Escape from X chromosome inactivation is an intrinsic property of the mouse Jarid1c locus

Although most genes on one X chromosome in mammalian females are silenced by X chromosome inactivation, some genes "escape" X inactivation and are expressed from both active and inactive Xs. How these escape genes are transcribed from a largely inactivated chromosome is not fully understood, but underlying genomic sequences are thought to be involved. We developed a transgene approach to ask whether an escape locus, that includes the mouse Jarid1c gene, is autonomous or is instead influenced by X chromosome location. Two BAC clones carrying Jarid1c and adjacent X-inactivated transcripts were randomly integrated into mouse XX embryonic stem cells. Four lines with single copy, X-linked transgenes were identified and we established that each integrated into regions that are normally X inactivated. As expected for genes that are normally subject to X inactivation, BAC transgene transcripts Tspyl2 and Iqsec2 were X inactivated. However, allelic expression and RNA/DNA FISH indicate that transgenic Jarid1c escapes X inactivation. Therefore, transgenes at four different locations on the X recapitulate endogenous inactive X expression patterns. We conclude that escape from X inactivation is an intrinsic feature of the Jarid1c locus and functionally delimit the Jarid1c escape domain as the 112 kb representing the maximum overlap of the BACs tested. Additionally, although extensive chromatin differences normally distinguish active and inactive loci, unmodified BACs are capable of directing proper inactive X expression patterns. Therefore, these studies establish that primary DNA sequence alone, in a chromosome position-independent manner, is sufficient to determine X chromosome inactivation status. This transgene approach will enable further dissection of key elements of escape domains and allow rigorous testing of specific genomic sequences on inactive X expression.