2003 silver fleece awards 1. clonaid- claim to have cloned humans –says that cloning will...
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2003 Silver Fleece Awards
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
The recipients represent “an egregious example of people feeding a line of bull to the public.”
Source- AP news, Friday, March 14, 2003
X chromosome inactivation-Review- 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”
• 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
X chromosome inactivation
Dosage compensation comparisons
2-fold increasein males
2-fold decreasein females
Stably inactivate one Xchromosome
2X
1X
2X
1X
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 steps-
1. 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
Xist
Xist
Blocking factor prevents Xist RNA stablization and spreading
Active
Inactive
Xist RNA interacts with stabilizing factors
Xist RNA expressedby both X chromosomes
Xist RNA
Inactivated X chromosome
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
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- Step 3
Xist is necessary and sufficient for X inactivation (using 450kb YAC)
insert Xist transgene on autosome results in inactivated autosome
X-inactivation- observations
mouse autosome
11
12
13
21
24
12
13
14
p
q
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- observations
Mechanism in mammals
If mutate Xist promoter- preferential X inactivation on chromosome with mutation– possibly due to failure to
compete with blocking factor
X-inactivation
Preferential inactivation
Delete Xist exons 1-5- mutant chromosome chosen but not inactivated
Prevent inactivation
delete
Xist Gene
Blocking factors
Random inactivation
• 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)
Mechanism in mammals
In Extraembryonic tissues, paternal X is always inactivated
X-inactivationX-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???
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
X-inactivation
X-inactivation is reprogrammed during
development
Random X inactivation in extraembryonic tissue!
Clerc and Avner, Science 290:1518, 2000
Thus, an epigenetic, non-erased tagging must occur normally with male X.
Science 295:345, 2002
Recent information- The factor CTCF may also be involved in X-chromosome 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
Recall CTCF is involved in genomic imprinting
Identifiction of the X-inactivation region (X