Never mind cholesterol, tobacco, ultraviolet overdose. The No. 1 risk factor in highly developed countries is aging. Topping the list of age-related ailments are heart disease, cancer and incipient blindness.
People today in technologically advanced societies are experiencing ever-increasing life spans. A century ago, in 1900, life expectancy averaged 50 years. Today, it's past 70, and counting. But paradoxically, the blessings of living longer go hand in hand with the increased incidence of age-related macular degeneration (AMD). Between 6 million and 10 million Americans are blind from AMD, and hundreds of thousands of cases are diagnosed in the U.S. every year. If things continue, the next 25 years will nearly double the incidence of AMD.
It figures. AMD blights a region in the middle of the retina. This central spot features an oval dimple or depression - the macula.
"The retina is a receptor for light," pointed out vision biochemist John Crabb, at the Cleveland Clinic Foundation. "Exposure to light," he explained, "can cause photo-oxidation in the retina. There's normally a lot of oxidative stress in the retina. Every day the tips of the photoreceptors are shed and phagocytosed by the retinal pigment epithelium [RPE]. It may also contribute to debris called drusen' - German for stones, crystals, nodules."
The photoreceptor cells are dense enough to define very fine print. Reading, sewing and watching TV remain some favored activities of very elderly individuals, but macular degeneration deprives them of those last sedentary enjoyments. There is no cure for AMD's progressive loss of vision.
Crabb and cell biologist Joe Hollyfield are co-senior authors of a paper in the current Proceedings of the National Academy of Sciences (PNAS), released online Oct. 21, 2002, and titled: "Drusen proteome analysis: An approach to the etiology of age-related macular degeneration."
"With this PNAS paper," Crabb told BioWorld Today, "we've firmed up the link between oxidative damage and AMD - at least in the formation of drusen proteins. What's new is we've found evidence of oxidative modifications on proteins. Before," he continued, "it was only suspected that oxidative damage had something to do with AMD, while smokers were known to double their risk of the disease. Just this last year, a report came out that showed high-dose daily doses of antioxidant vitamins reduces the progression of drusen formation in select individuals in the population."
Genetic Basis For Macular Degeneration?
"I think the basis of the selection," he surmised, "is genetic. Some people get lung cancer when they smoke and others don't. When some people smoke, their risk for AMD increases; in others it doesn't. We don't know what the genetic connection is. We used to think that maybe there's one gene that causes AMD but now we think many genes may contribute to it. As we get older our ability to resist oxidative stress apparently decreases, and in the process we have modifications to our genes and proteins." Crabb made the point, anent its genetic etiology, that "AMD has a 100 percent concordance in identical twins. If one twin has AMD, the other one is going to get it."
He described how he and his co-authors carried out their drusen analysis. "There were two things that were done here," he recounted. "One was the development of a method to isolate drusen. [Hollyfield] is responsible for that. He developed a way to dissect the human eye and pluck these little deposits off of the membranes, with capillary-type tweezers. I analyzed the protein content in the drusens. For the proteomic approach, we had some of the most up-to-date liquid chromatography and mass spectrometry methods available.
"We identified 129 proteins in the drusen deposits. The way that process works, we extracted the lipids from the drusens and then, with proteases, digested the protein into small pieces. So we made peptides out of the proteins, then separated the peptides by capillary liquid chromatography and measured them in the mass spectrometer. We sequenced those peptides as they went through the process, compared them to sequence databases and identified what proteins we had seen.
"For years," Crabb went on, "there's been some controversy about where drusen comes from. Our results show that the proteins we identified originated from both the retinal pigment epithelium and the systemic circulation of the blood. So, my theory is that the oxidative modifications come primarily from the retina. That oxidative stress in the retina and the turnover in the photoreceptor generate a lot of lipid oxidation products. There's so much of it that the retinal pigment epithelium can't handle it all and they get spit out into the bloodstream where they can modify proteins. It also modifies proteins in the RPE. So we have proteins from retinal tissue, as well as blood."
In their ongoing research, the Cleveland team is "trying to confirm that oxidative modification contributes to the formation of drusen. We've looked at only a few eyes from AMD patients: 18 donor eyes from people with normal vision; five eyes from patients with AMD," he said. "We need to examine more eyes from AMD donors. We'd like to see if we can discern differences in the protein population between AMD and normal donors. Then we want to further analyze and identify the oxidative modifications that are on proteins."
Seeking Drusen Therapies In Oxidative Behavior
Beyond this investigative program awaits the eventual application of these findings to preventive and therapeutic compounds against AMD that could emerge some day from their work. "Maybe there's a detoxifying protein that's produced in the retina," Crabb speculated, "which is inactivated as we get older. If we could figure out what that protein is, it could potentially become a therapeutic agent. Another possibility would be some scavenger that would trap these reactive oxygen species that seem to modify drusen proteins.
"The antibodies on which we have patents pending hopefully cover a method for predicting who might be susceptible to developing AMD," he pointed out. "It would allow us to pick up these modifications before the disease develops, then tip off the clinician that this patient might benefit from antioxidant vitamins," Crabb concluded, "or some other therapy that evolves down the line."