X Chromosome Activation Flashcards
Sex determination
Eutherian mammals = X and Y determinants
Female = XX. Large, many genes
Males = XY. Smaller, few genes
Describe Barr bodies
Dark areas at edge of nuclei cells
Proposed they are condensed chromatin from inactive X chromsome
Inactivation spreads from unique locus on X
Xic =
X-inactivation centre
Locus inactivation spreads form on X
No. Of Barr bodies normal female
1
No. Of Barr bodies normal male
0
No. Of Barr bodies XXY
Male Klinefelter syndrome
1
No. Of Barr bodies XXX
Normal female
2
No. Of Barr bodies XO
Female Turner syndrome
0
Cat patterning and inactive X chromosome
Male cats have X with either orange or black allele = no competition
Female have either black, orange or mosaic
both X’s the homozygous doesnt matter which silenced
If heterozygous then orange dominant
Can cause activation of random cells and end up part orange and black = why most tortoiseshell cats are female
Anhidrotic ectodermal dysplasia in humans
Mutation in X chromosome
Males = inherit the mutation
Females = could have mosaics depending on which chromosome activated in each cell
Symptoms:
- lack of sweat glands = males completely, females mosaicism
- sparse hair
-malformed teeth
What happens when x inactivation isn’t complete?
Many genes escape inactivation are present in regions that contain genes also present in Y chromosome
Pseudoautosomal regions present in 2 copies for either sex = need both for normal funcitons or haploinsufficent
Explains defects in humans wiht abnormal X numbers e.g. XXY
X inactivation should eliminate affects but the non-silenced genes still affects
Describe Klinefelter syndrome
Extra X in males = XXY Symptoms: - small testes (reduced testosterone) - Learning difficulties - infertility Additional x variants increase severity Can be treated with testosterone supplements in early development but doesnt cure Some symptoms develop pre-birth
Describe turner syndrome
Missing or disrupted X in females = XO Many genes haploinsufficient without both X’s All genes responsible not yet indentified Some symptoms include: - Poor breast development - no menstruation - low hairline - brown spots (nevi)
Xi
Inactivated chromsome
Xa
Activated chromosme
Xist
X inactivation specific transcript
Mechanism of x inactivation.
Begins in Xic
Produces RNA transcript for Xist
Xist in somatic cells only expressed in Xi
Essential for Xinactivaiton
in interphase nuclei Xist colocalises with Barr body
Stabilisation of Xist RNA mediates initiation of X inactivation
In each cell one chromosome expresses Xist - if more than one X it will inactivate as many as needed until 1 x left in
methylation locks in silencing. The more silenced the harder to reverse.
What are the 2 models for counting/sensing X chromosomes
- Protein expression occurs in X chromosomes. the protein binds to the X and acts as a promoter for Xist expression. Cooperative binding so there needs to be enough of it. Xist expression then causes chromosome to be silenced
- Autosomal factor binds to X chromosome and prevents proteins binding. The protein binds to a different x chromosome and silences it.
Both allow counting and make sure one silent, one active
Describe RNF12
Located 500bp from xist
Has ubiquitin ligase properties = high levels degrade xist repressor in embryonic somatic cells. A trans activating factor
Breaking down repressor means Xist active and begins to silence
As RNF12 nearby it gets silenced = reduces concentration of repressor
Causes one active and one silenced X chromsome
If XXX RNF12 repressor produced to level where 2 X’s silenced
Describe Tsix
Important to inactivation 3’ of Xist Loci that prevents Xist transcription On equivalent position to Xist on other strand = antisense transcription across promoter Changes chromatin structure Complex and not fully understood yet
Regulation of Xist by Tsix
Tsix regulated by long range cis active elements, pluipotency factors and Xite
Deletions skew silencing
Transcribing Tsix prevents machinery from transcribing Xist
Name other factors that regulate Tsix, RNF12 and Xist
Oct 4 = cell differentiation, producing stem cells
PRDM14 = developmental genes
SOX = differentiation
REX = imprinting
Name histone structures that chnage with inactivation
Trimethylates lysine 27 histone 3
Ubiquinates histone 2
Hypoacetylates histone 4
