'The importance of the [X chromosome] sequence goes beyond individual genes. We have also gained a deep insight into the way evolution has shaped the chromosomes that determine our gender to give them unique properties.'

"These detailed analyses of the X chromosome represent a monumental achievement for biology and medicine," says Francis S. Collins, M.D., Ph.D., director of National Human Genome Research Institute (NHGRI).

"They are exciting examples of what is being learned from the vast trove of sequence data produced by the Human Genome Project and made freely available to researchers around the world," Dr. Collins adds.

NHGRI is part of NIH, which led the U.S. component of the Human Genome Project along with the Department of Energy.

Complete DNA Sequence of Human X Chromosome

In the first study, an international team of more than 250 genomic researchers led by the Wellcome Trust Sanger Institute described an analysis of the complete DNA sequence of the human X chromosome. In humans and other mammals, sexual identity is governed by a pair of chromosomes referred to as "X" and "Y." Females have two X chromosomes, while males have one X chromosome and one Y chromosome.

One of the central goals of the effort to analyze the human genome is the identification of all genes, which are generally defined as stretches of DNA that code for particular proteins.

The new analysis confirmed the existence of 1,098 protein-coding genes on the X chromosome. Only 54 of the 1,098 genes have functional counterparts on the much smaller Y chromosome, which has been described as an "eroded" version of the X chromosome.

Interestingly, almost 10 percent of the genes on the X chromosome are part of a somewhat mysterious family of "cancer-testis antigens," which normally are expressed in the testis but also appear in certain cancers, making them possible targets for immunotherapy.

Deep Insight into Chromosome Evolution

The X chromosome's gene density is among the lowest for the human chromosomes that have been analyzed to date. Researchers say this may reflect a low density of genes on the ancestral chromosome that gave rise to the X chromosome, or it may indicate that genes coding for key proteins that are required in double dose were transferred from the X chromosome to other chromosomes during the course of mammalian evolution.

Despite its relatively low gene density, the X chromosome holds a prominent place in the study and understanding of human disease. This arises from the fact that any defects in genes on the X chromosome are often apparent in males, because the Y does not carry corresponding genes to compensate.

More than 300 diseases already have been mapped to the X chromosome, and though the X chromosome contains only 4 percent of all human genes, it accounts for almost 10 percent of inherited diseases caused by a single gene, which doctors often refer to as "Mendelian disorders." These "X-linked" disorders include red-green color blindness, hemophilia, varied forms of mental retardation and Duchenne muscular dystrophy.

"From studying such genes, we can get remarkable insight into disease processes," says Mark Ross, Ph.D., project leader at the Wellcome Trust Sanger Institute. "But the importance of the sequence goes beyond individual genes. We have also gained a deep insight into the way evolution has shaped the chromosomes that determine our gender to give them unique properties."

Chicken Connection

The research team compared the human X chromosome to the genome sequences of a variety of other organisms, including dog, rat, mouse and chicken. They found that the gene order of the human and dog X chromosomes were virtually identical.

Comparing gene order in the human and rodent sequences showed several segments had reshuffled in the rodent lineage, and an interesting, 9 million base pair region appears to have been deleted from the rodent chromosome after humans and rodents diverged from their common ancestor.

Of particular interest was the comparison of the human X chromosome to the sequence of the chicken. Most of the genes on the short arm of the human X are found on chicken chromosome 1, and most of the genes on the long arm of the human X are found on chicken chromosome 4. These findings support the idea that mammalian X and Y chromosomes evolved from an "ordinary" ancestral pair of identical chromosomes.

Unique Patterns of Gene Expression

The second study, which was supported by the NIH's National Institute of General Medical Sciences, focused on the activity of a large set of genes on the X chromosome.

Researchers at the Duke University Institute for Genome Sciences & Policy in Durham, N.C., and Pennsylvania State University in University Park surveyed the activity, or expression, of 471 genes on the X chromosomes of 40 women. To their surprise, they found that each woman's X chromosomes showed a unique pattern of gene expression.

More than 45 years ago, researchers discovered that most genes on one copy of a female's X chromosome are switched off -- a modification known as "X-inactivation." This mechanism thus reduced the level of female expression of genes on the X chromosome to the same level as that in an XY male.

Gene Expression at Variable Levels

Initially, it was thought the process resulted in a complete inactivation, or "silencing," of all of the genes on that copy of the chromosome in a female. However, in the late 1980s, researchers learned that some fraction of the genes remain active. The new work extends those findings to the complete set of X-linked genes.

Specifically, the researchers determined that due to the incomplete nature of X-inactivation, at least 15 percent of genes on the X chromosome produced proteins at higher, often variable, levels in females than in males. Furthermore, in some women -- but not in others -- an additional 10 percent of the X-linked genes are expressed at variable levels.

Much more work is needed to explore the implications of the new findings for human health and disease. However, "we now know that up to 25 percent of the X chromosome can be uniquely expressed in one sex relative to the other," says Duke's Huntington Willard, Ph.D., senior author of the study. "Such differences should be recognized as a potential factor to explain sex-specific traits, both in complex disease, as well as normal gender differences."