Condensed chromatin and cell inactivation by single-hit kinetics

Area of Science:

• Cell Biology
• Radiation Biology
• Genetics

Background:

• Mammalian cells exhibit extreme sensitivity to gamma rays during mitosis when chromatin is maximally condensed.
• Radiation-induced cell killing in mitotic cells follows single-hit inactivation kinetics.
• The relationship between chromatin condensation and radiation hypersensitivity requires further investigation.

Purpose of the Study:

• To investigate the correlation between chromatin condensation and radiation hypersensitivity in mammalian cells.
• To compare the radiosensitivity and chromatin density of wild-type (CHO K1) and radiosensitive mutant (xrs-5) cells at different cell cycle phases.

Main Methods:

• Synchronization of Chinese hamster ovary (CHO) K1 and xrs-5 cells using mitotic shake-off.
• Measurement of chromatin cross-section densities and radiosensitivities in mitotic and G1 phases.
• Analysis of alpha-inactivation coefficients to determine radiosensitivity.

Main Results:

• G1-phase CHO K1 cells showed dispersed chromatin with at least three times lower density than mitotic cells, correlating with a 10-fold decrease in radiosensitivity.
• Mitotic CHO xrs-5 cells exhibited denser chromatin and the highest radiosensitivity (alpha = 5.1 Gy(-1)).
• In G1 phase, CHO xrs-5 cells retained condensed chromatin, suggesting a defect in chromatin condensation/dispersion.

Conclusions:

• Radiation hypersensitivity in mitotic cells is linked to chromatin condensation status.
• CHO xrs-5 cells may have defects in chromatin condensation/dispersion, contributing to their radiosensitivity.
• Chromatin condensation status could influence intrinsic radiosensitivity in mammalian cells, relevant to radiotherapy dose fractionation.