Jpx RNA activates Xist by evicting CTCF.

In mammals, dosage compensation between XX and XY individuals occurs through X chromosome inactivation (XCI). The noncoding Xist RNA is expressed and initiates XCI only when more than one X chromosome is present. Current models invoke a dependency on the X-to-autosome ratio (X:A), but molecular factors remain poorly defined. Here, we demonstrate that molecular titration between an X-encoded RNA and an autosomally encoded protein dictates Xist induction. In pre-XCI cells, CTCF protein represses Xist transcription. At the onset of XCI, Jpx RNA is upregulated, binds CTCF, and extricates CTCF from one Xist allele. We demonstrate that CTCF is an RNA-binding protein and is titrated away from the Xist promoter by Jpx RNA. Thus, Jpx activates Xist by evicting CTCF. The functional antagonism via molecular titration reveals a role for long noncoding RNA in epigenetic regulation.

SOX9 and SRY binding sites on mouse mXYSRa/Enh13 enhancer redundantly regulate Sox9 expression to varying degrees.

Sox9 plays an essential role in mammalian testis formation. It has been reported that gene expression in the testes is regulated by enhancers. Among them, mXYSRa/Enh13-which is located at far upstream of the transcription start site-plays a critical role, wherein its deletion causes complete male-to-female sex reversal in mice. It has been proposed that the binding sites (BSs) of SOX9 and SRY, the latter of which is the sex determining gene on the Y chromosome, are associated with mXYSRa/Enh13. They function as an enhancer, whereby the sequences are evolutionarily conserved and in vivo binding of SOX9 and SRY to mXYSRa/Enh13 has been demonstrated previously. However, their precise in vivo functions have not been examined to date. To this end, this study generated mice with substitutions on the SOX9 and SRY BSs to reveal their in vivo functions. Homozygous mutants of SOX9 and SRY BS were indistinguishable from XY males, whereas double mutants had small testes, suggesting that these functions are redundant and that there is another functional sequence on mXYSRa/Enh13, since mXYSRa/Enh13 deletion mice are XY females. In addition, the majority of hemizygous mice with substitutions in SOX9 BS and SRY BS were female and male, respectively, suggesting that SOX9 BS contributes more to SRY BS for mXYSRa/Enh13 to function. The additive effect of SOX9 and SRY via these BSs was verified using an in vitro assay. In conclusion, SOX9 BS and SRY BS function redundantly in vivo, and at least one more functional sequence should exist in mXYSRa/Enh13.

Turnover of mammal sex chromosomes in the Sry-deficient Amami spiny rat is due to male-specific upregulation of Sox9.

Mammalian sex chromosomes are highly conserved, and sex is determined by SRY on the Y chromosome. Two exceptional rodent groups in which some species lack a Y chromosome and Sry offer insights into how novel sex genes can arise and replace Sry, leading to sex chromosome turnover. However, intensive study over three decades has failed to reveal the identity of novel sex genes in either of these lineages. We here report our discovery of a male-specific duplication of an enhancer of Sox9 in the Amami spiny rat Tokudaia osimensis, in which males and females have only a single X chromosome (XO/XO) and the Y chromosome and Sry are completely lost. We performed a comprehensive survey to detect sex-specific genomic regions in the spiny rat. Sex-related genomic differences were limited to a male-specific duplication of a 17-kb unit located 430 kb upstream of Sox9 on an autosome. Hi-C analysis using male spiny rat cells showed the duplicated region has potential chromatin interaction with Sox9. The duplicated unit harbored a 1,262-bp element homologous to mouse enhancer 14 (Enh14), a candidate Sox9 enhancer that is functionally redundant in mice. Transgenic reporter mice showed that the spiny rat Enh14 can function as an embryonic testis enhancer in mice. Embryonic gonads of XX mice in which Enh14 was replaced by the duplicated spiny rat Enh14 showed increased Sox9 expression and decreased Foxl2 expression. We propose that male-specific duplication of this Sox9 enhancer substituted for Sry function, defining a novel Y chromosome in the spiny rat.

Fetal ezrin expression affects macrophages and regulatory T cells in mouse placental decidua.

Ezrin is a cross-linker protein between membrane proteins and cytosolic actin, abundantly expressed in the placenta among the ERM protein family. Ezrin gene knockout mice exhibit fetal growth restriction after gestational day (GD) 15.5. This study aimed to clarify the effect of ezrin on immune cells that influence fetal growth and immune tolerance. Ezrin heterozygous knockout (Ez+/-) mice were interbred, and the gene expressions and immune cell distributions in the placentas of wild-type (Ez+/+) and ezrin knockout (Ez-/-) fetuses were analyzed. IL-6 expression in the placenta of Ez-/- fetuses was significantly higher than in Ez+/+ fetuses at GD 15.5. The mRNA expression of IL-6 in the uterine decidua attached to Ez-/- fetuses was higher compared to that attached to Ez+/+ fetuses but not in the junctional zone and labyrinth. Classical M1 and M2 macrophages in the decidua were analyzed by flow cytometry using CD86 and CD206 as markers. M1 macrophages increased in the decidua attached to Ez-/- mice compared to Ez+/+ mice, while M2 macrophages did not increase. CD4-positive T cells showed a reduction in the decidua attached to Ez-/- fetuses. Further analysis involved the subcutaneous administration of tacrolimus in pregnant Ez+/- mice from GD 8.5 to GD 15.5, which prevented the decrease in fetal body weight and decidual CD4-positive T cells in Ez-/- mice at GD 15.5. These results suggest that impaired expression of fetoplacental-derived ezrin induces inflammatory conditions in the uterine decidua through M1 polarization of macrophages, increased IL-6, and decreased CD4-positive T cells, including Treg cells.

Genetically humanized phenylketonuria mouse model as a testing tool for human genome editing in fertilized eggs.

Targeted genome editing has made significant advancements; however, safety and ethical issues have not been fully elucidated, resulting in strict control of the technique. We tested genome editing tools on gametes from a genetically humanized mouse model using a phenylketonuria (PKU) mouse model to gain insights into genome editing in human embryos. The human PKU mouse model PahhR111X mice was generated. The junctional region between exon 3 and intron 3 of Pah was replaced with a 120 bp corresponding human PAH sequence, including the pathogenic common variant c.331C > T in PahhR111X mice. PahhR111X mice successfully recapitulated the PKU phenotype and showed cognitive dysfunction and depressive-like behavior, which are observed in human patients with PKU. Genome editing was applied to fertilized eggs of PahhR111X mice utilizing sgRNA that targets the human sequence. Mice with the corrected allele exhibited normal serum phenylalanine levels. Through genome editing, we validated the utility of sgRNA. The genetically humanized mouse model suggested that germ-line genome editing of the pathogenic variant may be feasible for monogenic disorders by revealing the recovery of the phenotype; however, there are remaining issues with the tool, including its efficiency and accuracy. This genome editing protocol using a genetically humanized mouse model will provide insights for improving current issues and contribute to the establishment of heritable human genome editing protocols.

Stereoselective Synthesis of Spirolactone Analogues of Pyrrolomorpholine Alkaloids.

The synthesis of a spirolactone analogue of xylapyrraside B1, a potent antioxidant agent, is described. The key step is the stereoselective formation of the spirolactone skeleton via the formal [3+2] annulation of the isopropylidene-protected glyceraldehyde and δ-methylene lactone, mediated by trifluoroborane etherate. This study addresses the stereoselective synthesis of pyrrolomorpholine spiroketal alkaloids, enabling the production of these bioactive compounds and their analogues.

Humanization of a tandem repeat in IG-DMR causes stochastic restoration of paternal imprinting at mouse Dlk1-Dio3 domain.

The Dlk1-Dio3 imprinted domain, regulated by an intergenic differentially methylated region (IG-DMR), is important for mammalian embryonic development. Although previous studies have reported that DNA methylation of a tandem repeated array sequence in paternal IG-DMR (IG-DMR-Rep) plays an essential role in the maintenance of DNA methylation in mice, the function of a tandem repeated array sequence in human IG-DMR (hRep) is unknown. Here, we generated mice with a human tandem repeated sequence, which replaced the mouse IG-DMR-Rep. Mice that transmitted the humanized allele paternally exhibited variable methylation status at the IG-DMR and were stochastically rescued from the lethality of IG-DMR-Rep deficiency, suggesting that hRep plays a role in human IG-DMR for the regulation of imprinted expression. Moreover, chromatin immunoprecipitation analysis showed that TRIM28 was enriched in hypermethylated paternal hRep without ZFP57. Our results suggest that hRep contributes to the maintenance of human IG-DMR methylation imprints via the recruitment of TRIM28.

Foot morphology and correlation with lower extremity pain in Japanese children: A cross-sectional study of the foot posture Index-6.

BACKGROUND: Abnormal foot morphology in children and adolescents is a possible risk factor for lower extremity pain. Foot posture index-6 (FPI-6) is a valid and reliable tool to assess foot morphology. However, the normative data on the age distribution for FPI-6 in Asian children and adolescents are still minimal. Further, the correlation of FPI-6 with lower extremity pain is poorly understood. We aimed to investigate the normative distribution for FPI-6 and the relationship between FPI-6 scores and knee and heel pain in Japanese children. METHODS: We included 2569 Japanese children, aged 9-15 years, at a single school from 2016 to 2018. We summarized the age distribution of children and their mean bilateral FPI-6 scores. Additionally, we assessed the tenderness at the apophysis or tendon insertions at the knee and heel. We performed a cross-sectional analysis to investigate the correlations between FPI-6 scores and sex, age, and knee and heel pain for the data obtained each year. RESULTS: The mean FPI-6 score was 3.1 ± 2.4, 3.4 ± 2.0, and 3.2 ± 1.9 for the left foot and 3.0 ± 2.4, 3.2 ± 1.9, and 3.1 ± 1.9 for the right foot in 2016, 2017, and 2018, respectively. Boys tended to have higher scores than girls, and the FPI-6 score of the left foot was significantly higher than that of the right foot (p < 0.05). There was no correlation between FPI-6 scores and knee and heel pain. CONCLUSION: Children and adolescents between 9 and 15 years of age have neutral to slightly pronated foot morphology and an average FPI-6 score of 3.0-3.4. In addition, there was no relationship between foot morphology and knee and heel pain. This normative distribution for FPI-6 in Japanese children could serve as a reference value for future research and clinical evaluation.

Risk factors for the occurrence and protraction of patellar and patellar tendon pain in children and adolescents: a prospective cohort study of 3 years.

BACKGROUND: Patellar and patellar tendon pain is a common limitation to children's participation in social and physical activities. Some factors have been implicated in the occurrence and protraction of knee pain, but the causal relationship is unknown. The purpose of this study was to investigate whether participants' physical characteristics and activity level are risk factors for the occurrence and protraction of patellar and patellar tendon pain in children and adolescents. METHODS: A three-year prospective cohort study was conducted with healthy students who were aged 8-14 years old, in Japan. Height, weight, heel-buttock distance, straight leg raising angle, and dorsiflexion angle of the ankle joint were collected as individual physical factors at the beginning of each year. The presence of self-reported patellar and patellar tendon pain and the Hospital for Special Surgery Pediatric Functional Activity Brief Scale (HSS Pedi-FABS) was collected every month. Protraction was defined as either (1) pain lasting for more than three continuous months or (2) recurrent pain after more than three months of complete recovery. Participants who did not have any pain at the beginning of the observation period were included in the analysis. We analyzed the odds ratio (OR) of pain occurrence within a year of registration and protraction throughout the study period for all physical factors and HSS Pedi-FABS. RESULTS: We included 1133 participants in the analysis and 252 participants developed knee pain within a year. 34.8% of participants with pain experienced protraction during the follow-up period. A high HSS Pedi-FABS significantly predicted knee pain occurrence (OR 1.03, 95% confidence interval [CI] 1.01-1.05) and protraction (OR 1.03, 95% CI 1.00-1.05). In addition, younger children and girls were at a significantly higher risk of patellar and patellar tendon pain protraction (age, OR 0.81, 95% CI, 0.73-0.90; sex, OR 1.69, 95% CI, 1.09-2.64). Other physical factors did not significantly predict the occurrence or protraction of knee pain. CONCLUSIONS: This study showed that a greater physical activity level was a risk factor for the occurrence and protraction of patellar and patellar tendon pain in childhood. In addition, younger age and female sex predicted higher risk of protraction of pain.

Identification of the first promoter-specific gain-of-function SOX9 missense variant (p.E50K) in a patient with 46,XX ovotesticular disorder of sex development.

SOX9, a transcription factor, is expressed in the undifferentiated XX and XY gonads. SRY induces significant upregulation of SOX9 expression in XY gonads. Loss-of-function SOX9 variants cause testicular dysgenesis in 46,XY patients, while duplication of the total gene or the upstream regulatory region results in testicular development in 46,XX patients. However, gain-of-function (GoF) SOX9 variants have not been reported previously. We report the case of a 16-year-old female patient with a 46,XX karyotype who had masculinized external genitalia and unilateral ovotestis. Next-generation sequencing-based genetic screening for disorders of sex development led to the identification of a novel SOX9 variant (p.Glu50Lys), transmitted from the phenotypically normal father. Expression analysis showed that E50K-SOX9 enhanced transactivation of the luciferase reporter containing the testis enhancer sequence core element compared with that containing the wildtype-SOX9. This GoF activity was not observed in the luciferase reporter containing Amh, the gene for anti-Müllerian hormone. We genetically engineered female mice (Sox9E50K/E50K ), and they showed no abnormalities in the external genitalia or ovaries. In conclusion, a novel SOX9 variant with a promoter-specific GoF activity was identified in vitro; however, the disease phenotype was not recapitulated by the mouse model. At present, the association between the GoF SOX9 variant and the ovotestis phenotype remains unclear. Future studies are needed to verify the possible association.

Nitrogen Availability and Microbial Communities of Canopy Soils in a Large Cercidiphyllum japonicum Tree of a Cool-Temperate Old Growth Forest.

Canopy soils on large trees are important for supporting the lives of many canopy plants, and thereby increasing regional biodiversity. However, because of the less accessibility to canopy soils, there is insufficient knowledge on how canopy soils produce available nitrogen (N) for canopy plants through the activity of canopy soil microbes. Canopy soils usually have different soil properties from ground soils, so we hypothesized that canopy soils would have unique microbial communities compared to ground soils, but still provide available N for canopy plants. Here, we compared soil N availability, including net N mineralization and nitrification rate, and microbial communities between canopy soils (organic soils) collected at various heights of a large Cercidiphyllum japonicum tree and ground soils (organic and mineral soils) in a cool-temperate old-growth forest of Japan. The canopy soils had significantly different N availability (mass-based higher but volume-based lower) and microbial communities from the ground mineral soils. Among organic soils, the height of the soil had an impact on the microbial communities but not on the N availability, which agreed with our hypothesis. Despite the decrease in fungal abundance in the higher soils, the increase in certain components of the cellulose-decomposing fungi and oligotrophic bacteria may contribute to the available N production. Also, the abundance of ammonia-oxidizers did not change with the height, which would be important for the nitrification rate. Our study implied canopy soils could provide N to canopy plants partly through the functional redundancy within different microbial communities and constant population of ammonia-oxidizers.

The uterine epithelial loss of Pten is inefficient to induce endometrial cancer with intact stromal Pten.

Mutation of the tumor suppressor Pten often leads to tumorigenesis in various organs including the uterus. We previously showed that Pten deletion in the mouse uterus using a Pgr-Cre driver (Ptenf/fPgrCre/+) results in rapid development of endometrial carcinoma (EMC) with full penetration. We also reported that Pten deletion in the stroma and myometrium using Amhr2-Cre failed to initiate EMC. Since the Ptenf/fPgrCre/+ uterine epithelium was primarily affected by tumorigenesis despite its loss in both the epithelium and stroma, we wanted to know if Pten deletion in epithelia alone will induce tumorigenesis. We found that mice with uterine epithelial loss of Pten under a Ltf-iCre driver (Ptenf/f/LtfCre/+) develop uterine complex atypical hyperplasia (CAH), but rarely EMC even at 6 months of age. We observed that Ptenf/fPgrCre/+ uteri exhibit a unique population of cytokeratin 5 (CK5) and transformation related protein 63 (p63)-positive epithelial cells; these cells mark stratified epithelia and squamous differentiation. In contrast, Ptenf/fLtfCre/+ hyperplastic epithelia do not undergo stratification, but extensive epithelial cell apoptosis. This increased apoptosis is associated with elevation of TGFß levels and activation of downstream effectors, SMAD2/3 in the uterine stroma. Our results suggest that stromal PTEN via TGFß signaling restrains epithelial cell transformation from hyperplasia to carcinoma. In conclusion, this study, using tissue-specific deletion of Pten, highlights the epithelial-mesenchymal cross-talk in the genesis of endometrial carcinoma.

CRISPR/Cas9-mediated modulation of splicing efficiency reveals short splicing isoform of Xist RNA is sufficient to induce X-chromosome inactivation.

Alternative splicing of mRNA precursors results in multiple protein variants from a single gene and is critical for diverse cellular processes and development. Xist encodes a long noncoding RNA which is a central player to induce X-chromosome inactivation in female mammals and has two major splicing variants: long and short isoforms of Xist RNA. Although a differentiation-specific and a female-specific expression of Xist isoforms have been reported, the functional role of each Xist RNA isoform is largely unexplored. Using CRISPR/Cas9-mediated targeted modification of the 5' splice site in Xist intron 7, we create mutant female ES cell lines which dominantly express the long- or short-splicing isoform of Xist RNA from the inactive X-chromosome (Xi) upon differentiation. Successful execution of CRISPR/Cas-based splicing modulation indicates that our CRISPR/Cas-based targeted modification of splicing sites is a useful approach to study specific isoforms of a transcript generated by alternative splicing. Upon differentiation of splicing-mutant Xist female ES cells, we find that both long and short Xist isoforms can induce X-chromosome inactivation normally during ES cell differentiation, suggesting that the short splicing isoform of Xist RNA is sufficient to induce X-chromosome inactivation.

Xist RNA repeat E is essential for ASH2L recruitment to the inactive X and regulates histone modifications and escape gene expression.

Long non-coding RNA Xist plays a crucial role in establishing and maintaining X-chromosome inactivation (XCI) which is a paradigm of long non-coding RNA-mediated gene regulation. Xist has Xist-specific repeat elements A-F which are conserved among eutherian mammals, underscoring their functional importance. Here we report that Xist RNA repeat E, a conserved Xist repeat element in the Xist exon 7, interacts with ASH2L and contributes to maintenance of escape gene expression level on the inactive X-chromosome (Xi) during XCI. The Xist repeat E-deletion mutant female ES cells show the depletion of ASH2L from the Xi upon differentiation. Furthermore, a subset of escape genes exhibits unexpectedly higher expression in the repeat E mutant cells than the cells expressing wildtype Xist during X-inactivation, whereas the silencing of X-linked non-escape genes is not affected. We discuss the implications of these results to understand the role of ASH2L and Xist repeat E for histone modifications and escape gene regulation during random X-chromosome inactivation.

Single vs. repeated matrix metalloproteinase-13 knockdown with intra-articular short interfering RNA administration in a murine osteoarthritis model.

Purpose: Our aims were 1) to estimate the duration of short interfering RNA (siRNA) effect on matrix metalloproteinase-13 (Mmp-13) levels by a single intra-articular injection using a mouse knee osteoarthritis (OA) model and 2) to test whether repeated injections results in any additional suppressive effect on cartilage degradation compared to a single injection. Materials and Methods: OA was induced in 9 weeks old male C57BL/6 mice by destabilization of medial meniscus (DMM). Chemically modified siRNA targeted for Mmp-13 was injected into the knee joint at 1 week post-DMM surgery. Control group of knees received that for non-targeted genes. Synovial tissue was collected to measure Mmp-13 expression levels by quantitative polymerase chain reaction (qPCR) at 2, 3, and 6 weeks after surgery in each group. To test the effect of multiple injections, we created four experiment groups according to the number of injections. Histological assessment of articular cartilage was performed at 8 weeks post-DMM surgery. Results: In the Mmp-13 siRNA-treated group, expression levels of Mmp-13 mRNA were decreased by 40% compared to the control group at 2 weeks after surgery (p = 0.04), before returning to baseline at 3 weeks after surgery. A significant improvement in the histological score was observed in all Mmp-13 siRNA-treated groups compared to the control group (p < 0.05). However, no significant differences were seen between the single and multiple injection group. Conclusions: Our results suggested that the duration of siRNA effect in the knee joint lasts for at least 1 week, and that no further benefit is achieved by multiple injections.

Comparison of meniscal extrusion and osteophyte formation at the intercondylar notch as a predictive biomarker for incidence of knee osteoarthritis-Data from the Osteoarthritis Initiative.

BACKGROUND: Medial meniscal extrusion (ME) is a biomarker to predict later development of knee osteoarthritis (KOA). On the other hand, we have reported osteophyte formation at the posterior condylar notch of the femur served as a biomarker for the same purpose. The purpose of this study is to compare capacity of the two biomarkers in predicting KOA development. METHODS: Two cohort of knees were established utilizing publicly available data from the Osteoarthritis Initiative (OAI). No OA group (NOA) consisted of knees that were grade 0 or 1 on Kellgren and Lawrence grade (K/L) both at baseline and 48 months later, and pre-radiographic-OA group (PROA) consisted of knees that were grade 0 or 1 at baseline but grade ≥2 48 months later. Baseline MR images were evaluated in terms of ME and osteophyte formation at the posterior condylar notch. ME was evaluated both by meniscus subluxation index (MSI) indicating the ratio of the extruded width of the medial meniscus to the width of medial meniscal body and by the medial radial displacement (MRD) indicating actual extruded width. The size of the osteophyte was assessed using a semi-quantitative whole-organ magnetic resonance imaging score (WORMS). The predictive accuracy of KOA was assessed by the area under the receiver operating characteristic curve (AUC) and optimal cutoff was determined for each parameter. RESULTS: The AUC for MSI was 0.654 (0.561-0.748: 95% CI) and the cutoff value was determined as 17%. That for MRD was 0.677 (0.584-0.770) and the cutoff value was 2.2 mm. The AUC for the WORMS score at the posterior condylar notch was 0.667 (0.579-0.756) and the cutoff value was 2. CONCLUSIONS: Similar predicting capacity of KOA development was found both in ME and osteophyte formation at the posterior condylar notch. Using these simple parameter, mas-screening for KOA development would be possible.

Total Synthesis of (+)-Pyrenolide D.

An efficient approach to stereoselective construction of a spiro-γ-lactone core structure via BF3-promoted formal [3 + 2] annulation of aldehydo-aldose derivatives with γ-methylene-γ-butyrolactone has been developed. The spiro-γ-lactone derivative was then used in an efficient total synthesis of (+)-pyrenolide D. The developed chemistry paves the way for total synthesis of structurally diverse natural products containing spiro-lactone cores.

Genome-wide identification of polycomb-associated RNAs by RIP-seq.

Polycomb proteins play essential roles in stem cell renewal and human disease. Recent studies of HOX genes and X inactivation have provided evidence for RNA cofactors in Polycomb repressive complex 2 (PRC2). Here we develop a RIP-seq method to capture the PRC2 transcriptome and identify a genome-wide pool of >9000 PRC2-interacting RNAs in embryonic stem cells. The transcriptome includes antisense, intergenic, and promoter-associated transcripts, as well as many unannotated RNAs. A large number of transcripts occur within imprinted regions, oncogene and tumor suppressor loci, and stem cell-related bivalent domains. We provide evidence for direct RNA-protein interactions, most likely via the Ezh2 subunit. We also identify Gtl2 RNA as a PRC2 cofactor that directs PRC2 to the reciprocally imprinted Dlk1 coding gene. Thus, Polycomb proteins interact with a genome-wide family of RNAs, some of which may be used as biomarkers and therapeutic targets for human disease.

Xist Exon 7 Contributes to the Stable Localization of Xist RNA on the Inactive X-Chromosome.

To equalize X-linked gene dosage between the sexes in mammalian females, Xist RNA inactivates one of the two X-chromosomes. Here, we report the crucial function of Xist exon 7 in X-inactivation. Xist exon 7 is the second-largest exon with a well-conserved repeat E in eutherian mammals, but its role is often overlooked in X-inactivation. Although female ES cells with a targeted truncation of the Xist exon 7 showed no significant differences in their Xist expression levels and RNA stability from control cells expressing wild-type Xist, compromised localization of Xist RNA and incomplete silencing of X-linked genes on the inactive X-chromosome (Xi) were observed in the exon 7-truncated mutant cells. Furthermore, the interaction between the mutant Xist RNA and hnRNP U required for localization of Xist RNA to the Xi was impaired in the Xist exon 7 truncation mutant cells. Our results suggest that exon 7 of Xist RNA plays an important role for stable Xist RNA localization and silencing of the X-linked genes on the Xi, possibly acting through an interaction with hnRNP U.

Dynamic interplay and function of multiple noncoding genes governing X chromosome inactivation.

There is increasing evidence for the emergence of long noncoding RNAs (lncRNAs) as important components, especially in the regulation of gene expression. In the event of X chromosome inactivation, robust epigenetic marks are established in a long noncoding Xist RNA-dependent manner, giving rise to a distinct epigenetic landscape on the inactive X chromosome (Xi). The X inactivation center (Xic) is essential for induction of X chromosome inactivation and harbors two topologically associated domains (TADs) to regulate monoallelic Xist expression: one at the noncoding Xist gene and its upstream region, and the other at the antisense Tsix and its upstream region. The monoallelic expression of Xist is tightly regulated by these two functionally distinct TADs as well as their constituting lncRNAs and proteins. In this review, we summarize recent updates in our knowledge of lncRNAs found at the Xic and discuss their overall mechanisms of action. We also discuss our current understanding of the molecular mechanism behind Xist RNA-mediated induction of the repressive epigenetic landscape at the Xi. This article is part of a Special Issue entitled: Clues to long noncoding RNA taxonomy1, edited by Dr. Tetsuro Hirose and Dr. Shinichi Nakagawa.