Dogs in shades shed light on blindness

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Sausage dogs in designer sunglasses are helping scientists investigate a type of human blindness.

Like some humans, dachshunds suffer from a group of inherited eye diseases called cone-rod dystrophies (CRDs).

The disorders cause progressive loss of the retina's "cones" - the photoreceptor cells that respond to bright light and colours.

Destruction of the cones causes sufferers to become "day blind" and unable to see in bright conditions.

At first the eyes continue to work perfectly well in dim light, when other photoreceptors called "rods" come into play. But eventually the rods also deteriorate, leading to total blindness.

Scientists have now identified a genetic mutation in standard wire-haired dachshunds that may be responsible for the disorder.

They believe it could be having the same effect in humans, raising the prospect of new therapies that target the defect.

The dogs taking part in the research, conducted at the Norwegian School of Veterinary Science in Oslo, wear specially designed sunglasses to help them see in daylight.

Screening the dachshund's genetic blueprint revealed an abnormal chromosomal region that appeared most often in CRD-affected dogs.

Further investigation showed that part of a gene called nephronophthisis 4 (NPHP4) was lacking from day-blind animals.

The find excited the scientists, because NPHP4 had previously been implicated in human disease.

Dr Frode Lingaas, from the Norwegian School of Veterinary Science, said: "This gene has been associated with a combination of kidney and eye disease in human patients. Here, we found a mutation that affects only the eyes, suggesting that this gene might be a candidate for human patients with eye disease only."

The researchers, whose findings are reported online in the journal Genome Research, believe the mutant form of NPHP4 produces a defective protein that is missing a vital part. Normally this missing "domain" would interact with other proteins involved in eye function. The protein still retained another region with a kidney function role.

"The new information that the NPHP4 gene can be involved in eye diseases only can shed light on the etiology (causes) of some low-frequency eye diseases in people where similar mutations may be involved," said co-author Dr Kerstin Lindblad-Toh, from the Broad Institute of MIT (Massachusetts Institute of Technology) and Harvard University in Boston.

Knowing the mutation could make it possible to screen out new cases of the disorder in dogs, said Dr Lingaas.