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RAPID COMMUNICATION
Erythroid Krüppel-Like Factor Is Essential for b-Globin Gene Expression
Even in Absence of Gene Competition, But Is Not Sufficient to Induce
the Switch From g-Globin to b-Globin Gene Expression
By Louis-Georges Guy, Qi Mei, Andrew C. Perkins, Stuart H. Orkin, and Lee Wall
Different genes in the b-like globin locus are expressed at
specific times during development. This is controlled, in part,
by competition between the genes for activation by the
locus control region. In mice, gene inactivation of the erythroid Krüppel-like factor (EKLF) transcription factor results
in a lethal anemia due to a specific and substantial decrease
in expression of the fetal/adult-stage–specific b-globin gene.
In transgenic mice carrying the complete human b-globin
locus, EKLF ablation not only impairs human b-globin–gene
expression but also results in increased expression of the
human g-globin genes during the fetal/adult stages. Hence,
it may appear that EKLF is a determining factor for the
developmental switch from g-globin to b-globin transcription. However, we show here that the function of EKLF for
b-globin–gene expression is necessary even in absence of
gene competition. Moreover, EKLF is not developmental
specific and is present and functional before the switch from
g-globin to b-globin–gene expression occurs. Thus, EKLF is
not the primary factor that controls the switch. We suggest
that autonomous repression of g-globin transcription that
occurs during late fetal development is likely to be the
initiating event that induces the switch.
r 1998 by The American Society of Hematology.
I
the LCR.6,7,10 At each developmental stage, the array of proteins
that interact with the LCR and/or the globin genes favors
interaction with a particular gene. As switching takes place,
modifications in the transcription factor environment alter the
relative affinity between the LCR and the genes such that the
interaction switches from one gene to the next. Therefore,
competition between the globin genes partly dictates their
developmental pattern of expression.6,7,9 Identifying the transcription factors involved in LCR-gene interactions and/or the
switching process would be a major step toward elucidating the
mechanism regulating globin gene developmental specificity
and the LCR mode of action.
The erythroid Krüppel-like factor (EKLF) plays an important
role in transcription of the mouse fetal/adult- and human
adult-stage–specific b-globin genes. Upon binding to the CACC
box element in the promoters of these globin genes (Fig 1),
EKLF can act as a transcriptional activator.16-20 Mice in which
the EKLF gene is knocked out suffer from a severe anemia and
die at approximately 16 days postcoitus.21,22 The anemia is
N MAMMALS, EXPRESSION of globin genes is erythroid
specific and is developmentally regulated.1 The human
b-globin locus contains five active genes: e, which is transcribed in the embryonic yolk sac; Gg and Ag, which are
expressed in the fetal liver; and d (a minor contributor) and b,
which are transcribed in the bone marrow throughout adult life.
These five genes are arranged in their temporal order of
expression (58-e-Gg-Ag-d-b-38; Fig 1). There is only one
well-defined switch in globin gene expression from the mouse
b-like globin locus. Mice express ey, bh0, and bh1 globins
during embryonic life and in early fetal development. Thereafter, expression switches to the b-minor and b-major globins
genes, and these two genes continue to be transcribed into adult
life.
For high-level expression, the b-like globin genes are
dependent on a locus control region (LCR). The LCR in the
b-globin locus is signified by four DNase I hypersensitive site
regions situated upstream of the genes (Fig 1). This DNA
element has very strong erythroid-specific enhancer activity and
is characterized as an LCR by its ability to confer copy-number–
dependent, integration-site–independent, endogenous levels of
expression onto globin transgenes.2-4
In mice, transcription of a human g-globin transgene present
alone in cis with the LCR is downregulated, but only partially,
during fetal development.5-9 The human b-globin gene is
expressed at equivalent levels at all developmental stages in
transgenic mice when it is present alone with the LCR.6,7 On the
other hand, if g-globin and b-globin genes are linked together
with the LCR (LCR-g-b), both genes are appropriately regulated during development; the b-globin gene is not expressed in
the embryo and then is switched on near 11.5 days in fetal life,
whereas the g-globin gene is expressed in the embryo and then
is markedly downregulated during fetal life when the b-globin
gene is upregulated.6,7,10 These observations, in conjunction
with several other experiments,11-15 have suggested a competitive model to describe the developmental regulation of globin
genes. It has been postulated that a globin gene must interact
directly with the LCR through a looping mechanism for it to be
activated and that the LCR can only interact with one gene at a
time. Thus, the globin genes compete with each other to contact
Blood, Vol 91, No 7 (April 1), 1998: pp 2259-2263
From the Centre de recherche du Centre hospitalier de l’Université
de Montréal; the Institut du cancer de Montréal, Montreal, Quebec,
Canada; the Division of Hematology/Oncology, Children’s Hospital;
the Dana Farber Cancer Institute; Howard Hughes Medical Institute,
Boston, MA; the Department of Medicine, Université de Montréal,
Montreal, Quebec, Canada; and the Department of Physiology, Monash
University, Australia.
Submitted November 19, 1997; accepted January 5, 1998.
Supported by a grant from the Medical Research Council of Canada
to L.W., through a donation from the Fondation de l’Hôpital NotreDame, and by salary support from Défi corporatif Canderel to Q.M.
L.W. is a Scholar of the Fonds de la Recherche en Santé du Québec.
Address reprint requests to Lee Wall, PhD, Centre de recherche du
Centre hospitalier de l’Université de Montréal, Room 4605, Pavillon
Notre-Dame, 1560 Sherbrooke East, Montreal, Quebec, Canada H2L
4M1.
The publication costs of this article were defrayed in part by page
charge payment. This article must therefore be hereby marked ‘‘advertisement’’ in accordance with 18 U.S.C. section 1734 solely to indicate
this fact.
r 1998 by The American Society of Hematology.
0006-4971/98/9109-0043$3.00/0
2259
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2260
Fig 1. Structure of the human b-like globin locus and the b-globin
gene minimal promoter. The figure also shows EKLF that interacts
with the CACC motif element of the promoter.
caused by a major (approximately 20-fold) downregulation in
expression of the mouse b-major gene. Other erythroid-specific
genes, including other globin genes, are expressed at normal
levels, and erythropoiesis is apparently otherwise unaffected.
Therefore, the requirement for EKLF function appears to be
very specific to the fetal/adult-specific b-major globin gene in
mice. Similarly, in transgenic mice carrying the complete
human b-like locus, expression of the human b-globin gene is
severely hampered in an EKLF2/2 background.15,23 However, in
the absence of EKLF the human g-globin transgenes are
actually expressed at higher-than-normal levels during fetal life,
up to the time of embryonic lethality. This result indicates that
the switch in LCR interaction from the g-globin to the b-globin
gene that is expected to occur in early fetal development in mice
may not take place in the absence of EKLF.
A model to account for the above results would suggest that
EKLF is directly implicated in the interaction of the b-globin
gene with the LCR and that EKLF activity is developmental
GUY ET AL
specific. EKLF does interact with the erythroid-specific protein
GATA-1,24,25 for which there exist many DNA-binding sites in
the LCR.26-29 Thus, there is a potential for EKLF to be involved
in LCR-gene interactions. Whether or not the function of EKLF
is developmentally regulated has not been tested directly,
although it is known that the mRNA is present at early
stages.20,30 On the other hand, the fact that the b-globin gene is
expressed at all stages of development when it is alone with the
LCR (see above) questions the requirement for a developmentalspecific modifier acting on the b-globin gene and involved in
the switching process. EKLF function might, therefore, only be
necessary for competition against other globin genes. To
address these questions, we have looked at EKLF function in
the absence of gene competition and during development.
MATERIALS AND METHODS
Transgenic mice. EKLF1/2 mice and µD14 mice, which have been
described previously,22,31 were maintained on a C57B16 3 129 and
C57B16 3 CBA background, respectively. Fetuses were dissected out
at the indicated times following the observation of the spermatic plug.
DNA analyses. Fetal DNA was digested with HindIII, separated on
agarose gels, and blotted onto nylon membranes. The membranes were
hybridized to an Nco I-Sma I 550-bp fragment from the 58 region of
EKLF cDNA that distinguishes between the wild-type and the targeted
EKLF allele. The membranes were rehybridized with a 750-bp HindIII
fragment from HSS2 of the LCR that detects this same 750-bp fragment
from the µD transgene.
RNA analyses. RNA was isolated using Trizol reagent (GIBCOBRL, Gaithersburg, MD) as described by the manufacturer. RNase
protection assays were conducted as described32 with antisense probes
that span the 58 region of the mRNA.
Fig 2. Genotype determination. Fetus DNA digested with
HindIII was analyzed by Southern blot with an EKLF probe that
distinguishes between the knockout allele (10-kb fragment detected)
and the wild-type allele (6-kb fragment detected). The same blot was
reprobed with human LCR sequences that hybridize to a 0.75-kb
HindIII fragment from the transgene. The EKLF genotype and the
presence (tg) or absence (nt) of the
transgene is indicated at the top of
each lane.
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ROLE OF EKLF IN b-GLOBIN GENE SWITCHING
2261
RESULTS
The b-globin gene requires EKLF for expression even when it
is juxtaposed to the LCR and no gene competition is present.
In the complete b-like loci, the LCR is about 60 kb away from
the b-globin gene (Fig 1). There are several genes that compete
with the b-globin gene for interaction with the LCR, and this
competition plays a role in the developmental regulation of
these genes.6,7,10 Here, we aimed to test the function of EKLF in
b-globin–gene expression in the absence of gene competition.
We studied a transgenic mouse line carrying the human
microlocus construct33,34 in which the four hypersensitive site
regions of the LCR are directly linked in cis to the complete
human b-globin gene. In this construct, the LCR is within 4 kb
of the TATA box of the b-globin–gene promoter, as measured
from the middle of DNase hypersensitive sites 2 and 3 of the
LCR. The expression of this transgene is erythroid specific and
Fig 4. EKLF is also required for b-globin–transgene expression at
the embryonic stage. RNA isolated from 10.5-day yolk sac of embryos
with the various EKLF genotypes and transgenicity for the mD14
construct indicated at the top of the lanes was analyzed by RNase
protection with a human b-globin–transgene probe and a mouse
ey-globin probe. The position of the protected fragments detected by
each probe is indicated by the arrows to the right of the figure.
Fig 3. EKLF is required for b-globin–transgene expression in the
fetal-adult stage. Fourteen-and-a-half–day fetal liver RNA was isolated from mD14 transgenic (tg) animals or nontransgenic (nt) litter
mates with the EKLF genotype indicated at the top of each lane. The
RNA was analyzed by RNase protection using a human b-globin and a
mouse b-major probe. The RNA was also simultaneously analyzed
with a GATA-1 probe as a loading control. The position of the
protected fragments are indicated to the right of the figure.
position independent.33 We used the µD14 transgenic line,
which carries a single copy of the transgene and expresses
human b-globin at 43% the level of the endogenous mouse
b-major gene.31 µD14 transgenic mice were bred to mice
heterozygous for the EKLF inactivated allele. F1 animals that
were transgenic and EKLF1/2 were bred again to EKLF
heterozygous mice to obtain µD14 fetuses null for the EKLF
gene. The genotype of the animals was determined by Southern
blot using a probe that distinguishes the EKLF wild-type allele
from the knockout allele and a probe that is specific for the
human LCR region of the transgene (Fig 2). Human b-globin–
transgene expression was measured relative to mouse b-major
mRNA via RNase protection (Fig 3). Comparison of human
b-globin mRNA levels in fetal livers from EKLF knockout,
heterozygous, and wild-type animals showed that expression of
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2262
GUY ET AL
the transgene is as dependent on EKLF as the endogenous
mouse b-major gene. Both genes gave 15- to 20-fold lower
expression in the absence of EKLF. In comparison, GATA-1
expression was not affected by EKLF inactivation (Fig 3). Thus,
although the b-globin promoter is juxtaposed to the LCR and no
gene competition exists in the µD transgene, b-globin expression is still highly dependent on EKLF.
EKLF function is not developmental specific. When the
human b-globin gene is alone in cis with the LCR, it is
expressed at all developmental stages.7,10,35 Thus, we could
determine whether the EKLF protein is active at early embryonic stages by measuring the expression of the µD transgene in
the presence and absence of EKLF. RNA from yolk sacs (the
site of primitive erythropoiesis) was isolated from EKLF
wild-type, heterozygous, and null 10.5-day embryos. Human
b-globin–transgene expression was again measured by RNase
protection, but the embryonic stage-specific mouse ey-globin
mRNA served as the control in this case. Expression of the
transgene was strongly dependent on EKLF. Very little human
b-globin expression could be detected in EKLF2/2 embryos,
but the transgene was expressed at approximately 30% the level
of the mouse ey gene in wild-type and heterozygous embryos
(Fig 4, compare lane 3 with lanes 1 and 2). Results identical to
those shown in Fig 4 were obtained for a second set of embryos
in which two additional EKLF2/2 animals carried the transgene.
Thus, the dependence of b-globin gene expression on EKLF,
and hence EKLF function, is not developmental specific.
DISCUSSION
It had been suggested previously that EKLF function may be
developmental specific, partly because it is absent from K562
cells that express human embryonic and fetal globin genes, but
not the adult b-globin gene.20 If this were the case, EKLF might
serve as the predominant switch signal in the regulation of
globin genes. Previously, it had been shown that EKLF mRNA
and DNA-binding activity are indeed present in embryonic
erythroid tissues.20,22,30 However, it had not been excluded that
EKLF activity is controlled in a developmental-specific manner.
For example, EKLF function might depend on protein modifications and/or protein partners that are developmentally regulated.
However, as reported here, we have found that the b-globin
transgene in the µD construct is as dependent on EKLF for
expression in embryonic erythroid tissues as it is in fetal liver
(Fig 4). Thus, the function of EKLF is not developmental
specific. We conclude that EKLF cannot be responsible for the
initiation of globin gene switching, because it is present and
fully active before this process occurs. We cannot rule out that a
protein other than EKLF is involved in strengthening LCR—bglobin–gene interactions in a developmental-regulated fashion.
However, expression of the b-globin gene at all developmental
stages when present alone with the LCR6,7 suggests this may not
be the case.
Studies with several different transgene constructs have
shown that the g-globin gene is downregulated in later development, at least partly, irrespective of the presence of the b-globin
gene.6-9 We predict, therefore, that it may be this autonomous
downregulation of the g-globin genes that initiates the switch
from g-globin– to b-globin–gene expression. We suggest that
the b-globin gene is fully primed to interact with the LCR well
before it is actually expressed. However, the interaction between the b-globin gene and LCR cannot occur until the
g-globin genes begin to be downregulated and their interaction
with the LCR is weakened. Once this occurs, the LCR can
switch from the g-globin gene to the b-globin gene. By
competing for LCR interactions in this manner, the b-globin
gene may in turn help to downregulate the g-globin genes more
rapidly. In this sense g-globin downregulation appears to occur
more slowly in both heterozygous and homozygous EKLF
knockout mice than in wild-type animals, which results in
increased g-globin expression in fetal life.15 This suggests that
EKLF is involved in helping the LCR change from the g-globin
to the b-globin gene, presumably by stabilizing LCR—b-globin–
gene interactions.15 However, our results show that active
EKLF in itself is not sufficient to induce the process. We
therefore believe it is a combination of EKLF and the initiation
of the shutdown of the g-globin genes that is needed for the
switch from g-globin– to b-globin–gene expression to occur at
the proper time and rate during development.
ACKNOWLEDGMENT
We are grateful to R. Kothary for critical reading of the manuscript.
We thank Dr Tim Townes and Dr Jim Bieker for providing an EKLF
clone and Dr James Ellis for providing the µD14 transgenic mice.
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1998 91: 2259-2263
Erythroid Krüppel-Like Factor Is Essential for β-Globin Gene Expression
Even in Absence of Gene Competition, But Is Not Sufficient to Induce the
Switch From γ-Globin to β-Globin Gene Expression
Louis-Georges Guy, Qi Mei, Andrew C. Perkins, Stuart H. Orkin and Lee Wall
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