Identification of TSIX, encoding an RNA antisense to human XIST, reveals differences from its murine counterpart: implications for X inactivation

Abstract

X inactivation is the mammalian method for X-chromosome dosage compensation, but some features of this developmental process vary among mammals. Such species variations provide insights into the essential components of the pathway. Tsix encodes a transcript antisense to the murine Xist transcript and is expressed in the mouse embryo only during the initial stages of X inactivation; it has been shown to play a role in imprinted X inactivation in the mouse placenta. We have identified its counterpart within the human X inactivation center (XIC). Human TSIX produces a >30-kb transcript that is expressed only in cells of fetal origin; it is expressed from human XIC transgenes in mouse embryonic stem cells and from human embryoid-body-derived cells, but not from human adult somatic cells. Differences in the structure of human and murine genes indicate that human TSIX was truncated during evolution. These differences could explain the fact that X inactivation is not imprinted in human placenta, and they raise questions about the role of TSIX in random X inactivation.

Figure 1

PipMaker percent identity plot of the human XIC region in ES-10, showing the location of a novel transcript and homology to the comparable region in the murine genome. Nucleotides 0–80000 of the U80460 sequence (0–80 kb) are shown. Plotting of XIST exons (orange) is based on all known transcribed sequences (Brown et al. ; Hong et al. 2000). The black dot pattern shows the percent homology (50%–100%) with the comparable mouse Xic region. The XIST transcript is shown as a solid line with an arrow. The TSIX transcript (dotted line with arrow) is antisense to XIST and is initiated from four clustered transcription start sites, the most 5′ of which shown (see fig. 3). The purple vertical bar in XIST exon 1 represents the single CpG island identified by the Grail program (version 1.3). The small blue boxes indicate a CpG:GpC frequency of ⩾.6; the one near the TSIX start site is marked by a blue arrow. The locations of RT-PCR primers (a–cc) are shown in lowercase letters. The lack of homology outside of the XIST region has also been documented in a PipMaker plot by Nesterova et al. (, fig. 5e). MIR = mammalianwide interspersed repeats; SINE = short interspersed elements.

Figure 2

Strand-specific RT-PCR showing polyadenylation and orientation of transcripts. Top, Sense (XIST) transcript (cDNA synthesized using reverse primer). Bottom, Antisense (TSIX) transcript (cDNA synthesized using forward primer). RNA samples were total RNA, unless otherwise indicated. Lane 1, 100-bp marker; lanes 2 and 3, male; lanes 4 and 5, female; lanes 6–9, ES-10; lanes 10 and 11, human EBD LV cells; lanes 12 and 13, J1, mouse ES cell control. Samples were assayed using primer set g in the presence (+) or absence (−) of reverse transcriptase (RT). Note that ES-10 and LV cells have both sense and antisense transcripts with XIST exon 6 primers.

Figure 3

Identification of TSIX transcription start sites in ES-10 cells. Transcription start sites (arrows) were determined by 5′ RACE using ES-10 as template. The TSIX transcript is initiated from four clustered transcription start sites (bases 77657, 77865, 78267, and 78860 in the U80460 sequence). The location of repetitive elements are indicated as follows: black vertical lines denote Alu elements, gray boxes denote LTRs, diagonal lines denote LINE elements, and horizontal lines denote MER1 elements.

Figure 4

Dot plot showing the evolutionary breakpoint within the human XIC. The region shown is an extension of that in figure 1 and includes the region from 0–136 kb of U80460. The dark gray bars represent the XIST exons; the light bar shows the CpG island in XIST exon 1 (asterisk). The effective evolutionary breakpoint is shown as occurring at the start of the TSIX transcript; the actual breakpoint is difficult to determine. The areas circled and numbered 1–3 show the only conserved regions outside of the XIST gene. Region 1 is homologous to murine Tsix, and regions 2 and 3 are homologous to the murine testis gene (Tsx) exons 5 and 4, respectively, which are separated in the human genome by ∼40 kb. This figure, a PipMaker dot plot comparing human and mouse XIC sequences, is similar to the traditional dot plot of this region, shown by Lee et al. (, fig. 6); however, we now know that regions 2 and 3 are homologous to TSX and not TSIX.