Female induced pluripotent stem (iPS) cells, reprogrammed from human skin cells into cells that have the embryonic-like potential to become any cell in the body, retain an inactive X
chromosome, stem cell researchers at UCLA have found.

The finding could have implications for studying X chromosome-linked diseases such as Rett syndrome, caused by mutations in a gene located on the X chromosome.

"The findings differ from those seen in mouse skin cells that are reprogrammed into iPS cells, in which the inactive X chromosome reactivates," said Kathrin Plath, senior author of the study and a scientist with the Eli and Edythe Broad Center of Regenerative Medicine and Stem Cell Research at UCLA.

We knew from our studies that in reprogrammed mouse cells, the X chromosome becomes active again, said Plath, an assistant professor of biological chemistry and one of the first scientists in the world to reprogram mouse and human adult cells into iPS cells. “The question we wanted to ask is what happens in female human iPS cells.”

The study appears in the Sept. 3, 2010 issue of the journal Cell Stem Cell.

All female cells have two X chromosomes – one from each parent - and in early development, one X chromosome is permanently inactivated. The inactivation of the X chromosome ensures that females, like males, have one functional copy of the X chromosome in each cell of the body and that the cells develop normally Plath and her team took human skin cells from females of varying ages. The cells have one active and one inactive X chromosome. The research team added four transcription factors to reprogram the cells into iPS cells and examined the resulting iPS cells to uncover the status of the X chromosome. They found that one X chromosome remained inactive, making the reprogrammed cells similar to most female human embryonic stem cells, which have one active and one inactive X chromosome. More than 30 different iPS cells lines were analyzed in the study, with the same result, Plath said.

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