Pericentromeric structure of human X "isochromosomes": evidence for molecular heterogeneity.

Three different long-arm X isochromosomes and an isodicentric X chromosome were examined by in situ hybridization with X-chromosome-specific alpha-satellite probes and by quantitation of Southern blots hybridized with proximal short-arm probes. Each chromosome had a unique pericentromeric structure. The isodicentric X chromosome was clearly dicentric, showing two distinct alpha-satellite hybridization signals and duplication of short-arm material. Two isochromosomes showed a larger than normal, bifid alpha-satellite signal and also had duplications of different extents of short-arm material. The third X isochromosome could not be distinguished from a classical long-arm isochromosome; it did not have a short-arm duplication and it had a single alpha-satellite signal. These data indicate that rearrangements responsible for X isochromosome formation can occur at numerous locations in the pericentromeric region and that some X isochromosomes may involve duplications of substantial portions of the short arm.