Effect of chromosome size on aberration levels caused by gamma radiation as detected by fluorescence in situ hybridization.

Fluorescence in situ hybridization (FISH) is a powerful technique for detecting genomic alterations at the chromosome level. To study the effect of chromosome size on aberration formation, we used FISH to detect initial damage in individual prematurely condensed chromosomes (PCC) of gamma-irradiated G0 human cells. A linear dose response for breaks and a nonlinear dose response for exchanges was obtained using a chromosome 1-specific probe. FISH detected more chromosome 1 breaks than expected from DNA based extrapolation of Giemsa stained PCC preparations. The discrepancy in the number of breaks detected by the two techniques raised questions as to whether Giemsa staining and FISH differ in their sensitivities for detecting breaks, or is chromosome 1 uniquely sensitive to gamma-radiation. To address the question of technique sensitivity, we determined total chromosome damage by FISH using a total genomic painting probe; the results obtained from Giemsa-staining and FISH were nearly identical. To determine if chromosome 1 was uniquely sensitive, we selected four different sized chromosomes for paint probes and scored them for gamma-ray induced aberrations. In these studies the number of chromosome breaks per unit DNA increased linearly with an increase in the DNA content of the chromosomes. However, the number of exchanges per unit of DNA did not increase with an increase in chromosome size. This suggests that chromosome size may influence the levels of aberrations observed. Extrapolation from measurements of a single chromosome's damage to the whole genome requires that the relative DNA content of the measured chromosome be considered.