Genetics and epigenetics of the X chromosome

A consequence of Mendelian inheritance of X-linked traits is that women are more than equal to men in the face of X-linked diseases, protected as they are by the presence of two X chromosomes in their genome. This potentially beneficial inequality is diminished by the molecular mechanism known as X-chromosome inactivation (XCI), which triggers the transcriptional silencing of one of the X chromosomes in each female cell. The determination of which X to inactivate, a process that occurs during early embryogenesis, is random and clonally inherited. As a result, females are mosaic for the expression of X-linked genes. XCI is a highly regulated process involving large noncoding RNAs, chromatin remodeling, and nuclear reorganization of the X chromosome. It is a paradigm for epigenetic regulation and is frequently used as a biomarker for monitoring long-range gene reprogramming during cell differentiation and dedifferentiation. Our review analyses how XCI affects the expression of X-linked mutations, describes some of the most recent discoveries on the molecular mechanisms triggering XCI, and explores the therapeutic potentialities of the XCI process per se.