Evidence for X-linkage of steroid sulfatase in the mouse: steroid sulfatase levels in oocytes of XX and XO mice

Transcriptional changes in response to X chromosome dosage in the mouse: implications for X inactivation and the molecular basis of Turner Syndrome

Background

X monosomic mice (39,XO) have a remarkably mild phenotype when compared to women with Turner syndrome (45,XO). The generally accepted hypothesis to explain this discrepancy is that the number of genes on the mouse X chromosome which escape X inactivation, and thus are expressed at higher levels in females, is very small. However this hypothesis has never been tested and only a small number of genes have been assayed for their X-inactivation status in the mouse. We performed a global expression analysis in four somatic tissues (brain, liver, kidney and muscle) of adult 40,XX and 39,XO mice using the Illumina Mouse WG-6 v1_1 Expression BeadChip and an extensive validation by quantitative real time PCR, in order to identify which genes are expressed from both X chromosomes.

Results

We identified several genes on the X chromosome which are overexpressed in XX females, including those previously reported as escaping X inactivation, as well as new candidates. However, the results obtained by microarray and qPCR were not fully concordant, illustrating the difficulty in ascertaining modest fold changes, such as those expected for genes escaping X inactivation. Remarkably, considerable variation was observed between tissues, suggesting that inactivation patterns may be tissue-dependent. Our analysis also exposed several autosomal genes involved in mitochondrial metabolism and in protein translation which are differentially expressed between XX and XO mice, revealing secondary transcriptional changes to the alteration in X chromosome dosage.

Conclusions

Our results support the prediction that the mouse inactive X chromosome is largely silent, while providing a list of the genes potentially escaping X inactivation in rodents. Although the lower expression of X-linked genes in XO mice may not be relevant in the particular tissues/systems which are affected in human X chromosome monosomy, genes deregulated in XO mice are good candidates for further study in an involvement in Turner Syndrome phenotype.

Steroid sulfatase inhibitor alters blood pressure and steroid profiles in hypertensive rats

Our hypothesis is that the steroid sulfatase gene (Sts) may indirectly contribute to the modulation of blood pressure (BP) in rats with genetic hypertension. The steroid sulfatase enzyme (STS) catalyzes the conversion of estrone sulfate, dehydroepiandrosterone sulfate, cholesterol sulfate and glucocorticoid sulfates to their active nonconjugated forms. This causes the elevation of biologically active steroids, such as glucocorticoids, mineralcorticoids as well as testosterone, which may lead to increased BP. The main objective was to examine the effects of a steroid sulfatase inhibitor on blood pressure and steroid levels in rats with hypertensive genetic backgrounds. Three treatment groups, 5-15 weeks of age were used: controls, estrone and STS inhibitor (estrone-3-O-sulfamate), (n=8 per group). BP was taken weekly by tail cuff, and serum testosterone (T), estrogens (E), and plasma corticosterone (C) levels were measured by radioimmunoassay. BP was significantly reduced by the STS inhibitor in the strains with genetically elevated BP. Also the inhibitor alone significantly reduced plasma corticosterone in all strains compared to estrone treatment with a concomitant as well as significant rise in estrogens and reduction in testosterone and body weight.

Cloning and expression of the mouse pseudoautosomal steroid sulphatase gene (Sts)

Steroid sulphatase (STS) is an important enzyme in steroid metabolism. The human STS gene has been cloned and mapped to Xp22.3, proximal to the pseudoautosomal region (PAR). Using quantitative differences in STS activity among various mouse strains, a segregation pattern consistent with autosomal linkage was first reported, but more recent studies have linked Sts to the mouse PAR. Failed attempts to clone the mouse Sts gene using human reagants (STS cDNA and anti-STS antibodies) suggest a substantial divergence between these genes. However, partial amino-terminal sequence from purified rat liver Sts is very similar to its human counterpart, and several domains are conserved among all the sulphatases. We followed a degenerate-primer reverse transcriptase-PCR (RT-PCR) approach to amplify a conserved fragment of the rat Sts cDNA that was then used to clone the mouse Sts cDNA. This 2.3-kb cDNA revealed 75% similarity with rat Sts cDNA, while it was only 63% similar to human STS cDNA. Transfection of STS(-) A9 cells with the mouse Sts cDNA restored STS enzymatic activity. Sts was also mapped physically to the distal end of the mouse sex chromosomes, and our backcross studies placed Sts distal to the 'obligatory' cross-over in male meiosis.

Sex chromosomes, recombination, and chromatin conformation

We review what is known about the transcriptional inactivation and condensation of heteromorphic sex chromosomes in contrast to the activation of homomorphic sex chromosomes during meiotic prephase in animals. We relate these cytological and transcriptional features to the recombination status of the sex chromosomes. We propose that sex chromosome condensation is a meiotic adaptation to prevent the initiation of potentially damaging recombination events in nonhomologous regions of the X and Y chromosome.

The mammalian pseudoautosomal region

Despite being morphologically dissimilar, mammalian sex chromosomes pair in male meiosis. Molecular studies of the X and Y chromosomes in humans and mice have identified the pseudoautosomal region, a genetically unique region of shared, recombining sequences that fall within the meiotic pairing region. Complete meiotic and physical maps of the human pseudoautosomal region have been produced and the pseudoautosomal boundary has been cloned and sequenced. These studies have provided clues to mammalian sex chromosome function and evolution.

X-inactivation of the Sts locus in the mouse: an anomaly of the dosage compensation mechanism

The behaviour of the X- and Y-borne Sts locus has been studied in male and female mice. There was considerable heterogeneity in STS activity between inbred mouse strains, with a four fold difference in activity between the highest (101/H) and lowest (Ju/Ct) activity strains, which can be interpreted in terms of allelic differences. In all inbred strains male STS levels were higher than those of female STS levels and in the majority of strains tested male STS levels were nearly twice as high as female levels. Reciprocal crosses between C3H/HeH and the STS-deficient substrain, C3H/An, demonstrated that activities of the X- and Y-borne genes in males are essentially the same and this suggested that the lower STS level in females derives from X-inactivation of the locus. The possibility that hormonal differences could instead be responsible for the lower activity in females was ruled out by the findings that (a) castration of males did not reduce their STS levels and (b) sex-reversed males, X/X Sxr, had STS levels typical of females. Final proof that the mouse Sts locus can be subject to the X-inactivation process was provided by the observation that XX females had STS levels that were only slightly (20%) higher than those of XO females. The difference may indicate incomplete inactivation of the locus. Linkage data verifying the location of Sts on the distal end of the X chromosome are provided.(ABSTRACT TRUNCATED AT 250 WORDS)

X-linked genetic homologies between mouse and man

X-linked genes are conserved among all mammalian species, but the organization of genes on the X chromosome varies from one species to another. This review summarizes the evidence for established gene homologies between mice and human beings. It also describes genes that are possible homologies because of their locations in the human and murine X chromosomes and similarities in the phenotypes they produce. Based on current knowledge of homologous gene location, the human and murine X chromosomes appear to contain four highly conserved segments and differ in organization by only three to four simple chromosomal rearrangements.

Autosomal assignment of OTC in marsupials and monotremes: implications for the evolution of sex chromosomes

The OTC gene coding for ornithine transcarbamylase is sex linked and subject toXinactivation in humans and mice. We have used a rat cDNA probe to localize OTC byin situhybridization in marsupials and monotremes. The gene maps to an autosomal site in two distantly related marsupial species and in one monotreme (the platypus); the first demonstration that a geneX-linked in one mammalian species may be autosomal in another. Since the conservation of the mammalianXis thought to be a consequence of its isolation by the inactivation mechanism, we propose that an autosomal or pseudoautosomal segment containing OTC has been recruited into the inactivated region of theXrather recently in eutherian evolution while it remained autosomal, or was translocated to an autosome, in metatherian and prototherian mammals.

Steroid sulphatase levels are higher in males than in females of the root vole (Microtus oeconomus). Yet another rodent with an active Y-linked allele?

Steroid sulfatase (STS; EC 3.1.6.2) levels were assayed in cultured fibroblasts of root voles captured in the wild. Four independent experiments were performed using two different substrates (DHEAS and E1S). Evidence is presented that in this species, STS levels are significantly higher in males than in females (ratio 1.6:1). We discuss our findings on a comparative basis and suggest that in the root vole the STS gene(s) is X- and Y-linked (as in the mouse) and that it is subject to X-inactivation, or partially so, on one of the X chromosomes in the female.

Linkage of the steroid sulfatase gene to the sex-reversed mutation in the mouse

Dosage studies and the inheritance pattern of the gene for steroid sulfatase (Sts) in the mouse have previously provided indirect evidence for a functional Y-linked allele which recombines obligatorily with its X-linked allele in male meiosis. In this study, we have investigated the linkage relationship of Sts and the sex-reversed mutation (Sxr), a gene which is known to reside in the pairing region of the Y chromosome. The results clearly demonstrate that Sxr and Sts are linked in a region of obligatory recombination and Sts maps proximal to Sxr with most recombinants occurring proximal to the two genes.

Dosage of the Sts gene in the mouse

In this study we compared steroid sulfatase levels in XO, XX, and XY mice and carried out a clonal analysis in fibroblast cell cultures from mice heterozygous for the steroid sulfatase deficiency gene and heterozygous at the X-linked electrophoretic phosphoglycerate kinase locus. The combined results indicate that the murine steroid sulfatase locus is not dosage compensated and is not subject to X-inactivation. With respect to X-inactivation, it behaves in a somewhat different way from the closely linked sex-reversed gene and the human steroid sulfatase locus.

X-linkage of steroid sulphatase in the mouse is evidence for a functional Y-linked allele

In the human there is an X-linked gene affecting steroid sulphatase (STS) activity which, when deficient, is associated with X-linked congenital ichthyosis. The gene (STS) is located on the distal tip of the short arm and is only partially inactivated when it is on the inactive X-chromosome. In the mouse, the genetics of STS are not clear; the results of one study using XX:X0 oocyte comparisons indicated X-linkage, but three other studies using STS variants have produced segregation data compatible with autosomal linkage of murine STS. Here we present the results of STS assays of crosses of deficient C3H/An male mice to normal X0 animals which demonstrate X-linkage of STS in the mouse and indirectly indicate the existence of a functional STS allele on the Y-chromosome which undergoes obligatory recombination during meiosis with the X-linked allele.

Hormonal induction of steroid sulphatase in the mouse

By comparing steroid sulphatase levels per se, and also ratios to alpha-galactosidase, in 6 sets of mice - normal females, entire and castrated males both with and without exogenous testosterone administration - we obtained support for the contention that induction of this enzyme is in part controlled by male hormones.

Serologically H-Y antigen-negative XO mice

In a series of six independent experiments organ homogenates of 35 mice of the XX, XO or XY sex chromosome constitutions were absorbed using three different anti-H-Y antisera raised in inbred female LEW rats. Residual activities of absorbed antisera were tested in the Raji cell, complement-dependent, cytotoxicity test. Homogenates of various tissues, including the gonads, of XX and XO females were equally unable to absorb H-Y antibodies, indicating that tissues of these mice do not carry the H-Y antigen. In contrast, XY male homogenates fully absorbed H-Y antibodies of antisera at concentrations of 1/2 to 1/4. We discuss our findings with special attention to the problem of the existence of one or more H-Y antigens and, to the genetic regulation of the expression of this antigen.

Chromosome maps of man and mouse II

Chromosome displays and listings are presented showing loci whose position is known in both man and mouse, in similar manner to our previous report (Dalton et al. 1981). There is now evidence for at least 27 conserved autosomal segments with two or more loci in the two species. The human and mouse chromosome maps show the location of homologous genes. The mouse map also shows the positions of translocations used in gene location and of some other genes used in linkage studies on them.

A DNA fragment from the human X chromosome short arm which detects a partially homologous sequence on the Y chromosomes long arm

An X linked human DNA fragment (named DXS31 ) which detects partially homologous sequences on the Y chromosome has been isolated. Regional localisation of the two sex linked sequences was determined using a panel of rodent-human somatic cell hybrids. The X specific sequence is located at the tip of the short arm ( Xp22 .3-pter), i.e. within or close to the region which pairs with the Y chromosome short arm at meiosis. However the Y specific sequence is located in the heterochromatic region of the long arm ( Yq11 -qter) and lies outside from the pairing region. DNAs from several XX male subjects were probed with DXS31 and in all cases a double dose of the X linked fragment was found, and the Y specific fragment was absent. DXS31 detects in chimpanzee a male-female differential pattern identical to that found in man. However results obtained in a more distantly related species, the brown lemur, suggest that the sequences detected by DXS31 in this species might be autosomally coded. The features observed with these X-Y related sequences do not fit with that expected from current hypotheses of homology between the pairing regions of the two sex chromosomes, nor with the pattern observed with other X-Y homologous sequences recently characterized. Our results suggest also that the rule of conservation of X linkage in mammals might not apply to sequences present on the tip of the X chromosome short arm, in bearing with the controversial issue of steroid sulfatase localisation in mouse.

A sensitive and dependable assay for distinguishing hamster and human X-linked steroid sulfatase activity in somatic cell hybrids

A radioisotopic assay is described to distinguish between Chinese hamster and human steroid sulfatase activity in extracts prepared from hamster X human somatic cell hybrids. This assay is based on different pH optima and provides a sensitive and unambiguous biochemical marker for the short arm of the human X chromosome and as well as for otherwise genetically inactivated X chromosomes in rodent X human hybrids.

Developmental aspects of X chromosome inactivation in eutherian and metatherian mammals

The single active X principle has served for two decades as a focal point for research on the cyclic activation and inactivation of gene loci. Differences in X chromosome inactivation patterns of eutherian and marsupial mammals provide probes for investigating the mechanisms of the X inactivation process. In eutherian mammals, the X chromosome is inactivated early in meiotic prophase in males and remains inactive throughout the rest of spermatogenesis. During meiosis in females, the inactive X chromosome is activated so that both X chromosomes are active in oocytes. During the early cleavage divisions of female embryos, the paternally derived X is activated. It and the maternally derived X remain active until differentiation begins in early embryogenesis. At that time, the paternally derived X is inactivated in cells that give rise to extraembryonic membranes, whereas a random process determines which X chromosome is inactivated in cells that give rise to the embryo itself. Although less is known about developmental aspects of X inactivation in female marsupials, it is clear that the paternal X is preferentially inactive in postembryonic somatic cells. Furthermore, the paternal X is partially active at some loci in some cell types, indicating that it is not regulated as a single unit. The successful adaptation of a small (80-150 g), fecund marsupial to simple laboratory conditions now enables extensive experimentation on the large number of marsupials at various developmental stages. This capability, coupled with the application of newly developed cellular and molecular techniques to questions about X chromosome inactivation, shows great promise for advancing our understanding of the mechanisms that control the cyclic behavior of X chromosome activity.

Genetic analysis of murine arylsulfatase C and steroid sulfatase

SWR/J mice possess two- to threefold higher 4-methylumbelliferyl sulfate (4MUS), dehydroepiandrosterone sulfate (DHEAS) and estrone sulfate (E1S) sulfatase activities in liver and kidney extracts than do A/J mice. These interstrain activity differences are maintained throughout the 6- to 45-day postnatal period. Characteristics of the hepatic activities of SWR/J mice suggest that all three activities reside in the same enzyme. Biochemical properties of the SWR/J and A/J enzyme were not significantly different. Expression of hepatic enzyme activity is subject to regulation by an autosomal locus possessing two alleles with additive effects. Postnuclear E1S- and DHEAS-sulfatase activities are primarily microsomal. Although postnuclear hepatic 4MUS-sulfatase activity is predominantly microsomal, renal activity is primarily nonmicrosomal. Only that portion of 4MUS-sulfatase occurring in cell membranes appears capable of hydrolyzing E1S and DHEAS. The hepatic- and renal-specific subcellular distributions of 4MUS-sulfatase activity may reflect tissue differences in enzyme processing. Renal 4MUS-sulfatase activity is also controlled by an autosomal gene with two alleles having additive effects. Positive correlation between hepatic and renal 4MUS-sulfatase activities indicates that both activities are most likely influenced by the same gene.

Murine arylsulfatase C: evidence for two isozymes

SWR/J mice possess high arylsulfatase C, estrone sulfatase, and dehydroepiandrosterone sulfatase activities in liver, spleen, and kidney compared to A/J mice. This interstrain activity variation appears to be determined by at least 1 autosomal gene. Murine arylsulfatase C activity occurs in both hydrophobic and hydrophilic forms which differ with respect to certain biochemical properties and exhibit different subcellular distributions. The hydrophilic isozyme is a major component in kidney and brain extracts and a minor isozyme in liver and spleen extracts. The hydrophobic arylsulfatase C isozyme appears to be identical to steroid sulfatase. The hydrophilic arylsulfatase C isozyme does not possess steroid sulfatase activity; however, hydrophilic and hydrophobic arylsulfatase C share certain properties, suggesting that they may be structurally related. The autosomal gene(s) affects both arylsulfatase isozymes.

Variation in regulation of steroid sulphatase locus in mammals

Inactivation (lyonization) of one of the two copies of X-linked genes occurs in female mammals, thereby reducing the number of active copies to that of the male. It has been suggested that genes subject to lyonization would be expected to be preserved as a linkage group during mammalian evolution. A short region of the human X chromosome containing several genes, including that necessary for the expression of steroid sulphatase (STS), is exceptional in that it apparently escapes X-inactivation. As it is not apparent why the linkage of genes not subject to X-inactivation should be conserved, we have examined the expression of the STS gene in mice (it has been shown recently that this gene is X-linked). Enzyme levels were determined in normal males and females and in the progeny of crosses in which the sex reversing factor, Sxr, was segregating to produce XX males. We report here that in contrast to the situation in humans, the STS gene in mice is subject to the normal pattern of X-inactivation.