The XIST locus replicates late on the active X, and earlier on the inactive X based on FISH DNA replication analysis of somatic cell hybrids.
We have recently reported results of DNA replication analysis of three X-linked loci (FRAXA, F8C and XIST) on the X chromosomes in male and female fibroblasts using fluorescence in situ hybridization (FISH) (1). Although our findings that XIST replicates later on the active X than on the inactive X are similar to those of Boggs & Chinault (2) based on a FISH assay in female lymphoblasts, they are the opposite of observations recently reported by Hansen et al. (3) using a different technique. Because our conclusions about the inactive X were deduced from the behavior of the active X in male cells, we reexamined the time when these loci replicate on the human inactive X chromosome isolated from its homolog in somatic cell hybrids. We also studied the same chromosome as an active X in related hybrids. The results provide direct evidence that the expressed XIST locus on the inactive X replicates earlier than its repressed homolog on the active X and earlier than the FRAXA locus which is repressed on this chromosome. The silent XIST locus on the active X replicates late along with F8C which is also not transcribed in these cells. Possible reasons for the different results obtained by Hansen et al. (3) are discussed.
Torchia
B S
BS
Center for Medical Genetics, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21287-3914, USA.
Migeon
B R
BR
D000818:Animals; D002478:Cells, Cultured; D002874:Chromosome Mapping; D004261:DNA Replication; D005347:Fibroblasts; D005819:Genetic Markers; D014644:Genetic Variation; D006153:Guanosine Diphosphate / Q000235:genetics*; D006801:Humans; D006822:Hybrid Cells; D017404:In Situ Hybridization, Fluorescence / Q000379:methods; D008297:Male; D051379:Mice; D062085:RNA, Long Noncoding; D022661:RNA, Untranslated; D012867:Skin / Q000166:cytology / Q000378:metabolism; D014157:Transcription Factors / Q000235:genetics*; D014960:X Chromosome
D005819:Genetic Markers; D062085:RNA, Long Noncoding; D022661:RNA, Untranslated; D014157:Transcription Factors; C077885:XIST non-coding RNA; D006153:Guanosine Diphosphate