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Molecular Mechanisms of Facultative Heterochromatin Formation: An X-Chromosome Perspective

Authors
  • Żylicz, Jan J.
  • Heard, Edith
Type
Published Article
Journal
Annual Review of Biochemistry
Publisher
Annual Reviews
Publication Date
Jun 20, 2020
Volume
89
Pages
255–282
Identifiers
DOI: 10.1146/annurev-biochem-062917-012655
Source
Annual Reviews
Keywords
License
Yellow

Abstract

Facultative heterochromatin (fHC) concerns the developmentally regulated heterochromatinization of different regions of the genome and, in the case of the mammalian X chromosome and imprinted loci, of only one allele of a homologous pair. The formation of fHC participates in the timely repression of genes, by resisting strong trans activators. In this review, we discuss the molecular mechanisms underlying the establishment and maintenance of fHC in mammals using a mouse model. We focus on X-chromosome inactivation (XCI) as a paradigm for fHC but also relate it to genomic imprinting and homeobox (Hox) gene cluster repression. A vital role for noncoding transcription and/or transcripts emerges as the general principle of triggering XCI and canonical imprinting. However, other types of fHC are established through an unknown mechanism, independent of noncoding transcription (Hox clusters and noncanonical imprinting). We also extensively discuss polycomb-group repressive complexes (PRCs), which frequently play a vital role in fHC maintenance.

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