Missing X chromosome and ring chromosome 21 in a case of acute myelomonocytic leukemia.

The karyotypic picture of a female patient with acute myelomonocytic leukemia (A.M.M.L.) consisted in the loss of a sex chromosome and a ring chromosome 21. It is suggested that in A.M.M.L. the loss of sex chromosome may represent an early event, the monosomic cells being the object of further chromosome rearrangements, which involve more frequently a chromosome 21.     

Microdissection and microcloning of the mouse X chromosome.

A wild mouse (CD) karyotype in which all the chromosomes bar the X, 19, and Y, are fused as metacentrics has been used for the microdissection and microcloning of a specific mouse X chromosome region. Dissection of a proximal region of the X chromosome encompassing the genetic loci Hprt to Tfm and including mdx has yielded 650 clones. A number of the recovered clones containing sizable inserts have been confirmed as X chromosome specific. This X chromosome bank of clones provides a start point for the isolation of the mdx locus. It is now clear that microdissection and microcloning can be applied to all mouse chromosomes, including the X chromosome, yielding premapped banks of clones that will greatly aid in the isolation and characterization of important genetic loci.     

Nonrandom X chromosome inactivation in B cells from carriers of X chromosome-linked severe combined immunodeficiency.

X chromosome-linked severe combined immunodeficiency (XSCID) is characterized by markedly reduced numbers of T cells, the absence of proliferative responses to mitogens, and hypogammaglobulinemia but normal or elevated numbers of B cells. To determine if the failure of the B cells to produce immunoglobulin might be due to expression of the XSCID gene defect in B-lineage cells as well as T cells, we analyzed patterns of X chromosome inactivation in B cells from nine obligate carriers of this disorder. A series of somatic cell hybrids that selectively retained the active X chromosome was produced from Epstein-Barr virus-stimulated B cells from each woman. To distinguish between the two X chromosomes, the hybrids from each woman were analyzed using an X-linked restriction fragment length polymorphism for which the woman in question was heterozygous. In all obligate carriers of XSCID, the B-cell hybrids demonstrated preferential use of a single X chromosome, the nonmutant X, as the active X. To determine if the small number of B-cell hybrids that contained the mutant X were derived from an immature subset of B cells, lymphocytes from three carriers were separated into surface IgM positive and surface IgM negative B cells prior to exposure to Epstein-Barr virus and production of B-cell hybrids. The results demonstrated normal random X chromosome inactivation in B-cell hybrids derived from the less mature surface IgM positive B cells. In contrast, the pattern of X chromosome inactivation in the surface IgM negative B cells, which had undergone further replication and differentiation, was significantly nonrandom in all three experiments [logarithm of odds (lod) score greater than 3.0]. These results suggest that the XSCID gene product has a direct effect on B cells as well as T cells and is required during B-cell maturation.     

Cytogenetic and molecular studies on a recombinant human X chromosome: implications for the spreading of X chromosome inactivation.

A pericentric inversion of a human X chromosome and a recombinant X chromosome [rec(X)] derived from crossing-over within the inversion was identified in a family. The rec(X) had a duplication of the segment Xq26.3----Xqter and a deletion of Xp22.3----Xpter and was interpreted to be Xqter----Xq26.3::Xp22.3----Xqter. To characterize the rec(X) chromosome, dosage blots were done on genomic DNA from carriers of this rearranged X chromosome using a number of X chromosome probes. Results showed that anonymous sequences from the distal end of the long arm to which probes 4D8, Hx120A, DX13, and St14 bind as well as the locus for glucose-6-phosphate dehydrogenase (G6PD) were duplicated on the rec(X). Mouse-human cell hybrids were constructed that retained the rec(X) in the active or inactive state. Analyses of these hybrid clones for markers from the distal short arm of the X chromosome showed that the rec(X) retained the loci for steroid sulfatase (STS) and the cell surface antigen 12E7 (MIC2); but not the pseudoautosomal sequence 113D. These molecular studies confirm that the rec(X) is a duplication-deficiency chromosome as expected. In the inactive state in cell hybrids, STS and MIC2 (which usually escape X chromosome inactivation) were expressed from the rec(X), whereas G6PD was not. Therefore, in the rec(X) X chromosome inactivation has spread through STS and MIC2 leaving these loci unaffected and has inactivated G6PD in the absence of an inactivation center in the q26.3----qter region of the human X chromosome. The mechanism of spreading of inactivation appears to operate in a sequence-specific fashion. Alternatively, STS and MIC2 may have undergone inactivation initially but could not be maintained in an inactive state.     

The X chromosome of monotremes shares a highly conserved region with the eutherian and marsupial X chromosomes despite the absence of X chromosome inactivation.

Eight genes, located on the long arm of the human X chromosome and present on the marsupial X chromosome, were mapped by in situ hybridization to the chromosomes of the platypus Ornithorhynchus anatinus, one of the three species of monotreme mammals. All were located on the X chromosome. We conclude that the long arm of the human X chromosome represents a highly conserved region that formed part of the X chromosome in a mammalian ancestor at least 150 million years ago. Since three of these genes are located on the long arm of the platypus X chromosome, which is G-band homologous to the Y chromosome and apparently exempt from X chromosome inactivation, the conservation of this region has evidently not depended on isolation by X-Y chromosome differentiation and X chromosome inactivation.     

Detailed ordering of markers localizing to the Xq26-Xqter region of the human X chromosome by the use of an interspecific Mus spretus mouse cross.

Five probes localizing to the Xq26-Xqter region of the human X chromosome have been genetically mapped on the mouse X chromosome using an interspecific cross involving Mus spretus to a contiguous region lying proximally to the Tabby (Ta) locus. Pedigree and recombinational analysis establish the marker order as being Hprt-FIX-c11-G6PD-St14-1. The size of this contiguous region is such that the X-linked muscular dystrophy (mdx) mouse mutation probably maps within this segment. This in turn suggests that it is highly improbable that the mouse mdx locus represents a model for Duchenne muscular dystrophy (DMD). It is, however, compatible with the idea that this mutation may correspond in man to Emery Dreifuss muscular dystrophy. The high frequency of restriction fragment length polymorphisms found in this interspecific system for all the human cross-reacting probes examined up until now, using only a limited number of restriction enzymes, suggests that the Mus spretus mapping system may be of great potential value for establishing the linkage relationships existing in man when conserved chromosomal regions are concerned and human/mouse cross-reacting probes are available or can be obtained.     

Incomplete X chromosome dosage compensation in chorionic villi of human placenta.

Studies of glucose-6-phosphate dehydrogenase (G6PD) in heterozygous cells from chorionic villi of five fetal and one newborn placenta show that the locus on the allocyclic X is expressed in many cells of this trophectoderm derivative. Heterodimers were present in clonal populations of cells with normal diploid karyotype and a late replicating X chromosome. The expression of the two X chromosomes was unequal, based on ratios of homodimers and heterodimers in clones. Studies of DNA, digested with Hpa II and probed with cloned genomic G6PD sequences, indicate that expression of the locus in chorionic villi is associated with hypomethylation of 3' CpG clusters. These findings suggest that dosage compensation, at least at the G6PD locus, has not been well established or maintained (or both) in placental tissue. Furthermore, the active X chromosome in these human cells of trophoblastic origin can be either the paternal or maternal one; therefore, paternal X inactivation in extraembryonic lineages is not an essential feature of mammalian X dosage compensation.     

Evidence of evolutionary up-regulation of the single active X chromosome in mammals based on Clc4 expression levels in Mus spretus and Mus musculus

Previous studies have shown that the chloride channel gene Clc4 is X-linked and subject to X inactivation in Mus spretus, but that the same gene is autosomal in laboratory strains of mice. This exception to the conservation of linkage of the X chromosome in one of two interfertile mouse species was exploited to compare expression of Clc4 from the X chromosome to that from the autosome. Clc4 was found to be highly expressed in brain tissues of both mouse species. Quantitative analyses of species-specific expression of Clc4 in brain tissues from mice resulting from M. spretus × laboratory strain crosses, demonstrate that each autosomal locus has half the level of Clc4 expression as compared with the single active X-linked locus. In contrast expression of another chloride channel gene, Clc3, which is autosomal in both mouse species is equal between alleles in F₁ animals. There is no evidence of imprinting of the Clc4 autosomal locus. These results are consistent with Ohno’s hypothesis of an evolutionary requirement for a higher expression of genes on the single active X chromosome to maintain balance with autosomal gene expression [Ohno, S. (1967) Sex Chromosomes and Sex-Linked Genes (Springer, Berlin)].     

The X chromosome and systemic sclerosis

PURPOSE OF REVIEW: Similar to the majority of autoimmune rheumatic diseases, systemic sclerosis is characterized by a striking female predominance superimposed on a predisposing genetic background. At least two genetic mechanisms have been proposed that play a role in susceptibility to systemic sclerosis; firstly the maintenance of immune tolerance via genes on the X chromosomes and, secondly, fetal microchimerism. Based on these lines of evidence, experimental efforts have been most recently dedicated to investigating the role of X chromosome abnormalities (i.e. monosomy rates and inactivation patterns) in autoimmunity. We will review herein the most recent data on the role of the X chromosome in systemic sclerosis onset and discuss the potential implications. RECENT FINDINGS: Women with systemic sclerosis manifest an enhanced rate of X monosomic cells in peripheral blood compared with healthy age-matched women. Furthermore, a severely skewed X chromosome inactivation pattern is found in women with systemic sclerosis. SUMMARY: These observations, reproduced in other female-predominant autoimmune diseases, strongly support the role of the X chromosome in conferring susceptibility to tolerance breakdown and open novel scenarios to emphasize the unknown etiopathogenesis of systemic sclerosis. The implications of these findings will be discussed.     

X chromosome inactivation and autoimmune diseases

The pathogenesis of autoimmune diseases (AIDs) is characterized by a female preponderance. The causes for this sex imbalance are based on several hypotheses. One of the most intriguing hypotheses is related to an X chromosome inactivation (XCI) process. Females are mosaics for two cell populations, one with the maternal and one with the paternal X as the active chromosome. Skewed XCI is often defined as a pattern where 80% or more of the cells show a preferential inactivation of one X chromosome. The role of skewed XCI has been questioned in the pathogenesis of several AIDs, such as autoimmune thyroid diseases and rheumatoid arthritis.     

Mammalian X chromosome inactivation evolved as a dosage-compensation mechanism for dosage-sensitive genes on the X chromosome

How and why female somatic X-chromosome inactivation (XCI) evolved in mammals remains poorly understood. It has been proposed that XCI is a dosage-compensation mechanism that evolved to equalize expression levels of X-linked genes in females (2X) and males (1X), with a prior twofold increase in expression of X-linked genes in both sexes ("Ohno's hypothesis"). Whereas the parity of X chromosome expression between the sexes has been clearly demonstrated, tests for the doubling of expression levels globally along the X chromosome have returned contradictory results. However, changes in gene dosage during sex-chromosome evolution are not expected to impact on all genes equally, and should have greater consequences for dosage-sensitive genes. We show that, for genes encoding components of large protein complexes (≥ 7 members)--a class of genes that is expected to be dosage-sensitive--expression of X-linked genes is similar to that of autosomal genes within the complex. These data support Ohno's hypothesis that XCI acts as a dosage-compensation mechanism, and allow us to refine Ohno's model of XCI evolution. We also explore the contribution of dosage-sensitive genes to X aneuploidy phenotypes in humans, such as Turner (X0) and Klinefelter (XXY) syndromes. X aneuploidy in humans is common and is known to have mild effects because most of the supernumerary X genes are inactivated and not affected by aneuploidy. Only genes escaping XCI experience dosage changes in X-aneuploidy patients. We combined data on dosage sensitivity and XCI to compute a list of candidate genes for X-aneuploidy syndromes.