Molecular cloning, chromosomal localization, and expression of the murine SALL1 ortholog Sall1

SALL1 has been identified as one of now three human homologs of the region specific homeotic gene spalt (sal) of Drosophila, which encodes a zinc finger protein of characteristic structure. Mutations of SALL1 on chromosome 16q12.1 cause Townes-Brocks syndrome (TBS, OMIM no. 107480). In order to facilitate functional studies of this gene in a model organism, we searched for the murine homolog of SALL1. Here we report the genomic cloning, chromosome mapping, and partial expression analysis of the gene Sall1. Sequence comparison, Northern blot hybridization as well as the conserved chromosome location on the homologous mouse chromosome indicate that we have indeed isolated the murine homolog of SALL1.     

Embryonic expression of the murine homologue of SALL1, the gene mutated in Townes--Brocks syndrome

SALL1 is one of three human homologues of the Drosophila region-specific homeotic gene spalt (sal). Mutations of SALL1 on chromosome 16q12.1 cause Townes--Brocks syndrome (TBS) which is characterized by defects in multiple organ systems including limbs, ears, kidneys and anus. Here, we have analyzed the expression of the mouse homologue of SALL1 (Sall1) during early embryogenesis. Sall1 expression is very prominent in the developing brain and the limbs. Other sites of expression include the meso- and metanephros, lens, olfactory bulbs, heart, primitive streak and the genital tubercle. Hence, Sall1 expression to a large degree reflects the structures affected in human TBS.     

Interaction of the developmental regulator SALL1 with UBE2I and SUMO-1

Mutations in the SALL1 gene on chromosome 16q12.1 cause Townes-Brocks syndrome (TBS). This autosomal dominantly inherited disorder is characterized by typical malformations of the thumbs, the ears, and the anus, and also commonly affects the kidneys and other organ systems. SALL1 has recently been shown to localize to chromocenters and other heterochromatin foci in murine fibroblasts and to interact with the telomere-repeat-binding factor TRF1/PIN2. Here, we show that the ubiquitin-conjugating enzyme 2I (UBE2I), the human homolog of S. cerevisiae UBC9, and the small ubiquitin-like modifier-1 (SUMO-1) interact with SALL1 in the yeast two-hybrid system. The interaction of SALL1 and UBE2I was confirmed in a glutathione S-transferase (GST) pull-down experiment. In an in vitro assay, it could be demonstrated that SALL1 is covalently modified by at least two SUMO-1 molecules in the presence of UBA2/AOS1 and UBE2I. Mutation of lysine 1086 of SALL1 to arginine abrogates SALL1 sumoylation, suggesting the presence of a polymeric SUMO-1 chain in the wild type state.     

SALL3, a new member of the human spalt-like gene family, maps to 18q23

spalt (sal) of Drosophila melanogaster is an important developmental regulator gene and encodes a zinc finger protein of unusual but characteristic structure. Two human sal-like genes have been isolated so far, SALL1 on chromosome 16q12.1 and SALL2 on chromosome 14q11.1-q12.1. Truncating mutations of SALL1 have been shown to cause Townes-Brocks syndrome and are thought to result in SALL1 haploinsufficiency. Sequence comparison of SALL1 to the related genes Msal in mouse and Xsal-1 in Xenopus laevis suggested that SALL1 was not the human orthologue of Msal and Xsal-1. By database searching and genomic cloning, we isolated an EST and a corresponding human cosmid clone, which contain coding sequence of a human gene highly similar to mouse Msal. This gene, named SALL3, was found to be expressed in different regions of human fetal brain and in different adult human tissues. The chromosomal localization of SALL3 at 18q23 suggests that haploinsufficiency of this gene might contribute to the phenotype of patients with 18q deletion syndrome.     

csal1 is controlled by a combination of FGF and Wnt signals in developing limb buds

While some of the signaling molecules that govern establishment of the limb axis have been characterized, little is known about the downstream effector genes that interpret these signals. In Drosophila, the spalt gene is involved in cell fate determination and pattern formation in different tissues. We have cloned a chick homologue of Drosophila spalt, which we have termed csal1, and this study focuses on the regulation of csal1 expression in the limb bud. csal1 is expressed in limb buds from HH 17 to 26, in both the apical ectodermal ridge and the distal mesenchyme. Signals from the apical ridge are essential for csal1 expression, while the dorsal ectoderm is required for csal1 expression at a distance from the ridge. Our data indicate that both FGF and Wnt signals are required for the regulation of csal1 expression in the limb. Mutations in the human homologue of csal1, termed Hsal1/SALL1, result in a condition known as Townes-Brocks syndrome (TBS), which is characterized by preaxial polydactyly. The developmental expression of csal1 together with the digit phenotype in TBS patients suggests that csal1 may play a role in some aspects of distal patterning.     

Knockdown of SALL4 Protein Enhances All-trans Retinoic Acid-induced Cellular Differentiation in Acute Myeloid Leukemia Cells

All-trans retinoic acid (ATRA) is a differentiation agent that revolutionized the treatment of acute promyelocytic leukemia. However, it has not been useful for other types of acute myeloid leukemia (AML). Here we explored the effect of SALL4, a stem cell factor, on ATRA-induced AML differentiation in both ATRA-sensitive and ATRA-resistant AML cells. Aberrant SALL4 expression has been found in nearly all human AML cases, whereas, in normal bone marrow and peripheral blood cells, its expression is only restricted to hematopoietic stem/progenitor cells. We reason that, in AMLs, SALL4 activation may prevent cell differentiation and/or protect self-renewal that is seen in normal hematopoietic stem/progenitor cells. Indeed, our studies show that ATRA-mediated myeloid differentiation can be largely blocked by exogenous expression of SALL4, whereas ATRA plus SALL4 knockdown causes significantly increased AML differentiation and cell death. Mechanistic studies indicate that SALL4 directly associates with retinoic acid receptor α and modulates ATRA target gene expression. SALL4 is shown to recruit lysine-specific histone demethylase 1 (LSD1) to target genes and alter the histone methylation status. Furthermore, coinhibition of LSD1 and SALL4 plus ATRA treatment exhibited the strongest anti-AML effect. These findings suggest that SALL4 plays an unfavorable role in ATRA-based regimes, highlighting an important aspect of leukemia therapy.     

Genomic cloning, chromosomal mapping, and expression analysis of Msal-2

Mutations of SALL1 related to spalt of Drosophila have been found to cause Townes-Brocks syndrome, suggesting a function of SALL1 for the development of anus, limbs, ears, and kidneys. No function is yet known for SALL2, another human spalt-like gene. The structure of SALL2 is different from SALL1 and all other vertebrate spalt-like genes described in mouse, Xenopus, and Medaka, suggesting that SALL2-like genes might also exist in other vertebrates. Consistent with this hypothesis, we isolated and characterized a SALL2 homologous mouse gene, Msal-2. In contrast to other vertebrate spalt-like genes both SALL2 and Msal-2 encode only three double zinc finger domains, the most carboxyterminal of which only distantly resembles spalt-like zinc fingers. The evolutionary conservation of SALL2/Msal-2 suggests that two lines of sal-like genes with presumably different functions arose from an early evolutionary duplication of a common ancestor gene. Msal-2 is expressed throughout embryonic development but also in adult tissues, predominantly in brain. However, the function of SALL2/Msal-2 still needs to be determined. Received: 1 June 1999 / Accepted: 26 August 1999     

Cloning and expression analysis of SALL4, the murine homologue of the gene mutated in Okihiro syndrome

SALL4 is one out of four human homologues of the DROSOPHILA region-specific homeotic gene SPALT(SAL). Heterozygous mutations of SALL4 on chromosome 20q13.13--> q13.2 cause the autosomal dominant Okihiro syndrome which is characterized by radial limb defects, Duane anomaly and hearing loss. We have partially cloned the murine homologue of this gene, named SALL4, and completed the coding sequence by comparison to available EST and genomic sequences in the GenBank database. This comparison also revealed the chromosomal location of SALL4 on mouse chromosome 2H3 and suggested that a predicted testis expressed gene TEX20 at the very same locus is most likely not a gene on its own but part of the SALL4 3' UTR. We analyzed the expression of SALL4 during early embryogenesis by whole mount in situ hybridization and in the adult mouse by Northern blotting. In adult tissues, SALL4 expression is only found in testis and ovary. During embryonic development, SALL4 expression is widespread in early embryos and becomes gradually confined to the head region and the primitive streak. Prominent expression in the developing midbrain, branchial arches and the limbs suggests an important function of SALL4 during development of these structures as expected from the observation in Okihiro syndrome patients.     

The murine homolog of SALL4, a causative gene in Okihiro syndrome, is essential for embryonic stem cell proliferation, and cooperates with Sall1 in anorectal, heart, brain and kidney development

Mutations in SALL4, the human homolog of the Drosophila homeotic gene spalt (sal), cause the autosomal dominant disorder known as Okihiro syndrome. In this study, we show that a targeted null mutation in the mouse Sall4 gene leads to lethality during peri-implantation. Growth of the inner cell mass from the knockout blastocysts was reduced, and Sall4-null embryonic stem (ES) cells proliferated poorly with no aberrant differentiation. Furthermore, we demonstrated that anorectal and heart anomalies in Okihiro syndrome are caused by Sall4 haploinsufficiency and that Sall4/Sall1 heterozygotes exhibited an increased incidence of anorectal and heart anomalies, exencephaly and kidney agenesis. Sall4 and Sall1 formed heterodimers, and a truncated Sall1 caused mislocalization of Sall4 in the heterochromatin; thus, some symptoms of Townes-Brocks syndrome caused by SALL1 truncations could result from SALL4 inhibition.     

TLK1B mediated phosphorylation of Rad9 regulates its nuclear/cytoplasmic localization and cell cycle checkpoint

Breast cancer is the most frequent malignancy in women and drug resistance is the major obstacle for its successful chemotherapy. In the present study, we analyzed the involvement of an oncofetal gene, sal-like 4 (SALL4), in the tumor proliferation and drug resistance of human breast cancer. Our study showed that SALL4 was up-regulated in the drug resistant breast cancer cell line, MCF-7/ADR, compared to the other five cell lines. We established the lentiviral system expressing short hairpin RNA to knockdown SALL4 in MCF-7/ADR cells. Down-regulation of SALL4 inhibited the proliferation of MCF-7/ADR cells and induced the G1 phase arrest in cell cycle, accompanied by an obvious reduction of the expression of cyclinD1 and CDK4. Besides, down-regulating SALL4 can re-sensitize MCF-7/ADR to doxorubicin hydrochloride (ADMh) and had potent synergy with ADMh in MCF-7/ADR cells. Depletion of SALL4 led to a decrease in IC50 for ADMh and an inhibitory effect on the ability to form colonies in MCF-7/ADR cells. With SALL4 knockdown, ADMh accumulation rate of MCF-7/ADR cells was increased, while the expression of BCRP and c-myc was significantly decreased. Furthermore, silencing SALL4 also suppressed the growth of the xenograft tumors and reversed their resistance to ADMh in vivo. SALL4 knockdown inhibits the growth of the drug resistant breast cancer due to cell cycle arrest and reverses tumor chemo-resistance through down-regulating the membrane transporter, BCPR. Thus, SALL4 has potential as a novel target for the treatment of breast cancer.     

SALL4 is directly activated by TCF/LEF in the canonical Wnt signaling pathway

The SALL4 promoter has not yet been characterized. Animal studies showed that SALL4 is downstream of and interacts with TBX5 during limb and heart development, but a direct regulation of SALL4 by TBX5 has not been demonstrated. For other SAL genes, regulation within the Shh, Wnt, and Fgf pathways has been reported. Chicken csal1 expression can be activated by a combination of Fgf4 and Wnt3a or Wnt7a. Murine Sall1 enhances, but Xenopus Xsal2 represses, the canonical Wnt signaling. Here we describe the cloning and functional analysis of the SALL4 promoter. Within a minimal promoter region of 31bp, we identified a consensus TCF/LEF-binding site.The SALL4 promoter was strongly activated not only by LEF1 but also by TCF4E. Mutation of the TCF/LEF-binding site resulted in decreased promoter activation. Our results demonstrate for the first time the direct regulation of a SALL gene by the canonical Wnt signaling pathway.     

Sal-like 4 (SALL4) suppresses CDH1 expression and maintains cell dispersion in basal-like breast cancer

In cell cultures, the dispersed phenotype is indicative of the migratory ability. Here we characterized Sal-like 4 (SALL4) as a dispersion factor in basal-like breast cancer. Our shRNA-mediated SALL4 knockdown system and SALL4 overexpression system revealed that SALL4 suppresses the expression of adhesion gene CDH1, and positively regulates the CDH1 suppressor ZEB1. Cell behavior analyses showed that SALL4 suppresses intercellular adhesion and maintains cell motility after cell–cell interaction and cell division, which results in the dispersed phenotype. Our findings indicate that SALL4 functions to suppress CDH1 expression and to maintain cell dispersion in basal-like breast cancer.