The spalt-related gene of Drosophila melanogaster is a member of an ancient gene family, defined by the adjacent, region-specific homeotic gene spalt. Both genes have three widely spaced sets of C2H2 zinc finger motifs, but spalt-related has a fourth pair of C-terminal fingers resembling the Xenopus homolog, Xsal-1. The spalt-related gene is not expressed in early development as is spalt, but is expressed from mid-embryogenesis to the adult stage, but not in ovaries. Expression is localized to the nervous system. spalt-related is more promenant that spalt in the embryonic brain, but spalt is expressed at a higher level than spalt-related in the posterior spiracles. There is some degree of differential expression in the ventral cord of the CNS (Barrio, 1996).

Neuronal and mesodermal cell types are generated in separate cell lineages during the larval development of Caenorhabditis elegans. The gene sem-4 is required in both types of lineages for the normal development of neuronal and mesodermal cell types. The sem-4 gene encodes a protein containing seven zinc finger motifs of the C2H2 class, four of which are arranged in two pairs widely separated in the primary sequence of the protein. These pairs of zinc fingers are similar to pairs of zinc fingers in the protein encoded by the Drosophila homeotic gene spalt and in the human transcription factor PRDII-BF1. Analysis of sem-4 alleles suggests that different zinc fingers in the SEM-4 protein may function differentially in neuronal and mesodermal cell types. It is proposed that sem-4 interacts with different transcription factors in different cell types to control the transcription of genes that function in the processes of neuronal and mesodermal cell development (Basson, 1996).

Vulval cell-fate determination in C. elegans requires the action of numerous gene products, including components of the Ras/Raf/MAPK signaling cascade and the hox gene lin-39, an Antennapedia class homeodomain, most similar to those of the Drosophila homeotic genes Deformed and Sex combs reduced. sem-4 encodes a zinc finger protein, homologous to Drosophila Spalt, with roles in the fate specification of sex myoblasts, coelomocytes, and multiple neuronal lineages in C. elegans. By characterizing three new alleles of sem-4, identified in a screen for vulval-defective mutants, it has been determined that loss of sem-4 activity results in abnormal specification of the secondary vulval cell lineages. sem-4 interactions with other genes involved in vulval differentiation were analyzed and it was determined that sem-4 does not function directly in the Ras-mediated signal transduction pathway but acts in close association with and upstream of lin-39 to promote vulval cell fate. sem-4 regulates lin-39 expression and it is proposed that sem-4 is a regulator of lin-39 in the vulval cell-fate determination pathway that may act to link lin-39 to incoming signals (Grant, 2000).

Members of the spalt (sal) gene family encode zinc-finger proteins that are putative tumor suppressors and regulate anteroposterior (AP) patterning, cellular identity, and, possibly, cell cycle progression. The mechanism through which sal genes carry out these functions is unclear. The Caenorhabditis elegans sal gene sem-4 controls the fate of several different cell types, including neurons, muscle and hypodermis. Mutation of sem-4 transforms particular tail neurons into touch-neuron-like cells. In wild-type C. elegans, six touch receptor neurons mediate the response of the worm to gentle touch. All six touch neurons normally express the LIM homeobox gene mec-3 (Drosophila homolog: Lim3). A subset, the two PLM cells, also express the Hox gene egl-5, an Abdominal-B homolog, which is required for correct mec-3 expression in these cells. The abnormal touch-neuron-like-cells in sem-4 animals express mec-3; a subset also express egl-5. The following observations are reported: (1) that ectopic expression of sem-4 in normal touch cells represses mec-3 expression and reduces touch cell function; (2) that egl-5 expression is required for both the fate of normal PLM touch neurons in wild-type animals and the fate of a subset of abnormal touch neurons in sem-4 animals, and (3) that SEM-4 specifically binds a shared motif in the mec-3 and egl-5 promoters that mediates repression of these genes in cells in the tail. It is concluded that sem-4 represses egl-5 and mec-3 through direct interaction with regulatory sequences in the promoters of these genes; that sem-4 indirectly modulates mec-3 expression through its repression of egl-5, and that this negative regulation is required for proper determination of neuronal fates. It is suggested that the mechanism and targets of regulation by sem-4 are conserved throughout the sal gene family: other sal genes might regulate patterning and cellular identity through direct repression of Hox selector genes and effector genes (Toker, 2003).

Hox genes appear to be targets not only of sem-4 but also of other sal genes. Drosophila sal might negatively regulate Sex combs reduced (Scr) and other Drosophila Hox genes. Loss of sal function in Drosophila BX-C minus embryos produces some limited ectopic expression of the Hox gene Scr. Mutations in sal enhance the phenotypes of Polycomb group (PcG) mutants. These genes are known to be negative regulators of Hox genes. Loss of sal function affects AP patterning in Drosophila. Mutations in sal incompletely transform both head and tail structures into trunk-like structures: sal activity has been shown to promote head development. Hox genes in mammals might also be targets of sal family genes. Patients with TBS, which is caused by mutations in SALL1, display characteristic features of syndromes associated with mutations in HOX genes (Toker, 2003).

LIM homeobox genes, such as mec-3, might also be conserved targets of sal genes. The closest mammalian homolog to mec-3 is the human LIM homeobox gene Lhx5. Lhx5 and the human SALL1 gene appear to be expressed in different sets of cells in the developing thalamus, which constitutes a very small portion of the entire brain. SALL1 and Lhx5 are not expressed in most other regions of the fetal brain. Their expression in separate thalamic cells could indicate that SALL1 restricts Lhx5 expression in the thalamus (Toker, 2003).

The mechanism through which sem-4 negatively regulates its targets is probably conserved. SEM-4, SALL1 and mouse sall1 are transcriptional repressors. SALL1 and mouse sall1, fused to heterologous DNA binding domains, behave as repressors in mammalian cell culture assays. It is suggested that these genes bind directly to regulatory regions of their targets (Toker, 2003).

Drosophila and mammalian studies have suggested that sal genes might function as PcG genes. Mutations of Drosophila sal cause limited ectopic expression of the Hox genes Ubx and Scr, and sal mutations enhance mutations in the PcG genes polyhomeotic and Polycomb-like. Human SALL1 localizes to chromocenters in mammalian cells and mouse sall1 interacts with components of chromatin remodeling complexes. One additional speculation is that Drosophila sal might bind to a 138 bp silencing sequence in the Polycomb response element in Abd-B, the egl-5 ortholog. Two sites have been identified that match the SEM-4 binding sequence in this Drosophila silencing element (Toker, 2003).

A fundamental challenge of evolutionary and developmental biology is understanding how new characters arise and change. The recently derived eyespots on butterfly wings vary extensively in number and pattern between species and play important roles in predator avoidance. Eyespots form through the activity of inductive organizers (foci) at the center of developing eyespot fields. Foci are the proposed source of a morphogen, the levels of which determine the color of surrounding wing scale cells. However, it is unknown how reception of the focal signal translates into rings of different-colored scales, nor how different color schemes arise in different species. Several transcription factors, including butterfly homologs of the Drosophila Engrailed/Invected and Spalt proteins have been identified. These are deployed in concentric territories corresponding to the future rings of pigmented scales that compose the adult eyespot. A new Bicyclus anynana wing pattern mutant, Goldeneye, has been isolated in which the scales of one inner color ring become the color of a different ring. These changes correlate with shifts in transcription factor expression, suggesting that Goldeneye affects an early regulatory step in eyespot color patterning. In different butterfly species, the same transcription factors are expressed in eyespot fields, but in different relative spatial domains that correlate with divergent eyespot color schemes. These results suggest that signaling from the focus induces nested rings of regulatory gene expression that subsequently control the final color pattern. Furthermore, the remarkably plastic regulatory interactions downstream of focal signaling have facilitated the evolution of eyespot diversity (Brunetti, 2001).

To distinguish between different potential mechanisms of eyespot development and evolution, candidate genes involved in eyespot color pattern formation were sought. A screen was performed for gene products that are expressed during the period of scale cell differentiation (12 to 36 hours after pupation) and that have patterns that are correlated with the concentric rings of Bicyclus anynana eyespots. Among the various proteins and transcripts surveyed (these included Cubitus interruptus, Schnurri, SMAD, Brinker, aristaless, dachshund, and teashirt), only the Engrailed/Invected (Engrailed and/or Invected, hereafter denoted by En/Inv) and Spalt (Sal) transcription factors are expressed in patterns of scale-forming cells that correlate with eyespot formation. All identified proteins are expressed in cells in the region of the focus at the center of each eyespot field. Remarkably, a second domain of En/Inv expression arises in the 16 hour pupal wing in a distinct ring of cells outside of the focal region and at the periphery of each eyespot field. In addition, Sal is expressed in rings of cells between the focal region and the ring of En/Inv-expressing cells. Based upon physical landmarks of the developing wing and by comparison of the relative size and position of the concentric rings of gene expression patterns with the colored rings of the adult eyespot, correlations between protein expression patterns and the three colored rings of B. anynana eyespots were found. The En/Inv, Sal, and Dll expression in the focus corresponds to the white center in the adult eyespot. The territory marked by Sal and Dll expression, but not En/Inv expression, appears to correspond to the domain of the black ring of scales in the adult eyespot. Additionally, the outer ring of En/Inv expression correlates with the position of the gold ring of scales in the adult wing. A gene product for which the pattern of expression correlates with the outermost dark-brown ring of scales has not been identified (Brunetti, 2001).

From observations of the temporal and spatial relationships between En/Inv, Sal, and Dll expression, two important inferences can be made: (1) the switch from synchronous coincident expression of these three proteins in the center of the eyespot field to their asynchronous, nonoverlapping expression in the outer rings of the field suggests that they are under different regulatory controls when the foci are first established than when the eyespot field is elaborated; (2) the sequential appearance of the rings, in particular the expression of En/Inv in cells just outside of the Sal domain, suggests that one mechanism for generating concentric patterns of gene expression may be to exclude the expression of one gene from another's domain (Brunetti, 2001).

The transplantation of eyespot foci between species or of selected lines of B. anynana differing in eyespot color composition induces eyespot patterns characteristic of the host animal, suggesting that the response to the focal signal (not the signal itself) is different between species. It is possible that the differences in cells' responses to focal signaling could arise as a result of changes in the expression patterns of regulators. Alternatively, direct responses to focal signaling may be similar between species, but the regulators may interact with different downstream genes involved in scale pigmentation and structure. To determine when during development differences arise between the eyespot color schemes of various species, the expression patterns were compared of En/Inv, Sal, and Dll in B. anynana (Nymphalidae, Satyrinae), Precis coenia (Nymphalidae, Nymphalinae), Vanessa cardui (Nymphalidae, Nymphalinae), and Lycaeides melissa (Lycaenidae, Lycaeninae). In each of the examined species, which represent two different families of butterflies and three different genera within the Nymphalidae, the expression patterns of En/Inv, Sal, and Dll are different, yet they mark territories in the pupal wing that often correlate with color pattern schemes on the adult wing. For example, in P. coenia, the Sal territory in the pupal wing marks the entire area encompassed by the adult eyespot. In addition, the coexpression of En/Inv, Sal, and Dll in P. coenia forewings in an asymmetric patch of scales at the center of the pupal eyespot corresponds to the white/blue scales at the center of the adult eyespot. The coexpression of the same genes in scale-building cells outside of this central spot correlates with the black ring of scales on the adult. In V. cardui, a species closely related to P. coenia, En/Inv is expressed in an outer ring of scale-building cells that correlates with the black ring of scales in the adult eyespot. However, in L. melissa, a crescent of En/Inv expression correlates with the future position of orange scales on the adult, and En/Inv and Sal coexpression correlates with the metallic-looking patch of scales at the center of the eyespot field (Brunetti, 2001).

From comparative data, it is concluded that eyespot color pattern diversity is generated by regulatory differences at two distinct stages of eyespot development that evolve independently of each other: (1) during the focal signaling stage, through the generation of different combinations and patterns of expression of regulatory genes such as en/inv, sal, and Dll; and (2) during the scale differentiation stage, through differences in the response of pigmentation genes to the upstream regulators (Brunetti, 2001).

These results indicate that at least one tier of spatially regulated transcription factors is interposed between focal signaling and scale color differentiation. How the graded distribution of a focal signal is translated into the concentric territories of En/Inv, Sal, and Dll expression is therefore of special interest. In B. anynana, it is suggested that this occurs through response thresholds of, and negative cross-regulation among, genes regulated by the signal. For example, one of the simplest explanations for the exclusion of En/Inv and Sal expression from each other's territories outside of the focus could be the repression of one gene by the product of the other. The reciprocal effects of the Goldeneye mutation on En/Inv and Sal expression are strongly suggestive of negative crossregulation. The establishment, through negative crossregulation, of distinct spatial domains of downstream genes in response to a single activator is a common theme illustrated by the subdivision of the Drosophila embryonic mesoderm and neuroectoderm and of the proximodistal axis of Drosophila limb fields. In P. coenia, however, the nested nonexclusive expression of Sal and En/Inv suggests that here these genes do not crossregulate. Rather, the nested expression pattern outside of the focus is most simply explained by different threshold responses of these two genes to the focal signal; these responses are analogous to the threshold responses of genes to long-range signals in the Xenopus mesoderm and the Drosophila imaginal wing field (Brunetti, 2001).

The deployment of En/Inv, Sal, and Dll in all of the species examined also raises some interesting possible scenarios regarding the origin and diversification of eyespots and the evolution of the underlying genetic regulatory system that controls eyespot pattern formation. It has been proposed that eyespots have a single origin and are derived from simpler spot patterns of uniform color that evolved into organizing centers. Because all three proteins are deployed in color-correlated patterns in this well-diverged group of butterflies, it is likely that these genes were recruited into the developmental program early during the evolution of eyespots. Furthermore, it is intriguing that while the three proteins have distinct expression patterns during scale differentiation, they are coexpressed during focus formation. It is tempting to speculate, on the basis of the data presented here, that the evolution of eyespots in response to diverse selective environments involved the modification of the deployment of genes that were originally expressed in simpler spot patterns into additional concentric patterns organized around and by cells in the center of the eyespot field (Brunetti, 2001).

The medaka fish (a Japanese freshwater poeciliid fish) gene spalt encodes a zinc-finger transcription factor, which is expressed in all known hedgehog signaling centers of the embryo and in the organizer region at the midbrain-hindbrain boundary. The spalt expression domains expand in response to ectopic hedgehog activity and narrow in the presence of protein kinase A activity, an antagonist of hedgehog signaling, indicating that spalt is a hedgehog target gene. These results also suggest a signaling mechanism for anterior-posterior patterning of the vertebrate brain that controls spalt expression at the midbrain-hindbrain boundary in a protein kinase A dependent manner, likely to involve an unknown member of the hedgehog family (Koster, 1997).

Xsal-1 is a Xenopus homolog of spalt. The frog protein has three double zinc fingers like SAL, but in addition, it has a fourth double finger and an additional single finger. Xsal-1 is expressed in lateral axon tracts, in the midbrain, hindbrain and limbs. The frog gene is regulated by signals from the notochord and floor plate, and might function in neuronal cell specification (Hollenmann, 1996).

The spalt gene family is characterized by unique double zinc finger motifs and is conserved among various species from Drosophila to humans. A new Xenopus member of this family, Xsal-3, has been identified. It is 38% homologous at the amino acid level to the previously reported Xenopus homolog of the spalt gene, Xsal-1. Alternatively spliced Xsal-3 transcripts give rise to RNAs coding either two or three double zinc fingers, and the longer form is expressed maternally. Xsal-3 is expressed in the neural tube, the mandibular, hyoid, and branchial arch, and the pronephric duct, which is different from the expression pattern of Xsal-1. These findings suggest that Xsal-3 may have distinct roles in early Xenopus development (Onuma, 1999).

XsalF, a frog homolog of the Drosophila homeotic selector Spalt, plays an essential role for the forebrain/midbrain determination in Xenopus. XsalF overexpression expands the domain of forebrain/midbrain genes and suppresses midbrain/hindbrain boundary (MHB) markers and anterior hindbrain genes. Loss-of-function studies show that XsalF is essential for the expression of the forebrain/midbrain genes and for the repression of the caudal genes. Interestingly, XsalF functions by antagonizing canonical Wnt signaling, which promotes caudalization of neural tissues. XsalF is required for anterior-specific expressions of GSK3ß and Tcf3, genes encoding antagonistic effectors of Wnt signaling. Loss-of-function phenotypes of GSK3ß and Tcf3 mimic those of XsalF while injections of GSK3ß and Tcf3 rescue loss-of-function phenotypes of XsalF. These findings suggest that the forebrain/midbrain-specific gene XsalF negatively controls cellular responsiveness to posteriorizing Wnt signals by regulating region-specific GSK3ß and Tcf3 expression (Onai, 2004).

Msal is a mouse homolog that contains alternatively seven and nine zinc fingers, each of which contains tha SAL box motif KTTKGNLK. The additional zinc finger pair is conserved in mouse and Xenopus and expressed as an alternatively spliced product. A glutamine-rich putative transactivation domain close to the amino-terminus is conserved in the mouse homolog. msal is expressed in the developing neuroectoderm of the brain, the inner ear and the spinal cord and in urogenital ridge-derived structures. A weaker and transient expression is seen in early embryos in the branchial arches and in notochord, limb buds and heart (Ott, 1996).

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, a SALL2 homologous mouse gene, Msal-2, has been isolated and characterized. 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. The function of SALL2/Msal-2 still needs to be determined (Kohlhase, 2000a).

Two human sal-like genes have been isolated to date: 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 suggest that SALL1 is not the human orthologue of Msal and Xsal-1. By database searching and genomic cloning, an EST and a corresponding human cosmid clone have been isolated that contain the coding sequence of a human gene highly similar to mouse Msal. This gene, named SALL3, is 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 (Kohlhause, 2000b).

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. A chick homolog of Drosophila spalt, csal1, has been cloned. csal1 is expressed in limb buds from HH stages 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. These data indicate that both FGF and Wnt signals are required for the regulation of csal1 expression in the limb. Mutations in the human homolog 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 (Farrell, 2000).

Townes-Brocks syndrome (TBS) is a rare autosomal-dominant malformation syndrome with a combination of anal, renal, limb and ear anomalies. Cytogenetic findings suggest that the gene mutated in TBS maps to chromosome 16q12.1, where SALL1 (previously known as HSAL1), a human homolog of Drosophila spalt (sal), is located. No phenotype has yet been attributed to mutations in vertebrate sal-like genes. The expression patterns of sal-like genes in mouse, Xenopus and the fish Medaka, and the finding that Medaka sal is regulated by Sonic hedgehog, prompted an examination of SALL1 as a TBS candidate gene. SALL1 mutations have been shown to cause TBS in a family with vertical transmission of TBS and in an unrelated family with a sporadic case of TBS. Both mutations are predicted to result in a prematurely terminated SALL1 protein lacking all putative DNA binding domains. TBS therefore represents another human developmental disorder caused by mutations in a putative C2H2 zinc-finger transcription factor (Kohlhase, 1998).

Townes-Brocks syndrome (TBS) is an autosomal dominant developmental disorder characterized by anal and thumb malformations and by ear anomalies that can affect the three compartments and usually lead to hearing loss. The gene underlying TBS, SALL1, is a human homolog of the Drosophila spalt gene, which encodes a transcription factor. A search for SALL1 mutations undertaken in 11 unrelated affected individuals (five familial and six sporadic cases) led to the detection of mutations in nine of them. One nonsense and six different novel frameshift mutations, all located in the second exon, were identified. These mutations establish that TBS results from haploinsufficiency. The finding of de novo mutations in the sporadic cases is consistent with the proposed complete penetrance of the disease. Moreover, the occurrence of the same 826 C-to-T transition in a CG dimer, in six sporadic cases (i.e., six of the eight mutations identified in sporadic cases), reveals the existence of a mutation hotspot. Six different SALL1 polymorphisms were identified in the course of the present study, three of which are clustered in a particular region of the gene that encodes a stretch of serine residues. Finally, the chromosome 16 breakpoint of a t(5;16)(p15.3;q12.1) translocation carried by a TBS-affected individual was mapped at least 180 kb telomeric to SALL1, thus indicating that a position effect underlies the disease in this individual (Kohlhase, 1999).

Townes-Brocks syndrome (TBS) is an autosomal dominant developmental disorder characterized by anal and thumb malformations and by ear anomalies that can affect the three compartments and usually lead to hearing loss. A search for SALL1 mutations undertaken in 11 unrelated affected individuals (five familial and six sporadic cases) led to the detection of mutations in nine of them. One nonsense and six different novel frameshift mutations, all located in the second exon, have been identified. Together with the previously reported mutations, they establish that TBS results from haploinsufficiency. The finding of de novo mutations in the sporadic cases is consistent with the proposed complete penetrance of the disease. Moreover, the occurrence of the same 826C to T transition in a CG dimer, in three sporadic cases from the present series and three sporadic cases from the other series (i.e., six of the eight mutations identified in sporadic cases), reveals the existence of a mutation hotspot. Six different SALL1 polymorphisms have been identified in the course of the present study, three of which are clustered in a particular region of the gene that encodes a stretch of serine residues. Finally, the chromosome 16 breakpoint of a t(5;16)(p15.3;q12.1) translocation carried by a TBS-affected individual was mapped at least 180 kb telomeric to SALL1, thus indicating that a position effect underlies the disease in this individual (Marlin, 1999).

spalt 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 suggests that SALL1 is not the human ortholog of Msal and Xsal-1. By database searching and genomic cloning, an EST and a corresponding human cosmid clone, have been isolated that 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 (Chen, 1999).

SALL1 is a mammalian homolog of the Drosophila region-specific homeotic gene spalt (sal); heterozygous mutations in SALL1 in humans lead to Townes-Brocks syndrome. A mouse homolog of SALL1 (Sall1) has been isolated and it has been found that mice deficient in Sall1 die in the perinatal period and that kidney agenesis or severe dysgenesis are present. Sall1 is expressed in the metanephric mesenchyme surrounding ureteric bud; homozygous deletion of Sall1 results in an incomplete ureteric bud outgrowth, a failure of tubule formation in the mesenchyme and an apoptosis of the mesenchyme. This phenotype is likely to be primarily caused by the absence of the inductive signal from the ureter, as the Sall1-deficient mesenchyme is competent with respect to epithelial differentiation. Sall1 is therefore essential for ureteric bud invasion, the initial key step for metanephros development (Nishinakamura, 2001).

Drosophila spalt is downstream of the wingless signal in the tracheal system and sal deletion results in the absence of dorsal trunks in the trachea. Though murine Wnt4 is essential for kidney development, a normal ureter-mesenchyme interaction occurs in its mutants, which are different from phenotypes of Sall1-deficient mice. Furthermore Sall1 is not expressed in trachea and lungs, and Sall1-deficient mice apparently have no lung defects. Therefore, the simple analogy of Drosophila does not apply to mammals (Nishinakamura, 2001).

SALL4, a human homolog to Drosophila spalt, is a novel zinc finger transcriptional factor essential for development. SALL4 and its isoforms (SALL4A and SALL4B) were cloned. Through immunohistochemistry and RT-PCR, it was demonstrated that SALL4 was constitutively expressed in human primary acute myeloid leukemia (AML), and the leukemogenic potential of constitutive expression of SALL4 was directly tested in a murine model. SALL4B transgenic mice developed myelodysplastic syndrome (MDS)-like features and subsequently AML that was transplantable. Increased apoptosis associated with dysmyelopoiesis was evident in transgenic mouse marrow and colony-formation (CFU) assays. Both isoforms could bind to beta-catenin and synergistically enhanced the Wnt/beta-catenin signaling pathway. These data suggest that the constitutive expression of SALL4 causes MDS/AML, most likely through the Wnt/beta-catenin pathway. The murine model provides a useful platform to study human MDS/AML transformation, as well as the Wnt/beta-catenin pathway's role in the pathogenesis of leukemia stem cells (Ma, 2006).

Mutations in SALL4, the human homolog of the Drosophila homeotic gene spalt (sal), cause the autosomal dominant disorder known as Okihiro syndrome. This study shows 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 is reduced, and Sall4-null embryonic stem (ES) cells proliferat poorly with no aberrant differentiation. Furthermore, 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 (Sakaki-Yumoto, 2006).

Vertebrate placodes are regions of thickened head ectoderm that contribute to paired sensory organs and cranial ganglia. The transcription factor Spalt4 (also known as Sall4) is broadly expressed in chick preplacodal epiblast and later resolves to otic, lens and olfactory placodes. Ectopic expression of Spalt4 by electroporation is sufficient to induce invagination of non-placodal head ectoderm and prevent neurogenic placodes from contributing to cranial ganglia. Conversely, loss of Spalt4 function in the otic placode results in abnormal otic vesicle development. Intriguingly, Spalt4 appears to initiate a placode program appropriate for the axial level but is not involved in later development of specific placode fates. Fgfs can regulate Spalt4, since implantation of Fgf2 beads into the area opaca induces its expression. The results suggest that Spalt4 is involved in early stages of placode development, initiating cranial ectodermal invagination and region-specific gene regulatory networks (Barembaum, 2007).