Seeking Beta: Experimental Considerations and Theoretical Implications Regarding the Detection of Curvature in Dose-Response Relationships for Chromosome Aberrations

Robbing Peter to Pay Paul: Competition for Radiogenic Breaks During Rejoining Diminishes Curvature in the Dose Response for Simple Chromosome Exchanges

The large majority of chromosome damage produced by ionizing radiations takes the form of exchange aberrations. For simple exchanges between two chromosomes, multi-fluor fluorescence in situ hybridization (mFISH) studies confirm that the dose response to X rays or gamma rays is quasilinear with dose. This result is in seeming conflict with generalized theories of radiation action that depend on the interaction of lesions as the source of curvature in dose-response relationships. A qualitative explanation for such "linearization" had been previously proposed but lacked quantitative support. The essence of this explanation is that during the rejoining of radiogenic chromosome breaks, competition for breaks (CFB) between different aberration types often results in formation of complex exchange aberrations at the expense of simple reciprocal exchange events. This process becomes more likely at high radiation doses, where the number of contemporaneous breaks is high and complex exchanges involving multiple breaks become possible. Here we provide mathematical support for this CFB concept under the assumption that the mean and variance for exchange complexity increase with radiation dose.

A Bi-Exponential Repair Algorithm for Radiation-Induced Double-Strand Breaks: Application to Simulation of Chromosome Aberrations

Background: Radiation induces DNA double-strand breaks (DSBs), and chromosome aberrations (CA) form during the DSBs repair process. Several methods have been used to model the repair kinetics of DSBs including the bi-exponential model, i.e., N(t) = N₁exp(−t/τ₁) + N₂exp(−t/τ₂), where N(t) is the number of breaks at time t, and N₁, N₂, τ₁ and τ₂ are parameters. This bi-exponential fit for DSB decay suggests that some breaks are repaired rapidly and other, more complex breaks, take longer to repair. Methods: The bi-exponential repair kinetics model is implemented into a recent simulation code called RITCARD (Radiation Induced Tracks, Chromosome Aberrations, Repair, and Damage). RITCARD simulates the geometric configuration of human chromosomes, radiation-induced breaks, their repair, and the creation of various categories of CAs. The bi-exponential repair relies on a computational algorithm that is shown to be mathematically exact. To categorize breaks as complex or simple, a threshold for the local (voxel) dose was used. Results: The main findings are: i) the curves for the kinetics of restitution of DSBs are mostly independent of dose; ii) the fraction of unrepaired breaks increases with the linear energy transfer (LET) of the incident radiation; iii) the simulated dose–response curves for simple reciprocal chromosome exchanges that are linear-quadratic; iv) the alpha coefficient of the dose–response curve peaks at about 100 keV/µm.