Introduction to Epigenetics II

KEY CONCEPT
- Many biological processes, including X chromosome inactivation and genomic imprinting, are mediated through epigenetic mechanisms such as DNA methylation. 
The process of X chromosome inactivation in female (eutherian) mammals is a random process between the maternally and paternally derived X chromosomes. The X-inactivation center, or Xic, serves as the locus that ultimately determines X-inactivation. A key gene that is transcribed from the Xic is known as Xist (X inactive-specific transcript). Xist is a nontranslated RNA molecule that acts in cis to silence the X chromosome from which it is transcribed. The X-inactivation process is mediated by epigenetic processes, including DNA methylation, that maintain the inactive X in a silent state.
Genomic imprinting also relies on epigenetic processes, especially DNA methylation, for marking specific maternally or paternally derived genes. The expression of these genes during early development contributes to many biological phenotypes, including embryonic and postnatal growth. Moreover, aberrations of imprinting can lead to a number of imprinting diseases, such as Prader–Willi and Angelman syndromes.
Epigenetic processes may also directly impact proteins as well as nucleic acids, and an important example of this concept is prions. Prions are proteinaceous structures that can act as infectious agents. In fact, prions can cause human diseases such as Creutzfeldt-Jakob Disease (CJD), which is an example of the growing list of infectious diseases that are mediated through
epigenetic modifications of proteins.