A FISH study of chromosome fusion in the ICF syndrome: involvement of paracentric heterochromatin but not of the centromeres themselves.

We have used double fluorescence in situ hybridisation to study the involvement of centromeres and paracentromeric heterochromatin in the chromosome abnormalities seen in the ICF syndrome. To detect centromeres, we used a probe which labelled alphoid satellite DNA, and for the paracentromeric heterochromatin a probe for classical satellite II. Our results show that it is always the paracentromeric heterochromatin of the relevant chromosomes that becomes decondensed in this syndrome and which fuses to produce multiradial configurations. However, the centromeric regions, identified by their content of alphoid satellite DNA, appear never to become decondensed and always remain outside the regions of chromosome fusion in the multiradials.