Image-Based Quantitative Single-Cell Method Suggests Increase of Global Chromatin Accessibility in Tumor Compared to Non-tumor Cell Lines.

The phenotypic plasticity of cancer cells has recently emerged as an important factor of treatment failure. The mechanisms of phenotypic plasticity are not fully understood. One of the hypotheses is that the degree of chromatin accessibility defines the easiness of cell transitions between different phenotypes. To test this, a method to compare overall chromatin accessibility between cells in a population or between cell populations is needed. We propose to measure the chromatin accessibility of a cell by total fluorescence signal from nuclei stained with DNA-binding fluorescent molecules. This method is based on the existing data that some small molecules bind nucleosome-free DNA more easily than nucleosomal DNA. Thus, nuclear fluorescence of these molecules is proportional to the amount of nucleosome-free DNA, serving as a measure of chromatin accessibility. We optimized the method using several DNA binding molecules and known chromatin modulating agents. Using a set of tumor and non-tumor cells of different origins we observed the tendency to the higher chromatin accessibility of tumor versus non-tumor cells. Chromatin accessibility was also increased upon oncogene-induced transformation of mouse and human cells.