By David N. Leff
It¿s not just the neurodegenerative diseases ¿ such as Alzheimer¿s and Parkinson¿s ¿ that mark the passage from prime of life to advancing years. Another affliction of advancing years is opacifying of the eyeballs¿ lenses ¿ that is, age-related cataracts.
¿Age-related cataract is a multifactorial disease,¿ observed biological physicist George Benedek, at Massachusetts Institute of Technology. ¿Many different mechanisms can play a role there. And there¿s no genetic component to it,¿ he added. ¿Just the consequence of aging. It¿s a lifelong process.¿
Benedek is senior author of a paper in today¿s Proceedings of the National Academy of Sciences (PNAS), dated Feb. 29, 2000, which deals with an entirely different type of cataract. It bears the title: ¿Molecular basis of a progressive juvenile-onset hereditary cataract.¿
¿This particular cataract is extremely rare and unusual,¿ Benedek told BioWorld Today. ¿Very few families have it. What¿s special and interesting about juvenile cataract is that there¿s a genetic modification that occurs, which is not present in normal adult cataracts. Its course is relatively rapid compared to age-related cataract. In affected children, their eyes are born clear, but then the lenses become more and more opaque until at about 4 or 5 years of age, or older, these punctate [dotted] cataracts are so serious they have to have the lens surgically removed.¿
Clinical pediatric ophthalmologist David Walton is a senior surgeon at the Massachusetts Eye and Ear Infirmary in Boston, and a co-author of the PNAS paper. ¿These opaque dots in the lens are rather dramatic,¿ he told BioWorld Today. ¿Like crystalline nuggets, they are easily seen. Each one is perhaps a tenth of a millimeter in size. Within the lens ¿ which itself is possibly as big as a dime ¿ hundreds of them may be scattered all over.¿
Not Just Impaired Eyesight, But Blinding Light
Walton explained how these cataracts impair vision in their young patients. ¿Legal blindness is 20-200 ¿ that¿s the big E¿ on top of the chart. It means on the order of 5 percent visual acuity. And these children generally have about 20-60, or 20-80. That means 50 percent of normal visual acuity by age 6. But what they have even more,¿ he pointed out, ¿is tremendous intolerance to light. That¿s the scattering of light caused by these opaque little nuggets in the lens. Lens-removal surgery is often done to relieve the light sensitivity as much as to improve their vision.¿
The lens of a human eye consists of highly concentrated solutions of proteins called crystallin. Last year, other scientists determined that juvenile cataract results from a point mutation in one such protein, called gamma-D. In its amino-acid chain, an arginine is swapped for a cysteine. (See BioWorld Today, Feb. 4, 1999, p. 1.)
Benedek pointed out that that group ¿had just identified the mutation by linkage analysis in the gamma-D crystallin gene. And that¿s where they left it. Moreover, what was not known was why that mutation leads to opacification. What our PNAS paper did,¿ he said, ¿was answer the second question. We have actually expressed the mutant protein ¿ in an E. coli host cell ¿ and shown thereby that whatever properties are different are indeed at the protein level.
¿How does that substitution lead to opacity? The short answer,¿ Benedek went on, ¿is that this mutant cysteine group is able to form a linkage with another cysteine group on another gamma-D crystallin molecule, and then crosslink one another. The point is that in the native form, there is only one cysteine available per protein for that crosslinking, so the most you can form is a dimer ¿ a duplex molecule. That¿s not big enough to scatter light very effectively. That¿s why the lens is transparent in a normal person¿s eyes.
¿But in the mutant,¿ Benedek continued, ¿you have two sites, two cysteines, that get exposed on the protein¿s surface. And as a result, one can link with one protein, another with a second protein, and a third with a third. At that point, they form not mere dimers, but oligomers. Those larger multi-molecules, if they¿re big enough, will scatter light sufficiently to make the region where they¿re clustering, turbid ¿ clouded.¿
This newly described mechanism, the PNAS paper mentions, suggests that ¿a pharmacological agent that blocks the reactive thiol should prevent the formation of this cataract.¿ Benedek explained the role of that thiol:
¿The cysteines have a thiol group ¿ a sulfur-hydrogen chemical moiety. Two of these thiols can link with one another to form a sulfur-sulfur bond. If we can prevent that bond from forming,¿ he observed, ¿keep it in the sulfur-hydrogen form, then it can¿t form the sulfide linkage. That¿s the principle of such a therapeutic compound. But actually putting such a reagent into a living human eye ¿ and finding a safe one to do it,¿ he added, ¿is a hundred miles away.
¿What we¿re just simply suggesting is that if an agent blocks the reactive thiol, that would prevent forming the sulfur-sulfur bond. And to do that is a big deal. We can¿t do it for the simple reason that it would involve using human beings. We¿re just a physics lab here at MIT.¿
Wanted: Transgenic Mice To Scope Drug Design
¿If one wanted to follow up on this story,¿ Benedek observed, ¿you would want to create the arginine-to-cysteine mutation in a transgenic mouse. Knock one gene out and put the other one in. That¿s the sort of thing NIH or a biotech company could do. We don¿t have the resources or the expertise.
¿Right at the moment,¿ he went on, ¿we¿re satisfied with where we¿ve come at this point. There are other forms of juvenile cataracts also, which we would like to use the same method on as in this case. That is, expressing the mutant protein, and discovering how it produces opacity. Here we showed that it forms aggregates by virtue of these two cysteine groups.¿