DEFICIENT TRANSCRIPTION OF XIST FROM TINY RING X-CHROMOSOMES IN FEMALES WITH SEVERE PHENOTYPES

The severe phenotype of human females whose karyotype includes tiny ri ng X chromosomes has been attributed to the inability of the small rin g X chromosome to inactivate. The XIST locus is expressed only from th e inactive X chromosome, resides at the putative X inactivation center , and is considered a prime player in the initiation of mammalian X do sage compensation. Using PCR, Southern blot analysis, and in situ hybr idization, we have looked for the presence of the XIST locus in tiny r ing X chromosomes from eight females who have multiple congenital malf ormations and severe mental retardation. Our studies reveal heterogene ity within this group; some rings lack the XIST locus, while others ha ve sequences homologous to probes for XIST. However, in the latter, th e locus is either not expressed or negligibly expressed, based on reve rse transcription-PCR analysis. Therefore, what these tiny ring chromo somes have in common is a level of XIST transcription comparable to an active X. As XIST transcription is an indicator of X chromosome inact ivity, the absence of XIST transcription strongly suggests that tiny r ing X chromosomes in females with severe phenotypes are mutants in the X chromosome inactivation pathway and that the inability of these rin gs to inactivate is responsible for the severe phenotypes.