can send PDF copy of Nature article. Just notify me. RCJ
Scientists map Down syndrome chromosome
The smallest of the human chromosomes is also associated with epilepsy, Lou
Gehrig's disease and Alzheimer's
Tuesday, May 9, 2000
By RICK CALLAHAN of The Associated Press
In an achievement that could point the way to treatments for a host of
illnesses, scientists have mapped chromosome 21, the smallest human chromosome
and the one associated with Down syndrome, epilepsy, Lou Gehrig's disease and
It is the second human chromosome whose DNA has been been fully deciphered.
Chromosome 22 was mapped last fall.
"Another volume has just been placed on the shelf. Now we really have to
roll up our sleeves and assess what these genes are doing there, what role they
play in causing disease," said Francis Collins, chairman of the Human Genome
Project at the National Institutes of Health in Bethesda, Md.
The German- and Japanese-led team that mapped chromosome 21 is part of the
Human Genome Project, an international effort to decipher chemically the entire
human genetic blueprint.
The project also announced Monday that it is moving into the final phase of
cataloging the human genes.
Having collected about 90 percent of the estimated 3 billion letters in the DNA
code, the project is moving on to a final "finishing" phase in which it will
attack the difficult sections that remain and proofread what has been acquired.
"We have made a really important transition and we're very excited about
it," said Eric Lander, director of the Whitehead Institute Center for Genomic
Research, one of 16 laboratories involved in the Human Genome Project.
Chromosome 21 contains relatively few genes, but they are in a complex
tangle. Humans are typically born with 23 pairs of chromosomes, which are made
up of genes.
"When one stares at a sequence like this, it makes you realize how complex
it really is," said Huntington F. Willard, chairman of genetics at Case Western
Reserve University. He was not involved in the research.
"It's not just a simple string of 225 genes," Willard said. "It's really a
mess, a hornet's nest, a hodgepodge of duplications, altered sequences and
arrangements that determine the health and welfare of our species."
The findings will be published in Thursday's issue of the journal Nature.
The researchers found that chromosome 21 contains far fewer genes than the 545
in chromosome 22, the second-smallest chromosome.
The relative sparseness of genes in chromosome 21 could mean the total
number of genes in human DNA is less than 40,000 and not the 100,000 or more
that scientists had thought, said Andre Rosenthal, a professor at the Institute
for Molecular Biotechnology in Jena, Germany.
That means "we are not so different from Drosophila," the fruit fly, "or
yeast," Rosenthal said.
The map of chromosome 21 is 99.7 percent complete. Technical limitations
prevented a complete mapping, said David Patterson, a U.S. scientist involved
in the project.
Patterson said the map might allow researchers to home in on specific genes
in the chromosome that cause mental retardation in people with Down syndrome
and then perhaps develop drugs to treat such patients.
"Once we can find the genes that are important for learning problems, what
we hope is that we'll be able to understand what those genes do and somehow
compensate for having an extra copy of the gene," said Patterson, president of
the Eleanor Roosevelt Institute in Denver and chairman of the science advisory
board of the National Down Syndrome Society.
Down syndrome occurs when a person is born with an extra, third copy of
Down syndrome, the most common form of genetically caused mental retardation,
occurs in about one in 700 live births. Down syndrome also can cause congenital
heart disease and Alzheimer's by age 40.
Rudolph Tanzi, a professor of neurology at Harvard Medical School, said the
chromosome map could eventually shed light on why people with Down syndrome
have a low rate of breast, lung and gastrointestinal cancers. It could be that
an extra copy of chromosome 21 has tumor-suppressing qualities.
"One can argue that an extra dose of a gene will usually be a bad thing,
but once in awhile it can be a good thing. It can be protective," Tanzi said.
The Human Genome Project expects to have a rough draft of the entire human
genetic blueprint done this summer.
The public project, expected to finish its work by 2003, is competing
against a private company, Celera Genomics Corp. of Rockville, Md., which hopes
to sell the information to pharmaceutical companies and others.