Immunolocalization of fibroblast growth factor receptors 1 and 2 in mouse palate development

Genome-wide Gene-by-Sex Interaction Studies Identify Novel Nonsyndromic Orofacial Clefts Risk Locus

Risk loci identified through genome-wide association studies have explained about 25% of the phenotypic variations in nonsyndromic orofacial clefts (nsOFCs) on the liability scale. Despite the notable sex differences in the incidences of the different cleft types, investigation of loci for sex-specific effects has been understudied. To explore the sex-specific effects in genetic etiology of nsOFCs, we conducted a genome-wide gene × sex (GxSex) interaction study in a sub-Saharan African orofacial cleft cohort. The sample included 1,019 nonsyndromic orofacial cleft cases (814 cleft lip with or without cleft palate and 205 cleft palate only) and 2,159 controls recruited from 3 sites (Ethiopia, Ghana, and Nigeria). An additive logistic model was used to examine the joint effects of the genotype and GxSex interaction. Furthermore, we examined loci with suggestive significance (P < 1E-5) in the additive model for the effect of the GxSex interaction only. We identified a novel risk locus on chromosome 8p22 with genome-wide significant joint and GxSex interaction effects (rs2720555, p_2df = 1.16E-08, p_GxSex = 1.49E-09, odds ratio [OR] = 0.44, 95% CI = 0.34 to 0.57). For males, the risk of cleft lip with or without cleft palate at this locus decreases with additional copies of the minor allele (p < 0.0001, OR = 0.60, 95% CI = 0.48 to 0.74), but the effect is reversed for females (p = 0.0004, OR = 1.36, 95% CI = 1.15 to 1.60). We replicated the female-specific effect of this locus in an independent cohort (p = 0.037, OR = 1.30, 95% CI = 1.02 to 1.65), but no significant effect was found for the males (p = 0.29, OR = 0.86, 95% CI = 0.65 to 1.14). This locus is in topologically associating domain with craniofacially expressed and enriched genes during embryonic development. Rare coding mutations of some of these genes were identified in nsOFC cohorts through whole exome sequencing analysis. Our study is additional proof that genome-wide GxSex interaction analysis provides an opportunity for novel findings of loci and genes that contribute to the risk of nsOFCs.

Genetics and signaling mechanisms of orofacial clefts

Craniofacial development involves several complex tissue movements including several fusion processes to form the frontonasal and maxillary structures, including the upper lip and palate. Each of these movements are controlled by many different factors that are tightly regulated by several integral morphogenetic signaling pathways. Subject to both genetic and environmental influences, interruption at nearly any stage can disrupt lip, nasal, or palate fusion and result in a cleft. Here, we discuss many of the genetic risk factors that may contribute to the presentation of orofacial clefts in patients, and several of the key signaling pathways and underlying cellular mechanisms that control lip and palate formation, as identified primarily through investigating equivalent processes in animal models, are examined.

Transmission analysis of TGFB1 gene polymorphisms in non-syndromic cleft lip with or without cleft palate

OBJECTIVES

Transforming growth factor beta1 (TGF-β1) plays a significant role in craniofacial development. Previous linkage studies reported that the TGF-β1-locus at 19q13.1 harbour predisposing genes for non-syndromic oral clefts. In the present study case parents triads were evaluated to find the transmission effects of genetic variants in TGF- β1 towards non-syndromic cleft lip or palate (NSCL/P).

METHODS

Using allelic discrimination method148 families (case-parent triads) were assessed for single nucleotide polymorphisms (SNPs) in TGF-β1 gene. The SNPs were checked for mendelian errors and Hardy-Weinberg equilibrium (HWE). Transmission disequilibrium test and haplotype frequencies were estimated.

RESULTS

The TGF-β1 SNPs showed very low minor allele frequencies (MAFs) and observed heterozygosity (Hobs). The transmission disequilibrium test (TDT) and parent-of-origin likelihood ratio tests (PO-LRT) were not significant for any of the SNPs tested. Strong linkage disequilibrium (r² = 0.722) was found between rs1800469 and rs1800470 SNPs. Haplotype analysis ignoring parent of origin showed strong evidence of excess transmission but it is not significant (p-value = 0.293).

CONCLUSION

Transmission of minor alleles were not observed from either parent indicating that the TGF-β1 gene polymorphisms by themselves do not confer risk for non-syndromic oral clefts but, rather, modify the stability and the activation process of TGF-β1. As the number of families included in the study are less, results must be considered still preliminary and require replication using more families.

Gene Expression during Palate Fusion in vivo and in vitro

Failure of secondary palate fusion during embryogenesis is a cause of cleft palate. Disappearance of the medial epithelial seam (MES) is required to allow merging of the mesenchyme from both palatal shelves. This involves complex changes of the medial edge epithelial (MEE) cells and surrounding structures that are controlled by several genes whose spatio-temporal expression is tightly regulated. We have carried out morphological analyses and used a semi-quantitative RT-PCR technique to evaluate whether morphological changes and modulation in the expression of putative key genes, such as twist, snail, and E-cadherin, during the fusion process in palate organ culture parallel those observed in vivo, and show that this is indeed the case. We also show, using the organotypic model of palate fusion, that the down-regulation of the transcription factor snail that occurs with the progression of palate development is not dependent on fusion of the palatal shelves. Abbreviations: dsg1, desmoglein1; EMT, epithelial-mesenchymal transition; MEE, medial edge epithelium; MES, medial epithelial seam; RT-PCR, reverse-transcriptase polymerase chain-reaction.

Further evidence suggesting a role for variation in ARHGAP29 variants in nonsyndromic cleft lip/palate

BACKGROUND

Nonsyndromic cleft lip with or without cleft palate (NSCL/P) is a common birth defect of complex etiology. Several genes have been implicated in the etiology of NSCL/P, although only a few have been replicated across datasets.

METHODS

ARHGAP29 was suggested as a candidate gene for NSCL/P as it is located in close proximity to ABCA4 (1p22), a gene previously identified in a genome-wide association study of NSCL/P.

RESULTS

Rare, potentially damaging, coding variants in ARHGAP29 were found in NSCL/P cases, and its expression was detected during murine craniofacial development. In this study, we investigated whether variations in ARHGAP29 were associated with NSCL/P in our family based dataset. Five single-nucleotide polymorphisms (SNPs) flanking and within ARHGAP29 were genotyped in our NSCL/P datasets consisting of simplex and multiplex families of non-Hispanic white (NHW, primarily European) and Hispanic ethnicities. Results showed strong association of three ARHGAP29 SNPs with NSCL/P in the NHW families. Two intronic SNPs (rs1541098 and rs3789688) showed strong association with NSCL/P in all NHW families (p = 0.0005 and p = 0.0002, respectively), and simplex NHW families (p = 0.003 for both SNPs). A SNP in the 3' untranslated region (rs1576593) also showed strong association with NSCL/P in all NHW families (p = 0.002), and the multiplex subset (p = 0.002). ARHGAP29 SNP haplotypes were also associated with NSCL/P. Evidence of gene-gene interaction was found between ARHGAP29 and additional cleft susceptibility genes.

CONCLUSION

This study further supports ARHGAP29 as a candidate gene for human NSCL/P in families of Caucasian descent.

Type 1 fibroblast growth factor receptor in cranial neural crest cell-derived mesenchyme is required for palatogenesis

Cleft palate is a common congenital birth defect. The fibroblast growth factor (FGF) family has been shown to be important for palatogenesis, which elicits the regulatory functions by activating the FGF receptor tyrosine kinase. Mutations in Fgf or Fgfr are associated with cleft palate. To date, most mechanistic studies on FGF signaling in palate development have focused on FGFR2 in the epithelium. Although Fgfr1 is expressed in the cranial neural crest (CNC)-derived palate mesenchyme and Fgfr1 mutations are associated with palate defects, how FGFR1 in palate mesenchyme regulates palatogenesis is not well understood. Here, we reported that by using Wnt1(Cre) to delete Fgfr1 in neural crest cells led to cleft palate, cleft lip, and other severe craniofacial defects. Detailed analyses revealed that loss-of-function mutations in Fgfr1 did not abrogate patterning of CNC cells in palate shelves. However, it upset cell signaling in the frontofacial areas, delayed cell proliferation in both epithelial and mesenchymal compartments, prevented palate shelf elevation, and compromised palate shelf fusion. This is the first report revealing how FGF signaling in CNC cells regulates palatogenesis.

Perturbation of Fgf10 signal pathway in mouse embryonic palate by dexamethasone and vitamin B12 in vivo

BACKGROUND/PURPOSE

The Fgf10 signaling pathway plays an important role in early stages of mouse embryonic palatal development, which is associated with cell proliferation and differentiation. The objective of this study was to assess whether dexamethasone and vitamin B(12) affected the Fgf10 signal pathway of mouse embryonic palate.

MATERIALS AND METHODS

Immunohistochemical studies were performed for expression of Fgf10, Fgfr2b, and sonic hedgehog and for cell proliferation and apoptosis of mouse embryonic palate.

RESULTS

The expression of Fgf10, Fgfr2b, and sonic hedgehog was changed in mouse embryonic palate after dexamethasone and vitamin B(12) treatment, resulting in reduced and restored proliferation of mesenchymal cells.

CONCLUSIONS

Dexamethasone and vitamin B(12) affected the Fgf10 signaling pathway and cell proliferation of mouse embryonic palate. Cell apoptosis was not altered after dexamethasone and vitamin B(12) exposure.

Palate morphogenesis: current understanding and future directions

In the past, most scientists conducted their inquiries of nature via inductivism, the patient accumulation of "pieces of information" in the pious hope that the sum of the parts would clarify the whole. Increasingly, modern biology employs the tools of bioinformatics and systems biology in attempts to reveal the "big picture." Most successful laboratories engaged in the pursuit of the secrets of embryonic development, particularly those whose research focus is craniofacial development, pursue a middle road where research efforts embrace, rather than abandon, what some have called the "pedestrian" qualities of inductivism, while increasingly employing modern data mining technologies. The secondary palate has provided an excellent paradigm that has enabled examination of a wide variety of developmental processes. Examination of cellular signal transduction, as it directs embryogenesis, has proven exceptionally revealing with regard to clarification of the "facts" of palatal ontogeny-at least the facts as we currently understand them. Herein, we review the most basic fundamentals of orofacial embryology and discuss how functioning of TGFbeta, BMP, Shh, and Wnt signal transduction pathways contributes to palatal morphogenesis. Our current understanding of palate medial edge epithelial differentiation is also examined. We conclude with a discussion of how the rapidly expanding field of epigenetics, particularly regulation of gene expression by miRNAs and DNA methylation, is critical to control of cell and tissue differentiation, and how examination of these epigenetic processes has already begun to provide a better understanding of, and greater appreciation for, the complexities of palatal morphogenesis.

Biological mechanisms in palatogenesis and cleft palate

Clefts of the palate are common birth defects requiring extensive treatment. They appear to be caused by multiple genetic and environmental factors during palatogenesis. This may result in local changes in growth factors, extracellular matrix (ECM), and cell adhesion molecules. Several clefting factors have been implicated by studies in mouse models, while some of these have also been confirmed by genetic screening in humans. Here, we discuss several knockout mouse models to examine the role of specific genes in cleft formation. The cleft is ultimately caused by interference with shelf elevation, attachment, or fusion. Shelf elevation is brought about by mesenchymal proliferation and changes in the ECM induced by growth factors such as TGF-betas. Crucial ECM molecules are collagens, proteoglycans, and glycosaminoglycans. Shelf attachment depends on specific differentiation of the epithelium involving TGF-beta3, sonic hedgehog, and WNT signaling, and correct expression of epithelial adhesion molecules such as E-cadherin. The final fusion requires epithelial apoptosis and epithelium-to-mesenchyme transformation regulated by TGF-beta and WNT proteins. Other factors may interact with these signaling pathways and contribute to clefting. Normalization of the biological mechanisms regulating palatogenesis in susceptible fetuses is expected to contribute to cleft prevention.

Morphological and immunohistochemical studies on cleft palates induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin in mice

Morphological and immunohistological examinations were performed to reveal the mechanisms of cleft palate induction by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). ICR strain mice 8-10 weeks of age were used in the study. TCDD was administered in olive oil on gestation day (GD) 12.5 with gastric tubes at 40 microg/kg. From GD 13.5 to 16.5, palates were examined by scanning electron microscopy (SEM), hematoxyline-eosin (HE) staining, and immunohistochemical staining of FGFR1/2, TGF-beta3, MSX1 and LHX8. In the control group, both of the palatal shelves began elevating on GD 14.0 and finished within 6 h. After the elevation, all of the shelves had completely fused with each other on GD 14.5. In the TCDD-treated group, palatal shelves elevated 1 day later than in the control group. However, all palates had elevated by GD 15.0. After the elevation, the shelves contacted each other and fused; however, they were separated on GD16.0. HE staining showed that medial edge epithelium (MEE) was thinner in the TCDD group than in the control group. MEE observed under a high magnification (x2500) exhibited filopodia-like filaments and the cells were bulged in the control group. In contrast, in the TCDD group, no filaments were observed and the cells were flat with unclear boundaries. Immunohistologically, there were no characteristic findings except for FGFR1. FGFR1 was not expressed in the TCDD group after the fusion phase (GD 14.5). TCDD induces many morphological and molecular changes to MEE cells and causes cleft palates.

FGF signalling in craniofacial development and developmental disorders

The Fgf signalling pathway is highly conserved in evolution and plays crucial roles in development. In the craniofacial region, it is involved in almost all structure development from early patterning to growth regulation. In craniofacial skeletogenesis, the Fgf signal pathway plays important roles in suture and synchondrosis regulation. Mutations of FGF receptors relate to syndromatic and non-syndromatic craniosynostosis. The Fgf10/Fgfr2b signal loop is critical for palatogenesis and submandibular gland formation. Perturbation of the Fgf signal is a possible mechanism of palatal cleft. Fgf10 haploinsufficiency has been identified as the cause of autosomal dominant aplasia of lacrimal and salivary glands. The Fgf signal is also a key regulator of tooth formation: in the absence of Fgfr2b tooth development is arrested at the bud stage. Fgfr4 has recently been identified as the key signal mediator in myogenesis. In this review, these aspects are discussed in detail with a focus on the most recent advances.

The Teratogenic Sensitivity to 2,3,7,8-Tetrachlorodibenzo-p-dioxin Is Modified by a Locus on Mouse Chromosome 3

In an effort to understand how genetics can influence individual sensitivity to environmentally induced disease, we performed a linkage analysis to identify murine loci in addition to the Ahr locus that influence the incidence of cleft palate and hydronephrosis in developing mice exposed to the pollutant 2,3,7,8-tetrachlorodibenzo-p-dioxin (dioxin). Administration of 64 microg/kg dioxin to C57BL/6J (B6) dams at embryonic day 9 (E9) led to palatal clefting and hydronephrosis in nearly 100% of embryos by E17. In contrast, similar exposure of CBA/J (CBA) dams led to cleft palate in only 8% and hydronephrosis in 69% of embryos. To determine the genetic basis for this strain-dependent sensitivity, linkage analyses on the progeny of a B6CBAF1 intercross and a CBAxB6CBAF1 backcross were performed. The incidences of cleft palate and hydronephrosis were assessed and genomic DNA from embryos was analyzed at informative simple sequence length polymorphism (SSLP) markers. One locus segregating with dioxin-induced cleft palate was identified (p < 0.01) and designated as chemically mediated teratogenesis number 1 (Cmt1). The Cmt1 locus is located on chromosome 3.

Regional regulation of palatal growth and patterning along the anterior-posterior axis in mice

Cleft palate is a congenital disorder arising from a failure in the multistep process of palate development. In its mildest form the cleft affects only the posterior soft palate. In more severe cases the cleft includes the soft (posterior) and hard (anterior) palate. In mice a number of genes show differential expression along the anterior-posterior axis of the palate. Mesenchymal heterogeneity is established early, as evident from Bmp4-mediated induction of Msx1 and cell proliferation exclusively in the anterior and Fgf8-specific induction of Pax9 in the posterior palate alone. In addition, the anterior palatal epithelium has the unique ability to induce Shox2 expression in the anterior mesenchyme in vivo and the posterior mesenchyme in vitro. Therefore, the induction and competence potentials of the epithelium and mesenchyme in the anterior are clearly distinct from those in the posterior. Defective growth in the anterior palate of Msx1-/- and Fgf10-/- mice leads to a complete cleft palate and supports the anterior-to-posterior direction of palatal closure. By contrast, the Shox2-/- mice exhibit incomplete clefts in the anterior presumptive hard palate with an intact posterior palate. This phenotype cannot be explained by the prevailing model of palatal closure. The ability of the posterior palate to fuse independent of the anterior palate in Shox2-/- mice underscores the intrinsic differences along the anterior-posterior axis of the palate. We must hitherto consider the heterogeneity of gene expression and function in the palate to understand better the aetiology and pathogenesis of non-syndromic cleft palate and the mechanics of normal palatogenesis.

Shox2-deficient mice exhibit a rare type of incomplete clefting of the secondary palate

The short stature homeobox gene SHOX is associated with idiopathic short stature in humans, as seen in Turner syndrome and Leri-Weill dyschondrosteosis, while little is known about its close relative SHOX2. We report the restricted expression of Shox2 in the anterior domain of the secondary palate in mice and humans. Shox2-/- mice develop an incomplete cleft that is confined to the anterior region of the palate, an extremely rare type of clefting in humans. The Shox2-/- palatal shelves initiate, grow and elevate normally, but the anterior region fails to contact and fuse at the midline, owing to altered cell proliferation and apoptosis, leading to incomplete clefting within the presumptive hard palate. Accompanied with these cellular alterations is an ectopic expression of Fgf10 and Fgfr2c in the anterior palatal mesenchyme of the mutants. Tissue recombination and bead implantation experiments revealed that signals from the anterior palatal epithelium are responsible for the restricted mesenchymal Shox2 expression. BMP activity is necessary but not sufficient for the induction of palatal Shox2 expression. Our results demonstrate an intrinsic requirement for Shox2 in palatogenesis, and support the idea that palatogenesis is differentially regulated along the anteroposterior axis. Furthermore, our results demonstrate that fusion of the posterior palate can occur independently of fusion in the anterior palate.

Rescue of an in vitro palate nonfusion model using interposed embryonic mesenchyme

The authors previously established an in vitro palate nonfusion model on the basis of a spatial separation between prefusion embryonic day 13.5 mouse palates (term gestation, 19.5 days). They found that an interpalatal separation distance of 0.48 mm or greater would consistently result in nonfusion after 4 days in organ culture. In the present study, they interposed embryonic palatal mesenchymal tissue between embryonic day 13.5 mouse palatal shelves with interpalatal separation distances greater than 0.48 mm in an attempt to "rescue" this in vitro palate nonfusion phenotype. Because no medial epithelial bilayer (i.e., medial epithelial seam) could potentially form, palatal fusion in vitro was defined as intershelf mesenchymal continuity with resolution of the medial edge epithelia bilaterally. Forty-two (n = 42) palatal shelf pairs from embryonic day 13.5 CD-1 mouse embryos were isolated and placed on cell culture inserts at precisely graded distances (0, 0.67, and 0.95 mm). Positive controls consisted of shelves placed in contact (n = 6). Negative controls consisted of shelves placed at interpalatal separation distances of 0.67 mm (n = 6) and 0.95 mm (n = 7) with no interposed mesenchyme. Experimental groups consisted of embryonic day 13.5 palatal shelves separated by 0.67 mm (n = 11) and 0.95 mm (n = 12) with interposed lateral palatal mesenchyme isolated at the time of palatal shelf harvest. Specimens were cultured for 4 days (n = 19) or 10 days (n = 23), harvested, and evaluated histologically. All positive controls at 4 and 10 days in culture showed complete histologic palatal fusion. All negative controls at 4 days and 10 days in culture remained unfused. Five of six palatal shelves separated at 0.67 mm interpalatal separation distance with interposed mesenchyme were fused at 4 days, and all five were fused at 10 days. At an interpalatal separation distance of 0.95 mm with interposed mesenchyme (n = 12), no palates (zero of four) were fused at 4 days, but seven of eight were fused at 10 days. These data suggest that nonfused palatal shelves can be "rescued" with an interposed graft of endogenous embryonic mesenchyme to induce fusion in vitro.