X-chromosome inactivation

Give some examples of x-linked recessive diseases Give some classic patters of inheritance in a pedigree diagram

*Examples:* Colour blindness Haemophilia A/B DMD X-linked agammaglobulinemia G6PD Carrier females who have only one copy of the mutation do not usually express the phenotype, although differences in X chromosome inactivation can lead to varying degrees of clinical expression in carrier females since some cells will express one X allele and some will express the other. Dosage is very important

What is the Lyon hypothesis? How did Lyon discover this?

*his is that dosage compensation in mammals is by inactivation of all but one X-chromosome in cells with more than one X-chromosome* *The Barr body* (visible in some female cells) is an inactivated X chromosome Specifically; - Random inactivation - Which X-chromosome is switched off is heritable in somatic cells - The inactivation occurs in early development

Why do we require x-chromosome inactivation?

Everything is doubled apart from X/Y In females, there are two X chromosomes - therefore they require dosage compensation One way of getting around this would be to halve the expression of all genes. Another way would be to silence one of the X-chromosomes.

Give some epigenetic marks associated with X-inactivation?

Histone modifications Xist RNA coats the chromosome Histone marks associated with transcriptional activity are lost

How many x-chromosomes are inactivated in:? Male Turners Females Trisomy X

If a male, no X is inactivated If Turner's, no x is inactivated If a female, one X chromosome If a trisomy X, 2 or if more 2, 2 X chromosomes get switched off So essentially we switch off all but one x-chromosome.

What is hypohidrotic ectodermal dysplasia and how does this manifest in females?

In X-linked recessive inheritance, a female with one altered copy of the gene in each cell is called a carrier. Since females operate on only one of their two X chromosomes (X inactivation) a female carrier may or may not manifest symptoms of the disease. If a female carrier is operating on her normal X she will not show symptoms. If a female is operating on her carrier X she will show symptoms. In about 70 percent of cases, carriers of hypohidrotic ectodermal dysplasia experience some features of the condition. These signs and symptoms are usually mild and include a few missing or abnormal teeth, sparse hair, and some problems with sweat gland function. Some carriers, however, have more severe features of this disorder.

How many genes does the x-chromosome have? What are there general functions?

It contains about 5% of the 1000 genes (haploid genome) Genes on the X-chromosome encode for house keeping and specialised functions They are generally well conserved between species Only one X-chromosome is required to be active, otherwise there would be double the dose.

What is the concept of non-random x-inactivation (leading to skewed x-inactivation)?

Many females who have x-linked disease do not manifest the disease. They're mosaics. This results in women having 90% functional X. So, randomly, different X-chromosomes get inactivated. If you have a heterozygous X-linked recessive trait and 50% inactivate the mutation (they're fine) and 50% inactivate the active. Women end up having the majority okay cells. Cellular mosaicism created by X-in in females is often advantageous, protecting carriers of X-linked mutations from the severe clinical manifestations seen in males This is seen in the disease AED, Anhidrotic ectodermal dysplasia (Hypohydrosis + other clinical features). It's X-linked recessive inheritance.

How is the silencing of one chromosome selected? What is the default state for the x-chromosome?

One chromosome will be active (Xa) and one will be inactive (Xi) As above, in people with more than one X, still only one x becomes Xa. Therefore, the default state is inactivation. X-chromosome inactivation is a random process which occurs very early in gastrulation. This would suggest women should suffer from x-linked disorders 50% of the time, but in reality this is much lower. In very early development - any of the X-chromosomes could become active. Some Xi genes remain expressed (~12-20%), thus this provides some protection against defective genes. This could be due to the X-inactivation center... The XIC sequences on XX pair up - and this is probably related to the mechanism by which the chromsomes count

List the general features of an X-linked dominant disease

Remember, affected fathers cannot pass on this trait - and when sons are affected the mother must have the disease.

Give some examples of diseases with x-linked dominant inheritance patterns

Rett Syndrome Fragile X Epilepsy with mental retardation (EFMR) Alports syndrome As in autosomal dominant inheritance, only one copy of the disease allele on the X-chromosome is required for an individual to be susceptible to an X-linked dominant disease. Both males and females can be affected, although males are more severely affected because they only carry one copy of the genes found on the X-chromosome. For example, females with Alports (one copy) simply get haematuria. With males, Alports syndrome is caused. Moreover, some X-linked dominant disorders are lethal in males.

With reference to tortoiseshell cats - how do we know that daughter cells remember which X-chromosome is inactivated?

The daughters of one cell remembers which X-chromosome is inactivated. Moreover, this occurs in early development. To remember this inactivation, the cells must remember the epigenetic change. To demonstrate this - you have to look at tortoiseshell cats. Calico and tortoiseshell cats are females heterozygous for 2 alleles of an X-linked coat colour gene O (ginger) and o (black). The orange and black sectors are caused by X inactivation The skin is a mosaic of cells where some have the O allele active (making ginger pigment) and some do not (making black pigment). This can only happen in cats with two X chromosomes (females) White is a separate genetic determinant (dominant) Moreover, when a cat was cloned which was calico, despite identical DNA their coat patterning was totally different. This means in successive generations, the epigenetic control mechanisms change, thus different X-chromosomes are switched off.

What is a pseudo-autosomal region on x-chromosome? Give an example.

There is regions PAR1 and PAR2. These are homologous sequences of nucleotides on the X and Y chromosomes These regions of sex chromosomes get inherited just like autosomes Moreover, crossing over between the X and Y chromosomes is normally restricted to the pseudo autosomal regions, thus, pseudo autosomal genes exhibit an autosomal rather than sex linked pattern Therefore, females can inherit an allele originally present on the Y chromosome of their father. Moreover, in females these regions escape inactivation so there's 2 active alleles in a female.

What is TSIX? What is its main role? Which chromosome is it found? On the active or inactive one?

This is an antisense transcript to Xist, partially overlapping in sequence. It is a non-coding RNA gene as well. It is transcribed from the XIC Tsix has been proposed to be the regulator of Xist, a switch that determines whether Xist is on or off. Before the onset of X inactivation, Tsix is expressed from both X chromosomes. At the onset of X inactivation, Tsix expression becomes monoallelic, is associated with the future active X and persists until Xist is turned off. Tsix is not found on the inactive X once cells enter the X-inactivation pathway. Tsix has features suggesting a role in regulating the early steps of X inactivation, but not the silencing step Xist is only expressed from the future inactive X chromosome in females and is able to "coat" the chromosome from which it was produced. Many copies of Xist RNA bind the future inactivated X chromosome. Tsix prevents the accumulation of Xist on the future active female X chromosome to maintain the active euchromatin state of the chosen chromosome. Tsix is a negative regulator of Xist; X chromosomes lacking Tsix expression (and thus having high levels of Xist transcription) are inactivated much more frequently than normal chromosomes. *It's possible that Tsix RNA is randomly expressed at a lower level on one chromosome - thus this chromosome accumulates Xist and becomes inactive*

What is XACT?

This long non-coding RNA (lncRNA) was identified in 2013 and coats Xa Stands for X active coating transcript. It coats the active X chromosome. In the absence of XIST, XACT is expressed from both X chromosomes in humans but not in mice, suggesting a unique role for XACT in the control of human XCI initiation. lncRNA's are generally associated with gene silencing - suggesting a role for this molecule

How do females manifest haemophilia?

This mosaic pattern is also true for a female with haemophillia, who typically has a less severe (close to normal) phenotype - having ~50% of liver cells working which can still ensure normal clotting.

What is XIC and where is it located? What is its function

X-inactivation center (XIC) The XIC is located at Xq13. Inactivation begins at this site and then propogates along the whole chromosome in heterchromatin spreading. The 2.1(2)P region shows differential histone H4 hyperacetylation in undifferentiated female and male embryonic stem (ES) cells and has been suggested as a possible regulatory element in X inactivation. P1 and P2 are the somatic Xist promoters, and P0 is a postulated Xist promoter in undifferentiated ES cells and early embryos. Chromosomal translocations which place the XIC on an autosome lead to inactivation of the autosome, and X chromosomes lacking the XIC are not inactivated.

What is XIST?

X-inactive specific transcript (Brown et. al., Nature, 1991) It is a major component of the XIC. This is an RNA gene on the X-chromosome of placental animals. It acts as a major effector of the X-inactivation process. The XIST RNA is a large transcript, expressed on the inactive chromosome and not on the active one. It is processed in a similar way to mRNAs, through splicing and polyadenylation. It remains untranslated. X chromosomes lacking Xist will not be inactivated, while duplication of the Xist gene on another chromosome causes inactivation of that chromosome Spreading of Xist RNA in cis triggers epigenetic changes leading to X-chromosome inactivation. It leads to organise chromatin into a closed state. Interestingly, differentiated cells that have already undergone X-inactivation, loss of XIST does not cause reactivation. Therefore, XIST is required for the establishment of X-inactivation, not the maintenance.