Chromosome nactivationChromosome nactivationBabak NamiDepartment of Medical GeneticsSelçuk University

definition• X-inactivation (also called lyonization) is a

process by which one of the two copies of the X chromosome present in female mammals is inactivated.

• The inactive X chromosome is silenced by • The inactive X chromosome is silenced by packaging into transcriptionally inactive heterochromatin.

• X-inactivation occurs so that the female, with two X chromosomes.

• X-inactivation don’t occurs for X chromosome and in males (Dosage compensation).

• The choice of which X chromosome will be inactivated is random in mammals such as mice and humans.

• once an X chromosome is inactivated it will • once an X chromosome is inactivated it will remain inactive throughout the lifetime of the cell and its descendants in the organism.

• Unlike the random X-inactivation in placental mammals, inactivation in marsupials applies exclusively to the paternally derived X chromosome.

HistoryIn 1959 Susumu Ohno showed that the two X-chromosomes of mammals were different: one appeared like the autosomes; the other was condensed and heterochromatic.

In 1961, Mary Lyond proposed the random inactivation of In 1961, Mary Lyond proposed the random inactivation of one female X chromosome. The Lyon hypothesis also accounted for the findings that one copy of the X chromosome in female cells was highly condensed.

Ernest Beulter studying heterozygous females for G6PD deficiency proposed that there were two red cell populations of erythrocytes in such heterozygotes: deficient cells and normal cells, depending on whether the inactivated X chromosome contains the normal or defective G6PD allele.

Mechanism• At an early stage of embryo development, the cell

randomly "chooses" and X chromosome for inactivation.

• all females present a mosaic of cells within one of two cell lines, one line of cells expressing the maternal X and the other expressing the paternal X maternal X and the other expressing the paternal X chromosome.

• The selection is not random in the case of the human female's extraembryonic membranes (that go on to form the amnion, placenta, and umbilical cord).

• In all the cells of the extraembryonic membranes, it is father's X chromosome that is inactivated.

Mechanism

• X chromosome inactivation begins at a specific site on the X chromosome called the X Inactivation Center.

• Sequences at the X inactivation center (XIC), present on the X chromosome, control the present on the X chromosome, control the silencing of the X chromosome.

• The XIC contains two non-translated RNA genes, Xist and Tsix, which are involved in X-inactivation.

• The XIC also contains binding sites for both known and unknown regulatory proteins.

Mechanism• Xist (X-inactive specific transcript), a large (17 kb in

humans) transcript, is expressed on the inactivechromosome and not on the active one.

• It is processed similarly to mRNAs, through splicing and polyadenylation, however, it remains untranslated. It has been suggested that this RNA untranslated. It has been suggested that this RNA gene evolved at least partly from a protein coding gene that became a pseudogene.

• The inactive X is coated with this transcript, which is essential for the inactivation.

• X lacking Xist will not be inactivated, while translocation of the Xist gene on another chromosome causes inactivation of that chromosome.

Mechanism• The inactive X chromosome is coated by Xist RNA,

whereas the Xa is not:

The figure shows confocal microscopyimages from a combined RNA-DNA FISHexperiment for Xist in fibroblast cellsexperiment for Xist in fibroblast cellsfrom adult female mouse, demonstratingthat Xist RNA is coating only one of theX-chromosomes. RNA FISH signals fromXist RNA are shown in red color,marking the inactive X-chromosome (Xi).DNA FISH signals from Xist loci areshown in yellow color, marking bothactive and inactive X-chromosomes (Xa,Xi). The nucleus (DAPI-stained) is shownin blue color.

Mechanism• IST RNA is only expressed in cells containing at least two

Xs and is not normally expressed in male cells:

Expression of the XIST gene in males, females and somatic cell hybrids.hybrids.Slot blot of total cellular RNA isolated from human lymphoblastoid cell lines or mouse-human somatic cell hybrids retaining either the active or inactive human X chromosome, hybridized with the 14A XIST cDNA proble. The probe hybridizes only to RNA samples from cell lines which contain an inactive X chromosome.

Mechanism• During the inactivation process, the future Xa

ceases to express Xist, whereas the future Xi dramatically increases Xist RNA production.

• On the future Xi, the Xist RNA progressively coats the chromosome, spreading out from the coats the chromosome, spreading out from the XIC; the Xist RNA does not localize to the Xa.

• The silencing of gene along the Xi occurs soon after coating by Xist RNA.

Mechanism• TSIX is also a long (40 kilobase), noncoding RNA, but

it is transcribed in the opposite direction across the entire XIST gene. Like XIST, TSIX only acts on the chromosome that produces it.

• When TSIX transcription is reduced on one X, XIST• When TSIX transcription is reduced on one X, XISTexpression increases and leads to inactivation of that same X chromosome (Flair up or Gaza getirme!!).

• In contrast, overexpression of TSIX prevents increases in XIST expression and blocks inactivation of same X.

• These observations suggest that expression of TSIX is required to antagonize XIST on the future active X.

Mechanism• The inactive X chromosome does not express the

majority of TSIX gene, unlike the active X chromosome.

• This is due to the silencing of the Xi by repressive heterochromatin, which coats the Xi DNA and prevents the expression of most genes.prevents the expression of most genes.

• Compared to the Xa, the Xi has high levels of DNA methylation , low levels of histone acetylation, low levels of histone H3 lysine-4 methylation, and high levels of histone H3 lysine-9 methylation, all of which are associated with gene silencing.

• Additionally, a histone variant called macro H2A is exclusively found on nucleosomes along the Xi.

The Medical Importance of X Chromosome Inactivation

• First:• most sex-linked traits are recessively inherited.

Therefore, females heterozygous for a mutant gene on the X chromosome will usually not be gene on the X chromosome will usually not be symptomatic.

• in females, a recessive mutation on the X chromosome can be masked by a dominant wild-type allele.

The Medical Importance of X Chromosome Inactivation

• Second:• males have only one X chromosome per cell, a

mutant gene on an X chromosome will not be masked. masked.

• Males are called hemizygous for these X-linked genes. Thus, males with a mutant X-linked gene will express the diseased condition, while females with a mutant X-linked gene usually will not.

The Medical Importance of X Chromosome Inactivation

• Third:• there is no male-to-male transmission. This is

because of the sex determination scheme in humans. humans.

• If a man has a disease due to a mutation on the X chromosome, he cannot transmit it to his sons, since any male child of his will have inherited his Y chromosome, not his X.

The Medical Importance of X Chromosome Inactivation

• Fourth:• a female heterozygote is called a "carrier"

because she does not express the mutant X-linked gene even though she has it.linked gene even though she has it.

• The carrier has a 50 percent chance of passing the gene to each of her children.

• Sons who receive the gene will be hemizygotes and will manifest the trait. Daughters who receive the gene will be heterozygotes .

The Medical Importance of X Chromosome Inactivation

• Fifth:• a man who has the a trait caused by a mutant X

chromosome gene will pass the mutant gene only to his daughters. only to his daughters.

• They will most likely be carriers unless their mother is a carrier, in which case they have a 50% chance of manifesting the disease.

Nonrandom X inactivation

• Skewed means off to one side or nonrandom.• Skewed inactivation has been well documented

in several studies of female monozygotic twins heterozygous for X-linked disorders.

• In these pairs, one twin has skewed X inactivation toward the normal chromosome, and thus expresses the (defective) recessive phenotype while her twin has skewed inactivation toward the mutant chromosome and thus exhibits no symptoms.

Nonrandom X inactivation

• No cases have been documented in which monozygotic twins known to be heterozygous for an X-linked trait both manifest the trait or both display normal phenotype, thus a connection between the twinning and X chromosome inactivation has been suggested.