A TWIST in development

Retinoic acid induced specific changes in the phosphoproteome of C17.2 neural stem cells

Retinoic acid (RA), a vitamin A derivative, is an effective cell differentiating factor which plays critical roles in neuronal differentiation induction and the production of neurotransmitters in neurons. However, the specific changes in phosphorylation levels and downstream signalling pathways associated with RA remain unclear. This study employed qualitative and quantitative phosphoproteomics approaches based on mass spectrometry to investigate the phosphorylation changes induced by RA in C17.2 neural stem cells (NSCs). Dimethyl labelling, in conjunction with TiO2 phosphopeptide enrichment, was utilized to profile the phosphoproteome of self-renewing and RA-induced differentiated cells in C17.2 NSCs. The results of our study revealed that, qualitatively, 230 and 14 phosphoproteins were exclusively identified in the self-renewal and RA-induced groups respectively. Quantitatively, we successfully identified and quantified 177 unique phosphoproteins, among which 70 exhibited differential phosphorylation levels. Analysis of conserved phosphorylation motifs demonstrated enrichment of motifs corresponding to cyclin-dependent kinase and MAPK in the RA-induced group. Additionally, through a comprehensive literature and database survey, we found that the differentially expressed proteins were associated with the Wnt/β-catenin and Hippo signalling pathways. This work sheds light on the changes in phosphorylation levels induced by RA in C17.2 NSCs, thereby expanding our understanding of the molecular mechanisms underlying RA-induced neuronal differentiation.

Twist alters the breast tumor microenvironment via choline kinase to facilitate an aggressive phenotype

Twist (TWIST1) is a gene required for cell fate specification in embryos and its expression in mammary epithelium can initiate tumorigenesis through the epithelial-mesenchymal transition. To identify downstream target genes of Twist in breast cancer, we performed microarray analysis on the transgenic breast cancer cell line, MCF-7/Twist. One of the targets identified was choline kinase whose upregulation resulted in increased cellular phosphocholine and total choline containing compounds-a characteristic observed in highly aggressive metastatic cancers. To study the interactions between Twist, choline kinase, and their effect on the microenvironment, we used 1H magnetic resonance spectroscopy and found significantly higher phosphocholine and total choline, as well as increased phosphocholine/glycerophosphocholine ratio in MCF-7/Twist cells. We also observed significant increases in extracellular glucose, lactate, and [H +] ion concentrations in the MCF-7/Twist cells. Magnetic resonance imaging of MCF-7/Twist orthotopic breast tumors showed a significant increase in vascular volume and permeability surface area product compared to control tumors. In addition, by reverse transcription-quantitative polymerase chain reaction, we discovered that Twist upregulated choline kinase expression in estrogen receptor negative breast cancer cell lines through FOXA1 downregulation. Moreover, using The Cancer Genome Atlas database, we observed a significant inverse relationship between FOXA1 and choline kinase expression and propose that it could act as a modulator of the Twist/choline kinase axis. The data presented indicate that Twist is a driver of choline kinase expression in breast cancer cells via FOXA1 resulting in the generation of an aggressive breast cancer phenotype.

Protein kinase Cα-mediated phosphorylation of Twist1 at Ser-144 prevents Twist1 ubiquitination and stabilizes it

Twist1 is a basic helix-loop-helix transcription factor that plays a key role in embryonic development, and its expression is down-regulated in adult cells. However, Twist1 is highly expressed during cancer development, conferring a proliferative, migratory, and invasive phenotype to malignant cells. Twist1 expression can be regulated post-translationally by phosphorylation or ubiquitination events. We report in this study a previously unknown and relevant Twist1 phosphorylation site that controls its stability. To identify candidate phosphorylation sites in Twist1, we first conducted an in silico analysis of the Twist1 protein, which yielded several potential sites. Because most of these sites were predicted to be phosphorylated by protein kinase C (PKC), we overexpressed PKCα in several cell lines and found that it phosphorylates Twist1 on Ser-144. Using a combination of immunoblotting, immunoprecipitation, protein overexpression, and CRISPR/Cas9-mediated PKCα knockout experiments, we observed that PKCα-mediated Twist1 phosphorylation at Ser-144 inhibits Twist1 ubiquitination and consequently stabilizes it. These results provide evidence for a direct association between PKCα and Twist1 and yield critical insights into the PKCα/Twist1 signaling axis that governs cancer aggressiveness.

Twist1-Haploinsufficiency Selectively Enhances the Osteoskeletal Capacity of Mesoderm-Derived Parietal Bone Through Downregulation of Fgf23

Craniofacial development is a program exquisitely orchestrated by tissue contributions and regulation of genes expression. The basic helix–loop–helix (bHLH) transcription factor Twist1 expressed in the skeletal mesenchyme is a key regulator of craniofacial development playing an important role during osteoskeletogenesis. This study investigates the postnatal impact of Twist1 haploinsufficiency on the osteoskeletal ability and regeneration on two calvarial bones arising from tissues of different embryonic origin: the neural crest-derived frontal and the mesoderm-derived parietal bones. We show that Twist1 haplonsufficiency as well Twist1-sh-mediated silencing selectively enhanced osteogenic and tissue regeneration ability of mesoderm-derived bones. Transcriptomic profiling, gain-and loss-of-function experiments revealed that Twist1 haplonsufficiency triggers its selective activity on mesoderm-derived bone through a sharp downregulation of the bone-derived hormone Fgf23 that is upregulated exclusively in wild-type parietal bone.

Emerging role of Twist1 in fibrotic diseases

Epithelial–mesenchymal transition (EMT) is a pathological process that occurs in a variety of diseases, including organ fibrosis. Twist1, a basic helix–loop–helix transcription factor, is involved in EMT and plays significant roles in various fibrotic diseases. Suppression of the EMT process represents a promising approach for the treatment of fibrotic diseases. In this review, we discuss the roles and the underlying molecular mechanisms of Twist1 in fibrotic diseases, including those affecting kidney, lung, skin, oral submucosa and other tissues. We aim at providing new insight into the pathogenesis of various fibrotic diseases and facilitating the development of novel diagnostic and therapeutic methods for their treatment.

New Pattern of Sutural Synostosis Associated With TWIST Gene Mutation and Saethre-Chotzen Syndrome: Peace Sign Synostosis

The authors present a new and unique pattern of sutural fusion "peace sign synostosis" (PSS) characterized by synostosis of the metopic, bicoronal, and sagittal sutures and associated with abnormalities of the TWIST1 gene known to be associated with Saethre-Chotzen syndrome (SCS). To do so, we performed a retrospective review of patients with bicoronal, metopic, and at least partial anterior sagittal synostoses at the Children's Hospital of Philadelphia and Seattle Children's Hospital. Patients' demographics, genetic analysis, perioperative and clinic notes were reviewed. Five patients were identified with PSS and abnormalities of TWIST1 consistent with SCS. One patient, with the longest follow-up of 7 years, underwent 5 intracranial procedures and required a ventriculoperitoneal (VP) shunt. The remaining 4 patients underwent posterior cranial vault distraction as the initial procedure, followed by anterior cranial vault remodeling. Two patients required a VP shunt. To conclude, synostosis of the metopic, bicoronal, and sagittal sutures (PSS) appears to be associated with SCS and produces a characteristic skull morphology that can be readily identified on physical examination. Early data suggest a high rate of reoperation, increased necessity for a VP shunt, and potential complications. Of note, this novel phenotype had not been previously observed at our respective institutions, reported in the literature, or observed in association with TWIST1 abnormalities as described in association with SCS.

TWIST and ovarian cancer stem cells: implications for chemoresistance and metastasis

The transcription factor TWIST1 is a highly evolutionally conserved basic Helix-Loop-Helix (bHLH) transcription factor that functions as a master regulator of gastrulation and mesodermal development. Although TWIST1 was initially associated with embryo development, an increasing number of studies have shown TWIST1 role in the regulation of tissue homeostasis, primarily as a regulator of inflammation. More recently, TWIST1 has been found to be involved in the process of tumor metastasis through the regulation of Epithelial Mesenchymal Transition (EMT). The objective of this review is to examine the normal functions of TWIST1 and its role in tumor development, with a particular focus on ovarian cancer. We discuss the potential role of TWIST1 in the context of ovarian cancer stem cells and its influence in the process of tumor formation.

The basic helix-loop-helix (bHLH) transcription factor DEC2 negatively regulates Twist1 through an E-box element

Differentiated embryo chondrocyte 2 (DEC2/Sharp-1/Bhlhe41), a basic helix-loop-helix (bHLH) transcription factor, has been shown to regulate the transcription of target genes by binding to their E-box elements. We identified a possible DEC2-response element (consensus E-box: CACGTG) in the promoter region of Twist1. Forced expression of DEC2 significantly repressed Twist1 promoter activity under normoxia and under hypoxia as assessed by a luciferase reporter assay. In addition, over-expression of DEC2 repressed Twist1 mRNA expression assessed by quantitative real-time PCR. Site-directed mutagenesis studies showed that mutagenesis of the consensus E-box sequence eliminated the ability of DEC2 to reduce the Twist1 promoter activity. Chromatin immunoprecipitation (ChIP) assays confirmed that the DEC2-mediated repression is primarily achieved by binding to the E-box in the Twist1 promoter. Knockdown of DEC2 by siRNA significantly attenuated the repression of Twist1 expression. DEC2 and Twist1 exhibit inversed protein expression patterns during development of mouse tongue embryo tissue. Given the fact that DEC2 protein is emerging as an important regulator in a vast array of cellular events, including cell differentiation, maturation of lymphocytes and the molecular clock, our study elucidates an important mechanism by which DEC2 regulates cellular function by modulating the expression of Twist1.

The mesenchymal architecture of the cranial mesoderm of mouse embryos is disrupted by the loss of Twist1 function

The basic helix-loop-helix transcription factor Twist1 is a key regulator of craniofacial development. Twist1-null mouse embryos exhibit failure of cephalic neural tube closure and abnormal head development and die at E11.0. To dissect the function of Twist1 in the cranial mesoderm beyond mid-gestation, we used Mesp1-Cre to delete Twist1 in the anterior mesoderm, which includes the progenitors of the cranial mesoderm. Deletion of Twist1 in mesoderm cells resulted in loss and malformations of the cranial mesoderm-derived skeleton. Loss of Twist1 in the mesoderm also resulted in a failure to fully segregate the mesoderm and the neural crest cells, and the malformation of some cranial neural crest-derived tissues. The development of extraocular muscles was compromised whereas the differentiation of branchial arch muscles was not affected, indicating a differential requirement for Twist1 in these two types of craniofacial muscle. A striking effect of the loss of Twist1 was the inability of the mesodermal cells to maintain their mesenchymal characteristics, and the acquisition of an epithelial-like morphology. Our findings point to a role of Twist1 in maintaining the mesenchyme architecture and the progenitor state of the mesoderm, as well as mediating mesoderm-neural crest interactions in craniofacial development.

Twist contributes to hormone resistance in breast cancer by downregulating estrogen receptor-α

The role of estrogen receptor-α (ER) in breast cancer development, and as a primary clinical marker for breast cancer prognosis, has been well documented. In this study, we identified the oncogenic protein, TWIST1 (Twist), which is overexpressed in high-grade breast cancers, as a potential negative regulator of ER expression. Functional characterization of ER regulation by Twist was performed using Twist low (MCF-7, T-47D) and Twist high (Hs 578T, MDA-MB-231, MCF-7/Twist) expressing cell lines. All Twist high expressing cell lines exhibited low ER transcript and protein levels. By chromatin immunoprecipitation and promoter assays, we demonstrated that Twist could directly bind to E-boxes in the ER promoter and significantly downregulate ER promoter activity in vitro. Functionally, Twist overexpression caused estrogen-independent proliferation of breast cells, and promoted hormone resistance to the selective estrogen receptor modulator tamoxifen and selective estrogen receptor down-regulator fulvestrant. Importantly, this effect was reversible on downregulating Twist. In addition, orthotopic tumors generated in mice using MCF-7/Twist cells were resistant to tamoxifen. These tumors had high vascular volume and permeability surface area, as determined by magnetic resonance imaging (MRI). Mechanistically, Twist recruited DNA methyltransferase 3B (DNMT3B) to the ER promoter, leading to a significantly higher degree of ER promoter methylation compared with parental cells. Furthermore, we demonstrated by co-immunoprecipitation that Twist interacted with histone deacetylase 1 (HDAC1) at the ER promoter, causing histone deacetylation and chromatin condensation, further reducing ER transcript levels. Functional re-expression of ER was achieved using the demethylating agent, 5-azacytidine, and the HDAC inhibitor, valproic acid. Finally, an inverse relationship was observed between Twist and ER expression in human breast tumors. In summary, the regulation of ER by Twist could be an underlying mechanism for the loss of ER activity observed in breast tumors, and may contribute to the generation of hormone-resistant, ER-negative breast cancer.

Twist expression associated with the epithelial-mesenchymal transition in gastric cancer

This study aimed to investigate the expression of Twist in gastric cancer tissues and its correlation between Twist and the epithelial-mesenchymal transition (EMT). By means of RT-PCR and Western blot, the mRNA and protein expressions of Twist, E-cadherin, and Vimentin in 61 gastric cancer tissues and adjacent normal tissues were detected. The positive rates of Twist, E-cadherin, and Vimentin mRNA expression in gastric cancer tissues were 73.9. 40.6, and 60.9 %, respectively; compared to the expression of these genes in adjacent normal tissues (2.9, 75.4, and 27.5 %), the differences were significant (p < 0.05). The E-cadherin protein expression level in gastric cancer tissues was significantly lower than that in the adjacent normal tissues (p < 0.05). After the transfection of Twist siRNA into the MKN45 cells, the protein expression of Twist was significantly reduced (p < 0.05), the protein expression of E-cadherin was significantly increased, and the number of cells that passed through the Transwell chamber was significantly lower than that in the non-transfected control group as well as the transfected control group (p < 0.05). Twist may be associated with the epithelial-mesenchymal transition in gastric cancer and the tumorigenesis, invasion, and metastasis of gastric cancer.

Distinct Caenorhabditis elegans HLH-8/twist-containing dimers function in the mesoderm

BACKGROUND

The Caenorhabditis elegans basic helix-loop-helix (bHLH) factor HLH-8, the single Twist ortholog in the nematode genome, plays important roles in mesoderm development, including M lineage patterning and differentiation of vulval and enteric muscles. HLH-8 cooperates with HLH-2, the bHLH E/Daughterless ortholog, to regulate downstream target genes, but it is not known whether HLH-2 is an obligate partner for all HLH-8 functions.

RESULTS

Using hlh-2 loss-of-function alleles and RNAi, we discovered that HLH-2 is required in the vulval muscles but not in M patterning or enteric muscle development. Additionally, we found that expressing tethered HLH-8/HLH-8 dimers in hlh-8 null animals rescued M patterning and enteric but not vulval muscle development.

CONCLUSIONS

These results support a model whereby HLH-8/HLH-8 homodimers function in M lineage patterning and enteric muscles and HLH-8/HLH-2 heterodimers function in the M-derived vulval muscles. Interestingly, the different dimers function in the same M lineage cells and the switch in dimer function coincides with vulval muscle differentiation. The use of distinct Twist dimers is evolutionarily conserved, and C. elegans provides a paradigm for future dissection of differential promoter regulation by these dimers at a single cell resolution.

Normal and disease-related biological functions of Twist1 and underlying molecular mechanisms

This article reviews the molecular structure, expression pattern, physiological function, pathological roles and molecular mechanisms of Twist1 in development, genetic disease and cancer. Twist1 is a basic helix-loop-helix domain-containing transcription factor. It forms homo- or hetero-dimers in order to bind the Nde1 E-box element and activate or repress its target genes. During development, Twist1 is essential for mesoderm specification and differentiation. Heterozygous loss-of-function mutations of the human Twist1 gene cause several diseases including the Saethre-Chotzen syndrome. The Twist1-null mouse embryos die with unclosed cranial neural tubes and defective head mesenchyme, somites and limb buds. Twist1 is expressed in breast, liver, prostate, gastric and other types of cancers, and its expression is usually associated with invasive and metastatic cancer phenotypes. In cancer cells, Twist1 is upregulated by multiple factors including SRC-1, STAT3, MSX2, HIF-1α, integrin-linked kinase and NF-κB. Twist1 significantly enhances epithelial-mesenchymal transition (EMT) and cancer cell migration and invasion, hence promoting cancer metastasis. Twist1 promotes EMT in part by directly repressing E-cadherin expression by recruiting the nucleosome remodeling and deacetylase complex for gene repression and by upregulating Bmi1, AKT2, YB-1, etc. Emerging evidence also suggests that Twist1 plays a role in expansion and chemotherapeutic resistance of cancer stem cells. Further understanding of the mechanisms by which Twist1 promotes metastasis and identification of Twist1 functional modulators may hold promise for developing new strategies to inhibit EMT and cancer metastasis.

A newly described bovine type 2 scurs syndrome segregates with a frame-shift mutation in TWIST1

The developmental pathways involved in horn development are complex and still poorly understood. Here we report the description of a new dominant inherited syndrome in the bovine Charolais breed that we have named type 2 scurs. Clinical examination revealed that, despite a strong phenotypic variability, all affected individuals show both horn abnormalities similar to classical scurs phenotype and skull interfrontal suture synostosis. Based on a genome-wide linkage analysis using Illumina BovineSNP50 BeadChip genotyping data from 57 half-sib and full-sib progeny, this locus was mapped to a 1.7 Mb interval on bovine chromosome 4. Within this region, the TWIST1 gene encoding a transcription factor was considered as a strong candidate gene since its haploinsufficiency is responsible for the human Saethre-Chotzen syndrome, characterized by skull coronal suture synostosis. Sequencing of the TWIST1 gene identified a c.148_157dup (p.A56RfsX87) frame-shift mutation predicted to completely inactivate this gene. Genotyping 17 scurred and 20 horned founders of our pedigree as well as 48 unrelated horned controls revealed a perfect association between this mutation and the type 2 scurs phenotype. Subsequent genotyping of 32 individuals born from heterozygous parents showed that homozygous mutated progeny are completely absent, which is consistent with the embryonic lethality reported in Drosophila and mouse suffering from TWIST1 complete insufficiency. Finally, data from previous studies on model species and a fine description of type 2 scurs symptoms allowed us to propose different mechanisms to explain the features of this syndrome. In conclusion, this first report on the identification of a potential causal mutation affecting horn development in cattle offers a unique opportunity to better understand horn ontogenesis.

TWIST1 Gene: First Insights in Felis catus

TWIST1 is thought to be a novel oncogene. Understanding the molecular mechanisms regulating the TWIST1 gene expression profiles in tumor cells may give new insights regarding prognostic factors and novel therapeutic targets in veterinary oncology. In the present study we partially isolated the TWIST1 gene in Felis catus and performed comparative studies. Several primer combinations were used based on the alignments of homologous DNA sequences. After PCR amplification, three bands were obtained, purified and sequenced. Several bioinformatic tools were utilized to carry out the comparative studies. Higher similarity was found between the isolated TWIST1 gene in Felis catus and Homo sapiens (86%) than between Homo sapiens and Rattus norvegicus or Mus musculus (75%). Partial amino acid sequence showed no change in the four species analyzed. This confirmed that coding sequences presented high similarity (~96%) between man and cat. These results give the first insights regarding the TWIST1 gene in cat but further studies are required in order to establish, or not, its role in tumor formation and progression in veterinary oncology.

Twist modulates breast cancer stem cells by transcriptional regulation of CD24 expression

The cancer stem cell paradigm postulates that dysregulated tissue-specific stem cells or progenitor cells are precursors for cancer biogenesis. Consequently, identifying cancer stem cells is crucial to our understanding of cancer progression and for the development of novel therapeutic agents. In this study, we demonstrate that the overexpression of Twist in breast cells can promote the generation of a breast cancer stem cell phenotype characterized by the high expression of CD44, little or no expression of CD24, and increased aldehyde dehydrogenase 1 activity, independent of the epithelial-mesenchymal transition. In addition, Twist-overexpressing cells exhibit high efflux of Hoechst 33342 and Rhodamine 123 as a result of increased expression of ABCC1 (MRP1) transporters, a property of cancer stem cells. Moreover, we show that transient expression of Twist can induce the stem cell phenotype in multiple breast cell lines and that decreasing Twist expression by short hairpin RNA in Twist-overexpressing transgenic cell lines MCF-10A/Twist and MCF-7/Twist as well as in MDA-MB-231 partially reverses the stem cell molecular signature. Importantly, we show that inoculums of only 20 cells of the Twist-overexpressing CD44(+)/CD24(-/low) subpopulation are capable of forming tumors in the mammary fat pad of severe combined immunodeficient mice. Finally, with respect to mechanism, we provide data to indicate that Twist transcriptionally regulates CD24 expression in breast cancer cells. Taken together, our data demonstrate the direct involvement of Twist in generating a breast cancer stem cell phenotype through down-regulation of CD24 expression and independent of an epithelial-mesenchymal transition.

Unique CCT repeats mediate transcription of the TWIST1 gene in mesenchymal cell lines

TWIST1, a basic helix-loop-helix transcription factor, plays critical roles in embryo development, cancer metastasis and mesenchymal progenitor differentiation. Little is known about transcriptional regulation of TWIST1 expression. Here we identified DNA sequences responsible for TWIST1 expression in mesenchymal lineage cell lines. Reporter assays with TWIST1 promoter mutants defined the -102 to -74 sequences that are essential for TWIST1 expression in human and mouse mesenchymal cell lines. Tandem repeats of CCT, but not putative CREB and NF-kappaB sites in the sequences substantially supported activity of the TWIST1 promoter. Electrophoretic mobility shift assay demonstrated that the DNA sequences with the CCT repeats formed complexes with nuclear factors, containing, at least, Sp1 and Sp3. These results suggest critical implication of the CCT repeats in association with Sp1 and Sp3 factors in sustaining expression of the TWIST1 gene in mesenchymal cells.

Minireview: transcriptional regulation in development of bone

Regulation of gene expression by transcription factors is one of the major mechanisms for controlling cellular functions. Recent advances in genetic manipulation of model animals has allowed the study of the roles of various genes and their products in physiological settings and has demonstrated the importance of specific transcription factors in bone development. Three lineages of bone cells, chondrocytes, osteoblasts, and osteoclasts, develop and differentiate according to their distinct developmental programs. These cells go through multiple differentiation stages, which are often regulated by specific transcription factors. In this minireview, we will discuss selected transcription factors that have been demonstrated to critically affect bone cell development. Further study of these molecules will lead to deeper understanding in mechanisms that govern development of bone.

Vascular gene expression and phenotypic correlation during differentiation of human embryonic stem cells

The study of the cascade of events of induction and sequential gene activation that takes place during human embryonic development is hindered by the unavailability of postimplantation embryos at different stages of development. Spontaneous differentiation of human embryonic stem cells (hESCs) can occur by means of the formation of embryoid bodies (EBs), which resemble certain aspects of early embryos to some extent. Embryonic vascular formation, vasculogenesis, is a sequential process that involves complex regulatory cascades. In this study, changes of gene expression along the development of human EBs for 4 weeks were studied by large-scale gene screening. Two main clusters were identified-one of down-regulated genes such as POU5, NANOG, TDGF1/Cripto (TDGF, teratocarcinoma-derived growth factor-1), LIN28, CD24, TERF1 (telomeric repeat binding factor-1), LEFTB (left-right determination, factor B), and a second of up-regulated genes such as TWIST, WNT5A, WT1, AFP, ALB, NCAM1. Focusing on the vascular system development, genes known to be involved in vasculogenesis and angiogenesis were explored. Up-regulated genes include vasculogenic growth factors such as VEGFA, VEGFC, FIGF (VEGFD), ANG1, ANG2, TGFbeta3, and PDGFB, as well as the related receptors FLT1, FLT4, PDGFRB, TGFbetaR2, and TGFbetaR3, other markers such as CD34, VCAM1, PECAM1, VE-CAD, and transcription factors TAL1, GATA2, and GATA3. The reproducibility of the array data was verified independently and illustrated that many genes known to be involved in vascular development are activated during the differentiation of hESCs in culture. Hence, the analysis of the vascular system can be extended to other differentiation pathways, allocating human EBs as an in vitro model to study early human development.

Noggin inhibits chondrogenic but not osteogenic differentiation in mesodermal stem cell line C1 and skeletal cells

Osteoblasts and chondroblasts are derived from common mesenchymal progenitors. Although bone morphogenetic protein induces mesenchymal differentiation into both osteogenic and chodrogenic lineage cells in vitro, its inhibitor, Noggin, is expressed exclusively during chondrogenic but not osteogenic differentiation in an embryonal carcinoma-derived mesodermal cell line, C1. We hypothesized that Noggin may regulate cell differentiation in a lineage-specific manner. To test this hypothesis, Noggin was overexpressed using recombinant adenovirus (Ad/Noggin) in mesodermal C1 cells to examine whether Noggin specifically inhibits chondrogenic differentiation. Noggin overexpression by recombinant adenovirus infection reduced Sox9, patched, Ihh, and type II, X, and XI collagen mRNA expression levels in C1 cell aggregates that were induced to differentiate into chondrocyte lineage by culturing in differentiation medium. In contrast, Noggin overexpression did not affect osteogenic differentiation in C1 cells because osteoblast phenotypic markers such as osteocalcin and alkaline phosphatase mRNA levels were not altered. We further examined whether Noggin also differentially affects chondrogenesis and osteogenesis in limb development by using organ cultures of long bone. Ad/Noggin infection into 15.5 d post conception limb skeletal rudiments that were cultured on filter membrane in vitro or on the chorioallantoic membranes in ovo inhibited the levels of chondrogenesis, which were evaluated based on alcian blue staining. These results suggest that Noggin specifically blocks chondrogenic differentiation, rather than osteogenic differentiation, in mesodermal stem cell line C1 and skeletal cells.

Genomics of osteoarthritis

PURPOSE OF REVIEW

This review addresses the progress in three major fields of "genomics of osteoarthritis" over the past year: genetic alterations thought to be important for the initiation and progression of osteoarthritis, differential gene expression analysis, and functional genomics of osteoarthritis.

RECENT FINDINGS

Distinct genetic risk factors may predispose different joint sites to osteoarthritis, and although clear loci for susceptibility genes for common osteoarthritis have yet to emerge from the epidemiological studies, new approaches are narrowing down known loci. The search for specific genes using cDNA array technology has further demonstrated its potential in arthritis research as a powerful tool that could further provide biological insights into disease mechanisms, osteoarthritis polymorphic subtypes, the molecular validation of animal models, and the monitoring of drug activity on gene expression levels. Gene expression analysis has further characterized the striking shift in the gene expression pattern during "dedifferentiation" of chondrocytes in vitro as well as added depth to the phenotype of differentiated versus undifferentiated mesenchymal stem cells. Several new molecules potentially relevant to the disease process were identified, among them beta2-microglobulin (B2M), clusterin, and chitinase-like molecule 2.

SUMMARY

Functional genomic approaches will in the future allow to complement traditional biochemistry and molecular biology. Although there are limitations to cDNA array technology, "molecular portraits" of osteoarthritic chondrocytes in vivo and in vitro can be produced to analyze whole or large biologic systems rather than just single aspects of it. This will stimulate the testing of new markers, which are needed for the diagnosis and monitoring of osteoarthritis.

Gene expression in chondrocytes assessed with use of microarrays

BACKGROUND

Despite considerable limitations such as low sensitivity and insensitivity to alternative splicing, posttranscriptional regulation, and posttranslational modification, cDNA array technology provides a powerful tool with which to obtain an overview of gene expression patterns, hardly achievable with other techniques. This has been shown to be true for the analysis of known genes as well as the discovery of new genes of interest.

METHODS

Samples of normal and late-stage osteoarthritic cartilage of human knee joints were analyzed with use of the Human Cancer 1.2 cDNA-array and TaqMan analysis.

RESULTS

In spite of a large variability of expression levels among different patients, significant expression patterns for many known genes of interest such as cartilage matrix proteins (e.g., collagen types II, VI, and XI; aggrecan; decorin; biglycan) and matrix-degrading proteases were detected. Of the latter, MMP-3 appeared to be strongly expressed in normal and early degenerative cartilage and downregulated in the late disease stages. This indicates that, in the late stages of cartilage degeneration, other degradation pathways might be more important, for example, those involving enzymes such as MMP-2 and MMP-13, both of which were upregulated in late-stage disease.

CONCLUSION

Most results have to be considered to be preliminary to a certain degree, as technical tools and interpretation approaches are still emerging and need more validation. Clearly, there is a major challenge to distill information and knowledge out of the obtained mass of data. However, these data will be one basis of a new world of biological understanding. These new insights will be network-based and no longer molecule-centered. Today, molecules have a biochemical and physiological context; tomorrow, biological networks will have molecules as constituents.

Functional and structural characterization of the human gene BHLHB5, encoding a basic helix-loop-helix transcription factor

The genes encoding basic helix-loop-helix (bHLH) transcription factors have been implicated in many aspects of neural development, including cell growth, differentiation, and cell migration. Using both genomic and cDNA mouse and human clones encoding a neural-specific bHLH protein, human BHLHB5 was cloned and mapped to a region on chromosome 8q13 that segregates with Duane syndrome. Genomic sequence analysis of human BHLHB5 and mouse Bhlhb5 revealed that they contain a single exon encoding 381- and 355-amino-acid bHLH proteins, respectively. Multiple amino acid sequence alignments of the Bhlhb5 family members revealed several conserved motifs and an identical 147-amino-acid carboxy-terminal region that contains a 60-amino-acid bHLH domain. A 27-bp trinucleotide repeat (CAG)(9) encoding polyserine was found in human BHLHB5, but only one CAG was found at the corresponding position in the mouse Bhlhb5 and hamster BETA3 genes. Northern blot analysis of human BHLHB5 revealed brain-specific expression with the highest abundance in the cerebellum. Mouse Bhlhb5 can strongly repress a human PAX6 promoter.

Functional genomics of osteoarthritis

Functional genomics is a challenging new way to address a complex disease like osteoarthritis on a molecular level. Despite osteoarthritis being ultimately a biochemical problem, mainly characterized by an imbalanced cartilage matrix turnover, a deeper understanding of molecular events within the tissue cells (i.e., the chondrocytes) will provide not only a better understanding of pathogenetic mechanisms but also new diagnostic markers and cellular targets for therapeutic intervention. This innovative technology represents a challenging approach complementing (not replacing) classical research in previously described and new disease-relevant genes: large-scale functional genomics will open up new areas of so far unrecognized molecular networks. This will include as yet unidentified players in the anabolic-catabolic balance of matrix turnover of articular cartilage as well as disease-relevant intracellular signaling cascades so far hardly investigated in articular chondrocytes. However, care must be taken not to over or misinterpret results and some major challenges must be overcome in order to properly utilize the potential of this technology in the field of osteoarthritis.

Assessment of the gene expression profile of differentiated and dedifferentiated human fetal chondrocytes by microarray analysis

OBJECTIVE

To study the changes in patterns of gene expression exhibited by human chondrocytes as they dedifferentiate into fibroblastic cells in culture in order to better understand the mechanisms that control this process and its relationship to the phenotypic changes that occur in chondrocytes during the development of osteoarthritis (OA).

METHODS

Human fetal epiphyseal chondrocytes (HFCs) were cultured either on poly-(2-hydroxyethyl methacrylate)-coated plates (differentiated HFC cultures) or in plastic tissue culture flasks as monolayers (dedifferentiated HFC cultures). Following 11 days of culture under either condition, poly(A+) RNA was isolated from the two cell populations and subjected to a gene expression analysis using a microarray containing approximately 5,000 known human genes and approximately 3,000 expressed sequence tags (ESTs).

RESULTS

A > or =2-fold difference in the expression of 62 known genes and 6 ESTs was observed between the two cell types. The differences in expression of several of the genes detected by the microarray hybridization were confirmed by Northern analyses. Two transcription factor genes, TWIST and HIF-1alpha, and a cellular adhesion protein gene, cadherin 11, were markedly regulated in response to differentiation and dedifferentiation. Expression of these genes was also detected in adult normal and OA cartilage and chondrocytes. Analysis of the gene expression profile of HFCs revealed a complex pattern of gene expression, including many genes not yet reported to be expressed by chondrocytes.

CONCLUSION

Chondrocytes in monolayer become dedifferentiated, acquiring a fibroblast-like appearance and changing their pattern of gene expression from one of expression of chondrocyte-specific genes to one that resembles a fibroblastic or chondroprogenitor-like pattern. Changes in gene expression associated with the process of dedifferentiation of HFCs in vitro were observed in a wide variety of genes, including genes encoding extracellular matrix proteins, transcription factors, and growth factors. At least 3 of the genes that were regulated in response to dedifferentiation were also found to be expressed in adult normal and OA articular cartilage and chondrocytes.

Phenotypic findings due to trisomy 7p15.3-pter including the TWIST locus

We report on a three-month-old boy with a 46,XY,der(Y)t(Y;7)(p11.32;p15.3) karyotype and growth deficiency, postnatal microcephaly with large fontanels, wide sagittal and metopic sutures, hypertelorism, choanal stenosis, micrognathia, bilateral cryptorchidism, hypospadias, abnormal fingers and toes, and severe developmental delay. FISH studies showed partial trisomy 7p resulting from a de novo unbalanced translocation. The application of molecular probes from the TWIST gene region (7p15.3-p21.1) and probes from the pseudoautosomal region (PAR) demonstrated that the 7p15.3-pter fragment was translocated onto Yp with the breakpoint within approximately 20 kb from the Yp telomere. We discuss the possible role of the TWIST gene in abnormal skull development and suggest that trisomy 7p cases with delayed closure of fontanels can be a result of TWIST gene dosage effect.

Human Dermo-1 has attributes similar to twist in early bone development

Basic helix-loop-helix (bHLH) transcription factors are implicated in cell lineage determination and differentiation. Dermo-1 encodes a bHLH transcription factor that shares extensive homology with another bHLH transcription factor, Twist. We have cloned and characterized human Dermo-1 from two different bone cytoplasmic DNA (cDNA) libraries. Dermo-1 mRNA and protein expression were examined in human embryo and adult tissue sections. Dermo-1 is expressed in a subset of mesodermally and ectodermally derived tissues. We further examined expression of Dermo-1/Twist in human tissues and cell lines. In addition, we observed Dermo-1 expression in response to basic fibroblast growth factor in osteoblastic cell lines. To evaluate the functionality of the human Dermo-1 transcription factor in osteoblast metabolism, we made stable osteoblastic cell lines that over- and underexpress human Dermo-1. These cell lines were analyzed and compared with previously published data of similar cell lines transfected with Twist. Our results demonstrate that Dermo-1 caused changes similar to Twist in the osteogenic properties of osteoblastic cells, such as morphology, bone marker gene expression, and biochemical response to cytokines. However, Dermo-1 expression also has unique effects in regulating the mechanism of proliferation, on alkaline phosphatase enzyme activity, and in temporal expression patterns. We speculate that expression of Twist and Dermo-1 maintains cells in an osteoprogenitor or preosteoblast-like state, respectively, and prevents premature or ectopic osteoblast differentiation. Therefore, Twist and Dermo-1 must be sequentially downregulated in order to initiate the cascade of events responsible for osteogenic cell differentiation. These results indicate that, during osteoblast development, Dermo-1 may inhibit osteoblast maturation and maintain cells in a preosteoblast phenotype by utilizing mechanisms similar but not identical to those utilized by Twist.

TWIST, a basic helix-loop-helix transcription factor, can regulate the human osteogenic lineage

Basic helix-loop-helix (bHLH) transcription factors have been shown to play an important role in controlling cell type determination and differentiation. TWIST, a member of the bHLH transcription factor family, is involved in the development of mesodermally derived tissue, including the skeleton. We examined the role of human TWIST in osteoblast metabolism using stable expression of sense and antisense TWIST in human osteoblast HSaOS-2 cells. Changes in morphology and osteogenic phenotype characterized these stable clones. Cells that overexpressed TWIST exhibited a spindle shaped morphology, reduced levels of alkaline phosphatase, a reduced proliferation rate, and failed to respond to basic fibroblast growth factor (bFGF). In contrast, those that underexpressed TWIST demonstrated a cuboidal epithelial-like morphology characteristic of differentiated osteoblasts. TWIST antisense cells exhibited increased levels of alkaline phosphatase and type I collagen mRNA, initiated osteopontin mRNA expression, and had a reduced proliferation rate. These results indicate that TWIST overexpressing cells may de-differentiate and remain in an osteoprogenitor-like state, and antisense TWIST cells progress to a more differentiated mature osteoblast-like state. Therefore, the level of TWIST can influence osteogenic gene expression and may act as a master switch in initiating bone cell differentiation by regulating the osteogenic cell lineage.

TWIST gene mutation in a patient with radial aplasia and craniosynostosis: further evidence for heterogeneity of Baller-Gerold syndrome

The term Baller-Gerold syndrome was coined by Cohen [1979: Birth Defects 15(5B): 13-63] to designate the phenotype of craniosynostosis and radial aplasia. It is thought to be a rare autosomal recessive condition, which, in some patients, presents with additional abnormalities, such as polymicrogyria, mental retardation or anal atresia. A phenotypic overlap of Baller-Gerold and Roberts-SC phocomelia syndrome was noted when a patient with bicoronal synostosis and bilateral radial hypoplasia was found to have premature centromere separation, a finding characteristic of Roberts syndrome [Huson et al.,1990: J Med Genet 27:371-375]. Other cases of presumed Baller-Gerold syndrome were rediagnosed as Fanconi pancytopenia, Rothmund-Thomson syndrome or VACTERL association. These reports led to a narrowed redefinition of Baller-Gerold syndrome based on the exclusion of cytogenetic and hematopoetic abnormalities and the absence of additional malformations in patients with craniosynostosis and preaxial upper limb abnormalities. Here we report on a patient with unilateral radial aplasia and bicoronal synostosis without additional malformations and without chromosome breakage, who fits this narrow definition of Baller-Gerold syndrome. We identified a novel TWIST gene mutation in this patient, a Glu181Stop mutation predicting a premature termination of the protein carboxy-terminal to the helix 2 domain. This report provides further evidence that Baller-Gerold is of heterogeneous cause, and a thorough evaluation is indicated to identify a possibly more specific diagnosis, including Saethre-Chotzen syndrome. This differential diagnosis is of particular importance, as it is an autosomal dominant trait. Therefore, the recurrence risk for parents of an affected child can be 50% if one parent carries the mutation, as opposed to the 25% recurrence risk for autosomal recessive inheritance. Offspring of the affected patient also have a 50% risk to inherit the mutation, while the risk to bear an affected offspring for an autosomal recessive trait is very low.

A comprehensive screen for TWIST mutations in patients with craniosynostosis identifies a new microdeletion syndrome of chromosome band 7p21.1

Mutations in the coding region of the TWIST gene (encoding a basic helix-loop-helix transcription factor) have been identified in some cases of Saethre-Chotzen syndrome. Haploinsufficiency appears to be the pathogenic mechanism involved. To investigate the possibility that complete deletions of the TWIST gene also contribute to this disorder, we have developed a comprehensive strategy to screen for coding-region mutations and for complete gene deletions. Heterozygous TWIST mutations were identified in 8 of 10 patients with Saethre-Chotzen syndrome and in 2 of 43 craniosynostosis patients with no clear diagnosis. In addition to six coding-region mutations, our strategy revealed four complete TWIST deletions, only one of which associated with a translocation was suspected on the basis of conventional cytogenetic analysis. This case and two interstitial deletions were detectable by analysis of polymorphic microsatellite loci, including a novel (CA)n locus 7.9 kb away from TWIST, combined with FISH; these deletions ranged in size from 3.5 Mb to >11.6 Mb. The remaining, much smaller deletion was detected by Southern blot analysis and removed 2,924 bp, with a 2-bp orphan sequence at the breakpoint. Significant learning difficulties were present in the three patients with megabase-sized deletions, which suggests that haploinsufficiency of genes neighboring TWIST contributes to developmental delay. Our results identify a new microdeletion disorder that maps to chromosome band 7p21.1 and that causes a significant proportion of Saethre-Chotzen syndrome.