Dosage compensation

About: Dosage compensation is a research topic. Over the lifetime, 1920 publications have been published within this topic receiving 124589 citations.

XOL-1, primary determinant of sexual fate in C. elegans, is a GHMP kinase family member and a structural prototype for a class of developmental regulators

Abstract: Sex determination is the critical and universal developmental pathway underlying sexual reproduction. Its manifestations are pervasive and often conspicuous. Whereas the presence or absence of the Y chromosome dictates male or female development in mammals, sexual fate in the fruit fly Drosophila melanogaster and the free-living nematode Caenorhabditis elegans is determined genetically by the number of X chromosomes relative to the number of sets of autosomes. In mammals, the primary sex determining gene is SRY, which is present only on the Y chromosome and encodes an HMG domain-containing transcription factor. In the fruit fly, the primary sex determination gene Sex-lethal (Sxl; Maine et al. 1985) is a female-specific trans-acting gene regulator that binds tra transcripts and directs alternative splicing (Inoue et al. 1990). The SRY (Werner et al. 1995) and SXL (Handa et al. 1999) interactions with polynucleotides have been characterized structurally. In C. elegans, sexual differentiation is regulated by the expression levels of the developmental switch gene xol-1. High and low levels of xol-1 result in male (XO) and hermaphrodite (XX) development (Fig. ​(Fig.1),1), respectively. XOL-1 activity is absolutely required for proper sexual differentiation and male viability (Rhind et al. 1995), but its mechanism of action is unknown. Figure 1 Genetic control of sex determination and dosage compensation in C. elegans. xol-1 is the primary sex-determination switch gene and the direct molecular target of the X-chromosome counting mechanism. (Top) Male (XO). High xol-1 activity specifies male ... The cooperative activity of at least four X-linked genes, termed X-signal elements, represses expression of xol-1 (for review, see Meyer 2000a). By doubling the number of X-signal elements, an XX embryo reduces xol-1 expression by ∼10-fold (Rhind et al. 1995), facilitating hermaphrodite development. Two C. elegans X-signal elements have been characterized molecularly as follows: FOX-1 (Hodgkin et al. 1994; Nicoll et al. 1997; Skipper et al. 1999), an RNA-binding protein that may regulate alternate splicing of xol-1 RNA, and SEX-1 (Carmi et al. 1998), a nuclear receptor and likely a transcription factor. Although hermaphrodites and males differ in X chromosome number, the expression of most X-linked genes must be equal to ensure viability. This is accomplished through dosage compensation, which reduces expression of X-linked genes in hermaphrodites to male levels (for reviews, see Wood et al. 1997; Hansen and Pilgrim 1999; Meyer 2000b; Boag et al. 2001). High levels of XOL-1 in males correlate with low SDC-2 expression, preventing dosage compensation (Miller et al. 1988; Rhind et al. 1995). Conversely, low levels of XOL-1 in hermaphrodites correlate with high SDC-2 expression and the assembly on the X chromosome of the dosage compensation complex, which is composed of sdc, dpy, and mix-1 gene products (Nonet and Meyer 1991; Chuang et al. 1996; Lieb et al. 1996, 1998; Davis and Meyer 1997; Dawes et al. 1999; Chu et al. 2002). Other genes downstream of xol-1, such as her, tra, and fem, whose activities are inversely related in hermaphrodites and males, coordinate sexual differentiation (Goodwin and Ellis 2002). Null mutants of xol-1 are XO-lethal, inappropriately activating dosage compensation where only one X chromosome is present, whereas XOL-1 overexpression is XX-lethal, deactivating the dosage compensation pathway and elevating the expression of X chromosome genes to lethal levels in hermaphrodites (Rhind et al. 1995). XOL-1 is an acidic 51-kD nuclear protein (pKa 4.6), whose transcript is expressed at high levels only in pre-comma stage XO embryos (Rhind et al. 1995). XOL-1 transcripts are present in low levels throughout other larval stages in XO animals, but are nearly undetectable in XX larvae and adults of both sexes (Rhind et al. 1995). Currently, XOL-1 is annotated as a subtilisin-like protease on the basis of primary sequence (http://www.wormbase.org). BLAST searches of Genbank failed to identify any homologs that may have provided additional clues as to the function of XOL-1. Thus, we used x-ray crystallography to investigate the function of XOL-1, hypothesizing that the three-dimensional structure of the protein would yield insights into its nature, largely uncharacterized biochemically. The resulting crystal structure of XOL-1 (Fig. ​(Fig.2A)2A) unambiguously and unexpectedly defines XOL-1 as a member of the GHMP kinase family, a family of proteins known to be involved in small molecule metabolism, but not known to participate directly in sexual differentiation or dosage compensation. Figure 2 Comparison of XOL-1 and GHMP kinase structures. (A) The structure of XOL-1. Ribbon diagram of XOL-1 (PDB ID: 1MG7). Domain 1 consists of β-strands 2–7 and 12 (cyan) and α-helices 1–5 (yellow). Domain 2 consists of β-strands ... Galactokinase, homoserine kinase, mevalonate kinase, and phosphomevalonate kinase were originally identified as prototypic members of the GHMP kinase family and observed to contain a conserved Pro–Xaa3–Gly–Leu–Gly–Ser–Ser–Ala–Ala motif (Fig, 3A) that was hypothesized (Tsay and Robinson 1991; Bork et al. 1993), and later proved (Zhou et al. 2000; Krishna et al. 2001; Fu et al. 2002) to be involved in ATP binding. The nucleotide fold of GHMP kinases is distinct from those of other kinases, that is, P-loop, protein, and Hsp70-like kinases (Zhou et al. 2000; Bonanno et al. 2001; Fu et al. 2002; Romanowski et al. 2002; Yang et al. 2002), and binds ATP in both syn and anti-conformations (Zhou et al. 2000; Fu et al. 2002). GHMP kinases are found in bacteria, archaea, and eukaryotes and contain an unusual left-handed β–α–β fold similar to that observed in domain IV of elongation factor G (Zhou et al. 2000). In humans, deficiency of galactokinase, which participates in the conversion of galactose to glucose, contributes to cataract formation (Monteleone et al. 1971; Beutler 1972; Harley et al. 1972; Levy et al. 1972). Mutations in mevalonate kinase, an enzyme involved in the synthesis of sterols from acetate, are associated with mevalonic aciduria (Hoffmann et al. 1986; Schafer et al. 1992; Houten et al. 1999b) and hyperimmunoglobulinemia D/periodic fever syndrome (Drenth et al. 1999; Houten et al. 1999a, 2001; Cuisset et al. 2001; Rios et al. 2001; Simon et al. 2001). To date, no GHMP kinases have been shown to function in developmental pathways unrelated to metabolism.

Expression of an X-linked HMG-lacZ transgene in mouse embryos: implication of chromosomal imprinting and lineage-specific X-chromosome activity.

Abstract: X-chromosome activity in female mouse embryos was studied at the cellular level using an X-linked lacZ transgene which encodes beta-galactosidase (beta-Gal). Translation of maternal RNA in oocytes is seen as beta-Gal activity that persists into early cleavage-stages. Zygotic transcription of the transgene from the maternal X chromosome (Xm) is first found at about the 8-cell stage. By contrast, expression of the lacZ transgene on the paternal X chromosome (Xp) is not seen until later at the 16-32-cell stage. Preferential inactivation of Xp occurs in the mural trophectoderm, the primitive endoderm, and derivatives of the polar trophectoderm, but a small number of cells in these lineages may still retain an active paternal X chromosome. X inactivation begins at 3.5 days in the inner cell mass but contrary to previous findings the process is not completed in the embryonic ectoderm by 5.5 to 6.0 days. Regional variation in beta-Gal activity is also observed in the embryonic ectoderm during gastrulation which may be related to the specification of cell fates. Random inactivation of Xp and Xm ensues in all somatic tissues but the process is completed at different times in different tissues. The slower progression of X inactivation in tissues such as the notochord, the heart, and the embryonic gut is primarily due to the persistent maintenance of two active X chromosomes in a significant fraction of cells in these tissues. Recent findings on the methylation of endogenous X-linked genes suggest that the prolonged expression of beta-Gal might also be due to the different rate of spreading of inactivation along the X chromosome to the lacZ transgene locus in different tissues.

X chromosome inactivation, differentiation, and DNA methylation revisited, with a tribute to Susumu Ohno.

Abstract: X chromosome inactivation and DNA methylation are reviewed, with emphasis on the contributions of Susumu Ohno and the predictions made in my 1975 paper (Riggs, 1975), in which I proposed the “maintena

Is human X chromosome inactivation a sex-determining device?

Abstract: The evolutionary function of X chromosome inactivation is thought to be dosage compensation. However, there is, at present, little evidence to suggest that most X chromosome-linked genes require such compensation. Another view--that X chromosome inactivation may be related to sex determination--is examined here. Consider a hypothetical DNA sequence regulating a major structural gene concerned with the determination of maleness. If this regulatory sequence occurs in both X and Y chromosomes and if its copy number in the Y chromosome is significantly greater than in the X chromosome, then the male-determining properties of the Y chromosome could be attributed to this higher copy number. On the other hand, if the Y chromosome has the same copy number of this sequence as the X chromosome, it is difficult to see how determination of two sexes would occur under such circumstances because XX and XY genomes would then be indistinguishable in this regard. Such a situation seems to occur in the human species with respect to the banded krait minor satellite, a repetitious DNA sequence associated with sex determination. This apparent difficulty may be resolved if X chromosome inactivation renders regulatory as well as structural genes nonfunctional and thereby brings about a significant reduction in the effective copy number of X chromosome-linked DNA sequences concerned with sex determination. It is suggested that X chromosome inactivation brings about, in this manner, a critical inequality between XX and XY embryos and that sex determination in humans is a consequence of this inequality. An analogous situation appears to exist in certain insects in which inactivation of a haploid set of chromosomes (and presumably, therefore, a 50% reduction in the effective copy number of most genes) is associated with maleness. If this line of reasoning is correct, it would suggest that sex determination may be the primary function of X chromosome inactivation.

Unusual chromatin status and organization of the inactive X chromosome in murine trophoblast giant cells.

Abstract: Mammalian X-chromosome inactivation (XCI) enables dosage compensation between XX females and XY males. It is an essential process and its absence in XX individuals results in early lethality due primarily to extra-embryonic defects. This sensitivity to X-linked gene dosage in extra-embryonic tissues is difficult to reconcile with the reported tendency of escape from XCI in these tissues. The precise transcriptional status of the inactive X chromosome in different lineages has mainly been examined using transgenes or in in vitro differentiated stem cells and the degree to which endogenous X-linked genes are silenced in embryonic and extra-embryonic lineages during early postimplantation stages is unclear. Here we investigate the precise temporal and lineage-specific X-inactivation status of several genes in postimplantation mouse embryos. We find stable gene silencing in most lineages, with significant levels of escape from XCI mainly in one extra-embryonic cell type: trophoblast giant cells (TGCs). To investigate the basis of this epigenetic instability, we examined the chromatin structure and organization of the inactive X chromosome in TGCs obtained from ectoplacental cone explants. We find that the Xist RNA-coated X chromosome has a highly unusual chromatin content in TGCs, presenting both heterochromatic marks such as H3K27me3 and euchromatic marks such as histone H4 acetylation and H3K4 methylation. Strikingly, Xist RNA does not form an overt silent nuclear compartment or Cot1 hole in these cells. This unusual combination of silent and active features is likely to reflect, and might underlie, the partial activity of the X chromosome in TGCs.