The Questions of Developmental Biology

Second, knocking out one Xist locus in an XX cell prevents X inactivation from occurring on that
particular chromosome (Penny et al. 1996). Third, the transfer of a 450-kilobase segment
containing the mouse Xist gene into an autosome of male embryonic stem cells causes the random
inactivation of either that autosome or the endogenous X chromosome (Lee et al. 1996). The
autosome is "counted" as an X chromosome. Fourth, Xist appears to be involved in "choosing"
which X chromosome is inactivated. Female mice heterozygous for a deletion of a particular
region of the Xist gene will preferentially inactivate the wild-type chromosome (Marahrens et al.
1998). Xist expression is needed only for the initiation of X chromosome inactivation; once
inactivation occurs, Xist transcription is dispensible (Brown and Willard 1994).
It is still not known what Xist RNA does to inactivate the chromosome. The Xist RNA
works only in cis; that is, on the chromosome that made it. In the preimplantation mouse embryo,
both X chromosomes synthesize Xist RNA, but this RNA is quickly degraded. When the cells
begin to differentiate, the Xist RNA is stabilized on one of the two X chromosomes (Figure
5.24B-D; Sheardown et al. 1997; Panning et al. 1997). Xist RNA appears to be critical in the
counting, selection, and intrachromosomal spreading of X chromosome inactivation.
Maintaining X Chromosome Inactivation
Once Xist initiates the inactivation of an X chromosome,
the silencing of that chromosome is maintained in at least two
ways. The first way involves methylation. The Xist locus on the
active X chromosome becomes methylated, while the active Xist
gene (on the inactive X chromosome) remains unmethylated
(Norris et al. 1994). Conversely, the promoter regions of
numerous genes are methylated on the inactive X chromosome
and unmethylated on the active X chromosome (Wolf et al.
1984; Keith et al. 1986; Migeon et al. 1991).
This pattern may be reflected in the observation that the inactive X chromosomes of
humans and mice have hardly any acetylated histone 4 (Figure 5.25; Jeppesen and Turner 1993).
Conversely, the inactive X chromosome becomes associated with a higher concentration of the
histone 2A variant macroH2A1 (Costanzi and Pehrson 1998). These nucleosome changes may
create the heterochromatin that is characteristic of the Barr body.
We are still ignorant of the mechanisms by which the Xist transcript regulates the state of
the chromatin and by which the spreading of inactivation occurs. We still donot understand the
ways in which Xist transcription is linked to DNA methylation. We do not yet know how the
choice between the two X chromosomes is originally made, nor how the Xist RNA is transcribed
from a region surrounded by inactivated genes.* There is still much to learn about dosage