c-Myb), C/EBPs
even activate myeloid genes in
heterologous cell types, e.g., in
fibroblasts. Such combinatorial gene
switches permit plasticity during
growth and differentiation and limit
the number of regulators and
pathways required for cell type
specification. The concept of
concerted action of transcription
factors has now been confirmed by
many research groups and has been
extended to other hematopoietic
transcription factor interactions.
Chromatin remodeling
and lineage-specific gene
expression
A prerequisite for ectopic activation of
silent genes, such as myeloid genes
induced by Myb plus C/EBP in
fibroblasts, is to overcome the
repressive effects of chromatin. This is
accomplished by large protein
complexes that locally remodel
chromatin. An assay that we have
established to monitor activation of
endogenous, chromatin embedded genes
has helped to unravel the mechanism of
the collaboration between Myb and
C/EBPβ. It became evident that
C/EBPβ specifically interacts with the
SWI/SNF complex, and that this
interaction is required to activate a
group of myeloid genes. An aminoterminal
peptide which is contained
only in one particular isoform of
C/EBPβ (see below), is required for
SWI/SNF recruitment. Grafting the Nterminus
of C/EBPβ onto Myb
generates a chimeric transcription factor
that recruits SWI/SNF and activates
chromosomal genes, even in the
absence of C/EBPβ. This shows that
SWI/SNF recruitment is an important
feature of the Myb- C/EBPβ
collaboration. It is also the first
demonstration in vertebrates that the
SWI/SNF complex may be recruited by
transcription factors to remodel
chromatin at distinct sets of genes.
Cell growth arrest and
differentiation
In addition to inducing differentiation,
C/EBPs arrest cells in the G1 phase of
the cell cycle. To understand how
C/EBPs mediate both proliferation
arrest and differentiation, we
investigated whether oncoproteins
could interfere with distinct C/EBP
functions. Of various oncogenes
examined, only E7, from the high-risk
papillomavirus type 16 or 18 strains,
abrogated C/EBPα-induced growth
arrest. Remarkably, E7 did not
interfere with differentiation,
suggesting that the two C/EBP
functions can be separated (see
figure). Since C/EBPs are expressed
in mammary epithelium, cervical
epithelium and skin, our results imply
that elimination of C/EBP-mediated
proliferation arrest might contribute to
papilloma pathology. Furthermore, the
results suggest that C/EBPs act as
tumor suppressor proteins and,
therefore, are targets of tumorigenesis.
GBX2 is a homeobox target
gene of Myb
The product of c-Myb regulates genes
involved in stem cell self-renewal and
in progenitor differentiation. It is,
therefore, important to identify critical
Myb target genes and determine their
function. Recently, we isolated the
homeobox gene GBX2 as a target of
Myb. Ectopic expression of GBX2 in
precursor cells changes their
phenotype and growth properties
suggesting that GBX2 is involved in
hematopoiesis and the establishment
of the transformed phenotype by the
Myb onogene.
GBX2 gene expression is directly
induced by a leukemogenic version of
Myb, whereas its activation by c-Myb
depends on a co-activated receptor
tyrosine kinase or ras pathway. Thus,
leukemogenic Myb represents a gainof-function
derivative of its cellular
counterpart. Moreover, the results
suggest that a signaling cascade
regulates c-Myb function.
Activation of GBX2 by c-Myb
depends on signaling from the cell
surface. This is of particular interest
since the Drosophila melanogaster
homologue of GBX2, the unplugged
gene, is downstream of the FGF
receptor during tracheal development.
This implies that regulation of GBX2
expression is part of a conserved