By MingMing Yang
Physiologically speaking, men and women have clear, visible differences. From body shape to voice pitch, to the reproductive organs, we can easily distinguish between the two sexes. Psychologically, there are also known sex differences in personality and behaviours(Schmitt, 2017). Indeed, “Men are from Mars, Women are from Venus”, the differences between sexes are undeniable. However, this huge difference has a very tiny origin – the sex chromosomes, two out of the forty-six chromosomes, which determined who we are.
To visualize the complete set of genetic information of an organism, scientists developed a method called karyotyping, a process of pairing and ordering all the chromosomes of an organism, providing a genome wide snapshot of an individual’s chromosomes(O’Connor, 2008). In males, the sex chromosomes contain a X chromosome and a Y chromosome, whereas female show the presence of two X chromosomes (What is the Difference Between Male and Female Karyotypes. 2019).
The Y chromosome is one of the smallest chromosomes of the human genome. Compared to other human chromosomes, it has a limited number of genes, and most of them code for male-specific characteristics, contributing to male germ cell development and maintenance, thus determining sex in human. One of the most important genes identified is the SRY (Sex-determining Region on the Y chromosome) gene. It is located on the short arm of the Y chromosome and has been shown to be essential for initiating testis development and the differentiation of the indifferent, bipotential, gonad into the testicular pathway. SRY has also been proposed to be the master gene regulating the cascade of testis determination.(Quintana-Murci & Fellous, 2001).
Unlike the gene-poor Y chromosome, the X chromosome contains over 1,000 genes that are essential for proper development and cell viability. Comparing size of the X and Y chromosome on a karyotype, X chromosome is also significantly bigger than Y chromosome. This theoretically leads to a problem in females, as the presence of two copies of the X chromosome should result in a lethal double dose of X-linked genes. However, this is not the case. Mammalian females have evolved a unique mechanism of dosage compensation to correct this imbalance, by a process called X-chromosome inactivation. During development, one of the two X chromosomes of female mammals is transcriptionally silenced in a complex and highly coordinated manner, which then compact into a condensed structure called a Barr body. This region of cells is then stably maintained in a silenced state, without the expression of genes(Ahn & Lee, 2008).
X inactivation is triggered by the expression of the X inactive specific transcript (Xist). It is a long non-coding RNA that has the unique property of binding to and coating the chromosome from which it is transcribed. It is activated only in cells with more than one X, and therefore it is not expressed in male cells. Xist RNA is thought to recruit silencing factors that modify the chromatin, bringing about a mitotically stable heterochromatic configuration that can be propagated through subsequent cell divisions(Nesterova et al., 2008). Since it is difficult to study human embryos due to ethical concerns, the process of X inactivation is intensively studied in mice. An important gene identified that regulates the expression of Xist and thus X inactivation is Tsix, an antisense transcript of Xist. Genetic evidence from mouse models indicates that Tsix represses Xist expression in cis. Before the signal that initiates random X-chromosome inactivation is received, Xist and Tsix are both transcribed from all active X chromosomes in each female cell. Once inactivation is initiated during embryonic development, Tsix becomes turned off on one of the two Xs, which permits the upregulation of Xist from that locus and spread in cis from their site of synthesis to coat the entire X chromosome and establish transcriptional silencing. Tsix expression in another X chromosome persists and “protects” it from expressing Xist and being inactivated. Xist RNA continues to coat the silenced X chromosome throughout all subsequent cell divisions, where it contributes to the maintenance of silencing(Panning, 2008; Sun & Tsao, 2008).
X-inactivation is a random process in human embryonic tissues. In other words, the paternal and maternal X chromosome in all somatic cells of female have equal chances to be silenced, resulting in a mosaic of cells in every female, expressing exclusively the paternal or maternal X chromosome. This randomness gives females a chance to cope with X-linked mutations(Ahn & Lee, 2008). However, several processes can occur that disturb this randomness and lead to a predominance of maternal or paternal expression, also known as ‘skewed X-inactivation’. Sometimes, this process happens stochastically. Other times, there may be genetic modifiers or polymorphisms that bias the cell to choose a particular chromosome. This bias which reflects a disturbance to the process of randomness is known as primary non-random X-inactivation. Another reason explaining this phenomenon is related to the selection process, in which, when one X chromosome contains a gene or genes that provide a growth advantage or disadvantage, overall cell ratio may favour the expression of one or the other X after several cell divisions. This preserved randomness of the initial ‘choice’ but with skewing because of downstream selective effects is called secondary non-random X-inactivation(Sun & Tsao, 2008).
This skewed X-inactivation can affect females who are heterozygous for certain X-linked mutations, which in more severe cases, can manifest X-linked diseases that are usually only phenotypically seen in male like haemophilia and Duchenne muscular dystrophy. These carriers have a diseased phenotype which varies from normal to affected, depending on the degree of mosaicism. However, a direct association between clinical phenotype and the X‐inactivation phenotype is yet to be found(Brown, 1999; Ørstavik, 2006).
Interestingly, despite this chromosome wide inactivation, it has been discovered that around 15% of X-linked genes are devoid of Xist coating and escape from inactivation. Escape genes are important to human, as deficiency in these genes is thought to play a major role in phenotypes observed in Turner syndrome, a disease of which women only possess one X chromosome (X chromosome monosomy; 45,X). Females with this disease show severe phenotypes including ovarian dysgenesis, short stature, webbed neck, and other physical abnormalities(Berletch et al., 2011). Furthermore, escape from X inactivation can also cause phenotypes in individuals with additional copies of the X chromosome. The most common disorder of sex chromosomes in humans is the XXY aneuploidy, which affects one in five hundred males with Klinefelter syndrome. Affected males are usually infertile, and gynecomastia, sparse body hair and smaller testicular size are usually observed. Their secondary sexual characteristics cannot be fully developed due to lowered androgen production (Visootsak & Graham, 2006).
X-inactivation has been studied for a long time, but a lot of underlying mechanisms are still yet to be discovered. The study of X inactivation may also provide insight into cancer biology, as two active Xs have been found in many human breast and ovarian tumours(Liao et al., 2003). We should therefore remain hopeful in the future investigation of X chromosome.
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