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Genes of female X chromosome cataloged
CHICAGO — An international team of researchers said yesterday that they have cataloged all the genes on the female X chromosome, a feat expected to enable fresh insights into women's health and add a genetic component to the debate over differences between the sexes.
Described by the head of the Human Genome Project as "a monumental achievement for biology and medicine," the genetic map should help scientists better understand more than 300 X-linked diseases — such as hemophilia and Duchenne muscular dystrophy — that mothers pass on to their sons.
"From studying such genes, we can get remarkable insight into disease processes," said Mark Ross, project leader at the Wellcome Trust Sanger Institute in Hinxton, England.
The accomplishment was reported in the British journal Nature.
In humans and other mammals, sexual identity is governed by a pair of chromosomes known as X and Y. Every female has two X chromosomes, inherited from both parents, while all males have one X from their mother and one Y chromosome from their father.
With more than 1,000 genes and 160 million base pairs of DNA, the X looms like a giant next to the stunted Y chromosome that produces males. The Y chromosome — 78 genes, 23 million DNA subunits — was sequenced in 2003.
Because males carry only one X chromosome, they are particularly vulnerable to diseases carried by defective X genes. Women who carry such defective genes are usually protected by their backup copy of the X, but if their sons inherit that disease-causing X they are in trouble.
More than 300 diseases have been mapped to the X chromosome, and although it contains only 4 percent of all human genes, it accounts for almost 10 percent of inherited diseases caused by a single gene.
The sequencing was carried out by more than 250 researchers as part of the Human Genome Project. They worked at the Sanger Institute, Baylor College of Medicine in Houston, Washington University School of Medicine in St. Louis, the Max Planck Institute for Molecular Genetics in Berlin, the Institute of Molecular Biotechnology in Jena, Germany, and Applied Biosystems in Foster City, Calif.
In a companion paper in Nature, researchers at Duke and Penn State universities reported on the expression, or activity, of 471 genes on the X chromosomes belonging to 40 individual women. They were surprised to find such a large amount of variation.
Researchers also compared the human X chromosome to the equivalent in other animals, opening a window into the evolution of sex chromosomes. Until mammals began to replace reptiles and rule the Earth about 300 million years ago, the X and Y were of equal size and able to swap genetic material.
But over eons, the male-producing Y has been riddled by mutations, and the trading of genes trailed off. The chromosome withered away in size, and its function became limited to establishing maleness, ordering the building of male sexual organs and conferring the ability to produce sperm.
The X acquired more responsibilities and genes while the Y "slowly but surely dropped off the face of the Earth," said Dr. Steven Scherer, director of mapping at the Baylor College of Medicine Human Genome Sequencing Center.
"Although it contains a few important genes, it's almost like the appendix of the human genome."
Although the studies published yesterday are expected to galvanize X-chromosome research, it may take years for advances to be felt. Of the 1,000 genes identified, science has a good idea of what only 500 or so do.
"When the stories of all the chromosomes are eventually published, I expect this chromosome [X] to be the All-Star," Scherer said.
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