Researchers have mapped the physical structure of the nuclear landscape to better understand changes in genomic interactions occurring in cell senescence and aging.

Their findings have allowed them to reconcile the contradictory observations of two current models of aging: cellular senescence of connective tissue cells called fibroblasts and cellular models of an accelerated aging syndrome.

Cellular senescence is an irreversible state of cell cycle arrest and cells enter senescence in response to a variety of stresses. For example, oncogene activation triggers cell senescence as a mechanism to protect against unregulated cell proliferation and the creation of tumors. Cellular senescence is also thought to have a role in normal developmental processes and hence in aging.

In the first model, cellular senescence triggers large-scale spatial rearrangements of chromatin and the formation of dense nuclear domains called SAHF; senescence associated heterochromatic foci. Chromatin is the complex of DNA and proteins that forms the chromosomes in the nucleus. The second model uses fibroblast cells from people with a syndrome causing accelerated aging (Hutchinson-Gilford progeria syndrome, HGPS) and these cells show reduced compaction of chromatin and do not show the creation of SAHF domains. 

The Babraham Institute researchers measured the frequency of genome interactions occurring throughout the whole genome in senescent fibroblasts and compared them to studies on HGPS cells. This approach brought together scientists from two of the Institute's core research programs: epigenetics and nuclear dynamics.

Unexpectedly, they found that SAHF regions, thought to be highly condensed and structured, show a dramatic loss of local interconnectivity and internal structure in senescence chromatin and that this effect was also seen in the genomes from HGPS cells.

Looking in detail at the genomic events occurring during cell senescence allowed the researchers to resolve SAHF formation into two stages:

1) changes in local connectivity in the genome, similar to those found in HGPS and
2) the senescence-specific clustering of these regions, creating the SAHF domains.

Dr. Tamir Chandra, lead author and postdoctoral researcher based at both the Babraham Institute and Wellcome Trust Sanger Institute, said, "The seemingly opposite changes in chromatin behavior between cell senescence and cells from HGPS patients have been an obstacle to understanding their contribution to aging. Using physical interaction mapping, a direct measure of the genome architecture, our study suggests that the chromatin does initially change in a similar way in cell senescence and HGPS. We can now focus our studies on these early events common to both model systems." 

Professor Wolf Reik, Group Leader and Associate Director at the Babraham Institute and Associate Faculty at the Wellcome Trust Sanger Institute said, "There are probably important aspects of aging which are regulated or influenced by epigenetic mechanisms such as chromatin compaction. It is therefore important to understand dynamic changes of epigenetic marks during aging, how they come about, and what impact they have on altered cell function later in life". 

The research provides a common model of cellular aging supported by both the study of senescence and progeria. Having a better understanding of the biological events contributing to aging will result in benefits to health, wellbeing and independence in later life. 

Citation: Chandra, Ewels et al. (2015). Global reorganisation of the nuclear landscape in senescent cells. Cell Reports.  Funded by the BBSRC and the Wellcome Trust.