Cloning and characterization of a murine brain specific gene Bpx and its human homologue lying within the Xic candidate region

Nucleosome assembly protein 1-like 5 alleviates Alzheimer's disease-like pathological characteristics in a cell model

Alzheimer's disease (AD) remains one of the most common dementias of neurodegenerative disease-related diseases. Nucleosome assembly protein 1-like 5 (NAP1L5) belongs to the NAP1L protein family, which acts as a histone chaperone. However, the function and mechanism of NAP1L5 in AD are still unclear. Bioinformatics analysis, RT-qPCR, and Western blotting results showed that NAP1L5 was downregulated in the brain tissues of AD patients and a mouse cell model of AD. NAP1L5 overexpression alleviated (Amyloid-β precursor protein) APP metabolism and Tau phosphorylation. We further demonstrated that NAP1L5 regulated the AD-like pathological characteristics through the GSK3B/Wnt/β-Catenin signaling pathway. Moreover, we showed that the Wnt/β-Catenin signaling pathway, regulated by NAP1L5, was mediated by AQP1-mediated mechanism in N2a-APP695sw cell. In sum, these results suggested that NAP1L5 overexpression has neuroprotective effects and might act as potential biomarker and target for the diagnosis and treatment of AD.

Nap1l1 Controls Embryonic Neural Progenitor Cell Proliferation and Differentiation in the Developing Brain

The precise function and role of nucleosome assembly protein 1-like 1 (Nap1l1) in brain development are unclear. Here, we find that Nap1l1 knockdown decreases neural progenitor cell (NPC) proliferation and induces premature neuronal differentiation during cortical development. A similar deficiency in embryonic neurogenesis was observed in Nap1l1 knockout (KO) mice, which were generated using the CRISPR-Cas9 system. RNA sequencing (RNA-seq) analysis indicates that Ras-associated domain family member 10 (RassF10) may be the downstream target of Nap1l1. Furthermore, we found that Nap1l1 regulates RassF10 expression by promoting SETD1A-mediated H3K4 trimethylation at the RassF10 promoter. Nap1l1 KO defects may be rescued by RassF10 overexpression, suggesting that Nap1l1 controls NPC differentiation through RassF10. Our findings reveal an essential role for the Nap1l1 histone chaperone in cortical neurogenesis during early embryonic brain development.

The Ftx Noncoding Locus Controls X Chromosome Inactivation Independently of Its RNA Products

Accumulation of the Xist long noncoding RNA (lncRNA) on one X chromosome is the trigger for X chromosome inactivation (XCI) in female mammals. Xist expression, which needs to be tightly controlled, involves a cis-acting region, the X-inactivation center (Xic), containing many lncRNA genes that evolved concomitantly to Xist from protein-coding ancestors through pseudogeneization and loss of coding potential. Here, we uncover an essential role for the Xic-linked noncoding gene Ftx in the regulation of Xist expression. We show that Ftx is required in cis to promote Xist transcriptional activation and establishment of XCI. Importantly, we demonstrate that this function depends on Ftx transcription and not on the RNA products. Our findings illustrate the multiplicity of layers operating in the establishment of XCI and highlight the diversity in the modus operandi of the noncoding players.

X-Chromosome Inactivation: A Crossroads Between Chromosome Architecture and Gene Regulation

In somatic nuclei of female therian mammals, the two X chromosomes display very different chromatin states: One X is typically euchromatic and transcriptionally active, and the other is mostly silent and forms a cytologically detectable heterochromatic structure termed the Barr body. These differences, which arise during female development as a result of X-chromosome inactivation (XCI), have been the focus of research for many decades. Initial approaches to define the structure of the inactive X chromosome (Xi) and its relationship to gene expression mainly involved microscopy-based approaches. More recently, with the advent of genomic techniques such as chromosome conformation capture, molecular details of the structure and expression of the Xi have been revealed. Here, we review our current knowledge of the 3D organization of the mammalian X-chromosome chromatin and discuss its relationship with gene activity in light of the initiation, spreading, and maintenance of XCI, as well as escape from gene silencing.

Histone chaperones: assisting histone traffic and nucleosome dynamics

The functional organization of eukaryotic DNA into chromatin uses histones as components of its building block, the nucleosome. Histone chaperones, which are proteins that escort histones throughout their cellular life, are key actors in all facets of histone metabolism; they regulate the supply and dynamics of histones at chromatin for its assembly and disassembly. Histone chaperones can also participate in the distribution of histone variants, thereby defining distinct chromatin landscapes of importance for genome function, stability, and cell identity. Here, we discuss our current knowledge of the known histone chaperones and their histone partners, focusing on histone H3 and its variants. We then place them into an escort network that distributes these histones in various deposition pathways. Through their distinct interfaces, we show how they affect dynamics during DNA replication, DNA damage, and transcription, and how they maintain genome integrity. Finally, we discuss the importance of histone chaperones during development and describe how misregulation of the histone flow can link to disease.

A mechanistic role for the chromatin modulator, NAP1L1, in pancreatic neuroendocrine neoplasm proliferation and metastases

Background

The chromatin remodeler NAP1L1, which is upregulated in small intestinal neuroendocrine neoplasms (NENs), has been implicated in cell cycle progression. As p57^Kip2 (CDKN1C), a negative regulator of proliferation and a tumor suppressor, is controlled by members of the NAP1 family, we tested the hypothesis that NAP1L1 may have a mechanistic role in regulating pancreatic NEN proliferation through regulation of p57^Kip2.

Results

NAP1L1 silencing (siRNA and shRNA/lipofectamine approach) decreased proliferation through inhibition of mechanistic (mammalian) target of rapamycin pathway proteins and their phosphorylation (p < 0.05) in the pancreatic neuroendocrine neoplasm cell line BON in vitro (p < 0.0001) and resulted in significantly smaller (p < 0.05) and lighter (p < 0.05) tumors in the orthotopic pancreatic NEN mouse model. Methylation of the p57^Kip2 promoter was decreased by NAP1L1 silencing (p < 0.05), and expression of p57^Kip2 (transcript and protein) was upregulated. For methylation of the p57^Kip2 promoter, NAP1L1 bound directly to the promoter (−164 to +21, chromatin immunoprecipitation). In 43 pancreatic NEN samples (38 primaries and 5 metastasis), NAP1L1 was over-expressed in metastasis (p < 0.001), expression which was inversely correlated with p57^Kip2 (p < 0.01) on mRNA and protein levels. Menin was not differentially expressed.

Conclusion

NAP1L1 is over-expressed in pancreatic neuroendocrine neoplasm metastases and epigenetically promotes cell proliferation through regulation of p57^Kip2 promoter methylation.

Knockdown of nucleosome assembly protein 1-like 1 promotes dimethyl sulfoxide-induced differentiation of P19CL6 cells into cardiomyocytes

Transplantation of cardiomyocytes derived from stem cells is a promising option for cardiac repair. However, how to obtain efficient cardiomyocytes from stem cells is still a great challenge. Understanding of the mechanism that regulates the cardiac differentiation of stem cells is necessary for the effective induction of cardiomyocytes. A clonal derivative named P19CL6 cells can easily differentiate into cardiomyocytes with 1% dimethyl sulfoxide (DMSO) treatment, which offers a valuable model to study cardiomyocytes differentiation in vitro. In this study, the isobaric tags for relative and absolute quantitation (iTRAQ) proteomics were performed to identify proteins associated with cardiomyocytes differentiation of P19CL6 cells induced by DMSO. Out of 543 non-redundant proteins identified, 207 proteins showed significant changes during differentiation with ≥1.2-fold or ≤0.83-fold changes cut-offs. Nine proteins were confirmed by the quantitative real-time polymerase chain reaction (qRT-PCR) and Western blot analysis respectively. Notably, broad consistency was well showed between mRNA and protein expression for down-regulation of nucleosome assembly protein 1-like 1 (Nap1l1). Further study revealed that knockdown of Nap1l1 by stable transfection of shRNA vector significantly accelerated DMSO-induced cardiomyocytes differentiation of P19CL6 cells characterized by increases in expression of cardiac specific transcription factors, genes, and proteins (GATA4, MEF-2C, ANP, BNP, cTNT, and β-MHC). Therefore, Nap1l1 is a novel protein that regulates cardiomyocytes differentiation of P19CL6 cells induced by DMSO.

Histone chaperones: an escort network regulating histone traffic

In eukaryotes, DNA is organized into chromatin in a dynamic manner that enables it to be accessed for processes such as transcription and repair. Histones, the chief protein component of chromatin, must be assembled, replaced or exchanged to preserve or change this organization according to cellular needs. Histone chaperones are key actors during histone metabolism. Here we classify known histone chaperones and discuss how they build a network to escort histone proteins. Molecular interactions with histones and their potential specificity or redundancy are also discussed in light of chaperone structural properties. The multiplicity of histone chaperone partners, including histone modifiers, nucleosome remodelers and cell-cycle regulators, is relevant to their coordination with key cellular processes. Given the current interest in chromatin as a source of epigenetic marks, we address the potential contributions of histone chaperones to epigenetic memory and genome stability.

Nap1l2 promotes histone acetylation activity during neuronal differentiation

The deletion of the neuronal Nap1l2 (nucleosome assembly protein 1-like 2) gene in mice causes neural tube defects. We demonstrate here that this phenotype correlates with deficiencies in differentiation and increased maintenance of the neural stem cell stage. Nap1l2 associates with chromatin and interacts with histones H3 and H4. Loss of Nap1l2 results in decreased histone acetylation activity, leading to transcriptional changes in differentiating neurons, which include the marked downregulation of the Cdkn1c (cyclin-dependent kinase inhibitor 1c) gene. Cdkn1c expression normally increases during neuronal differentiation, and this correlates with the specific recruitment of the Nap1l2 protein and an increase in acetylated histone H3K9/14 at the site of Cdkn1c transcription. These results lead us to suggest that the Nap1l2 protein plays an important role in regulating transcription in developing neurons via the control of histone acetylation. Our data support the idea that neuronal nucleosome assembly proteins mediate cell-type-specific mechanisms of establishment/modification of a chromatin-permissive state that can affect neurogenesis and neuronal survival.

Isolation, X location and activity of the marsupial homologue of SLC16A2, an XIST-flanking gene in eutherian mammals

X chromosome inactivation (XCI) achieves dosage compensation between males and females for most X-linked genes in eutherian mammals. It is a whole-chromosome effect under the control of the XIST locus, although some genes escape inactivation. Marsupial XCI differs from the eutherian process, implying fundamental changes in the XCI mechanism during the evolution of the two lineages. There is no direct evidence for the existence of a marsupial XIST homologue. XCI has been studied for only a handful of genes in any marsupial, and none in the model kangaroo Macropus eugenii (the tammar wallaby). We have therefore studied the sequence, location and activity of a gene SLC16A2 (solute carrier, family 16, class A, member 2) that flanks XIST on the human and mouse X chromosomes. A BAC clone containing the marsupial SLC16A2 was mapped to the end of the long arm of the tammar X chromosome and used in RNA FISH experiments to determine whether one or both loci are transcribed in female cells. In male and female cells, only a single signal was found, indicating that the marsupial SLC16A2 gene is silenced on the inactivated X.

eXPRESSION: an in silico tool to predict patterns of gene expression

In embryological studies, expression pattern analyses are of special importance since genes that have temporally and spatially restricted expression are not only essential as lineage markers but are often causative in formation of specific fates. Further, where a molecule is expressed can be quite revealing in regard to its endogenous function. We present a gene discovery tool, termed eXPRESSION, that utilizes the public EST databases to identify genes matching desired transcriptional profiles. We first tested and validated the ability of eXPRESSION to discover tissue-specific genes in the adult mouse; empirically as well as with DNA microarrays and RT-PCRs. These studies showed that eXPRESSION predictions could identify genes that are specifically expressed in adult mouse tissues. Next, we developed a novel search strategy to find genes that are expressed in specific regions or tissues of the developing mouse embryo. With these tools, we identified several novel genes that exhibited a neural-specific or neural-enriched expression pattern during murine development. The data show that eXPRESSION is widely applicable and may be used to identify both adult and embryonic tissue- or organ-specific genes with minimal cost and effort.

Controlling X-inactivation in mammals: what does the centre hold?

Controlling gene expression is one of the most fundamental task of living organisms, from prokaryotes to higher eukaryotes, in order to develop, grow, and reproduce in an ever changing environment. In many cases, the expression status of a given gene is controlled independently of that of its neighbours through localised cis DNA elements responsible for the recruitment of specific factors and enzymatic activities. However, in a growing number of cases, genomic regions including several genes have been shown to be regulated in a coordinated manner. X-chromosome inactivation, the dosage compensation mechanism encountered in mammals, is one of the most Striking example of such coordinated gene regulation. This process, which occurs at the chromosome-wide level, affecting many hundreds of genes, is under the control of a unique, cis acting region, termed the X-inactivation centre, whose complexity is just beginning to be unravelled.

Xenopus nucleosome assembly protein becomes tissue-restricted during development and can alter the expression of specific genes

Nucleosome assembly proteins have been identified in all eukaryotic species investigated to date and their suggested roles include histone shuttle, histone acceptor during transcriptional chromatin remodelling and cell cycle regulator. To examine the role of these proteins during early development we have isolated the cDNA encoding Xenopus NAP1L, raised an antibody against recombinant xNAP1L and examined the expression pattern of this mRNA and protein. Expression in adults is predominantly in ovaries. This maternal protein remains a major component of xNAP1L within the embryo until swimming tadpole stages. xNAP1L mRNA is initially throughout the embryo but by gastrula stages it is predominantly in the presumptive ectoderm. Later, mRNA is detected in the neural crest, neural tube, eyes, tailbud and ventral blood islands. In order to test whether xNAP1L has a potential role in gene regulation we overexpressed this protein in animal pole explants and tested the effect on expression of a series of potential target genes. The mRNA encoding the transcription factor GATA-2 was markedly up-regulated by this overexpression. These data support a role for xNAP1L in tissue-restricted gene regulation.

Enox, a novel gene that maps 10 kb upstream of Xist and partially escapes X inactivation

Dosage compensation in mammals is accomplished by the transcriptional silencing of a single X chromosome in female cells, a process termed X inactivation. A cytogenetically defined region of the X chromosome, the X-inactivation center (Xic), is necessary in cis for this process. Although the precise nature of the Xic remains unknown, a key component, the Xist gene, has been shown to be essential for X inactivation. In XX somatic cells, Xist RNA is specifically transcribed from the inactive X chromosome, which is otherwise essentially heterochromatic. Previous studies aimed at defining the proximal limit of the Xic have indicated that it lies within 30 kb upstream of the Xist promoter. Here we describe a novel gene, Enox (expressed neighbor of Xist), that maps to an unmethylated CpG island 10 kb upstream of Xist. Enox transcripts are antisense relative to Xist, highly heterogeneous, and apparently noncoding. In female somatic tissue Enox partially escapes from X inactivation. We discuss the implications of these findings in relation to our understanding of the Xic.

SNPs in the CpG island of NAP1L2: a possible link between DNA methylation and neural tube defects?

Deletion of the murine X-linked Nap1l2 gene causes lethality from midgestation onwards. The affected embryos exhibit neural tube defects (NTDs) closely resembling spina bifida and anencephaly in humans. X-linked familial and spontaneous cases of NTD were analyzed for sequence alterations in the human NAP1L2. No differences were found in the familial cases. However, a number of single nucleotide polymorphisms (SNPs) within the 5' region of NAP1L2 were identified both in cases of spontaneous NTD and in normal controls. Most of these SNPs lead to the replacement of guanidines or cytosines within a CpG island that is conserved between the human and the mouse promoter regions. Demethylation in vitro activates Nap1l2 transcriptional activity, suggesting the importance of the CpG island in regulating the activity of the Nap1l2/NAP1L2 genes, and the potential importance of the polymorphisms in modifying their transcriptional activity. NAP1L2/Nap1l2 expression may therefore depend on the genetic-environmental factors that are frequently associated with NTDs.

Comparative sequence analysis of the X-inactivation center region in mouse, human, and bovine

We have sequenced to high levels of accuracy 714-kb and 233-kb regions of the mouse and bovine X-inactivation centers (Xic), respectively, centered on the Xist gene. This has provided the basis for a fully annotated comparative analysis of the mouse Xic with the 2.3-Mb orthologous region in human and has allowed a three-way species comparison of the core central region, including the Xist gene. These comparisons have revealed conserved genes, both coding and noncoding, conserved CpG islands and, more surprisingly, conserved pseudogenes. The distribution of repeated elements, especially LINE repeats, in the mouse Xic region when compared to the rest of the genome does not support the hypothesis of a role for these repeat elements in the spreading of X inactivation. Interestingly, an asymmetric distribution of LINE elements on the two DNA strands was observed in the three species, not only within introns but also in intergenic regions. This feature is suggestive of important transcriptional activity within these intergenic regions. In silico prediction followed by experimental analysis has allowed four new genes, Cnbp2, Ftx, Jpx, and Ppnx, to be identified and novel, widespread, complex, and apparently noncoding transcriptional activity to be characterized in a region 5' of Xist that was recently shown to attract histone modification early after the onset of X inactivation.

Control of neurulation by the nucleosome assembly protein-1-like 2

Neurulation is a complex process of histogenesis involving the precise temporal and spatial organization of gene expression. Genes influencing neurulation include proneural genes determining primary cell fate, neurogenic genes involved in lateral inhibition pathways and genes controlling the frequency of mitotic events. This is reflected in the aetiology and genetics of human and mouse neural tube defects, which are of both multifactorial and multigenic origin. The X-linked gene Nap1l2, specifically expressed in neurons, encodes a protein that is highly similar to the nucleosome assembly (NAP) and SET proteins. We inactivated Nap1l2 in mice by gene targeting, leading to embryonic lethality from mid-gestation onwards. Surviving mutant chimaeric embryos showed extensive surface ectoderm defects as well as the presence of open neural tubes and exposed brains similar to those observed in human spina bifida and anencephaly. These defects correlated with an overproduction of neuronal precursor cells. Protein expression studies showed that the Nap1l2 protein binds to condensing chromatin during S phase and in apoptotic cells, but remained cytoplasmic during G1 phase. Nap1l2 therefore likely represents a class of tissue-specific factors interacting with chromatin to regulate neuronal cell proliferation.

NAP-2: histone chaperone function and phosphorylation state through the cell cycle

We have recently cloned the human nucleosome assembly protein 2 (NAP-2). Here, we demonstrate that casein kinase 2 (CKII) from HeLa cell nuclear extracts interacts with immobilized NAP-II, and phosphorylates both NAP-2 and nucleosome assembly protein 1 (NAP-1) in vitro. Furthermore, NAP-1 and NAP-2 phosphorylation in crude HeLa cell extracts is abolished by heparin, a specific inhibitor of CKII. Addition of core histones can stimulate phosphorylation of NAP-1 and NAP-2 by CKII. NAP-2 is also a phosphoprotein in vivo. The protein is phosphorylated at the G0/G1 boundary but it is not phosphorylated in S-phase. Here, we show that NAP-2 is a histone chaperone throughout the cell cycle and that its cell-cycle distribution might be governed by its phosphorylation status. Phosphorylated NAP-2 remains in the cytoplasm in a complex with histones during the G0/G1 transition, whereas its dephosphorylation triggers its transport into the nucleus, at the G1/S-boundary, with the histone cargo, suggesting that binding to histones does not depend on phosphorylation status. Finally, indirect immunofluorescence shows that NAP-2 is present during metaphase of HeLa and COS cells, and its localization is distinct from metaphase chromosomes.

Characterization of a highly complex region in Xq13 and mapping of three isodicentric breakpoints associated with preleukemia

The chromosomal abnormality represented by an isodicentric X chromosome [idic(X)(q13)] is associated with a subset of acute myeloid leukemia (AML) and preleukemia observed in elderly females. A previous study localized the breakpoints of two acquired isodicentric X chromosomes associated with myelodysplasia to a 450-kb region proximal to the XIST gene. Here we report the construction and extensive characterization of a reliable 1-Mb P1 artificial chromosome and bacterial artificial chromosome contig covering a highly problematic region in Xq13 that includes the previously described isodicentric breakpoint region. In addition to mapping of the brain-specific gene (NAP1L2) and the phosphoglyceryl kinase alpha subunit 1 gene (PHKA1) and generation and mapping of a large number of STSs throughout the contig, we have mapped a putative transcriptional regulatory protein (HDACL1), and 35 ESTs. Sequencing data, Southern blot analysis, and fiber-FISH analysis have permitted characterization of extensive region-specific duplications and triplications in addition to an unusually high concentration of long interspersed repeat elements, both of which could be implicated in isodicentric chromosome formation and other Xq13 chromosome aberrations. FISH analysis of metaphase chromosomes from two previously unpublished AML patients and one preleukemic patient using cosmid clones and selected subclones allowed mapping of the idic(X)(q13) breakpoints to a 100-kb interval, consistent with the involvement of an X-linked gene in the genesis of this form of preleukemia, disruption of which may represent a preliminary step in progression to AML. Assembly and physical mapping of this complex 1-Mb contig establish a foundation for ongoing sequencing and gene identification projects in the region.

A phenotype map of the mouse X chromosome: models for human X-linked disease

The identification of many of the transcribed genes in man and mouse is being achieved by large scale sequencing of expressed sequence tags (ESTs). Attention is now being turned to elucidating gene function and many laboratories are looking to the mouse as a model system for this phase of the genome project. Mouse mutants have long been used as a means of investigating gene function and disease pathogenesis, and recently, several large mutagenesis programs have been initiated to fulfill the burgeoning demand of functional genomics research. Nevertheless, there is a substantial existing mouse mutant resource that can be used immediately. This review summarizes the available information about the loci encoding X-linked phenotypic mutants and variants, including 40 classical mutants and 40 that have arisen from gene targeting.

A developmental switch in H4 acetylation upstream of Xist plays a role in X chromosome inactivation

We have investigated the role of histone acetylation in X chromosome inactivation, focusing on its possible involvement in the regulation of Xist, an essential gene expressed only from the inactive X (Xi). We have identified a region of H4 hyperacetylation extending up to 120 kb upstream from the Xist somatic promoter P1. This domain includes the promoter P0, which gives rise to the unstable Xist transcript in undifferentiated cells. The hyperacetylated domain was not seen in male cells or in female XT67E1 cells, a mutant cell line heterozygous for a partially deleted Xist allele and in which an increased number of cells fail to undergo X inactivation. The hyperacetylation upstream of Xist was lost by day 7 of differentiation, when X inactivation was essentially complete. Wild-type cells differentiated in the presence of the histone deacetylase inhibitor Trichostatin A were prevented from forming a normally inactivated X, as judged by the frequency of underacetylated X chromosomes detected by immunofluorescence microscopy. Mutant XT67E1 cells, lacking hyperacetylation upstream of Xist, were less affected. We propose that (i) hyperacetylation of chromatin upstream of Xist facilitates the promoter switch that leads to stabilization of the Xist transcript and (ii) that the subsequent deacetylation of this region is essential for the further progression of X inactivation.

The mouse Tsx gene is expressed in Sertoli cells of the adult testis and transiently in premeiotic germ cells during puberty

Tsx is a gene of unknown function that was previously shown to be expressed specifically in the testis. In order to gain insight into the function of Tsx its pattern of expression was characterized with regard to both timing and cell type in the testis. Northern blot analysis of early postnatal testes showed not only that Tsx message was detectable shortly after birth, but that it increased substantially between 7 and 12 days postpartum (dpp), roughly coincident with the onset of meiosis in the mouse. Alternative Tsx transcripts, detected by RT-PCR, included a spliced form that first appeared at around 12 dpp. In situ hybridization revealed Tsx signal in the somatic Sertoli cells of the adult testis. Consistent with the data from Northern blots, in situ hybridization signal was first detectable in normal pubertal testes at 12 dpp. An anti-Tsx polyclonal antiserum specifically stained premeiotic germ cells in addition to Sertoli cells of pubertal testes at 16, 19, and 27 dpp. Tsx immunostaining in germ cells was nuclear, while Sertoli cells displayed signal throughout the cytoplasm and nucleus. In the adult, Tsx was detected exclusively in Sertoli cells. In contrast, in the adult testis of the oligotriche (olt) mutant, where spermatogenesis is blocked after meiosis, Tsx protein was still present in the spermatogonial nuclei of a subset of tubules. Taken together, these results demonstrate that Tsx expression is induced in both premeiotic germ cells and Sertoli cells during the first wave of spermatogenesis, but that expression is maintained at a detectable level only in Sertoli cells of the normal adult. The persistence of Tsx expression seen in spermatogonia of the adult olt mutant supports the hypothesis that during the first wave of normal spermatogenesis, the advent of a late-stage cell type, either elongating spermatid or spermatozoan, is responsible for extinguishing expression in spermatogonia in normal adult testis. To our knowledge, Tsx is the first gene to show a pattern of germ cell expression that is apparently specific to the pubertal testis.

The role of Xist in X-inactivation

The past year has seen important progress in our understanding of the role of the X inactive specific transcript gene (Xist) in the initiation and propagation of X-inactivation. A 35 kb Xist transgene had been shown to recapitulate the functions of the X-inactivation centre, progress has been made towards indentifying factors controlling the randomness of X-inactivation, and RNA stabilisation has been shown to play a role in Xist regulation at the onset of X-inactivation.

Cloning and localization of the murine Xpct gene: evidence for complex rearrangements during the evolution of the region around the Xist gene

The overall organization of the X-inactivation center (XIC/Xic) candidate region seems poorly conserved between human and mouse. The orientation of a region containing the X-inactive-specific transcript (Xist/ XIST) gene and three genes located 3' of Xist/XIST has been shown to be inverted between the two species, although the actual extent of this rearrangement is unknown. We have cloned and mapped the mouse homolog of the human XPCT (X-linked PEST-containing transporter) gene, which encodes a putative transmembrane transporter. Human XPCT is located about 200 kb outside of the XIC candidate region and 600 kb 5' of or telomeric to the XIST gene. The mouse Xpct gene, which lies approximately 300 kb 5' of and centromeric to Xist, displays 85% identity at the nucleotide level with the human gene, and the overall protein structure is conserved. The transcriptional orientation of mouse Xpct with respect to Xist is the opposite of that in human. Consequently, the evolution of the region between human and mouse appears to be highly complex, with structural rearrangements involving a region of up to 600 kb or more around the Xist gene.

X-chromosome inactivation in mammals

The inactive X chromosome differs from the active X in a number of ways; some of these, such as allocyclic replication and altered histone acetylation, are associated with all types of epigenetic silencing, whereas others, such as DNA methylation, are of more restricted use. These features are acquired progressively by the inactive X after onset of initiation. Initiation of X-inactivation is controlled by the X-inactivation center (Xic) and influenced by the X chromosome controlling element (Xce), which causes primary nonrandom X-inactivation. Other examples of nonrandom X-inactivation are also presented in this review. The definition of a major role for Xist, a noncoding RNA, in X-inactivation has enabled investigation of the mechanism leading to establishment of the heterochromatinized X-chromosome and also of the interactions between X-inactivation and imprinting as well as between X-inactivation and developmental processes in the early embryo.

The (epi)genetic control of mammalian X-chromosome inactivation

In mammals, the X chromosome is uniquely capable of complete inactivation. Research in the past two years has validated the long-held hypothesis that the 'X-inactivation center' (Xic) controls events of X inactivation and that its resident gene Xist is not only required but is at least partially responsible for the cis-restriction of X inactivation. Progress has also been made in identifying genes within the Xic. Although Xist remains the only known required element, evidence now suggests that a separate element for X counting must exist and that the Xic may be entirely contained within a 450 kb sequence. This small region may be sufficient for both initiation and establishment of X inactivation.