Mechanism and role of X inactivation Flashcards
epigenetic aspects of cloning
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)
Barr body
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
Random XCI results in
functional mosaicism in females
causes the variable expressivity of X linked diseases in heterozygote females
why XCI needed
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
different forms of dose compensation
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
non random XCI
in all cells of marsupials and in extraembryonic cells of placental mammals
paternal X is inactivated (epigenetic regulation)
random XCI
in somatic cells
statistically equal
rarely not equal
first count
MSCI
meiotic sex chromosome inactivation
XIC
x inactivation center
gene: XIST x inactive specific transcript
XIST
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
How many Xi in 46, XX 46, XY 47, XXY 49, XXXXX 49, XXXXY
46, XX- 1 xi 46, XY - o 47, XXY - 1 49, XXXXX - 4 49, XXXXY -3
Xist KO
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
what makes teh other Xist silent
Tsix is antisense partner of Xist
noncoding RNA transcribed in opposite direction
Cis acting
suppresses Xist on active X
stable transcriptional silencing of Xi is achieved by
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)
series of sequential steps of XCI
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
activates of XCI
activate XIST or repress TSIX
trans acting rnf12 (target Rex1 transcriptional factor for degradation)
Jpx, Ftx, Xite (all non coding RNA)
inhibitors of XCI
repress XIST and or activate TSIX
are pluripotency factors
OCT4, SOX2, nanog
Rex 1
XACT
a long non coding RNA covers active X chromsome only in human ES cells
not all genes are silenced in inactive X
examples
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
how gene might escape process of XCI
failure to initiate silencing
failure to maintain silencing
MSCI- meiotic sex chromosome inactivation
caused by other factors
differs from random and imprinted XCI
ATR, BRCA1
meiotic silencing by unpaired DNA
Possible co-evolution of genomic inprinting and
X inactivation
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
skewed X inactivation
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;
- primary non random
- 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
random XCI-
fortunate skewed XCI-
unfortunate skewed XCI-
male-
normal and mutant products
mainly normal product
mainly mutant product
only mutant product
microchimerism
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???
negative effects of microchimerism
Scleroderma
SLE
Multiple sclerosis
Primary biliary cirrhosis
Sjogren syndrome
Biliary atresia
Grave disease
Hashimoto thiroiditis
Juvenile dermatomyositis
Fetal loss
Neonatal lupus congenital heart block
positive effects of microchimerism
beta cell regeneration diabetes RA prevention of breast cancer tolerance in hematopoietic stem cell transplantation
