Female mammals carry two X chromosomes, while male mammals carry an X and a Y chromosome. The X chromosome carries a thousand genes, so in females, one of the X chromosomes must be silenced to avoid serious consequences; when the genes on one of the X chromosomes are not shut off, it can cause death in embryos. Researchers have now found that a protein called SPEN is crucial for the inactivation of an X chromosome, and have learned how it silences X chromosome genes. The work has been reported in Nature.
A molecule called Xist, which is a long non-coding RNA, was known to initiate the silencing of genes on the X chromosome. "The exact molecular mechanisms by which Xist mediates gene silencing have been a mystery for decades," said European Molecular Biology Laboratory (EMBL) in Heidelberg and Institut Curie graduate student François Dossin.
In this work, Dossin and researchers in the group of Edith Heard, Director General of EMBL, reduced the level of SPEN in embryonic stem cells. When they did that, it stopped X-inactivation from happening. The scientists also identified the location on the X chromosome where SPEN binds and exerts its effect.
When Xist is produced, it moves to the X chromosome where SPEN is bound. There, it links to SPEN, accumulating on the X chromosome. SPEN can also influence the genomic regions that regulate the expression of active genes. When gene silencing has occurred, SPEN can detach from the chromosome, and the genes remain permanently silenced for the lifetime of a cell.
"We dissected SPEN's role during X chromosome inactivation using a wide array of classical and cutting-edge approaches," said Dossin.
The researchers showed that one particular part of SPEN, which is called SPOC, takes the lead in silencing genes by interfering with the transcription of DNA into RNA. It also interacts with various proteins that function in the synthesis of RNA, and plays a part in the remodeling and modification of chromatin (a complex of DNA and the proteins that package it).
More work will be necessary to fully understand all of the details of X chromosome inactivation.
"We found that SPEN interacts with several pathways linked to gene silencing. Given that SPEN accounts for nearly all the silencing during X-inactivation, the next question to address is how much each of those pathways contributes to gene silencing," explained Edith Heard, Director General of EMBL.