A new study provides more insight into cataracts, the leading cause of vision loss and blindness in the elderly, finding that small pieces of a perfectly normal protein become toxic during the aging process.

A cataract results from deterioration in the highly ordered assembly of crystallin proteins in the eye lens. Normally, the ordered structure keeps lenses clear and able to efficiently transmit light. However, crystallins gradually break down during aging, causing the lens to become opaque and scatter light instead. Besides age, other risk factors such as diabetes, ultraviolet radiation, or drugs like corticosteroids can also contribute to cataracts.

Like cataracts themselves, the exact mechanisms governing their formation are cloudy, but Krishna Sharma and colleagues found that tiny bits of crystallin greatly contribute to this process.

They compared a range of human donor lenses and found that aged and cataract lenses accumulated about four times as many short (~10-20 amino acids) crystallin fragments compared to young lenses. These fragments could readily bind full-length crystallins, which disrupted their natural shape and organization and caused them to become insoluble.

Ironically, these tiny fragments are a by-product of the eye's efforts to stay healthy; when a crystallin becomes damaged, other proteins chew it up to remove it; but occasionally the process is incomplete, leaving tiny pieces that can cause greater damage.