Extreme reduction of chromosome-specific a-satellite domain is unusually common in human chromosome 21

Human centromeres contain large arrays of alpha-satellite DNA that are thought to provide centromere function. The arrays show size and sequence variation, but the extent to which extremely low levels of this DNA can occur on normal centromeres is unclear. Using a set of chromosome-specific alpha-satellite probes for each of the human chromosomes, we performed interphase fluorescence in situ hybridization (FISH) in a population-screening study. Our results demonstrate that extreme reduction of chromosome-specific alpha satellite is unusually common in chromosome 21 (screened with the alphaRI probe), with a prevalence of 3.70%, compared to 17,000 morphologically normal chromosomes studied. All of the low-alphoid centromeres are fully functional as indicated by their mitotic stability and binding to centromere proteins CENP-B, CENP-C, and CENP-E. Sensitive metaphase FISH analysis of the low-alphoid chromosome 21 centromeres established the presence of residual alphaRI as well as other non-alphaRI alpha-satellite DNA suggesting that centromere function may be provided by (1) the residual alphaRI DNA, (2) other non-alphaRI alpha-satellite sequences, (3) a combination of 1 and 2, or (4) an activated neocentromere DNA. The low-alphoid centromeres, in particular those of chromosome 21, should provide unique opportunities for the study of the evolution and the minimal DNA requirement of the human centromere