[Effect of gamma radiations on robertsonian translocations]

Centromeric breaks and the dissociation of Robertsonian translocations have been regarded as the possible cause of a few reported cases of mosaicism. In a previous study we suggested that the dissociative phenomenon is influenced by an instability of the point of fusion, when it is located on a heterochromatic region, which appears to be a preferential mitomycin C (MMC) breakpoint. In this study lymphocytes from 6 robertsonian translocation carriers were treated with gamma rays. In one case who was a mosaic, the MMC induced a statistically significant increase in the number of cells with the dissociation. In the remaining cases, centromeric breaks or complete fissions were observed in 1.8% of the cells. In addition, the analysis of the distribution of breakage points on heterochromatic or euchromatic regions showed that almost 50% of the breaks on translocation chromosomes were located on pericentromeric heterochromatin. We postulate that the dissociation which occurs in robertsonian translocations may be caused by instability of the point of fusion since it is located on a heterochromatic region, susceptible to breakage by clastogenic agents such as MMC and gamma rays.