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Electric eels spark hope on kidney stones

British scientists have discovered why electric eels do not get kidney stones but humans - and especially men - do.

After six years' work, a team at the Royal Postgraduate Medical School in London has discovered that a flaw in a gene on the female X chromosome leads to kidney stones - precipitates of calcium in the kidneys which harden into lumps and can cause excruciating pain while urinating.

Up to 1.5 million people in the UK aged over 25 are thought to suffer from the disorder, which can lead to complete kidney failure and costs millions of pounds every year in operations and treatments.

The researchers found that the genetic flaw causes the failure of a biochemical system which controls the movement of negatively-charged ions inside the kidneys. This means that positively-charged calcium ions are not dissolved into the urine.

A similar biochemical mechanism is used by electric eels to generate shocks: they shift negatively-charged ions rapidly around inside their cells to create a voltage, which is then used to ward off predators and stun prey. If an eel had the same genetic flaw as humans, it would lose its ability to give shocks.

Professor Rajesh Thakker, who led the team of scientists, said: "We started out by assuming there was a genetic basis, and looked for evidence." After interviewing 64 patients and 118 relatives from 11 families, the team determined that the cause was sex-linked: it is three times more common in men than in women. Also, if a man has kidney stones, his sons have a 45 per cent chance of developing them too.

Because the flaw is on a gene on the X chromosome, it is more likely to show up in men, who only have one such chromosome. Women have two, and so would need two faulty versions of the gene to develop the disorder.

The scientists' conclusion could lead to the development of drugs to treat the disorder, and should help doctors to predict the likelihood of patients' relatives developing the stones.

"What we really need now is to talk to more people ..." said Professor Thakker. "We think there may be up to ten [genes] involved, and we want to know just what part they all play." He added that the chances of stones recurring are between 70 and 80 per cent. "That's what makes it so important to really pin the causes down," he said.

In the meantime, the best way to prevent the disorder is to avoid hard water - which contains high levels of calcium - and dairy products.