In situ nick translation of the fragile X region

At the present time, the molecular nature of the fragile site at Xq27.3 is not well understood. To examine the sensitivity of this region to DNAase I, in situ nick translation was performed on metaphase chromosomes from a fragile X (fra(X] positive individual. In this technique DNAase I is used to nick regions of chromosomal DNA that are in "open" conformation. Biotinylated dUTP was incorporated by nick translation at these sites. The incorporation was identified by double antibody labeling and avidin-horseradish peroxidase staining. Spreads, which had been stained with this technique, were photographed and subsequently trypsin-Giemsa G banded (post-GTG banded) for chromosome identification. In 36 of 44 (82%) fra(X) positive male cells, the region distal to fra(X) (q27.3) was prominently stained in contrast to its light staining appearance in GTG preparations. The fragile site itself was outlined more clearly than can be achieved by GTG or homogeneous staining. When autosomal fragile sites were induced by the addition of 1.5 microM aphidicolin 17 hours prior to harvest, 24 of 27 (89%) fragile sites on the ends of autosomes were prominently stained in regions distal to the break. Because the fra(X) and autosomal fragile regions behaved similarly, this suggests that they have a similar conformation. Thus, while autosomal and Xq27.3 fragile sites are strongly induced by different means, the organization of these sites and the regions distal to them appear to be similar.