Xist and its role in X-chromosome inactivation

Compensation for the two-fold dosage difference in female versus male mammals for X-linked genes involves the formation of an extremely stable heterochromatin structure on one of the two X chromosomes in females. Expression of the untranslated RNA, XIST, is required in cis for the establishment of the heterochromatic state. It is unclear how this RNA recruits the factors involved in the chromatin alterations, but its unique association with the X chromosome is required for its function. Recent results in mouse have started to elucidate how expression of Xist is controlled, including the role of the antisense transcript Tsix. To determine the pattern of expression within the XIST region through human development I used germ-cell derived, embryonal carcinoma cells as a potential model since some lines do appear to have a limited capacity to recapitulate events in early human development when induced to differentiate in culture. Comparisons between N-Tera2Dl (male embryonal carcinoma cell line) and somatic cells reveal a complex expression pattern in the XIST region with the identification of an alternate XIST isoform as well as a downstream antisense transcript which appear to be developmentally regulated and specific to N-Tera2Dl. Although there were no regions of conservation outside of ZZST that would point to important regulatory elements, the presence of a developmentally-regulated antisense transcript in humans is suggestive of a possible function. To address this, an XIST transformant clone expressing both XIST and the overlapping downstream antisense transcript allowed me to analyze the effects of the antisense expression on sense XIST. However, unlike mouse Tsix, the human antisense does not appear to destabilize or affect sense XIST localization. I also examined the factors involved in XIST localization and have developed an inducible XIST cDNA construct to determine the domains within the transcript that are essential for localization. Initial deletion constructs have identified two regions within exon 1 that are essential for transcript accumulation and localization. This inducible system will also allow future experiments to address the role of cis-UNA elements and transcript levels on the transcript's association with the surrounding chromatin.Medicine, Faculty ofMedical Genetics, Department ofGraduat