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Dissection of environmentally-mediated epigenetic silencing

Lead Research Organisation: John Innes Centre
Department Name: UNLISTED

Abstract

Abstracts are not currently available in GtR for all funded research. This is normally because the abstract was not required at the time of proposal submission, but may be because it included sensitive information such as personal details.

Technical Summary

We intend to achieve a step change in our understanding of the mechanistic basis of epigenetic regulation. We will capitalize on the plant epigenetic silencing system, vernalization, which has many features that allow the complete dissection of different facets of epigenetic regulation. In addition, the silencing is quantitatively modulated by the environment enabling dissection of how external cues mediate epigenetic silencing. We will combine genetics, molecular biology and biochemical approaches with computational modelling to allow us to translate the extensive nuts and bolts information into an understanding of how the engine works. A particular strength of modelling will be its predictive nature and ability to distinguish between key components and those with subsidiary or redundant roles. The system we will use is vernalization, the cold-induced Polycomb-silencing of the target locus, FLC. We will dissect the many phases of vernalization: the triggering of FLC repression by prolonged cold; the nucleation and epigenetic stability of chromatin changes at FLC; and the spreading of the silencing yet spatial restriction to FLC. Our goal will be a full understanding of the complexity involved in the epigenetic silencing of this locus, described in a quantitative model that reveals how the silencing is induced by temperature and how individual components of the silencing network are integrated into a robust whole. This ambitious goal, which will uncover fundamental concepts important to gene regulation in many organisms, will be achieved through a tight integration of molecular analysis and computational modelling, enabling efficient cycling between experimentation, prediction and validation.

Planned Impact

unavailable

Publications

10 25 50
 
Description The project exploited the plant epigenetic silencing system, vernalization, to uncover fundamental principles underpinning epigenetic regulation. We described the sequence of events whereby prolonged cold induced an epigenetic switch in expression states of a gene encoding a floral repressor (encoded by the Arabidopsis gene FLC). Through a combination of genetics, molecular biology and biochemical approaches with computational modeling we achieved our goal, a step change in our understanding of the mechanistic basis of epigenetic regulation.

Our focused effort integrating the many facets of chromatin regulation on one target has meant that FLC has become widely recognized as a paradigm for understanding environmentally mediated epigenetic regulation.

We fully dissected the different phases of vernalization elaborating conserved features of epigenetic regulation generally. This involved characterization of chromatin regulation of FLC before cold, triggering of transcriptional silencing by prolonged cold, features of the nucleation of chromatin silencing during the cold, and spreading of chromatin silencing in post-cold growth and spatial restriction to FLC.

A major success has been to uncover the importance of bistable chromatin states that determine the expression of FLC before and after cold and how the digital switching of these states underpins the quantitative epigenetic memory involved in vernalization. Detailed analysis and modeling of the dynamics of histone changes at FLC showed that the quantitative accumulation of the silencing was a consequence of an increasing proportion of cells switching epigenetic states. We detailed the accumulation of H3K27me3, first at one site in the gene during the cold, and then covering the whole gene after transfer back to warm. This was undertaken in wild-type and many mutants to understand the requirements for nucleation, spreading and spatial restriction of H3K27me3 at FLC. Using proteomics we also detailed the Polycomb complexes required for the H3K27me3 dynamics and determined that their localization and activity was defined by a set of PHD-containing proteins that influence the methyltransferase activity of the complex. H3K36me3 was also shown to be the most likely histone modification to act as the opposing histone modification to H3K27me3. Characterization of the dynamics of the major histone variants at FLC was also completed.

Significant progress in characterizing important cis sequences was achieved by exploiting natural variation in vernalization response. Arabidopsis accessions that require different lengths of cold to satisfy their vernalization requirement revealed FLC cis polymorphism that influenced FLC epigenetic silencing. A small number of nucleotide polymorphisms, located in the FLC intragenic site where H3K27me3 accumulates during the cold, were shown to underpin a requirement for longer periods of cold. Modelling of the changing histone modifications showed that these polymorphisms impact a mechanism important for the stable epigenetic memory of the silenced state.
We also made considerable progress in understanding the role of non-coding transcripts in initiation and maintenance of epigenetic silencing. Our analyses suggest that FLC antisense transcripts play multiple roles in the vernalization process, but not to recruit PRC2 complexes as proposed by many studies. We showed that an early cold sensor step involved up-regulation of expression of FLC antisense transcripts. Detailed analysis of this regulation led us into the finding that an R-loop limits FLC antisense production in vivo. Plants expressing an FLC defective in production of the antisense transcripts accumulate H3K27me3 normally, but show altered decreases in H3K36me3 and attenuated cold-induced FLC transcriptional silencing. FLC cis polymorphism in the A. thaliana natural variants was also shown to influence splicing and polyadenylation of the antisense transcripts and their functional significance was confirmed by transgenic analysis.

The interconnection of different layers of regulation of FLC was further investigated through analysis of changes in interaction of FLC intragenic sequences and changes in nuclear organization of the FLC locus, both of which are influenced by vernalization, and both of which ensure robust expression or silencing of FLC as the plant grows in a noisy environment.
Exploitation Route Our findings show the power of integrating molecular with theoretical approaches to dissect epigenetic regulation. This should influence all academic and biotech investigators analysing epigenetic regulation in all organisms.
Sectors Agriculture

Food and Drink

Environment

URL http://www.jic.ac.uk/staff/caroline-dean/
 
Description GCRF databases and Resources phase 2
Amount £98,000 (GBP)
Organisation Biotechnology and Biological Sciences Research Council (BBSRC) 
Sector Public
Country United Kingdom
Start 11/2017 
End 07/2018
 
Description Mechanistic basis of nucleation and spreading underlying a Polycomb-mediated epigenetic switch (EPISWITCH)
Amount € 2,100,000 (EUR)
Funding ID 833254 
Organisation European Research Council (ERC) 
Sector Public
Country Belgium
Start 08/2019 
End 09/2024
 
Description Huxley Lecture 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Professional Practitioners
Results and Impact Invited to the University of Birmingham to deliver the Huxley Lecture entitled: "Epigenetic switching in seasonal timing".
Year(s) Of Engagement Activity 2016
 
Description Invited speaker at the Garnet Natural Variation meeting 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Professional Practitioners
Results and Impact Invited speaker at the Garnet Natural Variation meeting in Cambridge (12th -13th December 2016). Talk entitled: "Natural Variation in Vernalization".
Year(s) Of Engagement Activity 2016