Scientists at Geneva's University have succeeded in identifying one of the genes responsible for two forms of profound deafness.
Half of all serious deafness is caused by genetic mutation, and this latest discovery raises hopes that congenital deafness might one day be treatable.
The Geneva team, led by Professor Stylianos Antonarakis, concentrated its efforts on Chromosome 21, which it succeeded in sequencing last May.
"Once we had decoded this chromosome, our next step was to identify which genes caused which disorders in humans," Antonarakis told swissinfo.
He found that Chromosome 21, the smallest in the human genome, contains a gene, named Echos1, which can cause two kinds of profound deafness: one which is present from birth and another which develops during childhood.
As well as locating and identifying Echos1, Professor Antonarakis also observed a mutation in which a foreign piece of DNA inserts itself into the gene, deactivating it and causing deafness.
The team isolated the gene responsible and discovered this hitherto unknown mutation after comparing the genes of 200 people suffering from profound deafness and those of healthy Chromosome 21.
The mutation causes deafness because it prevents the gene from producing a protein present in the cochlea, the part of the inner ear containing the nerve endings essential for hearing.
"We do not know the exact function of this protein, and that will be the focus of our future research," Antonarakis says.
He added that it would take two or three years to understand the purpose of the protein and the mechanism of the disorder: "After that, we may be able to envisage new treatments."
Between three and five per cent of severe deafness is caused by this mutation in Chromosome 21.
"Chromosome 21 makes up just one per cent of the human genome, so you could say that it encodes one per cent of the genes associated with human disorders," Professor Antonarakis told swissinfo.
"But there are rare and common disorders that map to Chromosome 21."
The most notable of these is Down's Syndrome, in which the affected person has an extra 21st chromosome. But other disorders include forms of congenital blindness, epilepsy, manic depression and arteriosclerosis.
"We are going through these systematically to try to match the disease to the disorder. But this research has now really speeded up, because we have the sequence, because we know the infrastructure of the chromosome," Antonarakis says.
"It has brought us closer to understanding the mechanism of disorders, and once we understand the mechanism, we can introduce treatments," he adds.
Professor Antonarakis's discovery is published in this month's edition of journal, Nature Genetics.
By Roy Probert