"The prevailing thought was that genes that escaped X inactivation were pulled out of the core and expressed out there," said Calabrese.
The work of Calabrese's team complicates the current model of X-inactivation by finding indications of gene activity inside the Xist cloud and the presence of inactive genes outside the cloud, both of which would not have been thought possible in the prevailing model.
"It's kind of a subtle thing, but mechanistically it is a big difference," said Calabrese.
Inside the Xist cloud, sequencing discovered traces of DNase I sensitivity, a feature usually linked to transcription activity. While other markers associated with transcription were absent, the presence of DNase I sensitivity suggested that the nucleus did recognize the inactive X as usable DNA, but an unknown suppressive mechanism was preventing genes from being activated.
"We were surprised to see that. If they were totally silent, you would expect this to be not there This suggests that transcription factors or other proteins that bind DNA are still accessing the inactive X," said Calabrese.
The other surprising findings involve the 15 percent of "escaper" genes from the inactive X. Calabrese found evidence that active genes were found both inside and outside the Xist cloud, and that silenced genes that lay alongside active genes outside of the Xist cloud remained inactive.
"If X-inactivation was a strict nuclear barrier, then pulling a gene outside the barrier would turn it on, but it has got to be more than that because when an inactivated gene that is beside an escaper is outside this domain, it is still turned off," said Calabrese.
The presence of DNase I sensitivity within the Xist cloud and the finding of inactive genes outside of the cloud suggest that a site-specific mechani
|Contact: William Davis|
University of North Carolina Health Care