Double in situ hybridization in combination with digital image analysis: A new approach to study interphase chromosome topography

Double in situ hybridization with mercurated and biotinylated
chromosome specific DNA probes in combination with digital image
analysis provides a new approach to compare the distribution of
homologous and nonhomologous chromosome targets within individual
interphase nuclei. Here we have used two DNA probes representing
tandemly repeated sequences specific for the constitutive
heterochromatin of the human chromosomes 1 and 15, respectively,
and studied the relative arrangements of these chromosome targets
in interphase nuclei of human lymphocytes, amniotic fluid cells,
and fibroblasts, cultivated in vitro. We have developed a 2D-image
analysis approach which allows the rapid evaluation of large
numbers of interphase nuclei. Models to test for a random versus
nonrandom distribution of chromosome segments are discussed taking
into account the three-dimensional origin of the evaluated
2D-distribution. In all three human diploid cell types the
measurements of target-target and target-center distances in the
2D-nuclear image revealed that the labeled segments of the two
chromosomes 15 were distributed both significantly closer to each
other and closer to the center of the nuclear image than the
labeled chromosome 1 segments. This result can be explained by the
association of nucleolus organizer regions on the short arm of
chromosome 15 with nucleoli located more centrally in these nuclei
and does not provide evidence for a homologous association per se.
In contrast, evaluation of the interphase positioning of the two
chromosome 1 segments fits the random expectation in amniotic fluid
and fibroblast cells, while in experiments using lymphocytes a
slight excess of larger distances between these homologous targets
was occasionally observed. 2D-distances between the labeled
chromosome 1 and 15 segments showed a large variability in their
relative positioning. In conclusion our data do not support the
idea of a strict and permanent association of these homologous and
nonhomologous targets in the cell types studied so far.