Visualization of nonreciprocal chromosome exchanges in irradiated human fibroblasts by fluorescence in situ hybridization.

A long-standing controversy in radiation cytogenetics is the precise mechanism for the formation of chromosome exchanges. The classical breakage-and-reunion hypothesis suggested by Stadler and developed by Sax was challenged in 1959 by Revell, who proposed that radiation produced an "unstable lesion" which could interact with another such lesion to form an exchange. A difference between the predictions of these two theories is that the breakage-and-reunion hypothesis would allow the two broken ends of a chromosome to join with contemporary broken ends in different chromosomes, thereby producing nonreciprocal exchanges involving more than two chromosomes. This would not occur according to the Revell theory, which demands pairwise commital to the exchange. The ability to "paint" a whole chromosome using fluorescence in situ hybridization allows a discrimination between reciprocal and nonreciprocal chromosome exchanges. We scored metaphases in AG1522 human fibroblasts irradiated in G1 phase with 6 Gy and hybridized to chromosomes 1, 4, or 8. Of the complete exchanges involving one of these chromosomes, 26% were found to possess either one or three centromeres in the exchange pair. Since we could rule out any significant contribution of complex exchanges (i.e., those involving more than one break per chromosome), these one- and three-centromere exchange pairs must have arisen from a nonreciprocal exchange. Because an equal number of nonreciprocal exchanges would be expected to have a total of two centromeres, this suggests that approximately 50% of all exchanges at this dose were nonreciprocal. These data support the breakage-and-reunion hypothesis and are incompatible with the standard form of the exchange hypothesis of Revell.