Pax3 and the splotch mutations: structure, function, and relationship to teratogenesis, including gene-chemical interactions

Characterization of a novel missense mutation on murine Pax3 through ENU mutagenesis

N-ethyl-N-nitrosourea (ENU) mutagenesis has led to the elucidation of several regulator genes for melanocyte and skin development. Here we characterized a mutant from ENU mutagenesis with similar phenotype as that of Splotch mutant, including exencephaly, spina bifida and abnormal limbs in homozygotes as well as white belly spotting and occasionally loop-tail in heterozygotes. This novel mutant was named as Sp(xG). Through genome-wide linkage analysis in backcross progenies with microsatellite markers, the Sp(xG) was confined to a region between D1MIT415 and D1MIT7 on chromosome 1, where notable Pax3 gene was located. Direct sequencing revealed that Sp(xG) carried a nucleotide A894G missense transition in exon 6 of Pax3 gene that resulted in Asn to Asp substitution at amino acid 269 within the highly-conserved homeodomain (HD) DNA recognition module, which was the first point mutation found in this domain in mice. This N269D mutation impaired the transactivation capacity of Pax3 protein, but exerted no effect on Pax3 protein translation. The characterization of the new mutation expanded our understanding the transactivation and DNA-binding structure of Pax3 protein.

PC-TP/StARD2: Of membranes and metabolism

Phosphatidylcholine transfer protein (PC-TP, synonym StARD2) binds phosphatidylcholines, and catalyzes their intermembrane transfer and exchange in vitro. The structure of PC-TP comprises a hydrophobic pocket and a well-defined head group binding site, and its gene expression is regulated by peroxisome proliferator activated receptor-alpha. Recent studies have revealed key regulatory roles for PC-TP in lipid and glucose metabolism. Notably, Pctp(-/-) mice are sensitized to the action of insulin, and exhibit more efficient brown fat-mediated thermogenesis. PC-TP appears to limit access of fatty acids to mitochondria by stimulating the activity of thioesterase superfamily member 2, a newly characterized long-chain fatty acyl-coenzyme A thioesterase. Because PC-TP discriminates between phosphatidylcholines within lipid bilayers, it might function as a sensor that links metabolic regulation to membrane composition.

Identification of a novel nonsense mutation on the Pax3 gene in ENU-derived white belly spotting mice and its genetic interaction with c-Kit

In the course of a large-scale screening program of N-ethyl-N-nitrosourea mutagenesis, we isolated two semidominant mutation lines with white belly spotting, named as wps and wbs. Direct sequencing detected a nucleotide G-to-A transversion in exon 2 of the c-Kit gene in wps, which resulted in a missense D60N mutation. Another mutant, wbs, was mapped to chromosome 1 by genome-wide linkage analysis. In 93 meioses, the wbs locus was confined to a 5.2-Mb region between D1Mit380 and D1Mit215, including the Pax3 gene. A nonsense mutation K107X on the Pax3 coding region in wbs mice was identified, causing the loss of Pax3 protein in the homozygous mutant. We further demonstrated that Pax3 exhibited genetic interaction with c-Kit by intercrossing the wps and wbs mice. Further, Pax3 transactivated the c-Kit promoter in different cell lines. However, electrophoretic mobility shift assays showed that Pax3 did not bind to the c-Kit promoter, indicating that Pax3 may interact with c-Kit in an indirect way. This expands our understanding of the intricate regulatory network governing the melanocyte development.

Insights into prevention of human neural tube defects by folic acid arising from consideration of mouse mutants

Almost 30 years after the initial study by Richard W. Smithells and coworkers, it is still unknown how maternal periconceptional folic acid supplementation prevents human neural tube defects (NTDs). In this article, questions about human NTD prevention are considered in relation to three groups of mouse models: NTD mutants that respond to folate, NTD mutants and strains that do not respond to folate, and mutants involving folate-pathway genes. Of the 200 mouse NTD mutants, only a few have been tested with folate; half respond and half do not. Among responsive mutants, folic acid supplementation reduces exencephaly and/or spina bifida aperta frequency in the Sp(2H), Sp, Cd, Cited2, Cart1, and Gcn5 mutants. Prevention ranges from 35 to 85%. The responsive Sp(2H) (Pax3) mutant has abnormal folate metabolism, but the responsive Cited2 mutant does not. Neither folic nor folinic acid reduces NTD frequency in Axd, Grhl3, Fkbp8, Map3k4, or Nog mutants or in the curly tail or SELH/Bc strains. Spina bifida frequency is reduced in Axd by methionine and in curly tail by inositol. Exencephaly frequency is reduced in SELH/Bc by an alternative commercial ration. Mutations in folate-pathway genes do not cause NTDs, except for 30% exencephaly in folate-treated Folr1. Among folate-pathway mutants, neural tube closure is normal in Cbs, Folr2, Mthfd1, Mthfd2, Mthfr, and Shmt1 mutants. Embryos die by midgestation in Folr1, Mtr, Mtrr, and RFC1 mutants. The mouse models point to genetic heterogeneity in the ability to respond to folic acid and also to heterogeneity in genetic cause of NTDs that can be prevented by folic acid.

Activation of Pax3 target genes is necessary but not sufficient for neurogenesis in the ophthalmic trigeminal placode

Vertebrate cranial neurogenic placodes are relatively simple model systems for investigating the control of sensory neurogenesis. The ophthalmic trigeminal (opV) placode, for which the earliest specific marker is the paired domain homeodomain transcription factor Pax3, forms cutaneous sensory neurons in the ophthalmic lobe of the trigeminal ganglion. We previously showed that Pax3 expression in avian opV placode cells correlates with specification and commitment to a Pax3+, cutaneous sensory neuron fate. Pax3 can act as a transcriptional activator or repressor, depending on the cellular context. We show using mouse Splotch(2H) mutants that Pax3 is necessary for the normal neuronal differentiation of opV placode cells. Using an electroporation construct encoding a Pax3-Engrailed fusion protein, which represses Pax3 target genes, we show that activation of Pax3 target genes is required cell-autonomously within chick opV placode cells for expression of the opV placode markers FGFR4 and Ngn2, maintenance of the preplacodal marker Eya2, expression of Pax3 itself (suggesting that Pax3 autoregulates), neuronal differentiation and delamination. Mis-expression of Pax3 in head ectoderm is sufficient to induce FGFR4 and Ngn2 expression, but neurons do not differentiate, suggesting that additional signals are necessary to enable Pax3+ cells to differentiate as neurons. Mis-expression of Pax3 in the Pax2+ otic and epibranchial placodes also downregulates Pax2 and disrupts otic vesicle closure, suggesting that Pax3 is sufficient to alter the identity of these cells. Overall, our results suggest that activation of Pax3 target genes is necessary but not sufficient for neurogenesis in the opV placode.

Increased thymus- and decreased parathyroid-fated organ domains in Splotch mutant embryos

Embryos that are homozygous for Splotch, a null allele of Pax3, have a severe neural crest cell (NCC) deficiency that generates a complex phenotype including spina bifida, exencephaly and cardiac outflow tract abnormalities. Contrary to the widely held perception that thymus aplasia or hypoplasia is a characteristic feature of Pax3(Sp/Sp) embryos, we find that thymic rudiments are larger and parathyroid rudiments are smaller in E11.5-12.5 Pax3(Sp/Sp) compared to Pax3(+/+) embryos. The thymus originates from bilateral third pharyngeal pouch primordia containing endodermal progenitors of both thymus and parathyroid glands. Analyses of Foxn1 and Gcm2 expression revealed a dorsal shift in the border between parathyroid- and thymus-fated domains at E11.5, with no change in the overall cellularity or volume of each shared primordium. The border shift increases the allocation of third pouch progenitors to the thymus domain and correspondingly decreases allocation to the parathyroid domain. Initial patterning in the E10.5 pouch was normal suggesting that the observed change in the location of the organ domain interface arises during border refinement between E10.5 and E11.5. Given the well-characterized NCC defects in Splotch mutants, these findings implicate NCCs in regulating patterning of third pouch endoderm into thymus- versus parathyroid-specified domains, and suggest that organ size is determined in part by the number of progenitor cells specified to a given fate.

Gene-environment interactions in the causation of neural tube defects: folate deficiency increases susceptibility conferred by loss of Pax3 function

Risk of neural tube defects (NTDs) is determined by genetic and environmental factors, among which folate status appears to play a key role. However, the precise nature of the link between low folate status and NTDs is poorly understood, and it remains unclear how folic acid prevents NTDs. We investigated the effect of folate level on risk of NTDs in splotch (Sp(2)(H)) mice, which carry a mutation in Pax3. Dietary folate restriction results in reduced maternal blood folate, elevated plasma homocysteine and reduced embryonic folate content. Folate deficiency does not cause NTDs in wild-type mice, but causes a significant increase in cranial NTDs among Sp(2)(H) embryos, demonstrating a gene-environment interaction. Control treatments, in which intermediate levels of folate are supplied, suggest that NTD risk is related to embryonic folate concentration, not maternal blood folate concentration. Notably, the effect of folate deficiency appears more deleterious in female embryos than males, since defects are not prevented by exogenous folic acid. Folate-deficient embryos exhibit developmental delay and growth retardation. However, folate content normalized to protein content is appropriate for developmental stage, suggesting that folate availability places a tight limit on growth and development. Folate-deficient embryos also exhibit a reduced ratio of s-adenosylmethionine (SAM) to s-adenosylhomocysteine (SAH). This could indicate inhibition of the methylation cycle, but we did not detect any diminution in global DNA methylation, in contrast to embryos in which the methylation cycle was specifically inhibited. Hence, folate deficiency increases the risk of NTDs in genetically predisposed splotch embryos, probably via embryonic growth retardation.

Pax genes in embryogenesis and oncogenesis

The paired box genes are a family of nine developmental control genes, which in human beings (PAX) and mice (Pax) encode nuclear transcription factors. The temporal and spatial expressions of these highly conserved genes are tightly regulated during foetal development including organogenesis. PAY/Paxgenes are switched off during the terminal differentiation of most structures. Specific mutations within a number of PAX/Pax genes lead to developmental abnormalities in both human beings and mice. Mutation in PAX3 causes Waardenburg syndrome, and craniofacial-deafness-hand syndrome. The Splotch phenotype in mouse exhibits defects in neural crest derivatives such as, pigment cells, sympathetic ganglia and cardiac neural crest-derived structures. The PAX family also plays key roles in several human malignancies. In particular, PAX3 is involved in rhabdomyosarcoma and tumours of neural crest origin, including melanoma and neuroblastoma. This review critically evaluates the roles of PAX/Pax in oncogenesis. It especially highlights recent advances in knowledge of how their genetic alterations directly interfere in the transcriptional networks that regulate cell differentiation, proliferation, migration and survival and may contribute to oncogenesis.

Lineage-specific responses to reduced embryonic Pax3 expression levels

Pax3 is an essential paired- and homeodomain-containing transcription factor that is necessary for closure of the neural tube, and morphogenesis of the migratory neural crest and myoblast lineages. Homozygous loss-of-function mutation results in mid-gestational lethality with defects in myogenesis, neural tube closure and neural crest-derived lineages including melanocytes, Schwann cells and insufficient mesenchymal cells to septate the cardiac outflow tract. To address the function of Pax3 in later fetal stages and in specific adult tissues, we generated a floxed Pax3 allele (Pax3(flox)). An intermediate allele (Pax3(neo)) was produced via creation of the floxed allele, in which the TK-neo(R) cassette is present between exons 5 and 6. It was deduced to be a hypomorph, as Pax3 protein expression is reduced by 80% and homozygote hypomorphs die postnatally. To assess the consequences of reduced Pax3 levels on the various Pax3-expressing lineages and to determine the underlying cause of lethality, we examined Pax3 spatiotemporal expression and the resultant defects. Defective limb and tongue musculature were observed and lethality was due to an inability to suckle. However, the heart, diaphragm, trunk musculature, as well as the various neural crest-derived lineages and neural tube were all unaffected by reduced Pax3 levels. Significantly, elevated levels of the related Pax7 protein were present in unaffected neural tube and epaxial somatic component. The limb and tongue myogenic defects were found to be due to a significant increase in apoptosis within the somites that leads to a paucity of migratory hypaxial myoblasts. These effects were attributed to the hypomorphic effect of the Pax3(neo) allele, as removal of the TK-neo(R) cassette completely relieves the hypomorphic effect, as 100% of the Pax3(flox/flox) mice were normal. These data demonstrate a lineage-specific response to approximately 80% loss of Pax3 protein expression, with myogenesis of limb and tongue being most sensitive to reduced Pax3 levels. Thus, we have established that there are different minimum threshold requirements for Pax3 within different Pax3-expressing lineages.

Interacting proteins dictate function of the minimal START domain phosphatidylcholine transfer protein/StarD2

The Star (steroidogenic acute regulatory protein)-related transfer (START) domain superfamily is characterized by a distinctive lipid-binding motif. START domains typically reside in multidomain proteins, suggesting their function as lipid sensors that trigger biological activities. Phosphatidylcholine transfer protein (PC-TP, also known as StarD2) is an example of a START domain minimal protein that consists only of the lipid-binding motif. PC-TP, which binds phosphatidylcholine exclusively, is expressed during embryonic development and in several tissues of the adult mouse, including liver. Although it catalyzes the intermembrane exchange of phosphatidylcholines in vitro, this activity does not appear to explain the various metabolic alterations observed in mice lacking PC-TP. Here we demonstrate that PC-TP function may be mediated via interacting proteins. Yeast two-hybrid screening using libraries prepared from mouse liver and embryo identified Them2 (thioesterase superfamily member 2) and the homeodomain transcription factor Pax3 (paired box gene 3), respectively, as PC-TP-interacting proteins. These were notable because the START domain superfamily contains multidomain proteins in which the START domain coexists with thioesterase domains in mammals and with homeodomain transcription factors in plants. Interactions were verified in pulldown assays, and colocalization with PC-TP was confirmed within tissues and intracellularly. The acyl-CoA thioesterase activity of purified recombinant Them2 was markedly enhanced by recombinant PC-TP. In tissue culture, PC-TP coactivated the transcriptional activity of Pax3. These findings suggest that PC-TP functions as a phosphatidylcholine-sensing molecule that engages in diverse regulatory activities that depend upon the cellular expression of distinct interacting proteins.

Mouse mutants with neural tube closure defects and their role in understanding human neural tube defects

BACKGROUND

The number of mouse mutants and strains with neural tube closure defects (NTDs) now exceeds 190, including 155 involving known genes, 33 with unidentified genes, and eight "multifactorial" strains.

METHODS

The emerging patterns of mouse NTDs are considered in relation to the unknown genetics of the common human NTDs, anencephaly, and spina bifida aperta.

RESULTS

Of the 150 mouse mutants that survive past midgestation, 20% have risk of either exencephaly and spina bifida aperta or both, parallel to the majority of human NTDs, whereas 70% have only exencephaly, 5% have only spina bifida, and 5% have craniorachischisis. The primary defect in most mouse NTDs is failure of neural fold elevation. Most null mutations (>90%) produce syndromes of multiple affected structures with high penetrance in homozygotes, whereas the "multifactorial" strains and several null-mutant heterozygotes and mutants with partial gene function (hypomorphs) have low-penetrance nonsyndromic NTDs, like the majority of human NTDs. The normal functions of the mutated genes are diverse, with clusters in pathways of actin function, apoptosis, and chromatin methylation and structure. The female excess observed in human anencephaly is found in all mouse exencephaly mutants for which gender has been studied. Maternal agents, including folate, methionine, inositol, or alternative commercial diets, have specific preventative effects in eight mutants and strains.

CONCLUSIONS

If the human homologs of the mouse NTD mutants contribute to risk of common human NTDs, it seems likely to be in multifactorial combinations of hypomorphs and low-penetrance heterozygotes, as exemplified by mouse digenic mutants and the oligogenic SELH/Bc strain.

Thymus organogenesis and development of the thymic stroma

T-cell development occurs principally in the thymus. Here, immature progenitor cells are guided through the differentiation and selection steps required to generate a complex T-cell repertoire that is both self-tolerant and has propensity to bind self major histocompatibility complex. These processes depend on an array of functionally distinct epithelial cell types within the thymic stroma, which have a common developmental origin in the pharyngeal endoderm. Here, we describe the structural and phenotypic attributes of the thymic stroma, and review current cellular and molecular understanding of thymus organogenesis.

Impact of methylenetetrahydrofolate reductase deficiency and low dietary folate on the development of neural tube defects in splotch mice

BACKGROUND

The etiology of neural tube defects (NTDs) is multifactorial, with environmental and genetic determinants. Folate supplementation prevents the majority of NTDs, and a polymorphism in methylenetetrahydrofolate reductase (MTHFR) has become recognized as a genetic risk factor. The mechanisms by which folate affects NTD development are unclear. The Splotch (Sp) mouse is a well-characterized mouse model for studying spontaneous NTDs. To assess the potential interaction between folate metabolism and the Sp mutant in NTD development, we studied mice with both Sp and Mthfr mutations, as well as the interaction between Sp and low dietary folate.

METHODS

Wild-type, single Mthfr+/-mutant, single Sp/+mutant, and double mutant (Mthfr+/-, Sp/+) female mice were mated with males of the same genotype. Embryos were examined for NTDs on gestational day (GD) 13.5. To investigate the effects of folate deficiency on Sp mice, Sp/+female mice were fed a control diet (CD), a moderately folic acid-deficient diet (MFADD), or a severely folic acid-deficient diet (SFADD). They were mated with Sp/+males and the embryos were examined.

RESULTS

There were no differences in the incidence or severity of NTDs in embryos from double-mutant mating pairs compared to those from single Sp mutants. Embryos from Mthfr+/-dams did not exhibit NTDs. Diets deficient in folate did not influence the incidence or severity of NTDs in embryos from Sp/+mice.

CONCLUSIONS

We did not observe an interaction between Sp and Mthfr mutations, or between the Sp mutation and low dietary folate, in NTD development in Splotch mice.

Investigations into the etiology of neural tube defects

Neural tube defects (NTDs) are serious malformations affecting approximately 1 per 1000 births, yet the mechanisms by which they arise are unknown. There have been consistent efforts in many fields of research to elucidate the etiology of this multifactorial condition. While no single gene has been identified as a major independent risk factor for NTDs, candidate genes have been proposed that may modify the effects of maternal and/or embryonic exposures. Folate supplementation effectively reduces the occurrence of NTDs and, consequently, has focused much research on metabolism of folate-related pathways during pregnancy and development. Further understanding of normal development and how teratogens can perturb these orchestrated processes also remains at the fore of modern scientific endeavors. The composite of these factors remains fragmented; the aim of this review is to provide the reader with a summary of sentinel and current works in the body of literature addressing NTD disease etiology.

Effect of arsenite, maternal age, and embryonic sex on spina bifida, exencephaly, and resorption rates in the splotch mouse

BACKGROUND

This study examines interactions of a mutation in Pax3, embryonic sex, advanced maternal age, and arsenite exposure in the splotch (Sp) mouse model, with the aim of describing gene-environment interactions for neural tube defects and embryonic lethality.

METHODS

Splotch heterozygous C57BL/6J mice were crossed to produce offspring of three genotypes with a common maternal genotype that were exposed to either sodium arsenite on gestational day (GD) 8.0, or advanced maternal age (dams older than 12 months). Embryos were extracted on GD 12 and genotyped for both Pax3 and sex.

RESULTS

Arsenite treatment was a significant contributor to both exencephaly and spina bifida. Advanced maternal age resulted in a high exencephaly rate in Sp/Sp female embryos (but not other genotypes) and a high overall resorption rate. Arsenite treatment and advanced maternal age resulted in elevated sex ratios (male:female) for heterozygous and wild-type embryos. The sex ratio was highest for wild-type embryos and was lowered as the number of mutant Pax3 alleles increased. The sex ratio was not significantly different from 1.0 for splotch homozygotes. Control litters had spina bifida rates that were 95% in homozygous, 6% in heterozygous, and 0% in wild-type embryos.

CONCLUSIONS

If arsenite produces exencephaly by inactivating the Pax3 protein, then the fact that the exencephaly rate was increased in Sp/Sp embryos with no functional Pax3 indicates that arsenite may either induce this defect through additional pathways, or may alter the response via modifier genes. Genetic and environmental factors contributed to the determination of murine sex ratios, with female embryos being more susceptible to loss.

Quantification and localization of expression of the retinoic acid receptor-beta and -gamma mRNA isoforms during neurulation in mouse embryos with or without spina bifida

BACKGROUND

Previous studies observed that retinoic acid receptor-gamma (RARgamma) is expressed in the open caudal neuroepithelium but that RARbeta is expressed in the closed neural tube. Furthermore, retinoic acid (RA) induces RARbeta expression, a molecular event associated with neural tube closure, but treatment with RA at the appropriate gestation time causes failure of neural tube closure. Since there are four isoforms of RARbeta, perhaps the isoforms expressed in the closed neural tube and induced by RA are different. To investigate the hypothesis that the switch from RARgamma to RARbeta is mechanistically linked to neural tube closure, this study determined the concentrations and distributions of RARbeta and RARgamma isoforms in mouse embryos with RA-induced neural tube defects and in splotch (Sp) mutant embryos with spina bifida.

METHODS

Absolute concentrations of RARbeta and RARgamma isoforms were determined throughout primary neurulation (gestational day 8.5-10.0) in treated or untreated C57BL/6J mouse whole embryos by ribonuclease protection analysis. Treatment consisted of an oral dose of 100 mg/kg of all-trans-RA on gestational day 8.5. Spatial distributions of RARbeta and RARgamma were examined in RA-treated and Sp mutant embryos by in situ hybridization.

RESULTS

RARbeta2, gamma1, and gamma2 were expressed in untreated embryos and were induced 4.5-, 1.6-, and 4.0-fold, respectively, 4 hr after treatment with RA. In embryos with RA-induced spina bifida, RARbeta2 was expressed in the closed neural tube while RARgamma1 and RARgamma2 were expressed in the open caudal neuroepithelium. In splotch mice with spina bifida, the boundary between RARbeta and RARgamma did not correspond to the site of neural tube closure.

CONCLUSIONS

In RA-treated embryos, the relationship between RARbeta expression in the closed and RARgamma in the open caudal neuroepithelium was not altered. However, in splotch embryos with spina bifida, the juncture between RARbeta and RARgamma expression remained in the same anatomical position in the neuroepithelium irrespective of the neural tube closure status and suggests that the switch from RARgamma to RARbeta expression in the closing caudal neuroepithelium may not be causally linked to neural tube closure in the splotch mutant.

Gene expression profiling within the developing neural tube

The developing mammalian nervous system is subject to devastating congenital malformations with clinical significance that extends into the billions of health care dollars annually worldwide. Neural tube defects (NTDs) are among the most common of all human congenital defects, yet their etiology remains poorly understood. This is largely due to the complexity of the genetic factors regulating the intricate events involved in neurulation. Using mouse model systems and the application of modern molecular biological technologies, we have recently gained a greater appreciation for the factors that not only regulate normal neural tube closure (NTC), but those genetic factors that predispose an embryo to significant birth defects such as anencephaly or spina bifida. We have selected prominent murine mutants, both spontaneous and genetically modified, as well as the use of teratogenic agents, to examine the impact of altering the normal pattern of gene expression in the developing neural tube.

Msx2is an immediate downstream effector ofPax3in the development of the murine cardiac neural crest

The neural crest plays a crucial part in cardiac development. Cells of the cardiac subpopulation of cranial neural crest migrate from the hindbrain into the outflow tract of the heart where they contribute to the septum that divides the pulmonary and aortic channels. In Splotch mutant mice, which lack a functional Pax3 gene, migration of cardiac neural crest is deficient and aorticopulmonary septation does not occur. Downstream genes through which Pax3 regulates cardiac neural crest development are unknown. Here, using a combination of genetic and molecular approaches, we show that the deficiency of cardiac neural crest development in the Splotch mutant is caused by upregulation of Msx2, a homeobox gene with a well-documented role as a regulator of BMP signaling. We provide evidence, moreover, that Pax3 represses Msx2 expression via a direct effect on a conserved Pax3 binding site in the Msx2 promoter. These results establish Msx2 as an effector of Pax3 in cardiac neural crest development.