Transcript X chromosome inactivation- Review

2003 Silver Fleece Awards
The recipients represent “an egregious example of
people feeding a line of bull to the public.”
1. Clonaid- Claim to have cloned humans
– Says that cloning will “enable mankind to reach
eternal life”
– Claims that memories and personality will be
transferred into a brand new body
2. Urbana Nutrition, Inc. – Market “Longevity” as anti-aging
“There are no methods or products that actually slow, stop or reverse aging”
Leonard Hayflick, U. of California, San Francisco
Source- AP news, Friday, March 14, 2003
X chromosome inactivationReview- Panning an Jaenisch, RNA and the epigenetic regulation of X
chromosome inactivation. Cell 93:305, 1998
A. Introduction Barr bodies first described in females in 1949
 Turners syndrome (45,X) are Barr body negative;
Kleinfelters syndrome (47, XXY) are Barr body
positive
 Lyon hypothesis- one of the two X chromosomes in
female is inactivated; all but one is inactivated if
multiple X chromosomes - referred to as “dosage
compensation”
X chromosome inactivation
• Introduction X-chromosome inactivation occurs at day 3 of
embyrogenesis
 Inactivation process is random
 Inactivation state maintained throughout life
• A few genes remain active in the inactive X
chromosome, including XIST at Xq13
Dosage compensation comparisons
1X
1X
2X
2X
2-fold increase
in males
2-fold decrease
in females
Stably inactivate one X
chromosome
X chromosome inactivation in
flys and worms
 Distinct mechanisms to achieve dosage compensation
 C. elegans- Dosage compensation by reducing gene
activity by two fold on each X chromosome
 Mechanism- if one X-, XO-lethal gene is on resulting in
male determination
 Drosophila- Stimulate X gene transcription 2-fold in males
to equal levels from each X
– chromosomes in female
Mammals- X-inactivation is used to
compensate for 2 X chromosomes
Three steps1. choice- occurs in embryonic cells
 Xist is expressed from both X chromosomes
in female
 Xist encodes 15 kb polyadenylated
untranslated RNA that is unstable
 Xist is gene located within Xic
Mechanism of Xist-mediated silencing
Xist RNA expressed
by both X chromosomes
Xist
Xist
Xist RNA interacts with
stabilizing factors
Inactive
Xist
Active
Blocking factor prevents Xist
RNA stablization and spreading
Inactivated X chromosome
Xist RNA
X-inactivation- Step 2
2. initiation- begins at X-inactivation center
(Xic)
 Xist RNA spreads in cis to coat
chromosome
 Note that Xist does not interact directly with DNA,
but likely through a protein intermediate
• Xist gene on other X chromosome is
silenced
X-inactivation- Step 3
3. Spread- propagated bidirectionally from
Xic
 Xist methylation required for silencing of
Xist
 Dnmt KO- Male X and two female X are all
inactivated because Xist gene remains on and
Xist RNA coats chromosome
X-inactivation- observations
 Xist is necessary and
sufficient for X inactivation
(using 450kb YAC)
 insert Xist transgene on
autosome results in
inactivated autosome
13
p
12
11
12
13
14
q
21
24
mouse autosome
X-inactivation- observations
 but Xist is neither necessary or sufficient to
maintain X inactivation in somatic cell hybrids
 Thus initiation and maintenance of X
inactivation are likely distinct mechanisms
 Xist maintains inactive state in cis, not trans
X-inactivation
Xist Gene
Mechanism in
mammals
 If mutate Xist promoterpreferential X inactivation
on chromosome with
mutation
Random inactivation
Blocking factors
Preferential inactivation
– possibly due to failure to
compete with blocking factor
 Delete Xist exons 1-5mutant chromosome
chosen but not
inactivated
Prevent inactivation
delete
Mechanism in mammals
In Extraembryonic tissues, paternal X is always inactivated
• if paternally inherited mutant X – observe no X inactivation in
extraembryonic tissue in females
• if maternally inherited mutant X - WT phenotype in
extraembryonic tissue (i.e .WT X always inactive)
X-inactivation
X-controlling element (Xce) mapped to a 6 kb
region in Xist gene and is required for X inactivation
 Model- Mutually exclusive binding of blocking
factor to Xce on one X, and of initiator factor to Xist
on other X
 Marsupials and in mice extraembyonic tissues- paternal X
always inactivated in females
Reason???
X-inactivation
 Xist RNA may cause replication origins to
fire late, resulting in heterochromatin
formation
 modulate histone acetetylation
– The Drosophila mof gene is required for dosage
compensation and is an acetyltransferase
Clerc and Avner, Science 290:1518, 2000
X-inactivation is
reprogrammed during
development
Random X inactivation in
extraembryonic tissue!
Thus, an epigenetic, non-erased tagging must occur normally with male X.
Science 295:345, 2002
Recall CTCF is involved in genomic imprinting
Recent informationThe factor CTCF may also be involved in Xchromosome choosing. Science 295:345, 2002
CTCF binds and activates XIST anti-sense transcription (called
Tsix), which prevents Xist expression, which keeps that X
chromosome active
Identifiction of the X-inactivation region (X