Anomalies of chromatin condensation, such as fragmentation, uncoiling and pulverization, were observed in XP9UV25, a xeroderma pigmentosum fibroblast clone in which a high proportion of cells carried an end-to-end dicentric chromosome, dic (5;16) (p15.2;q24), that gives rise during propagation in culture to a variety of dicentric and monocentric derivatives. The coiling anomaly affected exclusively part of a rearranged chromosome, in particular the region previously involved in breakage events. The heterochromatic 16q region, which is a preferential breakpoint in the formation of dicentric and monocentric derivatives, was consistently the limit of the uncoiled or pulverized regions. This observation suggests that the anomalous chromatin behavior could derive from alteration of a region relevant for the correct condensation of the chromosome. In XP9UV25 the frequency of nuclei with associated micronuclei increased with time in culture, in parallel with that of mitoses with dicentric chromosomes. In situ hybridization with DNA probes specific for chromosomes 5 and 16 revealed hybridization signals in about 40% of micronuclei. Since the frequency of micronuclei is about ten times less than that of dicentrics, it is probable that only the rearranged chromosomes undergoing coiling anomalies are excluded in micronuclei.
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