THE SEVERE PHENOTYPE OF FEMALES WITH TINY RING X-CHROMOSOMES IS ASSOCIATED WITH INABILITY OF THESE CHROMOSOMES TO UNDERGO X-INACTIVATION

Mental retardation and a constellation of congenital malformations not usually associated with Turner syndrome are seen in some females with a mosaic 45,X/46,X,r(X) karyotype. Studies of these females show that the XIST locus on their tiny ring X chromosomes is either not present or not expressed. As XIST transcription is well correlated with inact ivation of the X chromosome in female somatic cells and spermatogonia, nonexpression of the locus even when it is present suggests that thes e chromosomes are transcriptionally active. We examined the transcript ional activity of ring X chromosomes lacking XIST expression (XISTE(-) ), from three females with severe phenotypes. The two tiny ring X chro mosomes studied with an antibody specific for the acetylated isoforms of histone H4 marking transcribed chromatin domains were labeled at a level consistent with their being active. We also examined two of the XISTE(-) ring chromosomes to determine whether genes that are normally silent on an inactive X are expressed from these chromosomes. Analyse s of hybrid cells show that TIMP, ZXDA, and ZXDB loci on the proximal short arm, and AR and PHKA1 loci on the long arm, are well expressed f rom the tiny ring X chromosome lacking XIST DNA. Studies of the ring c hromosome that has XIST DNA but does not transcribe it show that its A R allele is transcribed along with the one on the normal X allele. The se findings provide compelling evidence that (1) ring X chromosomes as sociated with severe phenotypes are unable to undergo X chromosome ina ctivation; (2) they represent chromosomal mutations affecting cis inac tivation; and (3) the severe phenotype is due to functional disomy res ulting from lack of dosage compensation for genes present within the r ing chromosome.