Random asynchronous replication of APP in H9 cells.

H9 cells were simultaneously hybridized with a probe against APP (red) and a FluorX-dCTP labeled BAC CTD-3154L15 (green). The blue color depicts nuclear DAPI staining. The H9 cells have a heterozygous deletion in chromosome 21 (from 21q22 to qter). The BAC CTD-3154L15 maps within this deleted region, and was used to mark one of the two copies of chromosome 21. The APP gene on chromosome 21 resides close to, but doesn't fall within, the deleted region. The images in A and B show nuclei from two different cells, from a same population, each of which has replicated a different allele early. (A) Nucleus of an H9 cell in which the double dot signals for APP and CTD-3154L15 are in close proximity, indicating that the APP allele residing in the intact chromosome has replicated early. (B) Nucleus of an H9 cell in which the double dot signal of APP does not have a signal for the BAC in its proximity, showing that the APP allele linked to the deleted chromosome has replicated early. In 45 cells counted, 24 cells replicated the intact chromosome early (like the cell in A), whereas 21 had replicated the allele linked to the deletion early (like the cell in B).