An international research team led by scientists at the Institute of Ophthalmology, UCL (University College London), has found high levels of zinc in the deposits in the eye which are a marker of age-related macular degeneration (AMD), the leading cause of blindness in the elderly in the developed world. The finding, published in the latest edition of Experimental Eye Research, contributes to a better understanding of AMD and could influence the development of effective treatments.
AMD is a form of macular disease which affects the eye's retina. An estimated half a million people in the UK have it, and 40 per cent of these are over 75. Despite the potentially devastating impact on patients’ quality of life, no successful therapy to stop or reverse the progression of AMD is available in the majority of cases.
An early sign of the disease is the formation of microscopic structures called ‘drusen’ in the eye. Exactly what the effects of these are and why they form is not yet fully understood. Explaining the research, Dr Imre Lengyel, UCL Institute of Ophthalmology, said: “We have discovered that drusen in eyes with AMD have very high levels of zinc.
“Zinc had previously been shown to contribute to the formation of plaques in the brains of patients with Alzheimer’s disease, so it was logical for us to test the idea that zinc might also contribute to the formation of plaque-like drusen in the eye as well. AMD can be considered as the Alzheimer’s disease of the eye, in that both involve the build-up of proteins and metals like zinc and copper into microscopic clumps.
“What is particularly important is that within the zinc that we found, there is a small pool – about 5-10 per cent – of what is known as ‘free’ or ‘loosely-bound’ zinc. Generally, zinc is essential to keeping a molecule’s shape, but mobilised zinc can cause lots of problems. However, since it is a small proportion of the overall zinc pool, it’s straightforward to target it. That’s what researchers are beginning to do with Alzheimer’s disease, developing methodologies and drugs that can capture this mobilised zinc and see if it slows down the degenerative process. This study shows that we could now potentially take a similar route for AMD treatment.”
The researchers looked at eyes with AMD that contained several large deposits and compared them to eyes from a similar age-group that had no known eye disease and no deposits in the macula. They analysed these using zinc sensing molecules, which glow when they bind with zinc. These glowing molecules only bind to the free or loosely-bound zinc which is particularly crucial in terms of the development of treatments.
Although AMD rarely results in central vision being lost, the condition can cause problems with reading, recognising people, seeing small objects and driving. The disease is more widespread in women and common risk factors are a positive family history and smoking. There are two forms of AMD – dry and wet. In dry AMD, visual cells stop functioning and die, whereas wet AMD is linked to new vessel growth and is the more aggressive form of the disease. There is currently no treatment for dry AMD, but there has been some progress for wet AMD, such as photo-dynamic therapy (PDT) and the use of drugs that stop new vessel growth. These are, however, only suitable for patients with advanced disease, are often temporary, and carry the risk of adverse reactions. The possibility of stopping or reversing drusen growth has the potential to arrest the progression of AMD at an early stage, before irreversible damage to the retinal cells occurs.
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