Lecture 24

Regulation of homeotic gene expression

Determination of parasegmental identity in Drosophila is mediated by the activities of the homeotic genes of the Antennapedia and Bithorax complexes.  Expression of the homeotic genes is confined to unique domains along the body axis whose anterior and posterior boundaries are maintained throughout development.  These unique domains of homeotic gene expression are actively maintained by repressive elements.  Homeotic genes contain enhancers that lack positional information and have the potential to be active in all parasegments.  Misexpression of homeotic genes outside these boundaries results in posterior homeotic transformations in all segments of the body axis.  Initial activation of the homeotic genes occurs in response to the developmental information provided by the maternal effect and segmentation genes.  Because the products of the segmentation genes are only transiently available, the maintenance of homeotic gene expression throughout development depends on other factors.  These other factors include the gene products from the Polycomb (PcG) and trithorax group (trx-G) genes.  In general, members of the Polycomb group are required to maintain the transcriptionally silent state in those cells in which the homeotic genes were initially repressed and the genes of the trithorax group are required for the continued expression in those cells in which the homeotic genes were originally active (reviewed in Francis and Kingston, 2001).  The hallmark feature of gene regulation by PcG and trxG proteins is that it can lead to a mitotically stable pattern of gene expression, often referred to as epigenetic regulation.

Epigenetic Regulation:

    Epigenetic regulation involves the maintenance of a particular state of gene expression, most commonly repression, throughout repeated mitosis, and frequently meiosis.  Changes in such heritable expression states occur without an alteration of the primary DNA sequence but do involve the assembly of specialized structures of chromatin.  It is becoming increasingly evident that epigenetic regulation of gene transcription plays a critical role in the regulation of gene expression in many biological processes.  Examples of epigenetic silencing include:  position-effect variegation (PEV) in Drosophila, cosuppression in plants, fungi and Drosophila, gametic imprinting in mammals, dosage compensation by X chromosome inactivation in mammals, telomeric position-effect (TPE) and mating type silencing in yeast.  Repression of target loci by the PcG in Drosophila is another example of epigenetic silencing.  A common theme that emerges amongst these diverse processes is the complex integration of epigenetic regulatory pathways with alterations in chromatin structure over target DNA loci.     

The Polycomb Group:

    Mutations in the PcG genes of Drosophila cause posterior homeotic transformations in all segments of the body axis similar to phenotypes of gain-of-function mutations of the homeotic genes. This similarity results from ectopic expression of the homeotic genes beyond their proper spatial boundaries in PcG mutants.  In embryos mutant for genes in the PcG, homeotic gene expression is correctly established, but it subsequently spreads beyond the normal boundaries resulting in posterior transformations.  This result shows that the PcG genes are not required for initial repression but maintenance of the silent state of the homeotic loci.

The Trithorax Group:

   Members of the trxG are required to stably maintain homeotic gene expression patterns established by the segmentation genes during embryogenesis.  The trxG genes exhibit antagonistic genetic interactions with PcG genes suggesting that trxG proteins function to counteract or modulate PcG-mediated silencing.  Although the action of the trxG is varied and complex, a unifying theme for the role of several trxG proteins lies in their chromatin remodelling activities that enhance the accessibility of transcriptional activators to DNA.  In addition to chromatin remodelling, trxG function is associated with histone modification. 

How is early transient repression of homeotic genes converted into permanent silencing?

   Using reporter gene constructs, several cis-regulatory elements necessary for maintaining the silent state of homeotic genes were identified.  The ability of these elements to maintain the silent state of downstream genes depends on the establishment of a repressive complex involving PcG proteins and their targets, the PcG response elements (PREs).  PREs are “cellular memory modules” that lock in transcription states (active or silent) determined early in embryogenesis and maintain them throughout development.  In fact, removal of a PRE from a silenced gene has been shown to result in a loss of silencing, even if the PRE is removed late in development.

The first problem is how do the PcG proteins, which are ubiquitously expressed and do not bind DNA, recognize specific regulatory targets only in domains where their repressive activity is required?

Is recruitment of the PcG sufficient to establish the transition from initiation to maintenance of silencing?

Once recruited to their targets, what are the molecular mechanisms of PcG-mediated transcriptional silencing?

How are the silencing complexes inherited through time and cell divisions?