X-INACTIVATION ANALYSIS AND DNA METHYLATION STUDIES OF THE UBIQUITIN-ACTIVATING ENZYME E1 AND PCTAIRE-1 GENES IN HUMAN AND MOUSE

Previously reported data on the X inactivation status of the ubiquitin activating enzyme E1 (UBE1) gene have been contradictory, and the iss ue has remained unsettled, Here we present three lines of evidence tha t UBE1 is expressed from the inactive X chromosome and therefore escap es X inactivation, First, by RNA in situ hybridization, UBE1 RNA is de tected from both the active and inactive X chromosomes in human female fibroblasts, Second, UBE1 is expressed in a large panel of somatic ce ll hybrids retaining inactive human X chromosomes, including two indep endent hybrids that did not require UBE1 expression for survival, And third, sites at the 5' end of UBE1 are unmethylated on both active and inactive X chromosomes, consistent with the gene escaping inactivatio n, In order to address whether other genes that escape inactivation ma p to the same region of the X chromosome, we have also examined the ex pression of genes mapping adjacent to UBE1, The gene for PCTAIRE-1 (PC TK1) maps within 5 kb of UBE1 and similarly escapes X inactivation by the somatic cell hybrid assay, whereas six other genes that are within 1 Mb of UBE1 in Xp11.23 are silenced on the inactive X chromosome, Co mparative mapping studies of the homologous loci in mouse establish th at Ube1-x and Pctk1 are also within close physical proximity on the mu rine X chromosome, and expression studies of the Pctk1 gene determine that, similar to Ube1-x, it is subject to X inactivation in mouse, Met hylation of CpG residues at restriction sites at the 5' end of both ge nes on the murine inactive X chromosome is consistent with both genes being subject to X inactivation in mouse, in contrast to their express ion status in humans.