X-chromosome inactivation and escape

Abstract

X-chromosome inactivation, which was discovered by Mary Lyon in 1961 results in random silencing of one X chromosome in female mammals. This review is dedicated to Mary Lyon, who passed away last year. She predicted many of the features of X inactivation, for e.g., the existence of an X inactivation center, the role of L1 elements in spreading of silencing and the existence of genes that escape X inactivation. Starting from her published work here we summarize advances in the field.

Figure 1

(a) Female mouse with the Cattanach insertion [Is(In7;X)1Ct] shows variegation of her coat colour due to spreading of XCI in the inserted portion of chromosome 7, which silences coat colour markers. (b) Nucleus from a female mouse fibroblast after Xist RNA-FISH (green). The inactive X is coated with Xist RNA.

Figure 2

Differences and commonalities in the distribution of genes (pink) that escape X inactivation on the mouse X chromosome (centromere to the left) between tissues in vivo: ovary (top), spleen (middle), and brain (bottom). Pink bars indicate the position of escape genes on schematics of the mouse X chromosome (centromere at left) (see Berletch et al. 2015).