(Received for publication, July 28, 1995; and in revised form, January 2, 1996)
From the
The Wilms' tumor gene, WT1, encodes a zinc finger transcription factor that can repress transcription of a number of genes. WT1 mRNA undergoes alternative splicing at two locations, yielding four different mRNA species and protein products. One alternative splice alters the zinc finger region of WT1, resulting in the addition of three amino acids, Lys-Thr-Ser (KTS), between zinc fingers 3 and 4, altering the binding of WT1 to DNA. Here, we show that the WT1 protein with and without the KTS tripeptide can repress transcription from the human full-length WT1 promoter. Repression of transcription by WT1 has been shown to require two WT1 binding sites. We examined WT1 repression of the human minimal WT1 promoter, which contains two potential WT1 binding motifs. WT1 lacking the KTS tripeptide (WT1-KTS) was unable to repress transcription from a minimal WT1 promoter of 104 base pairs, whereas WT1 containing the KTS tripeptide (WT1+KTS) repressed transcription from the minimal promoter. The ability of WT1+KTS to repress transcription where WT1-KTS could not provided a functional assay to define differential WT1 binding motifs based on the presence or the absence of the KTS tripeptides. We present data defining the differential consensus DNA binding motifs for WT1-KTS and WT1+KTS. We demonstrate that WT1 zinc finger 1 plays a role in the differential DNA binding specificity of WT1-KTS and WT1+KTS.
The human Wilms' tumor gene WT1 is located at 11p13 and encodes a zinc finger transcription factor that functions as a tumor suppressor (Call et al., 1990; Gessler et al., 1990). WT1 has a proline-rich amino terminus that mediates transcriptional repression (Madden et al., 1993) and four Cys-Cys His-His type zinc fingers in the carboxyl terminus that bind DNA (Call et al., 1990; Rauscher et al., 1990). WT1 mRNA undergoes alternative splicing at two sites, resulting in four mRNA species and protein products (Haber et al., 1991; Telerman et al., 1992). The mRNA splice isoforms occur in fixed ratios that are constant in all tissues that express WT1 during development and are conserved between species (Haber et al., 1991). One alternative splice in the WT1 mRNA involves the presence or the absence of exon 5, which is 51 nucleotides long, encodes 17 amino acids (Haber et al., 1991), and has no known homology to sequences in GenBank. Exon 5 is located between the proline-rich amino terminus and the zinc fingers and has no known function.
The second site of alternative splicing is the terminal 9
nucleotides of exon 9, which encodes the tripeptide KTS ()(Haber et al., 1991). This tripeptide is inserted
between exons 9 and 10, which encode zinc fingers 3 and 4. The WT1
isoform without the KTS tripeptide (WT1-KTS) represents 23% of
the mRNA. WT1-KTS binds a DNA motif similar to the early growth
response 1 gene (EGR-1) (Rauscher et al., 1990), the closest
homologue to the zinc finger region of WT1 (Call et al.,
1990). The WT1 isoform with the KTS tripeptide (WT1+KTS)
represents the majority of WT1 transcripts and binds DNA
motifs similar to those the WT1-KTS isoform binds, but no
consensus motif has been defined (Bickmore et al., 1992;
Drummond et al., 1994). WT1-KTS represses transcription
from the promoters of the EGR-1 (Madden et al., 1991),
platelet-derived growth factor a-chain (Gashler et al., 1992),
insulin-like growth factor II (Drummond et al., 1992),
transforming growth factor
1 (Dey et al., 1994),
colony-stimulating factor 1 (Harrington et al., 1993),
retinoic acid receptor-
(Goodyer et al., 1995), PAX2
(Ryan et al., 1995) and the insulin-like growth factor 1
receptor (Werner et al., 1994). WT1+KTS represses the
insulin-like growth factor II promoter (Drummond et al.,
1994). Recently WT1-KTS and WT1+KTS were shown to
autorepress the murine WT1 promoter (Rupprecht et
al., 1994), and stable transfectants expressing WT1 show reduction
in transcription from the human WT1 promoter (Malik et
al., 1994).
Our laboratory has previously characterized the human WT1 promoter (Fraizer et al., 1994). The WT1 promoter is GC-rich and has no CAAT or TATA boxes. Transcription initiates at multiple sites across the 652-bp promoter region (Fig. 1) (Campbell et al., 1994; Fraizer et al., 1994; Hofmann et al., 1993; Phelan et al., 1994). The promoter contains 11 Sp1 binding sites that are believed to be critical in initiating transcription (Hofmann et al., 1993). In addition, the human WT1 promoter contains 10 potential WT1 motifs of the consensus sequence GNGNGGGNG (Fraizer et al., 1994).
Figure 1: Sequence of the WT1 promoter. The sequence of the 652-bp WT1 promoter region in the CAT reporter plasmid is shown. The 104-bp minimal promoter region is shaded. Transcription start sites mapped to this region are shown in bold and marked with an arrow (Campbell et al., 1994; Fraizer et al., 1994; Hofmann et al., 1993; Phelan et al. 1994). The WT1 GNGNGGGNG binding motifs are underlined twice, and the additional 2 bp of our proposed expanded WT1 binding motif GNGNGGGNGNS is underlined once.
Here we report the autoregulation of human WT1, demonstrating that both WT1-KTS and WT1+KTS can repress transcription from the human full-length WT1 promoter. We determined that WT1+KTS but not WT1-KTS could repress transcription from a minimal 104-bp WT1 promoter construct that has half the transcriptional activity of the full-length WT1 promoter. Using the repression of the minimal WT1 promoter as a functional assay of DNA binding, we found that WT1+KTS and WT1-KTS bound similar but not the same sequences in the WT1 promoter. We show that the WT1-KTS and WT1+KTS binding sites differed in the sequences recognized by zinc finger 1. Using the refined WT1 consensus DNA binding motif, we have demonstrated that WT1 represses transcription of the proto-oncogenes c-myc and bcl-2 (Hewitt et al., 1995b).
Transfection of 10 µg of WT1-KTS cDNA expression plasmid with the full-length WT1 promoter reporter resulted in 65% transcriptional repression ( Fig. 2and 3A). Transfection of WT1-KTS cDNA expression plasmid with the minimal WT1 promoter reporter construct did not repress transcription (Fig. 2). The WT1+KTS cDNA expression vector repressed transcription from both the full-length and the minimal WT1 promoter reporters an average of 47% ( Fig. 2and 3B). WT1+KTS repression of the minimal WT1 promoter suggested that WT1+KTS bound sequences not recognized by WT1-KTS.
Figure 2: CAT assay. CAT assay demonstrating the transcriptional repression of the full-length (652-bp) WT1 promoter by WT1-KTS and WT1+KTS cDNA expression plasmids. Increasing amounts of WT1 cDNA expression plasmid reduced CAT activity from the WT1 promoter-CAT reporter plasmid.
Figure 4: EMSA of the minimal WT1 promoter region. EMSA with the minimal (104-bp) WT1 promoter region (Fig. 1) as the DNA probe and the WT1-KTS and WT1+KTS zinc finger fragments as the binding proteins. Each lane contains 2.5 pM probe DNA. Lanes 1 and 5, no protein; lanes 2-4, WT1-KTS; lanes 6-8, WT1+KTS. The amount of protein in each lane is as follows: lanes 2 and 6, 250 ng; lanes 3 and 7, 500 ng; lanes 4 and 8, 750 ng.
Figure 5:
EMSAs of
WT1 with the minimal promoter WT1 binding motifs. A,
EMSA with oligonucleotides with the two GNGNGGGNG motifs present in the
minimal WT1 promoter region. Lanes 1-4, WT1-290 oligonucleotide probe; lanes 5 and 6, WT1-309 oligonucleotide probe; lanes 1 and 6, no protein; lanes 2-5, 500 ng of
WT1-KTS protein; lane 2, 15 unlabeled WT1-290 oligonucleotide; lane 3, 15
unlabeled
EGR oligonucleotide; lanes 4 and 5, no competitor
oligonucleotide. B, WT1+KTS with the WT1 promoter WT1-290 oligonucleotide probe. Lanes
1-3, WT1-290 oligonucleotide. Lane 1, no
protein; lanes 2 and 3, 500 ng of WT1+KTS
protein; lane 3, 15
unlabeled EGR oligonucleotide
competitor. C, WT1+KTS with the WT1-309
oligonucleotide probe. Lane 1, no protein; lanes 2 and 3, 500 ng of WT1+KTS protein; lane 3,
15
unlabeled EGR oligonucleotide
competitor.
WT1 can repress the transcription of a number of genes (Werner et al., 1994, Goodyer et al., 1995; Harrington et al., 1993; Dey et al., 1994; Drummond et al., 1992; Gashler et al., 1992; Madden et al., 1991; Ryan et al. 1995; Hewitt et al. 1995b). Here we demonstrate that WT1 autoregulates the human WT1 promoter, in agreement with the data demonstrating repression of the murine WT1 promoter (Rupprecht et al., 1994). In the study reported here, WT1-KTS repressed transcription of the full-length human WT1 promoter 65% and WT1+KTS repressed transcription approximately 40%. In the process of studying the autoregulation of WT1, we determined that WT1+KTS but not WT1-KTS could repress transcription from the minimal promoter region. Drummond et al.(1992) had previously demonstrated that two WT1 binding sites were required for transcriptional repression by WT1. This suggested that the minimal promoter region contains WT1 binding motifs with different specificity and provided a functional assay to determine whether WT1+KTS binds DNA sequences not recognized by WT1-KTS. We found that WT1-KTS bound one site within the 104-bp minimal promoter region, whereas WT1+KTS bound two.
We propose that there are differences in the DNA binding motifs recognized by the WT1 splice isoforms WT1-KTS and WT1+KTS. Comparison of our EMSA results with the consensus motif GNGNGGGNG does not explain the differences in WT1-KTS and WT1+KTS binding. The consensus motif GNGNGGGNG was determined by x-ray crystallography studies of Zif268 (mouse EGR-1) (Paveletich and Pabo, 1991). This motif is the DNA binding site of the three zinc fingers of Zif268, which are only 64% identical to three of the four WT1 zinc fingers. Examination of the flanking sequences of the oligonucleotides used in EMSA leads us to propose a new DNA consensus motif that discriminates between WT1-KTS and WT1+KTS binding. We propose that WT1-KTS binds the motif GNGNGGGNGNG and that WT1+KTS binds GNGNGGGNGNS, where S = G or C. The difference between these two motifs is the 3` base; WT1-KTS recognizes only G, whereas WT1+KTS recognizes C or G. This motif is present twice in the EGR competitor oligonucleotide used in the EMSA (Table 1) and makes the EGR oligonucleotide a stronger competitor in EMSA. This expanded DNA binding motif adds two nucleotides to account for binding of zinc finger 1. Zinc fingers bind DNA as triplets (Paveletich and Pabo, 1991), but the DNA specificity of zinc finger 1 appears to be determined only by the central base of the 3` triplet, resulting in an 11-bp consensus sequence. The WT1-KTS and WT1+KTS DNA binding motifs differ in the bases recognized by zinc finger 1, not zinc fingers 3 or 4. This unexpected result is probably due to alterations in protein-DNA contact introduced by the insertion of KTS; crystallographic analysis may be required to elucidate the nature of WT1-DNA binding. The spacing between fingers is highly conserved among zinc finger transcription factors; WT1 is the only one described to have alternative splicing of amino acid residues between the zinc fingers.
Analysis of the WT1 promoter
region for these new WT1 DNA binding consensus sequences showed that
there were six potential WT1-KTS binding sites and eight
potential WT1+KTS binding sites (Table 2). Review of other
previously identified WT1 binding motifs confirmed the presence of
these 11-bp expanded WT1 binding motifs in the insulin-like growth
factor II, platelet-derived growth factor a-chain, retinoic acid
receptor-, and murine WT1 promoters (Drummond et
al., 1992; Gashler, et al., 1992; Goodyer et
al., 1995; Rupprecht et al., 1994). We identified two
additional target genes, c-myc and bcl-2, which
contain this 11-bp WT1 binding motif and are repressed by WT1-KTS
and WT1+KTS (Hewitt et al., 1995b).
Our WT1-KTS DNA binding motif differs from that described by Nakagama et al.(1995). However, we used a different assay than Nakagama et al.(1995) to define the WT1-KTS and WT1+KTS binding motifs. We believe that our functional assay is more sensitive, because it is based on in vivo binding whereas Nakagama et al.(1995) used only in vitro methods. EMSA conditions play a key role in the formation of DNA-protein complexes and do not necessarily mimic in vivo conditions. Our data also differ from those of Wang et al.(1993) in that the binding of WT1-KTS to only one WT1 binding motif did not activate transcription of the WT1 promoter and WT1-KTS neither activated nor repressed transcription from the WT1 minimal promoter. This discrepancy may be due to the type of promoter being assayed. The GC-rich WT1 promoter has no TATA box to direct transcriptional initiation from a single site. It should also be noted that other investigators examining TATA-less, GC-rich promoters (Drummond et al., 1992) have also failed to find transcriptional modulation by WT1 when only one WT1 binding site was present.
It has been argued that WT1 binding sites are required both
upstream and downstream of the transcriptional start site for
transcriptional repression by WT1 (Drummond et al., 1992). Our
data do not support this hypothesis; both WT1+KTS binding sites in
the minimal promoter were upstream of the major transcriptional start
site in the minimal promoter (Fig. 1). In both the EGR1 and
transforming growth factor 1 promoters (Madden et al.,
1991; Dey et al., 1994), the only WT1 binding sites are
upstream of the transcriptional start site. These promoters are also
repressed by WT1-KTS. It appears that multiple WT1 binding motifs
are required for transcriptional repression by WT1 but that the
multiple sites can be upstream of the transcriptional start site.
It is not clear how WT1 functions as a transcriptional repressor, although experiments by Madden et al.(1993) have demonstrated that the proline-rich exon 1 can mediate repression when fused to Gal4. It appears that two WT1 proteins are required to repress transcription, possibly by interfering with recruitment and/or stabilizing the formation of the initiation complex.
We defined differential consensus DNA binding motifs for WT1 based on a functional assay that can discriminate between WT1-KTS and WT1+KTS binding. By identifying different DNA binding motifs for different splice isoforms of WT1, we have increased the number of potential targets of WT1. Use of multiple WT1 binding sites within a promoter as well as the presence of both WT1 splice isoforms may allow for fine control of transcriptional regulation. Given that WT1+KTS is the predominant splice isoform and binds to sites not recognized by WT1-KTS, identification of the target genes of WT1+KTS may reveal how WT1 regulates cell growth and differentiation.