.
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In the past, it was thought that genetic defects on the X chromosome would only affect males and that females would be protected because they inherit two X chromosomes: if one is defective, the other can compensate for it. Advances in research have shown that this is not always the case. In some cases, females do manifest symptoms of X linked diseases, for example, in Duchenne muscular dystrophy. This section explains how this can happen.
Human females inherit two copies of every gene on the X chromosome, (one from the mother and one from the father), whereas males inherit only one (always from the mother). If both X chromosomes were active within a given female's cell, it would throw things off as the genes on the X chromosome would be expressed at twice the rate in female cells compared to males. To avoid this, one X chromosome is randomly selected and inactivated and one remains functional (active) in each cell. So, the cells of females have only one functioning copy of each X-linked gene - the same number as males. The inactivated X can be seen as a dense, stainable structure, called a Barr body (after its discoverer). X inactivation is also referred to as a method of dosage compensation.
Recent studies (NATURE VOL 434 17 MARCH 2005) clarify the process: The X developed a way to inactivate - silence - most of the genes on one of the two Xs in females, so that males and females would in large part have the same dosage of gene products. Early in female development, cells randomly choose either the maternal or paternal X to be the active X chromosome. The other one then transcribes large amounts of a large RNA from a gene in the middle of the chromosome, XIST. Only this chromosome becomes coated by XIST RNA, and thereby silenced by modification of its DNA and associated proteins. This choice is permanent, and has certain consequences. A famous example is the calico cat, the color pattern in its fur reflects the pattern of Xs inactivated in his or her cells during early development. Similarly, human females are mosaics of the X chromosomes from each parent, and the severity of an X-linked disease in a female depends on the percentage of the cells in which the mutant gene concerned is silenced or expressed. Research by Carrel and Willard shows that 75% of the inactivated genes are permanently silent, and about 15% permanently escape inactivation, meaning that they are expressed at twice the level in females as males. The remaining 10% are expressed in some inactive Xs but not others, indicating variability in human females that is likely to have medical relevance. Having twice the amount of any gene product could result in female-female or female-male differences: the possibilities, as yet undefined, are intriguing. About a 1% difference exists between males and females - a huge difference genetically (the difference between humans and chimpanzees is only 1.5%).
In fertilization, female cells receive two X chromosomes, one from the mother and one from the father. X inactivation occurs early in embryonic development. The X chromosome that becomes inactivated and converted into a Barr body is random. After inactivation has occurred, all the descendants of that cell will have the same X chromosome inactivated. Thus in some cells, the inactivation may silence the paternal X and in other cells, the maternal X chromosome may be inactivated. This produces a mosaic pattern in the female's cells.
X inactivation is a very complex and variable phenomena. For example, it appears that some X genes escape inactivation and are expressed at a higher rate in females.
Example: This mosaic effect can be see in the patchwork colored coat of a Calico Cat. The gene determining whether fur color is orange or black (that is, not orange) resides on the X. Females that carry the orange version on one X and the black version on the other X will end up with some orange areas and some black ones, depending on which X is inactivated in each cell. A different gene accounts for the white areas of fur. http://dhushara.freehosting.net/book/upd/aug201/xychr.htm
There are three basic steps involved:
=counting: each cell counts how many X chromosomes it has (this is done in males as well as females)
=specific X chromosomes are randomly selected for inactivation, leaving one X functional per cell
= inactivation takes place
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/S/SexChromosomes.html
If by mutation, there are extra X chromosomes, for example, a male with YXX, the extra X will be tagged and inactivated. In addition, a female with XXX would have two X chromosomes tagged and inactivated.
Failure to choose and completely shut down one X chromosome means immediate death for the developing embryo, due to a 'dosage' imbalance of genes.
At an early stage of embryo development, the cell randomly "chooses" and X chromosome for inactivation. This 'decision' is inherited by all of the descendants of that original cell. Thus, 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 chromosome. Males have only one cell line for the X chromosome because they receive only one X chromosome (from their mother) and one Y chromosome (from their father).
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.
X chromosome inactivation begins at a specific site on the X chromosome called the X Inactivation Center and then spreads towards the ends of the chromosome.
The chromosome becomes heterochromatic.
Aside: heterochromatic: Chromosomes are composed of chromatin. It consists of proteins (principally histones), DNA, and small amounts of RNA. In a metabolically inactive nucleus, chromatin is mainly found in a condensed form, called heterochromatin, in an active nucleus most of the chromatin is in an expanded form, euchromatin. A heterochromatic chromosome is also called an accessory chromosome (B chromosome) - any nonessential chromosome that is found in addition to the regular karyotype of a species. Accessory chromosomes are heterochromatic.
Heterochromatin consists of highly condensed chromosomal regions that stain darkly. This makes up the Barr body seen in the cell that represents the inactivated X chromosome.
Not all genes are inactivated - about 18 genes on the inactivated X chromosome normally escape inactivation.
Aside: Recall that human females inherit two copies of every gene on the X chromosome, whereas males inherit only one. There are 18 exceptions where X genes are also present on the Y chromosome and where males thus inherit two copies: the 9 pseudoautosomal genes and the 9 "housekeeping" genes found on the Y.
Let's look at these 18 genes in females. To yield an equal dosage of genes compared to males, these genes would need to remain active on both of the female's X chromosomes. As it turns out, these genes do escape inactivation in females and remain functional. Just how they manage this has yet to be discovered.
Most of these genes are also active on the active X chromosome.
One of these genes is inactive on the active X, and instead remains active on the inactive X.
This gene is called Xist (for X inactive specific transcript). An X chromosome must have the Xist gene in order to be inactivated.
Xist encodes a special large molecule of RNA (not like the mRNA type that is used in protein synthesis). This large RNA molecule has no open reading frames, thus makes no protein. Instead it makes a long RNA molecule that spreads out along the X chromosome and 'decorates' or paints the chromosome.
Transcription of the Xist gene continues on one of the X chromosomes, leading to an accumulation of Xist RNA and converting that chromosome into a condensed, inactive Barr body. Xist RNA accumulation inactivates all (or almost all) of the other hundreds of genes on that chromosome.
Transcription of Xist ceases on the other (active) X chromosome allowing all of its hundreds of other genes to be expressed. The shutdown of the Xist locus on the active X chromosome is done by methylating Xist regulatory sequences. DNA methylation usually results in gene repression so methylation permanently blocks Xist expression and permits the continued expression of all the other X-linked genes.
During the first steps of embryonic development of the female, the Xist locus on EACH of her two X chromosomes is expressed but the Xist RNA is quickly broken down. Then something happens to tip the balance in favor of one or the other of the X chromosomes that goes on to be fully coated with Xist RNA and inactivated.
If researchers delete the Xist gene, this abolishes a chromosome's capacity for X inactivation.
Likewise, if researchers insert an Xist gene into an autosome, it can induce inactivation on that autosome.
Dosage compensation of genes is widespread, however, the exact mechanisms involved vary widely depending upon which organism is examined.
There is some evidence that X inactivation is age-dependent. With advancing age, a relaxation of inactivation may take place and reactivation of some of the genes on the X chromosome may occur. This may be linked to health associated problems in older females.
Above based on: http://www.ultranet.com/~jkimball/BiologyPages/S/SexChromosomes.html#x_chromosome
From: http://8e.devbio.com/contents.php?sub=1&art=1&full=1
There are over 120 human genes located on the X chromosome that have no counterpart on the Y chromosome. The traits governed by these genes thus show sex-linked inheritance. This type of inheritance has certain unique characteristics.
First, most sex-linked traits are recessively inherited. Therefore, females heterozygous for a mutant gene on the X chromosome will usually not be symptomatic, since the other X chromosome has the wild-type (normal in the population) allele. Even though X chromosome inactivation occurs, the passage of substrates from one cell to the other (either in solution or through gap junctions) equalizes the material. So in females, a recessive mutation on the X chromosome can be masked by a dominant wild-type allele.
Second, males have only one X chromosome per cell, a mutant gene on an X chromosome will not be masked. Males are called hemizygous for these X-linked genes. Thus, males with a mutant X-linked gene will will express the diseased condition, while females with a mutant X-linked gene usually will not.
Third, there is no male-to-male transmission. This is because of the sex determination scheme in 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.
Fourth, a female heterozygote is called a "carrier" because she does not express the mutant X-linked gene even though she has it. (The terminology comes from an unfortunate link between genes and infectious diseases - as in a typhoid carrier, one who carries the germs for the disease but who is not made ill by them.) The carrier has a 50 percent chance of passing the gene to each of her children (since she will transmit either the wild-type X or the mutant X equally). Sons who receive the gene will be hemizygotes and will manifest the trait. Daughters who receive the gene will be heterozygotes (i.e., carriers).
Fifth, a man who has the a trait caused by a mutant X chromosome gene will pass the mutant gene 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.
Sixth, occasionally carriers will show mild symptoms of the trait (heterozygous manifestation) due to inactivation of the wild-type X chromosome significantly more than the mutant X chromosome. The pattern of X chromosome inactivation is generally random, but this means that there will be a certain percentage of heterozygous women who inactivate the wild-type X chromosome in a large percentage of their cells. For instance, at the time of X chromosome inactivation (about the 10th day of embryogenesis in humans), there may only be ten cells that form the liver. If a liver-specific gene on the X chromosome (like the gene encoding clotting factor VIII is mutant, there is a chance that a majority of these ten cells, most all will randomly inactivate the wild-type X and leave the mutant X to function. (This would be like flipping a coin ten times and getting tails in eight trials.) Such a carrier female will have a very low level of factor VIII and as a result will show symptoms of hemophilia (the disease caused in males when their X chromosome has a mutant gene for clotting factor VIII).
Since only half the human population has a Y chromosome, the Y chromosome does not have genes that are important to the vital body functions. Basically, it contains genes for testes formation and sperm maturation. Everyone has at least one X chromosome, however, and it contains numerous genes that are essential for normal development and body function. Among the genes on the X chromosome are those encoding dystrophin (whose loss of function mutations cause Duchenne or Becker's muscular dystrophy), Factor VIII (whose loss-of-function mutation results in hemophila), ornithine transcarbamylase (whose deficiency causes excess ammonia in the blood), and hypoxanthine phosphoribosyl transferase (whose deficiency causes mental retardation, uric acid stones, and a syndrome where the children attempt to mutilate themselves).
Skewed means off to one side or nonrandom. If the inactivation of X chromosomes is not random , and the majority of a woman's normal X chromosomes become preferentially inactivated, there may not be enough healthy X chromosomes left active to compensate for the defect and the X-linked traits can be manifested (in this case, disease symptoms).
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 (has symptoms of the disease), while her twin has skewed inactivation toward the mutant chromosome and thus exhibits no symptoms (allowing the healthy X chromosomes to support normal function). 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.
http://www.sccs.swarthmore.edu/users/99/shunt/embryo.html
In the context of neuromuscular disorders, it turns out that in the average case, given random X inactivation, there are enough healthy X chromosomes functioning (those escaping inactivation) to allow for normal muscle function. However, in cases of skewed X inactivation, there may not be enough healthy X chromosomes present and some degree of muscle disease (symptomatology) may be expressed. This phenomenon is rare but is seen in X linked muscular disorders like Duchenne Muscular Dystrophy and Becker Muscular Dystrophy. In one case study, a girl manifesting a Duchenne illness was found to have all of her paternal Xs functional and none of her maternal Xs (Reference: http://www.cnmcxcis.org/Page12.html). Females who show symptoms of these diseases are now referred to as "manifesting carriers." Symptoms may manifest at any age in the female.
Several questions about skewed X inactivation remain open. These include the cause of skewed inactivation and does skewed inactivation run in families (is it controlled by genetic factors)?
Parthenogenesis is a form of reproduction in which the egg develops into a new individual without fertilization. Researchers are trying to take advantage of X inactivation to clone human cells. Human eggs have only 23 chromosomes, but as we have learned, there is a brief period (quite long in biological terms) in early embryo development (before inactivation occurs), when female cells are formed and carry two functional X chromosomes. Scientists are trying to harvest eggs from women who have illnesses and are trying to "chemically fool" the eggs to begin to develop into an "embryo." This is done without fertilization (without using sperm). The hope is that stem cells can be harvested from the "embryo" before X inactivation occurs and can be grown into normal somatic cells (having 46 chromosomes). These cells could then be used to treat various diseases in the person.
Using parthenogenesis to treat heart disease:
For example, using parthenogenesis, a woman with heart disease could have eggs
harvested and create an "embryo." Her own line of embryonic stem cells could
be harvested and developed into healthy cardiac cells. These cardiac cells could
then be transplanted into her heart to repair damaged areas. This procedure
is "easier" in women than with men (because women have two Xs and undergo inactivation),
but experiments may someday allow a similar approach with men using just sperm
cells (and no eggs). Please see this
link for more excellent information.
.
X CHROMOSOME INACTIVATION: COUNTING, CHOICE AND INITIATION
Philip Avner and Edith Heard
NATURE REVIEWS GENETICS VOLUME 2 JANUARY 2001 59-67.
Abstract:
In many sexually dimorphic species, a mechanism is required to ensure equivalent levels of gene expression from the sex chromosomes. In mammals, such dosage compensation is achieved by X chromosome inactivation, a process that presents a unique medley of biological puzzles: how to silence one but not the other X chromosome in the same nucleus; how to count the number of Xs and keep only one active; how to choose which X chromosome is inactivated; and how to establish this silent state rapidly and efficiently during early development. The key to most of these puzzles lies in a unique locus, the X inactivation centre and a remarkable RNA - Xist - that it encodes.
In mammals, dosage compensation of X-linked genes is achieved by the transcriptional silencing of one of the two X chromosomes in the female during early development -- a process known as X inactivation. The early events in X inactivation are under the control of a key regulator, the X chromosome-inactivation centre or (Xic). Initiation of X inactivation involves a recognition step in which the number of X chromosomes in the cell is counted relative to cell ploidy so that only a single X chromosome is functional per diploid adult cell. One hypothesis postulates the existence of a blocking factor that is synthesized in limiting quantities sufficient for the binding of a single (Xic) per diploid cell. Initiation is also thought to include a process of choosing, whereby one of the two X chromosomes in the female cell might be preferentially selected for inactivation. Examples include the imprinted inactivation of the paternal X chromosome in extraembryonic tissues and the biased inactivation that results from allelic differences at the X chromosome-controlling element (Xce) locus in embryonic tissues
As defined cytologically, the Xic is a roughly 1 Mb region that contains several elements thought to have a role in X inactivation (BOX 1, below) and at least four genes (FIG. 1 - not shown). One of these, the X (inactive)-specific transcript (Xist) gene, encodes a large noncoding RNA that is relatively relatively poorly conserved. Xist has been shown to contribute to Xic function and is required for X inactivation. Other elements that lie within the Xic are candidates for involvement in the control of Xist expression, or for the mechanisms of counting and choice. One is the DXPas34 locus, which was originally identified as a result of its unique methylation profile on the active X chromosome (2). Another is the Tsix transcript, a noncoding transcript that is synthesized from the strand opposite to Xist and has been hypothesized to regulate the activity of Xist at the onset of X inactivation (3).
During random X inactivation, counting and choice must either precede, or be concomitant with, the onset of initiation and its earliest manifestation, the coating of the presumptive inactive X by Xist. Silencing of X-linked genes and replication asynchrony follow rapidly. Both seem to precede global histone hypoacetylation, the accumulation of a novel histone variant (macroH2A) and methylation of the inactive X, which seem to function as maintenance mechanisms for X inactivation (4-6). However, imprinted X inactivation, which occurs in certain mammals and by which the paternal X is preferentially inactivated, might differ in some respects from random inactivation.
In this article,we review recent results concerning the events that surround the initiation of X inactivation. We place an emphasis on the role of Xist and the events that occur upstream and downstream of Xist expression.
Box 1: Xic and the elements of X inactivation
Xist
The X inactive-specific transcript (Xist) gene is expressed exclusively from the inactive X chromosome, producing a 17-kb spliced, polyadenylated transcript that is retained in the nucleus. The Xist transcript seems to be the primary signal for spreading the inactive state along the chromosome. But Xist itself does not seem to be involved in counting. Some of the elements lying outside Xist that influence counting and choice in X inactivation (see Xic) might be involved in regulating its expression.
Xic
The X chromosome-inactivation centre (Xic) was originally defined through studies on structurally abnormal X chromosomes as a master control region, the presence of which is essential for X inactivation to occur. It is responsible for initiating X inactivation in cis: an X- chromosome fragment that carries a Xic can become inactivated, whereas one in which the Xic is missing cannot. The Xic is also involved in counting, whereby only a single X is kept active per two sets of autosomes in a cell, and all other Xic -carrying chromosomes are inactivated.
Xce
The X chromosome-controlling element (Xce) affects the choice of X to be inactivated (or to remain active). In females heterozygous for different Xce alleles, an X chromosome that carries a strong Xce allele is more likely to remain active than one that carries a weak Xce allele, thereby leading to skewed X inactivation. The degree of skewing is rarely more than 70:30%.Refined genetic mapping using microsatellite markers indicate that the Xce locus might be distinct from Xist, lying distal and 3' to Xist (38).
TsiX
TsiX is an element transcribed from the antisense strand relative to Xist. Tsix
is expressed in undifferentiated ES cells and early embryos, and has been proposed
to control Xist expression in cis at the onset of X inactivation (3,
35). TsiX antisense transcription spans the whole of the Xist gene, extending
well over 40 kb. The 5' end and promoter of the TsiX gene has been proposed
to be closely associated with the DXPas34 locus, although other (weaker) promoters
might be scattered across a large region 3' to Xist (34). Targeted deletion
of the 5' end of TsiX/DXPas34 leads to nonrandom inactivation of the deleted
X chromosome (35) and a failure of imprinted X inactivation (52). This might
indicate that this locus and/or the transcript that it produces influences X chromosome
choice and imprinting of the X chromosome.
DXPas34
The DXPas34 locus is a 3 kb CpG-rich region, containing a 34-mer minisatellite
repeat lying roughly 15 kb downstream of the 3' end of Xist. DXPas34 is hypermethylated
on the active X chromosome in somatic cells. The degree of hypermethylation
was thought to correlate with allelism at the Xce locus, although Xce lies outside
the DXPas34 region (2). The principal initiation site for TsiX transcription
has been reported to lie within DXPas34 (3).
http://woldlab.caltech.edu/
X inactivation is a complex and an important feature in human genetics. The overall process is known, however, the details are still being discovered. This research is at the cutting edge of molecular genetics.
X Inactivation [Review]
Annual Review of Genomics and Human Genetics
Volume 3, September 2002 (In Press)
[A review by one of the leaders in X inactivation research.]
Defect on X chromosome linked to repeated miscarriages
By LAURAN NEERGAARD
The Associated Press
Posted on Tue, Feb. 26, 2002
WASHINGTON - Scientists studying how people inherit muscular dystrophy have stumbled onto a genetic quirk that helps explain a different medical mystery: why so many women suffer recurrent miscarriages. . .
Miscarriages are common, occurring in about 20 percent of pregnancies. Most of those women later have a baby, but
1 percent to 2 percent of women suffer repeated miscarriages. Doctors discover a cause, such as uterine
abnormalities, for only half, leaving the rest with no explanation and no treatment.
Enter Eric Hoffman, an expert on the genetics of Duchenne muscular dystrophy, one of the deadliest inherited
diseases. It usually hits boys, but Hoffman diagnosed it in a girl. Her mother got 53 relatives to consent to genetic
testing to better understand why her daughter got sick.
The X and Y chromosomes are the sex chromosomes. Boys have an X and Y, and girls have two Xs, one inherited
from each parent. Cells need only one active X chromosome, so every cell of a girl's body turns off one of those Xs.
Typically, half the cells use the mother's X and the others use that of the father.
But Hoffman's patient was using only her father's X chromosome, and half the women in the girl's large extended
family had the same abnormality. More intriguing, the large family had only half the boys expected from normal
reproduction patterns -- and a lot of miscarriages.
"We said, `Uh-oh, what's going on?' " recalled Hoffman, genetics research chief at Children's National Medical Center
in Washington. Could this "skewed X inactivation," which otherwise did not harm the women, be a significant cause
of recurrent miscarriages?
Hoffman's staff tested 100 miscarriage sufferers for whom every other cause was ruled out. They found 14 percent
had the X flaw. In contrast, only one of 100 mothers who had never had a miscarriage had the genetic flaw.
A few of the women with the X flaw had had successful pregnancies -- three-fourths of the babies were girls. If they
did not inherit an active X chromosome, male fetuses were doomed, Hoffman explained.
Dr. Eric Hoffman is a key figure in genetic research and X inactivation.
He is linked into an excellent webpage that includes a genetics primer and a great deal of information on X inactivation.
The page is at: http://www.cnmcxcis.org/index.html
The origins of genomic imprinting in mammals
Frank Sleutels and Denise P. Barlow
Chapter 1, Advances in Genetics, Volume 46, Homology Effects,
Editors: C. -Ting Wu, Jay C. Dunlap, Academic Press, USA, 2002.
Abstract: Mammals are diploid organisms that inherit a complete chromosome set
from each parent. The vast majority of genes are equally expressed from both
parental chromosomes. However, in a small subset of genes, a process known as
genomic imprinting results in parental-specific gene expression. The imprinted
gene is expressed on one parental chromosome, but silent on the other. Parental
specific gene expression does not result from genetic changes, but instead results
from modifications to DNA or chromatin that are described as "epigenetic". Genomic
imprinting is thus an exceptionally good tool to study epigenetic gene regulation
because both the active and silent allele are retained in the same nucleus.
Historically, imprinting has only been viewed as a gene-silencing mechanism.
However, the actual data obtained from studies of imprinted genes, are not compatible
with this view and, instead, indicates that imprints were in many cases, acquired
as a gene-activating mechanism. To accommodate these findings, a model is proposed
whereby imprints can activate, or de-repress, genes previously silenced by an
epimutation.
"Chromosomal Silencing and Localization are Mediated by Different Domains of Xist RNA".
Anton Wutz, Theodore P. Rasmussen / Rudolf Jaenisch
Whitehead Institute for Biomedical Research, Nine Cambridge Center, Cambridge, Massachusetts 02142,USA.
Nature Genetics, vol. 30, no. 2, pp. 167-174 (February, 2002):
Abstract:
The gene Xist initiates the chromosomal silencing process of X inactivation in mammals. Its product, a noncoding RNA, is expressed from and specifically associates with the inactive X chromosome in female
cells. Here we use an inducible Xist expression system in mouse embryonic stem cells that recapitulates long-range chromosomal silencing to elucidate which Xist RNA sequences are necessary for chromosomal association and silencing. We show that chromosomal association and spreading of Xist RNA can be functionally separated from silencing by specific mutations. Silencing requires a conserved repeat sequence located at the 5' end of Xist. Deletion of this element results in Xist RNA that still associates with chromatin and spreads over the chromosome but does not effect transcriptional repression. Association of Xist RNA with chromatin is mediated by functionally redundant sequences that act cooperatively and are dispersed throughout the remainder of Xist but show little or no homology.
Published online: 7 January 2002, DOI:10.1038/ng820
The Sex Chromosomes and X Chromosome Inactivation
By: Huntington Willard
Chapter 61, pages 1191-1211, in: The Metabolic and Molecular Bases of Inherited Disease, vols 1-4.
edited by Charles R. Scriver, Arthur L. Beaudet, Williams S. Sly, and David Valle,
8th ed, ISBN 0-07-913035-6, New York, NY, McGraw-Hill, 2001.
A comprehensive review.
Every Cell Has a Sex
Chapter 2,
Theresa M. Wizemann and Mary-Lou Pardue, Editors
Exploring the Biological Contributions to Human Health: Does Sex Matter?
Committee on Understanding the Biology of Sex and Gender Differences
NATIONAL ACADEMY PRESS Washington, D.C. 2001
Abstract.
The biological differences between the sexes have long been recognized at the
biochemical and cellular levels. Rapid advances in molecular biology have revealed
the genetic and molecular bases of a number of sex-based differences in health
and human disease, some of which are attributed to sexual genotype - XX in the
female and XY in the male. Genes on the sex chromosomes can be expressed differently
between males and females because of the presence of either single or double
copies of the gene and because of the phenomena of different meiotic effects,
X inactivation, and genetic imprinting. The inheritance of either a male or
a female genotype is further influenced by the source (maternal or paternal)
of the X chromosome. The relatives roles of the sex chromosome genes and their
expression explains X chromosome-linked disease and is likely to illuminate
the reasons for heterogeneous expression of some diseases within and between
the sexes.
X-CHROMOSOME INACTIVATION: COUNTING, CHOICE AND INITIATION
Philip Avner and Edith Heard
NATURE REVIEWS - GENETICS, VOLUME 2, JANUARY 2001
Abstract:
In many sexually dimorphic species, a mechanism is required to ensure equivalent
levels of gene expression from the sex chromosomes. In mammals, such dosage
compensation is achieved by X chromosome inactivation, a process that presents
a unique medley of biological puzzles: how to silence one but not the other
X chromosome in the same nucleus; how to count the number of X's and keep only
one active; how to choose which X chromosome is inactivated; and how to establish
this silent state rapidly and efficiently during early development. The key
to most of these puzzles lies in a unique locus, the X inactivation centre and
a remarkable RNA-Xist-that it encodes.
[This is a very technical but comprehensive review article.]
Making sense (and antisense) of the X inactivation center
Huntington F. Willard and Laura Carrel
Proc. Natl. Acad. Sci. USA, Vol. 98, Issue 18, 10025-10027, August 28, 2001
Abstract:
The concept of the X inactivation center (Xic) as the master regulatory locus for X inactivation dates back to the mid-1960s (1, 2), not long after X chromosome inactivation itself was proposed as the mammalian dosage compensation mechanism (3-5). Defined genetically as the cis-acting [acting on the same side] locus required for an X chromosome to undergo inactivation early in female embryogenesis, the Xic defied molecular characterization for nearly 30 years until the discovery of the XIST gene (6), whose noncoding RNA product is transcribed from and remains intimately associated with the inactive X chromosome in female somatic cells (6-9). Xist in both humans and mice maps to the Xic region on the X chromosome and thus became a compelling candidate for a component of the Xic itself (10-12). Biology is rarely as simple as it first appears, however, and X inactivation embodies this principle fully! Only a few years ago, Lee and colleagues (13) described another component of the Xic just downstream of Xist, the Tsix gene, so-named because it consists of an antisense transcript of Xist and whose pattern of expression suggested a potential role as a regulator of Xist. Now, in this issue of PNAS, Stavropoulos et al. (14) extend these studies to address a fundamental question concerning the potential function of Tsix does its antagonistic relationship to Xist depend on its own noncoding RNA product or does it reflect actions at the DNA level, independent of transcription?
What evidence implicates Xist or Tsix as functional components of the Xic? The process of random X inactivation consists of multiple steps, responsible for assessing the number of X chromosomes in the developing mammalian embryo (counting), designating one or the other X as the future active or inactive X chromosome (choice), initiating and promulgating this choice along the length of the inactive X chromosome in cis [on the same side] to silence most of the genes on that X (initiation), and stably maintaining the inactive chromatin state through subsequent cell divisions (maintenance) (reviewed in refs. 15-17). [The choice step is predetermined in the imprinted form of X inactivation, in which the paternally inherited X is always the inactive X, as is seen in marsupials and in extraembryonic tissues in eutherian mammals (18).]
From: http://www.pnas.org/cgi/content/full/98/18/10025
"A Functional Role for Tsix Transcription in Blocking Xist RNA Accumulation but Not in X Chromosome Choice"
Nicholas Stavropoulos, Naifung Lu, and Jeannie T. Lee
Howard Hughes Medical Institute, Department of Molecular Biology, Massachusetts General Hospital, and Department of Genetics, Harvard Medical School, Boston, MA 02114
Proc. Natl. Acad. Sci. USA, vol. 98, no. 18, pp. 10232-10237 (August 28, 2001): 10.1073/pnas.171243598
Abstract:
In female mammals, upregulation of Xist triggers X chromosome inactivation in
cis. Upregulation is inhibited by sequences 3' to Xist contained within
the antisense locus, Tsix. Inhibition could depend on transcription of Tsix
and/or on DNA elements therein. Here we test the role of Tsix transcription
by augmenting the duration and strength of Tsix expression. We find that Tsix
hypertranscription is sufficient to block Xist RNA accumulation in a cis-limited
manner. We propose that Tsix transcription is necessary to restrict Xist activity
on the future active X and, conversely, that Tsix repression is required for
Xist RNA accumulation on the future inactive X. We also find that Tsix hypertranscription
does not affect X chromosome choice. Thus, choice is mediated by elements within
Tsix that are independent of promoter activity.
http://www.euchromatin.net/Stavropoulos1.htm
Protein May Play Role in Sex Chromosome Inactivation
Lee, J. T.
Science, December 7, 2001
The decision to inactivate one of the two X chromosomes, which occurs early in development, is a life-or-death decision that is made in the female eggs of all mammals, including humans. For reasons that are still not completely understood, failure to choose and completely shut down one X chromosome means immediate death for the developing embryo, due to a genetic 'dosage' imbalance.
Researchers studying the X inactivation process have now identified a molecule, called CTCF, which appears to be central to regulating X chromosome inactivation. The research team, which was led by Jeannie T. Lee, a Howard Hughes Medical Institute investigator at Massachusetts General Hospital, published its findings in the December 7, 2001, issue of Science.
http://www.hhmi.org/news/lee.html
X inactivation: Tsix and Xist as yin and yang.
Mlynarczyk, S. M. and B. Panning.
Current Biology 10, R899-903. 2000.
Abstract:
A new study shows that expression of Tsix, an antisense Xist gene, can be controlled by imprinting, and that high Tsix activity during X inactivation can protect the future active X chromosome from silencing by Xist. Tsix and Xist seem to have a yin and yang relationship, with opposite effects on X inactivation.
Molecular evidence for a relationship between LINE-1
elements and X chromosome inactivation: The Lyon
repeat hypothesis
Jeffrey A. Bailey, Laura Carrel, Aravinda Chakravarti, and Evan E. Eichler
6634-6639 / PNAS /June 6, 2000 /vol. 97 /no. 12
X inactivation is a chromosome-specific form of genetic regulation in which
thousands of genes on one homologue become silenced early in female embryogenesis.
Although many aspects of X inactivation are now understood, the spread of the
X inactivation signal along the entire length of the chromosome remains enigmatic.
Extending the Gartler-Riggs model [Gartler, S. M. and Riggs, A. D. (1983) Annu.
Rev. Genet. 17, 155-190], Lyon recently proposed [Lyon, M. F. (1998) Cytogenet.
Cell Genet. 80, 133-137] that a nonrandom organization of long interspersed
element (LINE) repetitive sequences on the X chromosome might be responsible
for its facultative heterochromatization. In this paper, we present data indicating
that the LINE-1 (L1) composition of the human X chromosome is fundamentally
distinct from that of human autosomes. The X chromosome is enriched 2-fold for
L1 repetitive elements, with the greatest enrichment observed for a restricted
subset of LINE-1 elements that were active less than 100 million years ago.
Regional analysis of the X chromosome reveals that the most significant clustering
of these elements is in Xq13-Xq21 (the center of X inactivation). Genomic segments
harboring genes that escape inactivation are significantly reduced in L1 content
compared with X chromosome segments containing genes subject to X inactivation,
providing further support for the association between X inactivation and L1
content. These nonrandom properties of L1 distribution on the X chromosome provide
strong evidence that L1 elements may serve as DNA signals to propagate X inactivation
along the chromosome.
From: http://www.pnas.org/cgi/reprint/97/12/6634.pdf
Silence of the Xs.(X chromosome inactivation)
Author: John Travis
Science News Online
Issue: August 5, 2000
A good, fairly non-technical review of the history of X inactivation research.
See: http://www.findarticles.com/m1200/6_158/64697776/p1/article.jhtml
Targeted Mutagenesis of Tsix Leads
to Nonrandom X Inactivation
Jeannie T. Lee, and Naifang Lu
Cell, Vol. 99, 47-57, October 1, 1999,
Summary
During X inactivation, mammalian female cells make the selection of one active
and one inactive X chromosome. X chromosome choice occurs randomly and results
in Xist upregulation on the inactive X. We have hypothesized that the antisense
gene, Tsix, controls Xist expression. Here, we create a targeted deletion of
Tsix in female and male mouse cells. Despite a deficiency of Tsix RNA, X chromosome
counting remains intact: female cells still inactivate one X, while male cells
block X inactivation. However, heterozygous female cells show skewed Xist expression
and primary nonrandom inactivation of the mutant X. The ability of the mutant
X to block Xist accumulation is compromised. We conclude that Tsix regulates
Xist in cis and determines X chromosome choice without affecting silencing.
Therefore, counting, choice, and silencing are genetically separable. Contrasting
effects in XX and XY cells argue that negative and positive factors are involved
in choosing active and inactive Xs.
A first-generation X inactivation profile of the human
X chromosome
Laura Carrel, Amy A. Cottle, Karrie C. Goglin, and Huntington F. Willard
PNAS / December 7, 1999 / vol. 96 / no. 25 /
Summary:
In females, most genes on the X chromosome are generally assumed to be transcriptionally
silenced on the inactive X as a result of X inactivation. However, particularly
in humans, an increasing number of genes are known to ''escape'' X inactivation
and are expressed from both the active (Xa) and inactive (Xi) X chromosomes;
such genes reflect different molecular and epigenetic responses to X inactivation
and are candidates for phenotypes associated with X aneuploidy. To identify
genes that escape X inactivation and to generate a first-generation X inactivation
pro- file of the X, we have evaluated the expression of 224 X-linked genes and
expressed sequence tags by reverse-transcription-PCR analysis of a panel of
multiple independent mouse/human somatic cell hybrids containing a normal human
XI but no Xa. The resulting survey yields an initial X inactivation profile
that is estimated to represent '10% of all X-linked transcripts. Of the 224
transcripts tested here, 34 (three of which are pseudoautosomal) were expressed
in as many as nine XI hybrids and thus appear to escape inactivation. The genes
that escape inactivation are distributed nonrandomly along the X; 31 of 34 such
transcripts map to Xp, implying that the two arms of the X are epigenetically
and/or evolutionarily distinct and suggesting that genetic imbalance of XP may
be more severe clinically than imbalance of Xq. A complete X inactivation profile
will provide information relevant to clinical genetics and genetic counseling
and should yield insight into the genomic and epigenetic organization of the
X chromosome.
X linked Inheritance: Hemophilia a clear diagram.
See: http://www.accessexcellence.org/AB/GG/x-linked.html
X Chromosome Inactivation Patterns in Normal Females
Omar Racchi et al
Blood Cells, Molecules, and Diseases (1998) 24(21) Nov 15: 439 447
ABSTRACT:
Since one of the two X chromosomes is randomly inactivated at an early stage of female
embryonic development, X linked markers have been used to study the origin and development of
various neoplastic disorders in affected heterozygous women; clonality assays have provided a useful tool to the understanding of the mechanisms underlying the development of neoplasia. Recently, a technique of clonal analysis has been devised that takes advantage of a highly polymorphic short tandem repeat within the X-linked human androgen receptor (AR) gene, resulting in a heterozygosity rate approaching 90%. The rapid expansion of the number of women now suitable for X inactivation analysis has however given rise to new controversies, one of the more troublesome being the possibility of a modification of the pattern of X chromosome inactivation pattern in blood cells of elderly women. In the present study we analyze with the AR assay a group of 166 healthy females aged between 8 and 94 years, with no history of genetic or neoplastic familial disorders. We failed to find any correlation between age and X chromosome inactivation pattern ( r = 0.17), even subdividing the subjects in different age groups according to the criteria used by other researchers, and therefore reaffirm that, when tested for with well-standardized and accurate criteria, extremely unbalanced inactivation of the X chromosome is a truly uncommon phenomenon in normal women.
From: www.scripps.edu/bcmd/pdfarea/issue_21_98/racchi.pdf
Genomic Imprinting.
C. Cristofre Martin.
Abstract
Genomic imprinting can be loosely defined as the gamete-of-origin dependent modification of phenotype. That is, the phenotype elicited from a locus is differentially modified by the sex of the parent contributing that particular allele. This process results in a reversible gamete-of-origin specific marking of the genome that ultimately produces a functional difference between the genetic information contributed by each parent.
In mammals, the term genomic imprinting has been restricted to describing mono-allelic gene expression or the inactivation of either the maternal or paternal allele of a particular locus.
From: http://www.ucalgary.ca/UofC/eduweb/virtualembryo/imprinting.html
X chromosome inactivation
in mammals.
Heard, E., Clerc, P. and Avner, P.
Annu. Rev. Genet. 31, 571-610 (1997).
A thorough review of the older X-inactivation literature.
The X Chromosome and the Female Survival Advantage: An Example of the Intersection between Genetics, Epidemiology and Demography.
Kaare Christensen, Karen-Helene Orstavik , James W.Vaupel
PDF file (no date or source listed)
This chapter will illustrate how a genetic observation combined with demographic
insight and a modified genetic - epidemiological design (a twin study) provides
evidence that part of the sex difference in survival can be attributed to the
fact that females have two X chromosomes and males have only one, a result which
is of potential interest for genetics, epidemiology, and demography.
http://cph.georgetown.edu/Conference/Christensen.pdf
Genetic Mutation Seen In Young Breast Cancer Patients
NEW YORK Jan 28, 2002 (Reuters Health) - Instead of the normal 50:50 distribution, the pattern of inactivated paternal or maternal X chromosomes is skewed in young women with breast cancer.
The researchers who report this finding, in the January issue of the Journal of Medical Genetics, surmise that a germline mutation in an X-linked tumor suppressor gene might give a proliferative advantage to cells in which the activated X chromosome carries the mutation. This would both skew the inactivation pattern and increase the risk of cancer.
Dr. K. H. Orstavik, from Rikshospitalet University Hospital, Oslo, Norway, and colleagues performed PCR analysis to determine the pattern of X inactivation among 216 women identified with breast cancer from 1984 to 1994. When diagnosed with breast cancer, these women were 27 to 90 years of age. For comparison, X inactivation analysis was performed for a control group in the same age range.
Protein May Play Role in Sex Chromosome Inactivation
December 7, 2001 - The decision to inactivate one of the two X chromosomes,
which occurs early in development, is a life-or-death decision that is made
in the female eggs of all mammals, including humans. For reasons that are still
not completely understood, failure to choose and completely shut down one X
chromosome means immediate death for the developing embryo, due to a genetic
'dosage' imbalance.
Researchers studying the X inactivation process have now identified a molecule, called CTCF, which appears to be central to regulating X chromosome inactivation. The research team, which was led by Jeannie T. Lee, a Howard Hughes Medical Institute investigator at Massachusetts General Hospital, published its findings in the December 7, 2001, issue of Science.
see: http://www.hhmi.org/news/lee.html
http://www.molgen.mpg.de/%7Exteam/xlinks.html
Mechanisms of X inactivation:
See: http://www.ultranet.com/~jkimball/BiologyPages/S/SexChromosomes.html
Demonstration of X Inactivation:
a. In Calico cats The classic mosaic example is Calico cats. One of the genes for hair color is found on the X chromosome. The gene makes either orange or black (white is on a different chromosome).
Each colored patch on a calico has a different X turned on. If the patch is orange, then there are a bunch of cells there with the orange X on. A black patch has a bunch of cells with the black patch on.
This is why a male calico is so rare. Most males only have a single X and so either the orange or the black hair color gene, not both.
From: http://www.thetech.org/genetics/ask.php?id=141
b. The mosaic effect is seen in human females affected by anhidrotic ectodermal dysplasia in which a mutant gene on one X chromosome results in patches of skin with no sweat glands.
Potential Connections Between Female Monozygotic Twinning and
X chromosome Inactivation
Introduction
Monozygotic twins, frequently called identical twins due to the striking similarity these pairs usually exhibit, inspire
a particular interest because of the genetic implications of two individuals who are genetically identical. Many
potential reasons for monozygotic twinning events have been proposed, although it is not definitively known which
events are primarily responsible. Later monozygotic twinning events seem to favor female twins, thus placing an
interesting twist on this question. Medical cases in which female monozygotic twins display
discordant phenotypes for X-linked diseases has pointed a finger at X chromosome inactivation as a possible
player in the cell splitting that results in such twins.
See: http://www.sccs.swarthmore.edu/users/99/shunt/embryo.html
Manifesting carriers:
BUT GIRLS DON'T GET DUCHENNE -- OR DO THEY? When DMD 'Carriers' Have Trouble of their Own by Margaret Wahl
Quest: Volume 5, Number 6, December 1998
See: http://www.mdausa.org/publications/Quest/q56girlsdmd.html
Manifesting carriers: Excerpts from an article from MDA (USA)
Dr. Kate Bushby is a clinician and researcher based at our Newcastle Muscle Centre (Department of Human Genetics at the University of Newcastle upon Tyne).
Heidi Short: "I started falling over when I was about 12. I went to different doctors but as there was no family history of the disorder, they thought it was just a phase I was going through. I felt awful through my school years. I hated PE and missed several weeks from each term. It felt like having permanent flu. When my middle child - Sam - was diagnosed with DMD, one of the family care officers suggested I should be tested. It has helped a great deal to know. I feel that I've got some credibility now. People won't think it's all in my mind".
Tracy McMahan: "I always had a peculiar walk - my pelvis sticks out and my back arches - but when people commented on it I couldn't explain. It was just me! After my son Liam was born, I became a lot weaker; I couldn't climb the stairs and lift him up. The condition was diagnosed at Oxford by Doctor Hilton-Jones' team. I was much happier after that, knowing what I had. Now if I have any questions there are people to ring, like Jane my family care officer, and that's so comforting".
These are quotations from two women who are now known to be 'manifesting carriers of Duchenne MD'. Until recently, this condition has been little understood and rarely diagnosed. Women and girls have struggled to get their symptoms recognized and accurately identified. However, with new genetic technology and increasing knowledge about Duchenne and Becker muscular dystrophy, the situation is improving. Dr. Kate Bushby of Newcastle explains: "The most usual situation in Duchenne and Becker muscular dystrophy is that women carry the faulty gene and pass it on to their children, but only their sons show symptoms. Some women we know, however, do show symptoms relating to the genetic fault themselves and we call this being a manifesting carrier. Amongst women known to be carriers of Duchenne and Becker muscular dystrophy (DMD and BMD), one in ten might fall into this category".
What symptoms does a manifesting carrier have?
There are as wide a variety of presentations of manifesting carriers as there are of males with DMD and BMD. Some girls may have a muscular dystrophy that is as severe as boys with DMD. Other women may only have very mild problems with muscle weakness late in adult life. Some women get aches and pains in their muscles as their first complaint and may notice enlargement of their calves and other muscles. There are other manifestations of Duchenne muscular dystrophy, for example involvement of intellectual function and of heart muscle. Problems in these areas can be the only signs that someone is a manifesting carrier. These presentations are less common than a woman noticing a mild but progressive weakness involving usually first her legs but later on the arms as well. Most manifesting carriers notice some progression of their muscle problem with time; however it is rare for there to be any sudden deterioration.
Do all manifesting carriers come from families where people are known to have Duchenne or Becker muscular dystrophy?
No. It is very important to note that the diagnosis of a manifesting carrier may be made in someone who has absolutely no family history of DMD or BMD at all. In the days before it was possible to look at the dystrophin gene and protein by special laboratory tests, a female could only be suspected of being a manifesting carrier of Duchenne or Becker muscular dystrophy if she had a definite family history of the condition or had an affected son herself. Now that it is possible to look at muscle biopsy samples directly with dystrophin staining, the diagnosis has been made in women and girls who were previously thought to have a form of limb-girdle muscular dystrophy.
These people usually have no family history of any muscle problems whatsoever. It is an important diagnosis to make in these cases because, unlike most of the forms of limb-girdle muscular dystrophy, the risk of having a child who may be affected with DMD or BMD is relatively high. Like any carrier of Duchenne or Becker muscular dystrophy, a manifesting carrier has a one in four chance of having an affected son in any pregnancy. There is no clear evidence to suggest that being a manifesting carrier tends to run in families. So if one carrier in any family has muscle problems this does not seem to make it any more likely that other members of the family will also manifest any problems relating to being a carrier.
Why do some women run into problems like this?
It is thought that people who are manifesting carriers of any X-linked condition such as DMD and BMD generally have some problem in the mechanism which we know as 'X inactivation'. Because men only have one X chromosome and women have two, one copy of the X chromosome in every cell in a woman's body is 'switched off' very early in development. The process is usually random; on average half the cells have one X chromosome active and the other half the other. In people who are manifesting carriers of Duchenne and Becker muscular dystrophy most cells have the faulty X chromosome active. The X inactivation patterns can be studied in blood and most manifesting carriers show a nonrandom pattern. Some manifesting carriers may show an apparently random pattern in blood. It is thought that these women may have skewed inactivation only or mainly in their muscle cells.
How can the condition be diagnosed?
It is best to try and perform a number of tests to be absolutely sure that someone is a manifesting carrier. A muscle biopsy can be extremely helpful because it is relatively easy to look at patterns of dystrophin in muscle using antibodies to dystrophin. The sort of pattern which we often see in manifesting carriers is that of a mixture of staining patterns in different muscle fibres, some labeling nearly normally for dystrophin, others labeling weakly and sometimes others which can be completely negative. The finding of this without a major disruption of the proteins that link in with dystrophin can be diagnostic of a manifesting carrier of Duchenne or Becker muscular dystrophy. As additional proof one can also look at X inactivation patterns in blood because most manifesting carriers have a nonrandom pattern. It is also possible though not as easy to look for he problem in the dystrophin gene itself in manifesting carriers. This usually requires relatively specialized tests which are not available everywhere.
What follow-up should manifesting carriers be getting?
This really depends on how much they are affected by their problem: For children who are affected with muscular dystrophy, whether they are boys or girls, it is appropriate for them to be seen at a muscle clinic. Adults may also find it helpful to attend a specialist clinic where they will find physiotherapy and family care officer back-up which may help them with their problems. However, adults with relatively minor problems may not need regular follow-up, providing their family doctor is aware of their problems and can refer them to the appropriate clinic should their symptoms get worse.
Above excerpted from: http://www.muscular-dystrophy.org/about_your_condition/manifesting_carriers/index.html
.