Abstract
DXZ4 is an X-linked macrosatellite composed of 12-100 tandemly arranged 3-kb repeat units. In females, it adopts opposite chromatin arrangements at the two alleles in response to X-chromosome inactivation. In males and on the active X chromosome, it is packaged into heterochromatin, but on the inactive X chromosome (Xi), it adopts a euchromatic conformation bound by CTCF. Here we report that the ubiquitous transcription factor YY1 associates with the euchromatic form of DXZ4 on the Xi. The binding of YY1 close to CTCF is reminiscent of that at other epigenetically regulated sequences, including sites of genomic imprinting, and at the X-inactivation centre, suggesting a common mode of action in this arrangement. As with CTCF, binding of YY1 to DXZ4 in vitro is not blocked by CpG methylation, yet in vivo both proteins are restricted to the hypomethylated form. In several male carcinoma cell lines, DXZ4 can adopt a Xi-like conformation in response to cellular transformation, characterized by CpG hypomethylation and binding of YY1 and CTCF. Analysis of a male melanoma cell line and normal skin cells from the same individual confirmed that a transition in chromatin state occurred in response to transformation.