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 Researchers
Find New Evidence Against Common Down Syndrome Theory
BBC News, October 22, 2004
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Researchers at
Johns Hopkins University say they have found evidence to
disprove the commonly held notion that Down syndrome is caused
by abnormalities in the so-called Critical Down Syndrome Region
of the brain. The researchers say they now believe Down syndrome
is caused by a combination of genetic and developmental factors,
and that understanding these is crucial to treating people with
the disorder.
A particular genetic region long assumed to be a critical factor
in this condition is not as important as thought, says the Johns
Hopkins team.
The US researchers studied mice engineered to have the 'culprit'
genes believed to be responsible for causing Down syndrome.
They told the journal Science that the cause was much more
complicated.
Conflicting results
They believe Down syndrome arises from an interplay of complex
genetic and developmental factors.
In Down Syndrome, an extra copy of one chromosome is inherited,
giving a person three copies of chromosome 21 instead of the
usual two. This is known as Trisomy 21.
Previous studies have supported the idea that extra genes within
a critical region of chromosome 21 could be the root of the
problem.
Rare cases of Down syndrome occur when only a segment of
chromosome 21, and the genes housed within it, is triplicated.
But Dr Roger Reeves and his colleagues say this notion can be
disproved by measuring Down syndrome-like characteristics in
mice that are genetically engineered to possess the suspect
genes.
Children and adults with Down have a distinctive facial
appearance as well as possessing three copies of chromosome 21
in their cells.
The researchers bred mice with one, two, or three copies of the
critical region housed within chromosome 21, and compared them
to other mice expressing both visible and genetic Down
syndrome-like characteristics.
The mice bred to have copies of only the critical region had
facial and skeletal changes different to those seen in Down
syndrome.
Dr Reeves said: "These mice weren't normal but they weren't Down
syndrome mice either.
"Their faces were longer and narrower than normal, but Down
syndrome is characterized by shorter than normal facial bones.
"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."
Too simple
Peter Elliott, of the Down Syndrome Research Foundation, said
the idea that there was a Critical Down Syndrome Region was too
simplistic.
"Our research is based upon an understanding of all the genes
and how the extra 163 genes in Trisomy 21 affect the metabolism
and so affect growth, development, health, and well being."
Mr Elliott said it was clear that the extra 163 genes produced
extra chemical messengers which could disrupt the function of
any of the 22,450 genes contained in the cells of people with
Down.
"Although the interaction of 163 extra genes affecting 22,287
other genes is very complex, we can look at the most serious
outcomes, identify the particular gene at fault, then, develop a
therapy to counteract the effect of that particular gene.
"But research to find treatment therapies and a cure for Down
syndrome is very complex. Millions will be needed to find the
answers."
A spokesman from the Down Syndrome Association said: "It is
encouraging to see that research is continuing into the
condition of Down syndrome and producing results that help us
understand the genetic make-up of chromosome 21 even further."
He said although the results disagreed with the findings of
previous studies, they should be viewed as positive because they
suggested that further research might lead to developments in
treating specific health problems common to people with Down
syndrome.
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