Two domains unique to osteoblast-specific transcription factor Osf2/Cbfa1 contribute to its transactivation function and its inability to heterodimerize with Cbfbeta

Identification of NIBAN2-Regulated RUNX2 Alternative Splicing Presents Novel Strategies for Antagonizing Osteoporosis

Osteoporosis is characterized by excessive bone resorption and/or defects in bone formation. Identification of factors promoting osteoblast differentiation may provide potential targets for osteoporosis therapy. Through integral analyses of multiple datasets, NIBAN2 is found to be tightly associated with bone formation and osteoporosis. Indeed, NIBAN2 promotes osteoblast differentiation, and conditional Niban2 knockout in osteoblasts caused bone loss and insufficient mineralization. Mechanistically, NIBAN2 interacts with the HNRNPU-cored spliceosome complex and alters its components to regulate the alternative splicing of RUNX2, which ultimately cause an increase in functional RUNX2 (nuclear localization sequence complete) but a decrease in dysfunctional RUNX2 (exon 6 exclusive) to reinforce osteoblast differentiation. Most importantly, NIBAN2 expression level negatively correlates with RUNX2 spliced isoforms and bone loss in osteoporosis patients. NIBAN2 overexpression rescues bone loss in ovariectomized mice. Thus, this research identifies NIBAN2-regulated RUNX2 alternative splicing as a potential mechanism of osteoblast differentiation that may present strategies for antagonizing osteoporosis.

The impact of RUNX2 gene variants on cleidocranial dysplasia phenotype: a systematic review

Cleidocranial Dysplasia (CCD) is a rare genetic disorder characterized by skeletal abnormalities and dental anomalies, primarily caused by variants in the RUNX2 gene. Understanding the spectrum of RUNX2 variants and their effects on CCD phenotypes is crucial for accurate diagnosis and management strategies. This systematic review aimed to comprehensively analyze the genotypic and phenotypic spectra of RUNX2 variants in CCD patients, assess their distribution across functional regions, and investigate genotype-phenotype correlations. This review included 569 reported variants and 453 CCD patients from 103 articles. Of 569 variants, in-frame variants constituted 48.68%, while null variants accounted for 51.32%. Regarding locations, RUNX2 variants were predominantly located in the RHD (55.54%), followed by PST (16.34%), NMTS (6.33%), QA (4.75%), VWRPY (1.23%), and NLS (1.41%) regions while 10.19% were in non-coding regions. In-frame variants occurred primarily in the RHD (90.97%), while null variants were found across various regions of RUNX2. Data analysis revealed a correlation between variant location and specific skeletal features in CCD patients. Missense variants, predominantly found within the functionally critical RHD, were significantly associated with supernumerary teeth, macrocephaly, metopic groove, short ribs, and hypoplastic iliac wings compared to nonsense variants. They were also significantly associated with delayed fontanelle closure, metopic synostosis, hypertelorism, limited shoulder abduction, pubic symphysis abnormalities, and hypoplastic iliac wings compared to in-frame variants found in other regions. These findings underscore the critical role of the RHD, with missense RHD variants having a more severe impact than nonsense and other in-frame variants. Additionally, in-frame insertions and deletions in RUNX2 were associated with fewer CCD features, compared to missense, frameshift, and nonsense variants. Null variants in the NLS region exhibited weaker associations with delayed fontanelle closure, supernumerary teeth, Wormian bones, and femoral head hypoplasia than variants in other regions. Moreover, the NLS variants did not consistently alter nuclear localization, questioning the role of NLS region in nuclear import. In summary, this comprehensive review significantly advances our understanding of CCD, facilitating improved phenotype-genotype correlations, enhanced clinical management, and a deeper insight into RUNX2 functional domains. This knowledge has the potential to guide the development of novel therapeutic targets for skeletal disorders.

Reassessing the association: Evaluation of a polyalanine deletion variant of RUNX2 in non-syndromic sagittal and metopic craniosynostosis

The RUNT-related transcription factor RUNX2 plays a critical role in osteoblast differentiation, and alterations to gene dosage cause distinct craniofacial anomalies. Uniquely amongst the RUNT-related family, vertebrate RUNX2 encodes a polyglutamine/polyalanine repeat (Gln23-Glu-Ala17 in humans), with the length of the polyalanine component completely conserved in great apes. Surprisingly, a frequent 6-amino acid deletion polymorphism, p.(Ala84_Ala89)del, occurs in humans (termed 11A allele), and a previous association study (Cuellar et al. Bone 137:115395;2020) reported that the 11A variant was significantly more frequent in non-syndromic sagittal craniosynostosis (nsSag; allele frequency [AF] = 0.156; 95% confidence interval [CI] 0.126-0.189) compared to non-syndromic metopic craniosynostosis (nsMet; AF = 0.068; 95% CI 0.045-0.098). However, the gnomAD v.2.1.1 control population used by Cuellar et al. did not display Hardy-Weinberg equilibrium, hampering interpretation. To re-examine this association, we genotyped the RUNX2 11A polymorphism in 225 individuals with sporadic nsSag as parent-child trios and 164 singletons with sporadic nsMet, restricting our analysis to individuals of European ancestry. We compared observed allele frequencies to the non-transmitted alleles in the parent-child trios, and to the genome sequencing data from gnomAD v.4, which display Hardy-Weinberg equilibrium. Observed AFs (and 95% CI) were 0.076 (0.053-0.104) in nsSag and 0.082 (0.055-0.118) in nsMet, compared with 0.062 (0.042-0.089) in non-transmitted parental alleles and 0.065 (0.063-0.067) in gnomAD v.4.0.0 non-Finnish European control genomes. In summary, we observed a non-significant excess, compared to gnomAD data, of 11A alleles in both nsSag (relative risk 1.18, 95% CI 0.83-1.67) and nsMet (relative risk 1.29, 95% CI 0.87-1.92), but we did not replicate the much higher excess of RUNX2 11A alleles in nsSag previously reported (p = 0.0001).

Interplay of RUNX2 and KLF4 in initial commitment of odontoblast differentiation

Odontoblast differentiation is a key process in dentin formation. Mouse dental papilla cells (mDPCs) are pivotal in dentinogenesis through their differentiation into odontoblasts. Odontoblast differentiation is intricately controlled by transcription factors (TFs) in a spatiotemporal manner. Previous research explored the role of RUNX2 and KLF4 in odontoblast lineage commitment, respectively. Building on bioinformatics analysis of our previous ATAC-seq profiling, we hypothesized that KLF4 potentially collaborates with RUNX2 to exert its biological role. To investigate the synergistic effect of multiple TFs in odontoblastic differentiation, we first examined the spatiotemporal expression patterns of RUNX2 and KLF4 in dental papilla at the bell stage using immunostaining techniques. Notably, RUNX2 and KLF4 demonstrated colocalization in preodontoblast. Further, immunoprecipitation and proximity ligation assays verified the interaction between RUNX2 and KLF4 in vitro. Specifically, the C-terminus of RUNX2 was identified as the interacting domain with KLF4. Functional implications of this interaction were investigated using small hairpin RNA-mediated knockdown of Runx2, Klf4, or both. Western blot analysis revealed a marked decrease in DSPP expression, an odontoblast differentiation marker, particularly in the double knockdown condition. Additionally, alizarin red S staining indicated significantly reduced mineralized nodule formation in this group. Collectively, our findings highlight the synergistic interaction between RUNX2 and KLF4 in promoting odontoblast differentiation from mDPCs. This study contributes to a more comprehensive understanding of the regulatory network of TFs governing odontoblast differentiation.

The expression of Runx2 in the pathogenesis of periodontitis

OBJECTIVE

Runt-related transcription factor 2 (Runx2) plays an important role in bone metabolism; however, the relationship between Runx2 and periodontitis remains unclear. We investigated Runx2 expression in the gingiva of patients to explore its role in periodontitis.

METHODS

Gingival samples of patients were collected, including healthy samples (control group) and periodontitis samples (P group). Periodontitis samples were divided into three groups based on the periodontitis stage. Samples with stage I and grade B periodontitis were in the P1 group, stage II and grade B in the P2 group, and stage III or IV and grade B in the P3 group. Immunohistochemistry and western blotting were performed to detect Runx2 levels. The probing (PD) and clinical attachment loss (CAL) were recorded.

RESULTS

Runx2 expression levels in the P and P3 groups were higher than those in the control group. In addition, Runx2 expression was positively correlated with CAL and PD (r1 = 0.435, r2 = 0.396).

CONCLUSION

The high expression level of Runx2 in the gingiva of patients with periodontitis may be correlated with the pathogenesis of periodontitis.

Functional consequences of C-terminal mutations in RUNX2

Cleidocranial dysplasia (CCD) is a genetic disorder caused by mutations in the RUNX2 gene, affecting bone and teeth development. Previous studies focused on mutations in the RUNX2 RHD domain, with limited investigation of mutations in the C-terminal domain. This study aimed to investigate the functional consequences of C-terminal mutations in RUNX2. Eight mutations were analyzed, and their effects on transactivation activity, protein expression, subcellular localization, and osteogenic potential were studied. Truncating mutations in the PST region and a missense mutation in the NMTS region resulted in increased transactivation activity, while missense mutations in the PST showed activity comparable to the control. Truncating mutations produced truncated proteins, while missense mutations produced normal-sized proteins. Mutant proteins were mislocalized, with six mutant proteins detected in both the nucleus and cytoplasm. CCD patient bone cells exhibited mislocalization of RUNX2, similar to the generated mutant. Mislocalization of RUNX2 and reduced expression of downstream genes were observed in MSCs from a CCD patient with the p.Ser247Valfs*3 mutation, leading to compromised osteogenic potential. This study provides insight into the functional consequences of C-terminal mutations in RUNX2, including reduced expression, mislocalization, and aberrant transactivation of downstream genes, contributing to the compromised osteogenic potential observed in CCD.

Heterozygous pathogenic variants involving CBFB cause a new skeletal disorder resembling cleidocranial dysplasia

BACKGROUND

Cleidocranial dysplasia (CCD) is a rare skeletal dysplasia with significant clinical variability. Patients with CCD typically present with delayed closure of fontanels and cranial sutures, dental anomalies, clavicular hypoplasia or aplasia and short stature. Runt-related transcription factor 2 (RUNX2) is currently the only known disease-causing gene for CCD, but several studies have suggested locus heterogeneity.

METHODS

The cohort consists of eight subjects from five unrelated families partially identified through GeneMatcher. Exome or genome sequencing was applied and in two subjects the effect of the variant was investigated at RNA level.

RESULTS

In each subject a heterozygous pathogenic variant in CBFB was detected, whereas no genomic alteration involving RUNX2 was found. Three CBFB variants (one splice site alteration, one nonsense variant, one 2 bp duplication) were shown to result in a premature stop codon. A large intragenic deletion was found to delete exon 4, without affecting CBFB expression. The effect of a second splice site variant could not be determined but most likely results in a shortened or absent protein. Affected individuals showed similarities with RUNX2-related CCD, including dental and clavicular abnormalities. Normal stature and neurocognitive problems were however distinguishing features. CBFB encodes the core-binding factor β subunit, which can interact with all RUNX proteins (RUNX1, RUNX2, RUNX3) to form heterodimeric transcription factors. This may explain the phenotypic differences between CBFB-related and RUNX2-related CCD.

CONCLUSION

We confirm the previously suggested locus heterogeneity for CCD by identifying five pathogenic variants in CBFB in a cohort of eight individuals with clinical and radiographic features reminiscent of CCD.

Multiple roles of Runt-related transcription factor-2 in tooth eruption: bone formation and resorption

OBJECTIVE

The aim was to provide a comprehensive review of the current knowledge of the multiple roles of Runt-related transcription factor-2 (RUNX2) in regulating tooth eruption, focusing on the molecular mechanisms regarding tooth eruption mediated by RUNX2.

DESIGN

Relevant literatures in PubMed, Medline, and Scopus database were searched, and a narrative review was performed. The multiple roles of RUNX2 in regulating tooth eruption was reviewed and discussed.

RESULTS

Aberrant RUNX2 expression leads to disturbed or failed tooth eruption. Tooth eruption involves both the process of bone formation and bone resorption. RUNX2 promotes osteogenesis around the radicular portion of the dental follicle that provides the biological force for tooth eruption through inducing the expression of osteogenesis-related genes in dental follicle cells/osteoblasts. On the other hand, through indirect and direct pathways, RUNX2 regulates osteoclastogenesis and the formation of the eruption pathway.

CONCLUSION

RUNX2 exerts a pivotal and complex influence in regulating tooth eruption. This review provides a better understanding of the function of RUNX2 in tooth eruption, which is beneficial to illuminate the precise molecular mechanism of osteogenesis and bone resorption, aiding the development of effective therapy for the failure of tooth eruption.

Cleidocranial dysplasia and novel RUNX2 variants: dental, craniofacial, and osseous manifestations

Cleidocranial dysplasia (CCD) is a skeletal disorder affecting cranial sutures, teeth, and clavicles, and is associated with the RUNX2 mutations. Although numerous patients have been described, a direct genotype–phenotype correlation for RUNX2 has been difficult to establish. Further cases must be studied to understand the clinical and genetic spectra of CCD.

Identification of a Novel Splice Site Mutation in RUNX2 Gene in a Family with Rare Autosomal Dominant Cleidocranial Dysplasia

Background:

Pathogenic variants of RUNX2, a gene that encodes an osteoblast-specific transcription factor, have been shown as the cause of CCD, which is a rare hereditary skeletal and dental disorder with dominant mode of inheritance and a broad range of clinical variability. Due to the relative lack of clinical complications resulting in CCD, the medical diagnosis of this disorder is challenging, which leaves it underdiagnosed.

Methods:

In this study, nine healthy and affected members of an Iranian family were investigated. PCR and sequencing of all exons and exon-intron boundaries of RUNX2 (NM_001024630) gene was performed on proband. Co-segregation analysis was conducted in the other family members for the identified variant. Additionally, a cohort of 100 Iranian ethnicity-matched healthy controls was screened by ARMS-PCR method.

Results:

The novel splice site variant (c.860-2A>G), which was identified in the intron 6 of RUNX2 gene, co-segregated with the disease in the family, and it was absent in healthy controls. Pathogenicity of this variant was determined by several software, including HSF, which predicts the formation or disruption of splice donor sites, splice acceptor sites, exonic splicing silencer sites, and exonic splicing enhancer sites. In silico analysis predicted this novel variant to be disease causing.

Conclusion:

The identified variant is predicted to have an effect on splicing, which leads to exon skipping and producing a truncated protein via introducing a premature stop codon.

Molecular mechanisms of intermuscular bone development in fish: a review

Intermuscular bones (IBs) are slender linear bones embedded in muscle, which ossify from tendons through a process of intramembranous ossification, and only exist in basal teleosts. IBs are essential for fish swimming, but they present a choking risk during human consumption, especially in children, which can lead to commercial risks that have a negative impact on the aquaculture of these fish. In this review, we discuss the morphogenesis and functions of IBs, including their underlying molecular mechanisms, as well as the advantages and disadvantages of different methods for IB studies and techniques for breeding and generating IB-free fish lines. This review reveals that the many key genes involved in tendon development, osteoblast differentiation, and bone formation, e.g., scxa, msxC, sost, twist, bmps, and osterix, also play roles in IB development. Thus, this paper provides useful information for the breeding of new fish strains without IBs via genome editing and artificial selection.

Skeletal and molecular findings in 51 Cleidocranial dysplasia patients from Turkey

Loss or decrease of function in runt-related transcription factor 2 encoded by RUNX2 is known to cause a rare autosomal-dominant skeletal disorder, cleidocranial dysplasia (CCD). Clinical spectrum and genetic findings in 51 CCD patients from 30 unrelated families are herein presented. In a majority of the patients, facial abnormalities, such as delayed fontanel closure (89%), parietal and frontal bossing (80%), metopic groove (77%), midface hypoplasia (94%), and abnormal mobility of shoulders (90%), were recorded following clinical examination. In approximately one-half of the subjects, wormian bone (51%), short stature (43%), bell-shaped thorax (42%), wide pubic symphysis (50%), hypoplastic iliac wing (59%), and chef's hat sign (44%) presented in available radiological examinations. Scoliosis was identified in 28% of the patients. Investigation of RUNX2 revealed small sequence alterations in 90% and gross deletions in 10% of the patients; collectively, 23 variants including 11 novel changes (c.29_30insT, c.203delAinsCG, c.423 + 2delT, c.443_454delTACCAGATGGGAinsG, c.505C > T, c.594_595delCTinsG, c.636_637insC, c.685 + 5G > A, c.1088G > T, c.1281delC, Exon 6-9 deletion) presented high allelic heterogeneity. Novel c.29_30insT is unique in affecting the P1-driven long isoform of RUNX2, which is expected to disrupt the N-terminal region of RUNX2; this was shown in two unrelated phenotypically discordant patients. The clinical findings highlighted mild intra-familial genotype-phenotype correlation in our CCD cohort.

Differential Expression of MicroRNA-3148 in Patients with Osteoporosis and Its Impacts on the Osteogenic Differentiation of Rat Bone Marrow Mesenchymal Stem Cells

We aimed to explore the effects of rat bone marrow mesenchymal stem cells (BMSCs) on osteogenic differentiation via analyzing miR-3148 expression in patients with osteoporosis. Realtime quantitative PCR was conducted for assessing microRNA-3148 expression. BMSCs from SD rats were transfected with microRNA-3148 mimics and microRNA-3148 inhibitor via liposomal trans-fection method utilizing Lipo2000, followed by analysis of microRNA-3148 level. After 10-days of osteogenic differentiation induction, alkaline phosphatase (ALP) staining and alizarin red (ARS) staining were done to investigate the osteogenic differentiation potential. Simultaneously, qRT-PCR measured the expression of osteogenesis marker genes (BMP and Runx2) in each group. qRT-PCR analysis revealed a high expression of miR-3148 in the bone tissue and the serum samples from patients with osteoporosis in comparison with healthy individuals. In addition, miRNA-3148 mimics could retard the osteogenic differentiation of BMSCs, while microRNA-3148 inhibitor could prompt the procedure. MicroRNA-3148 was highly expressed in the skeletal tissues and the serum samples from patients with osteoporosis and it could restrain the differentiation of BMSCs into osteoblasts, suggesting that it might be a novel therapeutic target for treating osteoporosis.

Transcriptional Programming in Arteriosclerotic Disease: A Multifaceted Function of the Runx2 (Runt-Related Transcription Factor 2)

Despite successful therapeutic strategies in the prevention and treatment of arteriosclerosis, the cardiovascular complications remain a major clinical and societal issue worldwide. Increased vascular calcification promotes arterial stiffness and accelerates cardiovascular morbidity and mortality. Upregulation of the Runx2 (Runt-related transcription factor 2), an essential osteogenic transcription factor for bone formation, in the cardiovascular system has emerged as an important regulator for adverse cellular events that drive cardiovascular pathology. This review discusses the regulatory mechanisms that are critical for Runx2 expression and function and highlights the dynamic and complex cross talks of a wide variety of posttranslational modifications, including phosphorylation, acetylation, ubiquitination, and O-linked β-N-acetylglucosamine modification, in regulating Runx2 stability, cellular localization, and osteogenic transcriptional activity. How the activation of an array of signaling cascades by circulating and local microenvironmental factors upregulates Runx2 in vascular cells and promotes Runx2-mediated osteogenic transdifferentiation of vascular smooth muscle cells and expression of inflammatory cytokines that accelerate macrophage infiltration and vascular osteoclast formation is summarized. Furthermore, the increasing appreciation of a new role of Runx2 upregulation in promoting vascular smooth muscle cell phenotypic switch, and Runx2 modulated by O-linked β-N-acetylglucosamine modification and Runx2-dependent repression of smooth muscle cell-specific gene expression are discussed. Further exploring the regulation of this key osteogenic transcription factor and its new perspectives in the vasculature will provide novel insights into the transcriptional regulation of vascular smooth muscle cell phenotype switch, reprograming, and vascular inflammation that promote the pathogenesis of arteriosclerosis.

Evolution and expansion of the RUNX2 QA repeat corresponds with the emergence of vertebrate complexity

Runt-related transcription factor 2 (RUNX2) is critical for the development of the vertebrate bony skeleton. Unlike other RUNX family members, RUNX2 possesses a variable poly-glutamine, poly-alanine (QA) repeat domain. Natural variation within this repeat is able to alter the transactivation potential of RUNX2, acting as an evolutionary 'tuning knob' suggested to influence mammalian skull shape. However, the broader role of the RUNX2 QA repeat throughout vertebrate evolution is unknown. In this perspective, we examine the role of the RUNX2 QA repeat during skeletal development and discuss how its emergence and expansion may have facilitated the evolution of morphological novelty in vertebrates.

Intracellular Delivery of Recombinant RUNX2 Facilitated by Cell-Penetrating Protein for the Osteogenic Differentiation of hMSCs

Human mesenchymal stem cells (hMSCs) are a commonly used cell source for cell therapy and tissue engineering because of their easy accessibility and multipotency. Runt-related transcription factor 2 (RUNX2) is a master regulator of the osteogenic commitment of hMSCs. Either recombinant plasmid delivery or viral transduction has been utilized to activate RUNX2 gene expression for effective hMSC differentiation. In this study, recombinant RUNX2 fused with cell-penetrating 30Kc19α protein (30Kc19α-RUNX2) was delivered into hMSCs for osteogenic commitment. Fusion of recombinant RUNX2 with 30Kc19α resulted in successful delivery of the protein into cells and enhanced soluble expression of the protein. Intracellular delivery of the 30Kc19α-RUNX2 fusion protein enhanced the osteogenic differentiation of hMSCs in vitro. 30Kc19α-RUNX2 treatment resulted in increased ALP accumulation and elevated calcium deposition. Finally, implantation of hMSCs treated with 30Kc19α-RUNX2 showed osteogenesis via cell delivery into the subcutaneous tissue and bone regeneration in a cranial defect mouse model. Therefore, we suggest that 30Kc19α-RUNX2, an osteoinductive recombinant protein, is an efficient tool for bone tissue engineering.

Gain-of-function variants and overexpression of RUNX2 in patients with nonsyndromic midline craniosynostosis

Craniosynostosis (CS), the premature fusion of one or more cranial sutures, is a relatively common congenital anomaly, occurring in 3-5 per 10,000 live births. Nonsyndromic CS (NCS) accounts for up to 80% of all CS cases, yet the genetic factors contributing to the disorder remain largely unknown. The RUNX2 gene, encoding a transcription factor critical for bone and skull development, is a well known CS candidate gene, as copy number variations of this gene locus have been found in patients with syndromic craniosynostosis. In the present study, we aimed to characterize RUNX2 to better understand its role in the genetic etiology and in the molecular mechanisms underlying midline suture ossification in NCS. We report four nonsynonymous variants, one intronic variant and one 18 bp in-frame deletion in RUNX2 not found in our study control population. Significant difference in allele frequency (AF) for the deletion variant RUNX2 p.Ala84-Ala89del (ClinVar 257,095; dbSNP rs11498192) was observed in our sagittal NCS cohort when compared to the general population (P = 1.28 × 10^-6), suggesting a possible role in the etiology of NCS. Dual-luciferase assays showed that three of four tested RUNX2 variants conferred a gain-of-function effect on RUNX2, further suggesting their putative pathogenicity in the tested NCS cases. Downregulation of RUNX2 expression was observed in prematurely ossified midline sutures. Metopic sites showed significant downregulation of promoter 1-specific isoforms compared to sagittal sites. Suture-derived mesenchymal stromal cells showed an increased expression of RUNX2 over matched unfused suture derived cells. This demonstrates that RUNX2, and particularly the distal promoter 1-isoform group, are overexpressed in the osteogenic precursors within the pathological suture sites.

Post-Translational Regulations of Transcriptional Activity of RUNX2

Runt-related transcription factor 2 (RUNX2) is a key transcription factor for bone formation and osteoblast differentiation. Various signaling pathways and mechanisms that regulate the expression and transcriptional activity of RUNX2 have been thoroughly investigated since the involvement of RUNX2 was first reported in bone formation. As the regulation of Runx2 expression by extracellular signals has recently been reviewed, this review focuses on the regulation of post-translational RUNX2 activity. Transcriptional activity of RUNX2 is regulated at the post-translational level by various enzymes including kinases, acetyl transferases, deacetylases, ubiquitin E3 ligases, and prolyl isomerases. We describe a sequential and linear causality between post-translational modifications of RUNX2 by these enzymes. RUNX2 is one of the most important osteogenic transcription factors; however, it is not a suitable drug target. Here, we suggest enzymes that directly regulate the stability and/or transcriptional activity of RUNX2 at a post-translational level as effective drug targets for treating bone diseases.

Identification of RUNX2 variants associated with cleidocranial dysplasia

Background

Cleidocranial dysplasia (CCD) is a rare autosomal dominant disorder mainly characterized by hypoplastic or absent clavicles, delayed closure of the fontanelles, multiple dental abnormalities, and short stature. Runt-related transcription factor 2 (RUNX2) gene variants can cause CCD, but are not identified in all CCD patients.

Methods

In this study, we detected genetic variants in seven unrelated children with CCD by targeted high-throughput DNA sequencing or Sanger sequencing.

Results

All patients carried a RUNX2 variant, totally including three novel pathogenic variants (c.722_725delTGTT, p.Leu241Serfs*8; c.231_232delTG, Ala78Glyfs*82; c.909C > G, p.Tyr303*), three reported pathogenic variants (c.577C > T, p.Arg193*; c.574G > A, p.Gly192Arg; c.673 C > T, p.Arg225Trp), one likely pathogenic variant (c.668G > T, p.Gly223Val). The analysis of the variant source showed that all variants were de novo except the two variants (c.909C > G, p.Tyr303*; c.668G > T, p.Gly223Val) inherited from the patient’s father and mother with CCD respectively. Further bioinformatics analysis indicated that these variants could influence the structure of RUNX2 protein by changing the number of H-bonds or amino acids. The experimental result showed that the Gly223Val mutation made RUNX2 protein unable to quantitatively accumulate in the nucleus.

Conclusions

The present study expands the pathogenic variant spectrum of RUNX2 gene, which will contribute to the diagnosis of CCD and better genetic counseling in the future.

Identification of a novel mutation of RUNX2 in a family with supernumerary teeth and craniofacial dysplasia by whole-exome sequencing: A case report and literature review

RATIONALE

Supernumerary teeth are those that teeth in excess number than the normal count. It is usually associated with genetic syndromes when present in more numbers. Several causal genes, such as APC, NHS, TRPS1, EVC and RUNX2, have been identified. However, etiology of supernumerary teeth remains largely unclear.

PATIENT CONCERNS

A family with the clinical diagnosis of supernumerary teeth, short stature and craniofacial dysplasia was examined.

DIAGNOSES

Molecular genetic analysis found that mutation occurred in the RUNX2 gene. On the basis of this finding and clinical manifestations, the final diagnosis of cleidocranial dysplasia was made.

INTERVENTIONS

Whole exome sequencing (WES) of DNA samples was performed to identify the disease-causing mutation, including the affected child and mother as well as the healthy father.

OUTCOMES

A novel mutation of RUNX2 (c.473C>A; p.A158E) was identified in both patients, but not in normal family member and in-house database containing 3,000 Chinese Han individuals WES. This mutation was further confirmed by Sanger sequencing and predicted to be deleterious by several commonly used algorithms, including SIFT, PPT-2, MutationTaster and Proven. Furthermore, phenotype-genotype correlation analyses of all published 239 cases with different mutations in RUNX2 revealed significant association of supernumerary teeth and facial dysplasia with the Runt domain of the encoded protein.

LESSONS

This is the first WES study to identify genetic cause in Chinese patients with a novel RUNX2 mutation. Our findings expanded the mutation spectrum and clinical features of the disease and facilitated clinic diagnosis and genetic counseling.

Metabolomics profiling of cleidocranial dysplasia

OBJECTIVES

Cleidocranial dysplasia (CCD) is a rare autosomal-dominantly inherited skeletal dysplasia that is predominantly associated with heterozygous mutations of RUNX2. However, no information is available regarding metabolic changes associated with CCD at present.

MATERIALS AND METHODS

We analyzed members of a CCD family and checked for mutations in the RUNX2 coding sequence using the nucleotide BLAST program. The 3D protein structure of mutant RUNX2 was predicted by I-TASSER. Finally, we analyzed metabolites extracted from plasma using LC-MS/MS.

RESULTS

We identified a novel mutation (c.1061insT) that generates a premature termination in the RUNX2 coding region, which, based on protein structure prediction models, likely alters the protein's function. Interestingly, metabolomics profiling indicated that 30 metabolites belonging to 13 metabolic pathways were significantly changed in the CCD patients compared to normal controls.

CONCLUSIONS

The results highlight interesting correlations between a RUNX2 mutation, metabolic changes, and the clinical features in a family with CCD. The results also contribute to our understanding of the pathogenetic processes underlying this rare disorder.

CLINICAL RELEVANCE

This study provides the first metabolomics profiling in CCD patients, expands our insights into the pathogenesis of the disorder, may help in diagnostics and its refinements, and may lead to novel therapeutic approaches to CCD.

A novel Alu-mediated microdeletion in the RUNX2 gene in a Chinese patient with cleidocranial dysplasia

Cleidocranial dysplasia (CCD; OMIM: 119600) is a rare autosomal dominant skeletal dysplasia caused by RUNX2 gene mutations. The present study described a sporadic case with CCD. The clinical data of the proband with CCD was reported and genetic analysis was performed. The proband presented with typical CCD features including supernumerary impacted teeth, bilateral clavicle dysplasia, delayed closure of cranial sutures, and short stature; while his hands were normal. Sequencing analysis of the entire coding region of the RUNX2 gene revealed no pathogenic changes; however, copy-number analysis with the Affymetrix HD array found ~500 kb genomicmicrodeletion. Real-time quantitative PCR validated this microdeletion in the 1-4 exons of the RUNX2 gene. The junction point of the breaking DNA was located in the directly oriented AluSz6 and AluSx repetitive elements, indicating that this microdeletion might be generated through an Alu-Alu mediated mechanism. In addition, this microdeletion existed in 21.8% of the asymptomatic mother's peripheral blood cells, demonstrating that the mosaicism was not associated with CCD phenotypes. In summary, a pathogenic microdeletion in the RUNX2 gene located on chromosome 6 was responsible for CCD.

Mutant Runx2 regulates amelogenesis and osteogenesis through a miR-185-5p-Dlx2 axis

Regulation of microRNAs (miRNA) has been extensively investigated in diseases; however, little is known about the roles of miRNAs in cleidocranial dysplasia (CCD). The aim of the present study was to investigate the potential involvement of miRNAs in CCD. In vitro site-directed mutagenesis was performed to construct three mutant Runx2 expression vectors, which were then transfected into LS8 cells and MC3T3-E1 cells, to determine the impact on amelogenesis and osteogenesis, respectively. miRCURY LNA miRNA microarray identify miR-185-5p as a miRNA target commonly induced by all three Runx2 mutants. Real-time quantitative PCR was applied to determine the expression of miR-185-5p and Dlx2 in samples. Dual-luciferase reporter assays were conducted to confirm Dlx2 as a legitimate target of miR-185-5p. The suppressive effect of miR-185-5p on amelogenesis and osteogenesis of miR-185-5p was evaluated by RT-PCR and western blot examination of Amelx, Enam, Klk4, and Mmp20 gene and protein expression, and by Alizarin Red stain. We found that mutant Runx2 suppressed amelogenesis and osteogenesis. miR-185-5p, induced by Runx2, suppressed amelogenesis and osteogenesis. Furthermore, we identified Dlx2 as direct target of miR-185-5p. Consistently, Dlx2 expression was inversely correlated with miR-185-5p levels. This study highlights the molecular etiology and significance of miR-185-5p in CCD, and suggests that targeting miR-185-5p may represent a new therapeutic strategy in prevention or intervention of CCD.

Novel Mutation of the RUNX2 Gene in Patients with Cleidocranial Dysplasia

Cleidocranial dysplasia (CCD) is an autosomal dominant disorder linked to mutations in the Runt-related transcription factor 2, encoded by the RUNX2 gene, which is essential for osteoblast differentiation and skeletal development. Here, we describe a novel nonsense mutation (c.532C>T; p.Q178X) in RUNX2 identified in 3 affected members of a Polish family with CCD. The localization and transcriptional transactivation studies show that the mutated form of the protein has altered the subcellular localization and significantly decreased transactivation properties, respectively. Consequently, our data show that the c.532C>T mutation generates a defective RUNX2 protein and is genetically linked to the CCD phenotype.

Facing the facts: The Runx2 gene is associated with variation in facial morphology in primates

The phylogenetic and adaptive factors that cause variation in primate facial form-including differences among the major primate clades and variation related to feeding and/or social behavior-are relatively well understood. However, comparatively little is known about the genetic mechanisms that underlie diversity in facial form in primates. Because it is essential for osteoblastic differentiation and skeletal development, the runt-related transcription factor 2 (Runx2) is one gene that may play a role in these genetic mechanisms. Specifically, polymorphisms in the QA ratio (determined by the ratio of the number of polyglutamines to polyalanines in one functional domain of Runx2) have been shown to be correlated with variation in facial length and orientation in other mammal groups. However, to date, the relationship between variation in this gene and variation in facial form in primates has not been explicitly tested. To test the hypothesis that the QA ratio is correlated with facial form in primates, the current study quantified the QA ratio, facial length, and facial angle in a sample of 33 primate species and tested for correlation using phylogenetic generalized least squares. The results indicate that the QA ratio of the Runx2 gene is positively correlated with variation in relative facial length in anthropoid primates. However, no correlation was found in strepsirrhines, and there was no correlation between facial angle and the QA ratio in any groups. These results suggest that, in primates, the QA ratio of the Runx2 gene may play a role in modulating facial size, but not facial orientation. This study therefore provides important clues about the genetic and developmental mechanisms that may underlie variation in facial form in primates.

Analysis of novel RUNX2 mutations in Chinese patients with cleidocranial dysplasia

Cleidocranial dysplasia (CCD) is an autosomal dominant inheritable skeletal disorder characterized by cranial dysplasia, clavicle hypoplasia and dental abnormalities. This disease is mainly caused by heterozygous mutations in RUNX2, a gene that encodes an osteoblast-specific transcription factor. In the present study, mutational analyses of RUNX2 gene were performed on four unrelated Chinese patients with CCD. Four different RUNX2 mutations were detected in these patients, including one nonsense mutation (c.199C>T p.Q67X) and three missense mutations (c.338T>G p.L113R, c.557G>C p.R186T and c.673C>T p.R225W). Among them, two mutations (c.199C>T p.Q67X and c.557G>C p.R186T) were novel and the other two had been reported in previous literatures. Except for Q67X mutation located in the Q/A domain, other three mutations were clustered within the highly conserved Runt domain. Green fluorescent protein (GFP) and RUNX2 fusion protein analyses in vitro showed that nuclear accumulation of RUNX2 protein was disturbed by Q67X mutation, while the other two mutations (c.338T>G p.L113R and c.557G>C p.R186T) had no effects on the subcellular distribution of RUNX2. Luciferase reporter assay demonstrated that all the three novel RUNX2 mutations significantly reduced the transactivation activity of RUNX2 on osteocalcin promoter. Our findings enrich the evidence of molecular genetics that the mutations of RUNX2 gene are responsible for CCD.

RUNX2 repeat variation does not drive craniofacial diversity in marsupials

BACKGROUND

Runt-related transcription factor 2 (RUNX2) is a transcription factor essential for skeletal development. Variation within the RUNX2 polyglutamine / polyalanine (QA) repeat is correlated with facial length within orders of placental mammals and is suggested to be a major driver of craniofacial diversity. However, it is not known if this correlation exists outside of the placental mammals.

RESULTS

Here we examined the correlation between the RUNX2 QA repeat ratio and facial length in the naturally evolving sister group to the placental mammals, the marsupials. Marsupials have a diverse range of facial lengths similar to that seen in placental mammals. Despite their diversity there was almost no variation seen in the RUNX2 QA repeat across individuals spanning the entire marsupial infraclass. The extreme conservation of the marsupial RUNX2 QA repeat indicates it is under strong purifying selection. Despite this, we observed an unexpectedly high level of repeat purity.

CONCLUSIONS

Unlike within orders of placental mammals, RUNX2 repeat variation cannot drive craniofacial diversity in marsupials. We propose conservation of the marsupial RUNX2 QA repeat is driven by the constraint of accelerated ossification of the anterior skeleton to facilitate life in the pouch. Thus, marsupials must utilize alternate pathways to placental mammals to drive craniofacial evolution.

Control of the Osteoblast Lineage by Mitogen-Activated Protein Kinase Signaling

PURPOSE OF THE REVIEW

This review will provide a timely assessment of MAP kinase actions in bone development and homeostasis with particular emphasis on transcriptional control of the osteoblast lineage.

RECENT FINDINGS

ERK and p38 MAP kinases function as transducers of signals initiated by the extracellular matrix, mechanical loading, TGF-β, BMPs and FGF2. MAPK signals may also affect and/or interact with other important pathways such as WNT and HIPPO. ERK and p38 MAP kinase pathways phosphorylate specific osteogenic transcription factors including RUNX2, Osterix, ATF4 and DLX5. For RUNX2, phosphorylation at specific serine residues initiates epigenetic changes in chromatin necessary for decondensation and increased transcription. MAPK also suppresses marrow adipogenesis by phosphorylating and inhibiting PPARγ, which may explain the well-known relationship between reduced skeletal loading and marrow fat accumulation.

SUMMARY

MAPKs transduce signals from the extracellular environment to the nucleus allowing bone cells to respond to changes in hormonal/growth factor signaling and mechanical loading thereby optimizing bone structure to meet physiological and mechanical needs of the body.

A variant of Runx2 that differs from the bone isoform in its splicing is expressed in spermatogenic cells

Background. Members of the Runx gene family encode transcription factors that bind to DNA in a sequence-specific manner. Among the three Runx proteins, Runx2 comprises 607 amino acid (aa) residues, is expressed in bone, and plays crucial roles in osteoblast differentiation and bone development. We examined whether the Runx2 gene is also expressed in testes.

Methods. Murine testes from 1-, 2-, 3-, 4-, and 10-week-old male mice of the C57BL/6J strain and W∕W^v strain were used throughout the study. Northern Blot Analyses were performed using extracts form the murine testes. Sequencing of cDNA clones and 5′-rapid amplification of cDNA ends were performed to determine the full length of the transcripts, which revealed that the testicular Runx2 comprises 106 aa residues coding novel protein. Generating an antiserum using the amino-terminal 15 aa of Runx2 (Met¹ to Gly¹⁵) as an antigen, immunoblot analyses were performed to detect the predicted polypeptide of 106 aa residues with the initiating Met¹. With the affinity-purified anti-Runx2 antibody, immunohistochemical analyses were performed to elucidate the localization of the protein. Furthermore, bioinformatic analyses were performed to predict the function of the protein.

Results. A Runx2 transcript was detected in testes and was specifically expressed in germ cells. Determination of the transcript structure indicated that the testicular Runx2 is a splice isoform. The predicted testicular Runx2 polypeptide is composed of only 106 aa residues, lacks a Runt domain, and appears to be a basic protein with a predominantly alpha-helical conformation. Immunoblot analyses with an anti-Runx2 antibody revealed that Met¹ in the deduced open reading frame of Runx2 is used as the initiation codon to express an 11 kDa protein. Furthermore, immunohistochemical analyses revealed that the Runx2 polypeptide was located in the nuclei, and was detected in spermatocytes at the stages of late pachytene, diplotene and second meiotic cells as well as in round spermatids. Bioinformatic analyses suggested that the testicular Runx2 is a histone-like protein.

Discussion. A variant of Runx2 that differs from the bone isoform in its splicing is expressed in pachytene spermatocytes and round spermatids in testes, and encodes a histone-like, nuclear protein of 106 aa residues. Considering its nuclear localization and differentiation stage-dependent expression, Runx2 may function as a chromatin-remodeling factor during spermatogenesis. We thus conclude that a single Runx2 gene can encode two different types of nuclear proteins, a previously defined transcription factor in bone and cartilage and a short testicular variant that lacks a Runt domain.

MicroRNA-145 regulates osteoblastic differentiation by targeting the transcription factor Cbfb

Osteoblastic differentiation is regulated by various factors, including hormones and transcription factors. Runt-related transcription factor 2 (Runx2) is an essential player in osteoblastogenesis and transactivates its molecular target by creating a protein complex with its hetero-dimeric partner core binding factor beta (Cbfb). However, the molecular regulation of Cbfb expression remains unknown. Here, we identified miR-145 as a crucial regulator of Cbfb expression. The expression of miR-145 increased during osteoblastogenesis, indicating that miR-145 works as an inhibitor of osteoblastogenesis. Stable expression of miR-145 decreased endogenous Cbfb expression and inhibited osteoblastogenesis, in cooperation with miR-34c. Furthermore, miR-145 decreased bone regeneration in vivo. Our results indicate that miR-145 physiologically regulates osteoblast differentiation and bone formation via Cbfb expression by forming a regulatory microRNA network.

Role of Runx2 phosphorylation in prostate cancer and association with metastatic disease

The osteogenic transcription factor, Runx2, is abnormally expressed in prostate cancer (PCa) and associated with metastatic disease. During bone development, Runx2 is activated by signals known to be hyperactive in PCa including the RAS/MAP kinase pathway, which phosphorylates Runx2 on multiple serine residues including S301 and S319 (equivalent to S294 and S312 in human Runx2). This study examines the role of these phosphorylation sites in PCa. Runx2 was preferentially expressed in more invasive prostate cancer cell lines (PC3 > C4-2B > LNCaP). Furthermore, analysis using a P-S319-Runx2-specific antibody revealed that the ratio of P-S319-Runx2/total Runx2 as well as P-ERK/total ERK was highest in PC3 followed by C4-2B and LNCaP cells. These results were confirmed by immunofluorescence confocal microscopy, which showed a higher percentage of PC3 cells staining positive for P-S319-Runx2 relative to C4-2B and LNCaP cells. Phosphorylated Runx2 had an exclusively nuclear localization. When expressed in prostate cell lines, wild type Runx2 increased metastasis-associated gene expression, in vitro migratory and invasive activity as well as in vivo growth of tumor cell xenografts. In contrast, S301A/S319A phosphorylation site mutations greatly attenuated these Runx2 responses. Analysis of tissue microarrays from 129 patients revealed strong nuclear staining with the P-S319-Runx2 antibody in primary prostate cancers and metastases. P-S319-Runx2 staining was positively correlated with Gleason score and occurrence of lymph node metastases while little or no Runx2 phosphorylation was seen in normal prostate, benign prostate hyperplasia or prostatitis indicating that Runx2 S319 phosphorylation is closely associated with prostate cancer induction and progression towards an aggressive phenotype. These studies establish the importance of Runx2 phosphorylation in prostate tumor growth and highlight its value as a potential diagnostic marker and therapeutic target.

Novel mutation of RUNX2 gene in a patient with cleidocranial dysplasia

BACKGROUND

Cleidocranial dysplasia is a rare hereditary skeletal disorder due to heterozygous loss of function mutations in the RUNX2 gene that encodes runt-related transcription factor 2 (RUNX2). Here we report a 52 year-old woman with cleidocranial dysplasia due to a novel RUNX2 mutation.

CASE DESCRIPTION

A 52 year-old Han Chinese woman presented with short stature and skeletal dysplasia that was first noted during early childhood. She was 153 cm in height and 40 kg in weight. Her skull was deformed with hypertelorism, midface hypoplasia, protrusion of chin, and dental abnormalities. Radiological examination revealed shortened clavicles and depressed skull bone and that were consistent with the clinical diagnosis of cleidocranial dysplasia. There was no family history of a similar skeletal disorder. We sequenced the RUNX2 gene and discovered a novel heterozygous mutation in exon 3 (c.476 del G, p.G159fs175X) that is predicted to cause a frameshift and premature termination that leads to the loss of the final 347 amino acid residues. This severely truncated protein is expected to be inactive.

LITERATURE REVIEW

RUNX2 gene controls osteoblast differentiation and chondrocyte maturation. Around 90 RUNX2 mutations have been discovered in patients with cleidocranial dysplasia.

CLINICAL RELEVANCE

We identified a case of cleidocranial dysplasia due to a novel mutation of RUNX2 gene at exon 3 (c.476 del G).

Periostin in intrahepatic cholangiocarcinoma: pathobiological insights and clinical implications

Periostin is a modular glycoprotein frequently observed to be a major constituent of the extracellular milieu of mass-forming intrahepatic cholangiocarcinoma and other desmoplastic malignant tumors. In intrahepatic cholangiocarcinoma, as well as in desmoplastic pancreatic ductal adenocarcinoma, periostin is overexpressed and hypersecreted in large part, if not exclusively, by cancer-associated fibroblasts within the tumor stroma. Through its interaction with specific components of the extracellular tumor matrix, particularly collagen type I and tenascin-C, and with cell surface receptors, notably integrins leading to activation of the Akt and FAK signaling pathways, this TGF-β family-inducible matricellular protein appears to be functioning as a key extracellular matrix molecule regulating such critically important and diverse malignant tumor behaviors as tumor fibrogenesis and desmoplasia, invasive malignant cell growth, chemoresistance, and metastatic colonization. This review will discuss current evidence and basic molecular mechanisms implicating periostin as a mediator of intrahepatic cholangiocarcinoma invasive growth. In addition, its significance as a potential prognostic biomarker for intrahepatic cholangiocarcinoma patients, as well as future possibilities and challenges as a molecular target for cholangiocarcinoma therapy and/or prevention, will be critically evaluated.

Mitogen-activated protein kinase (MAPK)-regulated interactions between Osterix and Runx2 are critical for the transcriptional osteogenic program

The transcription factors Runx2 and Osx (Osterix) are required for osteoblast differentiation and bone formation. Runx2 expression occurs at early stages of osteochondroprogenitor determination, followed by Osx induction during osteoblast maturation. We demonstrate that coexpression of Osx and Runx2 leads to cooperative induction of expression of the osteogenic genes Col1a1, Fmod, and Ibsp. Functional interaction of Osx and Runx2 in the regulation of these promoters is mediated by enhancer regions with adjacent Sp1 and Runx2 DNA-binding sites. These enhancers allow formation of a cooperative transcriptional complex, mediated by the binding of Osx and Runx2 to their specific DNA promoter sequences and by the protein-protein interactions between them. We also identified the domains involved in the interaction between Osx and Runx2. These regions contain the amino acids in Osx and Runx2 known to be phosphorylated by p38 and ERK MAPKs. Inhibition of p38 and ERK kinase activities or mutation of their known phosphorylation sites in Osx or Runx2 strongly disrupts their physical interaction and cooperative transcriptional effects. Altogether, our results provide a molecular description of a mechanism for Osx and Runx2 transcriptional cooperation that is subject to further regulation by MAPK-activating signals during osteogenesis.

Runx family genes in a cartilaginous fish, the elephant shark (Callorhinchus milii)

The Runx family genes encode transcription factors that play key roles in hematopoiesis, skeletogenesis and neurogenesis and are often implicated in diseases. We describe here the cloning and characterization of Runx1, Runx2, Runx3 and Runxb genes in the elephant shark (Callorhinchus milii), a member of Chondrichthyes, the oldest living group of jawed vertebrates. Through the use of alternative promoters and/or alternative splicing, each of the elephant shark Runx genes expresses multiple isoforms similar to their orthologs in human and other bony vertebrates. The expression profiles of elephant shark Runx genes are similar to those of mammalian Runx genes. The syntenic blocks of genes at the elephant shark Runx gene loci are highly conserved in human, but represented by shorter conserved blocks in zebrafish indicating a higher degree of rearrangements in this teleost fish. Analysis of promoter regions revealed conservation of binding sites for transcription factors, including two tandem binding sites for Runx that are totally conserved in the distal promoter regions of elephant shark Runx1-3. Several conserved noncoding elements (CNEs), which are putative cis-regulatory elements, and miRNA binding sites were identified in the elephant shark and human Runx gene loci. Some of these CNEs and miRNA binding sites are absent in teleost fishes such as zebrafish and fugu. In summary, our analysis reveals that the genomic organization and expression profiles of Runx genes were already complex in the common ancestor of jawed vertebrates.

Wnt signaling regulates pulp volume and dentin thickness

Odontoblasts, cementoblasts, ameloblasts, and osteoblasts all form mineralized tissues in the craniofacial complex, and all these cell types exhibit active Wnt signaling during postnatal life. We set out to understand the functions of this Wnt signaling, by evaluating the phenotypes of mice in which the essential Wnt chaperone protein, Wntless was eliminated. The deletion of Wls was restricted to cells expressing Osteocalcin (OCN), which in addition to osteoblasts includes odontoblasts, cementoblasts, and ameloblasts. Dentin, cementum, enamel, and bone all formed in OCN-Cre;Wls(fl/fl) mice but their homeostasis was dramatically affected. The most notable feature was a significant increase in dentin volume and density. We attribute this gain in dentin volume to a Wnt-mediated misregulation of Runx2. Normally, Wnt signaling stimulates Runx2, which in turn inhibits dentin sialoprotein (DSP); this inhibition must be relieved for odontoblasts to differentiate. In OCN-Cre;Wls(fl/fl) mice, Wnt pathway activation is reduced and Runx2 levels decline. The Runx2-mediated repression of DSP is relieved and odontoblast differentiation is accordingly enhanced. This study demonstrates the importance of Wnt signaling in the homeostasis of mineralized tissues of the craniofacial complex.

YY1 represses the transcriptional activity of Runx2 in C2C12 cells

Runx2 is a major transcription factor that induces osteoblast differentiation by bone morphogenetic proteins (BMPs). Conversely, YY1 is a transcription factor that inhibits BMP2-induced cell differentiation. Until now, there has been no understanding of how osteoblast differentiation by Runx2 and YY1 is regulated. In this study we focused on the relationship between Runx2 and YY1. We confirmed that alkaline phosphatase staining is repressed by YY1. Runx2 interacted with YY1 through Runt and the C-terminus domain of Runx2. YY1 markedly repressed the Runx2-mediated enhancement of transcriptional activity on the osteocalcin and alkaline phosphatase promoters. Knockdown of YY1 enhanced BMP2- and Runx2-induced osteoblast differentiation. YY1 decreased Runx2 DNA binding affinity. The results indicate that YY1 represses osteoblast differentiation by an interaction with Runx2 and inhibits the transcriptional activity of Runx2.

Nuclear matrix-targeting of the osteogenic factor Runx2 is essential for its recognition and activation of the alkaline phosphatase gene

BACKGROUND

A good understanding of the mechanism of gene regulation that is involved in bone mineralization is critical for the design of anabolic treatments for bone deficiency diseases. Alkaline phosphatase (ALP) expressed by osteoblasts plays an important role in promoting bone mineralization by hydrolyzing pyrophosphate. However, the mechanism by which the expression of ALP is regulated during osteoblast differentiation has not been thoroughly investigated.

METHODS

Chromatin immunoprecipitation. EMSA and mutagenesis were used to identify the Runx2 binding sites on ALP gene and to analyze the role of nuclear matrix-localization of Runx2 on the recognition and activation of ALP gene.

RESULTS

Using chromatin immunoprecipitation, we determined that both ectopic and endogenous Runx2 bound to ALP intron 1 in a region containing a cluster of five putative core-sites. The third one (11C3) among those fives was bound most strongly in vitro by Runx2 and acted as a Runx2-dependent transcriptional enhancer. Furthermore, a Runx2 mutant lacking the nuclear matrix-targeting sequence (Runx2deltaNMTS) bound to the ALP gene less efficiently than the wild-type protein and a Runx2 mutant that is deficient in its ability to bind to DNA (Runx2K120A) accumulated largely in the nuclear matrix.

CONCLUSIONS

Nuclear matrix-localization of Runx2 influences its ALP gene recognition.

GENERAL SIGNIFICANCE

Our results showed for the first time that ALP is a direct target gene of Runx2 and illustrated that the recognition/binding and activation of the ALP by this transcription factor are dependent on its nuclear matrix-targeting.

Osteoblast-targeted overexpression of TAZ increases bone mass in vivo

Osteoblasts are derived from mesenchymal progenitors. Differentiation to osteoblasts and adipocytes is reciprocally regulated. Transcriptional coactivator with a PDZ-binding motif (TAZ) is a transcriptional coactivator that induces differentiation of mesenchymal cells into osteoblasts while blocking differentiation into adipocytes. To investigate the role of TAZ on bone metabolism in vivo, we generated transgenic mice that overexpress TAZ under the control of the procollagen type 1 promoter (Col1-TAZ). Whole body bone mineral density (BMD) of 6- to 19-week-old Col-TAZ mice was 4% to 7% higher than that of their wild-type (WT) littermates, whereas no difference was noticed in Col.1-TAZ female mice. Microcomputed tomography analyses of proximal tibiae at 16 weeks of age demonstrated a significant increase in trabecular bone volume (26.7%) and trabecular number (26.6%) with a reciprocal decrease in trabecular spacing (14.2%) in Col1-TAZ mice compared with their WT littermates. In addition, dynamic histomorphometric analysis of the lumbar spine revealed increased mineral apposition rate (42.8%) and the serum P1NP level was also significantly increased (53%) in Col.1-TAZ mice. When primary calvaria cells were cultured in osteogenic medium, alkaline phosphatase (ALP) activity was significantly increased and adipogenesis was significantly suppressed in Col1-TAZ mice compared with their WT littermates. Quantitative real-time polymerase chain reaction analyses showed that expression of collagen type 1, bone sialoprotein, osteocalcin, ALP, osterix, and Runx2 was significantly increased in calvaria cells from Col1-TAZ mice compared to their WT littermates. In vitro, TAZ enhanced Runx2-mediated transcriptional activity while suppressing the peroxisome proliferator-activated receptor gamma signaling pathway. TAZ also enhanced transcriptional activity from 3TP-Lux, which reflects transforming growth factor-beta (TGF-β)-mediated signaling. In addition, TAZ enhanced TGF-β-dependent nuclear translocation of Smad2/3 and Smad4. Taken together, these results suggest that TAZ positively regulates bone formation in vivo, which seems to be mediated by enhancing both Runx2 and TGF-β signaling.

A new phenotypic variant in cleidocranial dysplasia (CCD) associated with mutation c.391C>T of the RUNX2 gene

The RUNX2 gene is a physiological regulatory gene implicated in the development of cleidocranial dysplasia (CCD). A 13-month-old child presented with clinical features of CCD. At the age of 3 years the diagnosis was corroborated by clinical genetic assessment and DNA analysis, revealing a missense mutation p.R131C (c.391C>T) in RUNX2. At the age of 8 years the child was found to have a unique dental phenotype, represented by lack of supernumerary teeth and congenital absence of one tooth. A simple therapeutic approach was adopted, consisting of interceptive orthodontic treatment. The presence of this specific missense mutation in RUNX2, associated with the lack of typical supernumerary teeth may suggest a phenotype-genotype association.

Loss of RASSF1A synergizes with deregulated RUNX2 signaling in tumorigenesis

The tumor suppressor gene RASSF1A is inactivated through point mutation or promoter hypermethylation in many human cancers. In this study, we conducted a Sleeping Beauty transposon-mediated insertional mutagenesis screen in Rassf1a-null mice to identify candidate genes that collaborate with loss of Rassf1a in tumorigenesis. We identified 10 genes, including the transcription factor Runx2, a transcriptional partner of Yes-associated protein (YAP1) that displays tumor suppressive activity through competing with the oncogenic TEA domain family of transcription factors (TEAD) for YAP1 association. While loss of RASSF1A promoted the formation of oncogenic YAP1-TEAD complexes, the combined loss of both RASSF1A and RUNX2 further increased YAP1-TEAD levels, showing that loss of RASSF1A, together with RUNX2, is consistent with the multistep model of tumorigenesis. Clinically, RUNX2 expression was frequently downregulated in various cancers, and reduced RUNX2 expression was associated with poor survival in patients with diffuse large B-cell or atypical Burkitt/Burkitt-like lymphomas. Interestingly, decreased expression levels of RASSF1 and RUNX2 were observed in both precursor T-cell acute lymphoblastic leukemia and colorectal cancer, further supporting the hypothesis that dual regulation of YAP1-TEAD promotes oncogenic activity. Together, our findings provide evidence that loss of RASSF1A expression switches YAP1 from a tumor suppressor to an oncogene through regulating its association with transcription factors, thereby suggesting a novel mechanism for RASSF1A-mediated tumor suppression.

Monocytes induce STAT3 activation in human mesenchymal stem cells to promote osteoblast formation

A major therapeutic challenge is how to replace bone once it is lost. Bone loss is a characteristic of chronic inflammatory and degenerative diseases such as rheumatoid arthritis and osteoporosis. Cells and cytokines of the immune system are known to regulate bone turnover by controlling the differentiation and activity of osteoclasts, the bone resorbing cells. However, less is known about the regulation of osteoblasts (OB), the bone forming cells. This study aimed to investigate whether immune cells also regulate OB differentiation. Using in vitro cell cultures of human bone marrow-derived mesenchymal stem cells (MSC), it was shown that monocytes/macrophages potently induced MSC differentiation into OBs. This was evident by increased alkaline phosphatase (ALP) after 7 days and the formation of mineralised bone nodules at 21 days. This monocyte-induced osteogenic effect was mediated by cell contact with MSCs leading to the production of soluble factor(s) by the monocytes. As a consequence of these interactions we observed a rapid activation of STAT3 in the MSCs. Gene profiling of STAT3 constitutively active (STAT3C) infected MSCs using Illumina whole human genome arrays showed that Runx2 and ALP were up-regulated whilst DKK1 was down-regulated in response to STAT3 signalling. STAT3C also led to the up-regulation of the oncostatin M (OSM) and LIF receptors. In the co-cultures, OSM that was produced by monocytes activated STAT3 in MSCs, and neutralising antibodies to OSM reduced ALP by 50%. These data indicate that OSM, in conjunction with other mediators, can drive MSC differentiation into OB. This study establishes a role for monocyte/macrophages as critical regulators of osteogenic differentiation via OSM production and the induction of STAT3 signalling in MSCs. Inducing the local activation of STAT3 in bone cells may be a valuable tool to increase bone formation in osteoporosis and arthritis, and in localised bone remodelling during fracture repair.