The mobile nature of acrocentric elements illustrated by three unusual chromosome variants.

Variant chromosomes are polymorphic in areas that are rich in repeat sequences such as the pericentromeric regions or in the acrocentric short arm regions. The dynamic nature of these regions is evident in the polymorphisms they exhibit. In this paper three unusual variants are described: a chromosome 21 with additional material on its short arm, a chromosome 7 with an insertion in the short arm and a chromosome 2 with satellites at the end of the long arm. All three variants were shown to involve acrocentric elements using special banding techniques and fluorescence in situ hybridization. The 21 variant was found to be a tricentric with a 21 and two 15 alpha-, two classical and three acrocentric beta-satellite signals interspersed by AgNOR-positive regions. The telomeres were present at the two terminal ends. The insertion on chromosome 7 was found to be C-band positive and to contain acrocentric beta-satellite DNA. However, acrocentric alpha-satellite, classical satellite, whole-chromosome-painting or all-telomeres sequence probes did not hybridize to the insertion. The satellited region of chromosome 2 had two C-bands, a small positive all-centromeres probe signal, and two signals for the beta-satellite probe. Sandwiched between the beta-satellite sequences was an AgNOR-positive region. The telomeres were present at the two ends of the satellited chromosome 2. Chromosome 2 subtelomeric probes hybridized to the terminal ends of the short and long arm of chromosome 2. The common thread in these three variants is the involvement of acrocentric short arm elements. The acrocentric short arm elements are shown to move to other acrocentric or nonacrocentric chromosomes and relocate to both terminal and interstitial positions. The integrations are stable and heritable.