After five years of work, Johns Hopkins researchers
report that a particular genetic region long assumed to be
a critical factor in Down syndrome isn't nearly as
important as once thought.
Their report in the Oct. 22 issue of Science,
based on studies in genetically engineered mice, shreds a
30-year-old notion that genes in this region are largely
responsible for the condition's characteristic facial
features and some of its other common traits. Down
syndrome, which affects roughly one in 700 live births, is
the most common genetic cause of mental retardation and
congenital heart disease.
"The simplistic idea that just one of the hundreds of
genes on chromosome 21 affects development no longer holds
up," says Roger Reeves, professor of
molecular biology and
genetics in Johns Hopkins' Institute for Basic
Biomedical Sciences and
McKusick-Nathans Institute of Genetic
Medicine. "Now researchers can take a deep breath,
accept that the syndrome is complex and move forward."
Down syndrome occurs when three, instead of two,
copies of chromosome 21 are present in a fertilized egg,
although rare cases occur when a section of the chromosome,
rather than the whole chromosome, is found in triplicate in
a situation called segmental trisomy.
A small region of this replicated segment is found in
triplicate in all people with segmental trisomy and Down
syndrome's facial features, and so it had been dubbed the
"Down syndrome critical region" or DSCR. Proponents of
DSCR's presumed role had focused on its consistency in
people with segmental trisomy but largely ignored the fact
that no one with this condition has only that region in
triplicate, Reeves says.
To see whether DSCR is as critical as many suggested,
Lisa Olsen, then a graduate student, created
"chromosomally" engineered mice and found that mice with
three copies of just their DSCR equivalent actually had
facial and skeletal changes opposite of those seen in Down
syndrome.
"These mice weren't normal, but they weren't Down
syndrome mice, either," says Reeves, whose lab had already
spent 15 years studying the mouse version of DSCR. "Their
faces were longer and narrower than normal, but Down
syndrome is characterized by shorter than normal facial
bones."
DSCR doesn't seem to be required for Down
syndrome-like features to result, either, the researchers
report. Olson found that mice with just two copies of DSCR
but three copies of the rest of the chromosome did have the
shorter bones characteristic of Down syndrome.
To measure DSCR's effects in mice, former Johns
Hopkins Professor Joan Richtsmeier, now at Pennsylvania
State University, used mathematical models she developed to
compare the length, angles and positions of the facial
bones of the mice to Down syndrome's effects in people.
"Some genes in the region contribute to the effects on
facial bones, but, in triplicate alone, this region
produces different traits than those seen in Down
syndrome," Reeves says. "If anyone is going to try to treat
the problems seen in Down syndrome, we need to understand
what is really happening and when in development it
happens.
"Until very recently, we wouldn't have even thought it
possible to 'treat' Down syndrome problems," he adds. "The
task seems insurmountable — the genetic problem is
there from conception; it's in every cell. But now we're
beginning to identify 'developmental cassettes' in mice in
which specific problems caused by a triple genetic dose
might be modifiable — if we can figure out the key
players."
As part of this effort, Olson used a technique to
precisely duplicate a defined chromosome segment and
applied it to the DSCR section on mouse chromosome 16, the
analog of human chromosome 21. She also made a mouse
chromosome 16 that lacked DSCR entirely.
Then scientists in Hopkins' Transgenic Mouse Facility
inserted each of the engineered chromosomes into mouse
embryonic stem cells, creating stem cells with either three
copies of DSCR or only one copy of DSCR. These stem cells
were then used to create chimeras — animals whose
makeup comes partially from their original cells and
partially from the inserted engineered stem cells.
Physical inspection of these animals' offspring showed
that triple-DSCR mice were bigger than normal mice. Mice
with only one copy of DSCR were smaller than normal,
similar to a well-studied mouse version of Down syndrome
that has three copies of many more of the genes found on
human chromosome 21.
Breeding the single-DSCR with the well-studied Down
syndrome mouse produced a mouse with only two copies of
DSCR but three copies of all other genes on mouse
chromosome 16. That "hybrid" mouse was similar to its Down
syndrome parent but more mildly affected, the researchers
report.
Reeves' lab is now testing another long-standing but
poorly supported tenet of Down syndrome research by using
the mouse models to study the involvement of neural crest
cells, precursors to structures affected in Down syndrome,
including the face, heart and the nerves that serve the
intestines.
The research was funded by the National Institute of
Child Health and Human Development. Authors are Olson and
Reeves of Johns Hopkins, and Richtsmeier and Jen Leszl of
Penn State. Olson was supported by a fellowship from the
Howard Hughes Medical Institute and is now assistant
professor of biology, University of Redlands, Calif. Sarah
South and Gail Stetten, both of Hopkins, proved the DSCR
status of the mice.