Report
Teaser, summary, work performed and final results
Periodic Reporting for period 2 - ChromADICT (Chromatin Adaptations through Interactions of Chaperones in Time)
Teaser
The primary role of chromatin is to compact the genome within the cell nucleus, but it also provides a large repertoire of information in addition to that encoded genetically. Chromatin organization contributes to genome functions such as gene expression and stable...
Summary
The primary role of chromatin is to compact the genome within the cell nucleus, but it also provides a large repertoire of information in addition to that encoded genetically. Chromatin organization contributes to genome functions such as gene expression and stable transmission through cell divisions. Additionally Chromatin structure and organization changes over time and in different tissues and adapts to physiological conditions. These dynamics are critical for development and for the maintenance/plasticity of cell fate over the course of life.
Understanding how to build this organization to ensure the establishment, maintenance, and propagation of a cellular identity in a given cell lineage has been a challenge for the field. The major protein bricks in this organization are the histones. In metazoans, they exist as histone variants. These variants, along with post-translational modifications provide a marking for distinct chromosomal landmarks important for both genome stability and expression. This is exemplified by the presence of the centromeric H3 variant (CenH3) at the centromere, a key chromosomal region enabling proper chromosome segregation during cell division.
Mechanistic aspects of histone deposition have been documented over the past years. However, understanding at a molecular level how histone variant choice and targeting to particular loci is controlled remains a major challenge. As the architects and bricklayers of the genome, histone chaperones escort histones throughout their cellular life and they represent key candidates to link the choice of variants to particular chromatin assembly pathways, and possibly their targeting to particular loci.
Our objective is to understand how the dosage of histones and their chaperones can affect cell fate. Using advanced genomics and imaging technologies in model systems, our aim is to characterize the dynamics of molecular complexes in which they are engaged and the changes that occur at places they mark in the genome. By extending the approach to distinct developmental transitions we wish to apply our findings to whole organisms. Ultimately we hope to establish a system to intervene and control these events in real time in developing organism.
Work performed
We discovered that high levels of the histone chaperone HJURP dedicated to the placement of the centromeric H3 variant (CenH3) is critical for survival of particular cancer cells. This observation led us to propose a model in which these cancer cells become dependent on or “addicted†to the HJURP chaperone This dependency might open novel therapeutic approaches exploiting such addiction as an ‘Achilles heel’.
We used high resolution microscopy imaging to follow in the cell nucleus the dynamics of the histone variants H3.1 and H3.3 during duplication of the genome. We found that the two variants organize into clusters with distinct variation in size and density during S phase progression (see figure) and identified the histone chaperone ASF1 as an important factor for the spatial distribution parental histones during replication.
Taken together these two examples of results obtained within the project illustrate the importance of characterizing the relationships between histones and their dedicated chaperones, including their dosage and dynamics, to understand how these can impact on cell survival and maintenance of chromatin marks supported by histone variants.
Final results
By building on our findings we expect to characterize the critical role and function of histone variants and their chaperones in the regulation of cell fate plasticity. We are currently developing models enabling the control of histone variant and chaperone dosage and cell differentiation.