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'Inert' Female Chromosome May Be Active

Finding points to unsuspected source of differences between men, women

WEDNESDAY, March 16, 2005 (HealthDay News) -- Something is stirring in the supposedly inactive female chromosome that plays a major role in genetic differences between men and women, researchers report.

One pair of chromosomes out of the 23 found in human cells determines sex. Women have two full-sized X chromosomes, while men have one X and a smaller Y, which holds the genes for masculine traits. To even things up, one of the two X chromosomes is inactivated in women, according to standard genetics textbooks.

Those textbooks may now need to be revised, according to a report in the March 17 issue of Nature. Studies have detected activity by some of the genes on the supposedly inert X chromosome, researchers conclude.

"Our study shows that the inactive X in women is not at silent as we thought," said co-researcher Laura Carrel, an assistant professor of biochemistry and molecular biology at Pennsylvania State University College of Medicine.

It's too early in the game to say exactly which genes are active and what this all might mean medically, Carrel said, but the finding does hold interesting possibilities.

"The effects of these genes from the inactive X chromosome could explain some of the differences between men and women that aren't attributable to sex hormones," she said.

Carrel has been studying skin cells from a number of women, using laboratory systems she developed to compare the activity of 94 genes in both the active and inactive X chromosomes. She found that "25 of the genes we looked at were expressed in the inactivated X chromosome," with "an intriguing level of variability" in their activity from woman to woman.

"The surprising aspect was the discovery of the extreme variability seen between different women," said study co-author Huntington L. Willard, head of the Institute for Genome Sciences and Policy at Duke University, where most of the work was done. Carrel recently left Duke to start her own laboratory at Penn State.

"We expected that all men were alike [genetically], within reason, and all women were alike, within reason," Willard said. "Now we have this subset of genes on the X chromosome that were expressed at different levels -- not only different numbers of genes, but also different levels of expression."

While this is work that remains in the realm of basic science, "the potential big payoff is in trying to understand diseases that are found to occur in different frequencies in men and women," Willard said. "Also, understanding the habits of behavior in neurocognition, which are different in men and women. It has been thought to be due to hormonal or sexual differences. This work could suggest a basis for the difference that lies in the human genome."

Carrel said she is continuing and extending this research. One new aspect of her lab's work will be to study possible differences in the proteins whose production is governed by gene activity. "We want to find out whether there is a difference at the protein level, and whether there is possible applicability to disease," she said.

She also will be looking at X chromosomes in other body tissues, to see whether the same pattern of activity exists in them. So far, the genes' specific function does not seem to determine their activity or inactivity, she said. "What seems to matter more is where they are located on the chromosome," Carrel said.

More information

A basic guide to the X chromosome is provided by the National Library of Medicine.

SOURCES: Laura Carrel, Ph.D, assistant professor, biochemistry and molecular biology, Pennsylvania State College of Medicine, Hershey; Huntington L. Willard, Ph.D, director, Institute for Genome Sciences and Policy, Duke University, Durham, N.C.; March 17, 2005, Nature
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