Researchers
at The University of Hong Kong believe they have found the mouse gene
whose human counterpart, when damaged, may be responsible for inner
ear disorders.
About
one in 1,000 infants are born with hearing defects. Of these, half
have balance problems caused by abnormal development of the inner
ear and in two-thirds of cases the defects are genetic. The road to
the genes discovery began in 1995 when Principal Investigator
Prof Kathryn Cheahs PhD student, Keith Leung, accidentally produced
a mouse mutant with an inner ear defect.
Studying
how genes are controlled, Keith injected DNA into mouse eggs producing
a transgenic mouse carrying the foreign genes in its chromosome. One
of the mice and its offspring were noticed because of abnormal circling
behaviour and because they had yellow coats.
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Prof
Cheah in her laboratory at Hong Kong U. |
Tests showed they could not swim like normal mice, and they could
not tell which way was up. Because of this, they were nicknamed Yellow
Submarine (Ysb) mice. The behaviour was indicative of a balance
problem caused by some kind of defect in the inner ear, said
Prof Cheah. The Ysb mice were also found to be deaf. Chance played
a large part in producing the Ysb mouse, added Prof Cheah. When
the DNA was injected, it could have inserted itself in any of the
mouses 40 chromosomes. Most
of the time, the DNA insertion is harmless and doesnt disrupt
any of the genes, she said. The next step in the research was
to discover where the DNA had inserted itself, and which gene had
been affected. Eventually, the abnormality was traced to Chromosome
3.
The identity
of the Ysb gene has yet to come out in a publication but Prof Cheah
hopes to go public on the discovery before the end of the year. She
said: What happened was a very complicated event; the DNA had
caused part of the chromosome to break and to turn around in two directions
and basically rearrange. We wanted to find out the molecular change
and what gene had been disrupted. Prof Cheahs research
took a fortunate turn when she was contacted by Karen Steel and Charles
Tease of Britains Medical Research Council. They had another
mouse mutant with a similar inner ear defect, the product of exposure
to Xrays.
In
the collaboration that followed, the two mice were mated. The resulting
double mutant offspring had the same inner ear defect; confirmation
that the same gene was involved. Contrary to expectation, however,
the defect arose not because the gene had been destroyed but because
regulatory DNA had been disrupted. Said Prof Cheah: Regulatory
DNAs are like switches attached to genes which tell them when and
where to turn on in the body. In our mice, we had disrupted one of
the switches that directed the expression of the gene to the inner
ear.
Most
human genes have a counterpart in the mouse and are organised similarly
in the corresponding chromosomes. Potentially, this could mean the
same gene is damaged in families with a genetic defect that affects
the inner ear and hearing.
In
the long term, we may be able to repair the fault by gene or stem
therapies, said Prof Cheah. The difficult with congenital
defects is repairing them in the foetus. If
we could find a way of delivering this gene to restore growth of the
cells that are responsible for balance and hearing, we may also be
able to correct the defect.
Principal
Investigator
Prof Kathryn Cheah: hrmbdkc@hkusua.hku.hk
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Mouse
Chromosome 3 and the location of Gene X.
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