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Medical AI Inquiry: You was designed to assist in medical decision-making based on the provided user questions. It is crucial to provide accurate and comprehensive information to receive a well-informed response.  Labels: BACKGROUND, RESULTS. Meshes: .
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Is xIST-induced silencing of flanking genes achieved by additive action of repeat a monomers in human somatic cells? Context: The establishment of facultative heterochromatin by X-chromosome inactivation requires the long non-coding RNA XIST/Xist. However, the molecular mechanism by which the RNA achieves chromosome-wide gene silencing remains unknown. Mouse Xist has been shown to have redundant domains for cis-localization, and requires a series of well-conserved tandem 'A' repeats for silencing. We previously described a human inducible XIST transgene that is capable of cis-localization and suppressing a downstream reporter gene in somatic cells, and have now leveraged these cells to dissect the sequences critical for XIST-dependent gene silencing in humans. We demonstrated that expression of the inducible full-length XIST cDNA was able to suppress expression of two nearby reporter genes as well as endogenous genes up to 3 MB from the integration site. An inducible construct containing the repeat A region of XIST alone could silence the flanking reporter genes but not the more distal endogenous genes. Reporter gene silencing could also be accomplished by a synthetic construct consisting of nine copies of a consensus repeat A sequence, consistent with previous studies in mice. Progressively shorter constructs showed a linear relationship between the repeat number and the silencing capacity of the RNA. Constructs containing only two repeat A units were still able to partially silence the reporter genes and could thus be used for site-directed mutagenesis to demonstrate that sequences within the two palindromic cores of the repeat are essential for silencing, and that it is likely the first palindrome sequence folds to form a hairpin, consistent with compensatory mutations observed in eutherian sequences.
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Silencing of adjacent reporter genes can be effected by as little as 94 bp of XIST, including two 'monomers' of the A repeat. This region includes a pair of essential palindromic sequences that are evolutionarily well-conserved and the first of these is likely to form an intra-repeat hairpin structure. Additional sequences are required for the spread of silencing to endogenous genes on the chromosome. Final Decision: yes