X-chromosom inactivation phenomenon
In eutherian mammals, sex is determined by the sex chromosomes, the X and the Y. Males are XY and females are XX. The X chromosome contains numerous important genes. In contrast, only a very few functional genes are found on the Y chromosome, composed of different species-specific types of highly and moderately repetitive noncoding DNA. As a result, the majority of X-linked genes are present as one copy in males (XY) and as two copies in females (XX). It is believed that mammals have evolved a mechanism to equalize the expression levels of X chromosome genes in males and females. This occurs in early development by switching off or inactivating one of the two X chromosomes in females (Lyon, 1961).
Inactivation of the X-chromosome is the process, during which in early embryogenesis one of genetically equivalent X-chromosomes in mammal females becomes transcriptionally inactive (Xi) and undergoes a number of structural rearrangements. At the preimplantation stages of embryonic development, imprinted inactivation of the paternally inherited X-chromosome (Xp) takes place. This inactivation is further preserved in trophectoderm, extra-embryonic endoderm, and their derivatives. During implantation of the embryo, Xp is reactivated in the epiblast cells with further random inactivation of either paternal or maternal X-chromosome.
Random Inactivation of the X Chromosome in Eutherian Females
Random inactivation of the X chromosome in mammals is a multistep process (see Heard and Disteche, 2006). Before the inactivation, the number of X chromosomes per diploid autosomal set is counted and the future active and inactive X chromosomes are alternatively selected (choice). The stages of count, choice, and initiation inactivation is controlled by specific X-chromosome locus, X inactivation center (XIC), the main element of which is the gene Xist (X inactive specific transcript). At the moment of initiation, the untranslated RNA of Xist gene accumulates on the X chromosome, leading to its inactivation and condensation. Later the inactive state of X chromosome is stably maintained during the subsequent mitotic divisions. Besides Xist, two additional genes have been discovered in the mouse XIC, namely, Tsix and Xite, which produce nuclear untranslated RNAs. These two genes are involved in the X-chromosome counting and alternative selection of the future active and inactive X chromosomes and play an important role in the regulation of Xist gene expression.
The random inactivation process includes:
- count of X-chromosomes per haploid set;
- choice of which X-chromosome to inactivate;
- initiation X inactivation;
- proliferation and maintenance of inactivity through cell generations.
An important approach to understanding mechanisms controlling initiation of X inactivation and Xist/Tsix expression has been to study systems exhibiting primary non-random X inactivation. In such cases the randomness of X inactivation is skewed as a result of a bias in the X chromosome selected to be inactive. This is thought to be effected through biasing the choice of which Xist allele is expressed or repressed. The classical example of primary non-random X inactivation is the X controlling element (Xce) allelic system in mouse (Cattanach and Isaacson 1967). Studies to date indicate that non-random X inactivation is attributable to allelic differences close to but genetically separable from the Xist locus (Simmler et al 1993). The underlying sequence differences have not been identified so far.
Another example of primary non-random X inactivation is provided by interspecific F1 crosses between different species of the common vole Microtus arvalis. Crosses between M.arvalis and anothe species of «arvalis» group has active X-cromosome from M.arvalis (Zakian et al 1987). Again the precise mechanisms underlying this phenomenon are unknown but are thought to involve factors that influence the choice of which Xist allele is expressed at the onset of X inactivation. The Xist/Tsix locus is involved in regulating the initiation of X inactivation and is therefore likely to be central to understanding mechanisms of non-random X inactivation. As a result of the project we have identified candidate sequence variants in the Xist/Tsix locus that may be responsible for non-random X inactivation in F1 vole hybrids
Projects for X-inactivation studing
- The study of molecular and epigenetic mechanisms of X-inactivation (Human ES cells, vole TS, Xen cells);
- The study of Xist/Tsix regulation (Common voles);
- The study of chromatin modifications (Common voles);
- The study of gene expression from the inactive X-chromosome (Common voles);
- The study of X-chromosome inactivation in the meiosis sex body formation (Common voles);
- The study of X-chromosome inactivation in marsupials (American opossum);
- The study of early developmental genes maintainig the sterm cells proliferation (Common voles);
- The study of early developmental genes in human embrionic stem cells;
- The study of the reprogrammig vole somatic cell genom in case of the fusion with ES cells (mouse)
- Derivation of the human and vole indused pluripotens cells (IPs)