The extra copy of Chromosome
21 that causes Down's
syndrome throws a spanner into the workings of all the other chromosomes as
well, said a study that surprised its authors.
Using data from a set of twins, one with Down's and one without, researchers
found a major difference in how genes functioned between the two.
The twins were born from a single egg that split, which means their DNA code
started off identical.
Down syndrome facts
But the individual with Down's had an extra copy of Chromosome 21 – a
difference enabling the scientists to see how this might affect the same
The additional chromosome had a knock-on effect on all the 22 other
chromosomes, the team reported in the journal Nature.
"We were very surprised by this finding," said Audrey Letourneau
of the University of Geneva Medical School who co-authored the study.
"It seems that this small, extra chromosome has a major influence on
the entirety of the genome."
Chromosomes are made up of bundles of DNA called genes, which hold the
information for cell function.
Humans have 23 pairs of chromosomes – or a total of 46 per cell.
Genetically caused mental disease
People with Down's Syndrome, about one in every 750, have a third, extra
Chromosome 21 – the smallest of all the chromosomes with about1% of the DNA in
Down's is the world's leading genetically-caused mental disease, and also
carries a heightened risk of heart defects, leukaemia, immune-system
malfunction and premature Alzheimer's disease.
The results may help researchers "find substances that could reverse
this gene-expression dysregulation, i.e. drugs that revert the expression of
genes... back to normal," co-author Stylianos Antonarakis told AFP.
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