Proximity effects for chromosome aberrations measured by FISH.

A Monte Carlo simulation computer program for radiation-produced chromosome aberrations, based on the breakage-and-reunion model, was extended to include proximity effects due to localization of chromosomes and limited range for break-break interactions. Two adjustable parameters were used. One corresponds to total dose: the other determines proximity effects by specifying the number of 'interaction regions' in a cell nucleus. The use of additional adjustable parameters was avoided by assuming randomness of break induction and aberration production. FISH chromosome painting data were obtained from 1.9 Gy 60Co gamma-rays-irradiated human lymphocytes. The data were compared with the computer simulation results, taking individual chromosome lengths into account. With about 13 interaction regions, agreement between the experiment and the simulation was good, even when detailed categories of damage were scored. An estimated average dsb-dsb interaction distance, based on 13 interaction regions, is about 1.3 micron. Monte Carlo methods give useful quantitative estimates of relative aberration yields, with a minimum of adjustable parameters and the theoretical assumptions, and indicated proximity effects. Computer simulation of FISH experiments can be adapted to any number of colours, any scoring criteria and any method of grouping aberrations into categories. Simulation allows systematic extrapolation of aberration data on painted chromosomes to whole-genome aberration frequencies.