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Sexual Genetics > Mechanism and role of X inactivation > Flashcards

Mechanism and role of X inactivation Flashcards

1
Q

epigenetic aspects of cloning

A

donor nucleus has typically somatic, not egg type epigenome

epigenetic tags might be altered in the donor nucleus since it may have been copied several times (the epigenetic copying machinery is less precise than DNA copy)

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2
Q

Barr body

A 
inactive X
x chromosome inactivation
lyonization
early embryogenesis
random, clonally inherited
all but one x is active 
general phenomenon in animals

dose compensation cause males have only one x and females 2

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3
Q

Random XCI results in

A

functional mosaicism in females

causes the variable expressivity of X linked diseases in heterozygote females

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4
Q

why XCI needed

A

imbalance of sex chromsome linked genes
Y- few funcitonal genes - 50 genes
male specific genes (SRY, AZF)
which have homologous X (house keeping) PAR1

X- 1500 genes
msot of them do not have homologous on Y
twice more product would distrub metabolic balance

solution: Dose compensation- there are different possibilities in different animals

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5
Q

different forms of dose compensation

A

female expression from X is downregulated by 50%

in males expression level is upregulated from X

in females it is down or males it is upregulated

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6
Q

non random XCI

A

in all cells of marsupials and in extraembryonic cells of placental mammals
paternal X is inactivated (epigenetic regulation)

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7
Q

random XCI

A

in somatic cells
statistically equal
rarely not equal
first count

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8
Q

MSCI

A

meiotic sex chromosome inactivation

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9
Q

XIC

A

x inactivation center

gene: XIST x inactive specific transcript

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10
Q

XIST

A 
x inactive specific transcript
17kb non coding RNA
-a repeat-silencing, inactivation
-other parts - cis coating capacity
Remains in nucleus
expressed only in cells with 2 X's
coat the chromosme - exclusion of transcription machinary and recruitment of chromatin remodeling complexes
expressed from inactive chromosome
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11
Q 
How many Xi in
46, XX
46, XY
47, XXY
49, XXXXX
49, XXXXY
A 
46, XX- 1 xi
46, XY - o
47, XXY - 1
49, XXXXX - 4
49, XXXXY -3
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12
Q

Xist KO

A 
sex differences in their phenotype 
males normal (Xist is not expressed)
Female phenotype depends on whetehr the nonfuctional copy is father or mother

mother- phenotype normal so maternal x is active in all tissues

father- retarded development and death as embryo - maternal x is inactivated in all embryonic cells but neither x is inactivated in trophoblasts so lack of dose compensation

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13
Q

what makes teh other Xist silent

A

Tsix is antisense partner of Xist
noncoding RNA transcribed in opposite direction
Cis acting
suppresses Xist on active X

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14
Q

stable transcriptional silencing of Xi is achieved by

A

a) counting- at least 2 copies are needed, if there is only one copy XIST is not transcribed
b) choice- unstable XIST is transcribed from both Xs, and TSIX is transcribed only from the future Xa
c) initiation - TSIX prevents the stabilization of XIST synthesized from future Xa
d) spreading- XIST inactivates the other genes of Xi which leads to transcriptional silencing
e) x chromosome genes are expressed form Xa, but not from Xi (heterochromatic)

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15
Q

series of sequential steps of XCI

A

Xic- Xist RnA expression

Xist RNA accumulation- Pol II exlusion

PRC1 and 2 localization

x linked gene silencing

late replication

macro H2A localisation hypo-H4 acetylation

establishment of inactive x chromosome

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16
Q

activates of XCI

A

activate XIST or repress TSIX

trans acting rnf12 (target Rex1 transcriptional factor for degradation)
Jpx, Ftx, Xite (all non coding RNA)

17
Q

inhibitors of XCI

A

repress XIST and or activate TSIX

are pluripotency factors
OCT4, SOX2, nanog
Rex 1

18
Q

XACT

A

a long non coding RNA covers active X chromsome only in human ES cells

19
Q

not all genes are silenced in inactive X

examples

A

PAR genes- found on Y too no dosage difference

having functional y linked counterparts
having nonfucntional Y linked copy
others without y counerparts

organized in clusters

first inactivated later it is reversed

20
Q

how gene might escape process of XCI

A

failure to initiate silencing

failure to maintain silencing

21
Q

MSCI- meiotic sex chromosome inactivation

A

caused by other factors
differs from random and imprinted XCI

ATR, BRCA1

meiotic silencing by unpaired DNA

22
Q

Possible co-evolution of genomic inprinting and

X inactivation

A

dosage compensation occurs in all animals having heterogametic sex chromosomes

egg laying mammals used dosage compensation, too but it was not X inactivation

With the evolution of placenta pressure arose to imprint the genes
(see conflict hypothesis) (in egg laying mammals there is no imprinting)

it was acheived by parental imprinting of growth stimulatory genes (e.g. Igf2) located
on different chromosomes e.g. X chromosome

it resulted imprinted X inactivation seemed to be better than other form of dosage
compensation (it is found in marsupials)

Random X inactivation developed later in placental mammals

23
Q

skewed X inactivation

A

when the random inactivation is not random

deviation from 50% inactivation of each parental allele (x) is skewing (common criteria 75-80% of cells)

proposed mechanisms;

  1. primary non random
  2. secondary, acquired non random

In some X linked disease there is a strong selection in heterozygotes to inactivate the mutant allele bearing X

If there is no selection skewing influences the extent of disease

Associations were found between skewed XCI and some diseases: autoimmune disorders (sleroderma), cancers (breast cancer), recurrent spontaneous abortions

Majority of skewing observed in adults and is aquired secondarily

24
Q

random XCI-

fortunate skewed XCI-

unfortunate skewed XCI-

male-

A

normal and mutant products

mainly normal product

mainly mutant product

only mutant product

25
Q

microchimerism

A

Exchange of cells between the fetus
and mother

These cells may reside in all tissues
they may last for decades

Number 1000 -10000

A woman may have – her cells
her mother’s cells
her children’s cells

Physiological role???

26
Q

negative effects of microchimerism

A

Scleroderma
SLE
Multiple sclerosis
Primary biliary cirrhosis
Sjogren syndrome
Biliary atresia
Grave disease
Hashimoto thiroiditis
Juvenile dermatomyositis
Fetal loss
Neonatal lupus congenital heart block

27
Q

positive effects of microchimerism

A 
beta cell regeneration
diabetes
RA
prevention of breast cancer
tolerance in hematopoietic 
stem cell transplantation

Sexual Genetics (10 decks)

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