(190v) Rheological Response of Chromatin to DNA Damage

Rheological
Response of Chromatin to DNA Damage

Authors:
Daniel
B. Whitefield, Li Lan, Shelly Peyton, Kris Noel Dahl

Abstract

            The DNA
within the nucleus of a cell is packaged with histones and other proteins to
form chromatin. Chromatin acts as a viscoelastic polymer network that displays
power law behavior in response to perturbations including DNA damage and
stresses applied through cell motility or mechanotransduction. The effects of
DNA damage upon the rheology of the chromatin network is a controversial topic
in the literature of biophysics with some studies yielding results that support
increased mobility of chromatin after damage is induced while other studies
yield results that support decreased mobility. In this study, we implement a
technique that allows for site-specific probing of chromatin in two distinct
functional regimes. We utilize particle tracking microrheology to measure the
Mean Squared Displacements of fluorescently labeled proteins bound to these
distinct regions of chromatin and fit this to a power law model that allows us
to extract biophysical parameters, such as force propagation and chromatin
decondensation, that yield much more detailed information than simple
comparisons of motion. We find differences in mobility between the two regimes
in the absence of DNA damage. In the presence of DNA damage, the two regions display
similar mobility. We further report that the biophysical properties of damage
sites evolve over time. This manifests as a reduction of force propagation and an
increase in chromatin decondensation within these regions. We suggest that these
rheological changes could be advantageous to DNA damage repair mechanisms by
preventing broken ends of Double Strand Breaks from migrating away from each
other and allowing greater access of repair factors. Lastly, we report more
recent data comparing rheological changes in the chromatin networks of three
different breast cancer cell lines after treatment with two different
chemotherapy drugs as well as differences between cell lines. Overall, this
work begins to elucidate the rheological properties related to the DNA damage
response in different chromatin regimes, times post-damage, sources of damage,
and cell lines.