A gene thought to keep a single X chromosome turned on in mice plays no such role in humans, Johns Hopkins researchers report in the August issue of the American Journal of Human Genetics. The finding is likely to relegate the disproven gene to relative obscurity, at least in humans, says Barbara Migeon, M.D., of the McKusick-Nathans Institute of Genetic Medicine, whose laboratory found the human version of the gene in 2001. It also moves the search for the gene from the X chromosome to the 22 other types of chromosomes found in human cells, she adds.
In mammals, one of the two X chromosomes inherited by all females is turned off during development to prevent a dangerous double dose of proteins. A gene called Xist unquestionably turns off X chromosomes in mice, humans and other mammals. Because every cell needs one active X chromosome, Xist must be suppressed on one X in both females and males (which have an X and a Y chromosome). Which gene (or genes) does this is still in question, says Migeon.
In mice, researchers elsewhere pointed to the Tsix gene, because it suppressed Xist and was itself expressed only on the active X. However, studying cells from various human developmental stages, Migeon and her team discovered that human Tsix is expressed only on the inactive X chromosome, right alongside Xist. The two continue to be expressed together until after birth, when for reasons unknown Tsix gradually disappears.
"The difference is striking," says Migeon, also a professor of pediatrics. "In mice, researchers have suggested that Tsix was the gene in mammals that suppresses Xist and allows an X chromosome to remain active, but we've shown clearly that it does not do this in humans."
Migeon suggests instead that the mouse Tsix is involved in imprinting, a way cells determine which of two gene copies to use to make proteins that depends only on which parent the copy came from. In mice, X-inactivation in the placenta is imprinted -- the X from the mother is always "on." In other embryonic tissues, however, inactivation occurs randomly -- the X from either the mother or father could be on. In humans, X-inactivation is random for all tissues, including the placenta.
"Human and mouse Tsix are very different from one another," says Migeon. "Sequence differences and missing regions in human Tsix are a window on what's happening in the mouse and help explain why the gene doesn't have the same function in humans."
Much remains unknown about human Tsix, including what, if anything, it does in humans. However, Migeon will leave those mysteries for others to investigate, choosing instead to continue a 30-year quest to fully understand X-inactivation in human development.
"We expect to find a gene on one of the other chromosomes that turns off Xist in a random fashion," says Migeon. "It is difficult to envision how a gene on the X chromosome could, by itself, regulate the function of Xist on only one member of the X chromosome pair."
To track down Xist's true suppressor, Migeon and her colleagues are studying human cells with "trisomies" -- cells that have 23 pairs of chromosomes plus a third copy of one chromosome. In these cells, if the Xist-suppressing gene is on the chromosome with three copies, X-inactivation would be abnormal, Migeon says.
The studies were funded by the National Institutes of Health. Authors on the study are Migeon, Catherine Lee, Ashis Chowdhury and Heather Carpenter, all of Johns Hopkins.
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