Identification and characterization of alternatively spliced murine Rgs11 isoforms: genomic structure and gene analysis

The RGS proteins comprise a large family of proteins which were recently identified as negative Regulators of G-protein Signaling. They have been shown to act as GTPase Activating Proteins (GAPs) towards the G(alpha) subunits of heterotrimeric G-proteins. In addition to this GAP activity, which has been shown to occur through the RGS domain, RGS proteins are likely to possess other functions due to the existence of other domains in these molecules (De Vries and Farquhar, 1999; Hepler, 1999). Here, we report the molecular characterization of the murine Rgs11 gene. The gene encodes a protein with high homology to human RGS11 (79.9%), containing conserved DEP (Dishevelled/EGL-10/Pleckstrin) and GGL (G protein gamma-like) domains. The gene is comprised of at least 13 exons, spanning 8-9 kb. Spliced transcript variants were identified which are co-expressed with 5A3, a transcript that contains the largest ORF. Expression of mouse Rgs11 was found to be restricted to specific tissues with a unique pattern of expression observed in brain.

Genomics: saviour or millstone?

The Keystone Symposium on the impact of Genomics Development was held in Santa Fe, New Mexico, from 2 to 7 February 2001.

The mouse slalom mutant demonstrates a role for Jagged1 in neuroepithelial patterning in the organ of Corti

The Notch signalling pathway has recently been implicated in the development and patterning of the sensory epithelium in the cochlea, the organ of Corti. As part of an ongoing large-scale mutagenesis programme to identify new deaf or vestibular mouse mutants, we have identified a novel mouse mutant, slalom, which shows abnormalities in the patterning of hair cells in the organ of Corti and missing ampullae, structures that house the sensory epithelia of the semicircular canals. We show that the slalom mutant carries a mutation in the Jagged1 gene, implicating a new ligand in the signalling processes that pattern the inner ear neuro-epithelium.

A systematic, genome-wide, phenotype-driven mutagenesis programme for gene function studies in the mouse

As the human genome project approaches completion, the challenge for mammalian geneticists is to develop approaches for the systematic determination of mammalian gene function. Mouse mutagenesis will be a key element of studies of gene function. Phenotype-driven approaches using the chemical mutagen ethylnitrosourea (ENU) represent a potentially efficient route for the generation of large numbers of mutant mice that can be screened for novel phenotypes. The advantage of this approach is that, in assessing gene function, no a priori assumptions are made about the genes involved in any pathway. Phenotype-driven mutagenesis is thus an effective method for the identification of novel genes and pathways. We have undertaken a genome-wide, phenotype-driven screen for dominant mutations in the mouse. We generated and screened over 26,000 mice, and recovered some 500 new mouse mutants. Our work, along with the programme reported in the accompanying paper, has led to a substantial increase in the mouse mutant resource and represents a first step towards systematic studies of gene function in mammalian genetics.

Mice overexpressing human uncoupling protein-3 in skeletal muscle are hyperphagic and lean

Uncoupling protein-3 (UCP-3) is a recently identified member of the mitochondrial transporter superfamily that is expressed predominantly in skeletal muscle. However, its close relative UCP-1 is expressed exclusively in brown adipose tissue, a tissue whose main function is fat combustion and thermogenesis. Studies on the expression of UCP-3 in animals and humans in different physiological situations support a role for UCP-3 in energy balance and lipid metabolism. However, direct evidence for these roles is lacking. Here we describe the creation of transgenic mice that overexpress human UCP-3 in skeletal muscle. These mice are hyperphagic but weigh less than their wild-type littermates. Magnetic resonance imaging shows a striking reduction in adipose tissue mass. The mice also exhibit lower fasting plasma glucose and insulin levels and an increased glucose clearance rate. This provides evidence that skeletal muscle UCP-3 has the potential to influence metabolic rate and glucose homeostasis in the whole animal.

Identification of two new Pmp22 mouse mutants using large-scale mutagenesis and a novel rapid mapping strategy

Mouse mutants have a key role in discerning mammalian gene function and modelling human disease; however, at present mutants exist for only 1-2% of all mouse genes. In order to address this phenotype gap, we have embarked on a genome-wide, phenotype-driven, large-scale N-ethyl-N--nitrosourea (ENU) mutagenesis screen for dominant mutations of clinical and pharmacological interest in the mouse. Here we describe the identification of two similar neurological phenotypes and determination of the underlying mutations using a novel rapid mapping strategy incorporating speed back-crosses and high throughput genotyping. Two mutant mice were identified with marked resting tremor and further characterized using the SHIRPA behavioural and functional assessment protocol. Back-cross animals were generated using in vitro fertilization and genome scans performed utilizing DNA pools derived from multiple mutant mice. Both mutants were mapped to a region on chromosome 11 containing the peripheral myelin protein 22 gene (Pmp22). Sequence analysis revealed novel point mutations in Pmp22 in both lines. The first mutation, H12R, alters the same amino acid as in the severe human peripheral neuropathy Dejerine Sottas syndrome and Y153TER in the other mutant truncates the Pmp22 protein by seven amino acids. Histological analysis of both lines revealed hypo-myelination of peripheral nerves. This is the first report of the generation of a clinically relevant neurological mutant and its rapid genetic characterization from a large-scale mutagenesis screen for dominant phenotypes in the mouse, and validates the use of large-scale screens to generate desired clinical phenotypes in mice.

Implementation of a large-scale ENU mutagenesis program: towards increasing the mouse mutant resource

Systematic approaches to mouse mutagenesis will be vital for future studies of gene function. We have begun a major ENU mutagenesis program incorporating a large genome-wide screen for dominant mutations. Progeny of ENU-mutagenized mice are screened for visible defects at birth and weaning, and at 5 weeks of age by using a systematic and semi-quantitative screening protocol-SHIRPA. Following this, mice are screened for abnormal locomotor activity and for deficits in prepulse inhibition of the acoustic startle response. Moreover, in the primary screen, blood is collected from mice and subjected to a comprehensive clinical biochemical analysis. Subsequently, secondary and tertiary screens of increasing complexity can be used on animals demonstrating deficits in the primary screen. Frozen sperm is archived from all the male mice passing through the screen. In addition, tail tips are stored for DNA. Overall, the program will provide an extensive new resource of mutant and phenotype data to the mouse and human genetics communities at large. The challenge now is to employ the expanding mouse mutant resource to improve the mutant map of the mouse. An improved mutant map of the mouse will be an important asset in exploiting the growing gene map of the mouse and assisting with the identification of genes underlying novel mutations-with consequent benefits for the analysis of gene function and the identification of novel pathways.

Neuropilin-2 is required in vivo for selective axon guidance responses to secreted semaphorins

Neuropilins are receptors for class 3 secreted semaphorins, most of which can function as potent repulsive axon guidance cues. We have generated mice with a targeted deletion in the neuropilin-2 (Npn-2) locus. Many Npn-2 mutant mice are viable into adulthood, allowing us to assess the role of Npn-2 in axon guidance events throughout neural development. Npn-2 is required for the organization and fasciculation of several cranial nerves and spinal nerves. In addition, several major fiber tracts in the brains of adult mutant mice are either severely disorganized or missing. Our results show that Npn-2 is a selective receptor for class 3 semaphorins in vivo and that Npn-1 and Npn-2 are required for development of an overlapping but distinct set of CNS and PNS projections.

Regulatory elements in the minimal promoter region of the mouse Xist gene

The Xist gene plays a central role in regulating X chromosome inactivation and Xist transcription has recently been shown to be necessary for X inactivation in mouse. We are currently analysing regulation of the Xist gene in order to determine the mechanisms underlying initiation of Xist expression and X inactivation. Sequence comparisons indicate that a region of approximately 0.4 kb upstream of the the major transcriptional start site comprises the Xist minimal promoter. Analysis of reporter constructs demonstrates that the minimal promoter region is active both in embryonic stem (ES) cells and in differentiated derivatives, indicating that sequences either further upstream or downstream are required for appropriate developmental control of Xist transcription. We have examined the minimal promoter region in detail, and in addition to common promoter elements have identified two previously uncharacterised transcription-factor binding sites. Mutation of these sites in reporter constructs indicates that they are functionally important.

Functional genomics: going forwards from the databases

Extrapolating systematically from gene sequence to function is undoubtedly the major challenge facing industry and academia alike as we approach the end of the millennium. Many electronic and laboratory approaches are being developed to meet this challenge but the rate of evolution of these is not keeping pace with the speed of sequence generation.

Stabilization of Xist RNA mediates initiation of X chromosome inactivation

The onset of X inactivation is preceded by a marked increase in the level of Xist RNA. Here we demonstrate that increased stability of Xist RNA is the primary determinant of developmental up-regulation. Unstable transcript is produced by both alleles in XX ES cells and in XX embryos prior to the onset of random X inactivation. Following differentiation, transcription of unstable RNA from the active X chromosome allele continues for a period following stabilization and accumulation of transcript on the inactive X allele. We discuss the implications of these findings in terms of models for the initiation of random and imprinted X inactivation.

A comparison of the properties of Sox-3 with Sry and two related genes, Sox-1 and Sox-2

The Sox gene family consists of a large number of embryonically expressed genes related via the possession of a 79-amino-acid DNA-binding domain known as the HMG box. Partial clones for the first three Sox genes (al-a3) were isolated by homology to the HMG box of the testis-determining gene Sry and are now termed Sox-1, Sox-2 and Sox-3, Sox-3 is highly conserved amongst mammalian species and is located on the X chromosome. This has led to the proposal that Sry evolved from Sox-3. We present the cloning and sequencing of Sox-1, Sox-2 and Sox-3 from the mouse and show that Sox-3 is most closely relate to Sry. We also confirm that mouse Sox-3 is located on the X chromosome between Hprt and Dmd. Analysis of the distribution of Sox-3 RNA shows that its main site of expression is in the developing central nervous system, suggesting a role for Sox-3 in neural development. Moreover, we demonstrate that Sox-3, as well as Sox-1 and Sox-2, are expressed in the urogenital ridge and that their protein products are able to bind the same DNA sequence motif as Sry in vitro, but with different affinities. These observations prompt discussion of an evolutionary link between the genes and support the model that Sry has evolved from Sox-3. However our findings imply that if this is true, then Sry has undergone concomitant changes resulting in loss of CNS expression and altered DNA-binding properties.

Requirement for Xist in X chromosome inactivation

The Xist gene has been proposed as a candidate for the X inactivation centre, the master regulatory switch locus that controls X chromosome inactivation. So far this hypothesis has been supported solely by indirect evidence. Here we describe gene targeting of Xist, and provide evidence for its absolute requirement in the process of X chromosome inactivation.

Cloning and characterization of a new human Xq13 gene, encoding a putative helicase

We describe the cloning and characterization of a new human Xq13 gene (XH2), extending over a 220 kb genomic stretch between MNK and DXS56. The gene, which undergoes X-inactivation, contains a 4 kb open reading frame and encodes a putative NTP-binding nuclear protein homologous to several members of the helicase II superfamily. The murine homologue maps to the syntenic genetic interval, between Pgk1 and Xist. In situ hybridization studies in mouse reveal precocious, widespread expression of the murine homologue of XH2 at early stages of embryogenesis, and more restricted expression during late developmental stages and at birth. XH2 is a new member of an expanding family of proven and putative helicases, sharing six conserved, collinear domains. In particular, the XH2 protein shows homology with yeast RAD54. Type II helicases have been implicated in nucleotide excision repair and the initiation of transcription. This new gene, represents a potential candidate for several genetic disorders mapped to human Xq13.

Imprinting and X chromosome counting mechanisms determine Xist expression in early mouse development

In mice, X inactivation is preceded by in cis Xist expression. Initially, normal female embryos express the paternal Xist allele exclusively, preceding imprinted X inactivation in the trophectoderm. Later expression of Xist alleles is random, preceding random X inactivation in the epiblast lineage. In this study using uniparental embryos, we demonstrate that Xist expression is initially dictated solely by parental imprinting, causing expression of all paternal alleles. Maternal alleles remain repressed, irrespective of X chromosome number. At the compacting morula stage, this parental imprint is erased, and the mechanism counting the X chromosomes imposes appropriate Xist expression with respect to chromosome number. Our results also suggest that Xist expression may itself be regulated by a novel imprinted maternally expressed gene.

Evidence that random and imprinted Xist expression is controlled by preemptive methylation

The mouse Xist gene is expressed exclusively from the inactive X chromosome and may control the initiation of X inactivation. We show that in somatic tissues the 5' end of the silent Xist allele on the active X chromosome is fully methylated, while the expressed allele on the inactive X is completely unmethylated. In tissues that undergo imprinted paternal Xist expression and imprinted X inactivation, the paternal Xist allele is unmethylated, and the silent maternal allele is fully methylated. In the male germline, a developmentally regulated demethylation of Xist occurs at the onset of meiosis and is retained in mature spermatozoa. This may be the cause of imprinted expression of the paternal Xist allele. A role for methylation in the control of Xist expression is further supported by the finding that in differentiating embryonic stem cells during the initiation of X inactivation, differential methylation of Xist alleles precedes the onset of Xist expression.

X chromosome inactivation and the Xist gene

X chromosome inactivation in mammals was first described over 30 years ago. The biological problem is how to achieve gene dosage equivalence between XX females and XY males; the solution is to genetically silence one whole X chromosome in each cell of the early developing female embryo. The molecular mechanism by which this is achieved, however, remains a mystery. Recently, through the discovery of the Xist gene, it appears that we may be on the brink of learning how this unique phenomenon is mediated. Here, I discuss the developmental regulation of X inactivation and the candidacy of Xist as the X chromosome inactivation centre, with particular reference to its possible role in the initiation, spread and maintenance of X inactivation.

Deletion of Y chromosome sequences located outside the testis determining region can cause XY female sex reversal

An approach designed to map and generate mutations in the region of the short arm of the mouse Y chromosome, known to be involved in sex determination and spermatogenesis, is described. This relies on homologous Yp-Sxra pairing and asymmetrical exchange which can occur at meiosis in XY males carrying Sxra on their X chromosome. Such exchange potentially generates deficiencies and duplications of Yp or Sxra. Three fertile XY females were found out of about 450 XY offspring from XSxra/Y x XX crosses. In all three, despite evidence for deletion of Y chromosomal material, the Sry locus was intact. Each deletion involved a repeat sequence, Sx1, located at a distance from Sry. Since expression of Sry was affected these results suggest that long range position effects have disrupted Sry action.

YAC clone contigs surrounding the Zfx and Pola loci on the mouse X chromosome

Pulsed-field mapping of a number of DNA markers in the Pola-Zfx region of the mouse X chromosome has established a genomic restriction map extending over 1.4 Mb. A number of YAC clones from the Pola-Zfx region have been isolated from three mouse YAC libraries--first, a mouse C57BL/10 partial R1 YAC library constructed in a yeast strain carrying a rad52 mutation (Chartier et al. (1992) Nature Genetics 1: 132-136); second, a mouse C3H partial R1 library (Larin et al. (1991) Proc. Natl. Acad. Sci. USA 88: 4123-4127); and third, a mouse C57BL/6 partial R1 library (Burke et al. (1991) Mamm. Genome 1:65). Six YAC clones encompass the Zfx-Pola region, confirming the linkage of the Pola and Zfx loci and establishing a physical map order in this region of cen-Pola-DXCrc140-DXCrc57-Zfx-tel. The close linkage of Pola and Zfx in the mouse genome suggests that the POLA and ZFX loci must also be closely linked on the human X chromosome.

Physical mapping of 2000 kb of the mouse X chromosome in the vicinity of the Xist locus

A physical map encompassing approximately 2.0 megabases (Mb) in the region of the mouse X-inactivation center has been constructed. The map extends from the Gjb-1 locus to the Xist locus and demonstrates the order of probes inseparable by genetic analysis. The deduced locus order is as follows: Gjb-1, Ccg-1, DXCrc171, Rps4, Phka, DXCrc177, DXCrc318, Xist. Detailed physical mapping in the region between the Phka and Xist loci indicates the position of CpG-rich islands associated with the 5' end of genes. The DXCrc177 and DXCrc318 loci, both defined by probes derived from linking clones, are associated with CpG-rich islands. The map provides a framework for the isolation of underlying sequences in the mouse X-inactivation center region.

Expression of Xist during mouse development suggests a role in the initiation of X chromosome inactivation

The mouse Xist gene maps to the X inactivation center (Xic) region and is expressed exclusively from the inactive X chromosome. It is thus a candidate gene for the Xic. We show that the onset of Xist expression in mouse development precedes X chromosome inactivation and may therefore be a cause rather than merely a consequence of X inactivation. The earliest Xist expression in morulae and blastocysts is imprinted, resulting in specific expression of the paternal Xist allele. Imprinted Xist expression may thus be the cause of nonrandom inactivation of the paternal X in trophectoderm. Strong Xce alleles can act to reduce the effect of imprinted Xist expression in the trophectoderm. The imprint on Xist expression is lost shortly before gastrulation when random X inactivation occurs. Our data support a direct role for Xist in the initiation of X inactivation.

Detection of a molecular deletion at the DXS732 locus in a patient with X-linked hypohidrotic ectodermal dysplasia (EDA), with the identification of a unique junctional fragment

X-linked hypohidrotic ectodermal dysplasia (EDA) has been localized to the Xq12-q13.1 region. A panel of genomic DNA samples from 80 unrelated males with EDA has been screened for deletions at seven genetic loci within the Xq12-13 region. A single individual was identified with a deletion at the DXS732 locus by hybridization with the mouse genomic probe pcos169E/4. This highly conserved DNA probe is from locus DXCrc169, which is tightly linked to the Ta locus, the putative mouse homologue of EDA. The proband had the classical phenotype of EDA, with no other phenotypic abnormalities, and a normal cytogenetic analysis. A human genomic DNA clone, homologous to pcos169E/4, was isolated from a human X-chromosome cosmid library. On hybridization with the cosmid, the proband was found to be only partially deleted at the DXS732 locus, with a unique junctional fragment identified in the proband and in three of his maternal relatives. This is the first determination of carrier status for EDA in females, by direct mutation analysis. Failure to detect deletion of the other loci tested in the proband suggests that the DXS732 locus is the closest known locus to the EDA gene. Since the DXS732 locus contains a highly conserved sequence, it must be considered to be a candidate locus for the EDA gene itself.

High-resolution mapping of the X-linked hypohidrotic ectodermal dysplasia (EDA) locus

The X-linked hypohidrotic ectodermal dysplasia (EDA) locus has been previously localized to the subchromosomal region Xq11-q21.1. We have extended our previous linkage studies and analyzed linkage between the EDA locus and 10 marker loci, including five new loci, in 41 families. Four of the marker loci showed no recombination with the EDA locus, and six other loci were also linked to the EDA locus with recombination fractions of .009-.075. Multipoint analyses gave support to the placement of the PGK1P1 locus proximal to the EDA locus and the DXS453 and PGK1 loci distal to EDA. Further ordering of the loci could be inferred from a human/rodent somatic cell hybrid derived from an affected female with EDA and an X;9 translocation and from studies of an affected male with EDA and a submicroscopic deletion. Three of the proximal marker loci, which showed no recombination with the EDA locus, when used in combination, were informative in 92% of females. The closely linked flanking polymorphic loci DXS339 and DXS453 had heterozygosities of 72% and 76%, respectively, and when used jointly, they were doubly informative in 52% of females. The human DXS732 locus was defined by a conserved mouse probe pcos169E/4 (DXCrc169 locus) that cosegregates with the mouse tabby (Ta) locus, a potential homologue to the EDA locus. The absence of recombination between EDA and the DXS732 locus lends support to the hypothesis that the DXCrc169 locus in the mouse and the DXS732 locus in humans may contain candidate sequences for the Ta and EDA genes, respectively.

The product of the mouse Xist gene is a 15 kb inactive X-specific transcript containing no conserved ORF and located in the nucleus

The Xist gene maps to the X inactivation center region in both mouse and human, and previous analysis of the 3' end of the gene has demonstrated inactive X-specific expression, suggesting a possible role in X inactivation. We have now analyzed the entire mouse Xist gene. The mature inactive X-specific transcript is 15 kb in length and contains no conserved ORF. The Xist sequence contains a number of regions comprised of tandem repeats. Comparison with the human XIST gene demonstrates significant conservation of sequence and gene structure. Xist RNA is not associated with the translational machinery of the cell and is located almost exclusively in the nucleus. Together with conservation of inactive X-specific expression, these findings support a role for Xist in X inactivation, possibly as a functional RNA or as a chromatin organizer region.

Lymphoid development in mice congenitally lacking T cell receptor alpha beta-expressing cells

Vertebrate T cells express either an alpha beta or gamma delta T cell receptor (TCR). The developmental relatedness of the two cell types is unresolved. alpha beta + T cells respond to specific pathogens by collaborating with immunoglobulin-producing B cells in distinct lymphoid organs such as the spleen and Peyer's patches. The precise influence of alpha beta + T cells on B cell development is poorly understood. To investigate the developmental effects of alpha beta + T cells on B cells and gamma delta + T cells, mice homozygous for a disrupted TCR alpha gene were generated. The homozygotes showed elimination of alpha beta + T cells and the loss of thymic medullae. Despite this, gamma delta + T cells developed in normal numbers, and there was an increase in splenic B cells.

A candidate spermatogenesis gene on the mouse Y chromosome is homologous to ubiquitin-activating enzyme E1

The human X-linked gene A1S9 complements a temperature-sensitive cell-cycle mutation in mouse L cells, and encodes the ubiquitin-activating enzyme E1. The gene has been reported to escape X-chromosome inactivation, but there is some conflicting evidence. We have isolated part of the mouse A1s9 gene, mapped it to the proximal portion of the X chromosome and shown that it undergoes normal X-inactivation. We also detected two copies of the gene on the short arm of the mouse Y chromosome (A1s9Y-1 and A1s9Y-2). The functional A1s9Y gene (A1s9Y-1) is expressed in testis and is lost in the deletion mutant Sxrb. Therefore A1s9Y-1 is a candidate for the spermatogenesis gene, Spy, which maps to this region. A1s9X is similar to the Zfx gene in undergoing X-inactivation, yet having homologous sequences on the short arm of the Y chromosome, which are expressed in the testis. These Y-linked genes may form part of a coregulated group of genes which function during spermatogenesis.

Determination of a molecular map position for Hyp using a new interspecific backcross produced by in vitro fertilization

We have established a Mus spretus/Mus musculus domesticus interspecific backcross segregating for two X-linked mutant genes, Ta and Hyp, using in vitro fertilization. The haplotype of the recombinant X chromosome of each of 241 backcross progeny has been established using the X-linked anchor loci Otc, Hprt, Dmd, Pgk-1, and Amg and the additional probes DXSmh43 and Cbx-rs1. The Hyp locus (putative homologue of the human disease gene hypophosphatemic rickets, HYP) has been incorporated into the molecular genetic map of the X chromosome. We show that the most likely gene order in the distal portion of the mouse X chromosome is Pgk-1-DXSmh43-Hyp-Cbx-rs1-Amg, from proximal to distal. The distance in centimorgans (mean +/- SE) between DXSmh43 and Hyp was 2.52 +/- 1.4 and that between Hyp and Cbx-rs1 was 1.98 +/- 1.39. Thus closely linked flanking markers for the Hyp locus that will facilitate the molecular characterization of the gene itself have been defined.

Methylation status of CpG-rich islands on active and inactive mouse X chromosomes

Single copy probes derived from CpG-rich island clones from Eag I and Not I linking libraries and nine rare-cutter restriction endonucleases were used to investigate the methylation status of CpG-rich islands on the inactive and active X chromosomes (Chr) of the mouse. Thirteen of the 14 probes used detected CpG-rich islands in genomic DNA. The majority of island CpGs detected by rare-cutter restriction endonucleases were methylated on the inactive X Chr and unmethylated on the active X Chr, but some heterogeneity within the cell population used to make genomic DNA was detected. The CpG-rich islands detected by two putative pseudoautosomal probes remained unmethylated on both the active and inactive X Chrs. Otherwise, distance from the X Chr inactivation center did not affect the methylation profile of CpG-rich islands. We conclude that methylation of CpG-rich islands is a general feature of X Chr inactivation.

X-chromosome inactivation may explain the difference in viability of XO humans and mice

Only about 1% of human XO conceptuses survive to birth and these usually have the characteristics of Turner's syndrome, with a complex and variable phenotype including short stature, gonadal dysgenesis and anatomical defects. Both the embryonic lethality and Turner's syndrome are thought to be due to monosomy for a gene or genes common to the X and Y chromosomes. These genes would be expected to be expressed in females from both active and inactive X chromosomes to ensure correct dosage of gene product. Two genes with these properties are ZFX and RPS4X, both of which have been proposed to play a role in Turner's syndrome. In contrast to humans, mice that are XO are viable with no prenatal lethality (P. Burgoyne, personal communication) and are anatomically normal and fertile. We have devised a system to analyse whether specific genes on the mouse X chromosome are inactivated, and demonstrate that both Zfx and Rps4X undergo normal X-inactivation in mice. Thus the relative viability of XO mice compared to XO humans may be explained by differences between the two species in the way that dosage compensation of specific genes is achieved.

Conservation of position and exclusive expression of mouse Xist from the inactive X chromosome

X-chromosome inactivation in mammals is a regulatory phenomenon whereby one of the two X chromosomes in female cells is genetically inactivated, resulting in dosage compensation for X-linked genes between males and females. In both man and mouse, X-chromosome inactivation is thought to proceed from a single cis-acting switch region or inactivation centre (XIC/Xic). In the human, XIC has been mapped to band Xq13 (ref. 6) and in the mouse to band XD (ref. 7), and comparative mapping has shown that the XIC regions in the two species are syntenic. The recently described human XIST gene maps to the XIC region and seems to be expressed only from the inactive X chromosome. We report here that the mouse Xist gene maps to the Xic region of the mouse X chromosome and, using an interspecific Mus spretus/Mus musculus domesticus F1 hybrid mouse carrying the T(X;16)16H translocation, show that Xist is exclusively expressed from the inactive X chromosome. Conservation between man and mouse of chromosomal position and unique expression exclusively from the inactive X chromosome lends support to the hypothesis that XIST and its mouse homologue are involved in X-chromosome inactivation.

High-density molecular map of the central span of the mouse X chromosome

A total of 17 linking clones previously sublocalized to the central span of the mouse X chromosome have been ordered by detailed analysis through interspecific Mus spretus/Mus musculus domesticus backcross progeny. These probes have been positioned with respect to existing DNA markers utilizing a new interspecific backcross segregating for the Tabby (Ta) locus. The density of clones within this 11.5-cM interval is now, on average, one clone every 1000 kb. This high-density map provides probes in the vicinity of a number of important genetic loci in this region which include the X-inactivation center, the Ta locus, and the mottled (Mo) locus, and therefore provides a molecular framework for identification of the genes encoded at these loci.

Molecular genetic analysis of the Ta25H deletion: evidence for additional deleted loci

Seventeen linking clones sublocalized to the central region of the mouse X Chromosome (Chr) were screened against genomic DNA from male mice carrying the tabby-25H (Ta25H) deletion. Two of these linking clones, lambda EM131 and lambda EM169, were found to be deleted in Ta25H/Y animals. Genetic mapping through Mus musculus domesticus/Mus spretus interspecific backcross progeny, segregating for the original tabby (Ta) gene mutation, was utilized to order these markers and to define nearest flanking markers to the Ta25H deletion (lambda EM140 and lambda EM171). The size of the Ta25H deletion was thus estimated as up to 4.5 centiMorgans (cM). The order of markers, proximal to distal, was found to be lambda EM140/lambda EM131, mouse androgen receptor gene (Ar)/lambda EM169, Ta/lambda EM171. A putative CpG-rich island and a highly evolutionarily conserved DNA probe were isolated from the DXCrc169 locus which co-segregates with the Ta locus in this study.

The search for the mouse X-chromosome inactivation centre

The phenomenon of X-chromosome inactivation in female mammals, whereby one of the two X chromosome present in each cell of the female embryo is inactivated early in development, was first described by Mary Lyon in 1961. Nearly 30 years later, the mechanism of X-chromosome inactivation remains unknown. Strong evidence has accumulated over the years, however, for the involvement of a major switch or inactivation centre on the mouse X chromosome. Identification of the inactivation centre at the molecular level would be an important step in understanding the mechanism of X-inactivation. In this paper we review the evidence for the existence and location of the X-inactivation centre on the mouse X-chromosome, present data on the molecular genetic mapping of this region, and describe ongoing strategies we are using to attempt to identify the inactivation centre at the molecular level.

Construction and analysis of linking libraries from the mouse X chromosome

A hybrid cell line containing the mouse X chromosome on a human background has been used to construct linking libraries from the mouse X chromosome, and approximately 250 unique EagI and NotI clones have been identified. Seventy-three clones have been sublocalized onto the X chromosome using interspecific Mus spretus/Mus domesticus crosses and a panel of somatic cell hybrids carrying one-half of reciprocal X-autosome translocations. The average spacing of the linking clones mapped to date is about one every 2 Mb of DNA. Two clones from the central region of the chromosome have been physically linked by pulsed-field gel electrophoresis. A large number of clones contain conserved sequences, indicating the presence of CpG-rich island-associated genes. The clones isolated from these libraries provide a valuable resource for comparative mapping between man and mouse X chromosomes, isolation of X-linked disease loci of interest by reverse genetics, and analysis of the long-range structure and organization of the chromosome.

Age-related reactivation of an X-linked gene close to the inactivation centre in the mouse

Age-related reactivation of an X-linked gene which maps close toXce, the X chromosome inactivation centre, has been observed. In five female mice which carried the X-linked coat colour geneMobloon the reciprocal translocation T(X;16)16H (Searle's translocation), and the wild-type gene on the normal X chromosome, and therefore expressed theMoblophenotype due to the non-random inactivation characteristic of Searle's translocation, progressive darkening of the coat was observed as the animals aged. This is due to reactivation of the previously inactivated wild-type gene at theMolocus on the normal X chromosome. As theMolocus is located 4 cM distal toXce, the X chromosome inactivation centre, these observations provide evidence of age-related instability of inactivation of an X-linked gene close to the inactivation centre.

Expression of H-2 class I genes in murine extra-embryonic tissues

Murine major histocompatibility complex class I genes are transcribed at high levels in placental tissues, lower levels in yolk-sac tissues and at barely detectable levels in the embryo at Day 13.5 of gestation. Genes are expressed at equivalent levels whether inherited maternally or paternally, and the genetic background has no effect on class I gene transcription. These results show that potential alloantigens are expressed in extra-embryonic tissues intimately associated with maternal tissues and blood supply and yet fail to induce immunological rejection.

T-cell depletion of allogeneic bone marrow prevents acceleration of graft-versus-host disease induced by exogenous interleukin 2

Highly purified human recombinant interleukin 2 (IL-2) markedly accelerated lethal GVHD in the H-2-identical B10.BR----CBA combination, but had no effect when the donor cells were depleted of mature (Thy-1.2-positive) T lymphocytes, indicating a strong immunopotentiating effect of IL-2 on mature T cells causing GVHD. In the same donor-host combination, IL-2 did not influence the recovery from the post-transplantation bone marrow aplasia. The results suggest that IL-2 could be considered for adjuvant hormonal therapy to enhance immune recovery in recipients of T-cell-depleted allogeneic marrow.

Saccharide structures of the mouse embryo during the first eight days of development. Inferences from immunocytochemical studies using monoclonal antibodies in conjunction with glycosidases

Monoclonal anti-carbohydrate antibodies have been used in conjunction with glycosidases in immunofluorescence studies to derive information about the structures and in situ distribution of saccharides of the mouse embryo during the first 8 days of development. The salient findings are as follows: Branched poly-N-acetyllactosamine sequences of I-antigen type are detectable from the first day onwards and are widely distributed in cells of the endoderm, ectoderm and mesoderm. Linear poly-N-acetyllactosamine sequences of i-antigen type are detectable from the fifth day onwards in cells of all three lineages, but have a more restricted distribution than the sequences of I-type. Poly-N-acetyllactosamine sequences that are susceptible to digestion with endo-beta-galactosidase are the main carriers of the SSEA-1, C14 and the blood group B-like antigens, which have the following structures (Formula; see text) and are found in endoderm and ectoderm but not in mesoderm cells. In the trophoblast however, these antigens are borne on saccharides that are resistant to endo-beta-galactosidase. A proportion of the poly-N-acetyllactosamine structures in the endoderm and the ectoderm of the 5- and 6-day embryos may contain the following novel structures: (Formula; see text) in which antigenicities of SSEA-1 and C14 determinants are masked. There are several types of sialyl-oligosaccharides: those reactive with anti-Gd, which has a specificity for NeuAc alpha 2-3Gal beta 1-4GlcNAc sequence in the extraembryonic mesoderm and the heart; those reactive with anti-Pr2 but not with anti-Gd, which may correspond to other N-acetylneuraminic acid containing sequences such as NeuAc alpha 2-3Gal beta 1-3GalNAc or NeuAc alpha 2-6Gal in preimplantation embryos and in the yolk sac, neural ectoderm and mesenchyme of the 8-day embryo; those with other sialic acid forms or linkages that do not react with anti-Gd and Pr2; among these are sialosyl-i sequences in the extraembryonic ectoderm, and sialosyl-I sequences in most cell types during the first 8 days. The latter are the main poly-N-acetyllactosamine structures in the neural ectoderm of the 8-day embryo. The sequence Gal beta 1-3GlcNAc beta 1-3Gal beta 1-4Glc/GlcNAc, or cross-reactive structures, which bind FC10.2 antibody occur in the extraembryonic endoderm and yolk sac. The roles of specific carbohydrate structures as receptors during embryonic development and cell growth are important topics of current research.(ABSTRACT TRUNCATED AT 400 WORDS)

Saccharide structures of the mouse embryo during the first eight days of development

Monoclonal anti-carbohydrate antibodies have been used in conjunction with glycosidases in immunofluorescence studies to derive information about the structures and in situ distribution of saccharides of the mouse embryo during the first 8 days of development. The salient findings are as follows:

(a) Branched poly-N-acetyllactosamine sequences of I-antigen type are detectable from the first day onwards and are widely distributed in cells of the endoderm, ectoderm and mesoderm.

(b) Linear poly-N-acetyllactosamine sequences of i-antigen type are detectable from the fifth day onwards in cells of all three lineages, but have a more restricted distribution than the sequences of I-type.

(c) Poly-N-acetyllactosamine sequences that are susceptible to digestion with endo-β-galactosidase are the main carriers of the SSEA-1, C14 and the blood group B-like antigens, which have the following structuresand are found in endoderm and ectoderm but not in mesoderm cells. In the trophoblast however, these antigens are borne on saccharides that are resistant to endo-β-galactosidase.

Cell interactions in preimplantation embryos: evidence for involvement of saccharides of the poly- N-acetyllactosamine series

Roles of cell surface carbohydrates containing the 3-fucosyl-N-acetyllactosamine and poly-Nacetyllactosamine sequences (SSEA-1 and I antigens, respectively) in the compaction of mouse embryos have been investigated using the endo-β-galactosidase of Bacteroides fragilis to modify the surface of cleavage-stage embryos. Treatment with this enzyme abolished SSEA-1 activity and diminished I antigen activity on the embryonic cell surface. Embryos cultured in the presence of endo-β-galactosidase from the 2- to 4-cell stage onwards, or treated with the enzyme at the compacting 8-cell stage, continued to compact and proceeded to form blastocysts at the normal rate. However, when compacted 8- to 16-cell embryos were experimentally decompacted in calcium-free medium, treated for 1 h with endo-β-galactosidase and returned to normal culture medium, the time taken for 50 % of the embryos to recompact was prolonged five-fold. There was an even greater delay if these embryos were maintained in culture medium containing the enzyme. Blastocysts were eventually formed under both conditions. Thus, endo-β-galactosidase did not affect compaction unless the embryos were first decompacted. On the assumption that recompaction and de novo compaction occur by similar mechanisms, we propose that carbohydrate-binding molecules are involved which have high affinities for poly-Nacetyllactosamine structures and protect them from digestion by endo-β-galactosidase.

Cell interactions in preimplantation embryos: evidence for involvement of saccharides of the poly-N-acetyllactosamine series

Roles of cell surface carbohydrates containing the 3-fucosyl-N-acetyllactosamine and poly-N-acetyllactosamine sequences (SSEA-1 and I antigens, respectively) in the compaction of mouse embryos have been investigated using the endo-beta-galactosidase of Bacteroides fragilis to modify the surface of cleavage-stage embryos. Treatment with this enzyme abolished SSEA-1 activity and diminished I antigen activity on the embryonic cell surface. Embryos cultured in the presence of endo-beta-galactosidase from the 2- to 4-cell stage onwards, or treated with the enzyme at the compacting 8-cell stage, continued to compact and proceeded to form blastocysts at the normal rate. However, when compacted 8- to 16-cell embryos were experimentally decompacted in calcium-free medium, treated for 1 h with endo-beta-galactosidase and returned to normal culture medium, the time taken for 50% of the embryos to recompact was prolonged five-fold. There was an even greater delay if these embryos were maintained in culture medium containing the enzyme. Blastocysts were eventually formed under both conditions. Thus, endo-beta-galactosidase did not affect compaction unless the embryos were first decompacted. On the assumption that recompaction and de novo compaction occur by similar mechanisms, we propose that carbohydrate-binding molecules are involved which have high affinities for poly-N-acetyllactosamine structures and protect them from digestion by endo-beta-galactosidase.

Parental source of chromosome imprinting and its relevance for X chromosome inactivation

In imprinting, homologous chromosomes behave differently during development according to their parental origin. Typically, paternally derived chromosomes are preferentially inactivated or eliminated. Examples of such phenomena include inactivation of the mammalian X chromosome, inactivation or elimination of one haploid chromosome set in male coccids, and elimination of paternal X chromosomes in the fly Sciara. It has generally been thought that the paternal chromosomes bear an imprint leading to their inactivation or elimination. However, alteration of the parental origin of chromosomes, as in the study of parthenogenotes in mammals and coccids, shows that passage of chromosomes through a male germ cell or fertilization is not essential for inactivation or elimination. It appears that neither chromosome set is programmed to resist or undergo inactivation. Instead the two sets differ in relative sensitivity, and the question is whether the maternal set have an imprint for resistance, or the paternal set one for susceptibility. Very early in development of mammals both X chromosomes are active. This makes it simpler to envisage the maternal X bearing an imprint for resistance to inactivation, which persists through the early developmental period. Similar considerations also apply in coccids and Sciara. Thus, imprinting should be regarded as a phenomenon conferred on the maternal chromosomes in the oocyte. This permits simpler models for the mechanism of X-inactivation, and weakens the case for evolution of X-inactivation from an earlier form of inactivation during male gametogenesis. One may speculate whether imprinting affects timing of gene action in development.