Mapping and expression of the ubiquitin-activating enzyme E1 (Ube1) gene in the mouse

Inactive X chromosome-specific histone H3 modifications and CpG hypomethylation flank a chromatin boundary between an X-inactivated and an escape gene

In mammals, the dosage compensation of sex chromosomes between males and females is achieved by transcriptional inactivation of one of the two X chromosomes in females. However, a number of genes escape X-inactivation in humans. It remains poorly understood how the transcriptional activity of these ‘escape genes’ is maintained despite the chromosome-wide heterochromatin formation. To address this question, we analyzed a putative chromatin boundary between the inactivated RBM10 and an escape gene, UBA1/UBE1. Chromatin immunoprecipitation revealed that trimethylated histone H3 lysine 9 and H4 lysine 20 were enriched in the last exon through the proximal downstream region of RBM10, but were remarkably diminished at ∼2 kb upstream of the UBA1 transcription start site. Whereas RNA polymerase II was not loaded onto the intergenic region, CTCF (CCCTC binding factor) was enriched around the boundary, where some CpG sites were hypomethylated specifically on inactive X. These findings suggest that local DNA hypomethylation and CTCF binding are involved in the formation of a chromatin boundary, which protects the UBA1 escape gene against the chromosome-wide transcriptional silencing.

An evolutionary rearrangement of the Xp11.3-11.23 region in 3p21.3, a region frequently deleted in a variety of cancers

In searching for a tumor suppressor gene in the 3p21.3 region, we isolated two genes, RBM5 and RBM6. Sequence analysis indicated that these genes share similarity. RBM5 and-to a lesser extent-RBM6 also have similarity to DXS8237E at Xp11.3-11.23, which maps less than 20 kb upstream of UBE1. A homologue of UBE1, UBE1L, is located at 3p21. 3. FISH analysis showed that the distance between UBE1L and RBM5 in 3p21.3 is about 265 kb. DXS8237E and UBE1 on the X chromosome have the same orientation, whereas on chromosome 3 the orientation of RBM5 and that of RBM6 are opposite to the orientation of UBE1L. Presumably, part of the Xp11.3-11.23 region has duplicated to chromosome 3. Part of this region on chromosome 3 may subsequently have duplicated again within the same chromosomal region. Inversion at some stage of the evolution of the human genome would explain the change in orientation of the genes on chromosome 3 compared with that of the genes on the X chromosome.

Characterisation of the coding sequence and fine mapping of the human DFFRY gene and comparative expression analysis and mapping to the Sxrb interval of the mouse Y chromosome of the Dffry gene

DFFRY (the Y-linked homologue of the DFFRX Drosophila fat-facets related X gene) maps to proximal Yq11.2 within the interval defining the AZFa spermatogenic phenotype. The complete coding region of DFFRY has been sequenced and shows 89% identity to the X-linked gene at the nucleotide level. In common with DFFRX , the potential amino acid sequence contains the conserved Cys and His domains characteristic of ubiquitin C-terminal hydrolases. The human DFFRY mRNA is expressed in a wide range of adult and embryonic tissues, including testis, whereas the homologous mouse Dffry gene is expressed specifically in the testis. Analysis of three azoospermic male patients has shown that DFFRY is deleted from the Y chromosome in these individuals. Two patients have a testicular phenotype which resembles Sertoli cell-only syndrome, and the third diminished spermatogenesis. In all three patients, the deletions extend from close to the 3' end into the gene, removing the entire coding sequence of DFFRY. The mouse Dffry gene maps to the Sxrb deletion interval on the short arm of the mouse Y chromosome and its expression in mouse testis can first be detected between 7.5 and 10.5 days after birth when type A and B spermatogonia and pre-leptotene and leptotene spermatocytes are present.

Embryonic growth and the evolution of the mammalian Y chromosome. II. Suppression of selfish Y-linked growth factors may explain escape from X-inactivation and rapid evolution of Sry

The mammalian Y chromosome may be an attractor for selfish growth factors. A suppressor of the selfish growth effects would be expected to spread were it to have an appropriate parent-specific expression rule. A suppressor could act by boosting the resource demands of competing female embryos. This possibility may explain incidences of the escape from X-inactivation and provides a rationale for why these genes typically have Y-linked homologues. Alternatively, a suppressor could act to decrease the resource demands of males with the selfish Y. This possibility is supported by the finding that the size of male, but not female, human infants is negatively correlated to the number of X chromosomes. A protracted arms race between a selfish gene and its suppressor may ensue. Both the variation in copy number of Zfy and the unusually fast sequence evolution of Sry may be explained by such an arms race. As required by the model, human Sry is known to have an X-linked suppressor. Preliminary evidence suggests that, as predicted, rapid sequence evolution of Sry may be correlated with female promiscuity. The case for fast sequence evolution as the product of maternal/foetal conflict is strengthened by consideration of the rapid evolution of placental lactogens in both ruminants and rodents.

The human pseudoautosomal GM-CSF receptor alpha subunit gene is autosomal in mouse

The gene encoding the granulocyte macrophage colony stimulating factor receptor alpha subunit (CSF2RA) has previously been mapped to the pseudoautosomal region of the human sex chromosomes. In contrast, we report that the murine locus, Csf2ra, maps to an autosome in the laboratory mouse. By in situ hybridization and genetic mapping, Csf2ra maps at telomeric band D2 of mouse chromosome 19. This first instance of a pseudoautosomal locus in human being autosomal in mouse, indicates incomplete conservation between the human and mouse X chromosomes and suggests that the genetic content of the pseudoautosomal region may differ between species of eutherian mammals due to chromosomal rearrangements.

Maintenance of X inactivation of the Rps4, Zfx, and Ube1 genes in a mouse in vitro system

Several genes, including RPS4X (ribosomal protein subunit 4), ZFX (zinc finger on the X chromosome), and UBE1 (ubiquitin-activating enzyme), have been shown to be expressed from the inactive X chromosome of cultured human cells. By contrast, these genes are subject to X-chromosome inactivation in tissues from adult mice. We have now examined the inactivation status of these genes in cultured mouse cells to determine whether the differences in X-chromosome inactivation between species is due to an intrinsic difference between human and mouse X-chromosome genes or whether it is a function of gene reactivation in cell culture per se. The expression of three mouse X-chromosome genes, Rps4, Zfx, and Ube1 was examined by reverse transcriptase polymerase chain reaction (RT-PCR) in heterozygous cultured cells from a cross of a laboratory mouse by Mus spretus, which were selected to uniformly express the X chromosome from the laboratory mouse parent. No expression of the M. spretus alleles of these genes was observed in the cell line (Hobmski), which is consistent with the patterns of expression previously observed in mouse in vivo and indicates that these genes remain stably inactivated in an immortalized mouse cell line. By cytogenetic and RT-PCR analyses the Hobmski cell line was shown to retain a late-replicating X chromosome from M. spretus, which expressed the M. spretus allele of the X (inactive) specific transcript (Xist). The Hobmski cell line will be a useful resource for studying the features that maintain X-chromosome genes inactive.