2003 Silver Fleece Awards

1
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
2
X chromosome inactivation-Review- Panning an
Jaenisch, RNA and the epigenetic regulation of X
chromosome inactivation. Cell 93305, 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

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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

4
Dosage compensation comparisons
1X
1X
2X
2X
2-fold increase in males
2-fold decrease in females
Stably inactivate one X chromosome
5
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

6
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

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(No Transcript)
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Mechanism of Xist-mediated silencing
Inactive
Active
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Inactivated X chromosome
Xist RNA
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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

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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

12
X-inactivation- observations

• Xist is necessary and sufficient for X
  inactivation (using 450kb YAC)
• insert Xist transgene on autosome results in
  inactivated autosome

13
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

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X-inactivation

• Mechanism in mammals
• If mutate Xist promoter- preferential X
  inactivation on chromosome with mutation
• possibly due to failure to compete with blocking
  factor

• Delete Xist exons 1-5-
• mutant chromosome chosen but not inactivated

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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)

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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???

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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

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Clerc and Avner, Science 2901518, 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.
19
Science 295345, 2002
20
Recall CTCF is involved in genomic imprinting
Recent information- The factor CTCF may also be
involved in X-chromosome choosing. Science
295345, 2002
CTCF binds and activates XIST anti-sense
transcription (called Tsix), which prevents Xist
expression, which keeps that X chromosome active
21
Identifiction of the X-inactivation region (X