Dose-response curves for X-ray induced interchanges and interarm intrachanges in human lymphocytes using arm-specific probes for chromosome 1.

Frequencies of chromosomal aberrations were estimated in X-irradiated (0; 0.5; 1; 2; 3 and 4 Gy) human lymphocytes using arm-specific probes for chromosome 1 in combination with a pancentromeric probe in a triple colour FISH procedure. This allowed the construction of dose-response curves for both interchanges as well as interarm intrachanges. We found that 11.7-18.4% of aberrant chromosomes contained interarm intrachanges. Assuming a free interaction of randomly induced exchange breakpoints throughout the whole genome, interarm intrachanges occur about 8.7 times more frequently than expected. The close proximity of the two arms of a chromosome in the interphase nucleus appears to promote the formation of interarm intrachanges. Convincing evidence was obtained, by comparing the frequency of colour junctions that occur between 1p-1q and 1p-3q (the arms 1q and 3q are of similar size). With this approach, we observed 8.8 times more colour junctions between 1p-1q than between 1p-3q, illustrating the importance of proximity in the formation of chromosomal exchange aberrations. At doses higher than 2 Gy, a saturation of simple reciprocal aberrations, both for interchanges as well as for interarm intrachanges was observed. Furthermore, we found that symmetrical and asymmetrical simple interchanges as well as interarm intrachanges occur with equal frequencies in human chromosome 1.