Mapping of the distal boundary of the X-inactivation center in a rearranged X chromosome from a female expressing XIST.

A female patient with primary amenorrhea, immature secondary sexual characteristics, and tall stature was found to have a normal X chromosome and a rearranged X [rea(X)] chromosome that resembled an 'isochromosome' Xp, but retained the proximal portion of Xq. The rea(X) was interpreted as rec(X)dup p,inv(X)(p11.4q13). Replication studies demonstrated that the rea(X) was always the late-replicating and, therefore, presumably inactive X chromosome, which must contain the X-inactivation center. Consistent with this interpretation, fluorescence in situ hybridization demonstrated that the rea(X) retained the XIST gene, and reverse transcription polymerase chain reaction analysis showed that XIST was expressed in the patient's cells. By fluorescence in situ hybridization with previously mapped probes, the breakpoint of the rea(X) was located within an approximately 500-kb region located approximately 200 to 700 kb distal to the XIST locus. This is the closest breakpoint distal to XIST in an inactivated X chromosome and, therefore, defines a new distal boundary for the X-inactivation center in humans.