Topography of genetic elements of X-chromosome relative to the cell nucleus and to the chromosome X territory determined for human lymphocytes

Topography of three genetic elements - dystrophin (dmd) exons 5-7 (E-1), 46-47 (E-2) and centromere of chromosome X (N-x) were studied relative to cell nuclei and to chromosome X territories of spatially fixed human lymphocytes. Repeated three-dimensional (3D) dual color fluorescence in situ hybridization combined with high-resolution cytometry was used. In addition, the nuclear location of fluorescence weight centers (FWC), spatial volume, and maximal area per one section of chromosome-X territories were investigated. The larger (X-L) and smaller (X-s) homologous X-chromosomes were distinguished for each nucleus according to the 3D volume of their territories. The distributions of the [center of nucleus]-to-[genetic element] distances (radial distributions) of dmd exons E-1, E-2, centromere N-x and FWC were very similar for both homologous X-chromosomes of female lymphocytes as well as for the chromosome X of the human male. On the other hand, larger average mutual distances between all pairs of signals (E-1, E-2, N-x, FWC) and larger average maximal area were observed for the larger chromosome (X-L) in comparison with the smaller one (X-s). The territory of the larger homologue showed also more irregular surface. The most significant differences between homologous X-chromosomes were found for N-x-E-1, N-x-E-2 and E-1-E-2 distances that were in average about twice longer for X-L as compared with X-s. These parameters correlate to each other and can be used for the reliable determination of more (de)condensed X-chromosome territory. The longer E-1-E-2 distances for X-L indicate more open chromatin structure of the dystrophin gene on this chromosome in contrary to closed structure on X-s. Substantially shorter distances of the dystrophin exons from the centromeric heterochromatin in X-s as compared to X-L can be explained by silencing effect of centromeres as described in Nature 1 (2000) 137. (C) 2002 Elsevier Science B.V. All rights reserved.