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Molecular and Cellular Biology, January 2003, p. 62-69, Vol. 23, No. 1
0270-7306/03/$08.00+0 DOI: 10.1128/MCB.23.1.62-69.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.
Division of Stem Cell Regulation,1 Laboratory of Gene Expression and Regulation, The Institute of Medical Science, The University of Tokyo, Tokyo 108-8639,3 Department of Life Sciences, The University of Tokyo, Tokyo 153-8902, Japan2
Received 5 June 2002/ Returned for modification 19 July 2002/ Accepted 1 October 2002
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Humans and mice have at least three sal-related genes, respectively (SALL1, -2, and -3 for humans and Sall1, -2, and -3 for mice) (2, 6-8, 15). SALL1 is located on chromosome 16q12.1, and heterozygous mutations of SALL1 lead to Townes-Brocks syndrome, an autosomal-dominant disease with features of dysplastic ears, preaxial polydactyly, imperforate anus and, less commonly, kidney and heart anomalies (9). Mice deficient in Sall1 die in the perinatal period, and kidney agenesis or severe dysgenesis are present (14). Sall1 is expressed in the metanephric mesenchyme surrounding ureteric bud, and homozygous deletion of Sall1 results in an incomplete ureteric bud outgrowth and failure of tubule formation in the mesenchyme. Therefore, Sall1 is essential for ureteric bud invasion, the initial key step for metanephros development.
Another Sall family gene, SALL2 is located on human chromosome 14q12, possibly overlapping a region of loss of heterozygosity in ovarian cancers (1). Mouse Sall2 binds to polyomavirus large T antigen and is proposed to be a potential tumor suppressor (11). Although mouse Sall2 was reported to be expressed during development and abundantly in the adult brain (6), precise expression patterns and the physiological function of Sall2 have remained unknown. We now report generation of Sall2-deficient mice, and in these animals we found that Sall2 is dispensable for normal developmental processes. We also present phenotypes of mice lacking both Sall1 and Sall2.
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Generation of Sall2-deficient mice. The targeting vector was constructed by incorporating the 5' BamHI-EcoRI 6.0-kb fragment and the 3' SmaI-BamHI 1.7-kb fragment into a vector that contained the neomycin-resistant (Neor) gene (pMC1-NeopolyA) and a diphtheria toxin A subunit (pMC1-DTA) in tandem. The 5' fragment was subcloned into a NotI-XhoI site 5' of the Neor gene, and the 3' fragment was cloned into an EcoRV site 3' of the Neor gene. The construct was linearized with NotI.
E14.1 embryonic stem cells were plated on mitomycin C-treated primary embryonic fibroblasts and clones resistant to G418 (400 µg/ml) were screened by using Southern blots. The genomic DNA from clones was digested with EcoRI, electrophoresed through 0.7% agarose, transferred to nylon membrane (HybondN+; Amersham-Pharmacia), and hybridized to a radioactive probe. The probe used to screen the samples was a BamHI-BamHI 0.6-kb fragment downstream of the 3' homology (probe B). The samples were also digested with SpeI and XhoI and then hybridized with the 5' probe (probe A) to confirm the correct homologous recombination. A probe corresponding to the Neor sequence was also used to verify that only one copy of the vector was integrated into the genome. Of 120 clones, 6 were correctly targeted.
Recipient blastocysts were from C57BL/6J mice. Chimeric animals were bred with C57BL/6J females. Mutant animals studied were of F2 and F3 generations. Mice were genotyped by using Southern blots or genomic PCR. The primer sequences used for PCR were as follows: CACATTTCGTGGGTCACAAG, CTCAGAGCTGTTTTCCTGGG , and GCGTTGGCTACCCGTGATAT (188 bp for the wild-type Sall2 allele and 380 bp for the mutated allele). To screen Sall1 mutants, we used AGCTAAAGCTGCCAGAGTGC, CAACTTGCGATTGCCATAAA, and GCGTTGGCTACCCGTGATAT (288 bp for the wild-type Sall1 allele,and 350 bp for the mutated allele).
The probes used for Northern blots were as follows: the Sall1-XhoI Sall1 fragment (2.5 kb), the EcoRI-SmaI N-terminal Sall2 fragment (2.0 kb), the SmaI-HindIII C-terminal Sall2 fragment (1.4 kb), and the N-terminal Sall3 fragment (2.0 kb).
Histological examination and in situ hybridization. Samples were fixed in 10% formalin and processed for paraffin-embedded sections (6-µm thick), followed by double staining with hematoxylin and eosin.
In situ hybridization was done with digoxigenin-labeled antisense riboprobes as described previously (14). A 1-kb fragment of Sall2 cDNA corresponding C-terminal three zinc fingers was amplified by using PCR, subcloned into pCRII (Invitrogen), and sequenced. Antisense transcript was generated with SP6 polymerase. Other probes were as described previously (14). None of the sense probes yielded signals.
Peripheral blood count and renal parameter measurement.
Cardiac puncture was done in 12-week-old mice, and samples were processed with Celltac
(Nihon Koden, Japan) for peripheral blood counts.
Next, 10- to 14-month-old mice were used to measure blood urea nitrogen and creatinine in serum with an automatic analyzer 7150 (Hitachi, Tokyo, Japan). Urinary protein was measured by using Pretest (Wako, Tokyo, Japan) containing tetrabromophenol blue.
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FIG. 1. Generation of Sall2-deficient mice. (A) Targeting strategy of Sall12 locus. Positions of the zinc finger motifs are indicated by ovals. Restriction sites: B, BamHI; RI, EcoRI; Spe, SpeI; Sma, SmaI; Xh, XhoI. (B) Southern blot analysis of wild-type (+/+), heterozygous (+/-), and homozygous (-/-) Sall2-deficient mice. Tail DNA was digested with EcoRI and hybridized with probe B. (C) Genomic PCR of wild-type (+/+), heterozygous (+/-), and homozygous (-/-) Sall2-deficient mice. The 388-bp band was amplified from the mutant allele, and the 188-bp band was amplified from the wild-type Sall2 genome. The positions of the PCR primers are indicated by arrows in panel A. (D) Northern blotting analysis of Sall genes in Sall2-deficient embryos at 13.5 dpc. Note that N-terminal Sall2 probe gave no signal in Sall2-deficient mice. In the case of the C-terminal Sall2 probe, the Sall2 band was absent in Sall2-deficient mice (solid arrowhead), but a shorter band appeared that also hybridized with Neor probe (open arrowheads).
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FIG. 2. Expression of Sall2 in developing embryos. (A) Metanephros and spinal cord at 11.5 dpc (sp, spinal cord; mm, metanephric mesenchyme); (B) metanephros at 11.5 dpc (ub, ureteric bud; mm, metanephric mesenchyme); (C) metanephros at 13.5 dpc; (D) brain at 14.5 dpc. Scale bar, 100 µm.
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TABLE 1. Genotype analysis of mice from Sall2 heterozygous intercrossesa
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FIG. 3. Histology in Sall2-deficient mice at 13.5 dpc. (A and B) Kidneys in wild-type (A) and Sall2-deficient mice (B); (C and D) hearts in wild-type (C) and Sall2-deficient mice (D); (E and F) inner ears in wild-type (E) and Sall2-deficient mice (F). Scale bars, 100 µm.
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TABLE 2. Peripheral blood counts of Sall2-deficient mice
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Sall1 is expressed in the metanephric mesenchyme, and the expression of Sall1 was not altered in the absence of Sall2, findings consistent with the data in Fig. 1D (Fig. 4A and B).
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FIG. 4. In situ hybridization of molecular markers in 13.5-dpc metanephros of wild-type (left panels [A, C, E, and G]) and Sall2-deficient mice (right columns [B, D, F, and H]). Scale bars, 100 µm. (A and B) Sall1; (C and D) Pax2; (E and F) Wnt4; (G and H) Ret.
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Wnt4 is required for epithelialization of the induced mesenchyme but not for the initial induction by the ureter (20). Wnt4 is expressed in mesenchymal cells on the sides of the ureteric bud and correlates to the site where the first pretubular aggregates form (Fig. 4E). Sall2-deficient mice showed unaltered Wnt4 expression (Fig. 4F).
Mice deficient in the tyrosine-kinase type receptor, Ret, show a failure of ureteric bud invasion and subsequent failure of mesenchymal differentiation (12, 17-19). Ret was expressed in the ureteric bud in the wild type, and its expression in Sall2 mutant mice was unaltered (Fig. 4G and H). These results indicate that markers of metanephric mesenchyme and ureteric bud were not affected in the absence of Sall2 and that Sall2 is not required for normal kidney development.
There was no limb deformity, anorectal anomaly, or ear anomaly, all of which are characteristic of Townes-Brocks syndrome, which is caused by SALL1 mutation. We suggest that Sall2 is not essential for development and that Sall2 absence may be compensated for by other Sall genes, the expression of which overlaps with that of Sall2.
Human SALL2 is located on chromosome 14q12, possibly overlapping a region of loss of heterozygosity in ovarian cancers (1). In addition, mouse Sall2 binds to polyomavirus large T antigen and was proposed to be a potential tumor suppressor (11). Sall2 mutant mice, however, did not show spontaneous tumor formation for more than 1 year after birth. Tumor formation upon virus inoculation will be required to test the hypothesis that Sall2 is a tumor suppressor.
Kidney defects in mice lacking both Sall1 and Sall2. Mouse Sall1 is essential for the initial step for metanephros formation: ureteric bud attraction. Hence, kidney agenesis or severe dysgenesis was present in Sall1-deficient mice. Other organs, however, were not affected, although heterozygous mutations of human SALL1 lead to Townes-Brocks syndrome, with features of dysplastic ears, preaxial polydactyly, imperforate anus, and heart anomalies in addition to kidney anomalies. The relative importance of SALL1 over SALL2 and -3 may be higher in humans than in mice, and Sall1 deficiency may be compensated for by Sall2 and -3 in mice.
To address this question, we crossed Sall1 and Sall2 mutants and generated mice lacking both genes. Some pups from a double heterozygous cross were dead perinatally and had kidney abnormalities; most were Sall1 single mutants, but we did find some double homozygotes. To further confirm the phenotypes, we set up pairs of Sall1+/- Sall2-/- mice and found that double mutants were born at a Mendelian frequency (Table 3). All of the double mutants, as well as the Sall1 mutants, were dead perinatally, and they all had kidney abnormalities. Of 12 double mutants (25.0%), 3 had no kidneys or ureters bilaterally. Four mice (33.3%) had unilateral kidney agenesis and hypoplasia on the other side. Five mice (41.7%) had two small remnant kidneys. Histological examination of all of the residual kidneys in the double mutant newborn showed size reduction and multiple cysts, which are comparable to the Sall1 mutants (Fig. 5A to F). At 12.5 dpc, size reduction and impaired ureteric branching were observed in Sall1/2-null mutants, which is also comparable to findings in the Sall1 mutants (Fig. 5G and H and data not shown). Thus, the severity of the kidney impairment of the double mutants was comparable to that of Sall1 single mutants reported earlier (14), indicating that Sall2 absence does not exacerbate the kidney defects caused by Sall1 mutation.
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TABLE 3. Genotype analysis of mice from Sall1+/- Sall2-/- intercrossesa
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FIG. 5. Kidney development in Sall1/2 double deficient mice. (A and B) Kidney of wild-type newborn. (C and D) Kidney of Sall1/2 double deficient mice. The kidney is small and contains multiple cysts. (E and F) Kidney of Sall1-deficient mice, which shows a similar histology to Sall1/2 doubly deficient mice. (G) Metanephros in wild-type mice at 12.5 dpc. Branching is evident. (H) Metanephros in Sall1/2 doubly deficient mice at 12.5 dpc. Kidney size and ureteric branching are reduced. kid, kidney; ad, adrenal gland; u, ureter. Scale bars, 100 µm.
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TABLE 4. Renal function in adult animalsa
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